CPT PRACTICE 



The cone penetration test was first introduced in Europe about 

 50 years ago, but only in the last decade has its use in the United 

 States become popular. Initial application was in design of piles. 

 Increased sophistication of the CPT, particularly the development of the 

 electrical-friction cone, has led to greater use of CPT-obtained data. 



A recent advancement has been the piezocone, which measures pore 

 water pressure response in addition to the mechanical response of the 

 soil. Pore pressures are generated during penetration in the soil pore 

 water that are recorded as part of the cone pressure. Being able to 

 record this pore pressure and use it to make corrections during the 

 analysis of CPT data are especially important in soft soils and offshore 

 sands. 



Interpretation of piezocone data is an active area of research. 

 The presentation of CPT practice in this report is limited to the inter- 

 pretation of electrical-friction cone data and the application of the 

 data to geotechnical designs. Areas of interest are briefly discussed 

 in terms of data interpretation and geotechnical design. As will be 

 noted, interpretation and use of CPT data are different for sandy and 

 clayey soils. Details of data interpretation and use of the data for 

 design are presented in the Appendix. 



Data Interpretation 



The data gathered during a CPT are the cone pressure and side 

 friction. Values for these items are influenced by many variables, 

 including soil type, density, fabric, and stress states, among others. 

 No single unique theoretical relationship relates all the variables to 

 the cone data, but theories have led to better understanding and inter- 

 pretation of CPT data. However, empirical relationships are still the 

 primary means of interpreting test results. 



Soil Classification . Efforts to classify soils from CPT data were 

 first reported by Begemann (1965). He found that the ratio of sleeve 

 friction to cone pressure correlated well to median grain diameter. His 

 findings have been confirmed and improved by other researchers (see 

 Reference list). Today, soil classification charts are used widely to 

 identify soil types, and these charts are being expanded to describe and 

 classify problem soils, such as carbonates. These charts are dependent 

 on cone type; a chart recommended by Martin and Douglas (1981) for 

 electrical-friction cone is given in the Appendix. 



Relative Density . Relative density of sands is estimated from cone 

 pressure data. However, caution is warranted because the cone pressure 

 data are affected by other factors such as overburden pressure. Recent 

 research has shown that a much better interpretation of relative density 

 can be made if at least one tri axial shear test is performed to define 

 the relationship of relative density to friction angle for a particular 

 soil. A graph of relative density as a function of cone pressure and 

 overburden pressure (Schmertmann, 1978) is presented in the Appendix. 



