6707 Netherlands Drive Wilmington, NC 28405 O M 919 694 1114 *2005 512 789 9709 E m.epley@sonicsampdrill.com I www.royaleijkelkamp.com/en-us/ Mike Epley Drilling Sales Manager 18 APRIL 2024 WorldWide Drilling Resource® CPT and Soil Behavior Adapted from Information by Royal Eijkelkamp Cone penetration testing (CPT) is commonly used to identify and define soil layers. When pushing the cone during CPT, the data can provide an idea of the soil behavior, referred to as soil behavior type (SBT). CPT doesn’t provide a direct measurement of the physical properties such as grain size and material, but does provide a good indication of the soil behavior and mechanical characteristics, such as its strength and stiffness. Over time, various SBT classifications have been generated, starting with the classifications made by an engineer named H.K.S. Begeman in the 1950s, using the data from the mechanical friction jacket cone, where cone tip resistance was compared with sleeve friction. The problem with charts of this nature is they produce narrow bands, which makes classification difficult and only accounts for a limited range of soil types. A better idea for CPT-based soil classifications is to compare the cone tip resistance with the friction ratio. Typically, the cone tip resistance is high in sands and low in clays, and the reverse applies to the friction ratio: low in sands and high in clays. Over the years, a number of charts for soil classification have been presented, but the soil behavior classification developed by P.K. Robertson and R.G. Campanella in the late 1980s, is the most commonly used classification and identifies 12 different SBTs. This classification not only links the cone tip resistance to the friction ratio, it also places it in a chart separate the pore pressure parameter. This provides an instant verification of the results allowing the user to use the most appropriate method. For example, in very soft soils where testing has been performed with a subtraction cone, the friction values may be less reliable, so it may be better to perform a classification using the pore pressure parameter. Since the parameters are available as the test is being performed, the CPT operator will have an idea of the SBT during the test. In 1990, Robertson amended this classification to account for the fact both penetration resistance and sleeve resistance increase with depth, due to the increase in effective overburden stress. CPT data is typically presented against depth. However, soil classification data such as the SBT are normally presented in graphs, making it harder to estimate changes in SBT with depth and harder to analyze soil behavior alongside conventional CPT data. To overcome this, M.G. Jefferies and M.P. Davies proposed the soil behavior type index, which allows SBT to be presented as a continuous profile alongside CPT data. Although this does reduce the number of zones in the classification, it provides a useful visual classification. The boundaries in the various classification charts were proposed based on historical data from well-studied test sites. However, soil is not the same world over. Limited sampling alongside CPT soundings on a project allows operators to perform a site specific correlation of the CPT data to the soil encountered. This correlation could even result in modification of the classification boundaries in a particular chart to reflect the reality of the soils on a given project. Due to the high applicability of CPT data to analyze mechanical soil characteristics (such as strength and stiffness), SBT charts are a strong soil classification method and a useful tool for delineating soils based on their mechanical characteristics, offering a quick visual identification. As with all correlations, care must be taken when using a particular soil classification method to ensure it’s appropriate for the soil conditions of the site. ENV [The ad] looks good, as always. Gordon Ballard Ballard Drilling Co., Inc. Wadley, AL
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