Effect of penetration rate for clay ground

Drainage condition during cone penetration tests depends on the ratio of the steady penetration rate to the soil permeability. The standard penetration rate of 20 mm/s in the field test produces undrained condition for clay or clayey silt because of its low permeability. For sand and coarser materials, pore water pressure is not generated and, therefore, penetration takes place under drained condition. For silty soils with intermediate values of permeability, partial drainage can occur during penetration. Situations of partial drainage are generally very difficult to analyze and can involve significant uncertainties in interpretation of measurements.

For cone penetration tests in clay under undrained condition, cone resistance depends on penetration rate. Strain rate dependency of mechanical properties of cohesive soil may be responsible for this. In order to assess undrained strength of clay, penetration rate should be taken into account. The figure shows an example of variation of cone resistance with logarithm of penetration rate.