The geotechnics team has an international reputation in the understanding of fundamentals of soil behaviour. This has been achieved through research investigations spanning across length scales, from micro to macro, from soils to structures, from gravitational loads to cyclic, dynamic and blasts. Group members have developed a strong focus on mechanical characterisation through advanced laboratory element testing, continuum constitutive modelling and numerical simulations, physical modelling (1g conditions) and field investigations at real geotechnical system scale. Applications include deep excavations in stiff clay, consolidation and creep of soft soils, offshore foundations, ground improvement with linear flexible inclusions and static/dynamic soil-structure interaction.
Our laboratories possess several stress-path triaxial cells as well as a unique set of multiaxial soil-test apparatus: true triaxial (independent variation of three principal stresses with rigid boundaries), cubical cell (independent variation of three principal stresses with flexible boundaries) and hollow cylindrical torsional apparatus (independent variation of four stress variables). The group has pioneered developments in laboratory geophysics for studying transmission of shear and compressive waves in laboratory specimens.
A set of experimental projects using these multiaxial and geophysical apparatus, funded by UK Research Council (EPSRC), is generating data to support development of constitutive models for the evolution of anisotropy of soil stiffness which are being incorporated in commercial numerical codes. The group have also recognised the strategic importance of dynamic soil structure interaction and are pioneering scale model testing to explore the dynamic behaviour of offshore foundations. At the same time cyclic element testing is being carried out to explore the changing stiffness (static and dynamic) when soil is subjected to large numbers of small load cycles. Expertise into detailed micro-mechanical investigation of granular soils has been developed through external research collaborations with colleagues from UK, France and Japan.
Micromechanics of seismic wave propagation in granular materials under multiaxial loading space
Academic leads: Dr Erdin Ibraim, Dr Simon Hamlin, Dr M Lings, Prof D Muir Wood and Dr C O’Sullivan (Imperial College), Dr I Cavaretta
The dual problem of wave propagation in granular soils and estimation of small strain stiffness is tackled through sophisticated laboratory testing and advanced numerical modelling. Understanding shear wave propagation through soils is essential for site response analysis in earthquake engineering, interpretation of geophysical surveys and SASW (Spectral Analysis of Surface Waves) as well as interpretation of laboratory seismic tests using piezoceramic bender elements (widely used to determine small strain shear stiffness for static and dynamic research and industrial applications) and predictions of ground movements during and after construction.
Mechanics of soft-rigid soil mixtures
Academic leads: Dr Erdin Ibraim, Mr. Salman Rouhanifar
The number of scrap tyres is increasing rapidly in both developed and developing countries due to the steady rise in the number of vehicles. As a consequence, the accumulation of used tyres is gradually becoming a real societal problem, equally from an economic and environmental point of view. Recent research showed that this material can be considered as an alternative for some conventional materials in construction industries, including geotechnical structures. However, before extensive implementation, further research is still required in order to understand the behaviour of the soil/tyre chip composite mixtures, including internal interaction mechanisms resulted from the combination of two particular materials, one soft, tyre rubber, and one rigid, granular soil. Insight into the underlying particle-level mechanisms and their role on the macroscale behaviour is explored and the quality of soft-rigid soil mixtures produced by normal mixing and depositional procedures, including the mitigation of particle segregation phenomena studied.
Fibre reinforced soils: experiments and modelling
It is well known that the roots of surface vegetation contribute to the stability of slopes as tension-resisting elements by adding strength to the near-surface soils in which the effective stress is low. Whereas the development of the roots network requires time with limited control of its fabric architecture, the use and inclusion of short flexible randomly distributed fibres can provide an artificial replication of the effects of vegetation. Fundamental research on the assessment of the mechanical behaviour of fibre reinforced granular soils has been initiated and extensive laboratory experimental tests performed. Key results were obtained (PhD of Andrea Diambra) on fabrication and fibre orientation, anisotropy and the need of multiaxial loading investigation, soil liquefaction mitigation. A predictive continuum constitutive model is developed. A micromechanics study by using DEM is in an advanced stage of progress with Dr K Maeda. Set up of research collaborations with colleagues from the Advanced Composites Centre for Innovation and Science (ACCIS) are explored.
Sand behaviour in small to medium strain domains using a hollow cylinder torsional apparatus
Academic leads: Dr Erdin Ibraim
Geotechnical problems invariably involve rotation of principal stress and strain directions and its influence on soil stress-strain response should be understood for correct modelling and prediction of the in-situ behaviour. Either, the principal stress and strain directions may vary from soil element to soil element in the ground around a geotechnical structure or they can change continuously their orientation as it may be the case due to repetitive loading occurring, for example, during an earthquake. The Hollow Cylinder Torsional Apparatus (HCTA) presents unique testing capabilities to investigate and duplicate these complex stress conditions that occur in the field. In this project, an existing HCTA has been upgraded and essential technical developments have been carried out on the loading system, drainage and pressure control panel, measurement systems, test control and data acquisition. These improvements have been validated by a first experimental testing programme on reconstituted sand specimens (MSc of Yoon Boung Shik, Commissioning of a new Hollow Cylinder Apparatus, 2005). For a large variety of specimen densities and stress states, the apparatus is ready to perform drained/undrained tests under monotonic/dynamic loading conditions. The project also allowed the development of a unique local system device of measurement of very small specimen displacements (strains). The accurate soil response in the very small displacement (strain) domain permits the evaluation of soil stiffness. The soil displacements are measured locally, in the central part of the specimen, and are performed by high resolution submersible non contact sensors. During a complete test, the design of a complex mounting system allows the re-positioning of the sensors. These developments have also been validated recently (Christiaens, 2006).
Crushable soils: particle-continuum duality
The significance of particle crushing to the mechanical behaviour of granular materials has been well identified and is of interest to many disciplines including geotechnics, geology, geophysics, mining engineering and powder technology. In the particular field of geomechanics, such phenomena are significant in a wide range of practical applications involving shallow foundations, soil around driven piles and foundations of large offshore structures, embankment and dam stability, pavement and railway substructures. The particle crushing process alters the stiffness and strength of a granular soil and it is associated with significant volumetric contractions. This research follows a collaboration with Ecole Centrale de Nantes (Prof PY Hicher, Dr C Danno, Dr Ali Djouabdi) supported by The Royal Society (co-investigator with Prof David Muir Wood (PI), 2008-2010) and uses advanced testing techniques to understand the leading underpinning mechanism as well as investigates ways to incorporate the concept of particle crushing into continuum based constitutive models for granular materials.
Time effects in granular soils
Academic leads: Dr Erdin Ibraim, Dr B Cazacliu (LCPC Nantes)
Granular soils experience non negligible time effects with clear implications for geotechnical system design. However, the leading mechanism still remains unknown. As the structure of the granular material and its evolution appears to be fundamental, a different modelling approach able to characterise this structure and track its evolution is therefore necessary. A joint research collaboration with Dr B Cazacliu (LCPC Nantes) is in progress.
Advancing the mobilisation strain framework through advanced laboratory testing
Academic lead: Dr P Vardanega
This project is part of an ongoing study aimed at the improvement of simple constitutive models to describe the stress strain behaviour of soils. Data-base building, statistical analyses along with advanced tri-axial testing in the Geomechanics Laboratories are all needed for this to occur. Simple, reliable models of stress strain will allow for improved SLS design methods and feed directly into the Mobilised Strength Design Framework.
Permeability of asphaltic road materials
Academic leads: Dr P Vardanega
Asphaltic and granular mixtures are key road building materials. The propensity for these materials to transmit water has a direct link on the performance of the road pavement system. Research is ongoing to develop improved guidelines for engineers to better assess how permeable these different mixtures will be in service and what engineering properties are linked to their permeability.