Geotechnical Engineering

DHARM

744 GEOTECHNICAL ENGINEERING

The area ratio of the vane shall be kept as low as possible, and shall not exceed 15%,

calculated as follows:

Ar =

8 2

2

tD d d

D

()−+π

π

...(Eq. 18.6)

where, Ar = area ratio,

t = thickness of the blades of the vane,

D = overall diameter of the vane, and

d = diameter of central vane rod including any enlargement due to welding.

A diagrammatic vane test arrangement is shown in Fig. 18.13.

Torque

measuring

device

G.L.

Intermediate

guides at 5 m

intervals

Borehole

casing

Bottom

guide

Vane

Penetration rod

as required

Vane

Fig. 18.13 Arrangement for vane test, from bottom of bore hole (IS)

The vane is pushed with a moderately steady force up to a depth of four-times the

diameter of the bore hole or 50 cm, whichever is more, below the bottom. No torque shall be

applied during the thrust. The torque applicator is tightened to the frame properly. After

about 5 minutes, the gear handle is turned so that the vane is rotated at the rate of 0.1°/s. The

maximum torque reading is noted when the reading drops appreciably from the maximum.

For a rectangular vane the shear strength of the soil is computed from the following

formula:

τ =

T

πDH^2 [( / ) ( / )] 26 + D

...(Eq. 18.7)