The undrained shear strength of clay is an important parameter for the design of embankments, shallow foundations, and pile foundations. Among the various methods of testing undrained strength, the simple shear is essential when the shearing mode of the soil surrounding the pile is similar to that in a simple shear test. This study employs a series of undrained strength tests with a Berkeley simple shear apparatus. Three reconstituted natural clays and one artificial clay were tested with a range of coefficients of consolidation. The influence factors, including sample pre-consolidation pressure, saturation back pressure, shearing rate, height of specimen, consolidation stress, and lateral stress ratio, were investigated in undrained simple shear tests. The failure mode in simple shear and corresponding strength parameters are also examined. Based on the test data set, models for describing the undrained strength with simple shear for high plasticity clays are developed and compared to test data for normally consolidated reconstituted clay. Good agreement between models and intact Onsoy clay is also observed when allowance is made for the coefficient of consolidation and strength parameters of undisturbed clay and in situ stress state.
The failure mode should be taken into account in the analytical solution for pile base resistance. In order to determine the pile’s failure pattern in soft rock, the graphical model test method is used. That the compression zone below the pile base in soft rock is compatible with the spherical cavity expansion is shown. An improved calculation model for the ultimate base resistance may be developed by considering the force balance of the rigid cone under the pile-end in the ultimate stress state and the limit pressure required to expand a spherical cavity. To account for the influence of socket length, the limiting pressure for the spherical cavity expansion in infinite space is adjusted to that in semi-infinite space. The ultimate resistance of a base is computed by factoring in the length effect. The normalization of the base resistance-displacement relation is provided based on the findings of pressure chamber experiments on pile load-settlement, which are independent of socket length and overburden pressure. This semi-empirical technique is verified via the use of pressure chamber experiments and field testing of piles in soft rock.
The shaft capacity of driven piles in clay increases with time even after installation-induced consolidation is completed. However, existing studies have shown that these gains in capacity are variable and cannot be predicted reliably. Projects such as those involving life extension of existing offshore platforms (which often involve increased platform loads) as well as those considering foundation re-use provide the motivation for this paper, which investigates the potential for model scale testing to replicate the ageing characteristic of shaft friction in a laboratory environment. The study employs a number of model piles installed in reconstituted, high plasticity Onsoy clay in laboratory pressure chambers. The first time tension load tests on these piles, performed over the period of 1 year after installation, are compared with a similar series of tests on full-scale driven piles in Onsoy, Norway. Good agreement between laboratory and field experiments is observed when allowance is made for the significantly longer consolidation periods of the full-scale piles. This finding implies that ageing effects of shaft friction in clay can be investigated over relatively short time periods using model piles in laboratory testing chambers.
The shaft friction in clay is essential to bearing capacity of pipe piles. Reasonable selection of design methods and parameters is very important for offshore piles. This paper focuses on the current popular vertical load design methods based on undrained strength for pipe piles, especially for offshore piles. Based on the database, the accuracy and reliability of various pile design methods based on undrained strength su in clay were reviewed. The small-scale model tests were conducted to evaluate su-based methods. For the shortcomings of small-diameter piles in the database, a field test of full-scale offshore pile was carried out and analysed. By comparing the calculated and the measured capacity for each method, the reliability of various design methods is evaluated for large-diameter piles. For the design methods based on undrained strength, it demonstrates the importance and reliability of the determination of undrained strength parameters. In view of the vertical loading conditions of offshore large-diameter steel pipe piles, reasonable suggestions for design methods and parameters determination are given.
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