Simon J. Wheeler

On a 2D hydro-mechanical lattice approach for modelling hydraulic fracture

A 2D lattice approach to describe hydraulic fracturing is presented. The interaction of fluid pressure and mechanical response is described by Biots theory. The lattice model is applied to the analysis of a thick-walled cylinder, for which an analytical solution for the elastic response is derived. The numerical results obtained with the lattice model agree well with the analytical solution. Furthermore, the coupled lattice approach is applied to the fracture analysis of the thick-walled cylinder. It is shown that the proposed lattice approach provides results that are independent of the mesh size. Moreover, a strong geometrical size effect on nominal strength is observed which lies between analytically derived lower and upper bounds. This size effect decreases with increasing Biots coefficient.

Shallow seabed methane gas could pose coastal hazard

Abnormally high levels of methane gas in seafloor sediments could pose a major hazard to coastal populations within the next 100 years through their impact on climate change and sea level rise. Marine scientists have known for many years that biogenic methane (CH4) is generated in shallow seabed sediments on continental margins, especially in rapidly deposited muddy sediments with high organic matter content (see Methane Flux Control in Ocean Margin Sediments (METROL) project in Mienert et al., [2004]). Gassy sediments are found in river deltas, estuaries, and harbors, but also in deeper waters on continental shelves and slopes. Human activities can accelerate natural seafloor gas generation by increasing the supply of sediments and organic matter from rivers through deforestation and intensive farming, and also by the disposal of human waste at sea. When this extra organic matter becomes buried to about one meter beneath the seabed, biogeochemical processes start to convert it to CH4 [Floodgate and Judd, 1992]. The impact of this extra CH4 could be felt within the next 100 years, assuming a one-centimeter-per-year sediment accumulation rate.

A sequential method for selecting parameter values in the Barcelona basic model

The popularity of the Barcelona basic model (BBM) has grown steadily since its publication in 1990, due to its ability to capture key aspects of unsaturated soil behaviour. Nevertheless, the BBM is still rarely employed by practitioners, partly because of the absence of simple and objective methods for selecting parameter values from laboratory tests. One difficulty is that, in the BBM, individual aspects of the isotropic virgin behaviour are controlled by multiple parameters, while at the same time a single parameter controls more than one aspect of soil behaviour. This has led to iterative procedures where parameter values are adjusted in turn to match experiments, which requires significant experience and can lead to the selection of widely varying parameter values depending on the user. The proposed method streamlines parameter selection with a view to increasing the appeal of the BBM for practitioners. The method adopts a “sequential” procedure where the five parameters governing isotropic virgin beh...

Image Analysis of Strains in Soils Subjected to Wetting and Drying

A novel image based technique has been developed to measure strains of soil specimens subjected to wetting-drying by using vapor equilibrium with closed-loop ventilation. A digital SLR camera, fitted with a macro telephoto lens, and an advanced photo system type-C (APS-C) sensor have been used to take images of specimens during wetting-drying. By using the simple radial model of optical distortion, it has been shown that macro fixed focal length lenses introduce extremely small strain errors, which can be neglected. It has also been shown that APS-C sensors are preferable to large Full Frame sensors when the specimen image does not fit the entire sensor area and pixel density, rather than pixel resolution, becomes the key parameter controlling measurement precision. Images of specimens have been acquired at different times during wetting-drying of the soil and processed by using the Adobe Photoshop software to assess evolution of strains. In particular, the planar strain field has been measured by means of a virtual “optical” strain gage rosette tracking the same three points on the specimen image throughout equalization. Multiple optical strain gage rosettes at different scales and positions have been employed to assess the uniformity of the strain field. The method was validated by imposing six different values of suction on six distinct specimens of the same isotropically compacted bentonite, with five values producing soil shrinkage (drying) and one value producing soil swelling (wetting). An excellent repeatability of results was observed in terms of both water content and strains. In addition, for each suction level, almost identical strain equalization curves are obtained from the analysis of multiple optical strain gage rosettes of different sizes placed at different locations over the specimen image. This confirms uniformity of the strain field and excludes the presence of friction at the specimen base.

Probabilistic modelling of auto-correlation characteristics of heterogeneous slopes

Spatial variability of soil materials has long been recognised as an important factor influencing the reliability of geo-structures. This study stochastically investigates the influence of spatial variability of shear strength on the stability of heterogeneous slopes, focusing on the auto-correlation function, auto-correlation distance and cross-correlation between soil parameters. The finite element method is merged with the random field theory to probabilistically evaluate factor of safety and probability of failure via Monte-Carlo simulations. The simulation procedure is explained in detail with suggestions on improving efficiency of the Monte-Carlo process. A simple procedure to create cross-correlation between random variables, which allows direct comparison of the influence of each strength variable, is discussed. The results show that the auto-correlation distance and cross-correlation can significantly influence slope stability, while the choice of auto-correlation function only has a minor effect. An equation relating the probability of failure with the auto-correlation distance is suggested in light of the analyses performed in this work and other results from the literature.

Influence of anisotropy on yielding and critical states of an unsaturated soil

A programme of controlled suction triaxial testing was performed on unsaturated and saturated samples of speswhite kaolin prepared by two different methods of compaction: isotropic and anisotropic. Tests involved probing stress paths, to investigate the initial forms of the yield surface for isotropically compacted and anisotropically compacted samples, and how the yield surface was altered by stress paths involving plastic straining. Tests also included shearing to failure, to investigate critical state conditions. The results suggest that critical states are unaffected by differences of initial anisotropy or by subsequent changes of anisotropy. Critical states can be represented by a series of parallel critical state lines in the deviator stress-mean net stress plane or by a single critical state line in the deviator stress-mean Bishop’s stress plane. Similarly, constant suction cross-sections of the yield surface can be represented by different forms of distorted ellipse in the deviator stress-mean net stress plane or the deviator stress-mean Bishop’s stress plane.

Probabilistic Study of Rainfall-Triggered Instabilities in Randomly Heterogeneous Unsaturated Finite Slopes

Water infiltration destabilises unsaturated soil slopes by reducing matric suction, which produces a decrease of material cohesion. If the porosity of the soil is spatially heterogeneous, a degree of uncertainty is added to the problem as water tends to follow preferential paths and produces an irregular spatial distribution of suction. This study employs the finite element method together with Monte Carlo simulations to quantify the effect of random porosity on the uncertainty of both the factor of safety and failure size of an unsaturated finite slope during and after a rainfall event. The random porosity is modelled using a univariate random field. Results show that, under partially saturated conditions, the random heterogeneity leads to a complex statistical variation of both factor of safety and failure size during the rainfall event. At any given time, the uncertainty about failure size is directly linked to the uncertainty about the position of the wetting front generated by infiltration. Interestingly, the statistical mean of the failed area is smallest when the mean of the factor of safety is lowest. In other words, the slope becomes more likely to fail, but the size of the failure mass tends to be limited. The study also investigates the sensitivity of failure uncertainty to external hydraulic parameters (i.e. initial water table depth, rainfall intensity) and internal soil parameters (i.e. permeability and water retention characteristics). In general, the sensitivity increases when the effect of these parameters on the spatial variation of suction is stronger.

Normal Compression Planar Surfaces for Specific Volume and Degree of Saturation

The glasgow coupled model (GCM) predicts unique unsaturated normal compression planar surfaces for specific volume and degree of saturation when the soil states are at the intersection of the mechanical (M) and wetting retention (WR) yield curves. More generally, the model predictions for any stress path where plastic volumetric strains (compression) and plastic increases of degree of saturation occur simultaneously correspond to stress states on these two planar surfaces. The existence and form of these planar surfaces has been only validated against isotropic compression data on unsaturated samples of compacted Speswhite kaolin, but their suitability to represent one-dimensional (1D) loading conditions remains unclear. This paper investigates this aspect by deriving equivalent expressions for 1D normal compression conditions and then comparing them against the experimental response of unsaturated samples of compacted Jossigny silt subjected to 1D compression paths at different constant values of matric suction.

Transport-Structural Modelling of Corrosion Induced Cracking

Transport of corrosion products into pores and cracks in concrete must be considered when predicting corrosion induced cracking in reinforced concrete structures, since this transport significantly delays the onset of cracking and spalling by reducing the amount radial displacement displacement imposed on the concrete at the steel/concrete interface. We aim to model this process by means of a transport-structural approach, whereby the transport part is driven by a pressure gradient generated by the volumetric expansion due to the transformation of steel into corrosion products. This pressure driven transport was introduced in an analytical axisymmetric thickwalled cylinder model and a numerical network approach. The influence of cracking and permeability on corrosion induced cracking process with increasing inner displacement is investigated with these two approaches.

Modelling the Time Dependence of Transport Properties of Porous Materials

A network model for the time dependence of transport properties of porous materials is proposed. A statistical volume element (SVE) of a capillary pore network of porous materials is idealised by a discrete lattice model consisting of spheres and pipes representing pores and throats, respectively, which can be filled with either liquid water or gas. The time dependence of the transport properties is modelled by incrementally decreasing the radius of the water filled pipes and spheres accounting for the evolution of the microstructure. The microstructural model is applied to investigate the change of macroscopic transport properties due to microstructural changes.