The strength of bulk materials increases with the addition of moisture and reaches a peak at some point beyond which additional moisture reduces the bulk strength. This point at which maximum strength is achieved can be determined in a Jenike style shear testing machine to enable design for worst case conditions.
The basic idea of the Penetration Method has its origin in this phenomenon. A test sample in a cylindrical vessel is subjected to vertical vibration of acceleration X in 50/60 Hz exerted thereon. To judge liquefaction, a penetration bit of P gf/cm2 as the weight is placed on the surface of the sample. When the penetration depth of the bit exceeds a criterion D mm for T minutes, it is judged that the liquefaction occurred. Otherwise, the moisture content of the sample is less than the FMP.