In addition to document review and subsurface exploration, an important part of the site investigation is laboratory testing. The laboratory testing usually begins once the subsurface exploration is complete. The first step in the laboratory testing is to log in all of the materials (soil, rock, or groundwater) recovered from the subsurface exploration. Then the geotechnical engineer and engineering geologist prepare a laboratory testing program, which basically consists of assigning specific laboratory tests for the soil specimens. The actual laboratory testing of the soil specimens is often performed by experienced technicians, who are under the supervision of the geotechnical engineer. Because the soil samples can dry out or changes in the soil structure could occur with time, it is important to perform the laboratory tests as soon as possible.
Usually at the time of the laboratory testing, the geotechnical engineer and engineering geologist will have located the critical soil layers or subsurface conditions that will have the most impact on the design and construction of the project. The laboratory testing program should be oriented towards the testing of those critical soil layers or subsurface conditions. For many geotechnical projects, it is also important to determine the amount of ground surface movement due to construction of the project. In these cases, laboratory testing should model future expected conditions so that the amount of movement or stability of the ground can be analyzed.
Laboratory tests should be performed in accordance with standard procedures, such as those recommended by the American Society for Testing and Materials (ASTM) or those procedures listed in standard textbooks or specification manuals.
For laboratory tests, it has been stated (M. J. Tomlinson, ‘‘Foundation Design and Construction,’’ 5th ed., John Wiley & Sons, Inc., New York):
It is important to keep in mind that natural soil deposits are variable in composition and state of consolidation; therefore it is necessary to use considerable judgment based on common sense and practical experience in assessing test results and knowing where reliance can be placed on the data and when they should be discarded. It is dangerous to put blind faith in laboratory tests, especially when they are few in number. The test data should be studied in conjunction with the borehole records and the site observations, and any estimations of bearing pressures or other engineering design data obtained from them should be checked as far as possible with known conditions and past experience. Laboratory tests should be as simple as possible. Tests using elaborate equipment are time-consuming and therefore costly, and are liable to serious error unless carefully and conscientiously carried out by highly experienced technicians. Such methods may be quite unjustified if the samples are few in number, or if the cost is high in relation to the cost of the project. Elaborate and costly tests are justified only if the increased accuracy of the data will give worthwhile savings in design or will eliminate the risk of a costly failure.
In order to analyze the results of laboratory tests, the concept of the soil element must be introduced. Figure 6.7 shows an element of soil that can be divided into three basic parts:
1. Solids—the mineral soil particles
2. Liquids—usually water that is contained in the void spaces between the solid mineral particles
3. Gas—such as air that is also contained in the void spaces between the solid mineral particles
As indicated on the right side of Fig. 6.7, the three basic parts of soil can be
rearranged into their relative proportions based on volume and mass. Note that the
symbols as defined in Fig. 6.7 will be used throughout this section
Index tests are the most basic types of laboratory tests performed on soil samples.
Index tests include the water content (also known as moisture content), specific gravity tests, unit weight determinations, and particle size distributions and Atterberg limits, which are used to classify the soil.
Water Content (w). The water content (also known as moisture content) test is probably the most common and simplest type of laboratory test. This test can be performed on disturbed or undisturbed soil specimens. The water content test consists of determining the mass of the wet soil specimen and then drying the soil in an oven overnight (12 to 16 hr) at a temperature of 110C (ASTM D 2216-92, 1998). The water content (w) of a soil is defined as the mass of water in the soil (Mw) divided by the dry mass of the soil (Ms), expressed as a percentage (i.e., w 100 Mw/Ms).
Values of water content (w) can vary from essentially 0% up to 1200%. A water content of 0% indicates a dry soil. An example of a dry soil would be near-surface rubble, gravel, or clean sand located in a hot and dry climate, such as Death Valley, California. Soil having the highest water content is organic soil, such as fibrous peat, which has been reported to have a water content as high as 1200%.
Specific Gravity of Soil Solids (G). The specific gravity (G) is a dimensionless parameter that is defined as the density of solids (s) divided by the density of water (w), or G s /w. The density of solids (s) is defined as the mass of solids (Ms) divided by the volume of solids (Vs). The density of water (w) is equal to 1 g/cm3 (or 1 Mg/m3) and 62.4 pcf.
For soil, the specific gravity is obtained by measuring the dry mass of the soil and then using a pycnometer to obtain the volume of the soil. Table 6.5 presents typical values and ranges of specific gravity versus different types of soil minerals.
Because quartz is the most abundant type of soil mineral, the specific gravity for inorganic soil is often assumed to be 2.65. For clays, the specific gravity is often assumed to be 2.70 because common clay particles, such as montmorillonite and illite, have slightly higher specific gravity values.
Total Unit Weight (t ). The total unit weight (also known as the wet unit weight) should only be obtained from undisturbed soil specimens, such as those extruded from Shelby tubes or on undisturbed block samples obtained from test pits and trenches. The first step in the laboratory testing is to determine the wet density, defined as t M/V, where M total mass of the soil, which is the sum of the mass of water (Mw) and mass of solids (Ms), and V total volume of the soil sample as defined in Fig. 6.7. The volume (V) is determined by trimming the soil specimen to a specific size or extruding the soil specimen directly from the sampler into confining rings of known volume, and then the total mass (M) of the soil specimen is obtained by using a balance.
The next step is to convert the wet density (t) to total unit weight (t). In order to convert wet density to total unit weight in the International System of Units (SI), the wet density is multiplied by g (where g acceleration of gravity 9.81 m/ sec2) to obtain the total unit weight, which has units of kN/m3. For example, in the International System of Units, the density of water (w) 1.0 g/cm3 or 1.0 Mg/m3, while the unit weight of water (w) 9.81 kN/m3.
In the United States Customary System, density and unit weight have exactly the same value. Thus, the density of water and the unit weight of water are 62.4 pcf. However, for the density of water (w), the units should be thought of as lbmass (lbm) per cubic ft, while for unit weight (w), the units are lb-force (lbf) per cubic foot. In the United States Customary System, it is common to assume that 1 lbm 1 lbf.
Typical values for total unit weight (t) are 110 to 130 pcf (17 to 20 kN/m3).
Besides the total unit weight, other types of unit weight are used in geotechnical engineering. For example, the dry unit weight (d ) refers to only the dry soil per volume, while the saturated unit weight (sat) refers to a special case where all the soil voids are filled with water (i.e., saturated soil). Another commonly used unit weight is the buoyant unit weight (b) which is used for calculations involving soil located below the groundwater table. Table 6.6 presents various equations used to
calculate the different types of unit weights. Note in Table 6.6 that w water content and G specific gravity of soil solids. The void ratio (e) and degree of saturation (S) are discussed in the next article.
Phase relationships are the basic soil relationships used in geotechnical engineering.
They are also known as weight-volume relationships. Different types of phase relationships are discussed below:
Void Ratio (e) and Porosity (n). The void ratio (e) is defined as the volume of voids (Vv) divided by the volume of solids (Vs). The porosity (n) is defined as volume of voids (Vv) divided by the total volume (V). As indicated in Fig. 6.7, the volume of voids is defined as the sum of the volume of air and volume of water in the soil.
The void ratio (e) and porosity (n) are related as follows:
The void ratio and porosity indicate the relative amount of void space in a soil.
The lower the void ratio and porosity, the denser the soil (and vice versa). The natural soil having the lowest void ratio is probably till. For example, a typical value of dry density for till is 2.34 Mg/m3 (146 pcf), which corresponds to a void ratio of 0.14. A typical till consists of a well-graded soil ranging in particle sizes from clay to gravel and boulders. The high density and low void ratio are due to the extremely high stress exerted by glaciers. For compacted soil, the soil type with typically the lowest void ratio is a well-graded decomposed granite (DG). A typical value of maximum dry density (Modified Proctor) for a well-graded DG is 2.20 Mg/m3 (137 pcf), which corresponds to a void ratio of 0.21. In general, the factors needed for a very low void ratio for compacted and naturally deposited soil are as follows:
1. A well-graded grain-size distribution
2. A high ratio of D100 /D0 (ratio of the largest and smallest grain sizes)
3. Clay particles (having low activity) to fill in the smallest void spaces
4. A process, such as compaction or the weight of glaciers, to compress the soil particles into dense arrangements
At the other extreme are clays, such as sodium montmorillonite, which at low confining pressures can have a void ratio of more than 25. Highly organic soil, such as peat, can have even higher void ratios.
with water. A totally dry soil will have a degree of saturation of 0%, while a saturated soil, such as a soil below the groundwater table, will have a degree of saturation of 100%. Typical ranges of degree of saturation versus soil condition are as follows:
Useful Relationships. A frequently used method of solving phase relationships is first to fill in the phase diagram shown in Fig. 6.7. Once the different mass and volumes are known, the various phase relationships can be determined. Another approach is to use equations that relate different parameters. A useful relationship is as follows:
The purpose of soil classification is to provide the geotechnical engineer with a way to predict the behavior of the soil for engineering projects. There are many different soil classification systems in use, and only three of the most commonly used systems will be discussed in this section.
Unified Soil Classification System (USCS). As indicated in Table 6.8, this classification system separates soils into two main groups: coarse-grained soils (more than 50% by weight of soil particles retained on No. 200 sieve) and fine-grained soils (50% or more by weight of soil particles pass the No. 200 sieve).
The coarse-grained soils are divided into gravels and sands. Both gravels and sands are further subdivided into four secondary groups as indicated in Table 6.8. The four secondary classifications are based on whether the soil is well graded, poorly graded, contains silt-sized particles, or contains clay-sized particles. These data are obtained from a particle size distribution, also known as a ‘‘grain size curve,’’ which is obtained from laboratory testing (sieve and hydrometer tests).
Figure 6.8 presents examples of grain size curves.
The Atterberg limits are used to classify fine-grained soil, and they are defined as follows:
Liquid Limit (LL). The water content corresponding to the behavior change between the liquid and plastic state of a silt or clay. The liquid limit is deter mined in the laboratory by using a liquid limit device. The liquid limit is defined as the water content at which a pat of soil, cut by a groove of standard dimensions, will flow together for a distance of 12.7 mm (0.5 in) under the impact of 25 blows in a standard liquid limit device.
Plastic Limit (PL). The water content corresponding to the behavior change between the plastic and semisolid state of a silt or clay. The plastic limit is also determined in the laboratory and is defined as the water content at which a silt or clay will just begin to crumble when rolled into a tread approximately 3.2 mm (0.125 in) in diameter.
The plasticity index (PI) is defined as the liquid limit minus the plastic limit (i.e., PI LL PL). With both the liquid limit and plasticity index of the finegrain soil known, the plasticity chart (Fig. 6.9) is then used to classify the soil.
There are three basic dividing lines on the plasticity chart, the LL 50 line, the A-line, and the U-line. The LL 50 line separates soils into high and low plasticity, the A-line separates clays from silts, and the U-line represents the upper-limit line (i.e., uppermost boundary of test data).
As indicated in Table 6.8, symbols (known as ‘‘group symbols’’) are used to identify different soil types. The group symbols consist of two capital letters. The first letter indicates the following: G for gravel, S for sand, M for silt, C for clay, and O for organic. The second letter indicates the following: W for well graded, which indicates that a coarse-grained soil has particles of all sizes; P for poorly graded, which indicates that a coarse-grained soil has particles of the same size, or the soil is skip-graded or gap-graded; M for a coarse-grained soil that has silt-sized particles; C for a coarse-grained soil that has clay-sized particles; L for a finegrained soil of low plasticity; and H for a fine-grained soil of high plasticity. An exception is peat, where the group symbol is PT. Also note in Table 6.8 that certain soils require the use of dual symbols.
AASHTO Soil Classification System. This classification system was developed by the American Association of State Highway and Transportation Officials (see Table 6.9). Inorganic soils are divided into 7 groups (A-1 through A-7), with the eighth group (A-8) reserved for highly organic soils. Soil types A-1, A-2, and A-7 have subgroups as indicated in Table 6.9. Those soils having plastic fines can be further categorized by using the group index (defined in Table 6.9). Groups A-1-a, A-1-b, A-3, A-2-4, and A-2-5 should be considered to have a group index equal to zero. According to AASHTO, the road supporting characteristics of a subgrade may be assumed as an inverse ratio to its group index. Thus, a road subgrade having a group index of 0 indicates a ‘‘good’’ subgrade material that will often provide good drainage and adequate bearing when thoroughly compacted. A road subgrade material that has a group index of 20 or greater indicates a ‘‘very poor’’ subgrade material that will often be impervious and have a low bearing capacity.
Organic Soil Classification System. Table 6.10 presents a classification system for organic materials. As indicated in Table 6.10, there are four major divisions, as follows:
1. Organic Matter. These materials consist almost entirely of organic material.
Examples include fibrous peat and fine-grained peat.
2. Highly Organic Soils. These soils are composed of 30 to 75% organic matter mixed with mineral soil particles. Examples include silty peat and sandy peat. 3. Organic Soils. These soils are composed of from 5 to 30% organic material. These soils are typically classified as organic soils of high plasticity (OH, i.e. LL 50) or low plasticity (OL, i.e., LL 50) and have a ratio of liquid limit (oven-dried soil) divided by liquid limit (not dried soil) that is less than 0.75 (see Table 6.8).
4. Slightly Organic Soils. These soils typically have less than 5% organic matter.
Per the Unified Soil Classification System, they have a ratio of liquid limit (ovendried soil) divided by liquid limit (not dried soil) that is greater than 0.75. Often a modifier, such as ‘‘slightly organic soil,’’ is used to indicate the presence of organic matter.
Also included in Table 6.10 is the typical range of laboratory test results for the four major divisions of organic material. Note in Table 6.10 that the water content (w) increases and the total unit weight (t) decreases as the organic content increases.
The specific gravity (G) includes the organic matter, hence the low values for highly organic material. The compression index (Cc) is discussed in Art. 6.5.6. Other Descriptive Terminology. In addition to the classification of a soil, other items should also be included in the field or laboratory description of a soil, such as:
1. Soil Color. Usually the standard primary color (red, orange, yellow, etc.) of the soil is listed.
2. Soil Texture. The texture of a soil refers to the degree of fineness of the soil.
For example, terms such as smooth, gritty, or sharp can be used to describe the texture of the soil when it is rubbed between the fingers.
3. Clay Consistency. For clays, the consistency (i.e., degree of firmness) should be listed. The consistency of a clay varies from ‘‘very soft’’ to ‘‘hard’’ based on the undrained shear strength of the clay (su). The undrained shear strength can be determined from the Unconfined Compression Test or from field or laboratory vane tests. The consistency versus undrained shear strength (su) is as follows:
4. Sand Density Condition. For sands, the density state of the soil varies from ‘‘very loose’’ to ‘‘very dense.’’ The determination of the density condition is based on the relative density (Dr in %).
5. Soil Moisture Condition. The moisture condition of the soil should also be listed. Based on the degree of saturation, the moisture conditions can vary from a ‘‘dry’’ soil (S 0%) to a ‘‘saturated’’ soil (S 100%).
6. Additional Descriptive Items. The soil classification systems are usually only applicable for soil and rock particles passing the 75-mm (3-in) sieve. Cobbles and boulders are larger than the 75 mm (3 in), and if applicable, the words ‘‘with cobbles’’ or ‘‘with boulders’’ should be added to the soil classification. Typically, cobbles refer to particles ranging from 75 mm (3 in) to 200 mm (8 in) and boulders refer to any particle over 200 mm (8 in).
Other descriptive terminology includes the presence of rock fragments, such as ‘‘crushed shale, claystone, sandstone, siltstone, or mudstone fragments,’’ and unusual constituents such as ‘‘shells, slag, glass fragments, and construction debris.’’
Soil classification examples are shown on the boring log in Fig. 6.5. Common types of soil deposits are listed in Table 6.11.
Shear Strength Tests
The shear strength of a soil is a basic geotechnical engineering parameter and is required for the analysis of foundations, earthwork, and slope stability problems.
This is because of the nature of soil, which is composed of individual soil particles that slide (i.e., shear past each other) when the soil is loaded.
The shear strength of the soil can be determined in the field (e.g., vane shear test) or in the laboratory. Laboratory shear strength tests can generally be divided into two categories:
1. Shear Strength Tests Based on Total Stress. The purpose of these laboratory tests is to obtain the undrained shear strength (su) of the soil or the failure envelope in terms of total stresses (total cohesion, c, and total friction angle, ).
These types of shear strength tests are often referred to as ‘‘undrained’’ shear strength tests.
2. Shear Strength Tests Based on Effective Stress. The purpose of these laboratory tests is to obtain the effective shear strength of the soil based on the failure envelope in terms of effective stress (effective cohesion, c, and effective friction angle, ). These types of shear strength tests are often referred to as ‘‘drained’’ shear strength tests. The shear strength of the soil can be defined as (Mohr-Coulomb failure law):
terms the shear strength of soils can be divided into two broad categories: granular (nonplastic) soils and cohesive (plastic) soils.
Granular Soil. These types of soil are nonplastic and include gravels, sands, and nonplastic silt such as rock flour. A granular soil develops its shear strength as a result of the frictional and interlocking resistance between the individual soil particles.
Granular soils, also known as cohesionless soils, can only be held together by confining pressures and will fall apart when the confining pressure is released (i.e., c 0). The drained shear strength (effective stress analysis) is of most importance for granular soils. The shear strength of granular soils is often measured in the direct shear apparatus, where a soil specimen is subjected to a constant vertical pressure ( n) while a horizontal force is applied to the top of the shear box so that the soil specimen is sheared in half along a horizontal shear surface (see Fig. 6.10). By plotting the vertical pressure ( ) versus shear stress at failure ( ƒ n ), the effective friction angle () can be obtained. Because the test specifications typically require the direct shear testing of soil in a saturated and drained state, the shear strength of the soil is expressed in terms of the effective friction angle ().
Granular soils can also be tested in a dry state, and the shear strength of the soil is then expressed in terms of the friction angle (). In a comparison of the effective friction angle () from drained direct shear tests on saturated cohesionless soil and the friction angle () from direct shear tests on the same soil in a dry state, it has been determined that is only 1 to 2 lower than . This slight difference is usually ignored and the friction angle () and effective friction angle () are typically considered to mean the same thing for granular (nonplastic) soils.
Table 6.12 presents values of effective friction angles for different types of granular (nonplastic) soils. An exception to the values presented in Table 6.12 are granular soils that contain appreciable mica flakes. A micaceous sand will often have a high void ratio and hence little interlocking and a lower friction angle. In summary, for granular soils, c 0 and the effective friction angle () depends on:
1. Soil Type (Table 6.12). Sand and gravel mixtures have a higher effective friction angle than nonplastic silts.
2. Soil Density. For a given granular soil, the denser the soil, the higher the effective friction angle. This is due to the interlocking of soil particles, where at a denser state the soil particles are interlocked to a higher degree and hence the effective friction angle is greater than in a loose state. It has been observed that in the ultimate shear strength state, the shear strength and density of a loose and dense sand tend to approach each other.
3. Grain Size Distribution. A well-graded granular soil will usually have a higher friction angle than a uniform soil. With more soil particles to fill in the small spaces between soil particles, there is more interlocking and frictional resistance developed for a well-graded than a uniform granular soil.
4. Mineral Type, Angularity, and Particle Size. Soil particles composed of quartz tend to have a higher friction angle than soil particles composed of weak carbonate.
Angular soil particles tend to have rougher surfaces and better interlocking ability. Larger-sized particles, such as gravel-sized particles, typically have higher friction angles than sand.
5. Deposit Variability. Because of variations in soil types, gradations, particle arrangements, and dry density values, the effective friction angle is rarely uniform with depth. It takes considerable judgment and experience in selecting an effective friction angle based on an analysis of laboratory data.
6. Indirect Methods. For many projects, the effective friction angle of the sand is determined by indirect means, such as the Standard Penetration Test and the Cone Penetration Test.
Cohesive Soil. The shear strength of cohesive (plastic) soil, such as silts and clays, is much more complicated than the shear strength of granular soils. Also, in general the shear strength of cohesive (plastic) soils tends to be lower than the shear strength of granular soils. As a result, more shear-induced failures occur in cohesive soils, such as clays, than in granular (nonplastic) soils.
Depending on the type of loading condition, either a total stress analysis or an effective stress analysis could be performed for cohesive soil. In general, total stress analysis (su or c and ) are used for short-term conditions, such as at the end of construction. The total stress parameters, such as the undrained shear strength (su), can be determined from an unconfined compression test or vane tests.
Figure 6.11 presents an example of the undrained shear strength (su) versus depth for Borings E1 and F1 excavated in an offshore deposit of Orinoco clay (created by sediments from the Orinoco River, Venezuela). The Orinoco clay can be generally classified as a clay of high plasticity (CH) and can be considered to be a relatively uniform soil deposit. The undrained shear strength was obtained from the Torvane device, laboratory vane, and unconfined compression test (UUC).
Note in Fig. 6.11 that there is a distinct discontinuity in the undrained shear strength (su) at a depth of 60 ft for Boring E1 and 40 ft for Boring F1. This discontinuity was due to different sampling procedures. Above a depth of 60 ft at Boring E1 and 40 ft at Boring F1, samplers were hammered into the clay deposit, causing sample disturbance and a lower shear strength value for the upper zone of clay. For the deeper zone of clay, a ‘‘WIP’’ sampling procedure was utilized, which produced less sample disturbance and hence a higher undrained shear strength.
Effective stress analyses (c and ) are used for long-term conditions, where the soil and groundwater conditions are relatively constant. Effective shear strength parameters are often obtained from laboratory triaxial tests, where a saturated soil specimen is sheared by applying a load to the top of the specimen (see Fig. 6.12).
During shearing, the pore water pressures (u) are measured in order to calculate the effective friction angle of the soil. Typical values of the effective friction angle () for natural clays range from around 20 for normally consolidated highly plastic clays up to 30 or more for other types of plastic (cohesive) soil. The value of for compacted clay is typically in the range of 25 to 30 and occasionally as high as 35. In terms of effective cohesion for plastic soil, the value of c for normally consolidated noncemented clays is very small and can be assumed to be zero for practical work. These effective friction angles () for cohesive soil are less than the values for granular soil (Table 6.12), and this is the reason there are more shear failures in cohesive than in granular soil.