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Wednesday, 21 November 2012 15:00

In Situ Vane Shear Test

Vane shear testing is one of the most common in-situ methods for the estimation of the undrained shear strength of the soil. The photos shown in this page are taken during the site characterization of a site in the Bay Area as part of the "Advanced Soil Mechanics Laboratory" course instructed by Professor R.B. Seed and M. Riemer at the University of California at Berkeley.

The Figure close beside illustrates the vane shear tip used in the tests. The tip has a diameter of 7.5cm and a height of 11.2cm. Typically the ration of height to diameter should be 2. The Vane is introduced into the borehole to the depth where the measurement of the undrained shear strength is required. Then it is rotated and the torsional force required to cause shearing is calculated. In the following Figure, you can see a manual vane shear. The blade is rotated at a specified rate that should not exceed 0.1degrees per second (practically 1degree every 10sec). The amount of rotation is specified in the green arrow whereas the red arrow has a device that measures the required Torque. The procedure and the equipment typically should follow the procedures suggested by the ASTM D2573-72.

The shear strength of the material is calculated from the Torque by dividing by a constant K which depends on the dimensions and the shape of the vane. More can be found in ASTM D 2573-72. It must be noted that when the strength is required for slope stability issues, the strengths factor proposed by Bjerrum (1972), which accounts for the different mode of failure, disturbance factor proposed by Bjerrum (1972), which accounts for the different mode of failure, disturbance and rate effects. The correlation of the correction factor as a function of the Plasticity Index (PI). It can be seen that for a Plasticity Index of 50, which is true for Young Bay Mud, μ=0.8. It can be seen that for a Plasticity Index of 50, which is true for Young Bay Mud, μ=0.8. The results of the corrected Vane Shear Strength measured in-situ are shown in the following plot. The plot is shear strength (corrected) vs time and the test was performed in a rate of 1degree per 10sec. You can see in the plot the peak shear strength and the residual. After the completion of the test the vane was rotated twice and then the residual strength was measured.

References:

  • ASTM D 2573-72, "Standard test method for Field vane shear test in cohesive soil". 
  • Course notes during the CE270L "Advanced Soil Mechanics Laboratory" instructed by Professor M. Riemer and R.B. Seed, at the Geoengineering Program of the University of California at Berkeley.

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