Imperial College London

Dr J Antonio H Carraro

Faculty of EngineeringDepartment of Civil and Environmental Engineering

Senior Lecturer in Experimental Geotechnical Engineering
 
 
 
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Contact

 

+44 (0)20 7594 6038antonio.carraro Website

 
 
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Location

 

528BSkempton BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

62 results found

Liu D, Harb Carraro JA, O'Sullivan C, 2022, USE OF COMBINED STATIC AND DYNAMIC TESTING TO QUANTIFY THE PARTICIPATION OF PARTICLES IN STRESS TRANSMISSION, Journal of Geotechnical and Geoenvironmental Engineering, ISSN: 0733-9410

Journal article

Liu D, Morimoto T, Carraro JAH, OSullivan Cet al., 2022, A semi-empirical re-evaluation of the influence of state on elastic stiffness in granular materials, Granular Matter, Vol: 24, Pages: 1-22, ISSN: 1434-5021

This study uses data acquired from three-dimensional discrete element method simulations to reconsider what measure of state can be used to predict stiffness in granular materials. A range of specimens with linear and gap-graded particle size distributions are considered and stiffness is measured using small amplitude strain probes. Analysis of the data firstly confirms that the void ratio, which is typically used as a measure of state in experimental soil mechanics, does not correlate well with shear stiffness. However, the empirical expressions developed by Hardin and his colleagues can capture variations in stiffness, provided an appropriate state variable is used. The study then highlights that the contribution of individual contacts to the overall stiffness is highly variable, depending on both the contact force transmitted and the particle size. Analyses explore how the stress transmission both within and between the different size fractions affects the overall stiffness. This heterogeneity in stiffness relates to the heterogeneity in the stress transmission amongst the different fractions. By considering the heterogeneity of stress distribution amongst different particle size fractions, a new semi-empirical stress-based state variable is proposed that provides insight into the factors that influence stiffness.

Journal article

Tastan EO, Harb Carraro JA, 2022, The effect of principal stress rotation and intermediate principal stress changes on the liquefaction resistance and undrained cyclic response of Ottawa sand, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 148, Pages: 1-12, ISSN: 0733-9410

In laboratory testing, the liquefaction resistance of sands is typically evaluated using cyclic triaxial and simple shear tests. These tests cannot be used in a rigorous manner to systematically assess the effects of principal stress rotation and intermediate principal stress changes on the undrained cyclic response of sands. In this study, the effect of these two factors on the liquefaction resistance of Ottawa sand was investigated using a cyclic hollow cylinder apparatus. At similar initial states of fabric and mean effective stress following K0 consolidation, the liquefaction resistance of Ottawa sand deposited underwater can: (i) decrease by 50% to 80 % as the major principal stress direction moves away from the vertical with Vc2= Vc3, or (ii) increase by 200% to 380%, as Vc2 increases while Vc1 remains vertical depending on the liquefaction criterion (strain levels). When the stress state defined by the imposed boundary condition deviates from axisymmetric compression, the combined effect on the liquefaction resistance is governed by principal stress rotation.

Journal article

Kazmi D, Serati M, Williams DJ, Olaya SQ, Qasim S, Cheng YP, Harb Carraro JAet al., 2022, Kaolin clay reinforced with a granular column containing crushed waste glass or traditional construction sands, International Journal of Geomechanics, Vol: 22, ISSN: 1532-3641

Installation of granular columns is a cost-effective and versatile in situ technique to improve the shear strength, settlement, and drainage behaviour of weak soils. It involves backfilling vertical boreholes in the ground with granular materials stiffer than the native soil, such as stone or compacted sand. However, the massive use and overexploitation of sand and natural aggregates have depleted their reserves in recent decades, causing damage to the environment, creating sand shortages and skyrocketing their price. Hence, it is essential to develop a sustainable alternative to natural aggregates to construct granular columns. The ever-increasing stockpiles of waste glass could be a potential replacement for natural sand in several geotechnical construction applications, noting that both materials have a similar chemical composition. Using crushed waste glass (CWG) as an alternative to traditional natural and manufactured (quarried) sands in granular columns could offer a multi75 pronged benefit by recycling non-biodegradable waste (glass) and by conserving a depleting natural resource (sand). Using a large direct shear (LDS) machine, this study investigated the shear strength behaviour of kaolin (to represent a typical weak soil) reinforced with a central granular column. Three different materials were separately used to backfill the column, including natural sand (NS), manufactured sand (MS) and CWG. The results revealed that the geocomposites containing the CWG column have the highest peak friction angle and relatively greater shear strength under high normal stresses, favouring the potential use of CWG as a green alternative to traditional sands in backfillinggranular columns, ultimately supporting resource conservation, waste recycling and the paradigm shift towards a circular economy.

Journal article

Quinteros VS, Harb Carraro JA, 2021, The initial fabric of undisturbed and reconstituted fluvial sand, Geotechnique, ISSN: 0016-8505

High-quality undisturbed samples of fluvial sand were obtained from the field using the ground freezing technique. In the laboratory, the in situ void ratio of these high-quality undisturbed frozen samples was replicated using four different reconstitution methods: dry deposition, moist tamping, water sedimentation (by spooning) and slurry deposition. The initial fabric of the specimens was evaluated using X-ray micro-computed tomography and advanced image analysis. Initial fabric features were assessed in terms of the particle orientation, anisotropy, void ratio distribution, and particles sizes within the specimens. Fabric analysis results suggest that none of the laboratory reconstitution techniques used captures the true three-dimensional initial fabric of undisturbed fluvial sand. However, the slurry deposition method managed to reproduce the inherent particle orientation, anisotropy, and the variations of void ratio and particle sizes of the undisturbed fluvial sand. This observation explains why previous rigorous studies on the macro-mechanical behaviour of sands deposited under water have systematically demonstrated that slurry deposition is the most suitable method to reconstitute in the lab natural sands deposited under water. This has major implications for geotechnical testing and analyses of liquefaction of sands deposited underwater such as fluvial, offshore and tailings sands.

Journal article

Liu D, O'Sullivan C, Carraro JAH, 2021, The influence of particle size distribution on the stress distribution in granular materials, Géotechnique, Pages: 1-37, ISSN: 0016-8505

This study systematically explores the effect of the shape of the particle size distribution on stress transmission in granular materials using three-dimensional discrete element method simulations. Extending prior studies that have focussed on bi-modal mixtures of coarser and finer grains, a broad range of isotropically compressed specimens with spherical particles and linear, fractal and gap-graded particle size distributions are considered. Considering isotropic stress conditions the nature of stress distribution was analysed by determining the mean effective particle stresses and considering the proportion of this stress transmitted by particles with different sizes. For gap-graded materials a contact-based perspective was adopted to consider the stress transmission both within and between the different size fractions. A clear correlation emerged between the cumulative distribution of particle sizes by volume and the cumulative distribution of particle sizes by mean effective stress for specimens with continuous PSDs. This correlation does not hold universally for gap-graded materials. In gap-graded materials the distribution of effective stress between the different size fractions depends upon the size ratio and the percentage of finer grains in the specimen. In contrast to specimens with continuous gradings, in the gap-graded specimens the distribution of effective stress amongst the different size fractions exhibited a marked sensitivity to density. Basic network analysis is shown to provide useful insight into effective stress transmission in the bimodal gap-graded materials.

Journal article

Barnett N, Rahman MM, Karim MR, Nguyen HBK, Harb Carraro JAet al., 2021, Equivalent state theory for sand with non-plastic fine mixtures: A DEM investigation, Geotechnique, Vol: 71, Pages: 423-440, ISSN: 0016-8505

DEM was used to simulate constant volume (undrained) triaxial compression tests for coarse particles (sand) mixed with non-plastic fines. The fines content (fc) of the mixtures tested was kept equal to 0, 0.05, 0.10 or 0.20. Accordingly, the critical state and micromechanical responses of these mixtures were evaluated. A focus on the influence of fc on sand behaviour was captured when fc<fthre, where fthre represents a threshold fines content, which corresponds to a transition between a fines-in-sand soil matrix to a sand-in-fines soil matrix. DEM was utilised to assess the micromechanical participation of fine particles within the sand skeleton (matrix). Such evaluations led to assessing the performance of the equivalent granular void ratio (e*), the equivalent granular state parameter (􀁜*), and ultimately their inherent parameter b, which represents the proportion of fines actively participating in the sand skeleton structure. It was observed that through capturing the stress partition of contact types within granular mixtures, a reasonable approximation of the active proportion of contacts within the sand matrix could be obtained leading to a DEM interpretation of the b parameter. The study therefore evaluated the concept and applicability of the equivalent state theory for sand-fines mixtures.

Journal article

Liu D, O'Sullivan C, Harb Carraro JA, 2021, Influence of particle size distribution on the proportion of stress-transmitting particles and implications for measures of soil state, Journal of Geotechnical and Geoenvironmental Engineering, Vol: 147, Pages: 04020182-1-04020182-14, ISSN: 0733-9410

It is generally accepted that the use of void ratio and bulk density as measures of soil8state have limitations in the case of gap-graded soils as the finer grains may not 9transmit stress. However, hitherto no one has systematically explored whether this 10issue also emerges for soils with continuous gradings. Building on a number of experimental and discrete element method (DEM) studies that have considered the idea of an effective void ratio for gap-graded or bi-modal soils, this contribution extends consideration of this concept to a broader range of particle size distributions. By exploiting high performance computers, this study considers a range of ideal isotropically compressed samples of spherical particles with linear, fractal and gap-graded (bimodal and trimodal) particle size distributions. The materials’ initial packing densities are controlled by varying the inter-particle coefficient of friction. The results show that even for soils with continuous particle size distributions, a significant proportion of the finer particles may not transmit stress and be inactive. Drawing on ideas put forward in relation to gap-graded soils, both a mechanical void ratio and mechanical bulk density that consider the inactive grains as part of the void space are determined. Even for the linear and fractal gradings considered here, the difference between the conventional measures and the mechanical measures is finite and density dependent. The difference is measurably larger in the looser samples considered. These data highlight a conceptual/fundamental limitation of using the global void ratio26as a measure of state in expressions to predict granular material behaviour

Journal article

Lim GT, Pineda J, Boukpeti N, Carraro JAH, Fourie Aet al., 2020, Reply to the discussion by Koutsoftas on “Effects of sampling disturbance in geotechnical design”, Canadian Geotechnical Journal, Vol: 57, Pages: 944-946, ISSN: 0008-3674

Journal article

Consoli NC, Carretta MS, Leon HB, Schneider MEB, Reginato NC, Harb Carraro JAet al., 2020, Behaviour of silty sands stabilised with cement subjected to hard environmental conditions, Proceedings of the ICE - Geotechnical Engineering, Vol: 173, Pages: 40-48, ISSN: 1353-2618

The present study evaluates the effect of three distinct amounts of fines, Portland cement and dry unit weights on the accumulated loss of mass (ALM), maximum shear modulus at small strains (G0) and tensile strength (qt) of stabilised sands subjected to wet-dry cycles. Tensile strength test results showed that addition of fines to a sand stabilised with cement increased its tensile strength, irrespective of the dry unit weight (γd) and cement amount present in the mixture. Increasing the amounts of fines of compacted cement-stabilised silty sand specimens subjected to wetting-drying cycles reduces ALM and increases G0 and qt of the mixtures. This may be due to the fact that specimens with larger amounts of fines have more contact points amongst particles, which provides better opportunities for the cement to develop more efficient bonds within the soil fabric, improving its mechanical performance. The increase in cement content and in γd of compacted cement-stabilized silty sand specimens reduced their ALM and increased G0 after each one of the twelve wet-dry cycles. The G0 and qt of cement-stabilized silty sand specimens with fines increases up to the sixth cycle, remaining practically constant after that, when these specimens are subjected to wetting-drying cycles.

Journal article

Consoli NC, Scheuermann Filho HC, Godoy VB, De Carli Rosembach CM, Harb Carraro Jet al., 2020, Durability of reclaimed asphalt pavement–coal fly ash–carbide lime blends under severe environmental conditions, Road Materials and Pavement Design, Vol: 21, Pages: 557-569, ISSN: 1468-0629

The sustainable use of industrial residue in enhancing the long-term performance of reclaimed asphalt pavement (RAP) has been proven to be effective under freeze–thaw and wet–dry conditions. This study focuses on coal fly ash (FA) and carbide lime (CL) as the enhancing agents. It evaluates how the durability and long-term performance of compacted RAP–FA–CL mixtures are impacted by dry unit weight and lime content. The tested mixture’s specimens were moulded in three layers through static compaction inside a cylindrical mould. Several single-level variables were used in the stabilisation process. Among these were: FA content of 25%, optimum water content of 9% (modified effort) and seven days of curing. Additionally, three target dry unit weights (17, 18 and 19 kN/m3 – the last of which was determined using the modified Proctor energy) and three percentages of lime content (3%, 5% and 7%) were used for a comparative analysis. The tested specimens’ accumulated loss of mass (after wetting–drying and freezing–thawing cycles) and splitting tensile strength were both evaluated as a function of the porosity/lime index. The experiments revealed that compacted RAP–coal FA–CL mixtures performed noticeably worse when subjected to freezing–thawing cycles than when subjected to wetting–drying cycles. These results indicate an increase in the breadth of the porosity/lime index, as it is shown to control the long-term performance of compacted RAP–coal FA–CL mixtures, in addition to controlling their mechanical response.

Journal article

Liu D, O'Sullivan C, Carraro JAH, 2020, Stress inhomogeneity in gap-graded cohesionless soils - A contact based perspective, Geo-Congress 2020, Pages: 341-348, ISSN: 0895-0563

Gap-graded cohesionless soils, comprising mixtures of fine and coarse grains, pose a particular challenge in soil mechanics. Reasoning and experimental data indicate that some of the finer grains may exist in the void space without transmitting any stress. A number of authors have proposed considering at least some of the volume of these particles along with the void space when calculating the void ratio in the case of low fines contents. The concept of a transitional fines content has been proposed, i.e., a fines content delineating materials whose behavior is dominated by the coarser grains and materials whose behavior is determined by the finer grains. This contribution uses discrete element method (DEM) simulations to explore the nature of stress transmission in gap-graded materials comprised of spherical particles. Partitioning the stress tensor by considering the contributions of the contacts between coarse particles, the contacts between coarse and fine particles, and the contacts between fine particles is shown to provide useful insight into the contribution of each type of particle to the overall stress transmission. In general, for the mixtures considered here, the coarse-coarse contacts transmit a greater range of forces and a greater average force. For the mixture with size ratio of 3.7, the range of contact force magnitudes transmitted by each contact type reduces with increasing fines content and increasing sample density. This sensitivity is more evident for the lower fines contents studied.

Conference paper

Quinteros S, Gundersen A, L'Heureux JS, Lunne T, Harb Carraro JA, Jardine Ret al., 2019, Øysand research site: Geotechnical characterisation of deltaic sandy-silty soils, AIMS Geosciences, Vol: 4, Pages: 750-783, ISSN: 2471-2132

This paper describes the geology and geotechnical engineering properties of the fluvial and 18 deltaic gravelly-sandy-silty sediments at Øysand, Norway. Geophysical and geotechnical site 19 investigations carried out between 2016 and 2018 at the site are presented. Field testing included state-20 of-the-practice and state-of-the-art soil characterisation techniques such as total sounding, seismic 21 cone penetration testing, seismic flat dilatometer, multichannel analysis of surface waves, electrical 22 resistivity tomography, ground penetrating radar, piezometers, thermistors strings, slug tests, and 23 permeability tests using a newly developed CPT permeability probe from NGI. Several sampling 24 techniques were used at the site to assess sample quality. Laboratory testing consisted of index tests 25 and advanced triaxial tests with bender elements to estimate shear strength and stiffness. Data 26 interpretation, engineering soil properties and state variables derived from this analysis are presented, 27 along with comments on data quality. Engineering problems investigated at Øysand so far, are related 28 to: the impact of using different CPTU types, sample quality assessment by obtaining soils with state-29 of-the-practice and state-of-the-art techniques (such as gel push sampler and ground freezing), and 30 frost heave susceptibility.

Journal article

Dominguez-Quintans C, Quinteros VS, Carraro JAH, Zdravkovic L, Jardine RJet al., 2019, Quality assessment of a new in-mould slurry deposition method for triaxial specimen reconstitution of clean and silty sands, 7th International Symposium on Deformation Characteristics of Geomaterials - IS Glasgow 2019

An innovative specimen reconstitution technique for sandy and silty soils that simulates underwater deposition is presented and evaluated. The technique is an upgraded version for triaxial testing of the well-established slurry deposition method. This novel setup integrates the reconstitution mould and the mixing tube into a single unit to avoid transferring the sample from the mixing tube to the mould. This subtle, but critical, modification enables reconstitution of very loose specimens as sample transfer disturbance, which can be significant, is eliminated. The quality of specimens prepared by the new reconstitution method was assessed by experiments on a clean sand from the UK (Ham River sand) and a silty sand from Norway (Øysand). The method, as any slurry-based procedure, is capable of producing homogeneous specimens with high initial degree of saturation, even in the absence of back pressure. The procedure is shown to be suitable for sands with or without fines. Moreover, the new method is able to achieve a wide range of initial void ratios, from very loose to very dense, without imposing any particle crushing in the latter case.

Conference paper

Chen S, Zdravkovic L, Carraro JAH, 2019, Thermally induced pore water pressure of reconstituted London clay, 7th International Symposium on Deformation Characteristics of Geomaterials - IS Glasgow 2019, Publisher: EDP Sciences, Pages: 1-5

Different forms of thermo-active structures have been proposed as a way of making use of the ground temperature to achieve renewable low-carbon heating and cooling in civil engineering construction. Such structures comprise piles, retaining walls or tunnel linings, and are used both as structural components and as conduits for utilising geothermal energy. In the scenario of the underground space in London, it is the thermo-active piles that have received most attention. However, little experimental evidence exists on the thermal behaviour of London clay to aid the design of thermo-active structures. This paper presents advanced laboratory testing on the reconstituted London clay to characterise the effect of temperature on its mechanical behaviour. Particular emphasis is given to thermally induced pore water pressures, as their evolution is not well understood. Tests are conducted in a temperature-controlled isotropic cell developed at Imperial College London. The emphasis of the current paper is on the temperature-based calibrations of different transducers. Soil specimens are isotropically consolidated and then subjected to undrained heating-cooling in the temperature range of 21 to 37 °C. Results obtained are compared with an existing laboratory study on another type of clay.

Conference paper

Lim GT, Boukpeti N, Fourie A, Pineda J, Harb Carraro JAet al., 2019, Mechanical behavior of intact and remoulded calcareous silts, 13th Australia New Zealand Conference on Geomechanics

Calcareous siltsare encountered in many offshore areas where oil and gas exploitationsaretak-ing place (e.g., Arabian gulf, southeastof Brazil, south east and north west of Australia). Understanding be-haviorof calcareous silts remainschallenging as undisturbed siltsamples are difficult to obtain, and most studies rely on remouldedsiltsamples. The purpose of this paperis to characterizethe mechanical behaviorof intact and remouldedoffshore calcareous silts from two different water depths. The comparisons are done based on microstructure characterizationusing scanning electron microscopy (SEM) images supported by in-dex tests, one-dimensional compression tests and undrained monotonic triaxial tests. The results have shown that,except for the critical state friction angle, the behaviour of remoulded silts differs from that of intact silts, due to the change in microstructure, which appears to be more compact with less intact open shellparticles.

Conference paper

Lim GT, Pineda J, Boukpeti N, Carraro JAH, Fourie Aet al., 2019, Effects of sampling disturbance in geotechnical design, Canadian Geotechnical Journal, Vol: 56, Pages: 275-289, ISSN: 0008-3674

This paper describes an experimental study of the effects of sampling disturbance in an Australian natural soft clay and the consequences of different sample quality on the representativeness of soil parameters used in geotechnical designs. The paper is divided into three sections. Laboratory test results obtained from specimens retrieved using three different tube samplers as well as the Sherbrooke (block) sampler are first described. Then, the sample quality assessment, using available indices proposed for soft soils, is presented. It is shown that sample quality varies with the stress paths and boundary conditions applied in laboratory tests. Finally, mechanical soil properties derived from specimens retrieved using the different samplers are used in the prediction of two classical problems in soil mechanics: the settlementand excess pore pressure response underneath an embankment as well as the settlement and bearing capacity of a shallow footing. These two examples are used here to highlight the consequences of poor sampling in practice.

Journal article

Liu D, O'Sullivan C, Carraro A, 2019, STRESS DISTRIBUTION IN TRIMODAL SAMPLES, 6th International Conference on Particle-Based Methods (PARTICLES) - Fundamentals and Applications, Publisher: INT CENTER NUMERICAL METHODS ENGINEERING, Pages: 58-67

Conference paper

Liu TF, Quinteros VS, Jardine RJ, Carraro JAH, Robinson Jet al., 2019, A unified database of ring shear steel-interface tests on sandy-silty soils

Characterisation of shearing behaviour at soil-structure interfaces is critical in the analysis and design of a wide range of geotechnical structures. Large-displacement ring shear interface testing employing preshearing stages has been recognised as a robust approach for characterising interface resistance, particularly when large relative displacements are developed between soil and interface during either installation or operation. Such tests are applied in practical design approaches, for example in the ICP method for driven piles (Jardine et al., 2005). This paper presents a database of interface shearing tests involving sandy-silty soils that contain low percentage (≤ 20%) of non-plastic fines, integrating outcomes from research and project studies conducted at Imperial College London and the Norwegian Geotechnical Institute using Bishop Ring Shear apparatuses. The outcomes enable a critical review of the potential effects of a wide range of factors, including: soil physical index properties (grading, fines content); interface characteristics (material type, surface roughness); and applied testing conditions (shear rate and normal effective stress). Trends identified from the datasets are integrated with previously reported studies to indicate the interface shear strength parameters that may be adopted for preliminary design with non-plastic sandy-silty soils on the basis of simple index tests.

Conference paper

Mashiri MS, Vinod JS, Sheikh MN, Carraro JAHet al., 2018, Shear modulus of sand-tyre chip mixtures, Environmental Geotechnics, Vol: 5, Pages: 336-344, ISSN: 2051-803X

This paper presents experimental results on the shear modulus of sand–tyre chip (STCh) mixtures. A series of bender element tests was carried out on specimens of sand mixed with varying proportions of tyre chips (TCh). Tests were carried out on STCh mixtures at a constant initial relative density of 50% for different initial effective confining pressures. The bender element test results indicate that the maximum shear modulus of the STCh mixtures increases with effective confining pressure and decreases with the gravimetric proportion of TCh, which ranged from 23 to 138kPa and 0 to 40%, respectively. However, the strain-controlled cyclic triaxial test results show that the shear modulus at large shear strains decreases with increasing single-amplitude shear strain as a function of the proportion of TCh in the mixture. The lower the proportion of TCh, the larger the degradation observed. The modified Hardin and Drnevich mathematical formulation adequately captures the variation in shear modulus of the STCh mixtures for a wide range of shear strain amplitudes.

Journal article

Lim GT, Pineda JA, Boukpeti N, Fourie A, Harb Carraro Jet al., 2018, Experimental assessment of sampling disturbance in calcareous silt, Géotechnique Letters, Vol: 8, Pages: 240-247, ISSN: 2045-2543

A small-scale physical modelling program using particle image velocimetry and digital image correlation techniques was designed to obtain a refined understanding of the disturbance experienced by calcareous silt during tube sampling. The influence of the sampler wall thickness (diameter-to-thickness ratio, B/t) and the tube penetration rate were evaluated by estimating displacement and strain fields around the tube sampler. The results indicate that sampling with a thin tube leads to a relatively narrow undisturbed zone located at the centre of the tube, with a width slightly smaller than the tube radius. Opposite to what is suggested for soft clays, tube sampling tends to densify the calcareous silt. This aspect has an important influence on the assessment of sample quality in calcareous silt for offshore and onshore projects.

Journal article

Consoli NC, Filho HCS, Godoy VB, Rosenbach CMDC, Carraro JAHet al., 2018, Durability of rap-industrial waste mixtures under severe climate conditions, Soils and Rocks, Vol: 41, Pages: 149-156, ISSN: 1980-9743

© 2018, Associacao Brasileira de Mecanica dos Solos. All rights reserved. The sustainable use of industrial wastes such as coal fly ash and carbide lime is an effective procedure to enhance the long-term performance of reclaimed asphalt pavement (RAP) under extreme freeze-thaw and wet-dry conditions. This study evaluates the impact of lime content (L) and dry unit weight (γ d ) on the durability and long-term performance of compacted RAP-fly ash-carbide lime mixes. For all mixtures tested, specimens were statically compacted inside a cylindrical mould to their target dry unit weights. Single-level variables used in the stabilisation process included: fly ash (FA) content of 25% (in relation to the RAP), optimum water content of 9% (modified compaction effort) and seven days of curing. Three target dry unit weights equal to 17, 18 and 19 kN/m 3 (the last one determined using the modified Proctor energy) as well as three different lime contents (3, 5 and 7%) were also used in the analysis. Both the accumulated loss of mass (ALM) after wetting-drying and freezing-thawing cycles and the splitting tensile strength (q t ) of the specimens tested were evaluated as a function of the porosity/lime ratio index (η/L iv ). Compacted RAP-fly ash-carbide lime mixtures performed better when subjected to wetting-drying cycles than to freezing-thawing cycles. The results indicate that the porosity/lime ratio index controls not only the mechanical response but also the long-term performance of compacted RAP-fly ash-carbide lime mixes, which substantially broadens the applicability of the index.

Journal article

Carraro JAH, Boukpeti N, Guadalupe-Torres Y, Joer Het al., 2018, Laboratory Testing, Encyclopedia of Maritime and Offshore Engineering, Editors: Randolph, Publisher: John Wiley and Sons, Ltd., ISBN: 9781118476352

Offshore infrastructure is extremely costly to design and build. Advanced and scientifically sound testing programmes can yield substantial savings to overall infrastructure costs due to the improved reliability that results from better and more accurate characterization of offshore sediments. Well-designed and carefully executed laboratory tests are paramount to the successful design and construction of offshore infrastructure. A variety of laboratory tests are available to the 21st century engineer to characterize the physical properties and mechanical response of offshore sediments and derive design parameters for offshore geomechanics analyses. To be competitive, engineers must be equally familiar with cutting-edge testing tools as well as conventional methods used in practice for decades. The most common and relevant laboratory tests available to assess fundamental aspects related to the composition and mechanical behavior of offshore sediments are discussed. These include basic characterization, soil fabric analyses and mechanical tests. Particular emphasis is placed on the underlying background and rationale associated with each test discussed.

Book chapter

Lim GT, Pineda JA, Boukpeti N, Carraro JAHet al., 2018, Predicted and measured behaviour of an embankment on PVD-improved Ballina clay, Computers and Geotechnics, Vol: 93, Pages: 204-221, ISSN: 0266-352X

This paper presents Class-A and Class-C predictions of the behaviour of an embankment built on soft Ballina clay improved with prefabricated vertical drains. Predictions were carried out using hand calculations and the finite-difference method. The latter approach allowed the variation of soil parameters and stress levels with depth to be considered in the analyses. An alternative systematic procedure for estimating soil parameters based on high-quality laboratory data is described. Class-A predictions highlighted some disagreement with the measured total settlements and pore pressure dissipation rates. For Class-C predictions, the choice of geotechnical parameters used in the analyses was guided by a systematic assessment of the stress states undergone by soil elements underneath the embankment centreline. This led to a better agreement between predicted and measured data, which demonstrates the potential of the proposed procedure for future analyses of embankment behaviour on soft Ballina clay.

Journal article

Harb Carraro J, Boukpeti N, Guadalupe-Torres Y, Joer Het al., 2017, Laboratory Testing, Encyclopedia of Marine and Offshore Engineering, Editors: Randolph, Publisher: Wiley

Offshore infrastructure is extremely costly to design and build. Advanced and scientifically sound testing programmes can yield substantial savings to overall infrastructure costs due to the improved reliability that results from better and more accurate characterization of offshore sediments. Well-designed and carefully executed laboratory tests are paramount to the successful design and construction of offshore infrastructure. A variety of laboratory tests are available to the 21st century engineer to characterize the physical properties and mechanical response of offshore sediments and derive design parameters for offshore geomechanics analyses. To be competitive, engineers must be equally familiar with cutting-edge testing tools as well as conventional methods used in practice for decades. The most common and relevant laboratory tests available to assess fundamental aspects related to the constituency and mechanical behavior of offshore sediments are discussed. These include basic characterization, soil fabric analyses and mechanical tests. Particular emphasis is placed on the underlying background and rationale associated with each test discussed.

Book chapter

Lim MF, Carraro JAH, Gourvenec S, 2017, Linking carbonate sand fabric and mechanical anisotropy from hollow cylinder tests: motivation and application, Geotechnical Frontiers 2017, Publisher: American Society of Civil Engineers, Pages: 317-326

In addition to density and stress, fabric is also a key state variable strongly affecting soil behavior. While fabric influence on mechanical behavior of soils has been investigated experimentally, the available database is limited in terms of boundary conditions and soil types tested. Offshore carbonate sediments are of special interest for offshore geotechnical analyses due to their prevalence in tropical waters and unique mechanical behavior that stems from their mostly biogenic origin. A key gap in the availability of experimental data on soil fabric relates to the anisotropy of offshore carbonate sediments. In practice, anisotropy studies (whether rigorously correlated to fabric or not) are typically carried out experimentally for simple boundary conditions such as idealized plane strain and axisymmetric states. In real geotechnical applications, stress paths subjected to soil elements in the field are far more complex, often involving the combined variations of both the orientation and magnitude of all three principal stresses. This paper presents a new multi- scale approach to assess soil fabric at the micro-scale level and relate it to the macro- mechanical response observed for generalized loading conditions. A new sampling method is illustrated that enables preservation and evaluation of the fabric of offshore sediments specimens following generalized stress disturbances imparted by a hollow cylinder apparatus. The link between fabric evolution and the observed stress-strain behavior of sand is discussed along with preliminary results. The approach is part of a broad framework that will be used to systematically study the evolution of soil fabric and anisotropy and their relationship to multi-directional loading scenarios.

Conference paper

Carraro JAH, 2017, Analysis of simple shear tests with cell pressure confinement, Geomechanics and Geoengineering: an international journal, Vol: 12, Pages: 169-180, ISSN: 1748-6025

The high cost of offshore infrastructure provides continuous encouragement for optimisation of design practices. Development of a more rational method to interpret results from simple shear tests with cell pressure confinement can reduce costs and improve reliability of offshore infrastructure. This paper addresses a commonly overlooked issue affecting design parameter selection: specimen shape varies from right cylinder to oblique cylinder after loading along a single shearing direction. Thus, horizontal stresses are not always equal to the cell pressure and their magnitude varies throughout the specimen’s lateral surface. An analysis is proposed that accounts for changing specimen geometry and lateral surface area during shearing and for the actual effect of cell pressure during testing. The analysis also describes how the intermediate principal stress can be assessed. Test results for medium dense silica sand are interpreted following de Josselin de Jong’s alternative shearing mechanism hypothesis. Conventional interpretation methods yield conservative design parameters for this soil. Failure states develop when the intermediate principal effective stress is halfway between major and minor principal effective stresses. Typical results for the soil tested show equipment performance meets standard direct simple shear requirements for shear strain rate, vertical stress and specimen height control.

Journal article

Lim GT, Boukpeti N, Carraro JAH, Pineda JAet al., 2015, Testing Tube Specimens from Soft Clay Deposits Containing Variable Amounts of Shells, 6th International Symposium on Deformation Characteristics of Geomaterials, Pages: 1097-1104

This paper presents the results of an experimental program aimed at characterizing tube specimens from an estuarine soft clay deposit, which contains variable amounts of shells. The characterization of the natural clay involved two non-destructive techniques for assessing specimen quality, namely computed axial tomography (CAT) and shear wave propagation using bender elements, as well as one-dimensional consolidation tests for evaluating soil compressibility. Image analysis of CT scans was used to quantify the volumetric shell fraction and its influence on the compressibility parameters derived from one-dimensional compression tests. The experimental results show that both the compressibility and the small-strain stiffness of the natural soft clay are influenced by the volumetric shell fraction.

Conference paper

Carraro JAH, 2015, Recycled materials to stabilise rammed earth: Insights and framework, Pages: 63-68

From a fundamental soil mechanics standpoint, rammed earth is an unsaturated compacted soil mixture that can have inter-particle bonding and reinforcing elements. The mechanical behavior of rammed earth can thus be interpreted using a rigorous geomechanics framework. In this paper, fundamental concepts required to analyse stabilised soil mixtures are outlined, with a particular focus on soil fabric, cementation and fiber-reinforcement issues. Assessment of specific gravity, particle shape, particle size distribution, plasticity, and volumetric indices of all fractions in a soil mixture can be useful for the rational (non-empirical) design of such materials, including rammed earth. Recent research on the beneficial use of waste materials in soil stabilisation is summarised with a focus on techniques aiming at increasing stiffness and strength, mitigating swell potential and improving thermal conductivity of stabilized soil mixtures. The successful use of waste materials such as carbide lime, scrap tire rubber, waste fibers, and various types of fly ash in soil stabilisation applications is highlighted. The potential use of alternative waste materials within a rational and mechanistic design framework that may be useful for rammed earth is discussed.

Conference paper

H Carraro JA, Bortolotto MS, 2015, Stiffness degradation and damping of carbonate and silica sands, Pages: 1179-1184

Stiffness and damping are some of the most important dynamic properties affecting wave propagation and small-strain response of soils. While these properties are useful in many geotechnical analyses, they are particularly relevant in analyses relying on the rigorous understanding of the mechanical behaviour of cyclically loaded soils, such as those affected by waves, wind loads and earthquakes. A fundamental understanding of such properties for offshore calcareous sediments still lacks as rigorous characterization of such properties is less established for these materials. In this study, a state-of-the-art resonant column apparatus is used to characterise the stiffness degradation and damping of a carbonate sand from the North West shelf of Australia. A silica sand with particle size distribution identical to that of the carbonate sand tested was also used to allow for the effect of soil mineralogy to be quantified. State variables such as density and mean effective stress were varied systematically to assess their effect on the stiffness degradation and damping ratio of the soils tested. Solid cylindrical specimens were subjected to mean effective stresses up to about 2MPas during the tests. Changes in particle size distribution were monitored to quantify particle breakage, which was shown to be higher for the carbonate sand than for the silica sand tested. At similar initial states of density and stress, increasing shear strains leads to a decrease in stiffness and corresponding increase in damping of the soils tested, as expected. However, the rate of stiffness degradation was always higher for the carbonate sand compared to its silica counterpart. At similar levels of stiffness degradation, the carbonate sand system-atically shows higher damping ratio than the silica sand.

Conference paper

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