Influence of particle contact models on soil response of poorly graded sand during cavity expansion in discrete element simulation

The discrete element method(DEM) has been extensively adopted to investigate many complex geotechnical related problems due to its capability to incorporate the discontinuous nature of granular materials. In particular, when simulating large deformations or distortion of soil(e.g. cavity expansion),DEM can be very effective as other numerical solutions may experience convergence problems. Cavity expansion theory has widespread applications in geotechnical engineering, particularly to the problems concerning in situ testing, pile installation and so forth. In addition, the behaviour of geomaterials in a macro-level is utterly determined by microscopic properties, highlighting the importance of contact models. Despite the fact that there are numerous contact models proposed to mimic the realistic behaviour of granular materials, there are lack of studies on the effects of these contact models on the soil response.Hence, in this study, a series of three-dimensional numerical simulations with different contact constitutive models was conducted to simulate the response of sandy soils during cylindrical cavity expansion. In this numerical investigation, three contact models, i.e. linear contact model, rolling resistance contact model,and Hertz contact model, are considered. It should be noted that the former two models are linear based models, providing linearly elastic and frictional plasticity behaviours, whereas the latter one consists of nonlinear formulation based on an approximation of the theory of Mindlin and Deresiewicz. To examine the effects of these contact models, several cylindrical cavities were created and expanded gradually from an initial radius of 0.055 m to a final radius of 0.1 m. The numerical predictions confirm that the calibrated contact models produced similar results regarding the variations of cavity pressure, radial stress, deviatoric stress, volumetric strain, as well as the soil radial displacement. However, the linear contact model may result in inaccurate predictions when highly angular soil particles are involved. In addition, considering the excessive soil displacement induced by the pile installation(i.e. cavity expansion), a minimum distance of11 a(a is the cavity radius) is recommend for practicing engineers to avoid the potential damages to the existing piles and adjacent structures.

Real-time determination of sandy soil stiffness during vibratory compaction incorporating machine learning method for intelligent compaction

An emerging real-time ground compaction and quality control, known as intelligent compaction(IC), has been applied for efficiently optimising the full-area compaction. Although IC technology can provide real-time assessment of uniformity of the compacted area, accurate determination of the soil stiffness required for quality control and design remains challenging. In this paper, a novel and advanced numerical model simulating the interaction of vibratory drum and soil beneath is developed. The model is capable of evaluating the nonlinear behaviour of underlying soil subjected to dynamic loading by capturing the variations of damping with the cyclic shear strains and degradation of soil modulus. The interaction of the drum and the soil is simulated via the finite element method to develop a comprehensive dataset capturing the dynamic responses of the drum and the soil. Indeed, more than a thousand three-dimensional(3D) numerical models covering various soil characteristics, roller weights, vibration amplitudes and frequencies were adopted. The developed dataset is then used to train the inverse solver using an innovative machine learning approach, i.e. the extended support vector regression, to simulate the stiffness of the compacted soil by adopting drum acceleration records. Furthermore, the impacts of the amplitude and frequency of the vibration on the level of underlying soil compaction are discussed.The proposed machine learning approach is promising for real-time extraction of actual soil stiffness during compaction. Results of the study can be employed by practising engineers to interpret roller drum acceleration data to estimate the level of compaction and ground stiffness during compaction.

Effect of constructing twin tunnels under a building supported by pile foundations in the Sydney central business district

In congested cities such as Sydney,competition for underground space escalates within the built environment because various assets require finite geotechnical strength and support.Specific problems such as damage to buildings may develop when high-rise buildings on piled foundations are subject to ground movements as tunnels are constructed.This paper focuses on the risks of tunneling beneath Sydney’s Martin Place and how buildings are subject to additional loads caused by tunneling.The key objective of this study is to improve the understanding of tunnel-rock-pile interactions and to encourage sustainable development.A finite element model is developed to predict the interaction between tunnel construction and piled foundations.The tunnel,rock,and pile components are studied separately and are then combined into a single model.The ground model is based on the characteristics of Hawkesbury Sandstone and is developed through a desktop study.The piles are designed using Australian Standards and observations of high-rise buildings.The tunnel construction is modeled based on the construction sequence of a tunnel boring machine.After combining the components,a parametric study on the relationship between tunnel location,basements,and piles is conducted.Our findings,thus far,show that tunneling can increase the axial and flexural loads of piles,where the additional loading exceeds the structural capacity of some piles,especially those that are close to basement walls.The parametric study reveals a strong relationship between tunnel depth and lining stresses,while the relationship between tunnel depth and induced pile loads is less convincing.Furthermore,the horizontal tunnel position relative to piles shows a stronger relationship with pile loads.Further research into tunnel-rock-pile interactions is recommended,especially beneath basements,to substantiate the results of this study.

Comparison of rectangular and circular bored twin tunnels in weak ground

The recent innovation of a rectangular tunnel boring machine(TBM),and its use in the Hongzhuan Road tunnel underpass by the China Railway Engineering Group(CREG),has revitalized shallow depth soft soil tunneling.This paper presents the findings of a numerical study using PLAXIS to determine the surface settlements and moments produced in tunnel linings for circular and rectangular twin tunnels.The effects of the relative positions of twin tunnels,critical distances,volume losses,depths of burial,and tunnel sizes for both circular and rectangular tunnels are the key parameters of this investigation.The results indicate that rectangular tunnels are suitable for shallow depths in weak ground as they have lesser settlement compared with circular tunnels.This is crucial for tunneling beneath important structures such as railway lines and existing roads.However,the maximum bending moment produced in the rectangular tunnel lining is higher than that for circular tunnels.The use of rectangular TBMs is an unconventional method in modern day tunneling;however,the analysis in this project recommends that tunnel industry engineers consider this method for shallow depth weak ground tunneling.