Dr Jubert Pineda

In this paper, a large number of one-dimensional tests, including constant water content compression and soaking under constant stress, are conducted. The microstructure evolution and deformation mechanism of the compacted loess under loading and wetting conditions are investigated with mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM) analysis. Experimental results show that, as the saturation of compacted loess increases at a constant moisture content, it will develop into a saturated consolidation process under further compression. At the microscopic level, the compression of the unsaturated compacted loess results from the collapse reduction of its macrospores, while the distribution of microspores is unaffected in compression. During increasing wetting under the constant vertical stress, the wetting deformation of compacted loess shows a trend of increasing and then decreasing with the increase of vertical stress, and the maximum wetting strain occurs near the compaction stress. Under wetting conditions, the bonds between particles and aggregations are weakened, and the particles and agglomerates collapse and slip, resulting in the reduction of macrospores and the increase of microspores. Also, the soil structure tends to be more uniform and stable after wetting. The creep of compacted loess is caused by the further slippage of particles under constant load and further compression of macrospores. In addition, the settlement law of compacted loess fill is summarized from the construction and post-construction period according to testing results.

An isotach elastoplastic constitutive model devised by Yang et al. (2016) and referred to as the Hunter Clay (HC) model attempts to capture a number of key behaviours of soft soils within a critical state framework, namely destructuration, fabric anisotropy and rate dependency, the latter often manifesting in creep settlement. Finite element implementation of the HC model is a useful means by which its application to practical problems can be facilitated. However, there are a number of significant challenges associated with the translation of isotach elastoplastic models into a finite element setting. In this paper, a detailed discussion of these challenges is undertaken and a new finite element implementation of the HC model is subsequently developed. This includes sophisticated numerical integration algorithms which employ automatic time substepping for solution of the governing finite element equations. The ability of the implemented HC model to predict the mechanical behaviour of soft soils under 1D compression is investigated via simulation of laboratory tests carried out on Ballina clay by Pineda et al. (2016) and Parkinson (2018).

This paper describes an experimental study aimed at evaluating the influence of soil microstructure on air permeability in compacted clay. Air permeability measurements, estimated using the gas pressure decay method, were carried out for a wide range of compaction states. The evolution of the air permeability during wetting and drying paths was also evaluated. The experimental results show that, for an increase in the as-compacted degree of saturation, air permeability may either increase or decrease depending on the as-compacted dry density. Air permeability increases with increasing the degree of saturation in loose specimens, whereas the opposite trend is observed for dense specimens. Microstructural analysis, carried out using mercury intrusion porosimetry (MIP) tests, shows a strong dependency of the air permeability on the as-compacted soil microstructure. This behaviour is also noticed in specimens that experienced a large variation in the degree of saturation during wetting and drying. Microstructural data indicate that air permeability is mainly controlled by large pores that display high connectivity. The degree of saturation plays a dual role in soil microstructure which, in turn, affects the air permeability. Denser specimens (dry density = 1·5 Mg/m3) show a reduction in keff due to the expansion of the clay aggregates with increasing the as-compacted degree of saturation. The increase in the as-compacted degree of saturation in loose samples (dry density =1·3 Mg/m3) produces an enhancement in the proportion of macro pores, thus increasing keff, as a consequence of modifications in the pore size distribution. A threshold value has been identified, above which further increase in degree of saturation causes a reduction in the proportion of macro pores, and therefore in keff. A new proposal for estimating the air permeability is proposed in this paper based on the determination of a pore size parameter obtained from MIP data. The proposed approach seems capable of describing the evolution of air permeability for the whole spectrum of compaction states, including specimens subjected to wetting and drying paths.

This paper presents the results of an experimental study aimed at evaluating the effects of soil microstructure on volume change and wetting-induced collapse of a compacted loess from Xi'an, China. One-dimensional (1D) compression tests are combined with Mercury Intrusion Porosimetry (MIP) tests and Scanning Electron Microscopy (SEM) analysis to examine the collapse behaviour for different compaction states and applied stresses. A phenomenon of partial collapse occurs upon full saturation (wetting), whose magnitude depends on the as-compacted suction, the as-compacted microstructure and the stress level applied. Following partial collapse upon full saturation some of the initially meta-stable microstructure of the compacted soil is preserved which leads to higher compressibility in subsequent loading stages. Additional collapse tests carried out under isotropic conditions show that partial collapse upon full saturation takes place only under zero-lateral deformation (1D) conditions due to the residual (¿locked-in¿) horizontal stresses maintained in the sample after compaction. Microstructural results and a simple macroscopic model for soil compaction are used to qualitatively explain the phenomenon of partial collapse observed in compacted loess.

This note presents a novel approach for controlling the degree of saturation during one-dimensional compression of unsaturated soils. This technique offers a simple and versatile way to study the hydromechanical response of unsaturated soils as well as the unsaturated¿saturated soil transition. By using a multi-stage approach, the same specimen can be used to evaluate the compressibility of unsaturated specimens at different degrees of saturation, which may reduce the long testing periods commonly required for unsaturated soil testing. The experimental results described in this paper show that the proposed technique is capable of controlling the degree of saturation within reasonable limits and provides an interesting approach to analyse the coupled hydraulic and mechanical behaviour of unsaturated soils.

The behaviour of soil, and in particular compacted clay fill, can have significant implications on the safe and reliable operation of man-made infrastructure. The mechanical behaviour of soil (e.g. volume change and shear strength) is widely recognised as being associated with the microstructural arrangement (fabric/structure). In the case of high plasticity clays, despite the large amount of research carried out, soil microstructure and its evolution along mechanical and hydraulic paths are still not well understood. This makes incorporation of microstructural analysis difficult in engineering practice and highlights the need for further research. A comprehensive microstructural analysis of Maryland clay, a high plasticity residual soil, based on mercury intrusion porosimetry tests, is presented in this paper. Experimental results obtained from undisturbed, reconstituted and compacted specimens subjected to different hydraulic and mechanical paths are described. As with mechanical investigations, the reconstituted state is proposed to be used routinely as a reference state for comparison of undisturbed and compacted soil. The microstructural evolution of the compacted clay, prepared on the wet side of standard Proctor optimum water content, with an initially high void ratio, is examined along the main drying path. Importantly, a monotonic suction increase from the as-compacted state is shown to have negligible effect on the distribution of macro-pores. However, a new insight is provided based on the evolution of the dominant micro-pore entrance diameter which is shown to reduce with increased suction. This micro-pore entrance diameter is shown to correspond with the theoretical suction back-calculated from a simple capillary tube model, up to a limit. It is observed that, under oedometric conditions, the as-compacted microstructure is erased during saturation (soaking) and resembles the reconstituted microstructure. For this particular material and preparation conditions, it is demonstrated that a bimodal microstructure is not recovered on drying from a saturated state.

This paper presents a study on the amplification of horizontal soil stresses during flat dilatometer test (DMT) blade penetration based on three-dimensional total and effective stress numerical analyses, while considering stress-flow coupling and large deformations. The focus here is on saturated clays, and the effect of soil stress history on the horizontal stress index is discussed in detail. The obtained results appear to be in good agreement with published and new field data, leading to the proposal of two new expressions for estimating the overconsolidation ratio and the earth pressure coefficient at rest directly from flat dilatometer tests in estuarine clays.

The paper presents the results of a comprehensive experimental programme carried out to study the effects of relative humidity cycling on the degradation of argillaceous rocks. Lilla claystone, a low-porosity Tertiary rock, was used for this purpose. Four aspects were analysed: (a) the influence of the number of relative humidity cycles; (b) the amplitude of relative humidity cycles; (c) the stress level; and (d) the effects of using liquid water or vapour during wetting paths. The application of relative humidity cycles induced a progressive degradation of the rock in terms of accumulative irreversible volumetric swelling, irreversible reduction in rock stiffness, and tensile strength. The irreversible expansion increased with the amplitude of the relative humidity change. However, it reduced with increase of the confining pressure. This irreversible behaviour accelerated when liquid water was used during the wetting paths. Microstructural analysis has shown that the degradation pattern of Lilla claystone was associated mainly with fissuring, as a consequence of non-uniform deformations of the clayey matrix. This phenomenon leads to the opening of fissures at the weaker interfaces of the clayey matrix with detrital, non-active minerals. A damage law derived in terms of the accumulated volumetric irreversible strain has been proposed to represent the progressive loss in volumetric and shear stiffness as well as the tensile strength.

This paper discusses some topics related to the sampling and laboratory testing currently ongoing on Ballina clay (NSW). Emphasis is made on particular aspects of natural soft clays frequently neglected in laboratory procedures that may affect its mechanical response. Preliminary results are shown to highlight the importance of sample disturbance, salinity and rate effects in Ballina clay. Ongoing research as well as future activities are discussed in the last section of the paper. Implications for the current state of practice as well as the development of new constitutive models for soft clays are highlighted.

© 2014, Emerald Group Publishing Ltd. All rights reserved. This note explores the influence of environmental effects, as those induced by cyclic changes in relative humidity, on the degradation of the shear strength parameters in Lilla claystone, a lowporosity clayey rock from northern Spain. The results of a comprehensive experimental programme, combining long-term relative humidity cycling tests with saturated direct shear tests, are described. A continuous monitoring of the evolution of volumetric strain during the previous relative humidity cycling is used to evaluate the swelling behaviour of the rock. Both undisturbed and degraded specimens are subjected to shearing at saturated conditions to determine the peak and post-rupture shear strength envelopes. The effects on rock brittleness and dilation angle are also analysed. Shear strength shows a strong dependence on the history of relative humidity cycling. In particular, the evolution of the peak shear strength parameters (c' and ¿') seem to be related to the accumulated irreversible strains developed during each cycle. A damage law, recently proposed by the authors, is used to represent the progressive degradation of the shear strength parameters as a function of the accumulated irreversible strains.

Mediterranean deltaic environments typically produce soil deposits in which silts dominate, granular intercalations are frequent and the mineralogy includes an important carbonate fraction. One question still open is what sampling methods are most appropriate to recover undisturbed samples from these deposits and how best to evaluate disturbance on samples recovered from them. In this paper we present results from a testing campaign on silty deposits in which several sampling methods (Sherbrooke, Osterberg, Shelby) were used to recover sandy and silty clay samples from a Holocene lagoon deposit. Sample quality is established on oedometric specimens using normalized void ratio change on reconsolidation, comparisons of laboratory and field shear wave velocity and the ratio of suction to vertical « In situ » effective stress. Normalized void ratio change and shear wave based criteria are in agreement, and more so if the laboratory measurements are taken just after resaturation. The ratio of suction to normalised vertical effective stress shows no clear relation with normalized void ratio change.

CAT (Computed Axial Tomography) has been sometimes used to obtain density maps of small core specimens of soft soil. Here it is applied to do the same with a Sherbrooke-type block sample recovered at depth from a silt layer. Sherbrooke block samples are considered as the one more closely related to the material 'in situ', because the sampling method excludes practically any other alteration process apart from deviatoric stress relief. Sherbrooke samples are large typically 25 cm in diameter and 35 in height. Because of this large specimen size the standard medical scanner used for the CAT test presented several artifacts (noise, rings, outliers) that made difficult the quantitative interpretation of the resulting images. The procedures employed to remove those artifacts from the images and obtain a map of density of the block sample are described here. Validation with independent laboratory measurements of density is shown to result in a good agreement. © 2015 Taylor & Francis Group.

The plastic anisotropy of reconstituted soft soils is described from the compression behaviour observed during radial loading paths in stress space. A unique relationship is established between the orientation of the yield surface and the corresponding normal compression line, which indicates that a stabilized fabric is maintained under continuous loading along radial stress paths. The equilibrium orientation angle of the plastic potential surface is obtained explicitly. A new rotational hardening law is proposed by considering the dependence of the evolution of anisotropic fabric with the current stress condition and plastic strain. An elastoplastic constitutive model for anisotropic soil is formulated within the framework of Critical State Soil Mechanics. Validation with experimental data and predictions from other models demonstrate the feasibility of the basic concept and the capacity of the proposed new model. © 2014 American Society of Civil Engineers.

Small-strain soil stiffness properties such as shear wave velocity, shear modulus, and material damping, are key subsoil parameters for an adequate analysis and design of unsaturated earth structures subject to static and non-static loading. Most conventional geotechnical testing techniques, however, are not able to capture this small-strain behavior and, hence, vastly underestimate the true soil stiffness, mainly due to inaccuracies in small-strain measurements. In the United States, a great deal of research efforts has been devoted to field and laboratory based measurements of soil suction, assessments of soil-water retention properties, and analyses of swell-collapse behavior. However, very few efforts have been focused on small-strain response of unsaturated soils and their dynamic characterization at small strains. This paper introduces a suction-controlled, proximitor-based resonant column device featuring a PCP-15U pressure control panel that allows for the implementation of the axis-translation technique via the independent control of pore-air and pore-water pressures in the specimen. A preliminary series of resonant column tests were conducted on statically compacted samples of silty sand for a range of constant suction states between 50 kPa and 400 kPa, at different net confining pressures. Results show the critical role of matric suction on the soil's small-strain response. The apparatus also features a full set of self-contained bender elements for simultaneous testing under both techniques. However, preliminary results from an on-going bender element testing program are beyond the scope of the present work. © 2011 Taylor & Francis Group, London.

The evolution of bar and shear wave velocities during a drying wetting cycle on a claystone is presented. There is a clear relationship with total suction, established by means of the material water retention curve. The stiffening effect of drying decreases with sample damage. The multiaxial bender element technique is firstly applied to rocklike materials with partial success.

A series of unconfined undrained triaxial tests were performed, on compacted and reconstituted samples of clayey soils, at different degrees of saturation. Pore water pressure changes were recorded. Stress-strain-suction relationships were studied and analyzed. The soil water retention curve (SWRC) or any scanning curve exhibit an S-inverted shape. At low suctions the curve has a small gradient which increases and tends to be constant at intermediate suctions. Two different stress-strain-suction behaviour patterns were identified depending on the initial position (before shearing) of the soil sample on the SWRC. Samples initially located in the zone of small gradient, of the SWRC, exhibited a stress-strain-suction behaviour which is similar to that of normally consolidated samples (i.e. strain hardening), whereas samples initially in the higher gradient zone, of the SWRC, exhibited a similar behaviour to that of over-consolidated samples (i.e. dilatancy occurred). On shearing, the behaviour of the soil at peak conditions was not dependant of the initial fabric. There was no influence of the suction history of the sample before shearing. Copyright ASCE 2006.