Abstract
Geotechnical engineers often deal with layered foundation soils. In this case, the soil bearing capacity assessment using the conventional bearing capacity theory based on the upper layer properties introduces significant inaccuracies if the top layer thickness is comparable to the rigid footing width placed on the soil surface. Under undrained conditions the cohesion increases almost linearly with depth. A few theoretical studies have been proposed in the literature in order to incorporate the cohesion variation with depth in the computation of the ultimate bearing capacity of the strip and circular footings. Rigorous solutions to the problem of circular footings resting on layered clays with linear increase of cohesion do not appear to exist. In this paper, numerical computations using FLAC code are carried out to assess the vertical bearing capacity beneath rough rigid circular footing resting on two-layered clays of both homogeneous and linearly increasing shear strength profiles. The bearing capacity calculation results which depend on the top layer thickness, the two-layered clays strength ratio and the cohesion increase rates with depth are presented in both tables and graphs, and compared with previously published results available in the literature. The critical depth for circular footing is found significantly less than for strip footing. Doi: 10.28991/cej-2021-03091689 Full Text: PDF
Highlights
For a surface strip footing resting on a single layer of homogeneous clay under undrained conditions, practitioners generally use Terzaghi’s expression to compute ultimate footing loads
The purpose of the present investigation is to take advantage of the numerical computation, which does not require in advance the specification of the failure mechanism surface needed for both limit equilibrium and limit analysis methods, to study the effect of two-layered clays with constant and linear increase of cohesion with depth on the bearing capacity beneath rigid circular footing subjected to axial static load
Comparison with Existing Solutions Before computing the bearing capacity of circular footings on two-layered undrained clays with constant and linearly increasing shear strength, first the case of single-layer was computed and compared with existing solutions available in the literature (Table 1): (i) The lower bound solutions on the basis of the three dimensional finite e0l.0e0m0 ent limit analysis given by Salgado et al [20], (ii) The method of characteristics solution of Houlsby and Martin [17], (iii) The lower bound limit analysis in conjunction with finite elements and linear programming reported by Khatri and Kumar [13] and (iv) The upper bound solution of Kusakabe et al [21]
Summary
For a surface strip footing resting on a single layer of homogeneous clay under undrained conditions, practitioners generally use Terzaghi’s expression to compute ultimate footing loads. Chi and Lin [5] utilized limit analysis and FLAC numerical simulation to investigate the bearing capacity of a footing on single thick stratum or two-layered cohesive soils They concluded that the partial punch-through shear failure was between the general shear failure and the full punch-through shear failure, depending on the strength ratio and the normalized layer thickness, the features of this failure mode was not apparent from the variations of bearing factor figures. The purpose of the present investigation is to take advantage of the numerical computation, which does not require in advance the specification of the failure mechanism surface needed for both limit equilibrium and limit analysis methods, to study the effect of two-layered clays with constant and linear increase of cohesion with depth on the bearing capacity beneath rigid circular footing subjected to axial static load.
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