Measurement of Properties
2.4 Rupture Strength
2.4.4 Soil Strength Measurements GeneralGeneral
Selection of Test Method
A number of factors required for the selection of the method for testing soils, including the following:
● Loading conditions: static or dynamic.
● Loading duration in the field: long-term (drained conditions) or short-term (undrained conditions).
● Parameter desired: peak or ultimate (residual) strength.
● Material suitability for undisturbed sampling and the necessity or desirability for in situ testing.
● Orientation of the field failure surface with that in the test; some cases are shown in Figure 2.47 and Figure 2.48. Stability analysis is often improperly based on compression tests only, whereas direct shear and extension tests are often required. Their strength values may differ significantly from the com-pressive parameters.
Testing Methods Summarized
● Soil laboratory static strength tests — Table 2.19.
● In situ static strength tests — Table 2.20.
● Laboratory dynamic strength tests — Table 2.33.
Passive Σ active driving forces and masses
Passive Active pressure = PA
= KA s Z σ1
σ3
S
FIGURE 2.47
Probable natural stress and strain restraint conditions: (a) Retaining wall influence of lateral yielding on stresses. (b) Mass slide of excavated slope. Influence of lateral yielding. (c) Stress–strain relations
corresponding to lateral yield conditions in (b). (d) Angle of friction relations corresponding to lateral yield conditions in (b). (From Burmister, D. M., ASTM Special Technical Publication No. 131, 1953. Reprinted with permission of the American Society for Testing and Materials.)
Skirt
Relevance of laboratory tests to shear strength along potential slip surface beneath offshore gravity structure. (From Kierstad and Lunne, International Conference on Behavior of Offshore Structure, London, 1979. With permission.)
TABLE 2.19
Summary of Soil Laboratory Static Strength Tests
Test (F also field test) Parameter Reference Comments Measured
Triaxial compression
CD φ_
, c_
Figure 2.28 Most reliable method for effective stresses CU φ, c, φ苶, c苶 Figure 2.49 Strength values higher than reality because
su Figure 2.50 disturbance causes lower w% upon reconsol-Table 2.21 dation (see footnote c in Table 2.21)
UU su Figure 2.29 Most representative laboratory value for
undrained shear strength in compression Triaxial extension φ苶, c苶, su Table 2.21 Normally consolidated clays yield values
approximately one-third those of compression tests because of soil anisotropy (Bjerrum et al., 1972) Plain strain compression φ Table 2.21 Values are a few degrees higher than those
or extension of normal triaxial test except for loose sands;
more closely approach reality for retaining structure (Lambe and Whitman, 1969) Direct shear box φ苶, c苶, φ苶r Figure 2.51 Values most applicable where test failure
surface has same orientation with field failure surface. Values generally lower than triaxial compression values for a given soil, but higher than triaxial extension. Most suitable test for determination of residual strengthφ苶 ,rfrom UD samples
Simple shear su, φ苶, c苶 Figure 2.34 Horizontal plane becomes plane of maximum shear strain at failure Unconfined compression Su 1/2 Uc Table 2.22 Strength values generally lower than reality Vane shear (F) su, sr Table 2.22 Applies shear stress on vertical planes Torvane (F) su, sr Table 2.22 Shear occurs in a plane perpendicular to
the axis of rotation
Pocket penetrometer (F) Su 1/2 Uc Table 2.22 Yields approximate values in clays.
Used primarily for soil classification by consistency
California bearing ratio (F) CBR value Figure 2.64 Used for pavement design. Empirical strength correlates roughly with Uc
Triaxial Shear Test Purpose
Total or effective stress parameters, either in compression or extension, are measured in the tri-axial shear apparatus. The test method is generally unsuited for measuring ultimate strength because displacement is limited and testing parallel to critical surfaces is not convenient.
Apparatus includes a compression chamber to contain the specimen (Figure 2.49) and a system to apply load under controlled stress or strain rates, and to measure load, deflec-tion, and pore pressures (Figure 2.50).
Specimens are usually 2.78 in. in diameter as extruded from a Shelly tube, or 1.4 in. in diameter as trimmed from an undisturbed sample. Specimen height should be between 2 and 2.6 times the diameter.
General Procedures
Rate of loading or strain is set to approximate field-loading conditions and the test is run to failure (Bishop and Henkel, 1962).
Confining pressures: Tests are usually made on three different specimens with the same index properties to permit defining a Mohr diagram. Test method variations are numer-ous. Specimens can be preconsolidated or tested at their stress conditions as extruded.
Tests can be performed as drained or undrained, or in compression or extension. The var-ious methods, parameters measured, and procedures are summarized in Table 2.21. CU tests are covered in ASTM D4767-02.
Direct Shear Test (ASTM D5607-02 for rock, ASTM D3080-03 for soils) Purpose
The purpose normally is to measure the drained strength parameters φ , c , and φr . TABLE 2.20
Summary of Soil In Situ Static Strength Tests
Test Parameters Reference Comments
Measured
Vane shear su, sr(direct test) Figure 2.56 Measures undrained strength by shearing two circular horizontal surfaces and a cylindrical verti-cal surface: therefore, affected by soil anisotropy SPT φ苶, su(indirect test) Section 2.4.5 Dris estimated from N and correlated with
soil gradation to obtain estimates of φ (Figure 2.93, Table 2.36)
Consistency is determined from N and correlated with plasticity to obtain estimates of Uc(Figure 2.94, Table 2.37)
CPT φ苶, su(indirect test) Section 2.4.5 Various theoretical and empirical relationships have been developed relating qcto φ苶, (Figure 2.61) suis expressed as in
Equation (2.50) where Nktis the cone-bearing capacity factor (deep foundation depth correction factor) Pore-water pressure u is measured by some cones (piezocones) Pressuremeters su(indirect test) Section 2.5.4 Affected by material anisotropy, suis expressed
as in Equation (2.77)
California bearing CBR value Section 2.4.5 Field values generally less than lab values
ratio because of rigid confinement in the lab mold
Inlet pressure
valve Deviator stress σd
σ3
σ3
Ram to apply load
Rubber membrane
Soil sample
Porous stone A
Manometer
Outlet valve
Sample drainage and water B pore-pressure connection Fluid
pressure Transparent material container
T T
FIGURE 2.49
Triaxial compression chamber arrangement.
FIGURE 2.50
The triaxial compression chamber, load application, and measurement system.
Apparatus
The test apparatus is illustrated in Figure 2.51.
Procedure
The specimen is trimmed to fit into the shear box between two plates, which can be per-vious or imperper-vious, depending upon the drainage conditions desired, and a normal load applied which remains constant throughout the test. The test is normally run as a consol-idated drained (CD) test (sample permitted to consolidate under the normal load), but it TABLE 2.21
Triaxial Test Methodsa
Test To Measure Procedure
Consolidated-drained (CD) or (S) Effective stress parameters φ_ , c_
Specimen permitted to drain and
compression test consolidate under confining
pressure until u 0.
Deviator stress applied slowly to failure while specimen drains during deformationb Consolidated-undrained (CU) or Total stress parameters φ, c Specimen permitted to drain and
(R) compression test (Figure 2.28) consolidate under confining
pressure until u 0.
Deviator stress applied slowly to failure, but specimen drainage not permitted Effective stress parameters φ_
, c_
Pore pressures are measured during test (see Pore-Pressure Parameters)
CK0U test su See Notes cand dbelow
Unconsolidated-undrained (UU) Undrained strength su Confining pressure applied but
or (Q) compression test (Figure 2.29) no drainage or consolidation
permitted to reduce test time.
Deviator stress applied slowly to failure with no drainage permitted
Extension tests as CD, CU, UU Lateral shear strength Maintain confining pressure constant and reduce axial stress, or maintain axial stress constant and increase confining
pressure until failure Plane strain compression or Parameter φin cohesionless Modified triaxial apparatus in
extension test granular soils which specimen can strain only
in axial direction and one lateral direction while its
dimension remains fixed in the other lateral direction
a See Table 2.19 for comments on test results and test comparisons.
b Backpressure is applied to the pore water to simulate in situ pore-water pressures, or to saturate partially sat-urated specimens.
c CKoU test: specimen anisotropically consolidated with lateral pressure at K0poand vertical pressure at po.
d SHANSEP procedure (Ladd and Foott, 1974) attempts to minimize the effects of sample disturbance and assumes normalized behavior of clay. Specimens are consolidated to pcstresses higher than po, rebounded to selected values of OCR, and then tested in undrained shear to establish the relationship between su/povs.
OCR for different modes of failure. Normalized behavior requires this relationship to be independent of pc. The suprofile is then calculated front the values of poand OCR vs. depth (Baligh et al., 1980).