Site Investigation - Elements of Soil Mechanics - page 177

gas supply

pressure control unit

air filled tube

water filled tube

upper guard cell

central measuring
probe

zone of soil under test

lower guard cell

Fig. 6.13 Pressuremeter test.

BS1377-9:1990, and examples on the determination of undrained shear strength and the settlement of
spread foundations in a sand are given in EN 1997-2:2007, Annex K.

6.6.6 Field vane test

In soft sensitive clays it is difficult to obtain samples that have only a slight degree of disturbance, and in
situ shear tests are usually carried out by means of the vane test (Fig. 6.14) . The apparatus consists of a
75 mm diameter vane, with four small blades 150 mm long. For stiff soils a smaller vane, 50 mm diameter
and 100 mm high, may be used. The vanes are pushed into the clay a distance of not less than three times
the diameter of the borehole ahead of the boring to eliminate disturbance effects, and the undrained
strength of the clay is obtained from the relationship with the torque necessary to turn the vane. The rate
of turning the rods, throughout the test, is kept within the range 6-12° per minute. After the soil has
sheared, its remoulded strength can be determined by noting the minimum torque when the vane is
rotated rapidly.
Figure 6.14 indicates that the torque head is mounted at the top of the rods. This is standard practice
for most site investigation work but, for deep bores, it is possible to use apparatus in which the torque
motor is mounted down near to the vane, in order to remove the whip in the rods.