Traditional techniques for assessing the quality of offshore soil samples, such as measuring the volume change during reconsolidation back to in situ stress (or recently the normalised void ratio change) are time consuming, often requiring one or several days to perform.
This paper assesses a rapid alternative technique for independently evaluating sample quality in soft clay, involving measurements of shear wave velocity, Vs using bender elements and soil suction, ur. It is found that the quality of samples indicated by Vs and ur. measurements are independently similar to those inferred from anisotropically consolidated undrained triaxial (CAUC) and oedometer tests. Sample quality may therefore be rapidly assessed in the field, leading to important economic and technical implications for the offshore industry.
The characterisation of soils for design and construction is strongly dependent on the removal of sufficiently high quality samples from the ground. Disturbed samples may result in poor estimates of strength and stiffness parameters from triaxial and oedometer tests, which may in turn lead to serious and costly design errors. Considerable attention has been devoted to the subject of sample disturbance over recent years, ranging from improvements in sampling techniques and equipment, to correcting for sample disturbance effects on disturbed samples. Although these improvements may decrease the effects of sampling disturbance, evaluation of sample quality remains essential if design parameters are to be considered reliable.
A number of different techniques may be used to evaluate sample quality, such as the measurement of volumetric strain1 and the normalised change in void ratio,¿e/e02. However, most of these techniques require reconsolidation back to in situ stresses before measurement, a process that may require a number of days testing. This assessment inefficiency is a particular problem for offshore sampling, where rapid assessment of sample quality may result in major economic saving.
This paper discusses the use of shear wave velocity, Vs and soil suction, up for rapidly evaluating sample quality. The proposed Lvs- Lu criterion of Donohue and Long3 is assessed by the addition of supplementary data from two Norwegian research sites at Onsøy and Nybakk-Kløfta. Samples of varying quality extracted using different sampling techniques were acquired from the two sites. The sampling techniques used at Onsøy were standard (steel), 75mm samples taken using the standard Japanese piston sampler and a prototype 100mm-diameter continuous sampler. Block samples were taken using the high quality Sherbrooke block sampling technique at the Nybakk site. All of the samples were tested by the same operators and the same techniques were used at the laboratories of University College Dublin.
Vs and ur Shear wave velocity, Vs
The seismic geophysical parameter - shear wave velocity, Vs, - has been recently used by a number of authors to assess sample disturbance. It has been observed that Vs and corresponding small strain shear modulus (equation 1), when measured in the laboratory after reconsolidation to in situ stresses, are generally lower than the in situ equivalent4, 5, 6. This reduction in stiffness has been ascribed to sampling disturbance.