The Cone Penetration Test (CPT) is an in-situ test that pushes a cone-tipped probe into the ground at a constant rate and measures tip resistance and sleeve friction. It provides continuous, high-resolution soil profiling and is a primary tool for liquefaction assessment and soil characterization.
The Cone Penetration Test (CPT) is an in-situ test in which a cone-tipped probe is pushed into the ground at a constant rate (typically 20 mm/s), and two primary measurements are recorded: tip resistance (q_c) and sleeve friction (f_s). The ratio of these two (friction ratio, R_f = f_s/q_c Γ 100%) provides an indication of soil type β low friction ratios indicate clean sands, high friction ratios indicate clays β making the CPT a continuous, high-resolution profiling tool. Modern CPT systems also include pore pressure measurement at the cone tip (uβ), forming the piezocone (CPTu) β which provides additional information on soil drainage characteristics and pore pressure response. The CPT has become the dominant in-situ test in many countries because it is faster, more repeatable, and provides more detailed profiling than the SPT.
The CPT is a primary tool for liquefaction assessment. The simplified liquefaction procedure, originally developed for SPT, has been extended to CPT: normalized tip resistance (q_c1N) is used to estimate the cyclic resistance ratio (CRR), which is compared to the cyclic stress ratio (CSR) to compute a factor of safety. The CPT-based method is generally considered more reliable than the SPT method because the CPT has better repeatability, finer vertical resolution, and a richer theoretical basis for interpretation. The CPT is also used for soil classification (using the Robertson chart or similar systems), for estimating soil unit weight, relative density, friction angle, and undrained shear strength, and for characterizing soil stratigraphy in detail. For critical projects β particularly those involving liquefaction potential, tailings dams, or major infrastructure β the CPT is typically the primary in-situ test, with SPT used as a supplement for sampling and correlation.
The CPT has important limitations. It cannot penetrate gravels, cobbles, boulders, or very stiff clays β for these materials, the CPT is stopped and alternative methods are required. It provides no sample for laboratory testing, which limits its use for characterizing soil behavior under controlled conditions. Interpretation requires experience, particularly for unusual soil types or cemented soils where standard correlations may not apply. In Iran, CPT is increasingly used for geotechnical investigation of important projects, though SPT remains more common because of historical practice and equipment availability. Modern practice increasingly combines CPT with other methods β shear wave velocity measurements (seismic CPT, SCPT), pore pressure dissipation tests, and sampling β to provide a comprehensive site characterization. For seismic hazard assessment, the combination of CPT, SPT, and Vs measurements provides a robust basis for evaluating liquefaction potential, cyclic softening, and site response. Software tools such as CLiq (from Geologismiki) and LiqSVs (from the University of Washington) implement the standard CPT-based liquefaction procedures and are widely used in practice. In Iran, the CPT-based liquefaction assessment is gaining adoption, particularly in projects with international consultants or where significant liquefaction risk is present.