Ha H. Bui

1-D Compression Behaviour of Acid Sulphate Soils Treated with Alkali-Activated Slag

Improvements of soft soils by mechanically mixing cementitious additives have been widely practised for construction of infrastructure. Mixing of additives improves strength and compressibility properties of soils through the development of soil structure. This study investigates the 1-D compression behaviour of alkali-activated slag treated acid sulphate soils (ASS) cured up to 365 days. The void ratio-logarithm of pressure (e-logσ′) behaviour of treated ASS, including the destructuration behaviour, with additive contents and curing time have been analysed. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses have been undertaken to explain the observed variations of the 1-D compression behaviour. This paper presents the results of these analyses in view of obtaining an insight into the 1-D compression behaviour of treated ASS with the help of mineralogical analysis.

Determining Soil Thermal Conductivity Through Numerical Simulation of a Heating Test on a Heat Exchanger Pile

Heat exchanger pile foundations have a great potential of providing space heating and cooling to built structures. This technology is a variant of vertical borehole heat exchangers. A heat exchanger pile has heat absorber pipes firmly attached to its reinforcement cage. Heat carrier fluid circulates inside the pipes to transfer heat energy between the piles and the surrounding ground. Borehole heat exchangers technology is well established but the heat exchanger pile technology is relatively new and requires further investigation of its heat transfer process. The heat transfer process that affects the thermal performance of a heat exchanger pile system is highly dependent on the thermal conductivity of the surrounding ground. This paper presents a numerical prediction of a thermal conductivity ground profile based on a field heating test conducted on a heat exchanger pile. The thermal conductivity determined from the numerical simulation was compared with the ones evaluated from field and laboratory experiments. It was found that the thermal conductivity quantified numerically was in close agreement with the laboratory test results, whereas it differed from the field experimental value.

A new SPH-based approach to simulation of granular flows using viscous damping and stress regularisation

The smoothed particle hydrodynamics (SPH) method was recently extended to simulate granular materials by the authors and demonstrated to be a powerful continuum numerical method to deal with the post-flow behaviour of granular materials. However, most existing SPH simulations of granular flows suffer from significant stress oscillation during the post-failure process, despite the use of an artificial viscosity to damp out stress fluctuation. In this paper, a new SPH approach combining viscous damping with stress/strain regularisation is proposed for simulations of granular flows. It is shown that the proposed SPH algorithm can improve the overall accuracy of the SPH performance by accurately predicting the smooth stress distribution during the post-failure process. It can also effectively remove the stress oscillation issue in the standard SPH model without having to use the standard SPH artificial viscosity that requires unphysical parameters. The predictions by the proposed SPH approach show very good agreement with experimental and numerical results reported in the literature. This suggests that the proposed method could be considered as a promising continuum alternative for simulations of granular flows.

Numerical Simulation of Granular Materials Based on Smoothed Particle Hydrodynamics (SPH)

Granular flows play an important role in many industrial processes. To optimize these processes, it is necessary to simulate the flow accurately. However, due to the complexity of the granular flows, there is no generally accepted theory or computational method at the present. This paper presents a new numerical approach based on the smoothed particle hydrodynamics method (SPH) to simulate granular materials, which are assumed to be elasto‐plastic. Herein, the Drucker‐Prager model with non‐associated plastic flow rule was employed to describe elasto‐plastic behavior of granular flows, and the accuracy of SPH approximation of governing equation is improved by adopting a procedure to renormalize the kernel derivative. Application of SPH to simulate granular flow in a silo was presented after a validation of SPH with experiment. It is shown that the current SPH model can capture overall behavior of granular flows.

EVALUATION OF PLASTICITY-BASED SOIL CONSTITUTIVE MODELS IN SIMULATION OF BRACED EXCAVATION

Numerical modelling continues to play a unique and intrinsic role in the process of geotechnical design. Of greatest concern are soil c onstitutive models that are employed within finite element software to predict soil behaviour. The objective o f this paper is to provide a numerical study of the Mohr- Coulomb and Hardening Soil constitutive models in simulation of a braced excavation. The Taipei Nation al Enterprise Centre (TNEC) basement construction process was well documented and the commercial finite element code, Plaxis, was selected for this numeric al comparative study. It was found that the Mohr- Coulomb soil model, a first order approximation, pr oduced an underestimation of the diaphragm wall deflection, whilst the Hardening Soil model provide d a good prediction of the observed in-situ diaphra gm wall deflections.

Seismic slope failure modelling using the mesh-free SPH method

In the past few decades, majority of dynamic behavior of slope have been conducted using the finite element method (FEM). However, earthquakes often cause large deformation and post-seismic soil deformation which are difficult to predict using the FEM due to mesh distortion issues. As an alternative numerical method, the smoothed particle hydrodynamics (SPH) has been recently applied to geotechnical field and showed to be a promising numerical technique to handle large deformation and post-failure behavior of geomaterials. In this paper, taking into consideration of this advantage, the SPH method is applied to simulate response of a slope subjected to seismic loading. Reliability of SPH results was assessed by comparing with experimental data available in the literature. It is shown that the SPH method could qualitatively predict slope failure behavior observed in the experiment.

Advanced Characteristics of Cement-Treated Materials with Respect to Strength Performance and Damage Evolution

AbstractCement-stabilized base (CSB), a cement-stabilized material for road pavement construction activities, generally has better essential properties than an unbound granular material, a commonly...

SEEPAGE FLOW-STABILITY ANALYSIS OF THE RIVERBANK OF SAIGON RIVER DUE TO RIVER WATER LEVEL FLUCTUATION

The Saigon River, which flows through the center of Ho Chi Minh City, is of critical importance for the development of the city as forms as the main water supply and drainage channel for the city. In recent years, riverbank erosion and failures have become more frequent along the Saigon River, causing flooding and damage to infrastructures near the river. A field investigation and numerical study has been undertaken by our research group to identify factors affecting the riverbank failure. In this paper, field investigation results obtained from multiple investigation points on the Saigon River are presented, followed by a comprehensive coupled finite element analysis of riverbank stability when subjected to river water level fluctuations. The river water level fluctuation has been identified as one of the main factors affecting the riverbank failure, i.e. removal of the balancing hydraulic forces acting on the riverbank during water drawdown.

Dynamic Modulus Measurements of Bound Cement-Treated Base Materials

One of the most common methods used in road-pavement construction is the stabilizing of the conventional pavement base course layer. This is achieved by adding cement or lime to gain better material performance. However, obtaining modulus input parameters from a cement-stabilized base course layer for pavement-response analysis under real traffic conditions has proven difficult in that, to date, only ambiguous results have been produced. Using the flexural modulus or elastic modulus in the response analysis has certain limitations in embracing real pavement behavior under traffic and temperature conditions. Accordingly, a more reliable modulus input parameter for pavement analysis under traffic (cyclic) loads is required to obtain more precise and reliable outputs. Moreover, there is, at present, no test protocol to determine a suitable modulus for a cement-stabilized base material under the cyclic loading regime. This study aims to examine the real dynamic responses of cement-stabilized base course materials with a view to adapting the asphalt mixture performance tester (AMPT), a specifically designed dynamic modulus test machine used on asphalt concrete material. The AMPT dynamic modulus test has as an advantage in that loading and temperature regimes based on real pavement conditions can be rationally simulated and directly applied to the test samples. As such, the dynamic moduli of a cement-stabilized base course material can be obtained under different temperature and loading rates. Moreover, the effects of the dynamic strain range, cement content, and curing duration on the dynamic responses of a cement-stabilized base course material may also be examined. Cement-stabilized base course materials of 4 %, 5 %, and 6 % cement contents (by mass) were used as the study materials. The findings of this study indicate that curing durations and cement contents significantly influence the dynamic modulus values of cement-stabilized base course materials. However, the dynamic modulus is insignificantly affected by the changes in temperature and loading rates within a specific range of testing conditions in this study. The test results also reveal that cement-stabilized base course materials under examination behave in the manner of an elastic material when subjected to an axial compressive deformation of 45–105 μstrains. This is because of the dynamic modulus having no impact upon changes in the dynamic strain ranges or on the magnitudes of cyclic loads. Moreover, the dynamic moduli from this study were found to be much higher than the elastic moduli suggested by previous studies. However, the flexural moduli, which are derived from standard flexural tests, demonstrated close values to those of the dynamic moduli obtained in this study. In the study, the dynamic modulus of cement-stabilized base course materials, derived from the dynamic modulus using AMPT, could more reasonably embrace the dynamic responses of a material under traffic-loading conditions. This leads to a somewhat more reliable modulus input for the cement-stabilized base course materials used in a rational pavement design and analysis method.

Flexural Properties of Cemented Granular Materials for Pavement Design

Cementation stabilization of unbound granular materials often offers a feasible solution for the strengthening of existing degraded unbound pavements. The primary determination mode of cemented pavement materials is fatigue cracking. Flexural properties including flexural modulus and tensile strain at break are incorporated into the fatigue criteria of cemented materials. However, there are no universally accepted testing protocols available for cemented pavement materials to determine the aforementioned properties in the laboratory. The four-point bending test is chosen in this paper to study the flexural properties of two different cemented pavement materials as it more closely simulates the stress/strain gradients generated in service. The results from this study revealed that, the strain at break could not be determined with sufficient precision as the tensile strain at the bottom of the specimen just prior to the point of fracture increases uncontrollably without further increase in tensile stress. Based on the response of cemented materials to different loading conditions, an equation is tentatively proposed for determination of the modulus for pavement design.