Modelling the behaviour of a disk of clay during pile driving using hypoplasticity

The bearing capacity of displacement piles in clay is usually seen to increase during the days following installation. This phenomenon is referred to as ‘set-up’ and occurs because during installation, the high strains imposed by the pile leave a mark on the state of the soil surrounding the pile (pore pressures, a.o.). Once installation has ended, the pore pressures generated during driving are free to dissipate, which allows the pile capacity to grow. This paper presents a numerical simulation of a plane strain soil disk adjacent to a pile which is being driven. Emphasis is placed on the simulation of the excess pore pressure during pile penetration and afterwards, during consolidation. The soil constitutive model used is hypoplasticity for clays coupled with intergranular strain, which allows capturing the soil small strain behaviour and dilatancy. The driving analysis is performed by using a dynamic integration scheme and the consolidation is coupled

Numerical investigation of the set-up around the shaft of a driven pile in clay

The context of this thesis is the installation of driven piles in clay. Pile driving is an installation method which consists of repeatedly striking the pile head with a mass until desired embedment is attained. Driving brings severe distortions to the soil which has to accommodate for the penetrating pile. At the end of installation, the soil is left in a distressed state, which progressively tends to equilibrium with time, leading to a change in pile capacity. This phenomenon referred to as pile set-up. The objective of this thesis was to implement a numerical model which could account for pile installation and subsequent set-up in clayey soils. Focus was placed on behaviour round the pile shaft. The particularity the developed model is that it accounts for the cycles of shaft-soil shearing occurring during driving. Supported by experimental evidence, the original model developed in this work outlines the following conclusions: • During installation, total stress at pile wall decreases with vertical distance from pile toe (the h/R effect) due to (a) stress relief away from the pile toe and (b) fatigue from the accumulation of driving blows; • After installation, the radial distribution of pore pressure presents a peak a few radii away from the pile shaft. This peak value increases with overconsolidation ratio (OCR) while the pore pressure at the pile wall decreases with OCR; • The shape of pore pressure distribution after installation leads to a short term minimum in radial effective stress (therefore in pile capacity) during set-up. Furthermore, the model yielded the following additional conclusions, which were outside of the scope of the available experimental data: • The soil effective stress response during installation and set-up is mainly governed by overconsolidation ratio, rather than soil strength or stiffness; • There is a critical hammer velocity for which the soil offers maximum adherence to the pile; • Although shaft capacity after set-up was comparable for the open-ended and the closed-ended piles, open-ended set-up time was four times shorter.(FSA - Sciences de l'ingénieur) -- UCL, 201

Numerical analysis of the set-up around the shaft of a closed-ended pile driven in clay

When a pile is driven into the ground, soil standing in the path of the pile is heavily distorted as it has to cede for the penetrating pile. At the end of installation, this soil is left in a distressed state, which progressively evolves to an equilibrium with time. As a result, for lightly overconsolidated clays, soil resistance generally decreases during driving but increases after installation; the latter phenomenon is referred to as set-up. Correct assessment of pile set-up is one of the most important considerations of displacement pile design, especially offshore. Indeed, underestimation of set-up leads to unnecessary expense and overestimation of set-up leads to a precarious overestimation of the pile capacity. This paper presents a numerical model, the aim of which is to predict set-up around the shaft of a driven pile in clay. The model was conceived after a careful literature review of the relevant field data, in order to support its assumptions. It is used to predict the stress and pore pressure distributions around the shaft of a driven pile during installation and subsequent equalisation. Numerical results are then compared to the literature results. Although the model underestimates the excess pore pressure created around the pile, it captures a key feature of displacement pile installation in clay: at the end of installation, the radial distribution of excess pore pressure presents a peak located a few radii away from the pile wall. This implies that, during equalisation, the radial effective stress at the pile wall decreases to a short-term minimum, leading to a short-term minimum in pile capacity, before eventually increasing

A Modified Case Method for Piles with Section Step Changes

Applied during pile driving, the Case Method offers an immediate estimate of the static resistance to driving (SRD) after each hammer blow. It has been used in its original form both in the onshore and offshore piling industry for more than 40 years to provide an indication of the pile static capacity. The Case Method requires measurements of force and velocity near the pile head as the hammer strikes the pile and produces an analytical estimate of the SRD, using a number of assumptions. One of them requires the pile to be of constant impedance (or cross section) along its length. However, for reason of economy, driven piles are often composed of several sections of different cross sections. The Case Method provides in that case an inaccurate estimate of the SRD. This article presents an improved version of the original Case Method which takes into account possible variations of impedance along the pile. A numerical validation shows that for piles displaying impedance changes, the modified Case Method presented herein provides an estimate closer to the actual SRD than the original Case Method. That conclusion is further validated by applying the modified Method to pile driving records and comparing its results to SRD estimates obtained through more reliable modelling

Influence of an impedance change on SRD computation by the Case Method

Applied during pile driving, the Case Method offers an immediate estimate of the soil resistance to driving (SRD) after each hammer blow. It has been used in its original form both in the onshore and offshore piling industry for more than 40 years to provide an indication of the pile static capacity. The Case Method requires measurements of force and velocity near the pile head as the hammer strikes the pile and produces an analytical estimate of the SRD, using a number of assumptions. One of them requires the pile to be of constant impedance (or cross section) along its length. However, offshore driven piles are often composed of several sections of different cross sections. The Case Method provides in that case an inaccurate estimate of the SRD. This article presents an improved version of the original Case Method which takes into account possible variations of impedance along the pile. For piles displaying impedance changes, the modified Case Method presented herein provides an estimate closer to the actual SRD than the original Case Method. This implies a more accurate estimation of the pile capacity and thus tends to eliminate the use of an additional safety factor when designing the pile

Modified Case Method for Piles with Section Step Changes

Applied during pile driving, the Case Method offers an immediate estimate of the static resistance to driving (SRD) after each hammer blow. It has been used in its original form both in the onshore and offshore piling industry for more than 40 years to provide an indication of the pile static capacity. The Case Method requires measurements of force and velocity near the pile head as the hammer strikes the pile and produces an analytical estimate of the SRD, using a number of assumptions. One of them requires the pile to be of constant impedance (or cross section) along its length. However, for reason of economy, driven piles are often composed of several sections of different cross sections. The Case Method provides in that case an inaccurate estimate of the SRD. This article presents an improved version of the original Case Method which takes into account possible variations of impedance along the pile. A numerical validation shows that for piles displaying impedance changes, the modified Case Method presented herein provides an estimate closer to the actual SRD than the original Case Method. That conclusion is further validated by applying the modified Method to pile driving records and comparing its results to SRD estimates obtained through more reliable modelling