Stephen Fityus

Numerical Modeling of Pore Pressure Influence on Fracture Evolution in Brittle Heterogeneous Rocks

Rock is a heterogeneous geological material. When rock is subjected to internal hydraulic pressure and external mechanical loading, the fluid flow properties will be altered by closing, opening, or other interaction of pre-existing weaknesses or by induced new fractures. Meanwhile, the pore pressure can influence the fracture behavior on both a local and global scale. A finite element model that can consider the coupled effects of seepage, damage and stress field in heterogeneous rock is described. First, two series of numerical tests in relatively homogeneous and heterogeneous rocks were performed to investigate the influence of pore pressure magnitude and gradient on initiation and propagation of tensile fractures. Second, to examine the initiation of hydraulic fractures and their subsequent propagation, a series of numerical simulations of the behavior of two injection holes inside a saturated rock mass are carried out. The rock is subjected to different initial in situ stress ratios and to an internal injection (pore) pressure at the two injection holes. Numerically, simulated results indicate that tensile fracture is strongly influenced by both pore pressure magnitude and pore pressure gradient. In addition, the heterogeneity of rock, the initial in situ stress ratio (K), the distance between two injection holes, and the difference of the pore pressure in the two injection holes all play important roles in the initiation and propagation of hydraulic fractures. At relatively close spacing and when the two principal stresses are of similar magnitude, the proximity of adjacent injection holes can cause fracturing to occur in a direction perpendicular to the maximum principal stress.

The shrink swell test

Although the assessment of the expansive potential of clay soils has been the subject of active research for the past 40 years, its treatment in routine geotechnical practice around the world remains inconsistent. This paper describes the shrink swell test, which is used routinely in Australian geotechnical practice as the principal method for the experimental assessment of the expansive potential of clay soils. The test procedure and its underlying assumptions are described and discussed in the context of the historical development of the test and its routine application. It is shown that the shrink swell test is a simple and economical means of assessing soil expansiveness, which is achieved largely through the adoption of several simplifying assumptions that effectively circumvent the measurement of soil suction. The significance of these assumptions is discussed, and it is concluded that the shrink swell test can be conveniently and reliably employed to guide the routine design of foundations in expansive soils.

Direct Shear Testing of a Marginal Material Using a Large Shear Box

In Australia, Q181C Test Method of Direct Shear Testing to estimate the Effective Angle of Internal Friction at Constant Volume Conditions for Granular (Coarse Grained) Materials is commonly applied to assess the suitability of backfills for reinforced earth walls. This paper presents a comparison between the results of three series of shear box tests on a typical ripped rock material, of marginal quality, that might be considered as a possible backfill material for a reinforced earth wall. Tests performed used 300 mm and 60 mm shear boxes, soil samples prepared to sub-19 mm and sub-4.75 mm sizes, and a range of shearing rates. The effect of pre-testing samples was also considered. The results show that accurate effective friction parameter measurements for coarse grained, granular backfill soils require the use of fresh soil specimens for each shearing test; the use of a large shear box that can accommodate soils with relatively large particles; and careful selection of shearing rates so that shearing takes place under drained conditions. If the above requirements are compromised, the measured effective friction angle is likely to be a significant under-estimate of the true effective friction angle, increasing the likelihood of the material failing to meet prescribed material quality standards.

The effect of a gap between the access tube and the soil during neutron probe measurements

The neutron probe is a tool employed for the measurement of water content in a soil mass. The presence of a gap between the soil and the neutron probe access tube, filled with either air or water, inevitably introduces a systematic error in neutron probe readings. In this study, experimental investigations and numerical analyses were carried out to evaluate the effects of this gap on neutron probe calibration. The numerical model was developed based on the multigroup neutron diffusion equations and the finite element method. The experiments were conducted in a heavy clay soil. The results show that an air gap of 2.5-30 mm between the soil and a 50-mm-diameter aluminium tube could lead to an underestimation of soil water content by 5-45%, but significant underestimation was apparent for air gaps <10 mm. It is also found that the neutron count is significantly overestimated if the gap around the access tube is filled with water rather than air, but this effect is most significant for larger gaps. The results of this research clearly indicate that a gap between the neutron probe access tube and the soil profile should be avoided during field installation, and that if a gap between the access tube and soil develops during service, a systematic error will be introduced into measurements.

The indirect estimation of saturated hydraulic conductivity of soils, using measurements of gas permeability. I. Laboratory testing with dry granular soils

A comprehensive knowledge of soil hydraulic conductivity is essential when modelling the distribution of soil moisture within soil profiles and across catchments. The high spatial variability of soil hydraulic conductivity, however, necessitates the taking of many in situ measurements, which are costly, time-consuming, and labour-intensive. This paper presents an improved method for indirectly determining the saturated hydraulic conductivity of granular materials via an in situ gas flow technique. The apparatus employed consists of a cylindrical tube which is embedded in the soil to a prescribed depth. Nitrogen at a range of pressures was supplied to the tube and allowed to escape by permeating through the soil. A 3-dimensional, axisymmetric, steady-state, finite element flow model was then used to determine the value of the soil intrinsic gas permeability which produces the best fit to the pressure–air flow data. Saturated hydraulic conductivities estimated from the application of the gas flow technique to 5 granular soils covering a wide range of permeabilities were in close agreement with values determined using a conventional permeameter. The results of this preliminary study demonstrate the potential of this approach to the indirect determination of saturated hydraulic conductivity based on measurement of gas flow rates in granular and structured soils.

The Grading Entropy-based Criteria for Structural Stability of Granular Materials and Filters

This paper deals with three grading entropy-based rules that describe different soil structure stability phenomena: an internal stability rule, a filtering rule and a segregation rule. These rules are elaborated on the basis of a large amount of laboratory testing and from existing knowledge in the field. Use is made of the theory of grading entropy to derive parameters which incorporate all of the information of the grading curve into a pair of entropy-based parameters that allow soils with common behaviours to be grouped into domains on an entropy diagram. Applications of the derived entropy-based rules are presented by examining the reason of a dam failure, by testing against the existing filter rules from the literature, and by giving some examples for the design of non-segregating grading curves (discrete particle size distributions by dry weight). A physical basis for the internal stability rule is established, wherein the higher values of base

Some Comments on the Entropy-Based Criteria for Piping

This paper is an extension of previous work which characterises soil behaviours using the grading entropy diagram. The present work looks at the piping process in granular soils, by considering some new data from flood-protection dikes. The piping process is divided into three parts here: particle movement at the micro scale to segregate free water; sand boil development (which is the initiation of the pipe), and pipe growth. In the first part of the process, which occurs during the rising flood, the increase in shear stress along the dike base may cause segregation of water into micro pipes if the subsoil in the dike base is relatively loose. This occurs at the maximum dike base shear stress level (ratio of shear stress and strength) zone which is close to the toe. In the second part of the process, the shear strain increment causes a sudden, asymmetric slide and cracking of the dike leading to the localized excess pore pressure, liquefaction and the formation of a sand boil. In the third part of the process, the soil erosion initiated through the sand boil continues, and the pipe grows. The piping in the Hungarian dikes often occurs in a two-layer system; where the base layer is coarser with higher permeability and the cover layer is finer with lower permeability. The new data presented here show that the soils ejected from the sand boils are generally silty sands and sands, which are prone to both erosion (on the basis of the entropy criterion) and liquefaction. They originate from the cover layer which is basically identical to the soil used in the Dutch backward erosion experiments.

Case Studies and Benchmark Examples for the Use of Grading Entropy in Geotechnics

The grading entropy concept can be adapted to the field of geotechnics, to establish criteria for phenomena such as particle packing, particle migration and filtering, through a quantified expression of the order/disorder in the grain size distribution, in terms of two entropy-based parameters. In this paper, the grading entropy theory is applied in some geotechnical case studies, which serve as benchmark examples to illustrate its application to the characterisation of piping, softening and dispersive soils, and to filtering problems in the context of a leachate collection system for a landfill site. Further, since unstable cohesive (dispersive) soils are generally improved by lime, the effect of lime addition is also considered, on the basis of some measurements and a further application of the grading entropy concept, which allows evolutions in the entropy of a soil to be considered as its grading is modified. The examples described support the hypothesis that the potential for soil erosion and particle migration can be reliably identified using grading entropy parameters derived from grading curve data, and applied through an established soil structure stability criteria and a filtering rule. It is shown that lime modification is not necessarily helpful in stabilizing against particle migration.

Water Content Measurement in Expansive Soils Using the Neutron Probe

Capacitance-type methods for measuring soil water content are known to be unreliable in expansive soils, as cracking disrupts the intimate contact between the soil and the measuring device. The neutron probe, which infers water content from the thermalisation of a cloud of neutrons, is potentially less affected by cracking. The effect of cracking on neutron probe measurements was investigated by a series of numerical simulations using an axisymmetric finite element model based on seven-group neutron-diffusion theory. The simulations employed a consistent soil cracking model based on Maryland clay, in which crack volumes are determined from the changes in void ratio in the shrinking bulk soil. The results show that the presence of cracks in a clay soil affects the inferred water content and that measurements affected by air-filled cracking under-predict not only the water content in the uncracked soil peds but also the average water content in the larger cracked soil mass. The reason for this under-prediction is understood by considering the spatial distribution of the thermalised neutrons in the cracked and uncracked soils. The fast neutrons emitted from the source are seen to diffuse preferentially along air-filled cracks, traveling a large distance from the detector before they become thermalised, thus reducing their likelihood of being back-scattered to the detector where they can be counted. The proximity of the first crack to the probe in the ground also affects the measurement. Water-filled cracks are seen to have the opposite (but lesser) effect to air-filled cracks. A comparison of a simple uniform width crack model to a more realistic model in which crack width varies with changing water content shows that the model is sensitive to crack distribution and that the linear calibration expressions that are typically employed for neutron probes are likely to be unreliable in cracked clay soils.

The Influence of Shape on the Inherent Rolling Potential of Loose Rocks

The likelihood that rolling of a rock will be initiated and/or sustained on a slope depends on many factors related to the characteristics of the block and the slope. However, all other things being equal, some solid shapes have a greater potential to roll on a slope than others. This paper describes the results of a systematic laboratory study to determine how shape affects the ease with which rolling of ball-like blocks can be initiated, and its likelihood of being sustained. A simple scheme is presented to group basic shapes with similar rolling tendencies. Through systematic tests with polyhedral blocks on a frictional ramp with a range of inclinations, different basic ball-shaped forms are compared in terms of the ease with which rolling can be initiated, and the likelihood that it will be sustained. The results show that “ball” shapes (with principal dimensions of roughly similar size) are more prone to rolling but that even between shapes within this group, such as cubes and octahedra, the tendency to roll is strongly influenced by other factors including the number of faces. The importance of the starting position on the initiation of rolling is also demonstrated and quantified.