Concept
The vane shear test is an in-situ geotechnical testing methods used to estimate the undrained shear strength of fully saturated clays without disturbance. The test is relatively simple, quick, and provides a cost-effective way of estimating the soil shear strength; therefore, it is widely used in geotechnical investigations. The results of the test are not reliable if clay contains silt or sand. Under special condition, the vane shear test can be also carried out in the laboratory on undisturbed soil specimens; however, the use of the vane shear test in in-situ testing is much more common.
Description and procedure
Vane shear test equipment
The vane shear test apparatus consists of a four-blade stainless steel vane attached to a steel rod that will be pushed into the ground. The height of vane is usually twice its overall widths and is often equal to 10 cm or 15 cm.
A typical vane shear test kit usually contains the following items:
- Torque wrench
- Drive head
- Extension rods, usually D20/D22x1000 mm
- Spanner for extension rod
- 2 or 3 Vane sizes. Commin sizes: 19x38 mm, 25x50mm, 30x60mm, 75.8x151.5 mm
- Transport Case
Vane shear test procedure
The test can be conducted either from the ground surface or from the bottom of a borehole or a test pit. If conducted from the bottom of a bore hole, the test area should be should be at the depth of least three times the borehole diameter lower that the borehole bottom in order to avoid the borehole disturbance effects.
The test starts by pushing the vane and the rod vertically into the soft soil. The vane is then rotated at a slow rate of 6° to 12° per minute. The torque is measured at regular time intervals and the test continues until a maximum torque is reached and the vane rotates rapidly for several revolutions.
At this time, the soil fails in shear on a cylindrical surface around the vane. The rotation is usually continued after shearing and the torque is measured to estimate the remoulded shear strength.
Undrained shear strength calculation
The undrained shear strength of the saturated soil is proportional to the applied torque and the dimensions of the vane. The undrained shear strength is calculated by equating the torque to the moments corresponding to the total shear strength over the sides and the ends of the cylindrical shear failure surface:
Torque = Ts + Te
Ts = moment of shear resistance force on the side of the cylindrical failure surface
Te = moment of shear resistance force at the two ends of the cylindrical failure surfac
Replacing the above equation with the test parameters, and solving for the undrained shear strength, we obtain:
where
C_u Undrained shear strength of the soil
T Maximum torque at failure
h height of the vane
d diameter of the vane
Standards
- ASTM D4767 - 04ASTM D2573 - 08 Standard Test Method for Field Vane Shear Test in Cohesive Soil Link
- EUROCODE 7 (1977)
References - External Links