Distribution Of Undrained Shear Strength Research Articles

Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea

The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations. In situ piezocone penetration tests (CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China’s first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments (HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays, the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays, while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.

Softening and consolidation around seabed pipelines: centrifuge modelling

Solutions for lateral breakout and axial response of submarine pipelines are well established if the undrained shear strength conditions of the soil are known and defined simply (such as uniform or increasing proportionally with depth). In reality, the geometry of the free surface and the distribution of undrained shear strength around a submarine pipeline post-lay are affected by the lay process. This is because of soil berms that form adjacent to the pipe, and remoulding and subsequent reconsolidation of the seabed. The effect of post-lay consolidation on the subsequent lateral and axial response of submarine pipelines has not been previously investigated through physical model testing. This paper presents results from centrifuge model tests describing lateral breakout behaviour of a pipe on soft clay as a function of (a) pipe installation conditions, (b) post-lay pipe weight and (c) consolidation prior to breakout. In addition, the effect of post-lay consolidation on axial pipe response is studied. The experimental results are compared with available numerical and analytical predictions. The results quantify the influence of the installation process, pipe weight and post-installation consolidation on the lateral breakout resistance and trajectory of the pipe and also the axial pipe response, and show how existing prediction methods can capture these effects.

피압에 의한 점토 지반의 용탈 특성

This paper performed consolidation tests on soft ground with and without artesian pressure conditions to find out characteristics of leaching effects using two types of one-dimensional column equipment(height : 1,100mm, outer diameter : 250mm). Artesian pressure of 5.5kPa was applied to the bottom of soft ground inside column equipment. Distribution of salinity and shear strength with soil depth were measured after the consolidation test. From the results, it was found that distribution of undrained shear strength and salt concentrations were similar at the top of clay ground irrespective of artesian pressure condition. However, at the bottom of clay ground, the values of undrained shear strength and salt concentration under artesian pressure were lower than those without artesian pressure. This result indicates that structure of soft soil with artesian pressure was weakened by salt leaching. Electronic resistance results showed that void ratio under artesian pressure condition was more reduced than that without artesian pressure condition.

Buried footings in random soils: comparison of limit analysis and finite element analysis

The limit analysis and the finite element method are powerful tools for analysing the bearing capacity of foundations. Previous research mainly focused on the foundations in uniform soils. In realistic conditions, soil properties are always varying spatially due to complex physical, chemical, and biological process in earth evolution. This paper investigates the bearing capacity and failure mechanism of footings buried at various depths in clays with spatially variable distribution of undrained shear strength using the lower bound limit analysis, the upper bound limit analysis, and the finite element analysis. Results show that the bearing capacity increases with increasing buried depths in spatially random soils, which is the same as in the uniform soils. The bearing capacity factors calculated using the finite element method, the lower bound limit analysis, and the upper bound limit analysis for a footing in spatially varied soils are all smaller than the corresponding values in uniform soils. The majority of the bearing capacity factors obtained from the finite element method is bounded by those obtained from the lower bound and the upper bound limit analysis. The shear planes show a clearly unsymmetrical manner in spatially varied soils using the three methods, which is different from the symmetrical shear plane in uniform soils.

Discussion of “Multivariate distribution for undrained shear strengths under various test procedures”

Care needs to be exercised when examining the empirical correlations for undrained shear strength proposed by Ching and Phoon (2013) because the manuscript does not adequately define the overconsolidation ratio (OCR), and the data in Table 2 that were used to develop the empirical correlations include values of OCR, as small as 1.1, 2.2, and 3.3 and as large as 50, 60, and 80. A clear distinction needs to be made among (i) OCR = p ′ / vo ′ , where for a natural soil deposit sublayer p ′ is the preconsolidation pressure and vo ′ is preconstruction effective vertical pressure; (ii) OCR = p ′ / vc ′ , where vc ′ is a consolidation pressure in the recompression range of a natural soil deposit sublayer with a preconsolidation pressure p ′ ; and (iii) OCR = vm ′ / vc ′ , where vm ′ , for a soil sublayer in the field or in the laboratory, is the maximum effective vertical pressure after loading and primary consolidation, and vc ′ is a consolidation pressure after unloading and primary rebound. In addition, the data in Table 2 include natural soil deposits as well as specimens reconstituted in the laboratory. In geotechnical engineering, there is a separate body of empirical literature on undrained shear strength of soft clay and silt deposits with values of p ′ / vo ′ typically less than 3 and rarely greater than 6 (Terzaghi et al. 1996, Arts. 20.1 to 20.7), and on undrained shear strength of stiff clays and shales (Terzaghi et al. 1996, Art. 20.8). This discussion only concerns soft clay and silt deposits because for these soils exists an adequate body of laboratory andfield undrained shear strengthdata as a functionof composition (e.g., expressed in termsof plasticity index, Ip), mode of shear (e.g., field vane (FV), direct simple shear (DSS), triaxial compression and extension (TC and TE, respectively), unconfined compression (UC), and mobilized in full scale construction), and rateof shearor time to failure (e.g., FVandUC fast, and field construction much slower).

Reply to the discussion by Mesri on“Multivariate distribution for undrained shear strengths under various test procedures”

A. OCR = p ′ / vo ′ , where for a natural soil deposit sublayer, p ′ is the preconsolidation pressure and vo ′ is preconstruction effective vertical pressure. B. OCR = p ′ / vc ′ , where vc ′ is a consolidation pressure in the recompression range of a natural soil deposit sublayer with a preconsolidation pressure p ′ . C. OCR = vm ′ / vc ′ , where vm ′ , for a soil sublayer in the field or in the laboratory, is the maximum effective vertical pressure after loading and primary consolidation and vc ′ is a consolidation pressure after unloading and primary rebound.

Multivariate distribution for undrained shear strengths under various test procedures

The undrained shear strength (su) of a clay is not a constant. In particular, suvalues of a clay evaluated by different test procedures are different because these tests may have different stress states, stress histories, degrees of sampling disturbance, and strain rates. This study constructs the multivariate probability distribution of sufrom various test procedures based on a large clay database. This multivariate distribution provides an estimation of the normalized undrained shear strength based on four factors: test mode, overconsolidation ratio, strain rate, and plasticity. Once this multivariate distribution is constructed, interesting questions can be answered systematically using simple Bayesian analysis, e.g., given the suvalue for one test procedure, what is the updated mean and coefficient of variation of the suvalue for another test procedure?

Finite element analysis of centrifuge tests on reinforced embankments on soft clay

Abstract The behaviour of reinforced embankments on soft clay has been explored using the technique of centrifuge modelling. Controlled in-flight embankment construction was carried out successfully in the centrifuge over a soft clay layer reinforced with an instrumented geotextile and the behaviour of the subsoil and the response of the reinforcement was observed. Analyses of idealized prototypes provided by the centrifuge tests have been carried out using the CRISP93 finite element program in which the stress-induced anisotropic behaviour of the one-dimensionally consolidated clay foundation was modelled. The results of the centrifuge model tests and their predictions using CRISP93 were in good agreement with each other. Also, the magnitude of tension induced in the reinforcement was found to be very sensitive to the magnitude and distribution of undrained shear strength of the clay foundation, indicating the importance of the effect of the anisotropy of the clay foundation on the behaviour of reinforced embankments.