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The feasibility of Carrillo's method is confirmed for consolidation problems of unsaturated soils under instantaneous loading in this study. In the plane strain and axial symmetry consolidation models, the coupled governing equations expressed by the excess pore-air and pore-water pressures are transformed into the equivalent uncoupled equations expressed by two functions of excess pore pressure using a decoupled method. Then, two rigorous theorems are used to prove that Carrillo's method is suitable for unsaturated soil consolidation under the plane strain and axial symmetry conditions. Finally, the mathematical formulas by Carrillo's method for unsaturated soil consolidation under plane strain and axisymmetric conditions are presented, and are verified against those of the existing analytical solutions. It turns out that the combinations of existing solutions to the conditions of a single direction flow by the proposed formulas can directly furnish the solutions to unsaturated soil consolidation with both horizontal and vertical flows. [ABSTRACT FROM AUTHOR]

Silty alluvial soils are highly compressible. Deep soil mixing is being increasingly used for remediating such ground conditions, which produces artificially cemented soil columns that gain strength with curing. This study investigated the one-dimensional compression and shear stiffness degradation behaviour of an alluvium in its reconstituted, undisturbed and artificially cemented states. The binder used to stabilise the soil was a low-carbon alkali-activated blast-furnace slag. Oedometer and triaxial data indicated that the nature of the soil structure evolved from being chiefly meta-stable when undisturbed to dominantly stable in its cemented state after 28 days of curing. A new framework has been presented to quantify continuous changes in structure within the alluvium during one-dimensional compression in its undisturbed and cemented states, with respect to its intrinsic properties. This better captured structure degradation during earlier stages of compression compared with previous frameworks and provided insights into defining the limits of meta-stable and stable components of structure within materials of higher strength. A new formulation has been developed for predicting shear stiffness degradation of samples with strain under triaxial conditions. This showed efficiency and good performance in modelling experimental data and was successfully used to quantify initial structure and degradation of structure within the undisturbed and cemented alluvium. [ABSTRACT FROM AUTHOR]

Vibroflotation is an established ground improvement technique for granular materials. It densifies loose sands by means of shear and compressive deformation processes imparted by the horizontal vibrations of a vibrator probe at the required soil depth. The field experience and empirical relations used for the design of vibroflotation processes prompt further study and analysis. Numerical simulations of the method help provide insight into the process. A coupled Eulerian–Lagrangian framework was used to model the compaction process. The compaction process was modelled based on the pilgrim step method for a depth range from 25 m to 10 m. A hypoplastic constitutive model was used to characterise the stress–strain behaviour of the fine sand. Numerical results were validated against in-field cone penetration test measurements made before and after the compaction of fine sand in east Germany. The validation was followed by a parametric study in which the following were evaluated in terms of their effects on the extent and degree of compaction: frequency, relative density, shape of vibrator, spacing of compaction points, nature of compaction process and type of sand. [ABSTRACT FROM AUTHOR]

It has been established that preloading of clay is an effective means to reduce creep of the soil under working load conditions. After an initial rebound during unloading, a long-term viscous response occurs involving both creep and swelling. A new model is proposed that allows modelling of the creep and swelling that occurs sequentially under constant load or relaxation. The model is verified with 58 published unloading tests under constant load and further evaluated with new one-dimensional tests with unloading stages under constant load and relaxation conditions. The model applies four viscoplastic parameters that are fit to the data by use of a Markov chain Monte Carlo algorithm to evaluate the posterior probability distribution of the parameters. The reinterpreted test results show that viscous swelling plays a prominent role in the unloading response and affects the creep strain rate significantly. The experimental data do not support the modelling of swelling as an isotache process but rather as a transient process triggered by unloading. Creep, on the other hand, can be modelled as an isotache response; yet after unloading the isotaches are distorted, leading to larger creep strain rates than suggested by non-linear fixed isotache models. The newly proposed model allows description of the one-dimensional unloading response under constant load and relaxation that vary as a function of the reference overconsolidation ratio relative to a reference isotache. [ABSTRACT FROM AUTHOR]

An attractive approach to reduce the carbon dioxide footprint of deep soil mixing (DSM) is to replace Portland cement-based binders by geopolymers based on metakaolin. Safe design requires a good understanding of the mechanical and hydraulic properties of the improved ground, but very little is known about metakaolin–soil mixtures. For instance, shrinkage during curing is a significant issue for metakaolin-based concretes but has not been previously studied in soilcretes. In this work the permeability and strength of sand- and silty sand-based metakaolin soilcretes are studied under different curing conditions. The development of microcracks induced by geopolymer shrinkage is confirmed through a microstructural study using mercury intrusion porosimetry, scanning electron microscopy and X-ray computed tomography. The influence of microporosity and binder filling on permeability and strength is clarified adapting well-established soil models. A modified Kozeny–Carman formulation is proposed for permeability. A mixture ratio model is calibrated to represent strength. In general, the metakaolin-stabilised materials present excellent mechanical and hydraulic properties, although these are very sensitive to curing conditions. [ABSTRACT FROM AUTHOR]

A research study was designed and conducted to evaluate the applicability of using crystalline silica-rich material as a co-additive with calcium-based stabilisers to treat sulfate-rich expansive soils. Extensive laboratory tests were conducted on lime-treated sulfate-rich soils, with and without crystalline silica admixture, to evaluate the improvements in engineering properties, including swell strain, unconfined compressive strength and resilient moduli properties. The influence of co-additive dosage, curing period and duration of moisture exposure was studied to gain insights into the behaviour of treated soils. Supplementary mineralogical and microstructural studies including X-ray diffraction, scanning electron microscope imaging and differential scanning calorimetry studies were performed on the treated soils to detect the formation of ettringite mineral and cementitious gel compounds. Test results indicated that the use of crystalline silica facilitated enhancement of both the strength before and after capillary soaking, and the resilient moduli properties, while suppressing ettringite mineral formation and associated free swell strains in the treated soils. Mineralogical and engineering studies showed the dominance of pozzolanic activity between lime and crystalline silica over ettringite formation and hence contributed to the effective stabilisation of sulfate-rich soils. Overall, the paper provides a comprehensive understanding of the role of crystalline silica admixture for mitigating deleterious sulfate heaving in lime-treated soils. [ABSTRACT FROM AUTHOR]

This paper presents a laboratory investigation into passive stabilisation of liquefiable sands by means of colloidal silica (CS). In order to examine the improvement of the mechanical behaviour of liquefiable sands stabilised with CS, an extensive laboratory testing programme comprising unconfined compressive strength and Brazilian tensile strength tests, as well as undrained monotonic and cyclic triaxial tests, was performed on a clean quartz sand treated with CS. The test results are compared to the corresponding results for the untreated sand. The effects of CS concentration, curing time, density and confining stress on the monotonic and cyclic response of the treated sand are examined, among others. It is shown that stabilisation of the tested sand with CS significantly improves both the undrained monotonic and cyclic strength. The influence of CS concentration on the monotonic response is reduced with increasing mean effective stress. Cyclic resistance, however, is shown to be practically unaffected by CS concentration. Furthermore, cyclic straining at 5 to 10% of double-amplitude axial strain does not influence the undrained shear strength of the stabilised sand. [ABSTRACT FROM AUTHOR]

This paper presents research initiated with the aim of assessing the feasibility of biocementing peat fens soil, a problematic soil underlying many of the embankments of the East Anglia railway network (UK). Biocementation of soil is an emerging, novel, ground improvement technique. It has recently attracted the interest of researchers worldwide because it has been proposed as potentially environmentally superior to chemical grouts and other common soil stabilisers – for example, cement or lime (linked to high carbon dioxide emissions). In this study, the authors screened and isolated non-pathogenic indigenous ureolytic microbial candidates with potential for biocementation from samples originating from peat fens in East Anglia, UK. Four strains were selected as the most suitable candidates, based on their growth rate and their viability in a wide range of temperatures, pH and soil moisture contents corresponding to typical seasonal field conditions. After a number of unconfined compressive strength (UCS) tests, one strain (Bacillus licheniformis) was selected as the most promising for this soil treatment and used for further study. Two different methods of implementation of the treatments were considered: namely, pressure flow soil column and electrokinetic injection. The UCS results supported by calcium carbonate measurements proved that biocementation did occur for both implementation methods and for a number of treatment combinations. Ongoing work on optimisation of treatments and implementation methods is being carried out towards the upscaling of the techniques for in situ implementation, which is planned for the next stage of the research. [ABSTRACT FROM AUTHOR]

Water infiltration into granular soils and the associated pore water pressure increase and reduction in shear strength can trigger landslides, instability of vertical cuts and failure of retaining walls. Water-repellent soils can reduce infiltration to maintain soil suction. Recent research has demonstrated the creation of synthetic water-repellent soils using chemical methods. This paper investigates a biological treatment for creating water-repellent sand by way of the growth of the fungus Pleurotus ostreatus. Water repellency was assessed using: (a) the water drop penetration test; (b) the molarity of ethanol drop test; and (c) the modified sessile drop method with contact angle (θ) determination by way of image analysis. Fungal-induced water repellency was found to be 'extreme' (θ > 110°) up to 4 weeks and 'severe' (θ > 105°) up to 12 weeks, even with no further supply of moisture or nutrients. A water-repellent layer was formed and maintained in saturated conditions, which is difficult to achieve using chemical methods. [ABSTRACT FROM AUTHOR]

Waste glass is a solid residue usually available near urban centres, where it is discarded after being used as a container for one of several products. Carbide lime (CL) is a by-product of the manufacture of acetylene gas. The present research evaluates the potential of combining these two wastes as a possible hydraulic cement to enhance the behaviour of sands. Pozzolanic reactions occur between silica in its amorphous phases (found in GG) and Ca++ (present in CL) in an alkaline environment. The effect of the amount of GG, the CL content and the porosity of the blend on the strength, stiffness and durability of compacted sand–GG–CL mixes is quantified for three distinct sands. An original parameter, termed the porosity/binder index (η/Biv), can be used to normalise the behaviour of the unconfined compressive strength (qu), the shear modulus at small strains (G0) and the accumulated loss of mass (ALM) of the compacted mixes, considering GG + CL as the binder. Results have shown unique normalised tendencies for qu, G0 and ALM with η/Biv for the three studied compacted sand–GG–CL blends, even though each sand has a distinct grain size distribution. [ABSTRACT FROM AUTHOR]

Spatially variable cement-treated ground can be considered, in design, as a uniform medium with appropriately reduced strength. Various strength reduction factors have been proposed for cement-treated ground under different loading conditions. However, the Tresca and Mohr–Coulomb models, which are widely used to represent the behaviour of the improved soil in geotechnical design, do not replicate the strain-softening behaviour and therefore may not lead to sufficiently cautious reduction factors. This study examines the effect of constitutive models on the strength reduction factor to account for spatial variability for tunnels with improved soil surrounds. The improved ground was simulated using three different constitutive models. The results show that the Mohr–Coulomb model gives the lowest critical stress release ratio, as the yield stress decreases much faster with effective stress than with the Tresca and cohesive Cam Clay (C3) models. However, the most conservative values of strength reduction factor needed to account for spatial variability are generally taken by either the C3 or Mohr–Coulomb model, depending on the drainage condition and geometric configuration. Hence using the highest strength reduction factor for each case allows spatial variability to be conservatively considered for cases where the Tresca or Mohr–Coulomb model is used. [ABSTRACT FROM AUTHOR]

Sands are naturally hydrophilic granular materials, yet, rendering them hydrophobic could lend them to a wide range of geotechnical applications. This study describes a powder-coating procedure performed after chemically modifying the surfaces of coarse, medium and fine sands and examines its effect on their hydrophobicity. The purpose is to render these granular materials more hydrophobic than what is conventionally achieved by chemical methods using a simple technique. The procedure consists of first silanising both the sands and silica powder at a similar concentration by means of an organosilane to modify their surface chemistry, then the silica powder is adhered to the sands at a mass mixing ratio to alter their hydrophobicity. Irrespective of the concentrations and mixing ratios, the powder-coating procedure enhances the hydrophobicity of sands in comparison to the sole use of the chemical method. Changes in the morphology of the sand grains, such as their particle size, particle shape and surface roughness, resulting from the powder-coating procedure are examined by means of dynamic image analysis, profilometry and scanning electron microscopy. The effects of surface chemistry, surface roughness and air on the hydrophobicity of the sands are discussed based on theoretical wetting models to analyse the experimental results. [ABSTRACT FROM AUTHOR]

Preloading with a surcharge is today commonly used together with prefabricated vertical drains for embankment construction on clayey soil to accelerate primary consolidation and increase strength. Because of considerable uncertainty related mainly to the rate of consolidation, there is a need to account for this in the vertical drain and surcharge design to ensure quality in the embankment construction. Addressing this issue, the paper presents a novel probabilistic design methodology that is compatible with the observational method. The procedure evaluates the suitable surcharge load to be used in combination with a vertical drain design in order to ensure that the established design criteria are satisfied with acceptable probability. [ABSTRACT FROM AUTHOR]

Quality control of jet grouting at large depth is a challenging task. An in-hole electrical resistivity method was recently shown to have high potential for assessing the diameter and integrity of jet grouting. To facilitate visualised inspection and practical use of the in-hole resistivity method for jet grouting, this study introduces a novel and practical tomographic approach. An analogous mapping approach from axially symmetric in-hole electrical resistivity tomography (ERT) to two-dimensional (2D) half-space surface ERT is proposed, allowing direct implementation of widely available 2D inversion software. A quantitative reduction method is also proposed to determine accurately the variation of column diameter from the inverted tomogram. Numerical simulations validate the proposed approach and show that the estimation error is within 10% when the electrode spacing is less than 1/5 of the column diameter. The spatial resolution and the effect of axially asymmetric condition are numerically investigated, the latter suggesting the importance of centralising electrodes in the soilcrete column. Two physical model tests are performed and verify the performance of the proposed method experimentally. [ABSTRACT FROM AUTHOR]

Microbial-induced calcium carbonate precipitation (MICP) is an innovative ground improvement technique which can enhance the strength and stiffness of soils and can also control their hydraulic conductivity. These engineering properties of MICP-treated soils are affected by the particle-scale behaviour of the precipitated calcium carbonate – namely, the composition, amount and distribution, which are controlled by the MICP process occurring at the particle scale. In this study, a microfluidic chip was designed and fabricated to improve understanding of MICP at the particle scale by observing the behaviour of bacteria and calcium carbonate crystals during this process. It was found that bacteria became evenly distributed throughout the microfluidic chip after the injection of bacterial suspension, grew during bacterial settling and detached during the injection of cementation solution. Bacteria aggregated during the cementation solution injection, and calcium carbonate crystals formed at narrow pore throats or open pore bodies either during or after the cementation solution injections. [ABSTRACT FROM AUTHOR]

In this study an effective stress model for cement-treated soil is employed to carry out a systematic study on cement-treated soil slab used to control the wall displacement in excavations with thick, soft clay layers. Deterministic analyses were first conducted to assess the drainage state of the treated soil slab. These indicate that, under timescales which are representative of those in actual construction, cement-treated soil slab conditions may range from nearly undrained to drained. Random finite-element analyses also show that the global undrained and drained behaviour of the treated soil slab differ significantly. Undrained loading leads to a rapid rise in stress with strain to a peak strength with subsequent softening. Drained behaviour, on the other hand, leads to a more compliant response, which suggests that the main issue in the drained condition is likely to be large displacement, rather than complete collapse. A rational design framework for cement-treated soil slab is proposed, which explicitly considers the distribution in global behaviours of the treated soil slab. This differs from current limit state approaches which are largely based on statistical properties of the input data, such as the unconfined compressive strength of sampled cores. [ABSTRACT FROM AUTHOR]

Microbial activities may sustainably improve earthen structures such as landfill covers. Typically, after consuming nutrient, bacteria produce hydrated exopolysaccharide to form a biofilm, which may cause bio-clogging in soil. Previous studies have mainly focused on the combined effects of biofilm and nutrient on the water permeability of saturated sand. The present study investigates biofilm effects on gas permeability of unsaturated sand at different degrees of saturation using a flexible wall permeameter. Soils with water ('Water'), nutrient only ('Nutrient') and a mixture of bacteria and nutrient ('Combined') were examined with three replicates. Statistical analysis and scanning electron microscopy (SEM) were adopted for further interpretation. Compared to 'Water', the gas permeabilities of 'Combined' and 'Nutrient' were consistently lower. The differences increase with decreasing saturation. At around 5% degree of saturation, the gas permeability of the 'Combined' and 'Nutrient' cases was 22% and 14% lower than the 'Water' case, respectively. Statistical analysis reveals that the gas permeability reductions in 'Combined' and 'Nutrient' cases were significant at a significance level of less than 0·05 (probability > 95%). The reduction was attributed to pore clogging by nutrient precipitation (for 'Nutrient') or both biofilm and nutrient precipitation (for 'Combined'), as evident from the SEM results. The effects of biofilm on reducing soil gas permeability can be overestimated if nutrient effects are ignored. [ABSTRACT FROM AUTHOR]

This study investigates the stiffness development by bender element tests on specimens of artificially cemented clay, considering the effects of cement content, total water content and curing period. Experimental results indicate that the small-strain shear modulus of cemented clay can be well determined by cement content and total water content. A high cement content or a low total water content results in a stiffness increment of cemented clay. The cementation effect of these two factors is taken into account in this study. However, stiffness of cemented clay increases with curing period; this effect can be well characterised by a hyperbolic function. The function is able to capture both short-term and long-term curing effects. Finally, the effects of mix ratio and curing time are combined, and a predictive model for the stiffness development of cemented clay is devised. The model is validated by experimental results on specimens of cemented clay from various sources. [ABSTRACT FROM AUTHOR]

The long-term storage of coal ash in impoundments can lead to concerns of structural stability as well as trace element migration to local surface water and groundwater sources. Microbial induced calcium carbonate precipitation (MICP) offers a potential approach for minimising leachability of heavy metal trace elements from coal ash by calcium carbonate cementation. In this study, a protocol for MICP treatment of coal ash has been experimentally developed. The MICP treatment is applied to three coal ashes from different power plants, and their response to the developed treatment protocol is assessed. Possible factors affecting the MICP treatment of coal ash are discussed in terms of efficacey and inhibition of the stabilisation process. For this purpose, several approaches such as shear wave velocity, electrical conductivity, pH measurement, acid washing, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy were implemented. The results indicated that carbon/carbide content of the fly ash material has an important role in the efficacey of the MICP treatment process. The most likely explanation is carbon/carbide aids in the nucleation of calcium carbonate precipitation. [ABSTRACT FROM AUTHOR]

This paper examines the interaction between the spatial variations in binder concentration (i.e. cement slurry concentration) and in situ water content, in cement-mixed soil, using field and model data as well as statistical analysis and random field simulation. The field data are first analysed to shed light on the spatial variation in the in situ water content, including its scale of fluctuation. A statistical model is then developed which takes into account the variation in binder concentration and in situ water content. This leads to a two-parameter model for the prediction of the mean, variance and probability distribution function of the strength of the cement-treated soil. The scale of fluctuation for the variation in binder concentration arising from imperfect mixing within a cement-mixed column is then examined using centrifuge model data. This indicates that the scale of fluctuation in binder concentration is much shorter in range than that of the in situ water content. The combined effect of these two scales of fluctuation is then studied by simulating the resulting random field using Monte-Carlo simulations. This indicates that the size of the sampling region has a significant effect on the scale of fluctuation that is captured. If the sampling region is of a similar size to the column diameter, the measured scale of fluctuation reflects that of the binder concentration. As the size of the sampling region increases, so does the measured scale of fluctuation. This explains the wide range of scales of fluctuation that have been reported for cement-treated soil. To capture both scales of fluctuation in core sampling, some boreholes should be sunk at close spacings of less than a column diameter, in order to capture short-range variation. [ABSTRACT FROM AUTHOR]

The strength anisotropy of fibre-reinforced sands in the multiaxial stress space has been investigated using a hollow cylinder torsional apparatus. Probing stress paths under constant cell pressure have been performed on both unreinforced and reinforced sand specimens to assess the influence of the orientation of the principal stress directions on the fibre strengthening contribution. For the first time, a deviatoric strength envelope for fibre-reinforced soils in the multiaxial stress space was identified. The addition of fibres produces an anisotropic increase and a distortion of the deviatoric strength envelope if compared to the unreinforced soil matrix. The fibre strengthening contribution is governed by the tensile strain domain developed and the fibre orientation distribution. Further observations on the effect of the addition of fibres on the volumetric response, principal stress and strain rate non-coaxiality, as well as the shear bands formation of the composite, are presented. An analytical model to capture the anisotropic fibre strengthening contribution is developed and discussed. [ABSTRACT FROM AUTHOR]

In spite of being one of the most popular ground improvement techniques, jet grouting is often viewed with suspicion, its properties underestimated and its role downgraded to provisional or subsidiary, mostly because of the lack of reliable methods to manage uncertainty. The random composition and scattered properties of the material observed at the small scale of laboratory tests lead designers to assume arbitrarily unjustified over-conservative strength. A methodology is herein introduced to quantify uncertainty, characterising the different factors of variability in the jet-grouted material and simulating their effects on representative structural elements. An experimental database representative of a broad range of situations, consisting of laboratory tests on samples cored from jet-grouted columns and sonic tomography scans performed on large blocks, is first examined to characterise the mechanical response of the jet-grouted material and its variability. Probabilistic and autocorrelation functions are then introduced to simulate the stochastic and spatially correlated variability and virtually reproduce realistic populations of samples. The virtual simulation of uniaxial compression tests on these samples with a three-dimensional finite-element method code leads to the statistical distribution of the strength being obtained and the mean and variance being computed. Repeating systematically this process for the variable parameters of the original functions and for different shapes and dimensions of the samples leads to general formulas expressing the statistical distribution of the uniaxial compressive strength. The inferred relations are finally combined to calculate correcting factors giving the characteristic strength of prismatic jet-grouted elements with variable dimensions and slenderness, starting from the distribution observed at the laboratory scale. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 paper demonstrate that the proposed method can be used for estimation of the lateral displacement caused by installing jet-grouted columns in clayey soils, despite minor discrepancies from the group column installation case between the predicted and measured values. These minor discrepancies can most likely be attributed to the omission of the effect of installation sequence and rheological properties of the grout in the calculation. [ABSTRACT FROM AUTHOR]

The Chapman-Richards model is adopted in this paper to best fit the Terzaghi's one-dimensional consolidation curve. The obtained formula fits the theoretical solution well, with a regression coefficient R2 of 0·9995 and an error of less than 2%. By adopting this model, a new method to predict the ultimate settlement and the coefficient of consolidation using monitoring settlement data is proposed. The accuracy of the proposed method is verified against oedometer test and field monitoring data. For the cases verified, the ultimate settlement predictions from the proposed method are more reliable than those from the Asaoka's method for the different range of settlement data. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 verified using subset re-sampling from the data set, thereby providing a basis for assessing sufficiency of sample size. Finally, a method of conservatively estimating population mean strength and variance from limited sample size based on a prescribed confidence level is also proposed. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of granular materials is known to be influenced by the addition of fibres. However, most previous research has been carried out on materials with relatively small grains (sands), and its application to larger grains is not well documented. This paper reports an investigation into the mechanical behaviour of a fibre-reinforced granular material with a relatively large grain size corresponding to one-third and one-fifth scale railway ballast. The investigation was carried out by means of triaxial tests incorporating a full-field, image-based deformation measurement technique; this enabled detailed observations to be made of each triaxial test specimen during shearing. The test data demonstrate the benefits of random fibre reinforcement for aggregates that have a relatively large grain size. Analysis taking into account the effect of fibre tension on the effective stresses experienced by the granular skeleton provides new insights into the mechanisms of reinforcement in larger sized granular materials. [ABSTRACT FROM AUTHOR]

Liquefied ground loses its stiffness and strength, and it flows laterally in sloping areas and backfill areas behind collapsed quay walls. A method of installing piles such as cement-treated ones into the ground to prevent the lateral flow has been proposed and studied. The authors are investigating the most effective arrangement of piles to optimise cost-effectiveness. It is proposed to irregularly displace the piles to prevent lateral flow in various directions. In the present study, centrifuge model tests were conducted to verify the efficacy of piles against the lateral flow of liquefied ground and the effect of the pile arrangement in detail. This study first demonstrated the similarity law for the lateral flow of liquefied ground in a centrifuge, where both the Froude and Reynolds numbers can be satisfied. Numerical analyses assuming the flow of liquefied ground to be fluid-like were conducted to consider the improvement mechanism and the effect of the pile arrangement. The results of the model tests and numerical analyses indicated that the improvement by piles dramatically reduced the lateral displacement and that the average total flow velocity was small in the irregular arrangement case. [ABSTRACT FROM AUTHOR]

Wind tunnel tests show that enzyme-induced carbonate precipitation (EICP) holds promise as a method for mitigation of fugitive dust emissions. Fugitive dust (wind-blown fine-grained soil) is a significant environmental problem in semi-arid and arid environments. Conventional methods for fugitive dust control, including the application of water, salt, or synthetic polymers, are either ineffective in arid climates, limited to short-term stabilisation, or very expensive. EICP employs hydrolysis of urea (ureolysis), a process catalysed by the urease enzyme, to precipitate calcium carbonate (CaCO3) in the presence of calcium ions. Urease is a widely occurring enzyme found in many plants and microorganisms. Wind tunnel experiments were conducted to evaluate the use of a topically applied EICP solution containing plant-derived urease to stabilise soil against fugitive dust emission. Three different soils were tested: a native Arizona silty sand, a uniform medium-grained silica sand and fine sand-sized mine tailings from southern Arizona. The wind tunnel tests established the treatment concentrations at which EICP was more effective in suppressing fugitive dust than specimens prepared by either thoroughly wetting the soil or treatment with a salt-urea solution. These tests demonstrate the potential of EICP treatment as a means of mitigating fugitive dust emissions. [ABSTRACT FROM AUTHOR]

Dimethyldichlorosilane has been used as a hydrophobising agent for various applications, one of which is the treatment of soils. In geotechnical engineering, synthetic water-repellent soils can be used in barriers or ground improvement because of their wettability properties. This note examines the effect of dimethyldichlorosilane concentration on the soil type and time-dependent effects by means of contact angles measurements using the sessile drop method. The results reveal that the presence of organic matter, residual water and other non-mineral matter hinders the effectiveness of the treatment. Treated clean soil particle surfaces from crushed rocks achieve an immediate and high degree of water repellency. [ABSTRACT FROM AUTHOR]

The strength of soil improved by deep mixing processes often exhibits significant spatial variability. This paper presents a statistical model for strength distribution in deep-mixed columns, based on the assumption that spatial variation in strength arises from spatial variation in binder mass fraction. The statistical inputs of the model are the probability density function and coefficient of variation of the binder distribution, both of which were obtained from centrifuge model data and were assumed to be independent of the amount of the binder introduced into the ground. The theoretical framework of the model is first developed, followed by an assessment of the probability density function for the binder distribution. The variation of binder mass fraction within a deep mixing column is fitted by a truncated normal distribution. The model is benchmarked against core strength data from three phases of deep mixing operation in a construction project. The simplicity of the model precludes consideration of the effects of detailed mixing blade configuration, in situ variation in soil properties as well as workmanship. In spite of this, the computed histograms of strength distribution gave a reasonable reflection of the measured strength distribution. This suggests that despite the variety and complexity of deep mixing processes, one may still be able to develop a simple and workable model of the strength distribution taking into account just a few important factors. [ABSTRACT FROM AUTHOR]

This technical note presents a laboratory modelling of a T-shaped soil-cement column for soft ground treatment under an embankment, and compares it with a conventional column. A thick sand layer was placed between the column-treated ground and the loading plate to simulate the embankment effect, and the stress, excess pore pressure and settlement of the ground were analysed. Since the T-shaped column had an enlarged column cap, it took more applied load than the conventional column, resulting in less stress on the soil. This effect reduced the soil settlement and differential settlement of the ground, and increased its consolidation rate. [ABSTRACT FROM AUTHOR]

In deep excavation construction, improved soil layers consisting of overlapping cement-admixed columns formed by deep mixing method or jet grouting are often used to stabilise an excavation in soft soils. The purpose of such soil layers is to resist lateral compression generated by movement of the retaining wall. Cement-admixed soils are well known to have high heterogeneity in strength. In this paper, the heterogeneity in strength and Young's modulus are studied using random finite-element analyses, considering three sources of variation: namely, a deterministic radial trend in strength and Young's modulus; a stochastic fluctuation component due to non-uniform mixing; and positioning errors arising from off-verticality of the mixing shafts. The results show that positioning errors have the largest effect on the strength of the slab as a whole, whereas the radial trend has the smallest effect, when normalised by the volume-average strength. Based on the results obtained, methods are proposed which allow equivalent homogeneous mass strength and modulus of the improved slab to be determined for a chosen percentile of exceedance or reliability index, which can be used in deterministic finite-element analyses. [ABSTRACT FROM AUTHOR]

Various analytical solutions have been proposed for a unit-cell consolidation with a vertical drain under surcharge loading. These solutions involve different assumptions to address various aspects of consolidation. There is a lack of generalised solution for analysing consolidation of soil assisted by a vertical drain under various loading conditions. This paper presents a simplified solution for consolidation under multi-ramp loading. Generalised governing equations of equal-strain consolidation are solved. Simultaneous radial and vertical flow conditions, as well as the combined effects of drain resistance and smear, are taken into account fully. An increase in total stress due to multi-ramp loading is reasonably modelled as a function of both time and depth. An analytical solution to calculate excess pore-water pressure at any arbitrary point in soil is derived by using the method of separation of variables. The conventional definition of the degree of consolidation is given in terms of the dissipation of excess pore-water pressure as a result of the maximum increase in total stress in soil. This definition is interpreted in relation to the ultimate ground surface settlement due to surcharge preloading. Its validity and accuracy are verified by comparing the special cases of the proposed solution with two available analytical solutions. The proposed solution is also validated by a well-documented case history with settlement and pore-water pressure measurements. Reasonably good agreement is obtained. A new degree of dissipation is defined in terms of the dissipation of excess pore-water pressure as a result of currently induced increase in total stress in soil. By using this definition, an equation is proposed to estimate the gain in undrained strength of soil due to consolidation for assessing the stability of surcharge fills more effectively and correctly. The loading path over time and the compressibility of smeared soil are shown to have a potentially important influence on the degree of consolidation and the degree of dissipation. [ABSTRACT FROM AUTHOR]

For fibre-reinforced granular soils, the efficiency of the fibres is governed by the local fibre-grain interaction mechanism. This local interaction mechanism is evaluated, in this paper, by using a modified version of the shear-lag stress theory. While this theory provides a description of the stress-transfer mechanism at fibre-matrix interface level, it also generates the stress distribution along the fibre. The proposed model explicitly accounts for the effects of the geometrical fibre and granular size characteristics, fibre stiffness, global stress level, soil density and the non-linearity of soil behaviour. An analytical expression for the ratio of strains in the fibre and in the composite, which is fundamental for any prediction of fibre contribution, is further derived. A discussion on the effects of the controlling parameters is presented, while the scale-up of the problem at the composite level is then conducted by using a continuum constitutive model appropriately modified to account for the strain ratio between the fibre and the composite. The model is validated against a series of triaxial compression tests on two different sands mixed with polypropylene fibres of different aspect ratios. [ABSTRACT FROM AUTHOR]

This study proposes and describes a novel approach for cementing sandy soils in marine environments by modifying the promising technique of microbially induced carbonate precipitation (MICP). In contrast to the usual MICP technique described in the literature, the method proposed herein relies on the calcium ions dissolved in seawater as the sole source of calcium for calcite formation. This proposed method involves flushing high-salinity-tolerant, urease-active bacteria followed by a mixture of urea and seawater through a porous sandy soil, leading to bacterial carbonate release from the urease reaction and precipitation of insoluble and semi-soluble carbonate salts including calcium carbonate and magnesium carbonate trihydrate. This precipitation method resulted in a physical stabilisation of sand that reached an unconfined compressive strength of up to 300 kPa, which is about two-fold higher (with same amount of crystals produced) than that of the MICP treatment in which highly concentrated calcium and urea solutions are used. Permeability was retained at about 30% for all MICP-treated samples, suggesting good drainage ability. This new exploration of MICP technology provides a high potential for using bio-cementation in marine environments, for applications such as mitigation of submarine sediment liquefaction and prevention of beach sand erosion and cliffs scouring. [ABSTRACT FROM AUTHOR]

Reactive magnesia (MgO) and carbide slag (CS) were used to activate ground granulated blastfurnace slag (GGBS) to stabilise a marine soft clay, and the stabilised clays were subjected to accelerated sodium sulfate attack. The results indicated MgO-GGBS stabilised clay was nearly inert to this sodium sulfate solution. The resistance of CS-GGBS stabilised clay to sodium sulfate attack was lower than that of MgO-GGBS stabilised clay, but higher than that of Portland cement stabilised clay. Unlike CS-GGBS stabilised clay, there was no calcium aluminate hydrate or alumino-ferrite monosulfate produced in MgO-GGBS stabilised clay, and hence no ettringite, which would cause deterioration, formed when exposed to sodium sulfate. [ABSTRACT FROM AUTHOR]

Electro-osmotic consolidation is an attractive, soft ground improvement technique. In this paper a theoretical model is proposed for electro-osmotic consolidation by coupling the seepage field, electric field, and the stress and strain field. The soil mass deformation, pore-water pressure and the electrical voltage are the basic variables in the governing equations. A three-dimensional numerical model based on the finite-element method was developed to simulate the electro-osmotic consolidation process, predicting the tempo-spatial variation of soil mass displacement. The numerical model is compared with previous analytical solutions. Three common electrode installation patterns are illustrated and analysed with the numerical model, and the results indicate that the triangular pattern can achieve the largest average surface settlement and the smallest differential settlement. The effect of the penetrating depth of the electrodes on the ground settlement is also investigated. An engineering field application is simulated to validate the developed numerical formulation. [ABSTRACT FROM AUTHOR]

A modified 150 mm Rowe cell equipped with pore water pressure measurement was used to capture the flow relationship during vacuum-assisted radial consolidation. Based on the measured data, a radial consolidation model incorporating the effects of vacuum preloading is proposed, based on a non-linear relationship between the flow velocity and hydraulic gradient. The predictions of the proposed consolidation model are then compared with the predictions based on Hansbo's Darcian and non-Darcian models. The agreement between the proposed model and the measured data is shown, and the advantages of the proposed model compared with the existing models are discussed. An embankment case history taken from the reclamation project at the Port of Brisbane, Australia, was analysed, based on the current solution, and compared with the field measurements. [ABSTRACT FROM AUTHOR]

Electro-osmotic consolidation is a potential method for soil improvement. An axisymmetric electro-osmotic consolidation model with coupled horizontal and vertical seepage is proposed and the analytical solution is derived without the equal strain hypothesis, which was used in previous models. The proposed analytical model can predict the distribution of pore-water pressure in the radial direction, and comparison with previous analytical models indicates that the degree of consolidation is overestimated if the model is simplified from an axisymmetric model to a planar model. A design chart is developed to facilitate the evaluation of applying electro-osmotic consolidation for soil improvement. [ABSTRACT FROM AUTHOR]

In the design of sand compaction piles, the increase in undrained shear strength of the soft clay around the sand piles is often ignored. This paper examines the long-term gain in strength in the soft clay using centrifuge modelling and finite-element analysis. Stress, pore pressure and strength measurements in the centrifuge models are used to benchmark the finite-element results. Parametric studies are then conducted using finite-element analyses to explore the effects of the friction angle and modulus of the soft clay on the trend and quantum of strength increase. The findings show that the ratio of improved strength to original shear strength decays as a function of the logarithm of radial distance. A method of estimating strength increase is proposed, based on two parameters that are related to the friction angle and modulus of the soil. For multiple piles driven with almost no time lag for pore dissipation, strength gain does not accumulate with successive piles. On the other hand, if there is sufficient time for pore pressure dissipation between the driving of successive piles, the strength gains from the successive piles can be accumulated. [ABSTRACT FROM AUTHOR]

The ICEDA nuclear waste storage facility is being constructed near Lyon, France. It is founded on a thick clay deposit using settlement-reducing piles as soil improvement, making it the first nuclear installation in France employing this technique. The high safety requirements of the project, combined with the use of an innovative system based on piles disconnected from the foundation, have necessitated detailed studies of the soil-structure behaviour. In addition to an extensive ground investigation, a full-scale in situ test was carried out over a period of 6 months. This paper presents some of the experience gained from the project, focusing on the findings from the full-scale test compared with a class A prediction. It illustrates the difficulty in choosing soil parameters that are representative of the soil formation as a whole, and the value that the full-scale test had in the design for ICEDA. [ABSTRACT FROM AUTHOR]

The use of soil conditioners during excavation with earth pressure balance machines offers a number of advantages, including increased advancing rates, higher safety and settlement minimisation in a variety of ground conditions. Dispersing polymers and foams are added to optimise the strength and improve the deformation properties of the supporting soil paste. In this paper, possible effects of some commercially used products on clays are discussed. Whereas for most clays conditioning is based on lubrication (leading to reduced shear strength), for calcium bentonite a high polymer dosage can lead to the counterproductive effect of increasing the material's strength. In the case of kaolin, the change of mechanical behaviour is much more dramatic than simply reducing the strength; instead, the soil transforms into a viscous material with both liquid and solid properties. This unexpected phenomenon is characterised in detail and the working mechanisms of the used dispersants are identified. Shear-strength measurements at different polymer concentrations, with different kaolins and commercial clay dispersants are presented. The results suggest an enhanced particle-lubrication mechanism, arising from ion exchange and altered electrostatic forces between the clay surfaces and the surrounding water layer; this picture is confirmed by additional adsorption and microscopic measurements. [ABSTRACT FROM AUTHOR]

In this paper, a simple expression to predict the average diameter of columns created with single, double and triple fluid jet grouting is proposed considering both the energy of the eroding jet and the resistance of the soil. The injection system (single, double or triple) and the composition of the injected fluids are taken into account on the basis of theoretical analysis of the turbulent diffusion of submerged jets. Instead of separately considering the energy of the air jet shrouding the eroding fluid, a simple parameter is introduced which represents the beneficial effect of air in reducing the energetic dissipation on the jet external surface. In this way, a new expression of the specific kinetic energy of the jet at any distance from the nozzle is proposed, which is a relevant step forward if compared with that at the nozzle or at the pump previously proposed, as it takes into account both the system characteristics and the composition of the eroding jet (either grout or water). In the expression proposed to predict the diameter of jet grouting columns, soil resistance is considered by way of the results of the standard penetration test and cone penetration test, in terms of and respectively. The expression has been calibrated on a number of experimental data, and in most cases proved its efficiency in predicting columns' diameter with a scatter of less than ±20%. The field trials results highlight that jet grouting is more effective in coarse-grained soils, as clearly shown by an efficiency parameter proposed by the authors. Design charts are drawn to quickly estimate the average diameter for different soils, injection systems and input energies. [ABSTRACT FROM AUTHOR]

Two access embankments to a railway bridge, having a maximum height of 18 m, experienced a continuous and severe heave shortly after construction. Vertical displacements reached 120 mm in a 2-year period. The embankments were designed, by including soil-cement-treated transition wedges, to provide an increasingly rigid support as the trains approach the stiff bridge abutments. A grid of 10 m deep jet-grouting columns was also built, with the purpose of stabilising the embankments. Instead, a sustained swelling deformation, which extended to depths of 8-10 m, was activated. The compacted soil was low-plasticity clayey material, with a variable percentage of gypsum. The embankments suffered a massive ettringite-thaumasite attack, which was triggered by the simultaneous presence of cement, clay, sulfates and an external supply of water (rain). The paper describes the field extensometer and inclinometer records, the long-term laboratory tests performed, some mineralogical observations and the reactions leading to the growth of expansive crystals, and presents a model that simulates the measured heave. Forces acting against the bridge, which was seriously damaged, were estimated. Remedial measures include removal of the active upper zone of the embankments, and the construction in stages of slabs supporting the rails and founded on piles built on both sides of the embankments. The case is considered unique because of the magnitude of the reaction developed in the embankments, and its damaging action on the nearby bridge structure. [ABSTRACT FROM AUTHOR]