ABSTRACT
Numerous studies have been carried out extensively about the issue of sampling disturbance (Baligh 1985; Jamiokowski et al. 1985; Clayton et al. 1998; Tanaka & Tanaka 1999; Hight 2000). Most of these investigations mainly concentrated on the destructuring and restructuring of the microstructure of clays induced by sampling and the effects of different samplers. On the other hand, there is still a lack of study on the sampling disturbance due to the tube penetration process. In fact, tube penetration causes predominant shearing and distortion of the surrounding soil and will have significant level of disturbance within the whole sampling process (Baligh et al. 1987). Santagata & Germaine (2002) stated that while many studies have attempted to assess sampling
1 INTRODUCTION
One of the essential elements involved in the geotechnical analysis and design, which is vital to their success, is precise estimation of engineering properties of soils from results of laboratory testing in which the stresses, deformations and boundary conditions of soil specimens can be readily and precisely controlled and observed. In order to obtain highly representative and reliable soil design parameters by means of laboratory tests, the engineering properties of the soil such as the shear strength and compressibility must be determined through appropriate testing of undisturbed samples previously retrieved from the ground using some form of sampling procedure (Joyce 1982; Clayton et al. 1995). However, samples obtained from field sampling may suffer from sampling disturbance to certain degrees that generally cause wide discrepancies in properties between soil specimens tested in the laboratory and the in-situ soils. Sampling disturbances are due to ground boring, sampler penetration and retrieval, sample transportation and storage, sample extrusion and trimming prior to testing etc. These disturbances have been regarded as a significant adverse problem to the geotechnical engineers because it always leads to poor acquisition of realistic soil parameters. It follows that the requisite for
disturbance under specific conditions, only two models that rationally quantify the effects of sampling disturbance have been proposed. They are the “perfect sampling approach” (Ladd & Lambe 1963; Skempton & Sowa 1963), and the “ideal sampling approach” (Baligh et al. 1987). The “perfect sampling approach” considers only the disturbance due to the in-situ stress release while the disturbance simulated based on the “ideal sampling approach” has shown that soil elements located inside a sampler tube undergo a complex strain history, involving both shear and normal strains. Baligh et al. (1987) has provided many important insights into this problem in which strains are developed as a sampler is pushed into the ground and pointed out that the effect of disturbance due to penetration has the largest impact on the sampling disturbance and therefore further improvement on this stage is necessary. Experience suggests that the disturbances associated with the withdrawal of the sampler from the ground, and the subsequent extrusion of the sample from the sampler tube, will be small in comparison with the effects of displacing the soil around the sampling tube during driving, provided that good practice is adhered to (Clayton et al. 1998). Since then, however, no more extensive work has been found on this topic. Furthermore, the effect of influence zone during penetration of the sampler at the location of adjacent sampler has not been thoroughly investigated. This information is essential in order to get better understanding of the sampler-soil interaction problem so as to facilitate quantification and analysis of sampling disturbances and also will facilitate the geotechnical site characterization planning as well. This involves an educated decision about the sampling location at the site, from which a reasonably accurate estimate of the ground conditions can be obtained for designing economic, safe and reliable foundations.
