Fundamental Procedure for Experiments Soil Lab

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    Experiment 6

    Consolidation Test (Oedometer).

    General

    The standard oedometer consolidation test for saturated clays is the main feature of thisexperiment. The test is carried out by applying a sequence of vertical loads to a laterallyconfined specimen having a height of about one quarter of its diameter. The verticalcompression under each load is observed over a period of time, usually up to 24 hours. Since nolateral deformation is allowed it is a one-dimensional test, from which the one-dimensionalconsolidation parameters are derived.

    ObjectiveThe objective of the oedometer consolidation test is to determine consolidation characteristics ofsoils with low permeability. The test determines two important consolidation parameters ofclays, i.e. coefficeient of volume compressibility, m v, and coefficient of consolidation, c v

    TheoryThe one-dimensional consolidation test procedure was first suggested by Terzaghi. The test is

    performed in an oedometer. A schematic diagram of an oedometer is shown in Figure 6.2(a).The soil sample is placed inside a metal ring with a porous stone at the top of the sample andanother at the bottom. The samples are usually 63.5mm in diameter and 25.4mm thick. Load onthe sample is applied through a lever arm and compression is measured by a micrometer dialgauge. The sample is kept underwater during the test. Usually each load is kept for 24 hours.After that, conventionally, the load is doubled, thus doubling the pressure on the sample, whilemeasurement of the compression continues. At the end of the test, the dry weight of the testsample is determined.

    The general shape of the plot of deformation of the sample versus time for a given loadincrement is shown in Figure 6.1. The plot shows three distinct stages that may be described asfollows:Stage I: initial compression, which is mostly due to preloading.Stage II: primary consolidation during which, due to expulsion of pore water pressure, isgradually transferred into effective stress.Stage III: secondary consolidation after complete dissipation of excess pore water pressure -some deformation of the sample is caused by plastic readjustment of soil fabric.

    The aim of the consolidation test is to determine two important consolidation parameters for theclay sample :

    1. The coefficient of volume compressibility, m v (in m 2/MN) is given by the equation

    mv =(H1-H2

    H1 ) x (

    1000 p2-p1

    ) m 2/MN (6.1)

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    where H 1 is the height of the specimen at the beginning of the stage(i.e. at the end of the previous stage ) (in mm).

    H2 is the height of the specimen at the end of that increment (in mm). p1 is the pressure applied to the specimen for the previous loading stage (in kPa).

    p2 is the pressure applied to the specimen for the loading stage being considered.

    (in kPa)

    The required units are in m 2/MN.

    Figure 6.1 Time-deformation Plot During Consolidation for Given LoadIncrement (source: Das 1979)

    2. The coefficient of consolidation, c v ( in m 2/year).

    The coefficient of consolidation, c v, may be determined by finding the time required for90% consolidation of the sample (U = 0.9).For the condition of double drainage, which is the case in the oedometer test :

    When U = 0.9Tv= 0.848

    Since T v=cvth2 (6.2)

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    ∴ cv =T90t90

    h2 = 0.848 xh2

    t90

    cv =90

    2

    t

    365.25x24x60x)1000

    h(848.0

    m2/year

    cv =90

    2

    th446.0

    m2/year

    cv =90

    112.0t

    H2 m2/year (6.2)

    where T v is the time factor.t is the time elapsed since the start of the consolidation (in min.).h is length of the drainage path (H is the thickness of the clay sample at

    time t) (in mm).c

    v is the coefficient of consolidation (m 2/year).

    where H is the average specimen thickness for the load increments (in mm)

    i.e. H =2

    HH 21 +

    In the standard oedometer consolidation test with double drainage the height H of the specimenis equal to 2h. The details of the theory of compressibility of soil, students should refer to their textbook andhandout.

    Material

    Compacted decomposite granite

    Apparatus1. Casagrande type oedometer (Figure 6.2) which includes :

    a. a consolidation ring, internal diameter 75 mm, height 20 mm; b. a fixed ring, consolidation cell;c. a dial gauge reading to 0.01 mm having a travel of at least 10 mm;d. a loading device, (see Figure 6.4)

    2. Glass plate 100 mm x 100 mm (approx.),3. Apparatus for moisture content determination (experiment 9),4. Top pan weighing balance reading to 0.1 g,5. Vernier callipers,6. Packet of 75 mm diameter filter papers,7. Silicone grease or petroleum jelly,8. Set of standard weights,9. Stop watch or clock readable to 1 sec,10. Palatte knife.

    Procedure

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    1. Preparation of the sample (see Figure 6.2 (c) and (d))a. Weigh the consolidation ring and glass plate separately to an accuracy of 0.1g. (Form 6.1)

    b. Lubricate the inside of the ring with a thin smear of silicone grease or petroleum jelly.c. Measure the height of the ring to 0.05 mm at four equally spaced points using the vernier

    callipers and calculate the mean height.

    d. Measure the internal diameter of the ring to 0.1 mm in two perpendicular directionsusing the vernier calipers. Calculate the mean diameter and the area in mm 2.

    e. Extrude a small amount of soil from the compaction mould using the mechanicalextruder.

    f. Press the cutting ring, bevelled sharp cutting edge downwards, into the soil until its uppermost rim is just below the soil surface.

    g. Extrude more of the soil so that the bottom of the ring is well clear of the edge of themould.

    h. Trim off the top of the soil with the palatte knife.i. Cut off the soil below the base of the consolidation cutting ring with the spatula.

    j. Place the glass plate on the top surface and gently slide the specimen clear using a

    palate knife to assist the process.k. Invert the ring containing the soil sample and trim off the upper surface of the clay level

    with the bevelled edge of the consolidation ring with the spatulas.l. Any voids should be carefully filled with pieces of clay without compressing the sample.m. Weigh the glass plate, ring the clay sample to the nearest 0.1 g.

    Notes : the height of the ring can be accepted as the initial height of the clay sample.

    2. Preparation and assembly of consolidation apparatus.a. Put a wet filter paper onto the porous disc at the base of the consolidation cell and

    place the sample, contained in the ring, on it with the bevelled cutting edge of the ringuppermost.

    b. Cover the top of the sample with a second wet filter paper and use the retaining screwsto secure the collar of the consolidation cell to the base to hold the consolidation ringand sample firmly together.

    c. Place the top porous stone and loading plate on tope of the filter paper.

    3. Assembly in load framea. Place the consolidation cell in position on the cell platform of the oedometer.

    b. Connect the loading yoke of the oedometer with the top platen of the consolidation celland adjust the counter balance weight of the beam so that it is slightly above thehorizontal position.

    c. Place a 100 g weight on the top pan of the weight hanger to give a very small positivedownward load on the sample in the consolidation ring (seating load).

    d. Check the beam ratio value and set it to 9:1.e. Fill the consolidation cell with water at room temperature.f. Clamp the compression dial gauge in to position, allowing space for swelling as well

    as compression of the sample and record the initial dial gauge reading.g. Screw up the beam support jack so that the beam is held fixed, ready for the start of the

    test.

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    4. Test procedure Normally, in the consolidation test, a loading sequence is adopted to give a range ofcompression stresses suitable for the soil type and also for the effective pressure whichwill occur in situ due to the overburden and the proposed construction. The initial

    pressure should be large enough to ensure that the sample in the consolidation cell does

    not swell.A loading sequence of stages selected from the following range of pressures is consideredappropriate (see BS 1377, 1990, Part 5, p. 5 section 3.5.1.).6, 12, 25, 100, 200, 400, 800, 1600, 3200 kPa.A typical test comprises four to six increments of loading, each held constant for 24 hoursand each applied stress being double that of the previous stage.Unloading decrements are usually half the number of loading increments.

    The single stage consolidation test to be performed will be for a stress of 100 kPa.a. Determine the value of mass (in kg) needed on the weight hanger pan to produce a

    stress of 100 kPa on the specimen ( σ’vo) see Appendix A. b. With the screw jack support in supporting position, load the weight hanger with the

    necessary weights and set the dial gauge to zero. Remove the weight used for seatingload.

    c. Check that the timing device (stop watch or clock) is working correctly, note the timeof day and activate the timing device whilst at the same time lowering the beamsupport jack to allow the consolidation to begin.

    d. Take readings of the compression gauge at the following time sequence (minutes)0.25, 0.5, 1, 2, 4, 9, 16, 25, 36, 49, 64, 81, 100 and 121 min.(Form No. 6.2)A final reading, after approximately 24 hours can be taken by the technical staff.

    e. As the sample undergoes compression record the data and plot a graph of compressiondial gauge readings versus time . (Figure 6.5)After 24 hours, when the consolidation will be virtually complete, unload the sampleand record the following data.

    weight of consolidation ring + sample, wet

    weight of consolidation ring + sample, dry*

    *after drying to constant weight in an oven at 105 oC.

    From this data the final moisture content and void ratio of the sample may be

    determined.f. Determine the values of

    90t

    from the graph of compression vs time on Figure 6.5 by :

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    • Draw the straight line of best fit to the early portion of curve (usuallywithin the first 50% of compression) and extend it to intersect the ordinate ofzero time. This intersection represents the corrected zero point, denoted by d o.

    • Draw the straight line through the d o point which at all points has abscissae 1.15times as great as those on the best fit line drawn in (a). The intersection of this

    line with the laboratory curve gives the 90% compression point, d 90 .• Read off the value of t 90 from the lab. curve corresponding to the d 90.• Determine the value of the coefficient of volume compressibility, m v(m2/MN)

    (see Equation 6.1) from the settlement data for this loading.• Determine the value of the coefficient of consolidation, c v(m2/yr) (see Equation

    6.2).g. The record of data obtained from a full consolidation test with several stages of

    loading and unloading. Plot a graph of void ratio versus log 10 of applied pressures.

    For the single stage test you are required to plot only settlement versus time inorder to find t

    90 by Taylor’s curve fitting method. For the determination of t

    90 and c

    v

    for each stage over several stages a separate graph of settlement versus time willhave to be drawn for each stage.

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    Appendix A

    Calculation of mass (m) or equivalent mass (in kg) supported by the specimen

    σ’vo =A

    am9810 ×× kPa

    m =a9810

    A'vo×

    ×σ

    where σ’vo is the vertical stress applied to the specimen (kPa).m is the mass or equivalent mass, supported by the specimen (kg).a is the lever arm ratio (9:1).A is the area of the specimen in mm 2.

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    (a)

    (b)

    Figure 6.3 (a) Section of a Typical Consolidation Cell, and (b) Details of a Consolidation Cell.

    Cell

    Consolidation ring

    Loading cap

    Fixing nuts Porous plate

    Lateral restraint forring

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    Figure 6.4 Section of a Loading Device

    Figure 6.5 Consolidation Curve

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

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    Form 6.1

    Soil descri tionDIMENSIONS Inital Overall Final Specimen

    specimen change specimen PreparationMethod

    Diameter D mm

    Area A mm 2

    Height H mm H o

    Re-moulded

    Volume V cm 3

    WEIGHINGS Initial specimen Final specimen(a) (b) (c)

    Wet soil + rin + traDr soil + rin + tra

    Rin + tra

    Wet soil g mo mo

    Dry soil g md md

    Water

    Moisture content measured %

    Densit M /m3

    Dr densit M /m 3

    Voids ratio eo

    Degree of saturation % S o

    Height of solids H s mm

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    Consolidation Test - Oedometer

    Soil and Rock Mechanics Laboratory, Department of Building and Construction, City University of Hong Kong

    Form 6.2Soil descriptionMachine no. Specimen diameter mm Height mmCell no. Lever ratio :1 Area mm 2

    LOADING/UNLOADING*Increment no./date started

    Load kg/lb */pressure kPa

    Mean daily temperature oCElapsed Clock Gauge Cumulative Clock Gauge Cumulative Clock Gauge Cumulativetime time Reading compression time reading compression time reading compression

    h m s t min t x 0.01 mm ∆ H x 0.01 mm ∆ H x 0.01 mm ∆ H

    Cumulative correction Net cumulativecompression ∆ H

    * Delete as appropriate