Learning Objective
This course introduces the student to the many components that make up a thorough Geotechnical report. The various observations and tests are explained so that an inference to the ground properties can be achieved. The Geotechnical report is a complex scientific document that can be confusing to even highly experienced people.
This course introduces the student to the many components that make up a thorough Geotechnical report. The various observations and tests are explained so that an inference to the ground properties can be achieved. The Geotechnical report is a complex scientific document that can be confusing to even highly experienced people.
Owners, architects, engineers, and contractors must be able to understand the implications of the reports in 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 foundation and earthwork. The structural engineer is responsible for the design of an economical but sturdy building foundation. The architect may be forced to arrange building layouts to accommodate varying soil conditions. The contractor wants to bid a competitive but realistic price for the excavation and embankment work. For these reasons, all the essential players have a personal stake in the Geotechnical report. All these players must be able to understand the ramifications of the soil study. A foundation failure of a major structure is measured in millions of dollars and ruined careers. It can even lead to loss of life, such as the Teton Dam failure. An ignorant or misinformed contractor can lose millions in unanticipated costs. Such mistakes often lead to major lawsuits that drag on for years and cost everyone except the lawyers. Being able to understand and analyze the Geotechnical report to avoid costly mistakes 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 information regarding the soil conditions at a specific location. These reports are a wealth of information for the person that can properly interpret the information presented. Usually the soil report is primarily written to give the structural engineer the specific information needed to effectively design the structural foundations. Construction methods and contractor concerns are usually addressed as a minor side issue and in gross generalities. Only after a number of site visits and studying several soil reports will you begin to 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 more definitive the soils report the greater the confidence the engineer has in developing a foundation design and the contractor will price the work more competitively. While small, light structures with shallow footing may only need a cursory review, large complex projects require a definitive, comprehensive and thorough Geotechnical 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 changed conditions. There is always language that states that the report represents only the actual spots examined and conditions can vary. This expressed caution is very real and it is very common to find unexpected soil changes or buried obstructions that are discovered during construction. It is wise to be very leery of contracts that contain a no changed condition clause. Whether the owner is trying to hide something 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 try to visit the project site and do your own pot holing with a backhoe. This will give you a much better understanding 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 local contractors will gain an amazing knowledge of the regional geology and the construction methods that are the most effective. For instance, in one area I worked there was an extensive volcanic tuff that was very difficult to rip or blast. By knowing the tuff formation limits exactly gave us a huge advantage in bidding work in the area and kept us out of trouble. If possible, interview local contractors and equipment operators 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 know what is needed to build the designed foundation. Generally, construction methods are the chosen by the contractor so long as no damage is caused. For that reason, the soil reports tend to be very vague when addressing construction methods.
The design engineer wants to know what is under the surface to support the structures. To that end, the engineer needs to know several critical soil properties:
- What is the allowable soil bearing pressure?
- What is the expected foundation settlement?
- What is the active soil load?
- What is the passive soil loading?
- What is the sliding friction factor?
- What is the potential for differential settlement?
- What is the soil liquefaction potential during an earthquake?
- What are the seismic design accelerations?
- Where is the groundwater table?
- What is a permanent stable slope?
- Will piling be required?
- Can the native soil be used for backfill?
- What are the criteria for the pavement sections?
- Are hazardous wastes present?
- How corrosive are the soils?
- 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 costs and time can be entered into a competitive bid. To that end the contractor will search the soils report to determine the following:
- Where is the ground water and how much water must be pumped?
- Is there rock to be drilled and shot (sometimes blasting is not allowed)?
- Can the ground be ripped with a bulldozer?
- What excavation equipment and methods will be most effective?
- Is there enough space on the job to store backfill materials?
- Can the native material be used for backfill?
- Will the native material need to be processed (screened/crushed) for backfill?
- How much backfill must be bought and imported?
- How steep can the temporary excavation slopes be cut?
- Can obstructions be expected?
- What compactive effort and equipment is needed for backfilling?
- Will excavation shoring be required?
- What is the most effective shoring method?
- 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 is directed mostly to the design engineer and the contractor usually must make an interpretation of the information to develop a construction plan. This interpretation is often vital to the success of the project. If the report is ambiguous or fails to properly identify the ground conditions, the result is often a changed condition claim. These claims can entail lengthy delays, increased cost, disputes and lawsuits. It is important to read the entire report and understand that each geologist has a different style of describing the soils, as they are field sampling.
Recently, a geologist described the soil in one report as dense silty-sand. This classification was suspicious because the borehole was advanced with a carbide tipped rotary drill and hit refusal above the required excavation depth. Further investigation revealed that the soil was in fact unweathered massive granite rock. Because blasting was not allowed, the excavation costs were estimated at $50.00 per cubic yard versus $3.00 per cubic yard. This amounted to nearly a million dollars of added excavation cost to the estimate. A cursory review of the report could easily mislead the owner, engineer and contractor with disastrous results.
Geotechnical Report Contents:
The well-prepared Geotechnical report will be.comanized along the following outline:
- Introduction: This will identify the project by location and name. It will also briefly outline the scope of the investigation.
- Project Description: This will give an overview of the structures, with proposed foundation depths.
- Field Exploration and Testing: This will identify the methods and equipment used to bore and test the soils.
- Site conditions: This will describe the terrain, prior known land use, general area geology, groundwater, fault proximity, seismic shaking, landslides and other concerns such as sink hole or fracturing problems.
- Recommendations: The various explorations and tests are translated into specific loading criteria, settlements, dewatering requirements, seismic accelerations, pavement sections and site coefficients.
- Site observations: This is where the appeal is made that it is important to have Geotechnical experts review the actual excavation procedures and the warning that differing soil conditions may be found.
- Maps: A general area map with geology and faults is usually included. Site maps with boring log locations are included.
- Logs: This will contain the boring logs with soil densities, blow counts, ground water elevations, moisture, soil classifications and sample locations.
- Test results: This section includes sieve analysis, optimum moisture plots, direct shear tests, cone penetrometer, contaminants and other various tests that are deemed necessary.
- Seismic velocities: This procedure is used when hard rock is expected. The sound speed through rock is a good indicator of what methods will effectively excavate the ground.
The contractor must make an interpretation of the soils report in order to answer the 14 common questions. There are a number of clues in the report that will assist the contractor in making decisions to select the appropriate construction methods, productions and estimating earthmoving costs. There is no one indicator to show what the ground conditions really will be. The entire report must be read, absorbed and analyzed. The report will contain observations, recommendations and test results that must be individually interpreted. Sometimes apparently conflicting information will be presented. Also, the soil conditions 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 flat that 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:
- Soil boring equipment
- Recommendations for construction methods and slopes
- Soil boring logs
- Soil description (classification such as silty-sand)
- Soil properties (hard, stiff, dense, loose, etc.)
- Boring depth
- Boring refusal
- Blow counts
- Soil moisture content
- Soil dry density
- Particle distribution curves (sieve analysis)
- Atterburg limits (liquid limits and plasticity index)
- Compaction test (optimum moisture for compaction)
- Direct shear tests
- Seismic velocity lines
- Permeability
- Boring Equipment:
- Recommendations:
- Soil Boring Logs:
- The date of the boring. This is useful information. Water table can fluctuate seasonally.
- The boring log number, in this case: B-1. This will be plotted on a plan view so that the actual spot can be scaled to the work to be done.
- The ground elevation at the boring is given: 703.8 (MSL). MSL is for mean sea level. Marine construction often uses mean lower low tide elevation as a reference. The difference between these references can cause mistakes, so be sure of the reference elevation.
- The drill used was a hollow stem auger. This means that the ground was not rock, since and auger will not penetrate rock that requires blasting or heavy ripping.
- The soil is logged and classified by depth from the ground surface. Sometimes the log will also reference elevation.
- The blows/foot is number of times the 140 lbs drive weight dropped 30 inches needs to penetrate one foot. The 37 blows per foot confirm the description of dense or well-compacted soil.
- The moisture content range of 1.2 to 5.7% means the soil is very dry and will require a lot of water to achieve compaction.
- The dry density in the top 4 feet is 127.7 pound per cubic foot (PCF). This indicates a well-graded soil with a low void ratio. It can be expected to have significant cohesion and friction angle.
- 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. This soil will probably have little or no cohesion. That means it may not stand on a construction slope as steep as 1H to 1V.
- The soil classification descriptions and relative compactions are shown as loose, dense, etc.
- Note that no ground water is shown. If ground water is found it will be shown at the depth of encounter as an inverted triangle with a line under it. Sometimes this symbol is small and easy to miss.
- Soil Descriptions:
- Soil Properties:
- Boring Depth:
- Boring Refusal:
- Blow Counts:
- Soil Moisture Content:
- Dry Soil Density:
- Sieve Analysis:
- Clays and silts: These are soils where the grains are less than 0.005 inches in size (less than #200 sieve size).
- Sands: The grain sizes are between 0.25 and 0.005 inches in size. (#4 to #200 sieve size)
- Gravels: The grain sizes are between four and 0.25 inches in size. (4" to #4 sieve size)
- Cobbles: These are rounded rocks that are between 12 and 4 inches in size.
- Boulders: These can be over 20 feet in size.
- Rock: Massive formations that require blasting or heavy ripping to excavate.
- 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.
- ML – Silt
- CL – Lean Clay
- OL – Low Plasticity Organic Soil
- OH – High Plasticity Organic Soil
- MH – Elastic Silt
- CH – Fat Clay
- GW – Well Graded Gravel
- GP – Poorly Graded Gravel
- GM – Silty Gravel
- SW – Well Graded Sand
- SP – Poorly Graded Sand
- SM – Silty Sand
- Atterburg Limits:
- Compaction Test:
- Direct Shear Test:
- Seismic Velocity Lines:
- Permeability:
The bore hole or test pit is advanced with a variety of equipment. Bore holes advanced with an 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, carbide tipped 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 difference as to how easily the ground can actually be dug. A small rubber tired backhoe-loader will show refusal (unable to dig) on soils that can be readily excavated with large track mounted backhoes or heavy equipment.
If the report is concerned with settlement, liquefaction and suggests over-excavation; that usually means weak clayey soils. While these soils may be easy to dig, they may also be unsuitable for backfill and wet and sticky. Even though clayey soils may appear to be firm enough to drive on, they can start to pump with repeated heavy-wheeled traffic. This is when the moisture is worked toward the surface by the equipment pounding and turns the top layer to mud and the equipment will get stuck. If this condition is present it may be necessary to build aggregate haul roads. Rain and snowmelt will turn clays to mud and may be difficult to dry out enough to travel on.
Often the recommendations will include soil pressures for temporary shoring. If the recommended soil active pressures are below about 25 pcf, the ground should be firm enough to allow fairly steep construction excavation slopes. If the recommendations are in the order 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. Very high-recommended loading of about 80 pcf indicates ground water is present and/or oozing clay mud is present. Remember this is only one part of the information available. Further scrutiny of the report will often reveal that the recommendations are ultra-conservative.
The boring logs will detail the soil layers by depth from the surface or by elevations. The log will contain such information as: soil classification, relative denseness of the soil, sampling points, sample recovery, water content, dry unit weight, blow counts per foot and ground water depth, drill refusal and if well casing was needed. Often the soil descriptions are subjective by being based on the experience and judgement of the observing geologist. How dense 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. The boring logs are a good place to start to understand the soil properties. The boring logs should be 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 usually means the ground is too weak to stand on a normal construction excavation slope.
Below is an example of a well-presented boring log
Note that the log presents the following data.
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 is a 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 the soil and give it shear strength. Soils with clay or silt are often referred to as cohesive soils, as they are capable of developing significant shear strength.
Usually soils are described as loose, dense, hard, stiff, soft, etc. Usually, the relative density terms of: loose, dense are applied to sands and gravels. Terms such as soft and stiff are usually applied to clays and silts. Hard can mean rock or cemented soils, although if the soils are cemented the geologist will usually mention that fact. Loose or soft ground means that the ground 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 tightly packed but have little or no fines to cause cohesion. Cemented soils can be caliche, volcanic tuff, or pyroclastic ash. These soils can be some of the most difficult and expensive materials to 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 a ripper tooth.
Usually the boring logs are advanced well below the planned excavation depth to insure that accurate formational trends can be plotted and no weak layers are present near the foundation grade. When the borings are terminated above or just at the planned excavation depth, you must be very wary. This means you cannot be sure of what may be encountered at the bottom of the excavation. Is there going to be rock or water that must be handled? Did an obstruction halt the drilling effort?
This is usually indicated at the bottom to the boring log, if it occurs. If all refusals are well below the planned excavation depth, it will not be a major concern. If a small percentage of the borings have met refusal in the excavation limits it usually means that there are isolated obstructions. It is not possible to determine what is the obstruction unless the soils report or site history reveals the object. A small isolated boulder can stop an auger drill and be of little concern. On the other hand, it could be a ridge of solid rock that will be expensive to remove.
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 is counted. Obviously the more the blows the harder the soil. Loose or soft soils will show blow counts of less than 10. Blow counts of 10 to 50 blows per foot usually mean the ground will be fairly easily excavated. When the blow counts are over 50 but less than 100, ripping of the ground is very likely. When blow counts exceed 100, the ground may be very difficult to excavate and require blasting or hoe ram effort.
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. Supersaturated clay can be 50% water. Those are oozing mud that will not support the weight of even low ground 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 allows easy compaction effort. We have all seen sandcastles built on the beach with moist sand and cars can drive just above the surf. Higher up on the beach where the sand is dry, cars will get stuck and the sand can only be piled in a cone shape. As soon as a wave washes over the sandcastle, it is erased. Clean, dry sand and gravel will have little or no cohesion and will ravel to a slope of at least 1H to 1V. The natural undisturbed angle of repose of sand and gravel is about 1.5H to 1V slope. This is seen at a gravel mine where processed material is discharged from a conveyor and forms a cone shaped pile. The friction angle of the soil allows 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 to excavate dry ground efficiently. When the difference between the in-situ moisture and the optimum moisture is known, water delivery needs can be calculated. If 6% water must be added to the soil to reach optimum water and the soil weights 100 pcf, then 6 pounds of water must 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 the effective water very rapidly and the required usage can double. It is difficult to add water to clays because the permeability is low. These soils may require substantial mixing and kneading to uniformly introduce water. The general rule of thumb is 30 to 50 gallons of water are needed to compact each cubic yard of soil. A scraper fleet is capable of moving 1,000 cyd per hour. The water needs can be approximately a 1,000 gal/min. Water supply for such operations can be a major problem.
When the moisture content is more than about 15%, the ground is too wet. Equipment can bog 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 water table. These soils are usually free drain enough that no special effort is required to dry them out. On the other hand, overly wet clays can be a serious problem to handle. Wet clay will tend to stick to tires and truck beds greatly reducing load and haul efficiency. Extensive effort may be required to reduce the moisture for fill and compaction. The clay or silt may require spreading; disking and/or mixing with dry material. This can be a very expensive and time-consuming effort.
Normal soils will have dry densities between 95 and 130 pounds per cubic foot (pcf). Solid rock and heavy metallic ores can have densities exceeding 150 pcf. Loose cohesionless sand will usually have a density of about 90 pcf. Volcanic cinders can be as low as 50 pcf. Soil density 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 is fractured and/or weathered can often be easily excavated.
This test will show the grain size distribution of the various soils. A well-graded soil will have a uniform grain size distribution from gravel to clay. A poorly graded soil will have the grain concentrated around a single grain size. Dry sands and gravels that contain less than about 10% clay can be almost completely cohesionless, meaning that while they are easy to excavate, they ravel to a slope flatter than 1H to 1V. Over sized rocks such as cobbles and boulders should be indicated. The oversized material may have to be screened out for the soil to be used as backfill or embankment. If there is more than about 25% clay, the soil may be unsuitable 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 slow expensive 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:
The gradation result show above is an example of a well-graded material from 1.5-inch gravel to silt and clay. If the soil is dense, it can be expected to have significant cohesion and friction angle. The fines percentage of about 18% means that it will behave more like gravel than clay, 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 the grain sizes are concentrated between the #16 and #50 sieve and only 3.9% fines. This is clean sand and probably will have very little cohesion and may require a construction excavation slope of 1H to 1V or flatter. When dry it will be like dry beach sand. The lack of fines will make it a free draining soil that will take a lot of water and vibratory compactive effort to embank.
Soil Types:
The basic soil types are:
Soil Classifications:
The soil classifications are often shown as the combined ones when no single classification is accurate. For example, GW-GC stands for well graded gravel with clay. 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. The soil classifications refer only to the grain size of the soil and little of the other ground properties.
Relative denseness of the soil:
Here we are getting to the point where the soil properties are described. Usually the terms: loose, soft, dense and hard are applied to the soil. Loose may mean running soils that will not stand on slopes greater than 1.5 Horizontal to one Vertical. Soft may mean clay with high water content and stable construction slopes may be difficult to maintain. Dense soil is usually the best material to work with because it will support the equipment, be easily excavated and steep construction slopes can be established. Hard soils probably require ripping equipment and slower excavation productions.
This is a series of tests for plasticity index and liquid limit that are applied to clays. These are a concern since the specifications may require that the backfill and embankment materials have limited plasticity and liquid limit. This requirement is designed to control swell and settlement as the moisture content of the soil changes. This can force the disposal of the excavated soil and the import of more suitable soil. Often the specifications are silent as to the suitability of the native soils for backfill and embankment, leaving it up to the contractor to determine if the soil should be used. If field tests show the soil unsuitable, then the contractor pays the extra costs. Another common requirement is the sand equivalent (SE) of the backfill material. That is a measure of what percentage of the soil acts as sand. Sometimes the soil's report will not contain these tests, but the specifications will use them to define suitable backfill and embankment soils.
This is very valuable information. By comparing the optimum maximum compaction density with the in place, native density a reliable calculation can be made for how much the soil will shrink from the ground to compacted fill. Typically, the shrink is between 5 and 15% for most normal soils. Soils with high clay contents can shrink more than 25%. Blasted or ripped rock will 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% compaction the 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 of shrink to allow for settlement, over build and over compaction.
Below is an example of a maximum density test:
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 other methods to achieve the desired relative compaction in the soil. For instance, the method that the California DOT uses requires at least twice the compactive effort to achieve the same relative compaction. Find out what the specified compaction method is because it can make a big difference in cost.
This is very valuable information. This test shows the friction angle and the cohesion of the soil. 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 the gross soil density (moisture plus dry density), friction angle and cohesion a slip circle analysis can accurately predict the stable temporary slope the ground can be excavated. Unfortunately, this is a very difficult analysis to perform by hand. There are computer programs available to make the calculation quickly and easily. Some of these programs are inaccurate at steep temporary construction slopes. The author has created a program that is specifically written for construction slopes. This takes the guesswork out of designing construction 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:
The three dots are the specific test points plotted on the graph. The slope of the extrapolated line is the friction angle (it is usually measured with a protractor). The sloping line intersection with 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 recommended construction slope is 3/4H to 1 V. The most common class of soil is class "B" that is compact dense soil and the recommended construction slope is 1V to 1H. The last class is "C" that is for loose ground and the recommended slope is 1.5H to 1V. These recommendations are very conservative. OSHA also requires all excavations over twenty (20) feet deep to be analyzed by a registered engineer. It is also well worth your while to obtain a slip circle program and analyze all significant excavations.
When the geologist finds or expects formational rock to be present, often seismic lines will be run. This test places several microphones in a long line. Then a charge or hammer blows induces 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 sound through 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 sound velocity is a guide to ripping only. Very hard rock, such as granite, that is fractured and or weathered can sometimes be easily ripped. On the other hand, relatively soft rock that is monolithic, 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:
The top two feet has a velocity of only 1,134 feet per second (fps). That means it is probably topsoil or loose alluvium. The next layer with a velocity of 5,341 is rock. This can probably be ripped using a Cat D8 or larger crawler tractor. This rock may also be easily blasted unless it is caliche or volcanic tuff. The rock below 22 feet is definitely drill and shoot hard rock. The high velocity of 18,498 fps means that it is very hard, dense and probably the fracture jointing is widely spaced.
This test is run to determine how quickly ground water will travel through the soil. Clean sands and gravels can transmit huge volumes and require massive pumping to lower the water 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 are open-graded and very permeable. These aquifers can be capped with impermeable layers. There have been cases where dewatering wells were unnecessarily extended into aquifers and forced excessive pumping. These tests aid the experienced dewatering specialist in designing an appropriate dewatering system.
Soils that contain more than 30 to 40% clay will behave like clay because the clay will more than fill the natural voids of the larger particles. This means that the larger particles do not have direct bearing on each other and the failure plane is mostly through the clay fines. Clays can be some of the most difficult and undesirable soils to try to work with. Plasticity index and liquid limit are used to define Clays. Expansive or fat clays have plastic and liquid indexes about 50 or more. These materials will significantly swell and contract with the change in moisture content. This is often the cause of pavement failure, sidewalk buckling and foundation cracking. Clays can be plastic in that they can continue to settle under a foundation 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 relative compaction are critical to the soil properties. If there is too much water the clay becomes mud. If there is not enough moisture the clay becomes dust. Either condition causes the clay to be difficult to handle and compact. Even when the clay is at optimum moisture, it can be difficult to achieve the required degree of compaction. To add to the problem, it can be difficult to get the clay to dry out or accept additional water. Clays that contain high moisture content, above 20% or so may be unstable. The water instead of adding cohesion now becomes a lubricant, reducing the friction angle to nearly zero. This can result in catastrophic failure, with little or no warning. Rain or irrigation water that is allowed to pond can supersaturate the soil and cause a violent slide. Sometimes warning signs of impending failure will cracks forming in the ground parallel to the bank and/or sloughing, bulging and slumping of the bank slope. In short, clay is the least desirable structural and construction soil to work with.
Well-graded sands and gravels that contain 5% to 15% clay or silt fines can be the best materials to work with. They will excavate and compact easily. There are enough fines to hold the optimum moisture but will not continue to settle under structural loading. The fines and water will add cohesion and allow steep construction slopes.
Open or poorly graded dry sand and gravel can be difficult to work with. Open or poorly graded mean the grain size is concentrated at one sieve size. Clean dry beach sand is an example of such material and can be difficult to walk and drive through. It will not easily compact and added water quickly drains away. It also will ravel to its natural angle of repose while being excavated. When such sand is dredged from under the water it will ravel on slopes as flat as 10H to 1V. When artesian water flows upward through sand it can become quicksand. 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 may be 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 size of the fragments depends of a number of circumstances. The drill-hole spacing, the amount and type of explosive, what the rock natural grain and fracturing is in place. Some rock will easily break up to small sizes. Some rock will break only into large riprap sizing. Some softer rock may be degraded to a usable size by track walking. Hard rock will require crushing to make gravel sizes.
Disposal can be a problem if no one wants to take riprap sized rock. To reduce disposal costs over sized rock can be separated with a bar screen. Shot rock also makes an excellent basement fill for a large embankment, such as a highway fill.
Sometimes, the geotechnical sampler will have cores available for inspection. These cores will give you an idea of the texture, hardness, and natural joint spacing of the rock. Try to get a sample to hold, scratch it with a knife, and see if it breaks or crumbles easily. There is no better way to understand what the rock properties are than holding samples and observing the formation in place
Any soil can be in any state of consolidation and cementation. Sandstone was once sand dunes or river deposits that were compressed by over burden and cementing minerals leached into the formation over millions of years. Slate and shale were once mud on at lake or sea bottom. Lava is heat fused inert minerals that weather very slowly and is usually very hard, requiring blasting or hoe ramming to remove. There are thousands of minerals that make 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 a backhoe be performed in the presence of the estimator for the project. Review the specifications to determine if the soil is suitable for backfill. The sieve analysis may show there is oversize that must be screened out. There may be too much clay fines. The Atterburg properties or sand equivalent may make the in-situ soil unsuitable and the material must be over excavated and/or disposed of offsite and suitable material bought and imported.
Failure to understand the geotechnical report has caused many a financial disaster. The contractor must bid aggressively to win work. It is better to know the real conditions and avoid costly mistakes that can cause a project to lose money out of pocket by ignoring or misinterpreting the information contained in the report. Each geologist will present his or her findings in different ways. Some reports are more thorough than others for several reasons. It is 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 lower bids. This usually results in a long expensive lawsuit over changed conditions that only the lawyers win.
Each soils report will contain a great deal of information. There is no one test or observation that will tell the contractor what is the best construction method for each site. Each test and observation is a clue. There are always dozens or even thousands of bits of information that must be scrutinized, analyzed, and correlated. This process can take several people weeks to prepare an excavation plan for a large project such as a dam. Only then can methods and productions be reasonably estimated.
Geotechnical reports always include disclaimers and warnings that the ground conditions may vary 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 form the reports are individual interpretations of what that person might expect to find. No one is able to see exactly what is under the ground surface. All the tests and observations only indicate probable trends that are often subject for debate between expert geologists. The contractor must be an expert is his own right. Only the contractor can and will decide what methods and costs are to be bid for the excavation and embankment work. If the contractor makes a judgmental mistake, no else will offer to pay for any of the additional costs.
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