Vane shear test (VST)
Essential aspects of the equipment and test procedure
The vane shear test is particularly suitable for estimating the undrained shear strength of soft clayey soils (from very soft to soft and medium clays). The test essentially consists of inserting into the ground a rod, to which a set of four rectangular blades, arranged in a cruciform pattern, are welded, as shown in Figure 1.30a. Once the device is positioned at the desired depth, a torque is applied to the rod, forcing it to rotate. This rotation movement will induce shear failure in a cylindrical shape, as the soil is detached from the surrounding ground (Figure 1.30b). The torsional moment is transmitted to the rod by a mechanical device placed on the ground surface, which imposes a constant rotation speed on the vane and allows determination of the applied moment versus angle of rotation diagram, typically similar to that of Figure 1.30c.
The vane can be introduced into the ground on a pre-drilled borehole to the depth of the layer to be characterized or pushed from the ground surface. In this case, it is essential that the vane crosses the ground, namely coarser and more resistant layers, surrounded by a protective device, as shown in Figure 1.31a. Once the layer to be characterized is reached, the vane is then detached from the protective casing and is positioned at the testing point (Figure 1.31b). At this last stage, the insertion of the apparatus has to be carried out with particular care, avoiding any rotation that may disturb the ground. In general, the test should be performed five minutes after insertion into place. Table 1.12 summarizes the main aspects of this test.
Interpretation of test results for deriving cu
Considering an isotropic soil mass and assuming that the undrained shear strength of the soil is fully mobilized in the outer surface of the cylinder (lateral surface plus the two bases), such strength, c_{fv>} can be obtained from the following equation:
Figure 1.30 Vane shear test: a) vane; b) failure plans; c) torque versus angle of rotation diagram.
Figure 1.31 Advancement equipment for pushing the vane test: a) protective casing and vane shoe; b) vane ready for testing; c) general view of the vane.
Table 1.12 Main aspects regarding the vane shear test (ENV 1997-3: 1999).
Parameter |
Value |
HID relation |
2 |
Vane dimensions |
Maximum DxH = 100 mm x 200 mm |
Blade thickness |
Minimum DxH = 40 mmx80 mm 0.8 to 3 mm |
Rod diameter near the vane |
<16 mm |
Extension rod diameter |
>20 mm |
Testing procedure Rate of penetration in the ground near the testing point |
<20 mm/s |
Depth of penetration (push method) |
>5D |
Depth of insertion (pre-drilling method) |
>5 x borehole diameter |
Minimum vertical distance between tests |
0.5 m |
Time interval between insertion and testing |
2 to 5 min |
Vane rotation rate |
6°/min to l2°/min |
This equation results from a simple equilibrium equation of moments when the soil resistance is mobilized: the maximum applied torque, or torsional moment, M_{tft} and the integral of the undrained shear strength moment in each unit area of the outer cylindrical surface of the soil, in relation to the vertical axis of the apparatus.
The results of the application of Equation 1.28 are usually corrected by a dimensionless parameter, //, in order to obtain a more accurate estimate of undrained shear strength (Bjerrum, 1972):
In Figure 1.32 are included two proposals for the parameter, //, as a function of the plasticity index of the clay.
These corrections resulted from back-analyses of well-documented failure cases of embankments on soft clays. Both proposals were based on the same case histories. The most recent proposal by Azzouz et al. (1983) provides a lower corrective factor because the authors analyzed these failure cases, taking three-dimensional effects into account, which had been neglected in Bjerrum's original work, where a plane strain approach had been adopted.
Note that // is a reducing coefficient for most soils, which means that the undrained shear strength directly derived from the test (Equation 1.28) corresponds to an overestimation of the observed resistance. According to Bjerrum (1972), this deviation is essentially linked to the very high rate of application of the shear stresses in the test. The vane test values of c_{u }after correction are generally close to those determined in the laboratory by direct simple shear tests.
The vane test is the most widely used means of characterizing the so-called sensitivity of clays, which is defined as the ratio of undrained shear strength of the undisturbed soil, c_{u}, and that of the remolded soil, c_{ur}:
Figure 1.32 Corrections to the vane shear test results (Bjerrum, 1972; Azzouz et al., 1983).
Once the maximum torque has been determined, the vane is rapidly rotated ten times to remold the soil in the failure surface. Then, a new test is performed, similar to the initial one, both in the performance aspects and in the interpretation, leading to the value of the remolded strength, to be introduced into the denominator of Equation 1.30. This sensitivity parameter is useful for assessing soil susceptibility to loss of strength with strain.