Pile Groups In Soft Clay Research Articles

Thermally induced mechanical interactions of energy pile groups subjected to cyclic nonsymmetrical thermal loading

Prior to this study, the long-term thermally-induced mechanical interactions of cyclic nonsymmetrical thermally loaded energy pile groups in soft clay have not been systematically elucidated, particularly with a limited understanding of the interactions among operating and non-operating energy piles, soil, and caps. Observing such a challenge, this paper presents a three-dimensional (3D) thermomechanical finite-element model (FEM), which considers the thermal-dependent behavior at the pile-soil interface and is validated using typical centrifuge tests reported in the literature. Special attention is given to the influence of soil-pile thermal expansion coefficient ratio, pile-soil stiffness ratio, and slab-soil stiffness ratio on the abovementioned interaction. The results indicate that a larger soil-pile thermal expansion coefficient ratio intensifies the interaction between energy piles and the surrounding soil, causing cyclic variations in thermally mobilized friction. With increasing pile-soil stiffness ratios, the differential thermomechanical responses among different piles within a group are enhanced because the toe and shaft resistances mobilized by the cap through nonoperating energy piles are weakened in the interaction process among energy piles, nonoperating energy piles, and the cap. Moreover, the increase in the slab-soil stiffness ratio exacerbates this interaction, strengthening the integrity of the pile groups.

Influence of pile head restraint on the performance of floating elevated energy pile groups in soft clay

Thermal elongation of energy piles (EPs) depends on the end restraints to displacement. The EP head is restrained by a working load and non-energy piles (NEPs) within an energy pile group, depending on the cap rigidity. The existence of both EPs and NEPs in non-symmetrical thermally loaded energy pile groups imposes different restraints on EP heads, resulting in different pile elongations. This would induce load redistribution between the piles, thereby influencing the serviceability of energy pile groups in soft soil. In this paper, a 3-D numerical parametric study was conducted after back-analysing a centrifuge test on a floating elevated energy pile group subjected to cyclic non-symmetrical thermal loading in soft clay. Effects of the number of EPs and NEPs in a 2 × 2 group, the flexural stiffness of the cap, and the factor of safety (FS; i.e., working load level) were studied. The non-symmetrical thermally loaded pile group with 3EPs and a rigid cap subjected to the lowest FS showed the maximum settlement due to the largest thermomechanical loading distributed between the piles through the rigid cap. In contrast, a flexible cap over the 2EPs group induced the maximum tilting due to a closer distance between EPs and NEPs in a side tilting. The axial load reduction was significant mainly in the 1EP group during cooling and at a higher FS, resulting in tension along the EP, which should be considered in the design.

Measured and Predicted Response of Pile Groups in Soft Clay Subjected to Cyclic Lateral Loading

Measured and Predicted Response of Pile Groups in Soft Clay Subjected to Cyclic Lateral Loading

Comparing the Response of Static Loading Tests on Two Model Pile Groups in Soft Clay

Static load test program was performed on a single pile and two 16-pile groups with equal and different pile lengths. The soil profile consists of sand fill to 0.5 m depth placed on a thick deposit of soft, normally consolidated and compressible clay. The closed end steel pipe piles in 60 mm diameter were installed from 1.5 m through 2.1 m depth within soft clay deposit. The center-to-center distance of piles in group is about 3 times of pile diameter. The strain gages were installed at one level above and two through four levels below ground surface. Tests were carried out about 7 days after driving by method of a series of load increments placed every 5-min until plunging failure occurred. The load at plunging failure for the single pile, the equal-length pile group and the different-length pile group were about 3, 40 and 48 kN, respectively. The movements at start of failure were about 12, 18, and 17 mm, respectively. The analysis of strain gage measurements indicates that the load distribution on piles in the different-length pile group has become significantly uniform.

Nonlinear seismic behavior of pile groups in cement-improved soft clay

Centrifuge tests were performed to investigate the effects of ground improvement on the seismic behavior of pile groups in soft clay. The soil profile consisted of four lightly overconsolidated clay layers overlying a dense layer of sand. The pile groups had a symmetrical layout consisting of 2×2 piles spaced at 3.0 pile diameters and were driven into both unimproved soft clay and soft clay improved by a simulated Cement Deep Soil Mixing (CDSM) method. The centrifuge model was subjected to seven different earthquake events with peak accelerations ranging from 0.03 to 0.66g. The foundation level motions of the improved pile groups were different than the surface free-field motion. The foundation level motion for the unimproved pile group was, however, identical to that in the free-field. Higher peak accelerations were observed in the pile cap of the group with smaller CDSM block (GIS) compared to the unimproved pile group (GU) and the group with the largest CDSM block (GIL). Higher pile cap to the soil surface spectral ratios were also obtained for the GIS group in both short and long periods. Cement-Deep-Soil-Mixing was effective in reducing the peak displacements of the GIL pile cap. The peak displacements of the GIS pile cap remained about the same as the GU pile cap. As the size of the ground improvement increased, the fundamental period of the pile groups reduced. The estimated fundamental periods of the GIS and GU pile groups were, however, close to each other. Acceleration and displacement response spectra of the foundation level motions in comparison to the fundamental periods of the pile groups provided insight into the observed acceleration and displacement responses. The adhesion between soft clay and CDSM blocks helped to reduce the soft clay settlement in the vicinity of CDSM blocks compared to the free-field and the vicinity of unimproved pile group. More residual excess pore water pressure was, however, generated in the vicinity of CDSM blocks compared to the free-field and the corresponding location in the unimproved pile group, likely due to vibrations of the CDSM blocks and the piles.

Influence of flood-induced scour on dynamic impedances of pile groups considering the stress history of undrained soft clay

Flood-induced scour causes loss of lateral support at bridge foundations and thus modifies their dynamic impedances, which will in turn influence the dynamic behavior and the seismic response of bridges located in seismically active flood-prone regions. When evaluating the seismic performance of scoured bridges, however, few efforts have been made to analyze the dynamic impedances of their foundations, and possible changes in stress history of the remaining soils are also ignored. In reality, the remaining soils around pile foundations undergo an unloading process due to scour and their over-consolidation ratios are increased, which can be expected to change the properties of the remaining soils. Therefore, a three-dimensional numerical model, in which the pile-soil-pile interaction and the radiation problem are accounted for by means of the elastodynamic Green's functions that permit the mutual interactions between all the piles of the group consistently and simultaneously, is employed in this paper to investigate the influence of flood-induced scour on the dynamic impedances of pile groups in soft clay. Accordingly, the key soil parameter of the elastodynamic Green's functions, i.e., Young's modulus of soft clay, is modified in this model to take into account the stress history effect due to scour. A case study to evaluate the dynamic impedances of 3×3 and 4×4 pile groups with various scour depths and pile spacing, considering or ignoring stress history effect, is discussed in this paper. The results show that ignoring the effect of stress history of the remaining soft clay will overestimate the scour affected dynamic impedances of pile groups, which are found to be reduced significantly as the scour depth increases.

Ultimate Soil Pressures for Pile Groups in Soft Clay Subjected to Lateral Soil Movements

Studies were carried out to investigate the ultimate soil pressure acting on pile(s) undergoing lateral soil movement in soft clay. A special apparatus has been designed to apply a uniform rectangu...

Response of pile groups subjected to lateral loads

AbstractAn approach is presented for the prediction of linear and nonlinear load–deformation behaviour of laterally loaded pile groups. The individual pile response is obtained by the conventional p–y curve technique, while group interaction is modelled using the Mindlin's solution. Good agreement is observed when comparing the present method of analysis with the commonly used interaction factor approach for the computation of response of pile groups embedded in a homogenous, isotropic elastic half‐space. The predicted pile group behaviour also compares favourably with existing field data on laterally loaded pile groups in soft clay.

Procedures for analysis of laterally loaded pile groups in soft clay : Bogard, D; Matlock, H Proc Conference on Geotechnical Practice in Offshore Engineering, Austin, Texas, 27–29 April 1983P499–536. Publ New York: ASCE, 1983

Procedures for analysis of laterally loaded pile groups in soft clay : Bogard, D; Matlock, H Proc Conference on Geotechnical Practice in Offshore Engineering, Austin, Texas, 27–29 April 1983P499–536. Publ New York: ASCE, 1983