Volume 6 – Geotechnical Manual, Site Investigation and Engineering ...

Chapter 3 IN-SITU GEOTECHNICAL TESTING
Figure 3.13 Assumed Geometry of Shear Surface for Conventional Interpretation of the Vane Test
If it is assumed that the shear stress mobilized by the soil is linearly proportional to displacement, up
to failure, then another simple assumption (Skempton 1948), that the shear stress on the top and
bottom of the cylindrical shear surface has a triangular distribution, is sometimes adopted. For the
rectangular vane this leads to the equation:
T = D2 H
1+ D 2 4H c u (3.13)
For a vane blade where H = 2D:
T = 3.53D 3 c u (3.14)
giving only 4% difference in shear strength from that obtained using the uniform assumption.
Undrained Strength and Sensitivity
The conventional interpretation for obtaining the undrained shear strength from the recorded
maximum torque (T) assumes a uniform distribution of shear stresses both top and bottom along the
blades and a vane with height-to-width ratio H/D = 2 (Chandler, 1988), as given in Eq. 3-11 above,
regardless of units so long as torque T and width D are in consistent units (e.g., kN-m and meters,
respectively, to provide vane strength c uv in kN/m 2 ). The test is normally reserved for soft to stiff
materials with c uv < 200 kPa. (2 tsf). After the peak c uv is obtained, the vane is rotated quickly
through 10 complete revolutions and the remoulded (or "residual") value is recorded. The in-situ
sensitivity of the soil is defined by:
3-18 March 2009