Prediction of Punch-through for Spudcan Penetration in Sand Overlying Clay

Abstract

Installation of spudcans in sand overlying clay sites is often fraught with potential punch-through problems which can cause serious damage to the jack-up rigs. A reliable evaluation of the punch-through potential of the spudcan requires an accurate prediction of spudcan bearing capacity-depth profile at the site. This paper proposes a new design method, which improves on the methods recommended by SNAME (2002), to estimate the spudcan bearing capacity as it penetrates a soil profile with sand overlying clay by considering the actual failure mechanisms of the soil caused by a continuously penetrating spudcanThe proposed method generalizes the spudcan bearing capacity depth profile into three different phases namely, prepeak, peak and post-peak phases as the spudcan penetrates the sand layer into the underlying clay. The development of the bearing capacities in the first two phases is associated with the strength variance between the upper sand and lower clay soil layers as well as the ratio of sand layer thickness to spudcan diameter; whereas for the post-peak phase, the bearing capacity is significantly affected by the presence of a sand plug trapped underneath the penetrating spudcan. The solution for estimating the bearing capacity of each phase is provided.The performance of the proposed method is illustrated by back-analyzing experimental and field data. Comparisons with the SNAME (2002)'s recommended methods are also included. In general, the proposed method shows a significant improvement in predicting the spudcan bearing capacity-depth profile, which could not be achieved by the existing methods based on a wished-in-place shallow foundation assuming a single failure mechanism applied repeatedly at different penetration depths. By considering the presence of a trapped sand plug, the spudcan final penetration in the underlying clay is better predicted by the proposed method.The proposed method captures the essential characteristics of spudcan penetration behavior in sand overlying clay which, in turn, enables the potential jack-up rig punch-through failure and the corresponding spudcan rapid settlement to be better assessed.

Bearing capacity on sand overlying clay soils: a simplified conceptual model

“…Numerical predictions using the method of Teh (2007) Figure 13 indicates that the method of Teh (2007) gives a relatively good average prediction of the peak penetration resistance, although Fig. 13(b) and Table 3 indicate that the general trend of calculated to measured q peak ratios is skewed.…”

“…Centrifuge results additional to those used in the model calibration were included in this comparative study. These consisted of five model spudcan penetration tests carried out by the first author using the beam centrifuges at the University of Western Australia, two centrifuge model tests from Craig & Chua (1990), and 12 from Teh (2007). The experimental details of these additional tests, as interpreted by the authors, are provided in Table 2.…”

“…The accuracy of the q peak values calculated from the new method, as well as those of SNAME (2002), Teh (2007) and Okamura et al (1998), has been evaluated through retrospective simulation of centrifuge experiments with H s /D < 1 . 12.…”

“…By ignoring the properties of the upper sand layer (since even in the punching shear approach the frictional resistance through the sand is expressed in terms of the strength of the underlying clay), both methods provide oversimplified approaches whereby the ratio of q peak to the bearing capacity of the underlying clay increases as a simple quadratic function of the ratio of the upper sand thickness to the foundation diameter, H s /D. Two alternative prediction methods have been developed more recently by Okamura et al (1998) and Teh (2007). Okamura et al (1998) proposed a new limit equilibrium method with a failure mechanism that combines the concept of the projected area method and the punching shear method, as shown in Fig.…”

“…3; the peak penetration resistance is calculated as the sum of regions of full and reduced bearing capacity in the underlying clay and frictional resistance along a shear surface at angle ł to the vertical (noting that ł is not a dilation angle in this method, but a geometrical parameter used to represent the shear surface inclination). Teh (2007) provides semi-logarithmic design charts to calculate ł and two other model parameters.…”