CPT Correlations Calculator

The cone penetration test pushes an instrumented probe into the ground at a controlled 20 mm per second, logging tip resistance qc, sleeve friction fs and, in a piezocone, pore water pressure u2 measured just behind the cone shoulder. The standard cone has a 60 degree apex angle and a 10 square centimetre projected area, about 35.7 mm in diameter, with a 150 square centimetre friction sleeve, per ASTM D5778 and ISO 22476-1. The first correction applied is qt = qc + u2 times (1 - a), where a is the net area ratio of the particular cone, typically 0.70 to 0.85. This accounts for pore water pressure acting on the unequal shoulder area and is negligible in sands but can raise qc by 20 to 40 percent in soft clays where u2 is large. Raw resistances increase with depth simply because confining stress increases, so interpretation uses normalized parameters: Qt = (qt - sigma_v) divided by the effective vertical stress, the normalized friction ratio Fr = fs divided by (qt - sigma_v) expressed as a percentage, and the pore pressure ratio Bq = (u2 - u0) divided by (qt - sigma_v). Plotting Qt against Fr places the soil in one of the nine soil behaviour type zones of the Robertson chart. Property correlations follow: an effective friction angle near 17.6 + 11 log10(Qt) for sands, undrained shear strength from (qt - sigma_v) divided by a cone factor Nkt that ranges from about 10 to 20 with 14 a common default, relative density from normalized resistance, and overconsolidation ratio from roughly 0.33 Qt. Typical qc values run below 1 MPa in soft clay and 15 to 30 MPa in dense sand. The important limitations are that no sample is recovered for verification, that the chart reports behaviour type rather than grain size so a sensitive clay can plot unexpectedly, that Nkt and the modulus factor are regional and should be calibrated against laboratory or field tests, and that gravel, cobbles and cemented layers cause refusal or damage.

The cone penetration test began in the Netherlands, where thick soft deltaic deposits made sampling unreliable and a continuous in-situ profile was worth more than discrete boreholes. Around 1932 P. Barentsen of the Dutch Department of Public Works pushed a 35 mm gas-pipe-mounted cone into the ground by hand, measuring the force required at intervals, which is why the device was long known as the Dutch cone. The essential geometry of a 60 degree apex and a 10 square centimetre base has changed little since. The tip-resistance-only measurement gave stratigraphy but little basis for identifying soil type. The decisive addition was the friction sleeve. Around 1953 H. K. S. Begemann introduced the friction jacket cone, allowing sleeve friction to be measured separately and the friction ratio to be used as a soil identification index, an idea he developed further in publications through the mid-1960s. Electrical cones with strain-gauge load cells replaced the mechanical telescoping designs from the 1960s, with Fugro developing an influential design in 1965 that gave continuous analogue and later digital logging rather than readings every 20 cm. Pore pressure transducers were added in the late 1970s, creating the piezocone or CPTu. Peter Robertson and Richard Campanella published widely used interpretation charts in 1983, and Robertson normalized soil behaviour type charts of 1986 and 1990 became the reference framework, later updated with a stress-exponent formulation. The 1997 textbook by Tom Lunne, Robertson and John Powell and Paul Mayne NCHRP Synthesis 368 of 2007 consolidated the correlation literature. Seismic and resistivity modules now add shear wave velocity and conductivity to the same sounding.