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    Chapter 8

    Compaction Using Standard Effort

    8.1 Purpose

    Soil placed as engineering ll (embankments, foundation pads, road bases) must be compacted to the se-

    lected density and water content to ensure the desired performance and engineering properties such as shearstrength, compressibility, or permeability. Also, foundation soils are often compacted to improve their en-gineering properties. Laboratory compaction tests provide the basis for determining the percent compactionand water content needed in the eld, and for controlling construction to assure that the target values areachieved.

    In a geotechnical laboratory you would prepare at least four (preferably ve) specimens with watercontents bracketing the estimated optimum water content. A specimen having a water content close tooptimum would be prepared rst by trial additions of water and mixing and then water contents for the restof the specimens would be selected to provide at least two specimens wet and two specimens dry of optimum,and water contents varying by about 2%, but no more than 4%. In this laboratory exercise each group in yoursection will compact one of the specimens at a specic water content, as directed by the laboratory instructor,and the results from all the groups will be combined later.

    The data, when plotted, represents a curvilinear relationship known as the compaction curve. The valuesof optimum water content and standard maximum dry unit weight are determined from the compaction curve.

    These test methods apply only to soils (materials) that have 20% or less by mass of particles retained onthe No.4 (4.75 mm) sieve.

    8.2 Standard Reference

    ASTM D 698 - Standard test methods for laboratory compaction characteristics of soil using standard effort(12,400 ft-lbf/ft 3 (600 kN-m/m 3 )).

    8.3 Required Materials and Equipment

    Mold - A cylindrical metal mold having a 4.000 0.016 in (101.6 0.4 mm) average inside diameter,a height of 4.584 0.018 in (116.4 0.5 mm) and a volume of 0.0333 0.0005 f t 3 (944 14 cm 3 ).

    Rammer - with free fall of 12 0.05 in (304.8 1.3 mm) from the surface of the specimen. The massof the rammer is 5.5 0.02 lbm (2.5 0.01 kg).

    Sample extruder - A jack for extruding compacted specimens from the mold.

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    Balance - with 1 g readability.

    straight edge - for leveling off compacted sample

    mixing tools - for mixing the sample of soil with increments of water.

    8.4 Procedure

    8.4.1 Specimen preparation

    1. Obtain from your laboratory instructor a sample of the soil to be tested. You will need approximately2 kg.

    2. Without previously drying the sample, pass it through a No. 4 (4.7 mm) sieve. Determine the watercontent of the processed soil. See chapter 2 for the procedure.

    3. Double check the target water content for your specimen with the laboratory instructor.

    4. Calculate how much water should be added or subtracted from your sample to obtain the desired watercontent. Remember to account for the moisture already present in the sample and use the exact valuefor the mass of the soil, not the approximate number.

    5. To add water, spray it into the soil during mixing; to remove water, allow the soil to dry in air at ambienttemperature Mix the soil frequently during drying to maintain an even water content distribution.Thoroughly mix each specimen to ensure even distribution of water throughout and then place in aseparate covered container.

    8.4.2 Compaction

    1. Determine and record the mass of the mold or mold and base plate.

    2. Assemble and secure the mold and collar to the base plate. Place on the concrete oor of the laboratory,NOT on the counters.

    3. The specimen is compacted in 3 layers. Remember that after compaction the layers should be approx-imately equal in thickness and the last layer should extend above the top of the mold, but no more than14 in (6 mm). Place approximately 1/3 of the loose soil into the mold for each layer and spread into alayer of uniform thickness.

    4. Compact each layer with 25 blows. In operating the manual rammer, do not lift the guide sleeveduring the rammer upstroke. Hold the guide sleeve steady and within 5 o of vertical. Apply the blowsat a uniform rate of approximately 25 blows per minute and in such a manner as to provide complete,uniform coverage of the specimen surface. Usually this is achieved by moving the rammer along theperimeter of the mold and using 5 blows to cover the whole area. Then the pattern is repeated for 5times.

    5. After compaction of the rst two layers, trim any soil remaining on the mold walls or extending abovethe compacted surface and include it with the soil for the next layer. Before placing the next layer of soil scarify the surface of the compacted soil with a knife or other suitable tool to avoid separation of the layers at the joints later in the test.

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    8. Compaction Using Standard Effort 43

    6. If the third layer extends above the top of the mold by more than 14 in (6 mm) or below the top of thecompaction mold, the specimen should be discarded.

    7. Following compaction of the last layer, remove the collar and base plate from the mold. A knife maybe used to trim the soil adjacent to the collar to loosen the soil from the collar before removal to avoiddisrupting the soil below the top of the mold.

    8. Carefully trim the compacted specimen even with the top of the mold by means of the straightedgescraped across the top of the mold to form a plane surface even with the top of the mold. Initialtrimming of the specimen above the top of the mold with a knife may prevent the soil from tearingbelow the top of the mold. Fill any holes in the top surface with unused or trimmed soil from thespecimen, press in with the ngers, and again scrape the straightedge across the top of the mold.

    9. Determine and record the mass of the specimen and mold to the nearest gram.

    10. Remove the material from the mold using the sample extruder.

    11. Obtain a specimen for water content by using the whole specimen or a representative sample. Select a

    suitable container and record its weight.12. Weigh the container and the specimen.

    13. Place in the oven for 24 hours. If the entire specimen is used, break it up to facilitate drying.

    14. Record the weight of the oven dried specimen in the container.

    8.5 Calculations

    Post the following information as directed by the laboratory instructor: laboratory section (week day),group (color), date, mass of moist specimen in the mold, mass of mold, water content determination:

    mass of moist soil after compaction and can, mass of can, mass of oven dried specimen an can. Seesection 8.5 for a form to ll.

    Calculate the total unit weight of each specimen:

    t = M t g

    V m=

    (M sm M m )gV m

    (8.1)

    where:

    M t = mass of moist soilM sm = mass of the moist specimen and mold

    M m = mass of the moldV m = volume of the mold (944 cm3 )g = acceleration of gravity (9.807 m/ s2 )

    Calculate water content of each compacted specimen:

    w = M w gM s g

    = (M wsc M sc )

    (M sc Mc (8.2)

    where:

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    M w = mass of waterM s = mass of dry soilM wsc = mass of wet soil and canM sc = mass of dry soils and canM c = mass of canw = water content

    Calculate dry unit weight:

    d = t1 + w

    (8.3)

    Plot the values and draw the compaction curve as a smooth curve through the points (see example,Fig. 3). Plot dry unit weight to the nearest 0.1 lbf ft 3 , (0.2

    kNm 3 ) and water content to the nearest 0.1 %.

    From the compaction curve, determine the optimum water content and maximum dry unit weight.

    Plot the 100% saturation curve.

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    Chapter 9

    Measuring Suction with the Filter PaperMethod

    9.1 Purpose

    The lter paper method has long been used in soil science and engineering practice and it has recently beenaccepted as an adaptable test method for soil suction measurements because of its advantages over othersuction measurement devices. Basically, the lter paper comes to equilibrium with the soil either throughvapor (total suction measurement) or liquid (matric suction measurement) ow. At equilibrium, the suctionvalue of the lter paper and the soil will be equal. After equilibrium is established between the lter paperand the soil, the water content of the lter paper disc is measured. Then, by using lter paper water contentversus suction calibration curve, the corresponding suction value is found from the curve. This is the basicapproach suggested by ASTM Standard Test Method for Measurement of Soil Potential (Suction) UsingFilter Paper (ASTM D 5298). ASTM D 5298 employs a single calibration curve that has been used to inferboth total and matric suction measurements. The ASTM D 5298 calibration curve is a combination of both

    wetting and drying curves. Bulut (2001) demonstrates that the wetting and drying suction calibrationcurves do not match, an observation that was also made by Houston et al. (1994). In this test, the wettingcurve as shown in Figure 9.2 is used because the lter paper becomes wet during the test.

    9.2 Soil Suction Concept

    In general, porous materials have a fundamental ability to attract and retain water. The existence of thisfundamental property in soils is described in engineering terms as suction, negative stress in the pore water.In engineering practice, soil suction is composed of two components: matric and osmotic suction (Fredlundand Rahardjo 1993). The sum of matric and osmotic suction is called total suction. Matric suction comesfrom the capillarity, texture, and surface adsorptive forces of the soil. Osmotic suction arises from the

    dissolved salts contained in the soil water. This relationship can be formed in an equation as follows:

    h t = hm + h (9.1)

    where:

    h t = total suction (kPa)

    hm = matric suction (kPa)

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    h p = osmotic suction (kPa)

    Total suction can be calculated using Kelvins equation, which is derived from the ideal gas law usingthe principles of thermodynamics and is given as:

    h t = RT

    V lnP P o (9.2)

    where:

    h t = total suction

    R = universal gas constant

    T = absolute temperature

    V = molecular volume of water

    P/P o = relative humidity

    P = partial pressure of pore water vaporP o = saturation pressure of water vapor over a at surface of pure water at the same temperature.

    If equation 9.2 is evaluated at a reference temperature of 25 o , the following total suction and relativehumidity relationship can be obtained:

    h t = 137, 182 ln (P/P o) (9.3)

    It can be said, in general, that in a closed system under isothermal conditions the relative humidity maybe associated with the water content of the system such as 100% relative humidity refers to a fully saturatedcondition. Therefore, the suction value of a soil sample can be inferred from the relative humidity and

    suction relationship if the relative humidity is known. In a closed system, if the water is pure enough, thepartial pressure of the water vapor at equilibrium is equal to the saturated vapor pressure at temperature, T.However, the partial pressure of the water vapor over a partly saturated soil will be less than the saturationvapor pressure of pure water due to the soil matrix structure and the free ions and salts contained in the soilwater (Fredlund and Rahardjo 1993).

    In engineering practice, soil suction has usually been calculated in pF units (Schoeld, 1935) (i.e., suc-tion in pF = log10 |suction in cm of water |). However, soil suction is also currently being represented inlog(kP a ) unit system (Fredlund and Rahardjo 1993) (i.e., suction in log(kP a ) = log10 |suction in kPa |).The relationship between these two systems of units is approximately suction in log(kP a ) = suction in pF- 1. Matric suction can be calculated from pressure plate and pressure membrane devices as the differencebetween the applied air pressure and water pressure across a porous plate. Matric suction can be formed in arelationship as follows:

    hm = (ua uw ) (9.4)

    where:

    hm = matric suction

    ua = applied air pressure

    uw = free water pressure at atmospheric condition

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    9. Measuring Suction with the Filter Paper Method 49

    The osmotic suction of electrolyte solutions, that are usually employed in the calibration of lter papersand psychrometers, can be calculated using the relationship between osmotic coefcients and osmotic suc-tion. Osmotic coefcients are readily available in the literature for many different salt solutions. Table 1gives the osmotic coefcients for several salt solutions. Osmotic coefcients can also be obtained from thefollowing relationship (Lang 1967):

    = wvmw

    lnP P o

    (9.5)

    where:

    f = osmotic coefcient

    v = number of ions from one molecule of salt (i.e., v = 2 for NaCl, KCl, NH4 Cl and v = 3 for Na2 SO4 ,CaCl 2 , Na2 S2 O3 ,etc.)

    m = molality

    w = molecular mass of water

    w = density of water

    The relative humidity term (P/Po ) in eq. 9.5 is also known as the activity of water ( aw ) in physicalchemistry of electrolyte solutions. The combination of eq. 9.2 and eq. 9.5 gives a useful relationship thatcan be adopted to calculate osmotic suctions for different salt solutions:

    h = vRTm (9.6)

    9.3 Required Materials and Equipment

    Schleicher & Schuell No. 589-WH lter paper Sensitive balance with accuracy of 0.0001 g

    Constant temperature container (or cooler)

    moisture tins and glass jars

    PVC rings, electrical tape

    tweezers and gloves

    Oven and aluminum block

    9.4 Procedure

    A testing procedure for total suction measurements using lter papers can be outlined as follows:

    1. At least 75% by volume of a glass jar should be lled with the soil; the smaller the empty spaceremaining in the glass jar, the smaller the time period that the lter paper and the soil system requireto come to equilibrium.

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    2. A ring type support, which has a diameter smaller than lter paper diameter and about 1 to 2cm inheight, is put on top of the soil to provide a non-contact system between the lter paper and the soil.Care must be taken when selecting the support material; materials that can corrode should be avoided,plastic or glass type materials are much better for this job.

    3. Two lter papers one on top of the other are inserted on the ring using tweezers. The lter papersshould not touch the soil, the inside wall of the jar, and underneath the lid in any way.

    4. Then, the glass jar lid is sealed very tightly with plastic tape.

    5. Steps 1, 2, 3, and 4 are repeated for every soil sample.

    6. After that, the glass jars are put into the ice-chests in a controlled temperature room for equilibrium.

    Researchers suggest a minimum equilibrating period of one week (ASTM D 5298; Houston et al., 1994;Lee 1991). After the equilibration time, the procedure for the lter paper water content measurements canbe as follows:

    1. Before removing the glass jar containers from the temperature room, all aluminum cans that are usedfor moisture content measurements are weighed to the nearest 0.0001 g. accuracy and recorded.

    2. After that, all measurements are carried out by two persons. For example, while one person is openingthe sealed glass jar, the other is putting the lter paper into the aluminum can very quickly (i.e., in afew seconds) using tweezers.

    3. Then, the weights of each can with wet lter paper inside are taken very quickly.

    4. Steps 2 and 3 are followed for every glass jar. Then, all cans are put into the oven with the lids half-open to allow evaporation. All lter papers are kept at 105 5o C temperature inside the oven for atleast 10 hours.

    5. Before taking measurements on the dried lter papers, the cans are closed with their lids and allowedto equilibrate for about 5 minutes. Then, a can is removed from the oven and put on an aluminumblock (i.e., heat sinker) for about 20 seconds to cool down; the aluminum block functions as a heatsink and expedites the cooling of the can. After that, the can with the dry lter paper inside is weighedvery quickly. The dry lter paper is taken from the can and the cooled can is weighed again in a fewseconds.

    6. Step 5 is repeated for every can.

    9.5 Soil Matric Suction Measurements

    Soil matric suction measurements are similar to the total suction measurements except instead of insertinglter papers in a non-contact manner with the soil for total suction testing, a good intimate contact shouldbe provided between the lter paper and the soil for matric suction measurements. Both matric and totalsuction measurements can be performed on the same soil sample in a glass jar as shown in Fig. 1. A testingprocedure for matric suction measurements using lter papers can be outlined as follows:

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    9. Measuring Suction with the Filter Paper Method 51

    Figure 9.1: Assembly for total and matric suction measurements.

    9.6 Procedure

    1. A lter paper is sandwiched between two larger size protective lter papers. The lter papers used insuction measurements are 5.5cm in diameter, so either a lter paper is cut to a smaller diameter andsandwiched between two 5.5cm papers or bigger diameter (bigger than 5.5cm) lter papers are usedas protection.

    2. Then, these sandwiched lter papers are inserted into the soil sample in a very good contact manner(i.e., as in Fig. 1). An intimate contact between the lter paper and the soil is very important.

    3. After that, the soil sample with embedded lter papers is put into the glass jar container. The glasscontainer is sealed up very tightly with plastic tape.

    4. Steps 1, 2, and 3 are repeated for every soil sample.

    5. The prepared containers are put into ice-chests in a controlled temperature room for equilibrium.

    9.7 Calculations

    After obtaining all of the lter paper water contents, gure 9.2 is employed to get total suction and matricvalues of the soil samples.

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    Figure 9.2: Filter paper wetting calibration curve.

    Paper Suction Determination

    Sample No. Project

    Boring No. Location

    Depth

    Description of sample

    Date Tested by

    Total SuctionPaper

    Matric SuctionPaper

    Container NumberMass of container (g)Mass of wet paper + container (g)Mass of wet lter paper (g)Mass of hot container (g)Mass of dry lter paper (g)Mass of water in lter paper (g)Water content of lter paper (%)

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    Chapter 10

    Hydraulic Conductivity

    10.1 Purpose

    Hydraulic conductivity is the parameter that tells us how fast water can ow through soil. This quantity is

    measured to determine if a particular soil is a suitable material for a levee, dam or landll liner, or lter.During this laboratory both the constant head and the falling head methods will be used.

    10.2 Standard Reference

    ASTM D 2434 - Standard test method for permeability of granular soils (constant head).

    10.3 Fundamental Test Conditions

    The following test conditions are prerequisites for laminar ow of water through granular soils, under

    constant-head conditions:

    Continuity of ow with no soil volume change during a test.

    Flow with the soil voids saturated with water and no air bubbles in the soil voids.

    Flow is steady state with no change in hydraulic gradients.

    Direct proportionality of velocity of ow with hydraulic gradients below certain values, at whichturbulent ow starts.

    All other types of ow involving partial saturation of soil voids, turbulent ow, and unsteady state of

    ow are transient in character and yield variable and time-dependent coefcients of permeability; therefore,they require special test conditions and procedures.

    10.4 Constant head test

    10.4.1 Required Materials and Equipment

    Permeameter - Specimen cylinders with minimum diameter of 8 or 12 times the maximum particlesize. The permeameter should be tted with a porous disk at the bottom with a permeability greater

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    Figure 10.1: Schematic of constant head test set-up.

    than that of the soil specimen, but with openings small enough to prevent movement of the soil par-ticles. The permeameter should be tted with manometer outlets for measuring head loss, h , over alength, L, equivalent to at least the diameter of the cylinder.

    Sample - A representative sample of air-dried granular soil containing less than 10% of the materialpassing the No. 200 sieve.

    Constant head board - Board including manometer tubes with scales for measuring head of water anda water reservoir.

    Plastic tubing

    Stopwatch

    Thermometer

    10.4.2 Procedure

    1. Unscrew the three nuts on the top of the permeameter cell and remove the top. Make sure the #200mesh screen is covering the two ports on the inside middle of the cell.

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    10. Hydraulic Conductivity 55

    2. Place one of the porous stones in the bottom of the cell. Fill the cell with the soil sample. Place theother porous stone on top. The top of this stone should be about 1/4 inch below the top of the cell.

    3. Make sure the surface where the O-ring seals off the cell is clean and replace the top. Evenly tighteneach of the nuts on the top of the cell.

    4. Connect a tube coming out of the reservoir on the board to the water faucet.

    5. Place the other tube coming out of the reservoir over the sink so that water will be allowed to drainout.

    6. Connect tubes from points A and B on the permeameter cell to the two manometers. The distancebetween these two points on our sample is 10 cm.

    7. Connect a tube from the needle valve on the board to the lower ball valve on the cell.

    8. Fill the reservoir with water. Adjust the water tap so that the level in the cup remains the same andwater is draining into the sink. This gives us constant head.

    9. Let the water ow slowly from the reservoir, into the cell, through the bottom porous stone, soilsample, top stone, out of the top of the cell. The water will replace the voids within our sample.

    10. Get all of the bubbles out of the tubes by tapping them. De-air the lines going to the manometer tubes.Close the top ball valve and open the lower ball valve. Crack the petcock valve to purge out any airbubbles and then close it off.

    11. Open the top ball valve on the cell and watch water come out of the top of the cell. The needle valveon the board controls the ow out of the top of the cell.

    12. Record the differential reading between the two manometers.

    13. Using a beaker, collect 100 mL of water from the water coming out of the top of the cell. Time howlong it takes to get 100 mL using the stopwatch.

    14. Repeat this process collecting 200 and then 300 mL of water.

    15. Take the temperature of the water in the constant head cup. Our data will only be good for water atthis specic temperature.

    10.4.3 Calculations

    Determine the average time it took to collect 100 mL of water.

    Calculate the cross-sectional area of our soil sample. The diameter of the sample is 2.5 in or 6.35 cm.

    Calculate the coefcient of permeability, k.

    k = QLA ht

    (10.1)

    Where

    Q=volume of water collectedL=sample height

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    A=cross-sectional area of soil specimen

    h=differential reading between h0 and h1t=duration of water collection

    Calculate the corrected k value for the temperature you recorded.

    kcorrected = k T C 20 C

    (10.2)

    10.5 Falling head test

    10.5.1 Required Materials and Equipment

    Permeameter

    Sample - use the same sample prepared for the constant head test

    Calibrated stand pipe

    Plastic tubing

    Calipers

    Stopwatch

    10.5.2 Procedure

    1. Connect a tube between points A and B on the permeameter cell. We bypass the manometers this time.

    2. We have a new sample height for this test. Measure the height using the calipers from the top of thebottom porous stone to the bottom of the top stone.

    3. Attach a tube from the top ball valve on the cell to the calibrated stand pipe valve.

    4. Open both of these valves.

    5. Attach a tube from the water faucet to the lower ball valve on the cell.

    6. Let the water ow through the cell, out of the top, and into the stand pipe.

    7. Purge any bubbles from the tubes by opening the petcock valve on the permeameter cell.

    8. Allow water to ow slowly through the stand pipe and out of the funnel at the top.

    9. When there are no more bubbles, disconnect the hose from the faucet and close the ball valve at thesame time.

    10. Time how long it takes the water to drop a known distance in the stand pipe.

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    10. Hydraulic Conductivity 57

    Figure 10.2: Schematic of falling head test set-up.

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    10.5.3 Calculations

    Calculate the cross-sectional area of the tube. The inner diameter of the tube is 3/16 in.

    Calculate the coefcient of permeability, k.

    k = aLAt ln

    h0h1 (10.3)

    Where

    a=cross-sectional area of tube

    L=sample height

    A=cross-sectional area of soil specimen

    t=elapsed time

    h0 =initial head h1 =nal head

    Calculate the corrected k value for the temperature you recorded using 10.2 .

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    10. Hydraulic Conductivity 59

    Constant Head Data

    H =L =Temp =

    mL Collected Time(s)

    Falling Head Data

    L =h0 =h1 =Temp =

    Trial Number Time(s)

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    Chapter 11

    Instrumentation and Calibration

    11.1 Purpose

    The purpose of this laboratory exercise is to familiarize students with some of the basic instrumentation

    commonly used in geotechnical engineering laboratories to measure the mechanical properties of soils. Forthe purposes of this class we will assume that electronic instrumentation is the preferred method for allmeasurements in our experiments. We will familiarize ourselves with transducers and calibrate them forfuture usa in our later laboratories.

    Typically, the physical quantities that we need to measure when conducting experiments on soils aretemperature, force, displacement, and pressure. The choice of which sensor to use for a particular task,in a particular situation depends on two main considerations: a) technical characteristics of the sensor; b)a cost-benet analysis, which includes considerations on ease of use as well. Temperature is the easiestto measure accurately and for all experiments in this laboratory will be measured with a simple mercurythermometer or a hand held digital thermometer. In many research laboratories the instrumentation is kept ata carefully controlled, constant temperature, because many sensors are sensitive to temperature, even if theyare not meant to measure it. Commercial geotechnical laboratories are not usually equipped with constanttemperature rooms because the expense would not be justied in normal circumstances.

    We must consider cost, simplicity, technical characteristics and time to decide whether to use relativelyinexpensive instrumentation, such as mechanical dial indicators(for displacement and force measurements)and simple pressure gages, or the more expensive, more accurate and automated electronic instrumentation.These days, the cost of electronics is usually not an impediment to the use of electronic instrumentation forall the measuring needs in a soil mechanics laboratory.

    11.2 Transducers

    A transducer is a device that converts energy from one form to another. An electronic transducer has either

    an input or an output that is electrical in nature, such as a voltage or a current. In our case, we are interestedin a transducer that senses a physical change (force, displacement, pressure) and converts it to an electricalsignal, directly related to that physical change. We call this type of transducer a sensor. The principle of electronic instrumentation is to use an electrical sensor to detect change in a physical quantity and outputan electrical signal to a measuring device. This electrical signal becomes convenient to use as the inputsignal to a measurement system, such as an elaborate voltmeter, a chart recorder, an oscilloscope, or evenbetter, a computer that will record all the information over time, so it can be retrieved a later date. Therelationship between measured output voltage and physical quantity being sensed is the response function,usually expressed in terms of a linear relationship obtained through a calibration process.

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    A good example: you are driving a car at the speed indicated on your speedometer. The rotation rate of the cars wheel is detected by a sensor, which outputs a voltage that increases with increasing speed. Youcould then use a digital voltmeter to read the output of the sensor. Obviously, you would get a speedingticket, because the voltage alone does not tell you the speed. You also need a relationship between measuredvoltage and speed because you would rather see your speed directly in miles/hr. In order to make this

    conversion to miles/hr, one has to determine the correlation between the sensors output voltage and theknown speed. Usually, the voltage is measured at several different speeds in order to maximize the accuracyof the conversion relationship. Lastly, thanks to the relationship between voltage and speed, the voltmetercan be re-scaled in miles/hr rather than volts.

    In conclusion, we used a sensor to detect a certain physical quantity and then converted the electricalvoltage output from the sensor (through calibration against a known standard) to engineering units that aremore meaningful and useful to us.

    In general, when examining the technical characteristics of a sensor we consider:

    Precision, or the ability to detect small changes in the measured quantity reliably, and the ability tomeasure the same value under repeated identical conditions. Example: a typical measuring tape has aprecision of 1/ 4in., because that is the closest distance between two marks.

    Accuracy, the difference between the measured quantity and the true value, as dened by acceptedstandards. Example: when you weigh yourself with a cheap scale the precision may be 1 lb, but theneedle indicates a weight that may not be accurate.

    Range

    Stability

    Noise

    Temperature coefcient

    Linearity error

    Physical ruggedness, and size.

    11.3 Calibration

    You will calibrate two transducers this week: a force transducer and a displacement transducer. Both willbe used in future laboratories during the rest of the semester. Output accuracy of these transducers is onlyas good as the quality of the transducer, accuracy of the standard used in comparison, and the care taken incalibrating the transducer to the known standard. In this weeks laboratory we will calibrate both the force(in N) and displacement transducer (in mm), each to a known physical standard. We will vary the physical

    parameters over a known range of the instrument and record the voltage output at each point over that range.We can then generate a best-t line through those points by plotting the physical input parameter (mm or N)against the resulting voltage output reading from the transducer. Plot the input parameter in physical units onthe Y axis and the resultant output reading on the X axis. Using any appropriate software package, determinethe best t to your data. Usually, a linear t is the most appropriate and will have the form:

    Physical Quantity = CF * Output Voltage + ZEROWhere: CF is the calibration factor; ZERO, this constant ensures that when the physical quantity is

    zero (for example: no force applied) or in the initial position (example: we need to measure the differentialdisplacement, not the absolute position) the formula will give a zero reading. Notice that in the case of the

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    voltage readings. The process is conceptually the same as before, but you will be using the load frames toapply the load to the transducer.

    The procedure and safe usage of the load frames will explained in detail during class. Be very careful inapproaching the load frames and while hanging the weights from the loading arm. The counterweight is notpermanently attached to the frame mechanism and will fall to the ground (or on your head!) if the loading

    arm is lifted to abruptly. Add the weights by extending your arm and reaching towards the hanger rather thanbending and placing your head under the loading arm. Move slowly and be aware of your surroundings.Remember to transform the units from kgf to Newton (N).

    11.4 Report

    For the report, be sure to include:

    Memo to Mike Linger

    Include a table with the data and the plot on the same page. Display the equation of the best t lineand R2 value taken to 5 decimal places. One page for each transducer.

    Attach raw data.

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    11. Instrumentation and Calibration 65

    Calibration Data Sheet (LSCT)

    Date Tested by

    Transducer type Serial number

    ExcitationVoltage

    MicrometerReading

    Incrementaldisplacement

    Transduceroutput

    Change intransduceroutput

    (V) (in) (in) (mV) (mV)

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    Calibration Data Sheet (Force)

    Date Tested by

    Transducer type Serial number

    ExcitationVoltage

    Mass Added IncrementalApplied Force

    Transduceroutput

    Change inTransducerOutput

    (V) (kgf) (N) (mV) (mV)

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    Chapter 12

    Flow Nets

    12.1 Denitions

    Flow net a graphical representation of the 2 D ow of water through soils

    Flow line the ow path of a particle of water

    Equipotential line a line representing constant head

    12.2 Flow Net Facts

    The area between two ow lines is called a ow channel.

    The rate of ow in a ow channel is constant.

    The velocity of ow is normal to equipotential lines.

    The difference in head between two equipotential lines is called the potential drop or head loss.

    12.3 Drawing Flow Nets

    Identify prexed conditions, noting starting directions of lines.

    Draw trial family of ow lines (or equipotentials) consistent with prexed conditions.

    Keeping the lines you just drew, sketch rst trial ow net. Make all lines intersect other set of lines at90 degrees.

    Erase and redraw lines until all gures are square. Subdivide as desired for detail and accuracy.

    12.4 Rules for Sketching Flow Nets

    Flow lines must intersect equipotential lines at right angles.

    The area between ow lines and equipotential lines must be curvilinear squares. An inscribed circleshould be able to be drawn that touches each side of the square.

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    Figure 12.1: Budhu, 2000.

    Figure 12.2: Cedergren, 1989.

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    12. Flow Nets 69

    Figure 12.3: Cedergren, 1989.

    Flow lines cannot intersect other ow lines.

    Equipotential lines cannot intersect other equipotential lines.

    The more ow lines and equipotential lines drawn, the more accurate your results. However, the morelines, the more difcult it will be to draw the ow net. Drawing a few will allow you to obtain asuitable solution.

    12.5 Common Mistakes

    12.6 Example

    12.7 Example Problem

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    Figure 12.4: Cedergren, 1989.

    Figure 12.5: Budhu, 2000.

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    Chapter 13

    State of Stress: Mohrs Circle

    13.1 Purpose

    In order to describe the state of stress at a point, it is necessary to characterize both normal and shear stresses

    acting on any arbitrary plane passing through that point. A three-dimensional state of stress is fully charac-terized by six components (3 normal stresses and 3 shear stresses). In the case of geotechnical engineering,we often limit the analysis to two-dimensional states of stress and strain. Mohrs circle construction is oneof the simplest methods to examine the state of stress and strain in the soil.

    13.2 Two-Dimensional States of Stress

    The 2-D state of stress at a point can be represented by the normal and shear stresses acting on the faces of an innitesimal plane element, such as that in gure 13.1 . The normal stress x acts on the plane normalto the x-axis in the direction parallel to the x-axis. The shear stress xy act on the same plane, but in thedirection parallel to the y-axis. The directions of y and yx are dened similarly. By equilibrium it follows

    that xy = yx .In soils, normal stresses are usually compressive and, by convention, compression is taken as positive(notice that the opposite convention is used in continuum mechanics). Then, stresses x and y in gure13.1 are positive quantities and the shear stresses are also positive as marked.

    If the stresses x , y , xy are known, it is possible to calculate the magnitudes of the stresses ( m , mn )on some arbitrary plane at an angle with the x-axis 13.1 .

    13.3 Derivation of stress transformation equations

    The system of axes (m,n) in gure 13.1 is rotated by an angle in the positive (counterclockwise) directionfrom the system of axes (x,y). Assume segment BC has length of L, then:

    AB = L sin() (13.1a)

    BC = L cos() (13.1b)

    Use equilibrium of forces in the m-direction and n-direction to derive:

    m L = x cos(L cos ) + y sin (L sin ) + xy sin (L cos ) + xy cos(L sin ) (13.2a) mn L = x sin()(L cos ) + y cos (L sin ) + xy cos(L cos ) + xy sin (L sin ) (13.2b)

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    13.4 Mohrs Circle of Stress

    For the purpose of plotting Mohrs circles, and for this purpose alone , we adopt the convention that counter-clockwise shear stresses are taken as positive quantities. Thus, the counter-clockwise shear stress xy ingure 13.1 is positive and the clockwise shear stress yx is negative when constructing the Mohrs circle ingure ?? .

    13.4.1 The Pole Method

    The pole of a Mohrs circle is a unique point on the Mohrs circle characterized by an important property:

    if a line is drawn through the pole with the direction of any plane in the physical space, itintersects the Mohrs circle at a point that denes the state of stress on that plane.

    By reversing the denition we can locate the pole as the intersection of the line representing the physicalplane on which the stresses are acting (line QP or RP) and the Mohrs circle. Line QP is parallel to the plane

    on which zz and zx act and is parallel to the x-axis. The point P where the line intersects the Mohrs circleis the pole, P. Similarly, RP is parallel to the z-axis and it also identies the pole.We may now use the pole of the circle to calculate the state of stress on any plane through the material.

    For example, the normal and shear stresses ( , ) on the plane inclined at an angle to the x-axis. Wedraw a line, PN, trough P inclined at with line QP (and the x-axis). The stresses at point N are the stressesacting on that plane.

    13.5 Principal Stresses and Principal Planes

    The points at which the Mohrs circle crosses the -axis represent planes on which the shear stress is zero andthe normal stress is either a minimum or a maximum. These planes are known as principal planes ande thecorresponding stresses as principal stresses . From the geometry of the Mohrs circle the principal stressesoccur on two orthogonal planes, therefore the stresses must also be orthogonal (see gure 13.3 .

    In three-dimensional stress analysis there are three principal stresses and three principal planes. Thesewill be denoted by 1 , 2 and 3 , and it is usual practice to dene 1 2 3 ; 1 is the major principalstress, 2 is the intermediate and 3 is the minor.

    When the layering of soils is horizontal, the vertical and horizontal stresses ( v , v , h and h ) areprincipal stresses and the vertical and horizontal planes are principal planes. Since it is an axi-symmetricstate of stress (the horizontal stress is the same independent of the direction in the horizontal plane) v = 1and h = 2 = 3 .

    13.6 Mohrs Circles of Total and Effective Stress

    So far we havent differentiated between total and effective stresses. If the stresses acting on the element ingure ?? are total stresses, the effective stresses can be calculated using the principle of effective stress:

    = u

    The effective stress circle has the same diameter of the total stress circle, but it is translated to the left bythe amount of the pore pressure (see gure 13.4 ). By examining the circles, we note that:

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    13. State of Stress: Mohrs Circle 75

    Figure 13.3: Principal planes and principal stresses

    = u

    =

    Thus, for a given state of total stress, changes in pore pressure have no effect on the shear stresses.

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    Figure 13.4: Mohrs circles of total and effective stress.

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    Figure 13.6: Example 2.

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    13. State of Stress: Mohrs Circle 79

    A loading due to a proposed embankment is estimated to be equivalent to a uniform load of 100kPa as shown in gure13.7 . The unit weight of the foundation material is 16kN/m 3 , and K o is 0.5. The water table is at 5m depth.

    Draw the Mohrs circle for the initial stresses in the soil, before the construction of the embankment.

    Two exiting pipelines run parallel to the embankment at a depth of 3m, as shown in gure 13.7 . We want tomake sure that the stress increase in the soil due to the construction of the embankment is not going to damagethe pipelines. What is the increase in stresses at A and B? Draw the new Mohrs circles for both points.

    What is the magnitude and the direction of the principal stresses at point A and B after construction?

    Figure 13.7: Schematic of the embankment and location of the pipelines.

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    Chapter 14

    Direct Shear Test of Soils UnderConsolidated Drained Conditions

    The following method departs from the ASTM Standard D3080, copyright c American Society for Testing

    and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959

    14.1 Purpose

    This test method covers the determination of the consolidated drained shear strength of a soil material indirect shear. The test is performed by deforming a specimen at a controlled strain rate on or near a singleshear plane determined by the conguration of the apparatus. Generally, three or more specimens are tested,each under a different normal load, to determine the effects upon shear resistance and displacement, andstrength properties such as Mohr strength envelopes. Shear stresses and displacements are nonuniformlydistributed within the specimen. An appropriate height cannot be dened for calculation of shear strains.Therefore, stress-strain relationships or any associated quantity such as modulus, cannot be determined from

    this test.

    14.2 Terminology

    Relative Lateral Displacement-The horizontal displacement of the top and bottom shear box halves.

    Failure-The stress condition at failure for a test specimen. Failure is often taken to correspond to themaximum shear stress attained, or the shear stress at 15 to 20 percent relative lateral displacement.Depending on soil behavior and eld application, other suitable criteria may be dened.

    14.3 Apparatus

    Shear Device-A device to hold the specimen securely between two porous inserts in such a way thattorque is not applied to the specimen. The shear device shall provide a means of applying a normalstress to the faces of the specimen, for measuring change in thickness of the specimen, for permittingdrainage of water through the porous inserts at the top and bottom boundaries of the specimen, andfor submerging the specimen in water. The device shall be capable of applying a shear force to thespecimen in water. The device shall be capable of applying a shear force to the specimen along apredetermined shear plane (single shear) parallel to the faces of the specimen. The frames that holdthe specimen shall be sufciently rigid to prevent their distortion during shearing. The various parts

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    of the shear device shall be made of material not subject to corrosion by moisture or substances withinthe soil, for example, stainless steel, bronze, or aluminum, etc. Dissimilar metals, which may causegalvanic action, are not permitted.

    Shear Box, a shear box, either circular or square, made of stainless steel, bronze, or aluminum, withprovisions for drainage through the top and bottom. The box is divided vertically by a horizontal planeinto two halves of equal thickness which are tted together with alignment screws. The shear box isalso tted with gap screws, which control the space (gap) between the top and bottom halves of theshear box.

    Porous Inserts, Porous inserts function to allow drainage from the soil specimen along the top andbottom boundaries. They also function to transfer horizontal shear stress from the insert to the top andbottom boundaries of the specimen. Porous inserts shall consist of silicon carbide, aluminum oxide, ormetal which is not subject to corrosion by soil substances or soil moisture. The proper grade of insertdepends on the soil being tested. The permeability of the insert should be substantially greater thanthat of the soil, but should be textured ne enough to prevent excessive intrusion of the soil into thepores of the insert. The diameter or width of the top porous insert or plate shall be 0.01 to 0.02 in. (0.2

    to 0.5 mm) less than that of the inside of the ring. If the insert functions to transfer the horizontal stressto the soil, it must be sufciently coarse to develop interlock. Sandblasting or tooling the insert mayhelp, but the surface of the insert should not be so irregular as to cause substantial stress concentrationsin the soil.

    Device for Applying and Measuring the Normal Force-The normal force is applied by a lever loadingyoke which is activated by dead weights (masses) or by a pneumatic loading device. The device shallbe capable of maintaining the normal force to within 61 percent of the specied force quickly withoutexceeding it.

    Device for Shearing the Specimen-The device shall be capable of shearing the specimen at a uniformrate of displacement, with less than 65 percent deviation, and should permit adjustment of the rateof displacement from 0.0001 to 0.04 in./min (.0025 to 1.0 mm/min). The rate to be applied dependsupon the consolidation characteristics of the soils (see 9.12.1). The rate is usually maintained with anelectric motor and gear box arrangement and the shear force is determined by a load indicating devicesuch as a proving ring or load cell. 6.4.3 The weight of the top shear box should be less than 1 percentof the applied normal force: this may require that the top shear box be modied and supported bycounter force.

    Shear Force Measurement Device-A proving ring or load cell accurate to 0.5 lbf (2.5 N), or 1 percentof the shear force at failure, whichever is greater.

    Shear Box Bowl-A metallic box which supports the shear box and provides either a reaction againstwhich one half of the shear box is restrained, or a solid base with provisions for aligning one half of the shear box, which is free to move coincident with applied shear force in a horizontal plane.

    Miscellaneous Equipment, including timing device with a second hand, distilled or demineralizedwater, spatulas, knives, straightedge, wire saws, etc., used in preparing the specimen.

    14.4 Procedure

    1. Assemble the shear box.

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    14. Direct Shear Test of Soils Under Consolidated Drained Conditions 83

    2. Place moist porous inserts over the exposed ends of the specimen in the shear box; place the shear boxcontaining the undisturbed specimen and porous inserts into the shear box bowl and attach the shearbox.

    3. Connect and adjust the shear force loading system so that no force is imposed on the load measuringdevice.

    4. Properly position and adjust the horizontal displacement measurement device used to measure sheardisplacement. Obtain an initial reading or set the measurement device to indicate zero displacement.

    5. Place a moist porous insert and load transfer plate on the top of the specimen in the shear box.

    6. Place the normal force loading yoke into position and adjust it so the loading bar is horizontal.

    7. Apply a small normal load to the specimen. Verify that all components of the loading system are seatedand aligned. The top porous insert and load transfer plate must be aligned so that the movement of theload transfer plate into the shear box is not inhibited. Record the applied vertical load and horizontalload on the system.

    8. Attach and adjust the vertical displacement measurement device. Obtain initial reading for the verticalmeasurement device and a reading for the horizontal displacement measurement device.

    9. Select the appropriate displacement rate.

    10. Shear the specimen, until the shear resistance measured by the load transducer levels off indicatingthat the specimen has failed.

    14.5 Calculation

    Calculate the nominal shear stress:

    = F A

    where:

    = nominal shear stress ( lbf/in 2 ,kPa),

    F = shear force (lbf, N),

    A = initial area of specimen ( in 2 , mm 2 ).

    Calculate the normal stress acting on the specimen:

    n = N A

    where:

    n = normal stress ( lbf/in 2 ,kPa ),

    N = normal vertical force acting on the specimen (lbf, N),

    A = initial area of specimen ( in 2 , mm 2 ).

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    Chapter 15

    One-Dimensional Consolidation

    15.1 Purpose

    A surface load, for example due to the construction of a building, results in increased stresses in the under-

    lying soils. The increase in stress also causes settlements. When the soils are ne grained and saturated theincrease in total stress is carried by the water, as excess pore pressure. Since these soils have low hydraulicconductivity the excess pore pressure will dissipate slowly and the settlement will be delayed in time.

    The consolidation test, or oedometer test, is used to determine the parameters that can be used to estimateboth the magnitude and the time rate of the settlements. The test is performed on a cylindrical specimen,constrained laterally by a ring and allowed to compress under a constant load. The load is held on thesample for 24 hours or until all excess pore pressure is dissipated. During this time the change in height ismeasured. The load is usually doubled at the end of the 24 hour period and the process repeated. Usually 5or 6 load increments are applied and then data are taken during one unloading step. The measurements areused to determine the relationship between the effective stress and void ratio or strain, and the rate at whichconsolidation can occur.

    This test method uses conventional consolidation theory based on Terzaghis consolidation equation tocompute the coefcient of consolidation, cv . The analysis is based on the following assumptions:

    The soil is saturated and has homogeneous properties.

    The ow of pore water is in the vertical direction.

    The compressibility of soil particles and pore water is negligible compared to the compressibility of the soil skeleton.

    The stress-strain relationship is linear over the load increment.

    The ratio of soil permeability to soil compressibility is constant over the load increment.

    Darcys law for ow through porous media applies.

    15.2 Standard Reference

    ASTM D 2435 - Standard test method for one-dimensional consolidation properties of soils.

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    Figure 15.1: Consolidometer assembly.

    15.2.1 Required Materials and Equipment

    Load Device - two systems are available in our laboratory: (a) pneumatic load frame; (b) mechanicalload frame.

    Consolidometer - the consolidometer is the device holding the sample during the test. It is composedof various parts, shown in gure 15.1 .

    Porous disks - to allow loading of the sample and free drainage at one or both ends of the specimen.

    Specimen trimming device - The specimen ring has a sharp edge that can be used as a cutter and can beused for trimming the sample down to the inside diameter of the consolidometer ring with a minimum

    of disturbance .

    Deformation indicator - to measure change in specimen height. Usually, two independent measure-ment devices are used. One is a simple dial gauge and the other is a linear strain conversion transducer(or LSCT).

    Miscellaneous equipment - distilled or demineralized water, water content cups, spatulas, knives, andwire saws, used in preparing the specimen.

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    15. One-Dimensional Consolidation 89

    15.3 Preliminary Preparation

    It is a good idea to become familiar with the equipment before getting started with specimen preparation.In addition, look at the data sheet to make sure you remember to take all the measurements that are neededlater.

    15.3.1 The consolidometer and the dial gauge

    1. Unassemble the consolidometer and become familiar with its parts. Practice assembling the con-solidometer again, pay attention to the right t of the parts.

    2. Place the porous disk in distilled water and gently tap them to improve their saturation by driving airbubbles out.

    3. Weigh and measure the diameter and height of the consolidation ring. Take measurements at 3 differentlocation around the ring. Enter the values in the data sheet.

    4. Record the serial number of the LSCT on your data sheet and write down the calibration factor.

    5. Each member of the team must become familiar with the workings of a dial gauge. Our dial gaugescan measure up to 1 in displacement in increments of 0.001 in. The dial gauge typically has two dials:a large one with subdivisions from 0 to 99, and a small one with subdivisions from 0 to 9 (see gure15.2 ). The small dial indicates the 1/10 in for the reading and the large one gives the 1/1000 in reading.

    The dial gauge in gure 15.2 is measuring 0 in. The dial gauge starts at 0 when the rod is completelyextended and indicates 1 in when the rod is all up if you reading on the back numbers in the large dial.This means that the readings during the test will go backward from 1 in to 0 during the test since youwant the rod to be almost all up at the beginning of the test.

    I strongly recommend you do not use red numbers to take your dial readings. Play with the dial gaugeto gure out why.

    Always remember that reading a dial gauge may seem obvious, but every semester some group dataof this week-long test must be thrown away because someone failed to take the right reading. I am notkidding.

    15.3.2 The data acquisition software

    1. Now look at the software. Start the GGdatalogger. It should appear as in gure 15.3 . There are3 windows on the screen because 3 LSCTs will be connected to each laptop. Therefore 3 differentgroups will refer to the same laptop. Select one of the ports in the signal processing box (the black box under the laptop) and plug in your LSCT. Write the channel number in the data sheet. Be carefulbecause data acquisition channel numbers are not sequential. The plugs are labelled but you may need

    to look under the laptop.2. Before you can run the software you need to enter the le names for your data les with path. Right

    now we just need to test the system and we are not going to save the data, so pick any name you wantas long as you can remember what it is. Before you start the real test you will want to look at this leand then delete it. All the 3 paths must be lled, otherwise you get an error message.

    3. You also need to check that the data acquisition system is actually reading the right channels. Thechannels listed in the text box labeled I/O should match the channel numbers that will be plugged in(typically: 0,8,1). Make sure your channel number is listed.

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    Figure 15.4: Location of the hidden windows in the GGDataLogger and close-up of the hidden section.

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    15. One-Dimensional Consolidation 93

    1. Use the ring to trim the specimen to the desired size. Do not slam the ring down into the soil asa cookie cutter. Remember you are trying to minimize disturbance and this would most denitelydisturb the soil. The appropriate procedure requires you to trim the soil to a gentle taper in front of the cutting edge. After the taper is formed, advance the cutter a small distance. Continue until thespecimen protrudes from the ring.

    2. Trim the specimen ush with the at ends of the ring. Use a thin wire saw to minimize smearing.

    3. Weigh the ring and the specimen to determine the initial wet mass of the specimen, M T o (see datasheet).

    4. Obtain the natural water content of the soil from material trimmed adjacent to the test specimen. It isbest to use the traditional oven method. Fill in the appropriate information in the data sheet.

    5. Assemble the ring with specimen, porous disks, in the consolidometer.

    15.5 Procedure for pneumatic load frames

    The pneumatic load frame applies the desired load to the specimen by regulating the supply of air pressure.A different supply pressure results in a different load on the sample. Since all our consolidometers are thesame and the specimen have a nominal diameter of 2.5 in, the loads have already been transformed intostresses, as shown in table 15.1 . Notice that the frame operates in two modes: low load (below 100 lbf)or high load (above 100 lbf). The following procedure should be followed to apply the desired load to thesample. Refer to gure 15.6 to identify the different switches.

    1. Center the consolidometer below the cross arm of the pneumatic (gure 15.5 ) load frame.

    2. Apply a seating pressure of 5 kPa (100 psf). Refer to gure 15.6 to identify the switches described inthe following instructions.

    (a) Set the HIGH/LOW LOAD selector valve to LOW LOAD(b) Set the LOAD valve to OFF .

    (c) Set the regulator to the desired seating stress (see table 15.1 ).

    (d) Turn the LOAD valve to LOAD .

    3. Check that the load is sufcient by trying to gently roll the loading ball between the consolidometerand the cross arm of the frame. You should not be able to move it.

    4. Adjust the location of the LSCT and dial gauge. Remember the sample is supposed to decrease inheight and the cross bar should be moving down. When you adjust the deformation indicators youneed to insure they can measure the change in height of the specimen throughout the test. Therefore,try to place the displacement indicators with their rods almost all the way up, but not completely.

    5. Inundate the specimen with water. Check if the dial gauge reading is changing.

    6. Record the initial dial gauge reading on the appropriate location in the data sheet.

    7. Select the supply air pressure for the loading frame which matches the desired load for the incrementfrom table 15.1 .

    8. Wait until all the groups using the same data acquisition system have completed all the previous stepsand are ready to start the actual test.

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    Figure 15.5: Pneumatic load frame setup.

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    15. One-Dimensional Consolidation 95

    Desired stress on the specimen Load Air supply pressureLow load High load

    (psf) (kPa) (lbf) (N) (psi) (psi)100 4.79 250 11.97 8.52 37.9 22.12

    500 23.94 17.04 75.8 30.24 1,000 47.88 34.09 151.64 46.43 2,000 95.76 68.18 303.28 78.69 4,000 191.52 136.35 606.52 8.608,000 383.04 272.7 1213.03 16.32

    16,000 766.08 545.4 2426.06 31.8332,000 1532.17 1090.8 4852.12 62.6064,000 3064.34 2181.6 9704.24 123.61

    Table 15.1: Air pressure supply for pneumatic loading frame and corresponding applied pressure on thesample.

    9. Prepare the loading system for application of the rst load. Refer to gure 15.6 to identify the switchesdescribed in the following instructions.

    (a) Turn the LOAD valve to OFF . This actually locks the load to whatever it is supposed to be. Itshould not unload the sample.

    (b) Set the regulator to the desired stress (see table 15.1 ).

    (c) Stop

    10. Make sure everyone is ready to go.

    11. Start data acquisition. Wait a few seconds to see the readings on the graphs.

    12. Load the sample by turning the the LOAD valve to LOAD .

    13. Look at the dial gauge and LSCT readings indicate change in height for a few minutes.

    14. After about 5 minutes, change the rate to 10 s interval. Type 10 in the text box and hit enter.

    15. You are done for today. Return tomorrow to change the load.

    15.6 Procedure for mechanical load frames

    The mechanical load frames simply use weights to apply the load (gure 15.7 ). The loading arm multipliesthe load on the specimen by 10. The load required to obtain the desired stress can be calculated given thearea of sample. It is also listed in table 15.1 . Remember to divide by 10 to obtain the actual weight you needto apply on the loading arm. Be extremely every time you change the load. The counterweight is not securedto the upper arm of the loading frame and it is known to crash to the oor while the load is changed.

    1. Assemble the ring with specimen, porous disks, in the consolidometer.

    2. Place the consolidometer in the mechanical load frame and apply a seating pressure of 5 kPa (100 psf).

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    Figure 15.6: Close-up of the front panel of the pneumatic load frame.

    3. Adjust the location of the LSCT and dial gauge. Remember the sample is supposed to decrease inheight and the cross bar should be moving down. When you adjust the deformation indicators youneed to insure they can measure the change in height of the specimen throughout the test. Therefore,try to place the displacement indicators with their rods almost all the way up, but not completely.

    4. Inundate the specimen with water. Check if the dial gauge reading is changing.

    5. Record the initial dial gauge reading on the appropriate location in the data sheet.6. Select the weights you are going to use for the rst stress increment.

    7. Wait until all the groups using the same data acquisition system have completed all the previous stepsand are ready to start the actual test.

    8. Start data acquisition. Wait a few seconds to see the readings on the graphs.

    9. Load the sample.

    10. Look at the dial gauge and LSCT readings indicate change in height for a few minutes.

    11. After about 5 minutes, change the rate to 10 s interval. Type 10 in the text box and hit enter.12. You are done for today. Return tomorrow to change the load.

    15.7 Second and following days

    Follow these directions for all the tests running data acquisition on the same laptop.

    1. Check that the specimens are still covered in water. Add water if necessary.

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    2. Stop the software.

    3. Enter the new le names for all 3 groups.

    4. Reset the rate to 1.

    5. Write down the dial gauge readings for all 3 specimens.6. Change the load to the next increment.

    Turn the LOAD valve to OFF .

    Set the regulator to the desired air supply pressure (see table 15.1 ).

    If changing to the high load mode, turn the HIGH LOAD/LOW LOAD valve to HIGH LOAD .

    Stop

    7. Repeat with the other specimen.

    8. Figure out what weights should be added or removed from the mechanical loading frame stack. Donot do it.

    9. Start the data acquisition.

    10. Load the sample by turning the the LOAD valve to LOAD .

    11. Now add or remove the weights from the mechanical loading frame arm.

    12. Look at the dial gauge and LSCT readings indicate change in height for a few minutes.

    13. After about 5 minutes, change the rate to 10 s interval. Type 10 in the text box and hit enter.

    14. You are done for today. Return tomorrow to change the load.

    15.8 Last day

    To minimize swell during disassembly, rebound the specimen back to the seating load (5 kPa). Once heightchanges have ceased (usually overnight), dismantle quickly after releasing the nal small load on the speci-men. Remove the specimen and the ring from the consolidometer and wipe any free water from the ring andspecimen. Determine the mass of the specimen in the ring and subtract the tare mass of the ring to obtainthe nal wet specimen mass, M T f . The most accurate determination of the specimen dry mass and watercontent is found by drying the entire specimen at the end of the test. Determine the nal water content, wf ,and dry mass of solids, M d , using the entire specimen.

    15.9 Calculation

    The goal of this test is to determine the magnitude of settlement and the rate of settlement. For this purpose,the compression index, C c , the re-compression index or swelling index, C r , the coefcient of secondarycompression, C , the coefcient of consolidation, cv , and the pre-consolidation pressure zc must be ob-tained from the data. Consult your textbook for the appropriate methods to use for interpretation of thedata.

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    15. One-Dimensional Consolidation 99

    15.10 Report

    For the report, be sure to include:

    cv , C , and void ratio for each day

    Void ratio versus effective stress plot z c, C c , C r

    Displacement versus time for each day included in appendix

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    Incremental loading consolidation test data

    Date Tested by

    Ring mass (g) Ring diameter ( ) 1) 2) 3) Avg.

    Ring height ( ) 1) 2) 3) Avg.

    Ring + initial wet sample mass (g) Initial wet sample mass (g)

    Final dry sample mass (g) Final water content

    Cup ID Cup mass (g) Final wet sample + cup mass (g)

    Final wet sample mass (g) Final dry sample + cup mass (g)

    Final dry sample mass (g) Final water content

    Cup ID Cup mass (g) Wet trimmings + cup mass (g)

    Wet trimmings mass (g) Dry trimmings + cup mass (g)

    Dry trimmings mass (g) Trimmings water content

    LSCT serial No. LSCT calibration factor Channel

    LSCT Voltage range (V)

    Dial gauge initial reading Dial gauge readings on Red Black

    Day Date Load Initial dial gauge Final dial gauge File name Reading by:Time (kPa) reading reading

    1 12

    2 25

    3 50

    4 100

    5 200

    6 400

    7 100

    7b seat

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    Chapter 16

    Triaxial Unconned Compression Test

    16.1 Purpose

    This test will be used to quickly determine the undrained shear strength of saturated clays. In this test, no

    radial stress will be applied to the sample ( 3 =0), but the axial stress, ( 1 ) will be increased until the samplefails (can no longer support load). The load is applied quickly so that the pore water cannot drain, meaningthat the sample is sheared at a constant volume. Since we are not applying a radial stress, the principle of effective stresses gives:

    3 = 3 u = 0 u = u (16.1)

    Because soils cannot sustain tension, 3 must be positive, and therefore, the excess water stresses, u ,must be negative. The results from the UC test are used to:

    Estimate the short-term bearing capacity of ne-grained soils for foundations

    Estimate the short-term stability of slopes

    Compare the shear strengths of soils from a site to establish soil strength variability quickly and cost-effectively (the UC test is cheaper than others)

    Determine the stress-strain characteristics under fast (undrained) loading conditions

    16.2 Procedure

    16.2.1 Materials

    Soil sample

    Sample form and sample manual rammer for compaction Sample extruder

    Knife

    Sample holder

    Calipers

    Pressure chamber

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    Porous stones

    Transducers - force and displacement

    Shearing apparatus

    Computer - Geotech Data Logger Program

    Digital Voltmeter

    16.2.2 Procedure

    1. Refer to Chapter 3, Compaction Using Standard Effort, for sample preparation.

    2. The sample extruded may now be cut in two, lengthwise. Each group should take half of the originalsample.

    3. Place the sample in the sample holder.

    4. Each side of the sample holder cuts the sample to different diameters; one for a coarse preparation,and one for a nal sample preparation

    5. Carefully, (very carefully), shave the sample with the knife so that the nal product is a cylindricalsample with a diameter equal to that of the sample holder.

    6. Take the sample and lay it horizontally on the sample holder. Using the edge of the holder, cut the endsoff of the sample so that they are square. You want a sample with a height-to-diameter ratio between2 and 2.5

    7. Record the dimensions of the sample in several places and record the average height and diameter.

    8. Disassemble the compression chamber

    9. Place a porous stone on the bottom platen of the compression chamber. Then place the sample with aporous stone on top of it. Re-assemble the compression chamber, taking care to prevent the plungerfrom interfering with the sample.

    10. Place the compression chamber with the sample inside it onto the loading frame.

    11. Prepare the computer data acquisition system as directed.

    12. Record the serial numbers of the force and displacement transducers

    13. Begin data acquisition and then start applying a load to produce an axial strain at a rate of 1/2 to2%/min. Allow the computer to take the readings, and stop the data acquisition when the failure planein the sample is visible.

    14. Save the computer data, and make a sketch of the failed sample, noting the angles of the failure planes.

    15. Record a water content of the sample after completion of the test.

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    16. Triaxial Unconned Compression Test 103

    16.3 Calculations

    The undrained shear strength is given as:

    S u = P z2A

    = 12

    1 (16.2)

    where P z is the axial load applied to the sample and

    A = Ao

    (1 e1 ) (16.3)

    Note that because we are assuming no volume change, and we are axially deforming our sample, thecross sectional area of the sample changes as the strain increases.

    16.4 Report

    For the report, be sure to include:

    Force vs. time plot

    Displacement vs. time plot

    Stress versus strain plot

    versus plot, (including the Mohrs circle)

    Values for su and q u (ultimate stress, or s1 at failure)

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    Chapter 17

    Unconsolidated Undrained Triaxial Test

    17.1 Specimen preparation

    The test will be carried out on a compacted specimen of clayey sand. Prepare the compacted specimen

    following the directions on chapter 8 and as directed by your teaching assistant. The procedure is similar tothat used for the preparation of the unconned compression specimen. Once the soil is extruded, carefullydivide the sample into two parts and obtain one specimen from each part.

    17.1.1 Preparation of the specimen

    1. Trim the specimen to the desired dimensions: diameter 38mm (1.5in) and height 76mm (3in), usinga trimming device. The trimming apparatus should allow for convenient trimming of a cylindricalspecimen of constant cross section, using either a wire saw or a steel blade. Be careful during thetrimming and while handling the specimen because the material is prone to cracking and crumbling.The specimen should be slightly taller than the nal desired height to allow the removal of top andbottom slices prior to nal measurement and testing. Be particularly careful in trimming the specimenends to ensure they are perpendicular to the longitudinal axis of the specimen (otherwise alignmentbecomes a real problem).

    2. Place the specimen on a small piece of saran wrap while you are handling it to avoid loss of moisture.

    3. Obtain the moist weight of the specimen.

    4. Measure height to 0.1mm (0.01in) with a caliper. Care should be taken that the measuring does notpenetrate soft specimens. The average of 3 readings should be used.

    5. Measure the specimen diameter to 0.01mm (0.001in) in two perpendicular directions and three eleva-tions (center, near top and near bottom) to obtain an average.

    6. Measure the thickness of the membrane. Usually this is done by folding the membrane and measuringthe thickness of several layers at a time and then dividing by the number of layers.

    17.1.2 Fitting end caps and membrane

    1. Carefully place the specimen on the bottom cap and then the top cap on the specimen. If needed,also place porous stones. Make sure the specimen is centered and the assembly is aligned (VERYIMPORTANT!!).

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    Figure 17.1: Stages of membrane tting (from Head, 1992)

    2. Fit two o-rings over the membrane stretcher and roll them near the middle of its length. Fit a membraneinside the stretcher and fold back the ends, over and outside. Apply vacuum to the membrane stretcher.Make sure the membrane is actually being pulled out and adheres to the stretcher. It may take a littletugging, but be careful not to twist the membrane at this point.

    3. Carefully lower the membrane stretcher over the specimen until it is nicely centered. Then releasevacuum and allow the membrane to adhere to the specimen.

    4. Carefully release the membrane ends covering the caps and trying to minimize the amount of airentrapped in contact with the sample.

    5. Lower the stretcher so that the bottom is located at about mid-height of the lower cap. Roll down andoff one o-ring to seal the membrane.

    6. Raise the membrane stretcher all the way up so that the bottom is located at about mid-height of thetop cap. Roll down and off the o-ring.

    7. Carefully fold the membrane over the o-rings.

    17.2 Procedure1. Assemble the triaxial cell.

    2. Carefully check that the piston is aligned with the top cap.

    3. If not, undo the cell set up and try to gently adjust the alignment until it works. It is important that youdo so very gently as to minimize the disturbance to the sample.

    4. Place the cell in the loading frame.

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    17. Unconsolidated Undrained Triaxial Test 107

    5. Prepare the computer data acquisition system as directed.

    6. Record the serial numbers of the transducers.

    7. Slowly increase pressure in the cell to the desired conning pressure. Be careful, the loading rod maybe lifted by the pressure if it is not locked. Allow 10 minutes under conning pressure for the sample

    to equilibrate.

    8. Begin data acquisition and note the reading for the load transducer (this should be the zero reading,or no load is applied). Then start loading at a strain rate of approximately 1%/min. Before the loadingrod comes into contact with the top cap the load transducer is reading the upward force due to thechamber pressure and the friction between the rod and the seal.

    9. Allow the computer to take the readings, and stop the data acquisition when the axial strains reach15%.

    10. Save the computer data, and make a sketch of the failed sample, noting the angles of the failure planes.

    11. Record a water content of the sample after completion of the test.

    17.3 Calculations

    The deviator stress is given as:

    1 3 = P zA

    (17.1)

    where P z is the axial load applied to the sample (corrected for uplift and friction and:

    A = Ao

    (1 e1 ) (17.2)

    Note that because we are assuming no volume change, and we are axially deforming our sample, thecross sectional area of the sample changes as the strain increases.

    You also need to correct the deviator stress 1 3 for the effect of the membrane:

    ( 1 3 ) = 4E m tm 1

    D (17.3)

    where: E m is Youngs modulus of the membrane, use 1400 kPa; tm is the thickness of the membraneand 1 = H/H is the vertical strain.

    17.4 Report

    Include the following information in the report:

    Rate of strain, in percent per minute.

    The stress-strain curve, (1 3 ) vs. 1 .

    Axial strain at failure, in percent.

    The value of compressive strength and the major and minor principal stresses at failure.

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    4. Fill the sample form with sand to the top of the sample form. Lightly tap the sand in to the sampleform and rell the sample form if needed.

    5. Place the top loading plate on top of the sample form, pull the sample membrane over the top plateand secure it with an o-ring.

    6. Apply a vacuum to the sample by attaching the appropriate hose to the vacuum outlet.7. Remove the sample form. The sample should be a smooth cylinder from the top of the porous stone to

    the bottom of the top loading plate. If it is not a smooth cylinder, the results will be affected.

    8. Measure and record the length of the sample in three locations.

    9. Measure and record the diameter of the sample of three locations.

    10. Assemble the pressure chamber as directed by your instructor.

    11. Pressurize the chamber as directed by our instructor.

    12. Remove the vacuum.

    13. Raise the loading frame to reduce the distance top loading plate and the load cell extension.

    18.2.3 Procedure

    1. With the specimen encased in the rubber membrane, which is sealed to the specimen cap and baseand positioned in the chamber, assemble the triaxial chamber. Bring the axial load piston into contactwith the specimen cap several times to permit proper seating and alignment of the piston with the cap.When the piston is brought into contact, record the reading on the deformation indicator. During thisprocedure, take care not to apply an axial stress to the specimen exceeding approximately 0.5% of the estimated compressive strength. If the weight of the piston is sufcient to apply an axial stressexceeding approximately 0.5% of the estimated compressive strength, lock the piston in place above

    the specimen cap after checking the seating and alignment and keep locked until application of thechamber pressure.

    2. Place the chamber in position in the axial loading device. Be careful to align the axial loading device,the axial load-measuring device, and the triaxial chamber to prevent the application of a lateral forceto the piston during testing. Attach the pressure-maintaining and measurement device and ll thechamber with the conning air. Adjust the pressure- maintaining and measurement device to thedesired chamber pressure and apply the pressure to the chamber air. Wait approximately 10 min afterthe application of chamber pressure to allow the specimen to stabilize under the chamber pressureprior to application of the axial load. Note: Make sure the piston is locked or held in place by the axialloading device before applying the chamber pressure.

    3. The axial load-measuring device is located outside of the triaxial chamber. The chamber pressure willproduce an upward force on the piston that will react against the axial loading device. In this case,start the test with the piston slightly above the specimen cap, and before the piston comes in contactwith the specimen cap, either: ( 1) measure and record the initial piston friction and upward thrust of the piston produced by the chamber pressure and later correct the measured axial load, or (2) adjustthe axial load-measuring device to compensate for the friction and thrust. If the axial load-measuringdevice is located inside the chamber, it will not be necessary to correct or compensate for the upliftforce acting on the axial loading device or for piston friction. In both cases record the initial readingon the deformation indicator when the piston contacts the specimen cap.

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    x

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    18. Triaxial Consolidated Drained Compression Test 113

    Triaxial Unconned Compression Test Data

    Sample No. Project

    Boring No. Location

    Depth

    Description of sample

    Date Tested by

    Time Force Displacement(s) (mV) (mV)