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    Understanding the Geotechnical Report as an Engineeringand Construction Reference

    Course Outline

    1. Engineer and contractor requirements.2. Geotechnical report contents3. Analysis of data given in report4. Soil boring equipment5. Recommendations for construction methods and slopes6. Soil boring logs7. Soil description (classification such as silty-sand)8. Soil properties (hard, stiff, dense, loose, etc.)9. Boring depth10. Boring refusal11. Blow counts12. Soil moisture content13. Soil dry density

    14. Particle distribution curves (sieve analysis)15. Atterburg limits (liquid limits and plasticity index)16. Compaction test (optimum moisture for compaction)17. Direct shear tests18. Seismic velocity lines19. Permeability

    Learning Objective

    This course introduces the student to the many components that make up a thorough Geotechnicalreport. The various observations and tests are explained so that an inference to the ground propertiescan be achieved. The Geotechnical report is a complex scientific document that can be confusing to evenhighly experienced people.

    Owners, architects, engineers, and contractors must be able to understand the implications of the reportsin order to achieve an on time, on budget and failure free project.

    At the conclusion of this course, the student will learn:

    That major projects usually have Geotechnical Reports. How to interpret the Geotechnical report for consistency. How to evaluate soils for embankment suitability.

    How to determine the presence of ground water and how it will effect design and construction. How to interpret seismic velocity diagrams for excavation methods.

    To Understand that the Geotechnical Report is often a subjective opinion of an individual

    geologist.

    Course Introduction

    The Geotechnical report provides critical and vital information for the owner, architect, design engineer,and the contractor to use and evaluate. The owner wishes to assess the cost of the project foundationand earthwork. The structural engineer is responsible for the design of an economical but sturdy buildingfoundation. The architect may be forced to arrange building layouts to accommodate varying soilconditions. The contractor wants to bid a competitive but realistic price for the excavation and

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    embankment work. For these reasons, all the essential players have a personal stake in the Geotechnicalreport. All these players must be able to understand the ramifications of the soil study. A foundationfailure of a major structure is measured in millions of dollars and ruined careers. It can even lead to lossof life, such as the Teton Dam failure. An ignorant or misinformed contractor can lose millions inunanticipated costs. Such mistakes often lead to major lawsuits that drag on for years and cost everyoneexcept the lawyers. Being able to understand and analyze the Geotechnical report to avoid costlymistakes is in the best interest of all the project players.

    Course Content

    Geotechnical (soils) reports are prepared to provide the design engineer and contractor with informationregarding the soil conditions at a specific location. These reports are a wealth of information for theperson that can properly interpret the information presented. Usually the soil report is primarily written togive the structural engineer the specific information needed to effectively design the structuralfoundations. Construction methods and contractor concerns are usually addressed as a minor side issueand in gross generalities. Only after a number of site visits and studying several soil reports will you beginto really understand what is being presented by the reports.

    The soil report is usually made available at least in part to the contractor during the bidding stage. It is in

    all the parties best interest to accurately and unambiguously define the ground conditions. The moredefinitive the soils report the greater the confidence the engineer has in developing a foundation designand the contractor will price the work more competitively. While small, light structures with shallow footingmay only need a cursory review, large complex projects require a definitive, comprehensive and thoroughGeotechnical analysis.

    Often the report is issued for information only and is not a part of the construction contract documents.This is done so that the owner can try to limit liability for contractor interpretations and changedconditions. There is always language that states that the report represents only the actual spotsexamined and conditions can vary. This expressed caution is very real and it is very common to findunexpected soil changes or buried obstructions that are discovered during construction. It is wise to bevery leery of contracts that contain a no changed condition clause. Whether the owner is trying to hidesomething or only limiting risk exposure, this can lead to very expensive and time-consuming litigation

    where no one wins.

    Whenever possible, you should try to get as much local history of the project site as possible. Always tryto visit the project site and do your own pot holing with a backhoe. This will give you a much betterunderstanding of the actual soil conditions. The site may have been a dumping ground or historical site.An archaeological find or hazardous material can be very disruptive to the work. Over time localcontractors will gain an amazing knowledge of the regional geology and the construction methods that arethe most effective. For instance, in one area I worked there was an extensive volcanic tuff that was verydifficult to rip or blast. By knowing the tuff formation limits exactly gave us a huge advantage in biddingwork in the area and kept us out of trouble. If possible, interview local contractors and equipmentoperators as to what to expect. Valuable information can be quickly obtained from a friendly conversation.

    The objectives and the information required by the design engineer and the contractor differ dramatically.

    The design engineer needs to know what is needed to found the structure. The contractor wants to knowwhat is needed to build the designed foundation. Generally, construction methods are the chosen by thecontractor so long as no damage is caused. For that reason, the soil reports tend to be very vague whenaddressing construction methods.

    Engineer objectives:

    The design engineer wants to know what is under the surface to support the structures. To that end, theengineer needs to know several critical soil properties:

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    1. What is the allowable soil bearing pressure?2. What is the expected foundation settlement?3. What is the active soil load?4. What is the passive soil loading?5. What is the sliding friction factor?6. What is the potential for differential settlement?7. What is the soil liquefaction potential during an earthquake?8. What are the seismic design accelerations?9. Where is the groundwater table?10. What is a permanent stable slope?11. Will piling be required?12. Can the native soil be used for backfill?13. What are the criteria for the pavement sections?14. Are hazardous wastes present?15. How corrosive are the soils?16. Will there be voids, obstructions or unstable soils?

    Contractor questions:

    The contractor wants to know what the subsurface conditions are so that an accurate estimate of costsand time can be entered into a competitive bid. To that end the contractor will search the soils report todetermine the following:

    1. Where is the ground water and how much water must be pumped?2. Is there rock to be drilled and shot (sometimes blasting is not allowed)?3. Can the ground be ripped with a bulldozer?4. What excavation equipment and methods will be most effective?5. Is there enough space on the job to store backfill materials?6. Can the native material be used for backfill?7. Will the native material need to be processed (screened/crushed) for backfill?8. How much backfill must be bought and imported?9. How steep can the temporary excavation slopes be cut?

    10. Can obstructions be expected?11. What compactive effort and equipment is needed for backfilling?12. Will excavation shoring be required?13. What is the most effective shoring method?14. Will the ground stand long enough to use trench shores or shields for pipe trenching?

    Now that we have asked the questions, how do we glean answers from the soil report? The report isdirected mostly to the design engineer and the contractor usually must make an interpretation of theinformation to develop a construction plan. This interpretation is often vital to the success of the project. Ifthe report is ambiguous or fails to properly identify the ground conditions, the result is often a changedcondition claim. These claims can entail lengthy delays, increased cost, disputes and lawsuits. It isimportant to read the entire report and understand that each geologist has a different style of describingthe soils, as they are field sampling.

    Recently, a geologist described the soil in one report as dense silty-sand. This classification wassuspicious because the borehole was advanced with a carbide tipped rotary drill and hit refusal above therequired excavation depth. Further investigation revealed that the soil was in fact unweathered massivegranite rock. Because blasting was not allowed, the excavation costs were estimated at $50.00 per cubicyard versus $3.00 per cubic yard. This amounted to nearly a million dollars of added excavation cost tothe estimate. A cursory review of the report could easily mislead the owner, engineer and contractor withdisastrous results.

    Geotechnical Report Contents:

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    The well-prepared Geotechnical report will be organized along the following outline:

    1. Introduction: This will identify the project by location and name. It will also brieflyoutline the scope of the investigation.

    2. Project Description: This will give an overview of the structures, with proposedfoundation depths.

    3. Field Exploration and Testing: This will identify the methods and equipment used tobore and test the soils.4. Site conditions: This will describe the terrain, prior known land use, general area

    geology, groundwater, fault proximity, seismic shaking, landslides and other concernssuch as sink hole or fracturing problems.

    5. Recommendations: The various explorations and tests are translated into specificloading criteria, settlements, dewatering requirements, seismic accelerations,pavement sections and site coefficients.

    6. Site observations: This is where the appeal is made that it is important to haveGeotechnical experts review the actual excavation procedures and the warning thatdiffering soil conditions may be found.

    7. Maps: A general area map with geology and faults is usually included. Site maps withboring log locations are included.

    8. Logs: This will contain the boring logs with soil densities, blow counts, ground waterelevations, moisture, soil classifications and sample locations.

    9. Test results: This section includes sieve analysis, optimum moisture plots, directshear tests, cone penetrometer, contaminants and other various tests that aredeemed necessary.

    10. Seismic velocities: This procedure is used when hard rock is expected. The soundspeed through rock is a good indicator of what methods will effectively excavate theground.

    The contractor must make an interpretation of the soils report in order to answer the 14 commonquestions. There are a number of clues in the report that will assist the contractor in making decisions toselect the appropriate construction methods, productions and estimating earthmoving costs. There is noone indicator to show what the ground conditions really will be. The entire report must be read, absorbedand analyzed. The report will contain observations, recommendations and test results that must beindividually interpreted. Sometimes apparently conflicting information will be presented. Also, the soilconditions will be described in such broad generalities that it is nearly meaningless to the contractor.Often impractical construction solutions are offered, such as temporary construction slopes that are so flatthat the top of slope would extend beyond the easement limits.

    The basic clues that must be understood to make an educated judgement of the soil conditions are:

    1. Soil boring equipment2. Recommendations for construction methods and slopes3. Soil boring logs4. Soil description (classification such as silty-sand)5. Soil properties (hard, stiff, dense, loose, etc.)6. Boring depth7. Boring refusal8. Blow counts9. Soil moisture content10. Soil dry density11. Particle distribution curves (sieve analysis)12. Atterburg limits (liquid limits and plasticity index)13. Compaction test (optimum moisture for compaction)14. Direct shear tests15. Seismic velocity lines

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    16.Permeability

    1. Boring Equipment:

    The bore hole or test pit is advanced with a variety of equipment. Bore holes advanced withan auger means that the ground can be excavated with normal earthmoving equipment, i.e.backhoes, scraper, bulldozers and the like. Bores that are made with a pneumatic, carbidetipped drill or similar rock drill means that blasting or heavy ripping will be required.

    Test pits are usually dug with a backhoe. The size of the backhoe makes a huge differenceas to how easily the ground can actually be dug. A small rubber tired backhoe-loader willshow refusal (unable to dig) on soils that can be readily excavated with large track mountedbackhoes or heavy equipment.

    2. Recommendations:

    If the report is concerned with settlement, liquefaction and suggests over-excavation; thatusually means weak clayey soils. While these soils may be easy to dig, they may also beunsuitable for backfill and wet and sticky. Even though clayey soils may appear to be firmenough to drive on, they can start to pump with repeated heavy-wheeled traffic. This is whenthe moisture is worked toward the surface by the equipment pounding and turns the top layerto mud and the equipment will get stuck. If this condition is present it may be necessary tobuild aggregate haul roads. Rain and snowmelt will turn clays to mud and may be difficult todry out enough to travel on.

    Often the recommendations will include soil pressures for temporary shoring. If therecommended soil active pressures are below about 25 pcf, the ground should be firmenough to allow fairly steep construction excavation slopes. If the recommendations are in theorder of 25 to 35 pcf the ground may require flatter construction slopes. If the

    recommendations are higher than 35 pcf, impractical flattened slopes may be required. Veryhigh-recommended loading of about 80 pcf indicates ground water is present and/or oozingclay mud is present. Remember this is only one part of the information available. Furtherscrutiny of the report will often reveal that the recommendations are ultra-conservative.

    3. Soil Boring Logs:

    The boring logs will detail the soil layers by depth from the surface or by elevations. The logwill contain such information as: soil classification, relative denseness of the soil, samplingpoints, sample recovery, water content, dry unit weight, blow counts per foot and groundwater depth, drill refusal and if well casing was needed. Often the soil descriptions aresubjective by being based on the experience and judgement of the observing geologist. Howdense or hard the soil is often based on how quickly the drill can be advanced. The

    description is often based on the look, feel and sometimes smell or taste of the soil. Theboring logs are a good place to start to understand the soil properties. The boring logs shouldbe plotted on the drawings and on cross sections so the relationship of the excavation,structures is scaled to the soils and water table. If casing of the hole was necessary, it usuallymeans the ground is too weak to stand on a normal construction excavation slope.

    Below is an example of a well-presented boring log

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    8. The dry density in the top 4 feet is 127.7 pound per cubic foot (PCF). Thisindicates a well-graded soil with a low void ratio. It can be expected to havesignificant cohesion and friction angle.

    9. The soil between 5 and 8 feet of depth has a dry density of only 96.8 pcf.This indicates the soil is poorly graded and a low percentage of fines. Thissoil will probably have little or no cohesion. That means it may not stand on aconstruction slope as steep as 1H to 1V.

    10. The soil classification descriptions and relative compactions are shown asloose, dense, etc.

    11. Note that no ground water is shown. If ground water is found it will be shownat the depth of encounter as an inverted triangle with a line under it.Sometimes this symbol is small and easy to miss.

    4. Soil Descriptions:

    This will usually identify the soil by classification of particle sizing such as cobble, gravel,

    sand, silt, or clay, etc. Most soils are a combination of these classifications, meaning there isa gradation of material. Sand and larger grains are often referred as cohesionless soils.Without clay or silt fines sand, gravel and cobbles will not have cohesion (glue) to bind thesoil and give it shear strength. Soils with clay or silt are often referred to as cohesive soils, asthey are capable of developing significant shear strength.

    5. Soil Properties:

    Usually soils are described as loose, dense, hard, stiff, soft, etc. Usually, the relative densityterms of: loose, dense are applied to sands and gravels. Terms such as soft and stiff areusually applied to clays and silts. Hard can mean rock or cemented soils, although if the soilsare cemented the geologist will usually mention that fact. Loose or soft ground means that theground may not support a slope as steep as 1H to 1V. Dense sands or gravels are no

    guarantee that the gravels will stand on a steep excavation slope, as they may be tightlypacked but have little or no fines to cause cohesion. Cemented soils can be caliche, volcanictuff, or pyroclastic ash. These soils can be some of the most difficult and expensive materialsto excavate. They can have a relatively low density and show as being relatively soft rock.However, they tend to absorb blasting energy and almost impossible to penetrate with aripper tooth.

    6. Boring Depth:

    Usually the boring logs are advanced well below the planned excavation depth to insure thataccurate formational trends can be plotted and no weak layers are present near thefoundation grade. When the borings are terminated above or just at the planned excavationdepth, you must be very wary. This means you cannot be sure of what may be encounteredat the bottom of the excavation. Is there going to be rock or water that must be handled? Didan obstruction halt the drilling effort?

    7. Boring Refusal:

    This is usually indicated at the bottom to the boring log, if it occurs. If all refusals are wellbelow the planned excavation depth, it will not be a major concern. If a small percentage ofthe borings have met refusal in the excavation limits it usually means that there are isolatedobstructions. It is not possible to determine what is the obstruction unless the soils report or

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    site history reveals the object. A small isolated boulder can stop an auger drill and be of littleconcern. On the other hand, it could be a ridge of solid rock that will be expensive to remove.

    8. Blow Counts:

    This is a very important measure of soil properties. This is a standard penetration test where

    a 140-pound weight is dropped and the number of blows to advance the point one (1) foot iscounted. Obviously the more the blows the harder the soil. Loose or soft soils will show blowcounts of less than 10. Blow counts of 10 to 50 blows per foot usually mean the ground will befairly easily excavated. When the blow counts are over 50 but less than 100, ripping of theground is very likely. When blow counts exceed 100, the ground may be very difficult toexcavate and require blasting or hoe ram effort.

    9. Soil Moisture Content:

    Optimum compaction moisture is usually between 8 and 15% of the dry weight of the soil.Ground water will show about 25 to 40% moisture by weight of the dry soil. Supersaturatedclay can be 50% water. Those are oozing mud that will not support the weight of even lowground pressure tractors. I have seen tractors break through a surface crust and sink so fast

    that the operator barely has time to climb on top of the roll cage.

    Optimum water content in the soil causes some cohesion, helps to control dust, and allowseasy compaction effort. We have all seen sandcastles built on the beach with moist sand andcars can drive just above the surf. Higher up on the beach where the sand is dry, cars will getstuck and the sand can only be piled in a cone shape. As soon as a wave washes over thesandcastle, it is erased. Clean, dry sand and gravel will have little or no cohesion and willravel to a slope of at least 1H to 1V. The natural undisturbed angle of repose of sand andgravel is about 1.5H to 1V slope. This is seen at a gravel mine where processed material isdischarged from a conveyor and forms a cone shaped pile. The friction angle of the soilallows the gravel to form a pile.

    When soils contain less than about 5% moisture, it is too dry. If there is more than 5 to 10%

    clay or silt fines, dust control is a concern. Spraying or pre-wetting water is usually needed toexcavate dry ground efficiently. When the difference between the in-situ moisture and theoptimum moisture is known, water delivery needs can be calculated. If 6% water must beadded to the soil to reach optimum water and the soil weights 100 pcf, then 6 pounds of watermust be added. This calculates to 162 lbs/cyd or 22 gal/cyd. Additional water for evaporation,dust control, and waste must be added. Free draining sands and gravels will use lose theeffective water very rapidly and the required usage can double. It is difficult to add water toclays because the permeability is low. These soils may require substantial mixing andkneading to uniformly introduce water. The general rule of thumb is 30 to 50 gallons of waterare needed to compact each cubic yard of soil. A scraper fleet is capable of moving 1,000 cydper hour. The water needs can be approximately a 1,000 gal/min. Water supply for suchoperations can be a major problem.

    When the moisture content is more than about 15%, the ground is too wet. Equipment canbog down by pumping clays and silts after only a few passes and turn the haul road to mud.Clean sands and gravels are usually not a problem unless the excavation is below the watertable. These soils are usually free drain enough that no special effort is required to dry themout. On the other hand, overly wet clays can be a serious problem to handle. Wet clay willtend to stick to tires and truck beds greatly reducing load and haul efficiency. Extensive effortmay be required to reduce the moisture for fill and compaction. The clay or silt may requirespreading; disking and/or mixing with dry material. This can be a very expensive and time-consuming effort.

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    10. Dry Soil Density:

    Normal soils will have dry densities between 95 and 130 pounds per cubic foot (pcf). Solidrock and heavy metallic ores can have densities exceeding 150 pcf. Loose cohesionless sandwill usually have a density of about 90 pcf. Volcanic cinders can be as low as 50 pcf. Soildensity by itself tells little about the soil properties. Volcanic tuff can have a density of less

    than 100 pcf and be some of the most difficult material to excavate. Dense rock that isfractured and/or weathered can often be easily excavated.

    11. Sieve Analysis:

    This test will show the grain size distribution of the various soils. A well-graded soil will have auniform grain size distribution from gravel to clay. A poorly graded soil will have the grainconcentrated around a single grain size. Dry sands and gravels that contain less than about10% clay can be almost completely cohesionless, meaning that while they are easy toexcavate, they ravel to a slope flatter than 1H to 1V. Over sized rocks such as cobbles andboulders should be indicated. The oversized material may have to be screened out for the soilto be used as backfill or embankment. If there is more than about 25% clay, the soil may beunsuitable for backfill or embankment. Unless the soil is very dry, it is difficult to screen clay

    out of soils because it clogs the screen. Clay can be removed by washing but it is a slowexpensive process generating a lot of very dirty water that can be difficult to get rid of.

    Below is an example of a gradation test or sieve analysis:

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    The gradation result show above is an example of a well-graded material from 1.5-inch gravelto silt and clay. If the soil is dense, it can be expected to have significant cohesion and frictionangle. The fines percentage of about 18% means that it will behave more like gravel thanclay, with just enough fines to enhance the compactive effort.

    Below is another soil gradation analysis:

    This gradation shown above is a poorly graded or open graded soil. Note that most of thegrain sizes are concentrated between the #16 and #50 sieve and only 3.9% fines. This isclean sand and probably will have very little cohesion and may require a constructionexcavation slope of 1H to 1V or flatter. When dry it will be like dry beach sand. The lack offines will make it a free draining soil that will take a lot of water and vibratory compactive effortto embank.

    Soil Types:

    The basic soil types are:

    1. Clays and silts: These are soils where the grains are less than 0.005inches in size (less than #200 sieve size).

    2. Sands: The grain sizes are between 0.25 and 0.005 inches in size.(#4 to #200 sieve size)

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    3. Gravels: The grain sizes are between four and 0.25 inches in size.(4" to #4 sieve size)

    4. Cobbles: These are rounded rocks that are between 12 and 4 inchesin size.

    5. Boulders: These can be over 20 feet in size.6. Rock: Massive formations that require blasting or heavy ripping to

    excavate.7. Any combination of the above. Most soils usually contain more than

    one soil type, for instance, a soil classified as sandy-gravel is gravel-containing sand.

    Soil Classifications:

    8. ML Silt9. CL Lean Clay10. OL Low Plasticity Organic Soil11. OH High Plasticity Organic Soil12. MH Elastic Silt

    13. CH Fat Clay14. GW Well Graded Gravel15. GP Poorly Graded Gravel16. GM Silty Gravel17. SW Well Graded Sand18. SP Poorly Graded Sand19. SM Silty Sand

    The soil classifications are often shown as the combined ones when no singleclassification is accurate. For example, GW-GC stands for well graded gravel withclay. The soil classifications are another step in the process, but they do not tell us

    how hard or soft the ground is or what excavation difficulties are to be expected. Thesoil classifications refer only to the grain size of the soil and little of the other groundproperties.

    Relative denseness of the soil:

    Here we are getting to the point where the soil properties are described. Usually theterms: loose, soft, dense and hard are applied to the soil. Loose may mean runningsoils that will not stand on slopes greater than 1.5 Horizontal to one Vertical. Soft maymean clay with high water content and stable construction slopes may be difficult tomaintain. Dense soil is usually the best material to work with because it will supportthe equipment, be easily excavated and steep construction slopes can beestablished. Hard soils probably require ripping equipment and slower excavation

    productions.

    Atterburg Limits:

    This is a series of tests for plasticity index and liquid limit that are applied to clays. These area concern since the specifications may require that the backfill and embankment materialshave limited plasticity and liquid limit. This requirement is designed to control swell andsettlement as the moisture content of the soil changes. This can force the disposal of theexcavated soil and the import of more suitable soil. Often the specifications are silent as to

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    the suitability of the native soils for backfill and embankment, leaving it up to the contractor todetermine if the soil should be used. If field tests show the soil unsuitable, then the contractorpays the extra costs. Another common requirement is the sand equivalent (SE) of the backfillmaterial. That is a measure of what percentage of the soil acts as sand. Sometimes the soil'sreport will not contain these tests, but the specifications will use them to define suitablebackfill and embankment soils.

    Compaction Test:

    This is very valuable information. By comparing the optimum maximum compaction densitywith the in place, native density a reliable calculation can be made for how much the soil willshrink from the ground to compacted fill. Typically, the shrink is between 5 and 15% for mostnormal soils. Soils with high clay contents can shrink more than 25%. Blasted or ripped rockwill often swell as much as 25% as it will go from a void ratio of zero to 25% or more. If the in-situ dry density of the soil is 100 pcf and the optimum density is 120 pcf, at 90% compactionthe density of the embankment will be 108 pcf. The ratio of 100 pcf to 108 pcf shows an 8%shrink from in-situ to embankment. Usually it is a good idea to add a couple of percent ofshrink to allow for settlement, over build and over compaction.

    Below is an example of a maximum density test:

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    The optimum moisture is shown as 11.5% and the maximum density is shown as 118.5 pcf.The test method used is ASTM D 698. This method requires less compactive effort than othermethods to achieve the desired relative compaction in the soil. For instance, the method thatthe California DOT uses requires at least twice the compactive effort to achieve the samerelative compaction. Find out what the specified compaction method is because it can make abig difference in cost.

    Direct Shear Test:

    This is very valuable information. This test shows the friction angle and the cohesion of thesoil. The friction angle is a measure of the angularity of the soil particles to resist rolling.Cohesion is the measure of the bonding of the soil particles in shear. Combined with thegross soil density (moisture plus dry density), friction angle and cohesion a slip circle analysiscan accurately predict the stable temporary slope the ground can be excavated.Unfortunately, this is a very difficult analysis to perform by hand. There are computerprograms available to make the calculation quickly and easily. Some of these programs areinaccurate at steep temporary construction slopes. The author has created a program that isspecifically written for construction slopes. This takes the guesswork out of designingconstruction excavations and allows huge savings in excavation, backfill, and shoring costs.

    The program will indicate when speed shores, shields and shoring are appropriate.

    Below is an example of a direct shear test:

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    The three dots are the specific test points plotted on the graph. The slope of the extrapolatedline is the friction angle (it is usually measured with a protractor). The sloping line intersectionwith the left vertical axis defines the cohesion in psf. Sometimes the cohesion is given in psi,which for low cohesion is virtually worthless since 144 psf is only 1 psi.

    OSHA classifies all ground in four (4) categories. The first class is "Rock" which allows a

    vertical cut. The next class is "A" such as cemented soils and caliche and the recommendedconstruction slope is 3/4H to 1 V. The most common class of soil is class "B" that is compactdense soil and the recommended construction slope is 1V to 1H. The last class is "C" that isfor loose ground and the recommended slope is 1.5H to 1V. These recommendations arevery conservative. OSHA also requires all excavations over twenty (20) feet deep to beanalyzed by a registered engineer. It is also well worth your while to obtain a slip circleprogram and analyze all significant excavations.

    Seismic Velocity Lines:

    When the geologist finds or expects formational rock to be present, often seismic lines will berun. This test places several microphones in a long line. Then a charge or hammer blowsinduces sound waves into the ground. A computer analyses the time delays that are recorded

    at each microphone. This will show the depth to each stratum and the speed of soundthrough the stratums. The faster the sound travels through the ground the harder the rock.Soil with sound velocities of less than 3,000 feet per second (fps) is usually easily excavated.Rock over 10,000 fps cannot be ripped efficiently.

    Most rock that can be effectively ripped lies in the 3,000 to 8,000 fps ranges. The soundvelocity is a guide to ripping only. Very hard rock, such as granite, that is fractured and orweathered can sometimes be easily ripped. On the other hand, relatively soft rock that ismonolithic, such as volcanic tuff and caliche, can be practically impossible to rip efficiently.

    Below is a plot of seismic velocities plotted by depth from the surface:

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    The top two feet has a velocity of only 1,134 feet per second (fps). That means it is probablytopsoil or loose alluvium. The next layer with a velocity of 5,341 is rock. This can probably beripped using a Cat D8 or larger crawler tractor. This rock may also be easily blasted unless itis caliche or volcanic tuff. The rock below 22 feet is definitely drill and shoot hard rock. Thehigh velocity of 18,498 fps means that it is very hard, dense and probably the fracture jointingis widely spaced.

    Permeability:

    This test is run to determine how quickly ground water will travel through the soil. Cleansands and gravels can transmit huge volumes and require massive pumping to lower thewater table enough to work at the bottom of an excavation. Clays are impermeable and

    release the water slowly. Often the ground water is transferred through aquifers that areopen-graded and very permeable. These aquifers can be capped with impermeable layers.There have been cases where dewatering wells were unnecessarily extended into aquifersand forced excessive pumping. These tests aid the experienced dewatering specialist indesigning an appropriate dewatering system.

    Soils that contain more than 30 to 40% clay will behave like clay because the clay will morethan fill the natural voids of the larger particles. This means that the larger particles do nothave direct bearing on each other and the failure plane is mostly through the clay fines. Clayscan be some of the most difficult and undesirable soils to try to work with. Plasticity index and

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    liquid limit are used to define Clays. Expansive or fat clays have plastic and liquid indexesabout 50 or more. These materials will significantly swell and contract with the change inmoisture content. This is often the cause of pavement failure, sidewalk buckling andfoundation cracking. Clays can be plastic in that they can continue to settle under afoundation over a long term, causing cracks to develop long after the structure is completed.

    Clays vary from being a viscous fluid to dense and stable material. Water content and relativecompaction are critical to the soil properties. If there is too much water the clay becomesmud. If there is not enough moisture the clay becomes dust. Either condition causes the clayto be difficult to handle and compact. Even when the clay is at optimum moisture, it can bedifficult to achieve the required degree of compaction. To add to the problem, it can bedifficult to get the clay to dry out or accept additional water. Clays that contain high moisturecontent, above 20% or so may be unstable. The water instead of adding cohesion nowbecomes a lubricant, reducing the friction angle to nearly zero. This can result in catastrophicfailure, with little or no warning. Rain or irrigation water that is allowed to pond cansupersaturate the soil and cause a violent slide. Sometimes warning signs of impendingfailure will cracks forming in the ground parallel to the bank and/or sloughing, bulging andslumping of the bank slope. In short, clay is the least desirable structural and construction soilto work with.

    Well-graded sands and gravels that contain 5% to 15% clay or silt fines can be the bestmaterials to work with. They will excavate and compact easily. There are enough fines to holdthe optimum moisture but will not continue to settle under structural loading. The fines andwater will add cohesion and allow steep construction slopes.

    Open or poorly graded dry sand and gravel can be difficult to work with. Open or poorlygraded mean the grain size is concentrated at one sieve size. Clean dry beach sand is anexample of such material and can be difficult to walk and drive through. It will not easilycompact and added water quickly drains away. It also will ravel to its natural angle of reposewhile being excavated. When such sand is dredged from under the water it will ravel onslopes as flat as 10H to 1V. When artesian water flows upward through sand it can becomequicksand. Sands and gravels will usually have a friction angle of 30 degrees or more.

    Rock properties are often difficult to deduce from the geotechnical report. Although you maybe able to tell if the rock must be shot or ripped from the descriptions and recommendations,it usually does not describe what the rock fragments sizes will be when excavated. The sizeof the fragments depends of a number of circumstances. The drill-hole spacing, the amountand type of explosive, what the rock natural grain and fracturing is in place. Some rock willeasily break up to small sizes. Some rock will break only into large riprap sizing. Some softerrock may be degraded to a usable size by track walking. Hard rock will require crushing tomake gravel sizes.

    Disposal can be a problem if no one wants to take riprap sized rock. To reduce disposal costsover sized rock can be separated with a bar screen. Shot rock also makes an excellentbasement fill for a large embankment, such as a highway fill.

    Sometimes, the geotechnical sampler will have cores available for inspection. These coreswill give you an idea of the texture, hardness, and natural joint spacing of the rock. Try to geta sample to hold, scratch it with a knife, and see if it breaks or crumbles easily. There is nobetter way to understand what the rock properties are than holding samples and observingthe formation in place

    Any soil can be in any state of consolidation and cementation. Sandstone was once sanddunes or river deposits that were compressed by over burden and cementing minerals

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    leached into the formation over millions of years. Slate and shale were once mud on at lakeor sea bottom. Lava is heat fused inert minerals that weather very slowly and is usually veryhard, requiring blasting or hoe ramming to remove. There are thousands of minerals thatmake up millions of soil conditions. No two sites are exactly alike.

    Only through study of the site-specific geotechnical report and a site visit can any real

    judgement of construction methods be made. It is also recommended that potholing with abackhoe be performed in the presence of the estimator for the project. Review thespecifications to determine if the soil is suitable for backfill. The sieve analysis may showthere is oversize that must be screened out. There may be too much clay fines. The Atterburgproperties or sand equivalent may make the in-situ soil unsuitable and the material must beover excavated and/or disposed of offsite and suitable material bought and imported.

    Failure to understand the geotechnical report has caused many a financial disaster. Thecontractor must bid aggressively to win work. It is better to know the real conditions and avoidcostly mistakes that can cause a project to lose money out of pocket by ignoring ormisinterpreting the information contained in the report. Each geologist will present his or herfindings in different ways. Some reports are more thorough than others for several reasons. Itis common to find that underground conditions vary from what was discovered by the test

    borings. At rare times the owner will even deliberately misrepresent the facts to gain lowerbids. This usually results in a long expensive lawsuit over changed conditions that only thelawyers win.

    Each soils report will contain a great deal of information. There is no one test or observationthat will tell the contractor what is the best construction method for each site. Each test andobservation is a clue. There are always dozens or even thousands of bits of information thatmust be scrutinized, analyzed, and correlated. This process can take several people weeks toprepare an excavation plan for a large project such as a dam. Only then can methods andproductions be reasonably estimated.

    Geotechnical reports always include disclaimers and warnings that the ground conditions mayvary from those found in the borings and test pits. There is good reason for the disclaimers.

    Geotechnical work is still as much an art as it is a science. Any extrapolations derived formthe reports are individual interpretations of what that person might expect to find. No one isable to see exactly what is under the ground surface. All the tests and observations onlyindicate probable trends that are often subject for debate between expert geologists. Thecontractor must be an expert is his own right. Only the contractor can and will decide whatmethods and costs are to be bid for the excavation and embankment work. If the contractormakes a judgmental mistake, no else will offer to pay for any of the additional costs.