Hans Petter Jostad

Finite Element Analyses Applied in Design of Foundations and Anchors for Offshore Structures

Offshore structures for oil and gas exploitation are designed to accommodate severe environments with large cyclic loads. These structures are either founded directly on the seabed, or they are moored to anchors installed in the seabed soil. The permanent and cyclic loading, the foundation or anchor geometry, and the nonlinear soil behavior may be very complex, and many interrelated aspects must be considered in the geotechnical design of the foundations. Finite-element analyses (FEAs) are used increasingly to deal with these complexities and offer the potential to increase accuracy, efficiency, and reliability and reduce the uncertainty of the design process. This paper presents the major geotechnical aspects in the design of foundations for offshore structures and examples from finite-element analyses carried out at the Norwegian Geotechnical Institute (NGI) to deal with these aspects. A brief review of the procedure used at NGI to obtain soil stress-strain-strength relationships from cyclic laboratory ...

Effect of Skirt-Tip Geometry on Set-Up Outside Suction Anchors in Soft Clay

An important part of suction anchor design is the determination of the shear strength along the outside skirt wall. Previous work has suggested that when a suction anchor in clay is installed by applying underpressure inside the anchor, the external skin friction may be reduced compared to the skin friction expected for driven piles. The primary reason for this reduction is that the movement of soil at and beneath the caisson tip during installation will be influenced by whether the anchor is penetrated by weight or by underpressure. To further investigate the impact of installation by underpressure, additional finite element analyses have been performed where the skirt installation process has been better followed than in the previous analyses. The movement of soil around the caisson wall was studied for both a flat caisson tip and a tip with a tapered edge of 45° towards the outside of the anchor. The tapering was made to see if it would cause more of the displaced soil to move outside the anchor and thereby increase the mean total stresses and the shear strength along the outside anchor wall. The analyses were made with two separate wall roughness factors for a typical anchor in soft clay.Copyright

Effect of Strain-Softening in Design of Fills on Gently Inclined Areas with Soft Sensitive Clays

The effect of strain softening in geotechnical design of fills in areas with soft sensitive clays is studied by a large number of finite element analyses. The reduction in undrained shear strength with increasing shear strain after the peak value will reduce the maximum fill height before failure compared with a perfectly plastic material. The finite element program Plaxis together with the material model NGI-ADPSoft are used in this study. A non-local strain formulation is used in NGI-ADPSoft to overcome the crucial problem of mesh dependent results typical for this type of problems. The effect of brittleness is then fully controlled by input parameters. The material properties are taken from NGI’s database of undrained shear test results on high quality block samples. The effect of strain-softening is quantified by establishing a scaling factor Fsoftening that gives the ratio between the calculated capacity without and with the effect of softening. The purpose is then that the peak undrained shear strength of sensitive clays simply can be divided by this factor before used in conventional limit equilibrium analyses with a strain independent (perfectly plastic) assumption to indirectly account for the effect of brittleness.

How Well Do We Understand the Undrained Strain Softening Response in Soft Sensitive Clays

Many geomaterials exhibit a reduction in strength after attaining their peak strength states. This behavior, also known as strain softening, has commonly been attributed to reductions in the cohesion and/or friction angle of the materials. This holds true especially when these strength parameters are evaluated at a very large strain levels. However, for the strain levels achieved by standard undrained triaxial tests on soft sensitive clays show that the friction angle and cohesion of these materials remain almost unchanged, even when they display a significant reduction in post-peak shear strength under undrained condition. In such conditions, the increase in shear-induced pore pressure is observed to be responsible for the observed undrained softening in soft sensitive clays. This paper elaborates on this aspect based on the behavior of soft sensitive Norwegian clays.

Tailings Dam Stability

Open image in new window Tailings are waste materials from mining operations, which need to be disposed of and safely stored. They are commonly transported as slurry in pipes to the storage facility and surface impoundments through spigots. Different types of tailings dam construction and disposal method include tailings dams designed as water retention dams, and dams built using the upstream method, downstream method or centreline method. Several examples of recent tailings dam failures involved dams constructed by the upstream method, where the new embankments are founded on tailings, causing the dam to become progressively more dangerous as its height increases. From back-calculation of historical failures, two distinct failure mechanisms seem to be dominant. The first mechanism is related to the development of progressive failure in a weak soil layer in the dam foundation. The second dominant failure mechanism is related to static or dynamic liquefaction of loose tailings material at a critical state. Static or dynamic liquefaction of loose tailings may occur at a critical condition, where a rapid (undrained) small increase in the shear strain results in a large increase in pore pressure, reduced effective stresses and a dramatic reduction of shear strength. Typical for these types of failures is that they occur rapidly with no warning. There is often no sign of increased displacement rates or pore pressure increase in the days prior to dam failure. Failure is often initiated by a local instability at a critical location, where redistribution of stresses due to reduced shear strength upon further deformations rapidly develops into a global failure mechanism without any additional load actions.

Correction Factors for Undrained LE Analyses of Sensitive Clays

Correction factors to be used in conventional undrained stability calculations in order to account for post peak strain softening behaviour of sensitive clays, has been recommended based on an extensive sensitivity study with advanced finite element simulations. It is found that a correction of the material factor is preferred compared to a reduction of the shear strength. The input parameters to the sensitivity study that had the highest correlation with the required correction factor were the shear strength increase with depth and the brittleness, which is the rate of shear strength reduction with strain. In a large block sample database of sensitive Norwegian clays, there was no clear correlation between the brittleness and the sensitivity. Hence, classification of the clays based on the sensitivity is not recommended for evaluating the effect of strain softening on the capacity.

Failure Mechanism of Spreads in Sensitive Clays

Through detailed case studies from the literature it is suggested that a sensitive clay spread is formed by propagation of a failure surface in an intact slope and dislocation of the soil mass in horsts and grabens. It is proposed that the initiation and propagation of the failure surface can be explained by progressive failure mechanism. According to this failure mechanism, failure is initiated near the toe of the slope and the strain-softening stress-strain behaviour of sensitive clays is used to redistribute shear stress along the quasi-horizontal shear zone. The failure propagates inside the deposit reducing the horizontal stress. Active strength of the soil may be mobilised, explaining the dislocation of the soil mass above the shear zone in horsts and grabens. A numerical procedure is used to back calculate the 1994 spread at Sainte-Monique, Quebec, Canada, involving slightly over-consolidated sensitive clay. The initiation and extent of the failure surface observed on site are explained by a soil having large brittleness during shear and large-deformation shear strength close to the remoulded shear strength of the soil.

Risk Assessment for Quick Clay Slides – The Norwegian Practice

The approach for the assessment of the risk associated with quick clay slides in Norway is a qualitative/semi-quantitative procedure developed as part of work for The Norwegian Water Resources and Energy Directorate. Slide areas are classified according to “engineering scores” based on an evaluation of the topography, geology and local conditions (to qualify hazard) and an evaluation of the elements at risk, persons, properties and infrastructure exposed (to qualify consequence). The risk score to classify the mapped areas into risk zones is obtained from the relationship R S = H WS × C WS , where R S is the risk score, H WS is the weighted hazard score and C WS is the weighted consequence score. The risk matrix is divided in five risk classes. Guidelines for the implementation of the risk matrix are administered by NVE. In practice, the approach is used to make decisions on required mitigation measures to reduce the risk. The approach is simple and makes room for engineering experience and judgment. For detailed regional planning, slope stability calculations need to be made. Methods for quick clay slide stability calculations taking into account the brittle behaviour of the material are under development. This chapter provides an illustration of this development work.

Experience of Parameter Optimization of the High-Cycle Accumulation Model for Undrained Triaxial Tests on Sand

This paper discusses the performance of the high-cycle accumulation model (HCA) for describing the soil response in undrained cyclic triaxial tests of sand, evaluated against a comprehensive experimental database. The parameters for the HCA model calibrated for drained triaxial tests is examined for its applicability in undrained conditions. A modified approach for parameter optimization for undrained conditions is proposed and demonstrated to provide good match to the triaxial test results.

Effects of Sample Disturbance in the Determination of Soil Parameters for Advanced Finite Element Modelling of Sensitive Clays

The stress-strain response of sensitive clays tested in a laboratory setting can be significantly affected by disturbance effects caused by sampling, transport, storage and specimen preparation. Soil models for finite element analyses are commonly calibrated using the results from laboratory tests and, consequently, calibrated model parameters are likely to be affected by sample disturbance. For sensitive clays subjected to constant volume shearing, the stress-strain behavior is dependent on the direction of loading and, due to build-up of shear induced pore pressure, effective stresses will reduce with increasing strain in the post-peak regime. According to previous studies, peak strengths, strains at failure and post-peak behavior of sensitive clays are all significantly influenced by sample quality. Therefore, the relative quality of model predictions generated using a sensitive clay finite element model can also be expected to be notably affected by sample disturbance. In this study, the impact of sample disturbance on the determination of model input parameters for advanced finite element modelling of sensitive clays is addressed and critically discussed. Two advanced soil models are used for this purpose: the total stress based NGI-ADPSoft model, which is able to predict the anisotropic strain-softening behavior of saturated sensitive clays, and the effective stress based S-CLAY1S model, which is characterized by an anisotropic yield surface and is able to simulate soil destructuration. The practical implications of a thoughtful selection of the input parameters are evaluated through FE stability analyses of a sensitive clay slope.