8.4 Field Measurements of Dynamic Modulus
Direct measurement of soil or rock stiffness in the field has the advantage of minimal material disturbance. The measurements are not constrained by the size of a sample. Dissipation of energy during strain, i.e. material damping, requires significant strains to occur. In situ techniques are based on measurement of the velocity of propagation of stress waves through the soil. The P-waves or compression waves are dominated by the response of the pore fluid in the saturated soils. Consequently, most techniques measure S-waves or shear waves. If the velocity of the shear wave through a soil deposit is determined to be Vs, the shear modulus G is given as:
There are three techniques for measuring shear wave velocity in in-situ soil; cross- hole, down-hole, and uphole. All the three techniques require boring to be made in the in-situ soil. In the cross-hole method, sensors are placed at one elevation in one or more bore holes. A source of energy is triggered in another bore hole at the same elevation. The waves travel horizontally from the source to the receiving hole. The arrival of the S-waves is noted on the traces of the response of the sensors. The velocity of S-wave can be calculated by dividing the distance between bore holes by the time for a wave to travel between them. Thus, the measured travel time reflects the cumulative travel through layers with different wave velocities. Interpreting the data requires sorting out the contribution of the layers.