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

Chapter 3 IN-SITU GEOTECHNICAL TESTING
Interpretation
The vane test is routinely used only to obtain ‘undisturbed’ peak undrained shear strength, and
remoulded undrained shear strength. The undrained strength is derived on the basis of the following
assumptions:
1. Penetration of the vane causes negligible disturbance, both in terms of changes in effective
stress, and shear distortion;
2. No drainage occurs before or during shear;
3. The soil is isotropic and homogeneous;
4. The soil fails on a cylindrical shear surface;
5. The diameter of the shear surface is equal to the width of the vane blades;
6. At peak and remoulded strength there is a uniform shear stress distribution across the shear
surface; and
7. There is no progressive failure, so that at maximum torque the shear stress at all points on
the shear surface is equal to the undrained shear strength, c u .
On this basis (Fig. 3.13), the maximum torque will be:
T = D2 Hc u
2
D/2
+ 2 2δr-rc
0
u
(3.11)
= D2 Hc u
2
= D2 H
2
+ 4πr3 D/2
3 c u0
1+ D
3H c u
For a vane blade where H = 2D:
T = 3.667D 3 c u (3.12)
March 2009 3-17