Punch-through Failure Research Articles

Numerical investigation of punch-through mitigation in stiff-over-soft clays using skirted spudcan

Punch-through failure, a critical issue in offshore engineering, occurs when a foundation penetrates through strong-over-weak soil layers. This paper presents a study on the effectiveness of skirt addition to spudcans in addressing punch-through failure through numerical simulations using the axisymmetric Particle Finite Element Method (PFEM). The validity of the PFEM is verified by simulating a spudcan penetration centrifuge test. Subsequently, PFEM simulations are conducted to analyse the penetration behaviour of both generic and skirted spudcans in stiff-over-soft clays. Results from the simulations reveal the bearing responses of spudcans as they penetrate into clays, elucidate associated failure mechanisms, and underscore the significant of considering the spudcan-driving process in estimating punch-through failure mitigation. Findings indicate that, the skirt effectively mitigates punch-through failure by increasing plug height in the lower soft layer, thus reducing the rate of bearing capacity reduction. Furthermore, critical factors such as skirt length, upper layer thickness, lower layer strength gradient, and the strength ratio between lower and upper layers are assessed through comprehensive parametric studies to evaluate the efficacy of skirted spudcans in mitigating punch-through failure, with eventual goal to provide essential guidance for practical design considerations in offshore foundation systems.

Penetration Behavior of Jack-up Leg with Spudcan for Offshore Wind Turbine to Multi-layered Soils Using Centrifuge Tests

This study examined the jack-up spudcan penetration for a new type of offshore wind substructure newly proposed using the jack-up concept to reduce construction costs. The jack-up spudcan for offshore wind turbines should be designed to penetrate a stable soil layer capable of supporting operational loads. This study evaluated multi-layered soil conditions using centrifuge tests: loose sand over clay and loose sand–clay–dense sand. The penetration resistance profiles of spudcan recorded at the centrifuge tests were compared with the ISO and InSafeJIP methods. In the tests, a spudcan punch-through effect slightly emerged under the sand-over-clay condition, and a spudcan squeezing effect occurred in the clay-over-sand layer. On the other hand, these two effects were not critically predicted using the ISO method, and the InSafeJIP result predicted only punch-through failure. Nevertheless, ISO and InSafeJIP methods were well-matched under the conditions of the clay layer beneath the sand and the penetration resistance profiles at the clay layer of centrifuge tests. Therefore, the ISO and InSafeJIP methods well predict the punch-through effect at the clay layer but have limitations for penetration resistance predictions at shallow depths and strong stratum soil below a weak layer.

Case study of spudcan punch-through in layered soil with interbedded thin stiff clay

Punch-through failure may occur when the spudcan is installed in layered soil, which can result in a detrimental effect to the jack-up unit and jeopardize personnel. This paper introduced a practical case of spudcan punch-through which occurred in a kind of multi-layer soil which has a thin stiff layer and a transition layer locating between soft layers. The existence of the transition layer blurs the difference of soil strength. To explore the failure mechanism set behind the field case, LDFE analyses were conducted with the Coupled Eulerian-Lagrangian (CEL) method. The results indicated that the failure of soil changes between three failure mechanisms of general shear, punch-through and squeezing with the penetration of spudcan. The transition point of failure mechanism can be identified through the depth of failure surface based on the LDFE results. According to the failure mechanism, a modified approach was set out in estimating the spudcan penetration resistance in the multi-layer soil, which matches the field data well on the basis of the soil strength obtained by laboratory element tests.

Spudcan Deep Penetration in Multi-Layered Soils Incorporating Sand Relative Density

Installing a spudcan jack-up rig in sediments with an interbedded sand-over-clay soil profile is still challenging in the offshore industry due to possible punch-through failure. The current methods for predicting the punch-through of spudcan usually ignore sand relative density, which is one of the most important parameters for sand soil. For multi-layered soils with interbedded sand commonly met in the field, this paper aims to determine the effects of sand relative density on predicting the punch-through failure of spudcan. Modified Mohr-Coulomb (MMC) and extended Tresca models characterized by incorporating a variation of mobilized strength parameters were used to describe the mechanical behaviors of sand and clay. Besides four groups of centrifuge tests, one field case in the South China Sea was also numerically simulated to validate the large deformation finite element (LDFE) analyses conducted in this paper. The results showed that neglect of sand relative density may lead to underestimation of the potential for punch-through failure of spudcan.

On Cubic Spudcan Deep Penetration in Dense Sand Overlying Non-Uniform Clay

Installing spudcans for jack-up rigs in a layered seabed with an interbedded strong-over-soft layer is challenging in the offshore industry due to possible punch-through failure. The methods currently used to predict punch-through failure mainly focus on generic spudcans, usually ignoring the geometric features of the spudcan. The aim of this study is to determine whether the punch-through potential of a generic spudcan is applicable to a cubic spudcan of a BH12# jack-up rig installed in dense sand overlying non-uniform clay. We conducted a series of large deformation finite element (LDFE) analyses on dense sand overlying non-uniform clay using a generic spudcan and a cubic spudcan. The thickness of the sand layer was also varied to cover a range of practical interest. The special cubic spudcan was found to be less likely to induce punch-through failure on sand-over-clay sediments compared with the corresponding generic spudcan. Herein, we propose a method to predict the degree of post-peak bearing reduction, which is a key measure of the severity of punch-through failure.

Spudcan–pile interaction in sand-over-clay: centrifuge modelling

Large soil movements arising from the installation of jack-up spudcan foundations may induce detrimental effects on piles supporting adjacent permanent jacket platforms. Particularly, in a dense sand-over-clay profile, there is a risk of spudcan rapid penetration due to underlying clay bearing capacity failure, known as spudcan punch-through failure, which can cause further damage to piles in the proximity. This study investigates the foundation interaction between the spudcan and adjacent pile in a sand-overlying-clay soil profile by way of centrifuge modelling. Issues investigated include thickness of the upper sand layer with incidence of sand plug beneath the spudcan and spudcan–pile clearance. The experimental findings reveal that spudcan punch-through hazard and the magnitude of induced pile moments amplifies primarily due to the increase in the thickness of the upper sand. In addition, the magnitude of induced pile moment in a sand-over-clay profile is significantly higher than that in a single clay profile because of the post-peak bearing behaviour observed in stratified soil, yielding to a greater risk to pile integrity. Soil pressure profiles on the piles are also back-analysed. Two generalised induced soil pressure profiles are deduced, with the first profile representing the onset of spudcan punch-through and the second profile denoting the situation of post-punch-through failure.

Quantifying installation risk during spudcan penetration

Installation of a mobile jack-up rigs with spudcan foundations are sometimes challenge due to uncertain offshore environments. Prediction of penetration resistance during installation is often necessary to avoid hazards such as punch-through failure. When the field monitored data deviates with these predictions, engineers have to decide how severe this deviation is based on their experience. This paper provides a method that can quantify the installation risk by a set of percentile curves of penetration resistance against depth. The percentile curves can be quantified through combining a random large deformation finite element method with Monte-Carlo simulation. The spatial variability in seabed soils can be considered and the penetration resistance curves through the random soils can be obtained. This model was verified by a spudcan installation penetration into deeper seabed. Finally, four field cases in North-East Brazil and Gulf of Mexico are investigated. Results show that the installation risk during spudcan installation can be estimated by comparing the measured preloading and penetration depth values with the percentile curves, which indicates the proposed method is capable to help decision making during spudcan installation.

Mitigating punch-through on sand-over-clay using skirted foundations

This paper investigates the potential for using skirted foundations as an alternative to spudcans in an attempt to mitigate punch-through failure of jack-up rigs during installation and preloading in sand-over-clay deposits. The results from a series of numerical analyses on the skirt length were presented in combination with different conditions of sand-over-clay deposit. Three-dimensional large deformation finite element (LDFE) analyses were carried out using the coupled Eulerian-Lagrangian approach. The sand was represented with a modified Mohr-Coulomb model and the clay with an extended Tresca model. The results showed that the likelihood and severity of punch-through increased with (i) increasing the thickness of the sand layer, (ii) increasing the strength of the clay layer, and (iii) reducing the strength non-homogeneity of the clay layer. The degree of mitigation was increased with increasing the skirt length. An optimal skirt length of 0.25D was suggested accounting for the rig towing constraint. Prediction models have been proposed for assessing (i) the potential for punch-through failure, and (ii) the resistance profile for skirted foundations penetrating sand-over-clay deposits.

Evaluation of Stress-Dependent Methods for the Punch-Through Capacity of Foundations in Clay with Sand

AbstractThis paper compiled 159 centrifuge tests for foundations in clay with interbedded sand where punch-through failure was observed. The accuracy of initial- and failure-stress-dependent method...

A finite element approach for predicting the full resistance profile of a spudcan deeply penetrating in dense sand overlying clay

This paper presents a finite element approach to calculate the full resistance profile of a spudcan deeply penetrating in dense sand overlying clay, in which a potential for an installing spudcan to experience a sudden uncontrolled punch-through failure exists. A modified Mohr-Coulomb model characterized by incorporating a four-phase variation of the mobilized strength and dilation parameters with an equivalent accumulated plastic strain is developed and tested for the overlying dense silica sand. An extended Tresca model is used for the strain softening of the underlying clay. A series of large deformation finite-element (LDFE) analyses are carried out, varying the strength and dilation parameters as well as the spudcan geometries. A fairly good performance of the present approach is verified by validating against groups of centrifuge tests data, allowing the numerical study to be extended parametrically. The four-phase variation of the mobilized strength and dilation parameters involved in the progressive failure of the upper dense sand is parametrically studied and extended to cover the range of sand relative densities that are of practical interest. Additionally, comparisons with the typical existing LDFE analyses using both simple and sophisticated constitutive models are carried out. It shows that the present approach performs fairly well to calculate the full resistance profile of a spudcan deeply penetration in both thin and thick dense sand overlying clay, especially the peak and post-peak resistance, within around 5% of the corresponding centrifuge tests results.

Effect of sleeves and skirts on mitigating spudcan punch-through in sand overlying normally consolidated clay

Punch-through failure of mobile jack-up rigs is still a main concern for spudcan foundations. Using centrifuge model tests and numerical modelling, this paper examines the mitigating effect of top-mounted sleeves and a downward skirt on spudcan punch-through in medium dense sand overlying clay. The findings show that the top-mounted sleeves and downward skirt are both useful separately for reducing punch-through distance. The top-mounted sleeve functions by interfering with soil backfilling above the spudcan and maintaining confinement to the sides. This results in an earlier recovery in penetration resistance and reduces the peak-to-trough drop in resistance. The downward skirt appears to mobilise the soil further ahead of the spudcan, which reduces the peak penetration resistance and increases the rate of post-trough recovery in penetration resistance. A combination of a top-mounted sleeve with a downward skirt appears to confer a smaller peak-to-trough drop in resistance and higher rate of post-peak recovery, and thereby a much smaller punch-through distance. The results would suggest that, compared with the downward skirt, the top-mounted square sleeve with a similar area ratio to the existing lattice leg is supposed to be a more effective option in mitigating spudcan punch-through failure.

Bayesian prediction of peak resistance of a spudcan penetrating sand-over-clay

Assessing the potential for a punch-through failure during spudcan installation in sand-over-clay is crucial for reducing risk in the operations of mobile jack-up platforms. Typically, in the offshore industry, the peak penetration resistance and the depth at which it occurs are determined deterministically without rigorously considering the uncertainties in the soil. This paper proposes a probabilistic approach to estimate the peak resistance and the corresponding depth, as well as a Bayesian method of incorporating installation data to update the predictions. Instead of a single value in the deterministic analysis, a range of the potential peak resistances and depths can be estimated by accounting for the uncertainties in the soil, the spudcan's geometry and in the calculation method itself, with a database of 66 geotechnical centrifuge tests characterising the model. This prior probability is then updated using the monitored data, allowing a real-time update of the probabilities associated with candidate values of peak resistance and depth during the installation. The advantage of such a probabilistic updating model is shown in a retrospective simulation of a mobile jack-up platform in sand-over-clay conditions in the Gulf of Mexico. The results show that the prior estimation can be effectively refined by incorporating the monitored data. The proposed method provides a powerful tool for assisting decision-making during the installation of jack-ups offshore.

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

The presence of a thin soft clay layer inside a bed of sand may significantly reduce the bearing capacity of the sand layer, imposing a risk of punch-through failure. In this paper, finite element (FE) simulations are reported using a hardening soil (HS) model for sand. The FE model has been verified against centrifuge tests involving loose and dense sand layers overlying clay soil. The effects of sand stiffness, foundation roughness, sand friction angle, undrained clay strength, clay strength nonhomogeneity, and sand and clay layer geometries on the foundation peak capacities have been studied. Punch-through failure is initiated with an inclined sand plug being sheared and pushed into the underlying soft clay. During punch-through, the clay layer fails due to significant radial squeezing. Existing analytical models do not capture the combined failure mechanism of sand shearing and clay radial squeezing. A new analytical model is developed to estimate the peak punch-through capacity of a spudcan in sand with an interbedded clay layer, showing improved performance over the current industry guidelines.

Foundation punch-through in clay with sand: centrifuge modelling

This paper is concerned with the vertical penetration resistance of conical spudcan and flat footings in layered soils. Centrifuge tests are reported for a clay bed with strength increasing with depth interbedded with dense and medium dense sand. Both non-visualising (full-model) and visualising (half-model) tests were conducted with high-quality digital images captured and analysed using the particle image velocimetry technique for the latter. The load–displacement curves often show a reduction in resistance on passing through the sand layers, which creates a risk of punch-through failure for the foundations when supporting a jack-up drilling unit. For a given foundation, the peak punch-through capacity (qpeak) is dependent on the thickness of both the overlying clay and the sand layer. The failure mechanism associated with the peak resistance in the sand layer involves entrapment of a thin band of top clay above the sand layer that subsequently shears along an inclined failure surface before being pushed into the underlying clay. The top clay height when normalised by the foundation diameter affects the soil failure pattern in this layer and along with the sand layer thickness controls the severity of the punch-through failure (i.e. the additional penetration before the resistance returns to the peak value). Comparisons are made with current industry guidelines for predicting qpeak and the risk of punch-through failure for sand overlying clay. These methods are shown to be conservative in their prediction of qpeak but inconsistent in predicting punch-through.

On spudcan deep penetration using Eulerian finite element in multi-layer sediments: soil plug mechanism

ABSTRACTIn stratified deposits, where an interbedded strong layer overlays a weak layer, the punch-through failure may occur and a soil plug from the overlying stronger layer usually exists beneath the advancing spudcan. Due to the limited understanding of the soil plug mechanism which really exists in both the strong-over-soft and soft-over-strong strata, no consideration of the soil plug effects is included in the present methods recommended in the current industry guidelines. This paper reports numerical simulations of spudcan deep penetration in multi-layer sediments with an interbedded sand layer, and aims at providing a better understanding of the soil plug mechanism. An extended Tresca model incorporating the clay strain softening together with an extended Mohr–Coulomb model with incorporation of the progressive failure of sand is used in this study. By comparing with the existing large deformation finite-element (FE) results and the centrifuge tests results, the numerical analyses conducted in this study are verified and discussed. Based on the soil plug mechanism, an improved approach, in which the sand plug mechanism is incorporated in terms of the three geometric parameters, h1, h2, and d1, is further presented for prediction of the spudcan penetration resistance.

Effect of footing shape on penetration in sand overlying clay

This paper reports on a series of centrifuge model tests investigating the effect of shape on the penetration resistance of spudcan and conical footings on sand overlying clay. The effect of footing shape and geometry on single-layer soil has been studied intensely. However, there is still limited understanding for conical footings on sand over clay. In the present study, digital images were captured during penetration of various shapes of half-footing held against a transparent window of a strongbox. The images were then analysed using particle image velocimetry techniques. Experimental evidence has shown that, irrespective of the conical angle of the underside within the range of 7–21°, when the footing penetrates through sand into an underlying clay layer: (a) accumulated radial and deviatoric shear strains along the future failure surface counteract each other, resulting in similar peak resistance in the sand layer and (b) a trapped sand plug of constant height is pushed into the underlying clay layer. These observations serve to justify the previously proposed methods for predicting the full penetration resistance profile on sand overlying clay, which is required to predict the potential for, and severity of, punch-through failure.

Simplified numerical prediction of the penetration resistance profile of spudcan foundation on sediments with interbedded medium-loose sand layer

In strong-over-soft strata, there is a potential for an installing spudcan to experience a punch-through failure, where the spudcan would penetrate a certain distance at a short time after the peak penetration resistance. Such a sudden punch-through failure would seriously threaten safety of the offshore structures, even causing casualties. This paper reports results from numerical modeling of spudcan penetration in sediments with interbedded medium-loose sand overlying a clay layer. Three typical stratifications commonly encountered in the field are considered: (1) sand-uniform clay, (2) sand-nonuniform clay and (3) uniform soft clay-sand-uniform soft clay-uniform stiff clay. Further, a field case encountered in the South China Sea was provided and analyzed. The analyses are carried out using a large deformation finite-element (LDFE) approach of the Coupled Eulerian–Lagrangian (CEL) in ABAQUS/Explicit. An extended Tresca model with incorporation of strain softening through two additional parameters of δrem and ξ95 following Einav and Randolph [1] is used for the clay, whereas the Mohr–Coulomb model is adopted for the sand. In particular, a simplified method, in which no additional parameters are required as input data, is presented to predict the punch-through resistance profile of a spudcan foundation on sediments with interbedded medium-loose sand layer. By comparison with the previously published LDFE results, recent centrifuge tests data as well as measures from one field case in the South China Sea, the numerical analyses conducted in this study are verified and discussed, and better performance of the simplified method is shown for prediction of the penetration resistance profile of a spudcan foundation on sediments with interbedded medium-loose sand layer.

Assessing the punch-through hazard of a spudcan on sand overlying clay

A complete analytical method to describe the full load-penetration resistance profile of a mobile jack-up spudcan footing penetrating a sand over clay stratigraphy is described. It is based on both large deformation finite-element analyses and geotechnical centrifuge experiments. The coupled Eulerian–Lagrangian (CEL) approach is used to accommodate the large deformations of a spudcan footing penetrating sand overlying clay. Modified Mohr–Coulomb and Tresca models describe the sand and clay behaviour, with modifications accounting for the effects of strain softening on the response of the soil. The CEL results are shown to match centrifuge tests well, allowing the numerical study to be extended parametrically, and with confidence, to cover the range of layer geometries, sand relative densities and footing shapes that are of practical interest to offshore jack-ups. The results are used to (a) assess the performance of an existing model to predict the peak resistance in the sand layer (extending its range of application to medium dense to dense sands and to conical footings of angle 0° to 21°), and (b) develop an expression for the bearing capacity factor when the footing penetrates into the underlying clay. Using the analytical formulas proposed, retrospective simulations of centrifuge tests show that the method provides a reasonable estimate of the peak punch-through load, the behaviour in the underlying clay, as well as the punch-through distance; the latter being a basic reflection of the severity of a potential punch-through failure.

A comparison of full profile prediction methods for a spudcan penetrating sand overlying clay

Spudcans are the traditional footings used for offshore mobile jack-up rigs. However, the installation of spudcans in sand overlying clay may lead to punch-through failure, which can cause serious damage to the jack-up rig and endanger personnel. This article compares three new methods proposed in the literature and an interpretation of the International Organization for Standardization (ISO) guideline for predicting the full penetration resistance profile. The penetration resistance profile for each of the methods is characterised by two key calculations: the peak resistance in the sand and the bearing capacity within the underlying clay. The punch-through distance – an indicator of the potential for and severity of punch-through failure – is estimated from these calculations. In comparison with 71 geotechnical centrifuge tests, the ISO guideline provides poor predictions, consistently underestimating the peak resistance in the sand and the underlying bearing capacity in the clay. Although all three of the new methods provide a superior response, by assessing the accuracy, scatter and geometric skew of the predictions, two of the methods are shown to be biased in at least one of the key calculations used to define the penetration resistance profile, thus producing bias in the prediction of the punch-through distance. However, one method yields largely unbiased predictions.

Predicting the resistance profile of a spudcan penetrating sand overlying clay

Assessment of the risk of punch-through failure of spudcan foundations on sand overlying clay requires prediction of the full penetration resistance profile, from touchdown and through punch-through to equilibrium of the vertical resistance at depth in the underlying clay layer. This study uses the Coupled Eulerian–Lagrangian approach, a large deformation finite element analysis method, to model the complete penetration resistance profile of a spudcan on sand overlying clay. The sand is modeled using the Mohr–Coulomb model, while the clay is modeled using a modified Tresca model to account for strain softening. The numerical method is then used to simulate a series of spudcan penetration tests, performed in a geotechnical centrifuge, on medium dense sand overlying clay. The punch-through behavior observed in the experiments is replicated, and the penetration resistance profiles from numerical analyses are generally a reasonable match to the experimental measurements. The influences of the sand layer height to foundation diameter ratio, sand–clay interface shear strength, and strength gradient in clay on the penetration resistance profiles are explored in a complementary parametric study. The penetration resistance in the underlying clay layer is well predicted using a simple linear expression for the bearing capacity factor for the spudcan and underlying sand plug. This expression is combined with an existing failure stress dependent model for predicting peak resistance to form a simplified method for prediction of the full penetration resistance profile. This new method provides estimates of the vertical penetration that the spudcan will run during the punch-through event. It is validated against both medium dense and dense sand centrifuge tests.