Seed and Idriss (1971) developed a method for evaluating soil liquefaction. The safety factor against liquefaction (FL) equals the cyclic stress ratio applied by the design earthquake (CSR) divided by cyclic resistance ratio of sand on level ground (CRR). When FL is less than 1.0, liquefaction can occur. There is a large established database for computing CRR based on SPT and CPT data.

Recent research has been performed with the seismic DMT where earthquakes have occurred and some areas have liquefied. The DMT horizontal stress index (K_D) was found to be a significantly better predictor than SPT and CPT for identifying those liquefaction areas as discussed by Maugeri and Monaco (2006) and Grasso and Maugeri (2006). Why? The soil's horizontal resistance and stress history are more important than its vertical resistance when determining whether it will liquefy.

The soil's shear modulus degrades as shear strain levels increase. The maximum shear modulus occurs at a strain level of approximately 0.0001%. Traditionally, the shear wave velocity has been measured by downhole or crosshole seismic tests. The shear modulus is determined using the following equation:

G_max = ρ* V_s², where ρ is the soil mass density.

Robertson, Campanella, Gillespie and Rice (1985) showed that the cone penetrometer with a seismic geophone (SCPT) located above it measured shear wave velocities that were essentially identical to downhole and crosshole methods. The SCPT is really the downhole seismic method with the SCPT being a convenient way of getting the geophone into the soil. Martin and Mayne (1998) developed a seismic dilatometer (SDMT) that was similar to the SCPT. A separate electrical cable was used for the geophone.

Diego Marchetti improved the SDMT by storing the seismic wave data inside the SDMT, transmitting the data serially through the single DMT wire and eliminating the second electrical cable. He also added a second geophone located 0.5 m above the first one and created a true interval seismic test. From one horizontal strike, identically shaped waves are measured with both geophones. The offset in the shear wave arrival times, delta time, can be easily measured. The wave travel distances, delta distance, between the two geophones is also easily determined. And thus the shear wave velocity can be computed by dividing the delta distance by the delta time. The SDMT was demonstrated at the Second International Conference on the Flat Dilatometer. After generating multiple strikes with the seismic DMT at the same depth, we found that the shear wave velocity was ±1 meter per second. (superb accuracy!!)

A family of shear modulus degradation curves has been established by researchers. After determining G_max, one has to figure out which is the correct curve to use to get the complete degradation curve. However, a working strain shear modulus, G_ws, can be obtained from the DMT constrained modulus, M, and Poisson's ratio (Cavallaro, et. al., 2006). From case history data, Gws occurs at a shear strain between 0.05 and 0.1%. With these two data points the correct shear modulus degradation curve can be found.