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Module 1 : Site Exploration and Geotechnical Investigation

Lecture 4 : In-situ tests [ Section 4.2 : Pressuremeter test, Vane shear test, Plate load test ]


	Vane shear test
	From experience it has been found that the vane shear test can be used as a reliable in-situ test for determining the shear strength of soft-sensitive clays. It is in beds of such material that the vane shear test is the most valuable, for the simple reason that there is at present no method known by which the shear strength of clays can be measured. The vane shear test should be regarded as a method to be used under the following conditions:
	Where the clay is deep, normally consolidated and sensitive.
	Where only the undrained shear strength is required.
	It has been found that the vane shear test gives similar results to that as obtained from unconfined compression tests on undisturbed samples. It is necessary that the soil mass should be in saturated conditions if the vane test is to be applied. Vane shear test cannot be applied for partially saturated soils for which the angle of shearing resistance is not zero.
	Description of vane:
	The vane consists of a steel rod having at one end four projecting blades or vanes parallel to the axis, and situated at 90º intervals around the rod. A posthole borer is first employed to bore a hole up to a point just above the required depth. The rod is pushed or driven carefully until the vanes are embedded at the required depth. At the other end of the rod above the surface of the ground a torsion head is used to apply a horizontal torque and this is applied at a uniform speed of about 0.1º/sec until the soil fails, thus generating a cylinder of soil. The area consists of the peripheral surface of the cylinder and the two round ends. The first moment of these areas divided by the applied moment gives the unit shear value of the soil. In India the diameter used is 50mm and height of the blade is 100mm.
	Determination of Cohesion or Shear Strength of Soil:
	Consider a cylinder of soil generated by the blades of the vane when they are inserted into the undisturbed soil in-situ and gradually turned and rotated about the axis of the shaft or vane axis. The turning moment applied at the torsion head above the ground is equal to the force multiplied by the eccentricity.