4. MARCO DE REFERENCIA
5.1. MARCO TEORICO
Failure Criterion Strain (%) su(psi) su∕𝜎1 con′ 𝜙′(deg)
Peak pore pressure, umax 0.9 7.2 0.48 29.1
Reaching the Kf line 3.5 15.4 1.02 35.3
Δu = 0 or A = 0 4.7 20.9 1.39 35.3
Peak principal stress ratio,(𝜎′1∕𝜎′3)max 7.0 30.8 2.06 35.8
Limiting strain 10.0 45.4 3.03 35.5
Peak deviator stress,(𝜎1− 𝜎3)max 15.0 69.7 4.65 34.7
0 0 20 40 60 80 20 40 60 80 100 120 p′ = (σ′1 + σ′3)/2 (psi) q = ( σ1 – σ3 )/2 (psi) 140 σ3′con = 15 psi Remolded LMVD Silt
Maximum deviator stress
Maximum principal stress ratio
Maximum pore pressure A = 0 (∆u = 0) On Kf line
10% axial strain Kf line
α = 30 degrees
Figure 5.20 Failure criteria for ICU triaxial tests on low-plasticity silt.
test specimen can occur. To prevent this from occurring, a back pressure in excess of that required to obtain saturation (B = 1) should be used in CU triaxial tests on low-plasticity and nonplastic silts.
The dilative behavior of low-plasticity silt also increases the importance of the failure criterion used when determining undrained strength parameters. Widely divergent values of strengths for silt are found depending on the failure criterion that is used to interpret test results.
Six different failure criteria that have been used to interpret the results of CU triaxial tests on dilative silts are listed in Table 5.11 and are shown in Figure 5.20. The pros and cons of these criteria are:
• Peak pore pressure occurs at very small strain and leads to very low undrained strength.
• Reaching the Kf line is subject to variation because the stress path approaches the Kf line asymptotically, and the point of intersection is therefore subject to quite wide variation.
• The point where Δu = 0 is readily determined and is the last point in the test where cavitation is sure not to affect
the test results. This is the point where the pore pressure parameters A and ̄A are equal to zero.
• Peak principal stress ratio is subject to wide variation in strain and strength because, in this phase of the test, the principal stress ratio is very nearly constant.
• Using a value of strain, such as 10 percent, as the failure criterion allows for various and undetermined amounts of cavitation before failure and is therefore undesirable. • Using peak deviator stress as the failure criterion is impractical. Depending on the test pressures and test equipment, the deviator stress may increase throughout the test, never reaching a peak value determined by the strength of the specimen.
The results of CU triaxial tests on dilative, low-plasticity silts often exhibit significant scatter. The use of the A= 0 failure criterion, which is the same asΔu = 0 criterion, re- duces the scatter and aids in interpretation of the undrained shear strength (Torrey, 1982). As noted above, this failure criterion also has the benefit that it does not rely on nega- tive pore pressures as a component of the undrained shear strength.
56 5 SHEAR STRENGTH
The isotropically consolidated–undrained (ICU) triaxial test is known to result in greater undrained strengths for a given vertical consolidation stress than other laboratory test methods (Ladd and Foott, 1974). Results from anisotrop- ically consolidated–undrained (ACU) triaxial tests or K0 consolidated–undrained triaxial tests (CK0U-TC) or direct simple shear (DSS) tests provide lower undrained shear strengths for low-plasticity silts. Unfortunately, there are no well-documented case histories comparing laboratory undrained strengths for dilative silts with back-calculated values that can be used to judge which of these tests is most appropriate for use in design. It is therefore important to ap- proach evaluation of the undrained strengths of these mate- rials cautiously.
5.3.9 Effective Stress Strength Envelopes
Low-plasticity and nonplastic silts can be sampled using techniques that have been developed for clays, although the quality of the samples is not as good. Disturbance during sampling is a problem for all silts, and care to minimize disturbance effects is important, especially for samples used to measure undrained strengths. Sample disturbance has a much smaller effect on measured values of effective stress friction angle(𝜙′) than it has on undrained strength.
Effective stress failure envelopes for silts can be deter- mined readily using consolidated–undrained triaxial tests with pore pressure measurements, using test specimens trimmed from “undisturbed” samples. Due to the dilative na- ture of low-plasticity silts, the choice of the failure criterion does not have a great influence on the determination of the effective stress friction angle. As can be seen from the stress path shown in Figure 5.20 along with friction angles given in Table 5.11, all failure criteria, except for the maximum pore pressure criterion, define very nearly the same value of𝜙′.
Drainage may occur so slowly in triaxial tests that perform- ing consolidated–drained (CD) triaxial tests may be impracti- cal as a means for measuring drained strengths. Drainage oc- curs more rapidly in direct shear tests, which can also be used for determination of effective stress friction angles for silts.
5.3.10 Strengths of Compacted Silts
Laboratory test programs for silts to be used as fills can be conducted following the principles that have been established for testing clays. Silts are moisture sensitive, and compaction characteristics are similar to those for clays. Densities can be controlled effectively using relative compaction (RC =
𝛾d∕𝛾d max). Undrained strengths of both plastic and nonplastic silts at the as-compacted condition are strongly influenced by water content.
Nonplastic silts have been used successfully as cores for dams and for other fills. Their behavior during compaction
is sensitive to water content, and they become rubbery when compacted close to saturation. In this condition they deform elastically under wheel loads, without failure and without further increase in density. Highly plastic silts, like San Fran- cisco Bay Mud, have also been used as fills, but it is difficult to adjust the moisture contents of highly plastic materials to achieve the water content and the degree of compaction needed for a high-quality fill.
5.3.11 Undrained Strength Ratios for Silts
Correlations are not available for making reliable estimates of the undrained strengths of silts because values of su∕𝜎′1con measured for different silts vary widely. A few examples are shown in Table 5.12. Some of the variations among the values in Table 5.12 may be due to the use of different failure criteria.
Values of the undrained strength ratio for normally consoli- dated dilative silts, assuming that the change in pore pressure at failure(Δu) is equal to zero (in effect using the A = 0 fail- ure criterion), can be estimated for ICU, ACU, and DSS tests using the following equations. For ICU tests, the undrained strength ratio can be expressed as
su 𝜎′
1con
= sin𝜙′
1− sin 𝜙′ (5.10)
For ACU tests consolidated to K0conditions, and assuming that K0= 1 − sin 𝜙′, the undrained strength ratio can be
expressed as
su 𝜎′
1 con
= sin 𝜙′ (5.11)
For DSS tests, also consolidated to K0 conditions, the undrained strength ratio can be expressed as
su 𝜎′ 1 con = sin 𝜙′− 1 2sin 2𝜙′ (5.12)
As an example, if𝜙′= 37 degrees and K0is assumed to be equal to 1− sin 𝜙′, then the undrained strength ratios can be calculated (Table 5.13).
The values in Table 5.13 can be useful for evaluating labo- ratory test results. If laboratory values are in excess of those listed, there is a potential that negative pore pressures are be- ing relied upon for undrained shear strength.
Additional studies are needed to develop more refined methods of classifying silts and correlations that can be used to make reliable estimates of undrained strengths. Until more information is available, properties of silts should be based on conservative estimates, or conservative interpretations of the results of laboratory tests on the specific material.
5.3.12 Typical Values of𝜙′for Silts
Low-plasticity and nonplastic silts normally have higher effective stress friction angles than do high-plasticity silts
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