Study of partial drainage effects in a natural silty deposit using variable rate CPTU testing

CPTUs carried out at standard rate (2.0 cm/sec) occur under conditions that range from fully undrained in clay to fully drained in clean sand. For intermediate soils partially drained conditions may occur which affect the CPTUs response (qt, u2) and complicate interpretation. Variable rate CPTU testing has been pursued in the last 20 years as one of the methods to investigate partial drainage. The present paper presents an extensive programme of variable rate CPTU tests carried out in the silt deposit at the Halden research site in Norway. Tests were performed at penetration rates spanning 4 orders of magnitudes from 0.02 cm/s to 20 cm/s, with cone penetrometers with diameters of 25, 36 and 44 mm, as well as u1 and u2 pore pressure measurements. The results show a significant and systematic increase in cone resistance as penetration changes from undrained to partially drained, and a complex pore pressure response as a function of rate. Using input from parallel dissipation tests, the normalized velocity framework fits the cone resistance changes with drainage but is of limited applicability for the pore pressure changes. The variable rate testing scheme adopted in the investigations provides a useful and robust basis for the interpretation of the data.