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CHAPTER 5

Shear Strengths of Soil and Municipal Solid Waste

A key step in analyses of soil slope stability is measuring or estimating the strengths of the soils. Meaningful analyses can be performed only if the shear strengths used are appropriate for the soils and for the particular conditions analyzed. Much has been learned about the shear strength of soils within the past 60 years, oftenfrom surprising and unpleasant experience with the stability of slopes, and many useful research studies of soil strength have been performed. The amount of information that has been amassed on soil strengths is very large. The following discussion focuses on the principles that govern soil strength, the issues that are of the greatest general importance in evaluating strength, and strengthcorrelations that have been found useful in practice. The purpose is to provide information that will establish a useful framework and a point of beginning for detailed studies of the shear strengths of soils at particular sites.

angle of internal friction. Measuring or estimating the drained strengths of these materials involves determining or estimating appropriate values of . The most importantfactors governing values of for granular soils are density, confining pressure, grainsize distribution, strain boundary conditions, and the factors that control the amount of particle breakage during shear, such as the types of mineral and the sizes and shapes of particles.
Curvature of Strength Envelope

GRANULAR MATERIALS

The strength characteristics of all types of granular materials(sands, gravels, and rockfills) are similar in many respects. Because the permeabilities of these materials are high, they are usually fully drained in the field, as discussed in Chapter 3. They are cohesionless: The particles do not adhere to one another, and their effective stress shear strength envelopes pass through the origin of the Mohr stress diagram. The shear strength of these materials can becharacterized by the equation s tan (5.1) the effective normal the effective stress

Mohr’s circles of stress at failure for four triaxial tests on the Oroville Dam shell material are shown in Figure 5.1. Because this material is cohesionless, the Mohr– Coulomb strength envelope passes through the origin of stresses, and the relationship between strength and effective stress on the failure planecan be expressed by Eq. (5.1). can be determined for each of A secant value of the four triaxial tests. This value corresponds to a linear failure envelope going through the origin and passing tangent to the circle of stress at failure for the particular test, as shown in Figure 5.1. The dashed line in Figure 5.1 is the linear strength envelope for the test with the highest confining pressure. Thesecant value for an individual test is calculated as of

2

tan

1

1ƒ 3ƒ

45

(5.2)

where s is the shear strength, stress on the failure plane, and

where 1ƒ and 3ƒ are the major and minor principal stresses at failure. Secant values of for the tests on Oroville Dam shell material shown in Figure 5.1 are given in Table 5.1, and the envelope for 3 4480 kPa is shown in Figure 5.1.35

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5

SHEAR STRENGTHS OF SOIL AND MUNICIPAL SOLID WASTE

1500

Shear stress -τ - psi

1000

Oroville Dam shell Initial void ratio = 0.22 Max. particle size = 6 in. Specimen dia. = 36 in. 1 psi = 6.9 kPa

38.2 degrees = secant φ' for σ3 ' = 650 psi = 4480 kPa
Curved strength envelope

500

0 0 500 1000 1500 2000 2500

3000

Effective normal stress - σ ' - psi

Figure5.1 Mohr’s circles of shear stress at failure and failure envelope for triaxial tests on Oroville Dam shell material. (Data from Marachi et al., 1969.)

Table 5.1 Stresses at Failure and Secant Values of for Oroville Dam Shell Material Test 1 2 3 4
3

0

log10

3

pa

(5.3)

(kPa)

1

(kPa)

(deg) 46.8 43.4 39.8 38.2

210 970 2,900 4,480

1,330 5,200 13,200 19,100...