Recently, an offshore geotechnical investigation was conducted using in situ testing equipment at several proposed offshore platform sites. A motion compensated drilling system was used to drill and handle in situ cone electrometer, vane, and push sampling tools. Laboratory soil testing comprised classification, strength and consolidation tests. Geotechnical engineering analyses were made for pile supported structures.
Soil and foundation conditions varied from soft clays to hard clays and dense sands. Soils data from in situ cone electrometer tests are correlated with soil test results. The ratio of sleeve friction to cone resistance, termed the friction ratio, is correlated with general soil types. A soil classification chart is developed and shows that friction ratio can be used to identify soil types. Several approaches are presented relating cone resistance to untrained shear strength using the cone factor, Nk. The cone factor is related to plasticity of cohesive soils and compared with published results. Pile end bearing, computed directly from cone resistance, is compared to end bearing derived from typical pile capacity procedures. Sleeve friction measurements in both clay and sand are analyzed and compared to pile friction computed from pile capacity procedures. Using cone electrometer data for predicting pile drivability is briefly discussed.
Results showed that cone electrometer testing provides more accurate soils data than just samples alone. Combining sleeve friction and liquidity index may provide information on the stress history of cohesive soils. The cone factor is not a unique number for a given soil, it varies with the method used to determine the factor. Cone data can be used in pile drivability predictions. Recommendations are made about selecting suitable combinations of cone soundings and sample boreholes for offshore geotechnical investigations.
A major geotechnical study was recently made to determine regional and site specific marine soil conditions for proposed offshore platforms. Soil conditions were investigated by drilling boreholes, recovering soil samples, making in situ cone electrometer (CPT) soundings, and conducting in situ vane tests. Soil samples were tested in the laboratory for classification, strength, and compressibility. In situ test data and laboratory data were used in the engineering analysis for pile supported platforms.
Laboratory soils data are compared to data collected from the in situ cone electrometer and vane tests. Correlations are made between various cone parameters (friction ratio, cone factor, and cone resistance) and typical soil test results (plasticity, strength, and soil type). Unit pile capacity values derived from several pile capacity methods are compared to cone resistance and sleeve friction
Results from this study show that incorporating in situ cone electrometer and vane tests with soil samples improve the knowledge of the soil conditions at a particular offshore site.
The M/V SURVEYOR used during this study is a 249 ft. long (76 m), specially designed, geotechnical drilling vessel (13). The vessel is equipped with a six anchor mooring spread capable of anchoring in water depths up to about 800 ft. (245 m). A bow thruster assists in maneuvering, positioning, and anchoring.
