Manual on Subsurface Investigations (2019)

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130 C H A P T E R 8 Laboratory Testing of Soil and Rock Introduction Laboratory tests on soil and rock can be used to model existing in situ conditions as well as conditions that will exist at different stages of project development because the tests can systematically characterize the behavior of soil and rock in a controlled environment. A typical laboratory testing program usually includes index and performance tests. Index tests provide general information (e.g., particle size distribution, material consistency, texture, soil moisture conditions, plasticity). Performance tests measure specific material parameters that are required for analysis, design, and assessment of constructability (e.g., hydraulic conductivity, shear strength, compressibility, stiffness). The laboratory testing program should be designed to provide the necessary data to reduce uncertainties and thus mitigate geotechnical risks pertinent to the project. A larger number of tests and more sophisticated tests can be justified when the results can mitigate technical and financial risks or reduce construction costs. The specific number and types of laboratory tests required vary with each project and depend on the following factors: • Uncertainty and variability of subsurface conditions • Availability of complementary geophysical and in situ test data • Availability of laboratory test results from previous projects with similar subsurface conditions • Type of geotechnical system under consideration (e.g., bridge foundations, embankments, earth retaining structures) • Performance requirements for the system (i.e., strength and service limit states) • Type and magnitude of load to be applied (e.g., seismic, long-term vs. short-term condition, earth pressure) • Presence of problematic geomaterials (e.g., swelling soils, collapsible soils, organic materials) or features (e.g., slickensides, fissures) • Project scope and budget • Local practice and customs This chapter includes brief discussions of frequently used soil and rock properties and laboratory tests. The discussions assume that the reader will use the corresponding AASHTO and ASTM test standards in conjunction with the content presented herein. Quality Assurance To help ensure that results of the laboratory testing program are representative of subsurface conditions, it is important to (i) accurately track samples throughout the site investigation program; (ii) transport, store, and handle samples in a manner that minimizes disturbance; and (iii) characterize the amount of sample disturbance.

131 8.2.1 Sample Tracking During a geotechnical site investigation, it is imperative to properly track samples from the field to the laboratory. Field personnel who collect the soil and rock samples must label each of the samples with a unique identification number. After the samples are delivered to the laboratory for testing, laboratory technicians may assign a different identification number to each sample. The identification numbers assigned by the field and laboratory personnel must be documented in the laboratory testing report so that each sample tested can be tracked and information about the sample (e.g., sample descriptions) can be retrieved from the corresponding boring log when needed. A spreadsheet or a data management program can be used to track the samples and manage the sample identification information. It is also advisable to create a chain-of-custody form to document who has possession of the samples at any point in time and is thus responsible for their safekeeping. 8.2.2 Sample Transportation, Storage, and Handling Protocols for sample storage and handling are available in ASTM D4220. Disturbed samples used for index testing should be sealed in jars or plastic bags to prevent moisture loss. Undisturbed, thin-walled tube samples should be sealed with several layers of a nonshrinking, flexible, microcrystalline wax, and should be stored and transported vertically. Temperature extremes should be avoided if possible during storage and transport. Once the samples arrive at the laboratory, they should be stored in a room with a relative humidity of approximately 100 percent to further reduce the potential for moisture loss. Samples should be tested as quickly as possible after their arrival at the laboratory. Samples that are not scheduled to be tested within few days should be resealed, if necessary, to minimize moisture loss. Long-term storage of tube samples is not recommended, as the tube may corrode and the soil may adhere to the tube. Exposure of undisturbed samples to the atmosphere while preparing test specimens should be kept to a minimum. If possible, undisturbed specimens should be prepared in a room with a relative humidity of approximately 100 percent. Excess portions of tube samples should be removed prior to extrusion to minimize sample disturbance due to side friction. Preferably, the tube should be cut into predetermined lengths using a band saw (Ladd and DeGroot 2004). However, if the soil is layered, it may be necessary to extrude the sample from the full-length tube and assign appropriate tests to portions of the sample as it is extruded. Tube samples should always be extruded in the same direction as sampling to minimize sample disturbance. Further guidance on using thin-walled tube samples is available in AASHTO T 207 and ASTM D1587. 8.2.3 Sample Disturbance Disturbance may occur during any one or a combination of the following steps in the process of obtaining and testing samples: drilling, sampling, transportation, extrusion, handling, or trimming in the laboratory. Causes of sample disturbance may be grouped into five types as described by Hvorslev (1949): • Changes in stress state • Changes in water content and void ratio • Changes in soil structure • Chemical changes • Mixing and segregation of soil 8.2.3.1 Changes in Stress State Changes in the stress state within the sample cannot be avoided during drilling, sampling, extrusion, and trimming. Figure 8-1 shows the changes in effective stress that a sample may experience starting from the

132 time of drilling to the time just prior to testing. This reduction in effective stress may reduce the measured shear strength and increase the measured compressibility of the laboratory sample compared to the in situ condition. Source: Ladd and DeGroot (2004) Figure 8-1. Changes in effective stress during drilling, sampling, and specimen preparation 8.2.3.2 Changes in Water Content and Void Ratio Changes in water content and void ratio may occur during and after sampling. In a fully saturated soil, a change in void ratio is accompanied by a corresponding change in water content. However, in a partially saturated soil, the void ratio can change without a corresponding change in water content, and conversely the water content may change with only minor corresponding changes in void ratio. Changes in water content and void ratio may affect the measured laboratory engineering properties, and the results may not be representative of the in situ soils. 8.2.3.3 Changes in Soil Structure Soil structure and fabric can be disturbed during drilling and sampling. For sampling in boreholes, disturbance before sampling is usually limited to the upper part of the sample. The lower part of the sample may be disturbed during sampling. Disturbance after sampling can be minimized by carefully storing, transporting, and handling the sample. 8.2.3.4 Chemical Changes Disturbance associated with chemical changes is usually caused by one or any combination of the following factors: • Infiltration of wash water or drilling fluid into the sample • Oxidation after sampling and during specimen preparation • Contact with the sample containers • An electrical charge

133 Storing samples in untreated steel containers for long periods of time can increase the occurrence of chemical changes. Sample containers should be coated with lacquer, and sealing caps should be made of inert material or the same material as the container. 8.2.3.5 Mixing and Segregation of Soil Mixing and segregating soil is generally associated with sampling operations and can be minimized by exercising care and using proper drilling and sampling procedures. 8.2.4 Assessment of Sample Disturbance Sample disturbance can have a significant effect on the quality of laboratory test results, which depends on the type and degree of disturbance, the nature of the material, and the type of testing. In addition to proper sampling, transporting, storing, and handling procedures to minimize disturbance, the degree of sample disturbance should be assessed prior to running any test that is sensitive to disturbance. Two methods that are available to evaluate the degree of disturbance are x-ray radiography and the measurement of the volume change of the specimen during reconsolidation. 8.2.4.1 X-Ray Radiography X-ray radiography (ASTM D4452) can be used to qualitatively assess the following sample characteristics, some of which may be caused by disturbance: • Variations in soil type • Macrofabric features (e.g., bedding, varves, fissures, cracks, shear planes, voids) • Presence of inclusions (e.g., gravel, shells, calcareous soils, peat, drilling mud) • Warping of the sample due to friction on the inside of the sample tube Figure 8-2 presents an example of x-ray radiography that shows the bedding in the sample and a possible crack at 0.75 ft (0.22 m) from the bottom of the tube.

134 ’ Source: Geosyntec Consultants, Inc. Figure 8-2. Example of x-ray radiography 8.2.4.2 Volume Change During Reconsolidation A more quantitative method to assess sample disturbance is to measure the volume change that occurs during reconsolidation of laboratory consolidation and strength tests to the estimated in situ stress. Andresen and Kolstad (1979) developed the criteria shown in Table 8-1 based on measurements of the vertical strain during reconsolidation to in 1D consolidation tests. Lunne et al. (1997, 2006) found that the change in void ratio normalized by the initial void ratio was a better indicator of sample disturbance and proposed the criteria shown in Table 8-2 for evaluating the degree of disturbance. Table 8-1. Specimen quality rating system based on vertical strain εv @ σʹv0 (%) <1 1–2 2–4 4–8 >8 Rating A B C C E Description Very good to excellent Good Fair Poor Very poor Source: Andresen and Kolstad (1979)

135 Table 8-2 Specimen quality rating system based on change in void ratio Overconsolidation Ratio Δe/e0 @ σʹv0 1–2 <0.04 0.04–0.07 0.07–0.14 >0.14 2–4 <0.03 0.03–0.05 0.05–0.10 >0.10 Rating 1 2 3 4 Description Very good to excellent Good to fair Poor Very poor Source: Lunne et al. (2006) Index Property Testing Geotechnical index tests provide a simple and inexpensive way of developing a qualitative understanding of some of the engineering properties of soil and rock. This qualitative understanding is useful in determining the need for and scope of more sophisticated performance tests. 8.3.1 Particle Size Distribution The particle size distribution test determines the distribution of grain sizes in a soil specimen. The test consists of two components: mechanical sieve analysis (for course-grained particles) and hydrometer test (for fine-grained particles). The testing procedures for particle size distribution are available in AASHTO T 88. Knowing the grain size distribution for coarse-grained soils is valuable for classifying and estimating the performance characteristics of soils (e.g., strength, permeability, compressibility, stiffness). The mechanical sieve test is conducted by passing oven-dried materials through a series of sieves of predetermined opening sizes. The amount of soil retained on each sieve is weighed to determine the percentage of material retained or passing that sieve. The hydrometer test is based on Stokes law, which relates the velocity at which a spherical particle falls through a fluid medium to the diameter and specific gravity of the particle and the viscosity of the fluid. The particle size distribution of a soil is presented as a plot of the percentage by weight passing a sieve vs. the logarithm of the sieve opening diameter in millimeters. The shape of the grain-size curve is indicative of the grading. A uniformly graded soil has a grain-size curve that is nearly vertical, while a well-graded soil has a curve that is flatter and extends across several log cycles of particle size (Figure 8-3).

136 Source: Geosyntec Consultants, Inc. Figure 8-3. Example of particle-size distribution plot for a well-graded sand 8.3.2 Moisture Content The (gravimetric) moisture content expresses the mass of water in a specimen as a percentage of its dry mass. The moisture content is used to evaluate the liquidity index of fine-grained soils as well as numerous empirical correlations for assessing performance characteristics (e.g., strength, compressibility) of fine- grained soils. The three methods commonly used to determine moisture content of soils are (i) oven dried, (ii) microwave-oven dried, and (iii) field-stove dried. The procedures for conducting these tests are available in AASHTO T 265 (ASTM D2216, ASTM D4643, and ASTM D4959). The oven-dried method is the most accurate of the three methods. The microwave-oven and field-stove methods are rapid tests; if either of these methods is selected as the primary testing method, additional specimens should be tested periodically with the oven-dried method to calibrate test results. The microwave oven uses radiation that can release the water trapped in the soil structure, thus potentially resulting in higher moisture contents than would be obtained using the oven-dried method. 8.3.3 Atterberg Limits The Atterberg limits represent the moisture contents at which the consistency and plasticity of fine grained soils change significantly. Figure 8-4 presents the different Atterberg limit states: liquid limit, plastic limit, and shrinkage limit. The liquid limit represents the moisture content at which the soil transitions from a plastic to a liquid state. The plastic limit represents the moisture content at which the soil transitions from a plastic to a semisolid state. The liquid and plastic limits are determined in accordance with AASHTO T 89 and T 90, respectively, or ASTM D4318. The shrinkage limit represents the moisture content at which the soil transitions from a semisolid state to a solid state and represents the moisture content below which reduction in volume is negligible.

137 Source: after Holtz et al. (2011) Note: LL: liquid limit; PL: plastic limit; SL: shrinkage limit; PI: plasticity index; LI: liquidity index Figure 8-4. Atterberg limits Other indices are derived from the Atterberg limits, including the plasticity index (PI), the LI, and activity (A). The PI is a measure of soil plasticity calculated by subtracting the plastic limit (PL) from the liquid limit (LL): = − The PI is used in numerous empirical correlations to estimate the engineering properties (e.g., strength and compressibility) of fine-grained soils. The LI relates the in situ moisture content to the liquid and plastic limits: = ( − ) ( − )⁄ where = the in situ (i.e., natural) moisture content It also correlates well with engineering properties of fine-grained soils such as strength, compressibility, and sensitivity. Finally, the activity is defined as follows: = Clay fraction⁄ where clay fraction = the percent finer than 0.002 mm by weight The activity is an index of the type of clay mineral and thus the specific surface. High values of activity often indicate the potential for large volume changes upon wetting and drying of a soil. 8.3.4 Unit Weight Unit weight is defined as the weight of material per unit volume and is useful for a variety of engineering calculations involving gravity or inertial loads on soil. The total (or moist) unit weight ( ) is defined as the total weight of soil (weight of solids plus weight of water) per unit volume: = ⁄ where = the total weight of the specimen

138 = the total volume of the specimen The saturated unit weight ( ) is defined as the total unit weight under the condition of full saturation (i.e., S = 100%). The dry unit weight is the weight of solids per unit volume and is related to the total unit weight and the moisture content: = = (1 + )⁄⁄ where = the dry weight of the specimen Test procedures for measuring the unit weight of soils are described in ASTM D7263. 8.3.5 Specific Gravity The specific gravity ( ) is the ratio of the weight of a given volume of soil particles to the weight of an equal volume of distilled water at a standard temperature. The following are typical ranges of specific gravities for different soils: • 2.65 ≤ ≤ 2.7 for gravel, sand, and silt • 2.65 ≤ ≤ 2.8 for clay • 2.0 ≤ ≤ 2.6 for organic soils • 1.3 ≤ ≤ 2.0 for peat Some of the uses for specific gravity values include calculating phase relationships, identifying dominant minerals, analyzing hydrometer data, and estimating unit weight. The test procedures for the specific- gravity test are provided in AASHTO T 100, as well as ASTM D854 and ASTM D5550. 8.3.6 Organic Content The organic-content test helps with classifying soil and evaluating the performance characteristics of a soil. For example, soils with high organic content have the potential to retain significant amounts of water, which may contribute to high primary and secondary consolidation and low strength depending on the nature of organic material. Organic soils are typically distinguished from inorganic soils by their odor and dark gray to black color. The organic content of a soil is the ratio of the mass of organic matter in a soil to the mass of dry soil, expressed as a percentage. The procedures for this test are available in AASHTO T 194 and ASTM D2974. 8.3.7 Electrical Resistivity Electrical resistivity is a measure of a soil’s resistance to the flow of electricity (i.e., electrical current) and is useful for evaluating the corrosion potential of soils (e.g., Roberge 2000). The test is conducted in accordance with AASHTO T 288 and ASTM G57. The test is typically conducted for geotechnical structures that include buried metals (e.g., soil nail walls, anchored walls, steel piling). The relationship between soil resistivity and corrosion potential is shown in Table 8-3. If the soil resistivity is between 30 and 50 ohm-meters, a chloride ion content test and a sulfate ion content test should be conducted. If the tests indicate that the chloride ion content is greater than 100 parts per million (ppm) ([100 milligrams per kilogram or mg/kg]) or sulfate ion content is greater than 200 ppm (200 mg/kg), then the soil should be treated as corrosive (Loehr et al. 2017).

139 Table 8-3 Relationship between electric resistivity and corrosion potential Soil Resistivity (ohm-meters) Corrosivity Rating >200 Essentially noncorrosive 100–200 Mildly corrosive 50–100 Moderately corrosive 30–50 Corrosive 10–30 Highly corrosive <10 Extremely corrosive Source: Roberge (2000) 8.3.8 pH Test The pH test measures the alkalinity or acidity of subsurface or surface water environments. The acidity of the subsurface environment is a good indicator of the corrosion potential of the soil, and the pH test can be used as a screening test to determine when more advanced tests, such as electrical resistivity, should be conducted. For example, if the results from a pH test indicate that the pH of a soil is below 4.5, the soil is most likely corrosive, and confirmation with the resistivity test would be prudent. The procedures for the pH test are available in AASHTO T 289 and ASTM G51. Soil Classification The three most widely used soil classification systems in geotechnical engineering practice include the AASHTO Soil Classification System, USCS, and Visual-Manual Procedure for Description and Identification of Soils. 8.4.1 AASHTO Soil Classification System The AASHTO classification system uses information on the grain-size distribution and Atterberg limits. Soils are divided into two major groups: granular and silt-clay (Table 8-4). The first group consists of the granular materials with 35 percent or less passing the No. 200 (0.075-mm) sieve, and the second group is the silt-clay materials with more than 35 percent passing the No. 200 (0.075-mm) sieve. The sample classification is obtained by proceeding from left to right in Table 8-4. For Groups A-2 and A-4 through A-7, the chart shown in Figure 8-5 may be used as an aid to classification. The group index ( ) provides an index of the suitability of a soil for use as a subgrade material and is calculated as follows: = ( − 35) 0.2 + 0.005( − 40) + 0.01( − 15)( − 10) 0 The group index for Groups A-1, A-2-4, A-2-5, and A-3 is always zero; for Groups A-2-6 and A-2-7, the group index should be calculated using only the second term in the expression above. Group index values should always be rounded to the nearest whole number and shown in parentheses after group symbol (e.g., A-2-6(3) or A-4(5)). Additional details pertaining to this classification system are provided in AASHTO M 145 and ASTM D3282.

140 Table 8-4. AASHTO soil classification system General Classification Granular Materials (35% or less passing 0.075 mm) Silt-Clay Materials (More than 35% passing 0.075 mm) A-1 A-2 A-7 Group Classification A-1-a A-1-b A-3 A-2-4 A-2-5 A-2-6 A-2-7 A-4 A-5 A-6 A-7-5 A-7-6 Sieve analysis, % passing: 2.00 mm (No. 10) 0.425 mm (N0. 40) 0.075 mm (No. 200) 50 max 30 max 15 max -- 50 max 25 max -- 51 min 10 max -- -- 35 max -- -- 35 max -- -- 35 max -- -- 35 max -- -- 36 min -- -- 36 min -- -- 36 min -- -- 36 min Characteristics of fraction passing 0.425 mm Liquid limit Plasticity index -- 6 max -- NP 40 max 10 max 41 min 10 max 40 max 11 min 41 min 11 min 40 max 10 max 41 min 10 max 40 max 11 min 41 min 11 min Usual types of significant constituent materials Stone fragments, gravel, and sand Fine sand Silty or clayey gravel and sand Silty soils Clayey soils General rating as subgrade Excellent to good Fair to poor American Association of State Highway and Transportation Officials, 2018. Plasticity index of A-7-5 subgroup is equal to or less than LL – 30. Plasticity index of A-7-6 subgroup is greater than LL – 30 (see Figure 8.5).

141 Source: AASHTO T 145 Figure 8-5. AASHTO plasticity chart 8.4.2 Unified Soil Classification System Like the AASHTO classification system, the USCS is based on particle-size distribution and the Atterberg limits of the fine-grained portion of the soil. The division between coarse- and fine-grained soils is based on particle size. However, the boundary between coarse- and fine-grained soils is 50 percent passing the No. 200 (0.075 mm) sieve rather than 35 percent in the AASHTO system. Classification using the USCS proceeds from left to right using Table 8-5. Figure 8-6 may be used as an aid to classifying fine- grained and organic soils. Details of using the USCS are in ASTM D2487. 0 20 40 60 80 10010 30 50 70 90 LIQUID LIMIT 0 20 40 60 10 30 50 70 PL A ST IC IT Y IN D EX

142 Table 8-5. Unified soil classification system a. Based on the material passing the 3-in (75-mm sieve). b. If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name c. Gravels with 5–12% fines require dual symbols: GW-GM well-graded gravel with silt; GW-GC well-graded gravel with clay; GP-GM poorly graded gravel with silt; GP-GC poorly graded gravel with clay d. Sands with 5–12% fines require dual symbols: SW-SM well-graded sand with silt; SW-SC well-graded sand with clay; SP-SM poorly graded sand with silt; SP-SC poorly graded sand with clay e. Cu = D60/D10 Cc= (D30)2/(D10 x D60) n. PI ≥ 4 and plots on or above “A” line. f. If soil contains ≥ 15% sand, add “with sand” to group name. o. PI < 4 or plots below “A” line. g. If fines classify as CL-ML, use dual symbol GC-GM or SC-SM. p. PI plots on or above “A” line. h. If fines are organic, add “with organic fines” to group name. q. PI plots below “A” line. i. If soil contains ≥ 15% gravel, add “with gravel” to group name. j. If Atterberg limits plot in hatched area, soil is a CL-ML, silty clay. k. If soil contains 15–29% plus No. 200 add “with sand” or “with gravel,” whichever is predominant. l. If soil contains ≥ 30% plus No. 200, predominantly sand, add “sandy” to group name. m. If soil contains ≥ 30% plus No. 200, predominantly gravel, add “gravelly” to group name. Source: re-created from ASTM D2487 Criteria for Assigning Group Symbols and Group Names Using Laboratory Testsa Soil Classification Group Symbol Group Nameb Coarse-Grained Soils Gravels Clean Gravels Cu ≥ 4 and 1 ≤ Cc ≤ 3e GW Well-graded gravelf More than 50% retained on No. 200 sieve More than 50% of coarse fraction retained on No. 4 sieve Less than 5% of finesc Cu < 4 and/or 1 > Cc > 3e GP Poorly graded gravelf Gravels with Fines Fines classify as ML or MH GM Silty gravelf,g,h More than 12% finesc Fines classify as CL or CH GC Clayey gravelf,g,h Sands 50% or more coarse fraction passes No. 4 sieve Clean Sands Cu ≥ 6 and 1 ≤ Cc ≤ 3e SW Well-graded sandi Less than 5% finesd Cu < 6 and/or 1 > Cc > 3e SP Poorly graded sandi Sands with Fines Fines classify as ML or MH SM Silty sandg,h,i More than 12% finesd Fines classify as CL or CH SC Clayey sandg,h,i Fine-Grained Soils 50% or more passes the No. 200 seive Silts and Clays Liquid limit less than 50 Inorganic PI > 7 and plots on or above “A” linej CL Lean clayk,l,m PI < 4 or plots below “A” linej ML Siltk,l.m Organic Liquid limit – oven dried <0.75 OL Organic clayk,l,m,n Liquid limit – not dried < 0.75 OL Organic siltk,l.m,o Silts and Clays Liquid limit 50 or more Inorganic PI plots on or above “A” line CH Fat clayk,l,m PI plots below “A” line MH Elastic siltk,l,m Organic Liquid limit – oven dried < 0.75 OH Organic clayk,l,m,p Liquid limit – not dried < 0.75 Organic siltk,l,m,q Highly Organic Soils Primarily organic matter, dark in color, and organic odor PT Peat

143 Source: ASTM D2487 Figure 8-6. Plasticity chart 8.4.3 Visual-Manual Procedure The visual-manual procedure (ASTM D2488) provides a systematic process for grouping soils with similar physical properties. It is used to supplement the classification of soils using the USCS. One aspect of the visual-manual procedure is to determine the percentages of the soil constituents shown in Table 8-6 in the specimen. In addition, other characteristics of the soil are evaluated and described using the checklist presented in Table 8-7. Table 8-6. Soil constituents for visual-manual procedure Soil Type Description Boulder Particles of rock that will not pass a 12-in. (30-cm) square opening Cobble Particles of rock that will pass a 12-in. (30-cm) square opening and be retained on a 3-in. (75-mm) sieve Coarse Gravel Particles of rock that will pass a 3-in. (75-mm) sieve and be retained on a 0.75-in (19-mm) sieve Fine Gravel Particles of rock that will pass a 0.75-in. (19-mm) sieve and be retained on a No. 4 (4.75-mm) sieve Coarse Sand Soil that will pass a No. 4 (4.75-mm) sieve and be retained on a No. 10 (2.00-mm) sieve Medium Sand Soil that will pass a No. 10 (2.00-mm) sieve and be retained on a No. 200 (75-μm) sieve Fine Sand Soil that will pass a No. 40 (425-μm) sieve and be retained on a No. 200 (75-μm) sieve Silt Soil that will pass a No. 200 (75-μm) sieve and is nonplastic or very slightly plastic and exhibits little or no strength when air dry

144 Soil Type Description Clay Soil that will pass a No. 200 (75-μm) sieve and exhibits plasticity and considerable strength when air dry Organic A silt or clay with sufficient organic content to influence the soil properties Peat A soil composed primarily of vegetable tissue in various stages of decomposition usually with an organic odor, a dark brown to black color, a spongy consistency, and a texture ranging from fibrous to amorphous Source: ASTM D2488 Table 8-7. Checklist for visual-manual procedure 1. Group name 2. Group symbol 3. Percent of cobbles or boulders, or both (by volume 4. Percent of gravel, sand, or fines, or all three (by dry weight) 5. Particle-size range: Gravel – fine, coarse Sand – fine, medium, coarse 6. Particle angularity: angular, subangular, subrounded, rounded 7. Particle shape: flat, elongated, flat and elongated 8. Maximum particle size or dimension 9. Hardness of coarse sand and larger particles 10. Plasticity of fines: nonplastic, low, medium, high 11. Dry strength: none, low, medium, high, very high 12. Dilatancy: none, slow, rapid 13. Toughness: low, medium, high 14. Color (in moist condition) 15. Odor (mention only if organic or unusual) 16. Moisture: dry, moist, wet 17. Reaction with hydrochloric acid: none, weak, strong For intact samples Consistency (fine-grained soils only): Consistency (fine-grained soils only): very soft, soft, firm, hard, very hard Structure: stratified, laminated, fissured, slickensided, lensed, homogeneous Cementation: weak, moderate, strong Local name Geologic interpretation Additional comments: presence of roots or root holes, presence of mica, gypsum, etc., surface coatings on coarse-grained particles, caving or sloughing of auger hole or trench sides, difficulty in augering or excavating. Source: ASTM D2488 8.4.4 Additional Tests for Soil Classification In certain situations, it may be necessary to conduct more complex laboratory testing to determine the soil mineralogy, structure, fabric, and geochemistry. For those situations, there are new technologies

145 available, including electrical sensing, laser diffraction, particle optical sizing, electron microscopy, and x- ray diffraction (Abbireddy and Clayton 2009). Compaction Tests Compaction tests determine the maximum dry unit weight and the optimum moisture content of a soil. The maximum dry unit weight is defined as the maximum dry unit weight that can be achieved under a specified nominal compactive effort for a given soil. The optimum moisture content is defined as the moisture content corresponding to the maximum dry unit weight. Good compaction is desirable to increase the shear strength and stiffness of soils. Hydraulic conductivity depends on whether the soil is compacted with a moisture content less than (dry) or greater than (wet) the optimum moisture content. In general, lesser hydraulic conductivities are achieved when compacting wet of optimum. The two test methods used to characterize the compaction characteristics of soils are AASHTO T 99 (ASTM D698) and AASHTO T 180 (ASTM D1557). Table 8-8 summarizes the equipment and test procedures for the two compaction tests. The primary difference between the two test methods is in the amount of compactive effort used. Figure 8-7 is an illustration of the difference in compaction test results between standard and modified Proctor tests; the modified Proctor test yields a larger maximum dry unit weight and smaller optimum moisture content. Table 8-8. Summary of equipment and procedures for density tests Test Parameter AASHTO T 99 AASHTO T 180 (Standard Proctor) (Modified Proctor) Hammer Weight (lbf) 5.5 10 Drop Distance (in.) 12 18 Number of Layers 3 5 Blows/Layer for 4-in. Mold 25 25 Blows/Layer for 6-in. Mold 56 56 Energy (ft-lbf/ft³) 12,375 56,250 Notes: lbf: pound force; ft-lbf/ft3: feet pound force per cubic foot Source: Geosyntec Consultants, Inc.

146 Source: Geosyntec Consultants, Inc. Figure 8-7. Example of compaction curves Hydraulic Conductivity There are two types of laboratory devices commonly used to measure the hydraulic conductivity of soils: (i) a flexible-wall permeameter and (ii) a rigid-wall, compaction-mold permeameter. Tests conducted with both devices are based on measurements of 1D vertical flow through the specimen and assume that Darcy’s law is valid. Both devices are also intended for use when the anticipated hydraulic conductivity (K) is less than 10-3 cm/s. For soils with K > 10-3 cm/s, a simpler test procedure is available (AASHTO T 215). 8.6.1 Flexible-Wall Permeameter The flexible-wall permeameter test (ASTM D5084) is conducted in a device similar to a triaxial cell with the specimen enclosed in a flexible membrane that is sealed at the top cap and base. Porous end pieces are used at the top and bottom of the specimen to help ensure uniform flow. Either undisturbed or laboratory- compacted specimens may be tested. The permeameter cell permits a confining pressure to be applied to the specimen to simulate in situ stress conditions, and consolidation should be allowed to occur prior to measuring the hydraulic conductivity. The cell also permits the use of backpressure to facilitate saturation of the specimen prior to testing. Ideally, the hydraulic conductivity should be measured using hydraulic gradients similar to those anticipated in the field. However, for specimens with low hydraulic conductivity (less than approximately 10-6 cm/s), larger hydraulic gradients may be needed to reduce testing time. The flexible-wall permeameter test may be conducted using one of four test and interpretation variants: • Constant-head test (Method A) • Falling-head test with constant tailwater level (Method B) • Falling-head test with increasing tailwater level (Method C) • Constant rate of flow test (Method D) 0 5 10 15 20 25 Moisture Content, w (%) 100 105 110 115 120 125 D ry U ni t W ei gh t, dr y (lb /ft 3 ) Modified Proctor Standard Proctor Zero Air Voids Line

147 Detailed instructions for conducting and interpreting each type of test are provided in ASTM D5084. Permeation of the specimen should continue until at least four measurements of K have been made for which (i) the ratio of outflow to inflow is between 0.75 and 1.25 and (ii) the measured hydraulic conductivity is within ±25 percent for K > 10-8 cm/s or ±50 percent for K < 10-8 cm/s. 8.6.2 Rigid-Wall, Compaction-Mold Permeameter A rigid-wall, compaction-mold permeameter test (ASTM D5856) is conducted using a compaction mold modified to allow water to flow in and out of a specimen in a controlled way. The device is typically used to test laboratory-compacted samples. Unlike the flexible-wall permeameter, backpressure saturation cannot be used to facilitate of check saturation. As such, the rigid-wall, compaction-mold permeameter test is applicable to soils that can easily be saturated with water so that the material contains little or no air during testing. The test should be conducted using flow upward to One potential problem with this test is the potential for water to flow along the interface between the inside of the compaction mold and test specimen, particularly for soils that shrink when exposed to water. In this situation, the flexible-wall permeameter should be used. For soils that tend to swell when exposed to water, swell in the vertical direction may be prevented if desired via the use of swell rings or confining pressure. As with the flexible-wall permeameter, the hydraulic conductivity should be measured using hydraulic gradients similar to those anticipated in the field. Larger hydraulic gradients may be used to reduce testing time for specimens with low hydraulic conductivity (less than approximately 10-6 cm/s). When large hydraulic gradients greater than approximately 20 are used, special care should be taken to monitor for flow along the interface between the mold and specimen. The rigid-wall, compaction-mold permeameter test may be conducted using one of five test and interpretation variants: • Constant-head test (Method A) • Falling-head test with constant tailwater level (Method B) • Falling-head test with constant headwater level and rising tailwater level (Method C) • Falling-head test with decreasing headwater level and increasing tailwater level (Method D) • Constant rate of flow test (Method E) Detailed instructions for conducting and interpreting each type of test are provided in ASTM D5856. Permeation of the specimen should continue until at least four measurements of K have been made for which (i) the ratio of outflow to inflow is between 0.75 and 1.25 and (ii) the measured hydraulic conductivity is within ±25 percent for K > 10-8 cm/s or ±50 percent for K < 10-8 cm/s. In addition, the total inflow to the specimen during the test should be expressed in terms of the number of pore volumes and reported. Consolidation Consolidation tests assess the compressibility, stress history (i.e., preconsolidation stress), time rate of consolidation, creep characteristics, and swell potential of soil specimens. There are two different types of laboratory testing methods commonly used to evaluate the consolidation behavior of soils: the incremental load test (AASTO T 216 and ASTM D2435) and the constant rate of strain (CRS) test (ASTM D4186). The incremental load test subjects the specimen to a series of predetermined static loads—moving from lowest to highest load during the loading cycle and moving from highest to lowest during the unloading cycle. The load is generally doubled during the loading cycle and halved during the unloading cycle. During a CRS test, axial load is applied at a constant rate of deformation (i.e., strain), and axial force, axial displacement, and excess pore pressure are measured. The rate of deformation is adjusted to maintain

148 the ratio of excess pore pressure to vertical effective stress between approximately 3 and 15 percent. The CRS test has several advantages compared to the incremental load test, including better definition of the preconsolidation stress because vertical stress vs. vertical strain data are acquired continuously, shorter testing duration, and easier test automation (Fox et al. 2014). Conversely, the CRS test does not permit measurements of secondary compression and requires more complex test equipment. Figure 8-8 shows typical results obtained from a CRS consolidation test, including plots of the vertical strain ( ) and vertical coefficient of consolidation ( ) vs. vertical effective stress ( ). Figure 8-9 shows similar results from an incremental load test. The difference between the number of data points collected during CRS and incremental load tests is apparent by comparing the two figures. Source: Geosyntec Consultants, Inc. Figure 8-8. Example results from CRS consolidation test

149 Numerous graphical methods are available to define the preconsolidation stress ( ) from consolidation tests (Ku and Mayne 2013). The original and most popular approach is that attributed to Casagrande (1936). An example of the Casagrande approach is shown in Figure 8-9. A line is drawn tangent to the steepest slope of the curve (along the normally consolidated portion). This slope is called the virgin compression index ( ) and is defined as Δ Δ(log )⁄ . The easiest way to determine its value is visually by choosing void ratios over one log cycle of stress. In this example, select 1 tsf and 10 tsf, where the void ratios are 1.58 and 0.90, respectively. For these conditions, is simply the difference in void ratios, thus = 0.68. The next step is to choose the maximum point of curvature on the curve and draw three lines at this point: a horizontal line, a line tangent to the curve, and bisector of the two. The intersection of the latter with the line determines the most probable value of . For the example shown, the estimated = 0.70 tsf (67.0 kPa). Because the current effective overburden is = 0.43 tsf (41.2 kPa), the corresponding overconsolidation ratio (OCR) = 1.63. Source: Paul Mayne Figure 8-9. Example of Casagrande procedure to estimate preconsolidation pressure Shear Strength The shear strength of a soil is an essential engineering parameter that is used in a large number of transportation-related applications, including foundation capacity, stability of natural and cut slopes, and stability of earth retaining structures, among others. The shear strength is influenced by many factors: • Basic soil characteristics (e.g., mineralogy, grain size distribution, soil fabric) • Current state of stress (i.e., stress level and isotropic vs. anisotropic consolidation) • Stress history (i.e., OCR) • Stress path (i.e., compression vs. extension) • Drainage (i.e., drained vs. undrained) • Rate of loading (i.e., strain rate effects) The relative importance of these factors differs for coarse-grained and fine-grained soils. Therefore, laboratory tests should be conducted under conditions in the laboratory that simulate in situ conditions as closely as possible. The laboratory shear strength tests presented in this section include (i) miniature vane, (ii) DS, (iii) triaxial, and (iv) direct simple shear (DSS) tests.

150 8.8.1 Miniature Vane Test The miniature vane test is well suited for measuring the undrained shear strength of very soft to soft samples of clay with undrained shear strengths less than approximately 500 psf (23.9 kPa). The miniature vane test can also be conducted very quickly. The test is conducted by inserting a vane with a diameter between 0.5 and 1.0 in. (1.3 and 2.5 cm) into the specimen to a depth of at least twice the height of the vane. The vane is rotated at 60 to 90 degrees per minute, and the maximum measured torque is used to calculate the undrained shear strength. The measured undrained shear strength must be corrected for strength anisotropy and strain rate. Details of the test procedure are described in ASTM D4648. 8.8.2 Direct Shear Test The DS test is useful to obtain the drained shear strength of soils along a predetermined failure plane. Soil is placed in a split shear box and consolidated under a normal stress. The normal stress is kept constant, and the shear stress is increased at a constant rate of deformation to cause the specimen to shear along a predetermined horizontal plane. Detailed testing procedures for the DS test are available in AASHTO T 236 (ASTM D3080). Example results from a DS test illustrating the calculation of the drained friction angle ( ) from tests conducted at different normal stresses are presented in Figure 8-10. Although the DS test is simple and inexpensive, the test has several limitations. First, the normal and shear stresses are known only on the horizontal plane along which shearing occurs. Thus, it is not possible to determine the complete state of stress within the specimen to construct Mohr’s circle. Secondly, there is no direct control over specimen drainage. Shearing must be conducted slowly enough to ensure the soil is fully drained. For coarse-grained soils, this is easily achieved. But for fine-grained soils, the test is less practical for this reason. Finally, stresses within the specimen are not uniform and failure is forced along a horizontal plane, which may not be the plane of weakness within the specimen. Source: Geosyntec Consultants, Inc. Figure 8-10. Example DS test results 8.8.3 Triaxial Tests The triaxial test is the most common and versatile test available to determine the shear strength and stress- strain properties of soil. In the triaxial test, a cylindrical specimen is sealed in a rubber membrane, placed in a cell, and subjected to pressure (i.e., confining pressure). A typical triaxial cell configuration is shown

151 in Figure 8-11. The confining pressure is used to simulate the in situ stress conditions for the specimen. The use of an isotropic confining pressure is common, but it is also possible to apply an anisotropic confining pressure to more accurately simulate the in situ stress conditions. The most common way to shear the specimen is by triaxial compression test, in which an increasing axial load is then applied to the specimen until the specimen fails. In triaxial compression, the axial stress is the major principal stress ( ) and the intermediate ( ) and minor principal ( ) stresses are equal to the confining pressure. The difference ( − ) is referred to as the principal stress difference or deviator stress. Drainage of water from the specimen is controlled by connections to the bottom cap as shown in Figure 8-11. Alternatively, pore pressures within the specimen may be measured if no drainage is allowed. Source: Holtz et al. (2011) Figure 8-11. (a) Schematic of triaxial apparatus and (b) assumed stress conditions for triaxial compression Triaxial tests are generally classified based on the drainage conditions selected while (i) applying the confining pressure and (ii) shearing the specimen. There are three types of triaxial tests commonly conducted in geotechnical practice: (i) unconsolidated-undrained (UU), (ii) consolidated-undrained (CU), and (iii) consolidated-drained (CD). 8.8.4.1 Unconsolidated-Undrained Test The purpose of the UU test (AASHTO T 296 and ASTM D2850) is to measure the undrained shear strength of fine-grained soils. Drainage is not allowed while applying either the isotropic confining pressure or axial load. The axial load should be applied to cause an axial strain rate of approximately 1 percent per minute. Failure is defined as the maximum principal stress difference or the maximum principal stress difference at an axial strain of 15 percent, whichever occurs first. The undrained shear strength is equal to one-half of the principal stress difference at failure: = ( − ) 2⁄ Although the UU test is simple and inexpensive, the measured undrained shear strength reflects several errors related to the test procedure (Ladd and DeGroot 2004): • The rapid rate of shearing increases the measured undrained shear strength. • Isotropic consolidation and axial compression increase the measured undrained shear strength. • Sample disturbance decreases the measured undrained shear strength.

152 These errors often lead to significant variability in test results. Ladd and DeGroot (2004) note that the widespread use of the UU test in practice depends to a large extent on the fact that, on average, these errors tend to cancel. The unconfined compression test (AASHTO T 208 and ASTM D2166) is a variation on the UU test where the confining pressure is zero (i.e., = 0). Because no attempt is made to simulate the in situ stress conditions, the unconfined compression tests are strongly affected by sample disturbance and are considered the least reliable of the triaxial strength tests (Duncan et al. 2014). 8.8.4.2 Consolidated-Undrained Test The CU test (AASHTO T 297 and ASTM D4767) is a versatile test used to measure the undrained shear strength of soils. Because pore pressures generated during shearing are commonly measured, it is possible to interpret the CU test in terms of effective stresses and thus calculate drained strength parameters as well. Drainage is allowed during the application of the confining pressure, permitting the specimen to consolidate under the confining pressure, which helps to mitigate the effects of sample disturbance. Isotropic consolidation (CIU) using the estimated in situ vertical effective stress is the most common test variant. It is also possible to anisotropically consolidate the specimen (CK0U or CAU) to more accurately simulate in situ stress conditions. Using the estimated in situ effective stresses to consolidate the specimen is the recompression technique. An alternative to the recompression technique—the Stress History and Normalized Soil Engineering Properties (SHANSEP) method (Ladd and Foott 1974, Ladd 1991)—is beyond the scope of this manual. While applying the consolidation stresses, it is also advisable to check the saturation of the specimen. Axial compression is commonly used to shear the specimen. Draining is not permitted, and the resulting excess pore pressures induced by shear are measured. The axial load is applied at a rate slow enough to ensure that excess pore pressures are able to equilibrate within the specimen. Failure may be defined using one of several criteria: (i) maximum principal stress difference, (ii) principal stress difference at an axial strain of 15 percent, (iii) maximum effective stress obliquity [( ⁄ )max], or (iv) Skempton’s pore pressure parameter ̅ = 0 for dilative soils (Brandon et al. 2006). It is common to plot the results of the test using the effective stress path, defined as a vs. diagram where: = ( + ) 2⁄ = ( − ) 2⁄ An example of the effective stress paths from three CIU tests is shown in Figure 8-12. The undrained shear strength for each test is calculated as the value of q corresponding to one of the failure criteria listed above. It is also common to express the results as an undrained strength ratio by normalizing the undrained strength by the vertical effective stress used for consolidation of the specimen (i.e., ⁄ ). The drained (i.e., effective stress) friction angle ( ) may be determined from the slope of the line ( ) connecting the failure points for each test as shown: = If necessary, the effective cohesion intercept can be calculated from the y-intercept (a) of the vs. diagram: = ⁄

153 Finally, it is important to recognize that the measured undrained strength will differ for CIU and CK0U tests because of strength anisotropy. Interested readers should see Kulhawy and Mayne (1990) for details. Source: Geosyntec Consultants, Inc. Figure 8-12. Example CU test results 8.8.4.3 Consolidated-Drained Test The CD test (ASTM D7181) is conducted to obtain the drained shear strength parameters of coarse- grained soils. Because of the difficulty of obtaining undisturbed samples of coarse-grained soils, particularly below the groundwater table, specimens are often reconstituted to the desired void ratio or relative density. The specimen is allowed to consolidate (i.e., drain) under the applied (isotropic) confining pressure, and the change in volume is recorded. Drainage is also permitted during shearing, resulting in no excess pore pressures. The change in volume during shearing is also measured. Because the excess pore pressure is zero, the applied stresses are equal to the effective stresses. The drained friction angle ( ) may be determined from a vs. diagram in a manner similar to the CU test. Figure 8-13 is an example of results from a CD test. Source: Geosyntec Consultants, Inc. Figure 8-13. Example CD test results 0 2 4 6 8 101 3 5 7 9 Mean Effective Stress, p' = ( '1+ '3)/2 (ksf) 0 2 4 1 3 5 Sh ea r S tr es s, q = ( ' 1- ' 3) /2 (k sf ) ' = 3 6.3 ˚ 0 4 8 12 2 6 10 Sh ea r S tr es s, q = ( ' 1- ' 3) /2 (k sf ) 0 4 8 12 16 202 6 10 14 18 22 Mean Effective Stress, p' = ( '1+ '3)/2 (ksf) ' = 3 4.6 ˚

154 8.8.4 Direct Simple Shear Test The DSS test (ASTM D6528) is used to measure the undrained shear strength of fine-grained soils. The specimen is trimmed into a wire-reinforced membrane or a conventional membrane within a stack of TeflonTM-coated circular steel rings. A vertical stress is applied to simulate the in situ vertical effective stress (i.e., the recompression technique). The SHANSEP method may also be used if desired. The wire- reinforced membrane or metal rings prevent lateral deformation of the specimen, resulting in anisotropic consolidation under K0 conditions. An advantage of the test is that the consolidation phase is essentially a 1D consolidation test, from which compression parameters can be estimated. Once the specimen is consolidated under the applied vertical stress, a horizontal load is applied to induce simple shearing. Although there is no direct control over drainage, undrained tests may be conducted by adjusting the vertical stress to maintain a constant specimen height. Because the volume of the specimen remains unchanged, the specimen is considered to be shearing undrained. The change in the vertical stress required to maintain a constant height is assumed to be equal to the excess pore pressure generated during shearing. Figure 8-14 shows a plot of shear stress vs. vertical effective stress from a series of DSS tests. One of the limitations of the DSS test is that the state of stress in the specimen is not uniform and indeterminate. The undrained shear strength is commonly assumed to equal the maximum measured shear stress or the shear stress at a defined value of shear strain. The undrained shear strength measured in the DSS test is approximately two-thirds of the undrained shear strength obtained from CK0U triaxial compression tests (Kulhawy and Mayne 1990) and is approximately the average of the undrained strengths measured in triaxial compression and extension. As such, Ladd and DeGroot (2004) consider the undrained shear strength from the DSS test as the most appropriate single value to use for many stability problems. Source: Geosyntec Consultants, Inc. Figure 8-14. Example DSS test results 8.8.5 Interpretation of Laboratory Strength Data Ladd and DeGroot (2004) recommend that measured values of undrained shear strength from CU and DSS tests be adjusted or corrected to account for the following factors: • Undrained strength anisotropy • The shear stress on the failure plane at failure • Strain incompatibility between varying modes of failure along potential failure surfaces • Differences between triaxial and plane-strain conditions, and 3D effects Ladd and DeGroot (2004) provide additional details on each of these adjustments and their significance for various stability problems.

155 Dynamic Properties For transportation facilities subjected to seismic loads, laboratory tests may be used to measure the dynamic properties of soils for use in analysis and design. For analyses to evaluate the potential for ground motion amplification through near-surface materials, the resonant column test may be used to measure the (i) shear wave velocity ( ) and initial tangent shear modulus ( ) and (ii) the variation of shear modulus (G) and material damping ratio (D) with cyclic shear strain ( ). Liquefaction or cyclic softening may be a concern for loose sands and low-to-moderate plasticity silts, respectively, below the water table. In these situations, cyclic triaxial or cyclic DSS tests can be used to measure the cyclic strength of these soils. 8.9.1 Resonant Column Test The resonant column test (ASTM D4015) is a simple, robust technique to measure the shear modulus and damping ratio of soils at very small-to-intermediate shear strain levels. A cylindrical soil specimen is placed inside the resonant column device with the bottom cap assumed to be fixed and a torsional excitation introduced to the specimen via the top cap and drive system. The specimen is enclosed in a membrane sealed at the top cap and base similar to a triaxial test. An isotropic confining pressure can be applied to the specimen to simulate the in situ stress conditions. The torsional excitation of the specimen is varied over a range of frequencies to find the resonant frequency of the specimen in shear. Given the resonant frequency, may be easily calculated. The initial tangent shear modulus is calculated as follows: = where = the total mass density of the specimen The material damping ratio may be calculated using the half-power method or the free-vibration decay method. In the half-power method, the damping ratio is determined from the half-power points of the frequency response curve. Alternatively, the material damping ratio may be determined from free-vibration decay measurements using the logarithmic decrement. Accurate measurements of the material damping ratio are challenging, especially when the magnitude of the material damping ratio is small. Potential sources of error include (i) imperfect coupling between the test specimen and end platens, (ii) lack of fixity of the bottom pedestal, (iii) ambient noise, (iv) membrane effects, and (v) equipment-generated damping. The shear modulus and material damping ratio of soils are dependent on the level of cyclic shear strain induced in the specimen. The resonant column test is ideally suited to measuring this aspect of soil behavior. Examples of shear modulus reduction and material damping ratio curves measured using the resonant column test are shown in Figure 8-15.

156 Source: Geosyntec Consultants, Inc. Figure 8-15. Example results from resonant column test 8.9.2 Cyclic Triaxial Test Cyclic triaxial tests (ASTM D5311) are commonly used to evaluate the cyclic resistance of soils to liquefaction or cyclic softening during earthquakes. The testing equipment is similar to the equipment used for static triaxial compression tests except that a provision is made for applying a cyclic axial load. The test is usually conducted with stress control (i.e., the amplitude of the cyclic load is kept constant throughout the test and the resultant strains are measured). The amplitude of cyclic loading is usually expressed as the cyclic stress ratio, defined as the maximum shear stress within the specimen divided by the effective confining pressure. Cyclic loading is applied until initial liquefaction is observed (i.e., the excess pore pressure is equal to the confining pressure) or a threshold axial strain is obtained (e.g., 3 percent single- amplitude or 5 percent double amplitude). By conducting tests at different cyclic stress ratios, a cyclic strength curve can be developed that shows the number of cycles of loading required to cause liquefaction. There are several limitations of the cyclic triaxial test. Earthquake motion is better represented by a simple shear rather than axial loading condition; triaxial specimens are usually consolidated isotropically, while in the field the soil is anisotropically consolidated; and in a cyclic triaxial test, the direction of the

157 major principal stress instantaneously rotates 90°, from vertical to horizontal and then back, whereas in the field, the major principal stress will rotate smoothly and remain nearly vertical. Care should be taken when high cyclic stress ratios are tested. Large cyclic stress ratios may lift the top cap off the specimen or cause “necking” during the extension phase of loading. Necking decreases the cross-sectional area of the specimen, which causes significant stress concentrations. 8.9.3 Cyclic Direct Simple Shear Test Cyclic DSS tests are superior to cyclic triaxial tests in that the test better replicates field conditions and earthquake loading. Similar to the monotonic DSS test, the specimen is trimmed into a wire-reinforced membrane or a conventional membrane within a stack of TeflonTM-coated circular steel rings. A vertical stress is applied to simulate the in situ vertical effective stress. The wire-reinforced membrane or metal rings prevent lateral deformation of the specimen, resulting in anisotropic consolidation under K0 conditions. A cyclic, horizontal shear load is applied at constant amplitude and at a loading frequency ranging from 0.005 Hz to 1 Hz until a predetermined shear strain is reached, or a given number of cycles of loading have been applied. A single-amplitude shear strain of either 3 or 5 percent is usually considered to represent the initiation of liquefaction. During cyclic loading, no volume change is permitted, and it is assumed that undrained loading conditions exist. Thus, it is further assumed that the change in vertical stress required to maintain constant volume is equivalent to the change in pore pressure. Figure 8-16 shows an example of results from a cyclic DSS test. An advantage of the cyclic DSS test is that the specimens have a relatively uniform stress field and minimum pore water pressure redistribution. There are also several disadvantages to the cyclic DSS including the lack of test standardization to date, the inability to saturate specimens, and issues caused by the absence of imposed complementary shear stresses on the sides of the specimen. Source: Geosyntec Consultants, Inc. Figure 8-16. Example results from a cyclic DSS test

158 Laboratory Test Methods for Characterizing Subgrade Soils and Unbound Bases for Pavement Design There are three laboratory test methods commonly used to determine the design parameters for pavement subgrade soils and unbound bases: (i) resilient modulus test, (ii) CBR test, and (iii) Resistance (R)-value test. 8.10.1 Resilient Modulus Test Resilient modulus tests determine the resilient modulus ( ) from the relationship between applied stress and deformation of unbound pavement subgrade soils and granular bases. The test procedure consists of placing a cylindrical specimen in an oversized triaxial cell; applying an axial deviator stress of constant magnitude, duration, and frequency while maintaining a constant lateral confining stress; and measuring the recoverable or resilient strain. Typically, the test is conducted under different values of axial and lateral confining stresses. Details of the testing procedure are available in the National Cooperative Highway Research Program (NCHRP) 1-28A (NCHRP 2004). A schematic of the equipment used for this test is presented in Figure 8-17. The advantage of resilient modulus test over other methods used to characterize unbound pavement materials is that it measures a fundamental material property (material stiffness). The resilient modulus is a required input in the AASHTO mechanistic-empirical pavement design method (AASHTO 2015). Source: NCHRP (2004) Note: LVDT: linear variable differential transformer Figure 8-17. Schematic of the resilient modulus equipment 8.10.2 California Bearing Ratio Test The CBR test (AASHTO T 193 and ASTM D1883) evaluates the bearing capacity of unbound pavement materials at specified moisture and density conditions. The test consists of pushing a cylindrical piston with a cross-sectional area of 3 in.2 (7.6 cm2) into a soil specimen at a specified rate of 0.05 in. (1.3 cm) per

159 minute and measuring the load required to cause a penetration of 0.1 or 0.2 in. (0.25 or 0.5 cm). This load is expressed as a percentage of a standard load representing a CBR of 100 for the corresponding penetration depth. 8.10.3 Resistance-Value Test The primary purpose of the R-value test (AASHTO T 190 and ASTM D2844) is to evaluate the ability of unbound pavement material to resist lateral deformation when subjected to an axial load. The R-value test is also used to evaluate the suitability of expansive soils for use under pavements and the amount of overburden required to control the expansion. The testing procedure consists of applying an axial load to compacted specimens and measuring the resulting lateral pressure. The test specimens are compacted using special kneading compaction equipment and are tested using a stabilometer. The R-value is calculated from the ratio of applied vertical pressure to the developed lateral pressure using the following equation: = 100 − 1002.5 − 1 + 1 where = the applied vertical pressure of 160 psi = the transmitted horizontal pressure at = 160 psi = the turns displacement of the stabilometer fluid needed to increase the horizontal pressure from 5 psi to 100 psi Laboratory Tests for Rock Laboratory testing for rock has very limited applicability to the measurement of significant rock mass properties because rock samples small enough to be tested in the laboratory are usually not representative of the entire rock mass. Typically, laboratory tests are used in conjunction with field tests to give reasonable estimates of rock mass properties. Significant rock properties are defined as those properties that are of concern to the design and construction in rock: • Compressive strength • Shear strength • Hardness (durability) • Compressibility • Permeability Some common laboratory tests for rock include slake-durability, ultrasonic pulse velocity, point load, direct shear strength, compressive strength, and elastic moduli. 8.11.1 Moisture Content The oven-dried method for measuring the moisture content of soils (Section 8.3.2) may also be used to measure the moisture content of rock specimens. Because the moisture content of intact rocks is typically low, a larger sample size (>500 grams) is needed to enable sufficient precision in the calculated moisture content. Details of the test procedure are described in ASTM D2216.

160 8.11.2 Unit Weight The total and dry unit weights of rock specimens are measured using the same test procedures for soil specimens described in Section 8.3.4. The standardized test procedure is ASTM D7263. 8.11.3 Slake Durability of Weak Rock The purpose of the slake durability test (ASTM D4644) is to qualitatively estimate the durability of shales, clay-bearing rocks, and similar weak rocks when subjected to cycles of wetting and drying. The testing procedure consists of placing dried fragments of rock of known weight into a drum that is partially submerged in distilled water. The drum is rotated for 10 minutes to subject the rock fragments to repeated wetting and abrasion, and the fragments are subsequently dried. After two cycles of wetting, abrasion, and drying, the weight, shape, and size of the remaining rock fragments are recorded. The slake durability index is calculated using the following equation: = ℎ ℎ × 100 where = the durability index after two cycles Several classification systems based on the durability index are available to classify weak rocks. An example is the shale rating based on the durability index, point load strength for > 80, and plasticity index for < 80 (Franklin 1981). 8.11.4 Point-Load Strength Index Point-load tests (ASTM D5731) determine the point load strength index ( ) and strength anisotropy index ( ) for strength classification of rock materials. The point-load test was developed primarily for field use and should be used to test medium-strength rocks with compressive strengths greater than approximately 2,200 psi. The procedure consists of applying a load slowly to rock specimens using truncated conical platens until failure occurs. The point-load index is calculated as follows: = ⁄ where P = the failure load D = the distance between the platens when failure occurs To minimize bias, the initial distance between the platens should be approximately 2 in. (5 cm) If this is not possible, a size correction factor should be used to adjust the measured point-load index. For anisotropic rocks, the anisotropy index is defined as the ratio of measured perpendicular and parallel to the planes of weakness in the specimen. At least 10 to 20 specimens should be tested to obtain representative results, depending on the type of specimen that is used (i.e., rock core vs. irregular specimens). Note that although the point-load index is a strength, it is considered an index of rock strength and should not be used directly for analysis and design. The uniaxial compression test should be used for these purposes.

161 8.11.5 Direct Shear Test DS tests (ASTM D5607) determine the shear strength (friction angle and cohesion) of rock along a plane of weakness. The test can be conducted on intact rock and rock with natural or artificial (e.g., rock-concrete interface) discontinuities. The measured shear strength reflects the combined strength due to friction and the dilatancy and breaking of asperities along the plane of shearing. The relative amount of dilatancy and breaking of asperities will depend on the normal stress applied to the specimen. For rocks with multiple discontinuities, only one discontinuity can be tested per specimen. The test procedure and equipment for rock are similar to that of soil presented in Section 8.8.2. One difference is that high-strength gypsum cement is poured around the specimen in the holding rings to encapsulate it. Figure 8-18 shows results from DS tests conducted on two specimens of quartzite bedrock containing discontinuities along with the range of interpreted friction angles ( ). Source: Geosyntec Consultants, Inc. Figure 8-18. Example results from DS test on rock with discontinuities 8.11.6 Compressive Strength and Elastic Moduli of Intact Rock Core Specimens Compression tests (ASTM D7012) are used to measure the compressive strength and elastic properties of intact rock core specimens. Both the compressive strength and elastic moduli of intact rock are useful for developing parameters for rock mass strength and deformation as described in Chapter 10. ASTM D7012 describes four test variations: • Method A provides procedures for determining the undrained triaxial compressive strength without pore pressure measurement. • Method B provides procedures for measuring axial and lateral strains without pore pressure measurements; therefore, both the undrained triaxial compressive strength and elastic properties (i.e., Young’s modulus and Poisson’s ratio) can be determined. • Method C provides procedures for obtaining the unconfined, uniaxial compressive strength. • Method D provides provisions for measuring axial and lateral strains; therefore, both the unconfined, uniaxial compressive strength and elastic properties can be determined. 0 25 50 75 100 125 150 Normal Effective Stress, 'n (psi) 0 25 50 75 100 Pe ak S he ar S tr en gt h, pe ak (p si ) Sample A Sample B ' = 28. 9 ˚ ' = 18 .9 ˚

162 Similar to soil, the strength and deformation properties of rock are stress dependent. Methods A and B are used to simulate the in situ stress state of rock. Similar to the triaxial test for soils, Methods A and B for rock are used to obtain the angle of internal friction, angle of shearing resistance, and cohesion intercept. Methods C and D are similar to the unconfined compression test for soils. The Young’s modulus (E) calculated using Methods B and D can be expressed as the (i) tangent modulus at a fixed percentage (typically 50 percent) of the maximum strength, (ii) secant modulus at a fixed percentage (typically 50 percent) of the maximum strength, or (iii) average modulus in the linear portion of the stress-strain curve.

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