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Existing projects indicate that a large number of pile boreholes formed via cement fly ash gravel (CFG) pile construction in soft soil could produce a borehole group effect in the absence of timely backfilling. The borehole group effect could cause ground settlement and threaten the safety of adjacent buildings, tunnels, and pipelines. However, the mechanism of the borehole group effect, causing serious deformation in surrounding soil, has seldom been studied, and there exists no simple simulation method to predict the borehole group effect. In this study, a case involving CFG pile boreholes in a soft soil area causing a significant settlement of surrounding buildings was introduced. Based on this case, a centrifuge test was designed to simulate the influence of a single borehole and multiple boreholes on the surrounding soft soil. The mechanism of the borehole group effect was investigated via centrifuge tests and the finite-element method. In the case of a single borehole, a horizontal circumferential stress arch and vertical stress transfer arch were formed in the soil, which effectively limited the shrinking deformation of the borehole wall. In the case of a large number of boreholes with a small spacing, the horizontal and vertical stress arches around the boreholes affected and weakened each other. As a result, the shrinking deformation of the borehole group was greater than that of the single borehole. In other words, the borehole group effect was found to cause serious deformation in the surrounding soil. To overcome the difficulty in the simulation of a large number of pile boreholes in engineering, the borehole-type conversion and multihole merging method was proposed. The simulation results indicated that the simplified method was reasonable. [ABSTRACT FROM AUTHOR]

Time-dependent settlements of piled raft foundations embedded in consolidating clays were studied using three-dimensional (3D) finite-element analysis and critical state-based soil constitutive modified Cam-Clay (MCC) model. In this study, different small to large rectangular rafts connected to circular bored cast-in-situ piles resting on four different types of clays and with varying overconsolidation ratios (OCRs) (1, 2, and 5) for each clay under a single drainage condition were analyzed. The influence of various piled raft parameters (pile length, pile spacing, pile diameter, raft thickness) and soil parameters (Poisson's ratio of soil, soil critical state parameters, stress history of soil or OCR) on immediate and consolidation settlements of piled rafts were investigated. The detailed parametric study shows that critical state parameters and soil elastic properties have important roles in piled raft immediate and consolidation settlements. However, the pattern of parametric influence on immediate settlement and consolidation settlement is different. To capture the total settlement at the end of primary consolidation, a simple and quick calculation method for estimating the average soil subgrade modulus below the piled raft in consolidating clays is proposed through regression analysis and defining a combined influence parameter based on soil parameters, area ratio, and normalized loading. The results of the proposed method were compared with five case studies of piled raft foundations in various normally or overconsolidated clays showing satisfactory matches. It is believed that the proposed method will be very useful for design practitioners to get a quick estimate for the overall settlement of piled raft at the initial design stage. [ABSTRACT FROM AUTHOR]

Traditional bearing capacity theories for the ultimate capacity of shallow foundations assume that the thickness of the bearing stratum is infinite. The presence of a hard layer within a certain depth below the foundation can significantly influence the unit load supported by the soil. Therefore the original bearing capacity equations should be modified to account for this condition in determining the ultimate bearing capacity. In order to evaluate this phenomenon further, model square and circular footing tests were performed on a bed of well-graded sand. Test beds were prepared at three different relative densities corresponding to loose, medium, and dense conditions: Dr=24, 57, and 87%, using five different sand layer thicknesses, H; H/B values of 0.5, 1, 2, 3, and 4, where B is the footing width. Results of the model scale footing tests show that the bearing capacity factor, Nγ, should be modified up to H/B=3, instead of H/B=1, as previously suggested. The footing shape factor, sγ, should account for both shape and finite layering. This technical note gives a description of the test methods and material used and presents the test results in comparison to previous results. [ABSTRACT FROM AUTHOR]

This paper presents the results of laboratory model tests on the influence of soil confinement on the behavior of a model footing resting on granular soil. Confining cylinders with different heights and diameters were used to confine the sand. The ultimate bearing load of a circular footing supported on a three-dimensional confined sand bed was studied. The studied parameters include the cell height, cell diameter, the depth to the top of the cell, and the embedded depth of footing. Initially, the response of a nonconfined case was determined and then compared with that of confined soil. The results were then analyzed to study the effect of each parameter. The results indicate that the bearing load capacity of circular footing can be appreciably increased by soil confinement. It was concluded that such reinforcement resists lateral displacement of soil underneath the footing leading to a significant improvement in the response of the footing. For small cell diameters, the cell–soil footing behaves as one unit (deep foundation), while this pattern of behavior was no longer observed with large cell diameters. The recommended cell heights, depths, and diameters that give the maximum bearing capacity improvement are presented and discussed. [ABSTRACT FROM AUTHOR]

The authors focus on research into deep building foundations and its behavior under axial and lateral loading. They mention that the prediction of load-transfer curves, both for side resistance and for end bearing, allow for the analysis of deep foundations under a range of conditions. They talk about soil properties around deep foundations after installation and how the pile capacity under axial load differs depending on if the pile was driven into consolidated clay or loose cohesionless soil. They state that research can lead to an improvement of design methods of deep foundations.