Engineering geology


The vane shear test is an in-situ test for determining the undrained shear strength of soft to medium clays. Figure 1 is a schematic drawing of the essential components and test procedure. The test consists of forcing a four bladed vane into undisturbed soil and rotating it until the soil shears. Two shear strengths are usually recorded, the peak shearing strength and the remolded shearing strength.

Figure 1. Vane shear test equipment and procedure (after FHWA, 2002b).

These measurements are used to determine the sensitivity of clay, which is defined as the ratio of the peak undrained shearing strength to the remolded undrained shearing strength. Sensitivity, St, allows analysis of the soil resistance to be overcome during pile driving in clays which is useful for pile driveability analyses. It is necessary to measure skin friction along the steel connector rods which must be subtracted to determine the actual shear strength. The VST generally provides the most accurate undrained shear strength values for clays with undrained shear strengths less than 1 ksf (50 kPa). The test procedure has been standardized in AASHTO T 223-74 and ASTM D 2573.

Table 1

It should be noted that the sensitivity of a clay determined from a vane shear test provides insight into the set-up potential of the clay deposit. However, the sensitivity value is a qualitative and not a quantitative indicator of soil set-up. Classification of clayey soils based on sensitivity values is presented in Table 1.

Leave a Reply

%d bloggers like this: