The empirical procedures for offshore foundations design are directly influenced by the shear strength values selected for analyses. There is generally a large amount of scatter in measured undrained shear strength data from offshore borings because of various types of laboratory tests and sample disturbance. A comparison of strength data obtained on samples of cohesive soils from the Gulf of Mexico taken with a 2.25-in.-OD thin-wall tube percussion sampler and a 3.0-in.-OD thin-wall push sampler confirms that (1) the strength results for 3.0-in. push samples give the highest and most representative values with less scatter, (2) the unconsolidated-undrained triaxial compression test gives the best measure of undrained shear strength for cohesive soils, and (3) strengths interpreted from SHANSEP on push samples correlate well with conventional laboratory and in situ strength data.
Offshore geotechnical investigations around the world are frequently performed from an oil field supply vessel that is held on location by a four-point mooring spread. The drilling and sampling procedures first introduced in 1962 are usually performed with a skid-mounted drilling rig positioned over a center wellin the deck of the vessel, as described by McClelland (1972). Samples are most frequently taken with a wire-line activated hammer sampler which provides a simple and economical means of compensating for vessel motion during sampling. The deleterious effects of this sampling technique upon sample quality have been recognized for many years (Emrich, 1970; Schjetne and Brylawski, 1979; and Broms, 1980); however, the usually increased costs associated with acquiring higher quality samples has prolonged the use of percussion sampling.
Soil samples may be disturbed to varying degrees during the data gathering operations (drilling, sampling, and testing) because of a number of factors. The purpose of this paper is to review how the undrained shear strength values (S) interpreted for offshore foundation design are affected to varying degrees by:
boat motion and movement of the drill pipe;
drilling operations;
sampling technique;
stress relief during sample recovery;
sample extrusion;
sample handling, packaging, and transportation procedures;
sample storage;
laboratory testing procedures; and
geologic and physicochemical properties of the soil Undrained shear strength data for Gulf of Mexico clays sampled by conventional marine wire-line sampling equipment are presented and compared with data from samples obtained with a larger diameter push sampler and from in situ field vane tests.
Based on results of this comparison, the authors will recommend drilling, sampling, and testing procedures that should be standardized and carefully monitored by the geotechnical field engineer to reduce sample disturbance and improve the quality of measured undrained shear strength data used in foundation design.
Most offshore foundation design analyses are performed today by procedures based on the Mohr Coulomb failure criteria. Although the profession has long recognized the assumptions and approximations involved with this criteria, field evidence (Bishop and Bjerrum, 1960) revealed by a large number of actual failure of slopes and foundations indicate good agreement with results of analyses using this criteria.
