6 A series of vane shear tests has been performed in a stratum of inorganic clay that has a plasticity index of 50. The vane had a diameter of 50 mm and a height of 100 mm. The test results were as follows: Depth (m) Torque at Failure (N-m) 3.4 12.7 4.1 18.1 5.0 15.8 6.6 20.1 Compute the undrained shear strength, su, for each test, then combine this data to determine a single su value for this stratum.