Foundation punch-through in clay with sand: analytical modelling

Abstract

Severe punch-through of jack-up rig foundations can occur due to the presence of a stronger sand layer in a bed of relatively soft clay. Analytical estimation of the bearing capacity and leg load-penetration response on such multi-layer stratigraphies is challenging. Accurate mechanism-based models need to be established in each of the layers involved and the effects of the mechanisms in each of the layers on the response in the other layers must be captured. Based on the recently-developed failure stress-dependent punch-through models for sand-clay stratigraphies, an extended model is proposed for clay-sand-clay stratigraphies. Half-spudcan PIV (Particle Image Velocimetry) centrifuge tests and full-spudcan centrifuge tests are used in developing and validating the extended model. The centrifuge test results were discussed in the companion paper (Ullah et al., 2016) and this paper focuses on the analytical developments and prediction assessment. Both spudcan peak resistance (qpeak) and spudcan punch-through depth (dpunch) can be estimated using the model. The predictions by the extended model and by the current industry guidelines (ISO, 2012) are compared against the centrifuge test data. The extended model proposed in this paper outperforms the approaches suggested in the guidelines. An advantage of the proposed approach is that it can be used for either sand-clay or clay-sand-clay scenarios and exhibits excellent performance compared to the model testing dataset considered in this work for both cases. The resulting penetration resistance model is a useful design tool for routine punch-through risk assessment.

Optimization for the Assessment of Spudcan Peak Resistance in Clay–Sand–Clay Deposits

“…The predictive model proposed by Ullah et al [4] has been validated against the centrifuge tests reported by Ullah et al [19] and Hossain [1]. Based on the geometries and soil parameters summarized in Table 1, the peak resistances can mostly be predicted with an error bounded by ±20%, as shown in Figure 2 (solid grey circles) in terms of the ratio r between predicted and measured peak resistances.…”

“…The typical penetration resistance profile of spudcan in such deposits is shown in Figure 1, which is presented as the total penetration resistance q versus the penetration depth d measured from the seabed surface to the lowest point of the largest cross-section of spudcan. As the spudcan penetrates in the top clay layer, either a single-layer response (i.e., the penetration response in single-layer clay, without being affected by the underlying layers) or a squeezing response occurs depending on the distance between the spudcan base and the sand layer [1,4,5]. Once the squeezing response is triggered at the depth of d sq , the penetration resistance will increase sharply until reaching the peak value of q peak in the interbedded sand layer at the depth of d peak .…”

“…Therefore, accurate prediction of the peak resistance is a vital component for spudcan penetration in clay-sandclay deposits. Recently, deterministic design methods have been proposed by Ullah et al [4] and Zheng et al [5] based on centrifuge model tests and large-deformation finite-element (LDFE) analyses. The peak resistance of spudcan in the interbedded sand layer is calculated by modifying the failure-stress-dependent predictive model established by Lee et al [6] and Hu et al [7] for a spudcan on sand-over-clay deposits.…”

“…In this paper, a deterministic predictive model for the calculation of spudcan peak resistance in clay-sand-clay deposits is presented first, which is modified from that proposed by Ullah et al [4] to rectify the underestimated peak resistance depth. Accordingly, design formulas are proposed to account for the effects of the bearing capacity depth factor and the protruded soil plug in the bottom clay layer in the calculation of peak resistance.…”

Peak punch-through capacity of spudcan in sand with interbedded clay: numerical and analytical modelling

“…Recently, some studies were reported in this area through physical modelling in a centrifuge (Hossain 2014;Ullah et al 2016a). A comprehensive analytical model was presented in Ullah et al (2016b) for spudcan installation in a clay bed with interbedded sand (i.e. clay-sand-clay) where a method of predicting the complete load-penetration profile was proposed.…”