A NEW FALL CONE DEVICE FOR MEASURING THE UNDRAINED STRENGTH OF VERY WEAK COHESIVE SOILS

A new fall cone device capable of measuring undrained shear strength o f extremely soft cohesive soils is described and evaluated. The Automa ted MIT Fall Cone has a unique mechanical counterweight system that en ables the use of effective cone weights as low as 0.005 N. This weight is an order of magnitude lower than possible with a standard fall con e apparatus. The new device also uses a data acquisition system to rec ord cone penetration versus time throughout the test. An extensive exp erimental program was performed on a remolded marine clay (Boston Blue Clay) to evaluate the new fall cone apparatus. The program includes m easurements of cone velocity and penetration to study the dynamics of the new device. The cone velocities and penetration times agree with t heoretical predictions that account for the inertia of the counterweig ht system. The experimental program includes strength measurements usi ng various cone weights and geometries to check the validity of the st rength-penetration relationships over a wide range of liquidity index values (0.75 to 5.6).