Laboratory-scale cone penetration tests (CPTs) in silt were performed with x-ray micro tomography and analysed with three-dimensional digital image correlation. During insertion of the instrumented probe, these tools allow the identification of a contractant bulb of silt close to the tip of the probe surrounded by a larger bulb of dilating material. The results obtained (in particular the failure mechanisms observed) shed new light on the mechanics of cone penetration in silt and consequently reflect on the interpretation of in situ CPTs.
A. Gylland et al.
ABSTRACT 25A detailed characterisation of the quick clay underlying the NTNU research site at Tiller, 26 Trondheim is presented. The objective of the work is to provide guidance on quick clay 27 parameters to engineers and researchers working with similar clays in Scandinavia and 28North America especially on landslide hazard assessment. The material is lightly 29 overconsolidated and is characterised by its high degree of structure and very high 30 sensitivity (quick clay). Clay and water contents are both about 40%. The plasticity index 31 is low (5%). This relates to the low active minerals of the clay and silt fractions (illite / 32 chlorite and quartz / feldspars respectively).
Challenges in obtaining high quality samples of sensitive low-plasticity clay in an effective manner have been overcome by the development of the mini-block sampler. The starting point for the development of the new sampler was the Sherbrooke block sampler that was first introduced in Canada in 1979. Although the Sherbrooke block sampler can produce high quality samples, its use, particularly in industrial projects, has been limited due to the perceived high costs, practical difficulties, and the time required. This paper outlines details of the development and design of the mini-block sampler together with developed techniques for protection and transportation of the samples and preparation of specimens for laboratory testing. The sampler has been used successfully at five Norwegian clay sites. For two of the sites comparative Sherbrooke samples were available and it is shown that the quality of the mini-block samples is very high and is at least as good as that of the Sherbrooke samples. The work also demonstrates that rigging, preparation, and sampling with the mini-block sampler is fast, practical and is not labour intensive. Furthermore the resulting samples are easy to transport and handle, but still provide sufficient material for extensive laboratory testing.
A soil sample goes through stress changes during and after sampling. Sensitive clays are affected by sample disturbance and stress changes have a great effect on the quality. The reduction of in-situ total stresses to zero causes the soil sample to develop a negative pore pressure, which is also referred to as residual effective stresses. In an ideal situation, a block sample shall retain its residual effective stress during sampling and storage, which prevents it from swelling. To study this, an attempt was made to monitor the pore pressure variations inside a block sample of soft, sensitive, low-plasticity clay during and after sampling. The pore pressure was measured continuously during the storage period of 3 days and the results were compared with a similar work. The findings suggest that the residual effective stress in block samples may be reduced in a matter of minutes after sampling. Testing performed on reference samples corroborate these storage effects.
An approach for assessing the effects of sample quality is presented. Soil samples were taken using a 50 mm Swedish STII piston sampler and the Norwegian University of Science and Technology (NTNU) mini-block sampler from a soft clay test site. Differences in laboratory test results are identified for several stress paths, assisted by simulations made using an advanced constitutive model. Hitherto such comparisons have focused on differences in basic engineering properties such as strength and stiffness. The effect of choosing alternative model parameters from piston and block samples is demonstrated through the analysis of the long-term settlement of an embankment. The simulations show that substantially larger settlements and lateral displacements are predicted using parameters obtained from the piston samples. Furthermore, the magnitude of the differences is larger than expected. This demonstrates that for this application, relatively small differences in the assessed sample quality, using traditional laboratory data interpretation methods, are amplified when applied to a prototype boundary value problem. It is suggested that a little more care in sampling and testing can result in large cost savings as a result of the more reliable model parameters that can be extracted, particularly when the improved sampling is combined with the use of an advanced constitutive model.
A block sample goes through stress changes during and after sampling. Sensitive clays are affected by sample disturbance and stress changes have a great effect on sample quality. Reduction of the in-situ total stresses to zero causes a soil sample to develop a negative pore pressure. In an ideal situation, a block sample retains its negative pore pressure during sampling and storage, preventing it from swelling. To study this, five in-situ tests were conducted in which the pore pressure variations inside block samples were monitored during and after sampling. The pore pressure was measured continuously during a storage period of several days and the results were compared with tests on reconstituted clay in an attempt to simulate the same stress conditions as those in situ. Similar to the in-situ tests, the pore pressure inside the reconstituted samples was measured throughout the test. The findings suggest that the negative pore pressure in mini-block samples may be reduced in a matter of minutes or hours after sampling. Parallel testing on mini-block and reconstituted clay samples indicated that pore pressure dissipation had a negative effect on the preconsolidation pressure, constrained modulus and sample quality.
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