R G Robinson

Full flow penetrometers (FFPs) such as T-bar and ball penetrometer provide reliable estimates of undrained shear strength (su) of soft-soil deposits. Several studies are reported in the literature on the field application of these penetrometers for offshore applications typically for the deep-water environment. However, the lack of sampling provision creates problem for identifying the accurate soil type from the load–penetration response. Information obtained from the field samples is vital in determining the stratigraphy and conducting several basic laboratory tests. This study proposes a new sampling-cum-penetration testing device, which uses a split-spoon sampler for sampling with a spherical cutting shoe. The cutting shoe was modified to increase the bearing area for enhanced accuracy in soft deposits. Strength assessment study was conducted in the laboratory using remoulded kaolin clay at various undrained shear strengths. To investigate the occurrence of plugging and the extent of influence zone during penetration, image-based deformation measurement technique was adopted. The modified ball penetrometer shows a strength factor of about 14·3 in remoulded kaolin clay.

Consolidation parameters are essential for the design of a variety of geotechnical structures. These parameters are commonly determined by performing laboratory one-dimensional consolidation test. The conventional one-dimensional consolidation test takes about 10–14 days to complete one test. This paper describes an accelerated consolidation testing procedure, in which the standard √t method is used to decide the subsequent increment. To validate the suggested testing procedure, tests were conducted on five reconstituted and three undisturbed soil samples. Results from the suggested procedure and the conventional incremental load one-dimensional consolidation data are analysed and compared. The time required to complete the test using the accelerated consolidation method could be as low as 2–5 h for most of the soils compared to 10–14 days in the case of the conventional consolidation test. Soils with very low coefficient of consolidation, of the order of 10-9 m2/s, requires about 30 h (11 h during loading stage and 19 h during unloading stage) to complete the test.

Vacuum preloading of clay deposits is becoming an increasingly popular ground improvement technique. Although many studies have been reported in literature, the mechanism of vacuum preloading is still not properly understood. Soil under vacuum preloading is expected to undergo an inward lateral deformation, therefore the influence of lateral deformation on volumetric strain requires further analysis. This paper addresses this aspect through laboratory studies on reconstituted samples of kaolinite. The Rowe cell apparatus was modified to measure lateral deformation under various states of stress because lateral and vertical deformation of soil depends on its state of stress and associated lateral pressure. A method for predicting the volumetric and lateral strains under vacuum consolidation is proposed and then applied to two case studies in China.

Clay deposits often exhibit inherent anisotropy due to different microfabrics, resulting from both environmental conditions during deposition and stress conditions subsequently. Many anisotropic elastic-plastic models are developed for modeling the behavior of clays subjected to anisotropic stress conditions. In this study, the drained and undrained shearing behavior of kaolinite clay samples with different microfabrics are modeled using anisotropic S-CLAY1 model with a rotational hardening rule and the isotropic modified cam clay (MCC) model. The inherent anisotropy resulting from the difference in the microfabric of the clay samples, reflected as the initial value of the rotational hardening parameter of the S-CLAY1 model, is determined based on the shape of the undrained stress paths of the samples subjected to consolidated undrained triaxial tests. The simulated results are compared with the results of consolidated undrained and drained triaxial tests carried out on kaolinite samples with different microfabrics. Clay samples with different microfabrics are prepared artificially, in the laboratory, by remolding with different pore fluids. It is observed that while S-CLAY1 model predicts the undrained behavior of kaolinite samples better, the drained behavior is better predicted by the MCC model.

This note describes a faster and complete consolidation testing procedure using the inflection point method so as to obtain the void ratio-consolidation curve at the end-of-primary consolidation and the coefficient of consolidation. The testing procedure is similar to the conventional incremental load consolidation test, with the only difference being that the subsequent loading is applied once the degree of consolidation of U=70·15% is reached. The time and settlement corresponding to U=70·15% is obtained using the inflection point method, from which the end-of-primary consolidation and the coefficient of consolidation are evaluated. The validity of the proposed procedure is verified by performing tests on four reconstituted and three undisturbed soil samples. By adopting the proposed procedure, complete consolidation tests can be completed within 2·5 to 9 h depending on the coefficient of consolidation of the soils.

Microfabric plays an important role in the engineering behavior of soils. Although many studies are available in the literature on the effect of microfabric on the static behavior of soils, the effect on the cyclic behavior is less understood. In the present study, samples with different microfabric were prepared in the laboratory by reconstituting commercially available kaolin clay with different pore fluids under a consolidation pressure of 100 kPa. Consolidated undrained triaxial tests were carried out on these samples under static and cyclic loading conditions. Dispersed samples were found to have monotonic stress-strain behavior with a peak deviatoric stress and higher peak undrained shear strength than the flocculated samples. However, the dispersed samples were found to offer less resistance to cyclic loading. When subjected to cyclic loading, dispersed samples failed within a few cycles under a cyclic stress ratio (defined as the ratio of cyclic deviatoric stress to the undrained shear strength) close to 0.6, whereas in flocculated samples, sudden failure was not observed even at a higher cyclic stress ratio of 0.9, although strains and pore pressures accumulated to higher values. Postcyclic monotonic tests conducted on samples that did not fail under cyclic loading showed an apparent overconsolidation effect caused by cyclic loading in a similar manner, as reported in the literature. © 2011 American Society of Civil Engineers.

Land reclamation is a major civil engineering activity in Singapore. Due to depletion of suitable local fills and the cost of imported land, dredged and excavated clay fills, in spite of their poor engineering properties, are being evaluated as a fill material. To reduce double handling, it is desirable for the clay to be used directly in a lump form, instead of the more conventional slurry fill. While the performance of a slurry fill is relatively well understood, the behavior of lumpy fill is not. This paper reports the results of a laboratory study carried out on lumpy fill made of cubical clay lumps of size ranging from 12.5 to 50 mm. The study showed that the interlump voids are substantially closed at a consolidation pressure much lower than the preconsolidation pressure of the lumps. The study also shows that at a consolidation pressure of about 100 kPa, the permeability of a lumpy fill is reduced to an order similar to that for homogeneous clay. However, the shear strength profile obtained using the cone penetration test indicates that the fill is still highly heterogeneous under a pressure of 100 kPa. When the preconsolidation pressure of the lumps is exceeded, the strength profile becomes uniform. The degree of swelling of the lumps plays a significant role. For fully swollen lumps, the consolidation pressure required to close the interlump voids is considerably less than that if the lumps were not allowed to swell. The coefficient of secondary compression of the lumpy fill is comparable to the homogeneous clay indicating that secondary compression is not a serious issue. © ASCE.