Piezocone Profiling of a Deepwater Clay Site in the Gulf of Guinea

Abstract

Abstract Offshore soil sampling, conducted for the purpose of site characterization, is often plagued with sample disturbance issues and other difficulties related to the sampling process. Field tests, such as the piezocone penetration test (PCPT), are not subject to such difficulties and are routinely used to complement the sampling program. However, the conventional practice is to directly evaluate the in-situ undrained shear strength using an assumed or fitted bearing factor, Nkt. The hybrid cavity-expansion and critical state theory establishes the relationship between OCR and piezocone measurements. Once the stress history is known, the various modes of soil strength may be estimated through the use of appropriate S values. This paper presents interpretations of piezocone data at 12 separate locations in the Jubilee field development in the Gulf of Guinea. Separate and independent profiles of stress history and shear strength of the deep water clays found at this site are compared with the results of a detailed and extensive laboratory testing program. The experimentally determined SHANSEP parameter m, was found to be representative of the soil, but the strength ratio from DSS testing was judged to be too high. S values used in the interpretations were based on those reported in the literature for similar soils. Internal friction angles of 30° to 31° and an IR of 100 adequately predicted OCR with depth. The profiles derived from the PCPT data compared well with the results of CRS and DSS tests consolidated to in-situ pressures. The use of piezocone data in this manner allows the development of a site-specific family of undrained shear strength profiles that are continuous throughout the depth of testing. This profile can complement data from the soil sampling program which inherently has information gaps arising from the limitations of sampling and testing or as a result of uncertainty caused by sample disturbance. The interpretations are relatively quick and easy to perform and can increase the degree of confidence in the soil modeling parameters selected. Introduction Deepwater field development projects often include a scope for piezocone penetration tests (PCPT). These provide quick profiles of the subsurface soil stratigraphy. The cone tip resistance (qc), sleeve friction (fs), and induced penetration porewater pressures (u) are measured continuously and expediently with depth (z). These readings are post processed and interpreted to provide relevant geotechnical parameters. The PCPT results can be used to supplement the data obtained from cores (piston core and jumbo piston core) or from boreholes if they are present. This is often a very useful exercise because sampling from deepwater sediments is in itself a very costly process and sample disturbance is an ever present concern. Comparison of shear strengths as measured from soil samples and those derived from the PCPT measurements can clarify whether an observed reduction in strength at a particular depth is related to sample disturbance or not. The advantage is due to the test being conducted in-situ rather than in the laboratory. Site investigations for deepwater offshore projects generally include a soil sampling and testing program where the soil stress history and strength profiles with depth are defined. PCPT data is used to complement this program and is often very useful in providing information between sampling depths and through data gaps which may exist due to failed sample recovery or insufficient penetration. PCPT data also provides a separate and independent estimation of undrained shear strength which is routinely compared with the compiled results of various laboratory shear strength tests (i.e. UU and minivane test). Since no single soil test is singularly definitive of the " true" strength, PCPT data is a powerful tool in the arsenal of geotechnical engineers who are tasked with the rather elusive goal of characterizing and modeling soil profiles in deep water that are, very often, green-field regions.