This paper presents the results of extensive full-flow penetrometer testing conducted at the Bothkennar soft clay research site. The results of in situ penetration tests are compared to field vane tests and laboratory data obtained from high quality Laval and Sherbrooke block samples. Bearing capacity factors for the T-bar and ball, NT-bar and NBall relating to undrained shear strength, su, and remoulded shear strength, su-rem, are compared with theoretically derived N factors. Overall, the close relationship between net resistances from the T-bar and ball, qT-bar and q ball, and SuAVG, coupled with the ability to measured remoulded shear strength, su-rem, indicates that full-flow penetrometers could be advantageous over piezocone tests (CPTU) in site investigations for offshore foundation systems. It is still necessary, however, to develop empirical N factors, relating measured resistance to undrained shear strength, su and remoulded shear strength su-rem, until more rigorous numerical solutions for the T-bar and ball are developed.
The geotechnical design of many offshore foundation systems required accurate determination of the undrained and remoulded shear strength of a soil. Traditional practice has been to use piezocone tests (CPTU), couple with in situ vane tests or laboratory testing of retrieved samples to derive empirical strength correlations. With the gradual move into deeper waters and the difficulties obtaining undisturbed samples for laboratory testing, more reliance is being made on in situ methods of strength determination to obtain these parameters. Difficulties measuring resistance of soft soils in deep water with the CPTU and uncertainties about the failure mechanism led to the development of full-flow penetrometers, such as the T-bar and ball. The larger bearing area of these probes gives higher resolution of the measured resistance compared to the CPTU. Corrections to measured resistance due to the full-flow mechanism of these devices. Cyclic penetration tests, where the penetrometer is cycled up and down in a layer, can also be used to estimate the remoulded shear strength of a soil. Lunne et al1 carried out testing at two onshore and one offshore soft clay sites and found that N factors relating the measured resistance of the T-bar to undrained shear strength, sw had a slightly narrower deviation band than similar factors for the CPTU.
Before these new penetrometers can be adopted as standard tools in offshore site investigations, extensive research needs to be carried out in a range of well characterized soils to gain appropriate knowledge and confidence in their use. This paper presents the results of full-flow penetrometer tests conducted at the former UK Engineering and Physical Sciences Research Council (EPSRC) Soft Clay research site at Bothkennar in Scotland. N Factors relating the penetration resistance for the T-bar and ball laboratory measured undrained shear strengths sw obtained from Lavel and Sherbrooke bloack samples, as well as from field vane tests (FVT) have been computed and compared to similar factors calculated for the CPTU. These results are compared to the theoretical N factor derived from plasticity solutions which are currently used in geotechnical practice.