Geotechnical Engineering Online Library

This study proposes a double-rough-walled fracture model to represent the natural geometries of rough fractures. The rough surface is generated using a modified successive random additions (SRA) algorithm and the aperture distribution during shearing is calculated using a mechanistic model. The shear-flow simulations are performed by directly solving the Navier-Stokes (NS) equations. The results show that the double-rough-walled fracture model can improve the accuracy of fluid flow simulations by approximately 14.99%–19.77%, compared with the commonly used single-rough-walled fracture model. The ratio of flow rate to hydraulic gradient increases by one order of magnitude for fluids in a linear flow regime with increment of shear displacement from 2.2 mm to 2.6 mm. By solving the NS equations, the inertial effect is taken into account and the significant eddies are simulated and numerically visualized, which are not easy to be captured in conventional experiments. The anisotropy of fluid flow in the linear regime during shearing is robustly enhanced as the shearing advances; however, it is either increased or decreased for fluids in the nonlinear flow regime, depending on the geometry of shear-induced void spaces between the two rough walls of the fracture. The present study provides a method to represent the real geometry of fractures during shearing and to simulate fluid flow by directly solving the NS equations, which can be potentially utilized in many applications such as heat and mass transfer, contaminant transport, and coupled hydro-thermo-mechanical processes within rock fractures/fracture networks.

This study presents an application of artificial neural network (ANN) and Bayesian network (BN) for evaluation of jamming risk of the shielded tunnel boring machines (TBMs) in adverse ground conditions such as squeezing grounds. The analysis is based on database of tunneling cases by numerical modeling to evaluate the ground convergence and possibility of machine entrapment. The results of initial numerical analysis were verified in comparison with some case studies. A dataset was established by performing additional numerical modeling of various scenarios based on variation of the most critical parameters affecting shield jamming. This includes compressive strength and deformation modulus of rock mass, tunnel radius, shield length, shield thickness, in situ stresses, depth of over-excavation, and skin friction between shield and rock. Using the dataset, an ANN was trained to predict the contact pressures from a series of ground properties and machine parameters. Furthermore, the continuous and discretized BNs were used to analyze the risk of shield jamming. The results of these two different BN methods are compared to the field observations and summarized in this paper. The developed risk models can estimate the required thrust force in both cases. The BN models can also be used in the cases with incomplete geological and geomechanical properties.

The influence of towing speed on the effectiveness of the 4-sided impact roller using earth pressure cells (EPCs) is investigated. Two field trials were undertaken; the first trial used three EPCs placed at varying depths between 0.5 m and 1.5 m with towing speeds of 9–12 km/h. The second used three EPCs placed at a uniform depth of 0.8 m, with towing speeds of 5–15 km/h. The findings from the two trials confirmed that towing speed influences the pressure imparted to the ground and hence compactive effort. This paper proposes that the energy imparted to the ground is best described in terms of work done, which is the sum of the change in both potential and kinetic energies. Current practice of using either kinetic energy or gravitational potential energy should be avoided as neither can accurately quantify rolling dynamic compaction (RDC) when towing speed is varied.

This article presents an innovative method of bio-mediated soil improvement for increasing the shear strength of loose sand. The improvement is realized by mixing the loose sand with the inoculum of Rhizopus oligosporus, a kind of fungus widely used in food industry for making Indonesian tempeh. The objective of this article is to investigate the performance and mechanism of mixing tempeh inoculum as a binding agent of loose sand particles. The inoculum dosage, water content of loose sand, and curing time were examined for identifying the increment of unconfined compressive strength (qu) of the samples. The results showed that qu of the treated samples increased when the inoculum dosage was elevated. It shows that 5.24% inoculum could yield 68 kPa of qu, and 5% water content and 3 d curing time produced the maximum qu. Moreover, the mechanism of hypha and mycelium in binding the soil particles was clearly observed using a digital microscope and scanning electron microscope.

Analysis of soil-structure interaction is commonly conducted by dividing the infinite domain of the soil into two domains: interior and exterior domains. The interior domain is bounded in a small region, while the exterior domain is replaced by artificial boundary conditions. The choice of artificial boundary conditions is a critical issue in the analysis of soil-structure interaction problems. Perfectly matched discrete layer (PMDL) has been proved as a good approach for modeling the exterior domain. In this study, a modified version of the PMDLs, i.e. PMDLs with analytical wavelengths (AW-PMDLs), is used in the soil-structure interaction analysis in time domain, which essentially can be regarded as an extension of the analysis in frequency domain, being previously proven to be effective. Numerical verifications are implemented. The results demonstrate that the proposed method performs well in the analysis of soil-structure interaction problems in time domain.

Many experimental results have demonstrated the apparent discrepancy of a rock material between its flexural tensile strength measured using various bending methods and its tensile strength measured using direct tension method or Brazil disc (BD) method. To understand the physical mechanism for such discrepancy, numerical simulation using the realistic failure process analysis (RFPA) is carried out in this work to simulate the tensile failure of heterogeneous rocks. Direct tension and semi-circular bend (SCB) tests are simulated using RFPA for rock materials with different levels of inhomogeneity, which is characterized by the homogeneity index of the Weibull distribution used in RFPA. The numerical results show that the discrepancy in the tensile strength values is caused by the inhomogeneity of the rock material. Furthermore, non-local failure criterion is adopted to calculate the characteristic length of the rock materials used in the simulation. It is shown that below a certain value of the homogeneity index, both the characteristic length and discrepancy between two types of tensile strengths of rock decrease with increase of the homogeneity index up to a critical value, at which the discrepancy disappears and the rock material is essentially homogeneous.

In this paper, a coupled thermo-hydro-mechanical (THM) simulation in a faulted deformable porous medium is presented. This model involves solving the mass conservation, linear momentum balance, and energy balance equations which are derived from the Biot's consolidation theory. Fluid pore pressure, solid displacement, and temperature are chosen as initial variables in these equations, and the finite element method in combination with the interface element is used for spatial discretization of continuous and discontinuities (fault) parts of the medium to solve the equations. The main purpose of this study is providing precise formulations, applicability, and ability of the triple-node zero-thickness interface element in THM modeling of faults. It should be noted that the system of equations is solved using a computer code written in Matlab program. In order to verify the developed method, simulations of index problems such as Mandel's problem, and coupled modeling of a faulted porous medium and a faulted aquifer are presented. The modeling results obtained from the developed method show a very good agreement with those by other modeling methods, which indicates its accuracy.

Permeation grouting with cement agent is one of the most widely used methods in various geotechnical projects, such as increasing bearing capacity, controlling deformation, and reducing permeability of soils. Due to air pollution induced during cement production as well as its high energy consumption, the use of supplementary materials to replace in part cement can be attractive. Natural zeolite (NZ), as an environmentally friendly material, is an alternative to reduce cement consumption. In the present study, a series of consolidated undrained (CU) triaxial tests on loose sandy soil (with relative density Dr = 30%) grouted with cementitious materials (zeolite and cement) having cement replacement with zeolite content (Z) of 0%, 10%, 30%, 50%, 70% and 90%, and water to cementitious material ratios (W/CM) of 3, 5 and 7 has been conducted. The results indicated that the peak deviatoric stress (qmax) of the grouted specimens increased with Z up to 50% (Z50) and then decreased. The strength of the grouted specimens reduced with increasing W/CM of the grouts from 3 to 7. In addition, by increasing the stress applied on the grouted specimens from yield stress (qy) to the maximum stress (qmax), due to the bond breakage, the effect of cohesion (c′) on the shear strength reduced gradually, while the effect of friction angle (φ′) increased. Furthermore, in some grouted specimens, high confining pressure caused breakage of the cemented bonds and reduced their expected strength.

Cohesive non-swelling soil (CNS) cushion technology is widely used to solve swelling deformation problems in expansive soil areas. However, the swelling inhibition mechanism is still not fully understood. In this study, the inhibition effect on expansive soil using a CNS layer was studied by performing five types of laboratory model tests under unidirectional seepage. The results showed that CNS cushion technology produced a sound inhibition effect on the swelling characteristics of expansive soil. It was shown that the cations in the CNS layer moved downward and accumulated on the surface of solids and produced an electrical environment inside the expansive soil. In this process, the adsorbed hydrated cations participated in ion exchange with the expansive soil, leading to the modification effect on its swelling potential. Meanwhile, the adsorbed water membrane surrounding the expansive soil aggregates formed by the hydrated cations obstructed further adsorption of water molecules, which inhibited the swelling development of expansive soil. Therefore, the swelling inhibition mechanism can be attributed to three factors: (i) modification effect, (ii) electrical environment, and (iii) deadweight of the CNS layer. The combined contribution of modification effect and electrical environment can be considered as an electric charge effect, which mainly controls the swelling characteristics of expansive soil.

To estimate the required support pressure for stability of circular tunnels in two layered clay under undrained condition, numerical solutions are developed by performing finite element lower bound limit analysis in conjunction with second-order cone programming. The support system is assumed to offer uniform internal compressive pressure on its periphery. From the literature, it is known that the stability of tunnels depends on the overburden pressure acting over it, which is a function of undrained cohesion and unit weight of soil, and cover of soil. When a tunnel is constructed in layered undrained clay, the stability depends on the undrained shear strength, unit weight, and thickness of one layer relative to the other layer. In the present study, the solutions are presented in a form of dimensionless charts which can be used for design of tunnel support systems for different combinations of ratios of unit weight and undrained shear strength of upper layer to those of lower layer, thickness of both layers, and total soil cover depth.

The present study examines the effect of stabilization on the geo-environmental properties of crude oil contaminated kaolin clay. Lime and cement were mixed in a ratio of 1:2 and added to the simulated crude oil contaminated kaolin clay at different percentages (5%, 10%, 15%, and 20%) as a stabilizing binder. Parameters investigated include consistency limits, unconfined compressive strength (UCS), and direct shear, and compressibility and leaching characteristics of the untreated and stabilized soils. The experimental testing reveals a decrease in the consistency limits with addition of the stabilizing binder. Maximum UCS values occurred for 15% cement-lime stabilized kaolin clay at different curing periods (i.e. 0 d, 7 d, 14 d, and 28 d). By increasing the cement-lime content from 5% to 15%, the UCS values of the stabilized clay increase from 185 kPa to 350 kPa and from 785 kPa to 1160 kPa for uncured and 28 d-cured samples, respectively. Both the compression and recompression indices of the contaminated kaolin clay from the consolidation test decrease by 40% and 50%, respectively, with 20% stabilizing binder addition. The leachability of the contaminated clay also reduces with incorporation of cement and lime. According to the scanning electron microscope (SEM) test, addition of stabilizing binder transforms the dispersed structure of contaminated kaolin clay into a knitted flocculated structure. The study shows the effectiveness of cement-lime mix in stabilizing the contaminated kaolin clay and the possible use of stabilized contaminated kaolin clay as an alternative construction material.

Development of deep underground mining projects is crucial for optimum extraction of mineral deposits. The main challenges at great depth are high rock stress levels, seismic events, large-scale deformation, sudden failures and high temperatures that may cause abrupt and unpredictable instability and collapse over a large scale. In this paper, a ground control and management strategy was presented corresponding to the three stages of projects: strategic design, tactical design and operational design. Strategic design is results in preparing a broad plan and primary design for mining excavations. The tactical design is to provide detail design such as stabilisation methods. Operational design stage is related to monitoring and updating design parameters. The most effective ground control strategies in this stage are maintenance, rehabilitation, monitoring and contingency plan. Additionally, a new procedure for design of ground support systems for deep and hard rock was proposed. The main principles are: static and/or dynamic loading types, determination of loading sources, characterisation of geological conditions and the effects of orientation of major structures with openings, estimation of ground loading factor, identification of potential primary and secondary failures, utilisation of appropriate design analysis methods, estimation of depth failure, calculation of the static and/or dynamic demand ground support capacity, and selection of surface and reinforcement elements. Gravitational force is the dominant loading force in low-level stresses. In high stress level, failure mechanism becomes more complex in rock mass structures. In this condition, a variety of factors such as release of stored energy due to seismic events, stress concentration, and major structures influence on ground behaviour and judgement are very complicated. The key rock engineering schemes to minimise the risk of failures in high-stress levels at great depth involve depressurisation and quality control of materials. Microseismic and blast monitoring throughout the mining operations are required to control sudden failures. Proper excavation sequences in underground stopes based on top-down, bottom-up, centre-out and abutment-centre were discussed. Also, the performance of a ground support system was examined by field observation monitoring systems for controlling and modifying ground support elements. The important outcome of the research is that the proposed procedure of selecting ground support systems for static and dynamic situations was applied in several deep underground mines in Western Australia. Ground behaviour modes and failure mechanism were identified and assessed. Ground demand for static and dynamic conditions was estimated and an appropriate ground support system was selected and evaluated in site-specific conditions according to proposed method for ground support design at great depth. The stability of rock masses was confirmed, and the reliability of the design methodology for great depth and hard rock conditions was also justified.

To understand the deformational behaviours of geosynthetics-reinforced soil retaining walls (GRS RWs), a series of plane-strain shaking table tests was conducted on retaining wall models. The backfill of the models was made of poorly graded gravel. Deformations and strains in the gravelly backfill induced by seismic loading are recorded in real time, which are of importance to understand the seismic strength and stability of the GRS RW systems, as strain localisation development in the backfill and foundation is related to the degree of strength degradation of the system. In the present study, we aimed at quantifying the induced deformations of the GRS RW models due to shaking. Digital image correlation (DIC) technique was then employed to analyse and provide full-field deformation and motion images with the models. It is demonstrated that, unlike conventional contact devices that are yet limited to provide quantities of a singular and fixed location, DIC provides deformation and motion of the area of interests to reveal the evolution of localisation.

Large-scale slope destabilization could be aggravated due to swift urbanization and ever-rising demands of geoengineering projects such as dams, tunnels, bridges and widening roads. National Highway-58 connects Delhi to Badrinath in India, which passes through complex geomorphological and geological terrain and often encounters cut slopes susceptible to slope failures. In the present investigation, a detailed geotechnical appraisal is conducted along the road cut slopes from Rishikesh to Devprayag in the Himalayas. Twenty vulnerable road cut slopes were demarcated for detailed slope stability analysis using Phase2D finite element modeling simulator. Nonlinear generalized Hoek-Brown (GHB) criterion was adopted for stability analyses. Out of 20 slopes, five slopes (S6, S7, S18, S19 and S20) are unstable with factor of safety (FoS) less than or equal to 1, and thus needs immediate attention. The FoS values of four slopes (S2, S9, S13 and S17) lie between 1 and 1.3, i.e. marginally stable, and slopes S1, S3, S4, S5, S8, S10, S11, S12, S14, S15 and S16 are stable. Mohr-Coulomb (MC) criterion was also adopted to compare the slope stability analysis with GHB criterion. The FoS calculated from GHB criterion is close to that using MC criterion for lower values of FoS whereas for higher values, the difference is marked. For the jointed rock in the Himalayan region, the nonlinear GHB criterion gives better results as compared to MC criterion and matches with the prevailing field conditions. Accordingly, some suggestions are proposed to strengthen the stability of cut slopes.