Limiting cavity depth for spudcan foundations penetrating clay

Abstract

Centrifuge model tests and finite element (FE) analysis have been conducted to study the penetration of spudcan foundations in uniform clay with nominally constant strength with depth. In particular, the transition between shallow penetration, with soil heaving to the ground surface, and deep penetration, with a localised flow-round mechanism, has been investigated. This transition governs the onset of back-flow and hence the depth of soil lying on the installed spudcan, which in turn influences the bearing capacity and also the potential for suction to develop and hence the uplift capacity and moment resistance of the foundation. The maximum cavity depth above the spudcan prior to any back-flow is therefore a critical issue for spudcan assessment in clay. In the centrifuge model tests, a half-spudcan model penetrating against a transparent window has been used to visualise the soil flow mechanisms around the spudcan during penetration. The formation of a cavity above the spudcan is revealed by both centrifuge modelling and FE analysis. It is found that there are three distinct penetration mechanisms during spudcan installation: during initial penetration, an open cavity is formed with vertical walls; with further penetration, soil flows partially around the spudcan into the cavity; during deep penetration, the spudcan is fully embedded and the soil flow mechanism is entirely localised. Over the wide range of normalised soil strengths explored, the soil back-flow in the second stage was shown to be due to a flow failure that was triggered by the spudcan penetration and not by wall failure, that is, the collapse of the vertical sides of the soil cavity. This observation is supported by FE analysis. The cavity depth due to flow failure is much shallower than the criterion for wall failure that is incorporated in current design guidelines. Instead, a new design chart and expression is suggested with the normalised cavity depth expressed as a function of the soil shear strength, normalised by the effective unit weight of the soil and the spudcan diameter.

Deep-penetrating spudcan foundations on layered clays: numerical analysis

“…It can be seen that at shallow penetration ( Fig. 3(a)), the soil at the spudcan edge flows upwards to the surface and the soil beneath the central part of the spudcan is forced downwards to the lower layer, forming a cavity above the spudcan and deforming the layer interface, and with less surface heave compared to the case of a single layer clay (Hossain et al, 2005). The cavity remains completely open through the top layer and back-flow of soil around the spudcan into the cavity is initiated at a penetration depth of about d/ D ¼ 1 .…”

“…Details of the LDFE approach for analysing spudcan penetration have been given by Hossain et al (2005). Penetration of the spudcan is simulated by specifying incremental displacements.…”

“…Although spudcan base roughness was found to have little influence on the critical depth, the values for both cases are presented together. The relationship developed previously for uniform soil (s ut ¼ s ub ¼ s uH ) (Hossain et al, 2005) Table 1…”

“…Details of the analysis method are summarised briefly and the results are then validated through comparisons of the deformation fields and profiles of bearing resistance with results from the Small strain, smooth; Wang & Carter, 2002 Small strain, rough; Merifield & Nguyen, 2006 Small strain, rough; Edwards & Potts, 2004 Model tests, rough; Brown & Meyerhof, 1969 Model tests, rough; Meyerhof & Hanna, 1978 Model test, smooth; Desai & Reese, 1970 centrifuge model tests. An expression is developed to estimate the maximum depth of cavity that forms above the spudcan before back-flow of soil occurs, comparing the results with previous work in a single layer clay (Hossain et al, 2005). Profiles of bearing resistance for use in design are presented for a range of layer geometries, t/D, strength ratios, s ubs /s ut , normalised strengths, s ubs /ª9 b D, and for different rates of strength increase of the bottom layer, kD/s ubs (see Fig.…”

Investigation of the potential of bottom water jetting to ease spudcan extraction in soft clay

“…Immediate back-flow on the top of the spudcan was observed visually during testing, and it is expected that the deep failure mechanism, characterised by a symmetrical flow-around mechanism, occurred at an embedment ratio of about 0 . 7, as described by Hossain et al (2005). Test nomenclature: U, undrained; D, partially drained; nJ, no jetting; J, jetting; E, extraction.…”