Showing papers in "Geotechnique in 2017"

Estimation of lateral displacement induced by jet grouting in clayey soils

Abstract: The installation of jet-grouted columns in soft ground can induce lateral displacement resulting from injection of a great amount of water and grout into the ground, which may then lead on to further significant effects. This paper presents an approach to estimate the lateral displacement due to the installation of a jet-grouted column in clayey soils. Assuming that the installation of a jet-grouted column is represented by the expansion of a cylindrical cavity with a uniform radial stress applied at the plastic–elastic interface in a half-plane, the analytical solution developed by Verruijt is adopted in this study to calculate the lateral displacements attributable to the expansion of the cylindrical cavity. An empirical equation that accounts for the jetting parameters (jetting pressure, flow rate of the fluid and rod withdrawal rate) and soil properties (soil type and its undrained shear strength) has been developed to determine the radius of the plastic zone. Three case histories analysed in this pap...

The applicability of microbially induced calcite precipitation (MICP) for internal erosion control in gravel–sand mixtures

Abstract: Seepage-induced internal erosion in earth-filled embankment dams has been attracting the attention of civil engineering researchers and practitioners for decades. Microbially induced carbonate prec...

Statistical interpretation of soil property profiles from sparse data using Bayesian compressive sampling

Abstract: In geotechnical engineering, the number of measurement data obtained from in situ or laboratory tests is usually sparse, especially for projects of small or medium size. Interpretation from such sparse measurement data is challenging and may result in significant statistical uncertainty, which refers to inaccuracy of the statistical inference results caused by a limited number of data used in the statistical inferences. Consider, for example, a soil property profile (i.e. variation of a soil property with depths), which is usually interpreted from sparse measurement data and unavoidably contains significant uncertainty. Geotechnical design and analysis results are greatly affected by the interpreted soil property profile and its uncertainty. Quantification of the uncertainty contained in the interpreted soil property profile is therefore essential, especially for probability-based design and analysis. This paper aims to address this problem using a Bayesian compressive sampling (BCS) method. The proposed ...

Thermally induced group effects among energy piles

Abstract: The behaviour of conventional pile groups (e.g. closely spaced) that are subjected to mechanical loads has been shown to be different from the behaviour of single isolated piles. The so-called ‘gro...

Stress–strain response of soft clay to traffic loading

Abstract: Approximately 25% of China's 120 000 km of expressway, as well as many new metro lines and airports, rest on soft clay deposits. However, service settlements are proving larger than expected, especially in southeast China. This note describes laboratory experiments on K0-consolidated intact samples of soft clay taken near Wenzhou, south-east China, that explore whether cyclic traffic wheel loading contributes significantly to the observed settlement trends. Cyclic triaxial (CT) tests are reported together with cyclic hollow cylinder (CHCA) experiments that imposed cardioid-shaped 2τzθ − (Δσz − Δσθ) stress paths. Cyclic principal stress axis rotation is shown to have an important influence on vertical straining. Once a certain threshold has been exceeded, the resilient and permanent strains developed in the CHCA tests become progressively larger than their counterparts in CT tests conducted at the same vertical cyclic stress ratio, with trends that diverge progressively as vertical cyclic stress ratio incr...

Kinematic analysis of seismic slope stability with a discretisation technique and pseudo-dynamic approach: a new perspective

Abstract: Slopes are more vulnerable to instability when subjected to earthquake ground shaking. In order to account for the dynamic forces induced by ground shaking, a novel procedure is introduced in this paper to estimate slope stability under the ultimate limit state with a combination of pseudo-dynamic approach and discretisation technique. A pseudo-dynamic approach is adopted which allows the introduction of an arbitrary time history of seismic accelerations. In order to consider non-uniformity of soil properties of the slope, the discretisation technique is proposed with the aim of generating a potential failure mechanism with discretised points by forward difference ‘point-to-point’ method. Infinitesimal trapezoidal elements composed of successive discretised points and sloping surface are selected for kinematic analysis. In this way, the problem is decomposed into separate components, which aids computational effort. The upper-bound solutions of limiting surcharge loading and yield seismic acceleration are...

Quantification of the morphology of shelly carbonate sands using 3D images

Abstract: Shelly carbonate sands proliferate in regions of the world where construction of offshore structures is in high demand. These structurally weak sands have high intra-granular voids and complex angular grain shapes. To improve the understanding of the mechanical properties of the material, a detailed morphological quantification is required. This paper presents a three-dimensional characterisation of the morphology of shelly carbonate sands based on analyses of X-ray computed tomography images. Two sands from the Persian Gulf with distinct grading were investigated. An adaptive watershed segmentation technique was developed to identify the individual grains for subsequent grain-scale analysis, which overcomes the challenges posed by the intricate microstructure of these sands. Non-invasive measurements of size, intra-granular void and various shape parameters were carried out, and statistical analyses were conducted, to characterise the grains. The results help to better understand the mechanisms of grain ...

Determination of representative strength of deep cement-mixed clay from core strength data

Abstract: Deep cement mixing is widely used to improve soft clayey soils as part of stability intervention in deep excavation and tunnelling. The quality of the improvement is often evaluated by measuring the strength of cored samples post-installation. Although spatial variability in the strength of cement-mixed soil is well recognised, there is still no well-accepted framework for interpreting core strength data. In particular, it is still unclear as to how a representative strength should be derived from the core data. This issue is examined herein using data from two field cases. The core data are first normalised with respect to the duration between treatment and testing, to account for the increase in strength with time. Second, the robustness of two types of criteria is then examined and the sample minima are shown to be relatively sensitive to outlying data. A set of theoretical equations on the variability of sample attributes, such as mean, variance and representative strength, with sample size is then ve...

Particle breakage and the critical state of sands

Abstract: A series of triaxial tests were conducted to investigate the characteristics of the critical state loci (CSLs) of original and pre-crushed sands. The results of the original sand showed that the CS...

A large-strain radial consolidation theory for soft clays improved by vertical drains

Abstract: A system of vertical drains with combined vacuum and surcharge preloading is an effective solution for promoting radial flow, accelerating consolidation. However, when a mixture of soil and water is deposited at a low initial density, a significant amount of deformation or surface settlement occurs. Therefore, it is necessary to introduce large-strain theory, which has been widely used to manage dredged disposal sites in one-dimensional theory, into radial consolidation theory. A governing equation based on Gibson's large-strain theory and Barron's free-strain theory incorporating the radial and vertical flows, the weight of the soil, variable hydraulic conductivity and compressibility during the consolidation process is therefore presented.

Dry granular flow interaction with dual-barrier systems

Abstract: Multiple barriers are commonly installed along predicted geophysical flow paths to intercept large flow volumes. The main criterion for multiple-barrier design is volume retained. The velocity of t...

Shear wave velocity in sand: effect of grain shape

Abstract: Whether the shear wave velocity in sand is dependent on grain shape is a basic question of considerable interest, yet remains open owing to the lack of solid experimental evidence. This technical n...

Equivalent ‘smear’ effect due to non-uniform consolidation surrounding a PVD

Abstract: This paper investigated the effect of non-uniform consolidation within a prefabricated vertical drain (PVD) unit cell on the average degree of consolidation through laboratory model tests and finite-element analysis. By considering the permeability (k) changing with the void ratio (e), the results of the finite-element analysis showed that the effect of non-uniform consolidation has a considerable effect on the average degree of consolidation of a PVD unit cell. Further laboratory model tests of a PVD unit cell were conducted, and analyses of the model test results confirmed the effect of non-uniform consolidation. Finally, a method of considering the effect of non-uniform consolidation for the analysis of PVD-induced consolidation was established with an equivalent ‘smear’ effect concept.

Group action effects caused by various operating energy piles

Abstract: To date, no field data referring to various operating energy piles over timescales of practical geothermal applications have been made available to investigate the thermally induced ‘group action’....

Early detection of first-time slope failures using acoustic emission measurements: large-scale physical modelling

Abstract: Early warning systems for slope instability need to alert users of accelerating slope deformation behaviour to enable safety-critical decisions to be made. This study shows that acoustic emission (...

Evolution of deformation and breakage in sand studied using X-ray tomography

Abstract: Particle breakage of a granular material can cause significant changes in its microstructure, which will govern its macroscopic behaviour; this explains why the mechanisms leading to particle break...

The mechanics of a saturated silty loess with a transitional mode

Abstract: Problems of identifying unique normal compression and critical state lines have been found in gap-graded soils and also some silty soils, a mode of behaviour sometimes known as ‘transitional’. Loess is a typical silty soil that covers up to 631 000 km2 in China and is of great significance as a result of the large number of engineering projects under construction. To examine the potential transitional behaviour in loess, an extensive series of carefully controlled oedometer tests and triaxial tests on intact and reconstituted specimens were carried out. Loess specimens with a wide range of initial void ratios were reconstituted using different preparation methods. Intact specimens were taken from two depths, but their natural variabilities were not significant and could be neglected, apart from their difference in the initial void ratios. The results have highlighted the features of transitional behaviour in this loess, not only in reconstituted but also intact states, and in particular that there is an i...

Stability of intact slopes with tensile strength cut-off

Abstract: The Mohr–Coulomb yield criterion, typically used in slope stability analyses, predicts the uniaxial as well as the isotropic (triaxial) tensile strength. In this study, the tensile strength was reduced or eliminated from the yield condition in order to determine whether it has any consequences on the outcome of the stability analyses of slopes. It was discovered that the consequences on the slope stability factor are very significant. Slopes that are most affected by the tension cut-off are steep slopes, particularly in the presence of seepage. The difference in stability factor for steep slopes calculated with the classical Mohr–Coulomb strength envelope and a criterion with tensile strength cut-off can exceed 50% for steep slopes subjected to seepage (70% for vertical slopes). Tension cut-off used in the analysis permits deformation modes with the dilatancy angle exceeding the internal friction angle, and this is the likely cause of producing more critical mechanisms of slope collapse. The novelty of th...

Construction of simplified design p-y curves for liquefied soils

Abstract: In practice, laterally loaded piles are most often modelled using a ‘Beam-on-Nonlinear-Winkler-Foundation’ (BNWF) approach. While well calibrated p-y curves exist for non-liquefied soils (e.g. soft clay and sands), the profession still lacks reliable p-y curves for liquefied soils. In fact, the latter should be consistent with the observed strain-hardening behaviour exhibited by liquefied samples in both element and physical model tests. It is recognised that this unusual strain-hardening behaviour is induced by the tendency of the liquefied soil to dilate upon undrained shearing, which ultimately results in a gradual decrease of excess pore pressure and consequent increase in stiffness and strength. The aim of this paper is twofold. First it proposes an easy-to-use empirical model for constructing stress-strain relationships for liquefied soils. This only requires three soil parameters which can be conveniently determined by means of laboratory tests, such as a cyclic triaxial and cyclic simple shear tests. Secondly, a method is illustrated for the construction of p-y curves for liquefiable soils from the proposed stress-strain model. This involves scaling of stress and strain into compatible soil reaction p and pile deflection y, respectively. The scaling factors for stress and strain axis are computed following an energy-based approach, analogous to the upper-bound method used in classical plasticity theory. Finally, a series of results from centrifuge tests are presented, whereby p-y curves are back-calculated from available experimental data and qualitatively compared with that proposed by the authors.

Visualisation of seepage-induced suffusion and suffosion within internally erodible granular media

Abstract: A rigid-walled ‘transparent soil’ permeameter has been developed to study visually the mechanisms occurring during seepage-induced internal erosion in susceptible granular media under upward flow. ...

Plant age effects on soil infiltration rate during early plant establishment

Abstract: Infiltration rate affects slope stability by determining the rate of water transport to potential failure planes. This note considers the influences of vegetation (grass and willow) establishment a...

Undrained behaviour of granular material and the role of fabric in isotropic and K0 consolidations: DEM approach

Abstract: The behaviour of granular material was investigated by simulating an undrained triaxial compression test for three-dimensional assembly of ellipsoid particles over a wide range of void ratio, e, and mean confining stress, p′. The assembly was either isotropically consolidated or K0 consolidated prior to undrained shearing up to 30% axial strain to reach steady-state conditions. A unique steady-state line was obtained, irrespective of the consolidation path. The micro-mechanical quantities, such as coordination number and von Mises fabric in terms of second invariant of deviatoric fabric, FvM, also reached steady-state values. The normalised anisotropic fabric variable, A, and the trace of the joint stress–fabric tensor, KF, evolved during undrained simulation and reached the steady-state value at the end of shearing. This forms a unique relationship in KF–p′–e space and the projection of this relationship in e–log(p′) space is the classical steady-state line. This underpins the concept of fabric evolution...

The equivalent pier method for energy pile groups

Abstract: This study presents a method for estimating the average vertical displacement of energy pile groups subjected to thermal loads. The method consists of replacing any regular energy pile group with a...

Probabilistic stability analyses of undrained slopes with linearly increasing mean strength

Abstract: Based on recently published deterministic solutions as a benchmark, the random finite-element method is used here to investigate the influence of spatial variability on the undrained stability of normally consolidated random slopes, where the mean strength increases linearly with depth while the coefficient of variation remains constant. Results are presented in the form of charts that give the mean and standard deviation of a dimensionless stability number. Using the charts presented in this note, engineers can obtain a preliminarily assessment of the probability of failure of normally consolidated clay slopes.

Thermal effects in landslide mobility

Abstract: The paper describes a thermo-hydro-mechanical formulation to model thermally induced effects due to the irreversible work input generated during soil deformation. The model was implemented into a material point calculation procedure. The method is applied to the analysis of landslides. A simple slope stability is first analysed. Mechanical work essentially dissipates in shearing bands, which develop excess pore water pressures. The marked effect of soil permeability to control the slide motion after failure is described. Shear band thickness is also a relevant control variable. The problem posed by the non-realistic thickness of shear bands in numerical calculation is addressed by means of a numerical procedure that includes consideration of embedded shear bands where the strains are assumed to be localised. Balance equations describing local flow and thermal interactions between shear bands and the remaining material are formulated. The method is applied to model the instability and subsequent rapid moti...

Numerical investigation of the effects of tunnelling on existing tunnels

Abstract: Construction of the Crossrail tunnels just beneath the existing Central line tunnels at the northern side of Hyde Park provided the impetus for this paper. A basic three-dimensional (3D) finite-element (FE) model was developed to study a general case of a new tunnel (NT) crossing perpendicularly below an existing tunnel (ET). A series of 3D FE analyses was carried out and the results presented in this paper reveal some of the interaction effects. Changes in hoop forces, bending moments and lining deformations of the ET due to excavation of the NT are discussed. Conclusions are drawn about how the relative position of the excavation face of the NT in relation to the ET's axis affects the latter's behaviour. Cross-sectional and longitudinal deformations of the ET are discussed, leading to recommendations for field monitoring of similar interaction cases. Two parametric studies were also carried out to quantify the effects of the magnitude of the earth pressure balance machine face pressure and the longitudi...

A method for the design of embedded cantilever retaining walls under static and seismic loading

Abstract: A simple-to-use method is proposed for the design of embedded cantilever retaining walls in cohesionless soils under static and seismic conditions. Taking into account the displacement pattern generally exhibited by this type of structure, which consists of an approximately rigid rotation around a point located close to the wall base, it is assumed that active and passive pressures are fully mobilised up to a certain depth from the excavation level. Below this depth, a simplified distribution of earth pressure is considered, with values that are not predetermined by limit state. Analytical expressions are provided to calculate readily the net pressures acting on the wall and the associated internal forces arising in the structure. Comparisons are made with numerical and experimental results concerning embedded cantilever retaining walls under static and seismic conditions. In all the cases considered, a fairly good agreement is found between these results and those predicted by the proposed method. Some a...

A statistical review of the behaviour of concrete-face rockfill dams based on case histories

Abstract: The concrete-face rockfill dam (CFRD) has become a preferred dam type, but its design remains largely based on past experiences. Understanding the behaviour of CFRDs is critical to the design and s...

Site characterisation for the Ballina field testing facility

Abstract: A soft soil field testing facility has been recently established near Ballina, New South Wales, on the east coast of Australia, which is aimed at improving design and construction methods for transport infrastructure. Several sampling, laboratory and in situ testing campaigns have been performed to characterise the material properties of the soil. High-quality laboratory testing has been performed at one location through the soil profile and a range of geophysics, cone penetrometer, seismic dilatometer, shear vane and permeability tests have been carried out at other locations. The in situ tests have demonstrated that the stratigraphy and test data are reasonably uniform across the site. Seasonal groundwater variations cause the in situ stress state to vary with time. In situ test data have been compared with laboratory tests in order to estimate soil material properties at the locations of the in situ tests. The accuracy of published correlations was variable and site-specific correlations were found to ...

The influence of particle surface roughness on elastic stiffness and dynamic response

Abstract: Discrete-element method (DEM) simulations of planar wave propagation are used to examine the effect of particle surface roughness on the stiffness and dynamic response of granular materials. A new contact model that considers particle surface roughness is implemented in the DEM simulations. Face-centred cubic lattice packings and random configurations are used; uniform spheres are considered in both cases to isolate fabric and contact model effects from inertia effects. For the range of values considered here surface roughness caused a significant reduction in stiffness, particularly at lower confining stresses. The simulations confirm that surface roughness effects can at least partially explain the value of the exponent in the relationship between stiffness and mean confining stress for an assembly of spherical particles. Frequency domain analyses showed that the maximum frequency transmitted through the sample is reduced when surface roughness is considered. The assumption of homogeneity of stress and ...