Continuum-based numerical simulation of static and high-strain dynamic pile load testing adopting advanced soil models

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Piles are generally used to carry structural loads when the soil at the ground surface is low in strength or the loads are substantial. It is very common to conduct pile load testing to assess whether the piles will behave as predicted in the design stage. Static load testing (SLT) is considered to be the benchmark for assessing the performance of piles since it is known the most fundamental way of pile load testing. However, this kind of test is time consuming and expensive, and in cases such as offshore operations, SLT is generally not possible for many cases. In spite of this, powerful computer programs for pile testing simulation have been revolutionised and are available. Of these different methods, the dynamic load testing (DLT) method for assessing the static bearing capacity of piles is of major interest and importance. A dynamic pile test is based on the signal matching technique in which the pile-soil system is modelled using the CAse Pile Wave Analyses Program (CAPWAP). This program tries to calculate the tip and side resistance of embedded piles and produces a force versus time signal which matches the measured data. The signal matching analysis uses a one-dimensional wave equation analysis of piles based on the Smith model to differentiate between toe and shaft resistance, to ascertain the distribution of frictional resistance along the pile shaft to determine the tensile and compressive stresses during pile driving. However, this technique uses a mass–spring–dashpot system to model the soil media surrounding and below the toe which imposes some restrictions such as being user-dependant process and using constant uncommon soil parameters such as quake along the pile length, regardless of soil strata, which can be layered or uniform. Furthermore, using CAPWAP to analyse pile driving interrupts the continuity of different stages of pile modelling from simulating pile driving, quality control, and investigating settlement. GRLWEAP or CAPWAP generally should be used with a second software package such as PLAXIS in order to investigate any subsequent settlement or interaction. In order to overcome the aforementioned limitations and assess pile behaviour during load testing in more detail, so-called continuum numerical models using the finite element program PLAXIS are established. In these numerical models, wave propagation, the static and dynamic response of piles during load testing for solid concrete piles and open-ended tubular steel piles are evaluated. In fact, the numerical simulations in this study are a remarkable improvement compared to the previous numerical studies because when simulating pile load testing, different soil models such as the Mohr-Coulomb, hardening soil, hardening soil with small strain stiffness and hypoplastic with intergranular strain are utilised to carry out a more rigorous deformation analysis. To investigate the capability of the numerical model, the dynamic and static responses of a driven steel pipe pile monitored as part of a highway bridge construction project in New South Wales, Australia is simulated and numerically analysed using the finite element method. During these dynamic and static load testing simulations, a hardening soil model with small strain stiffness is used to obtain the best correlation between the large and small strains, while the pile is under a static load and being driven. The numerical predictions obtained using two-dimensional continuum finite element simulations are then compared with the corresponding predictions obtained from the CASE method and CAPWAP program to evaluate the predictions. Moreover, the total and static soil resistances as well as displacement and velocity traces obtained from numerical model are compared with the existing data acquired from the field measurements. The results indicate that the hardening soil model with small strain stiffness exhibits a reasonable correlation with the field measurements during static and dynamic loading. Evaluation of static and dynamic pile load testing based on the continuum based finite element model has many advantages for geotechnical engineers dealing with pile design, because an established continuum numerical model can assess pile testing under more realistic conditions. This model can also be used to evaluate the performance of piles under different loading conditions on a single pile or group of piles, and piles built close to existing structures. Furthermore, this method retains the continuity of different stages of modelling from simulating pile driving, quality control, and investigating settlement, while all these analyses are carried out using one appropriate finite element based software.
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