Measurement of Properties - Geotechnical Engineering Investigation - page 204

(NGI apparatus-Kjellman, 1951), or in a box which provides a rigid confinement as lateral
stress is applied (Roscoe, 1953). A vertical stress is applied to cause consolidation and a lat-
eral stress is applied to induce shear failure (Figure 3.34).

Procedure
The specimen is consolidated anisotropically under a vertical stress and then sheared by
the application of lateral stress. In the undrained test, zero volume change during shear
can be maintained by adjusting the vertical stress continuously during the test (Bjerrum
and Landva, 1966).

Principal Axis Orientation
Initially, the principal axes are in the vertical and horizontal directions. At failure, the hor-
izontal plane becomes the plane of maximum shear strain (the lower point on the failure
circle in Figure 3.47b) .

Undrained Shear Tests: Laboratory
Laboratory tests used to measure the undrained shear strength, and in some cases the
residual or remolded shear strength, are described in Table 3.22. Included are unconfined
compression (Figure 3.52), miniature vane ( Figure 3.53) (ASTM D4648-00), torvane

TABLE 3.22
Laboratory Tests for Undrained Shear Strength

Test Parameters Procedure Comments

Triaxial compression s u See Table 3.21 UU or CK 0 U test

Unconfined U c 2 s u Unconfined specimen Test yields stress-strain curve and U c .

compression ( Figure 3.29) ( σ 3 0) strained to Slight drainage may occur during

(Figure 3.52) failure by axially shear. Values usually conservative

applied load because of nonrecovered decrease

in effective stress occurring from
disturbance during extraction

Vane shear s u , s r Small two-bladed vane Test limited to clay soils of very

(miniature vane) is pushed about 2 in. soft to firm consistency

(Figure 3.53) into a UD-tube specimen (Table 3.37)

and then rotated, while
the torque indicated on
a gage is recorded to
measure peak and
ultimate strength

Torvane a ( Figure 3.54) s u , s r One-inch-diameter disk Test limited to clay soils of very

containing eight small soft to firm consistency. Tested

vanes is pushed about zone may be disturbed by sample

¼ in. penetration in UD trimming and cutting

sample and rotated while
torque is recorded

Pocket penetrometer a U c 2 s u Calibrated spring-loaded Penetration is limited to soils with

(Figure 3.55) rod (¼ in. diameter) is U c 4.5 tsf. Data are representative

pushed into soil to a for soils with PI 12. Below this

penetration of 6 mm and value the φ of granular particles

the gage read for U c increases strength to above the

measured value of s u

Simple shear s u See Section 3.4.4

a

These tests are considered generally as classification tests to provide an index of soil consistency (Table 3.37).