CPT for Soft Sediments and Deepwater Investigations

Abstract

Abstract As energy exploration moves into deeper water, there is a growing need for high quality in-situ testing of near surface seafloor sediments. The cone penetration test (CPT) has become the primary in-situ test for offshore site investigations, especially in deep water. This paper describes key features for improved accuracy for the CPT with particular emphasis on testing in deepwater. Recent developments in the design of cones and their deployment systems for deepwater and key features for interpretation are also described. Introduction There is a growing need for high quality in-situ testing of near surface seafloor sediments for site investigations in deepwater (>1,000m). In deepwater the near surface seafloor sediments are often very soft fine-grained soils with low shear strength. Projects in deepwater often require a variety of structures to be placed on and in the seafloor. Hence, a detailed evaluation of the soft seafloor sediments has become increasingly important. The cone penetration test (CPT) has become the primary in-situ test for offshore site investigations, although full-flow penetrometers (T-bar and Ball-cone) have also become popular in soft sediments. To measure the in-situ characteristics of soft near surface seafloor sediments requires careful attention to details in cone design and procedures. The following sections describe the main features that are important for accurate measurements using the CPT and briefly discuss key features in interpretation. Cone design Lunne et al. (1997) provided a detailed description of developments in CPT equipment, procedures, checks, corrections and standards. However, there are several major issues related to equipment design and procedure that are worth repeating and updating. Friction sleeve measurements: Sleeve friction values can be very helpful in indentifying soil behavior type (e.g. Robertson, 1990) and aiding in interpretation (e.g. Robertson, 2009). However, it has been documented (e.g. Lunne et al., 1986) that the CPT sleeve friction is generally less accurate than the cone tip resistance. The application and use of CPT sleeve friction values in soft sediments requires accurate measurements. The perceived lack of accuracy has often meant that the CPT sleeve friction values are either ignored or underutilized. The lack of accuracy in fs measurement is primarily due to the following factors (Lunne and Anderson, 2007);Pore pressure effects on the ends of the sleeveTolerance in dimensions between the cone and sleeveSurface roughness of the sleeveLoad cell design and calibration