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Dr M.D. Bolton
Dr S.P.G. Madabhushi
Prof. A.N. Schofield
Dr S.M. Springman
The 100 g-tonne beam centrifuge was used to its designed capacity in a study of the lateral loading of piles in normally consolidated soil, modelling the storm loading of the foundation system of the Shell URSA Tension Leg Platform now being constructed to operate in 1000 m of water in the Gulf of Mexico. Allowances for soil degradation due to undrained cyclic lateral loading, and group interaction effects, on the 120 m long 2.5 m diameter piles could be validated only through centrifuge models. Other industry-funded work in offshore engineering included further tests of spud-can footings on sand, using the 2 m drum centrifuge, and exploratory tests of the uplift resistance of pipes buried in clay as part of an investigation into the stability of submarine pipelines.
The 2 m and 0.8 m diameter drum centrifuges are providing fast turn-round of new experimental techniques. Foam filling tightly packed inside a foil lining can represent a tunnel element which is buried in soil prior to the foam being dissolved in situ at an advancing face. In this way stress comes to bear on the lining in a progression along the tunnel axis, and a settlement trough can be simulated(F2). The central turntable of the small drum centrifuge has been used to store clay slurry prior to it being distributed to the ring channel for consolidation. Tools have been devised to trim the soil surface, cut trenches, and pour embankments in flight.
Optic fibres communicating with photometric transducers buried in clay, have been used to sense chemical concentrations in pollution migration studies in the mini-drum(F16,F15). This technology will be employed in the new European NECER network of centrifuges in environmental research, which links the major geotechnical centrifuge centres in Europe, and for which Cambridge will provide leadership on sensors.
Research in soil dynamics and earthquake engineering continues with the development of new earthquake actuators for use on our geotechnical centrifuges. The Stored Angular Momentum (SAM) actuator was successfully tested on the 10m beam centrifuge(F7,F8). The seismic performance of embankments founded on deep saturated sand deposits was investigated. In a collaboration with the US Army Corps of Engineers, Vicksburg, the SAM actuator was tested on the newly commissioned US Army Centrifuge. Based on the experience of design and manufacture of the Mark I SAM actuator, an improved version is being manufactured which will fire earthquakes at up to 150 g on the Army Centrifuge. Our own SAM will be upgraded to provide up to 30 g of lateral sinusoidal shaking at 100 g of centrifuge acceleration, with a payload of 250 kg. In addition, an Electro-Magnetic Mini Earthquake (EMME) actuator has been commissioned to test small packages of soil under arbitrary vibration spectra.
Dr M.D. Bolton
Dr R.J. Lynch
Dr J.D. McKinley
Dr K. Soga
A poster on Contaminated Land Research at CUED which summarised the work of our Environmental Geotechnics Laboratory was presented at the inaugural meeting of NICOLE (a network for industrially contaminated land in Europe) at Hanover in May 1996. This included new sensors using optical fibres, new geophysical techniques, studies of organic and inorganic pollution transport through landfill liners, and both centrifuge and mathematical modelling of coupled consolidation and transport in mineral waste lagoons with sand capping, and of electrokinetic remediation.
A 4-channel fibre-optic photometric sensor system has been designed and built. This was used to measure the dispersion of a plume of dye as it moves through the soil. An analogous fibre-optic pH sensor for ground-water has been designed for use in electric fields, in which a conventional glass electrode device is unable to operate. It has been used to record a plume of sodium hydroxide moving through a bed of sand(F6), as part of a study of electro-kinetic remediation.
A collaboration with Unicam Ltd assisted the on-going investigation of the electrokinetic remediation of contaminated clay(F17). A limitation to the process is the eventual formation of a region of high electrical resistance. Inductively coupled plasma optical emission spectrometry has been used to investigate this region in the clay, and it was found to contain high levels of iron, aluminium and copper compounds, often originating as impurities in the clay, but mobilised by the applied electric potential and the changing pH of the clay.
Work continues on the study of contaminant transport coupled with consolidation associated with mineral waste disposal. The centrifuge investigation of the problem has been completed, whereas the finite element modelling still continues. The current version of the FE program uses the small strain assumption with the incremental updating of co-ordinates. Numerical simulations of various cases are analysed to present the effect of consolidation on the contaminant transport process. The numerical implementation of the alternative finite deformation approach is in progress.
Experimental investigation of the electrical properties of soils contaminated by heavy metals has started. Measurements of soil conductivity and dielectric constant will be made over a wide range of frequency (kHz to GHz). A new consolidation cell is under development to measure these properties under appropriate confining stresses.
The effect of pore fluid chemistry on permeability and sorption properties of clays is being investigated in the laboratory using kaolin clay and London clay. The effect of pH on sorption properties was found to be significant. The comparison between the sorption properties determined from batch tests on dis-aggregated samples and those from dispersion column tests suggests that there is a need to consider scale effects in order to predict sorption in the field.
Dr K. Soga
Work continues on the study of cyclic behaviour of soils using a resonant column device. The contribution of viscous damping to the overall damping was investigated by varying pore fluid viscosity and particle size(F20). The test results showed that the magnitude of the viscous damping increased with shear strain and was influenced by the particle size. It is hypothesised that viscous damping loss is caused by viscous drag of the fluid moving around the particle contacts and a semi-empirical equation has been proposed to describe the contribution of this microscopic behaviour to the damping of granular materials.
The dynamic properties of clays were investigated using a cyclic torsional shear device that can measure the stiffness degradation characteristics with strains ranging from 1% to 10-4 % or less(F21). The measured stiffness degradation curves were highly variable due to the large variation in the electrical and chemical bonds and the repulsion forces acting between clay particles. Changes in voids ratio and frequency are found to influence the stiffness degradation characteristics of certain clays.
The study on seismic wave velocity measurement also continues. The measured wave velocities are compared with results obtained using the resonant column method and in-situ wave velocity determinations(F16). The pulse transmission method gives a higher S-wave velocity than the resonant column method, especially for clay samples. The pulse transmission method gives good agreement with the in-situ velocity measured by the suspension type S-wave logging.
Structured natural soils exhibit large time-dependency with respect to deformation as a result of softening or destruction of the metastable soil structure. Strain rate controlled triaxial compression tests and drained triaxial creep tests were performed on undisturbed Pancone clay samples from Pisa, Italy to investigate the time (or rate) dependency of the clay. The test results were then used along with other published data to examine the rate dependency of yield stresses for soils with different degrees of meta-stability.
Dr M.D. Bolton
Dr K. Soga
Work continues on dynamic re-meshing and refinement in finite element analysis, and on the estimation of discretisation errors. A new high order quadrilateral element has been defined, which has been shown to possess some advantages for the analysis of soil deformations(F4). Our CRISP finite element program has been the subject of another phase of investment. The Sage company, working in collaboration with Cambridge University Technical Services Ltd, has been instrumental in refining the user interface. A new CRISP Consortium has now been put in place, which will draw expertise from a number of centres in the UK and abroad with the objective of updating the soil modelling and solution techniques in Sage-CRISP.
There is some progress on the finite element modelling of contaminant transport problems. Coupled consolidation and contaminant transport has been modelled and applied to the capping of compressible mineral wastes. Current numerical implementation includes electro-osmosis, consolidation and electrokinetic contaminant migration.
Dr M.D. Bolton
Dr J.D. McKinley
Dr K. Soga
The construction of embankments on very soft soils can be improved by inserting wick drains in the clay, possibly enhanced in their function by constructing the embankment in stages with time permitted for consolidation, by the laying down of a sheet of reinforcing material on the clay prior to embankment construction, or by a combination of these. Current decision-making is largely empirical, and can lead to excessive conservatism. Centrifuge model tests(F18) and associated finite element simulations(F19) have established that reasonably accurate predictions can now be made in these situations. Anisotropy of the soft clay will generally be a significant determinant of stability and deformation during construction.
Work on pile driving continues, in association with Giken Seisakusho Ltd. A photo-elastic cell was used to highlight the bulb of stress induced at the tip of a small driven pile section. Field tests in Japan of water-jetting in conjunction with the "silent method" of hydraulic pile driving, have been followed by the construction of a laboratory model package. Soil crushability is an important determinant of the ease of pile driving, and centrifuge tests have also been carried out on the penetration of crushable calcareous sands.
The assessment and improvement of construction activities is an increasingly important theme. The surface settlements produced during the advance of a tunnel face during construction, and the residual stresses in the lining, are predictable only through 3D FE analysis which carefully models the construction sequence(F3). Work continues on the simultaneous elimination of tunnelling subsidence by compensation grouting, which involves the planned injection of cement grout to compensate for ground loss. Thousands of injections are typically made from each of hundreds of sleeved tubes or TAMS (tubes à manchettes), as movement indicators record the onset of subsidence beneath buildings which have to be protected. A new £2.3M BRITE Euram project, COSMUS, has now started, bringing us into collaboration with Soletanche, Tractabel, Glötzl, CEA-LETI and EPFL with the objective of improving the monitoring, analysis and control of these sensitive operations.
An investigation has begun of real-time drilling data obtained from Soletanche's ENPASOL system, in order to investigate correlations between the drilling parameters and more fundamental soil properties. The current work concentrates on signal processing of the raw data to discriminate between noise and information. Other work has been done on the applicability and the limitations of artificial neural networks in recovering meaningful information from scattered data(F14).
Laboratory investigation of the Woolwich & Reading bed was conducted using samples taken from the Jubilee Line Extension project. The observed variation of measured soil properties supported the difficulty encountered during tunnelling through this heterogeneous stratum.
Dr M.D. Bolton
Dr S.M. Springman
A more up-to-date understanding of the interaction between soil and a retaining wall is now embodied within BS 8002 Code of Practice for Earth Retaining Structures, and the task of explaining and justifying the new calculation procedures has begun(F1).
Work has continued on the lateral loading of bridge abutments. A simplified mechanism has been proposed for the complex interaction between a road embankment, the soft soil upon which it stands, its retaining wall acting as a bridge abutment for an underpass, and the piles which support the abutment and carry the weight of the deck to a stratum below the soft clay(F5,F22). These geo-structural mechanisms are based on plastic deformation fields(F3), and offer a rational method of design to limit movements, without the introduction of arbitrary safety factors. The study of portal frame, or integral abutment bridges, has been reported(F23). In particular, centrifuge model data of the influence of cyclic loading, due at full scale to the thermal expansion and contraction of the rigidly attached bridge deck, has been collated and analysed.
Dr S.P.G. Madabhushi
Following the Kobe earthquake the ground accelerations recorded at the Port Island were analysed with respect to the design of reinforced soil structures. In the VELACS (Verification of Liquefaction Analyses by Centrifuge Studies) Extension Conference, Class A predictions were made of the seismic performance of saturated layers of sandy soils and embankments founded on saturated sand beds. The numerical predictions for the deformations in different classes of soil-structure interaction problem were investigated(F9).
Dr M.D. Bolton
Dr G.R. McDowell
The fundamental reappraisal of the behaviour of granular materials is continuing from a micro-mechanical perspective. Discrete element models have been developed which feature grain fracture, and void migration. The term "clastic mechanics" is now being used for the quasi-continuum behaviour of an aggregate of grains which fracture before they yield plastically. Elasticity and sliding friction at points of contact can lead to the observation of hysteresis in aggregates which are far from crushing. It has been proposed, however, that the phenomenon of "normal consolidation" in soils can best be explained via the progressive fracture of grains. It has been demonstrated that elementary propositions regarding size effects, and the influence of co-ordination number, lead to the dispersion of particle sizes along a limited fractal grain size distribution. A plot of linear compression versus logarithm of stress can be recovered through the assumption of a work equation, and a dimensionally consistent treatment of the elastic-plastic compression of soils has been derived, for the first time.
Dr M.D. Bolton
Dr S.P.G. Madabhushi
Some success has been achieved in the finite element analysis of the brain, with application to mechanical insults such as tumour growth, oedema, and hydrocephalus. The ventricles (cavities) within the brain are essential to the prolongation of symptom-free function after the onset of tumour growth, through their capacity freely to close up. However, the region of oedema which characteristically surrounds a tumour may offer as significant a volumetric expansion as the tumour itself. FE analysis suggests that the cause of this swelling must be leakage from malformed blood vessels within and around the tumour. Our collaboration with the Department of Neurosurgery continues.
Research was undertaken into the mechanics of revision Total Hip Arthoplastic (THA) surgeries. The prostheses in these surgeries are inserted into pre-compacted granulated bone graft. The initial mechanical strength of the bone graft as obtained in the present day surgical procedures was investigated in a pilot study(F12). The particle size distributions result
F1. BOLTON, M.D. Geotechnical design of retaining walls. Structural Engineer, 74, (21), 365-369 (1996).
F2. BOLTON, M.D., LU, Y.C., SHARMA J.S. Centrifuge models of tunnel construction and compensation grouting. Proceedings, International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, London (April 1996); Edited by R.J. Mair, R.N. Taylor, 471-477 (Balkema, 1996).
F3. DASARI, G.R., RAWLINGS, C.G., BOLTON, M.D. Numerical modelling of a NATM tunnel construction in London clay. Proceedings, International Symposium on Geotechnical Aspects of Underground Construction in Soft Ground, London (April 1996); Edited by R.J. Mair, R.N. Taylor, 491-496 (Balkema, 1996).
F4. EL-HAMALAWI, A., BOLTON, M.D., BRITTO, A.M. A cubic strain quadrilateral finite element. International Journal for Numerical and Analytical Methods in Geomechanics, 20, (4), 295-302 (1996).
F5. ELLIS, E.A. Soil-structure interaction of piled bridge abutments constructed on soft clay. Ground Engineering, 29, (6), 42-44 (1996).
F6. LYNCH, R.J., Bolton, M.D. Low-cost fiber optic pH sensor for the in situ measurement of groundwater. Proceedings, International Symposium on Optical Science, Engineering and Instrumentation, Session on Soil and Water Monitoring and Remediation, Denver, C0, 2835-2847. SPIE Proceedings 2835 (August 1996).
F7. Madabhushi, S.P.G. Preliminary centrifuge tests using the stored angular momentum (SAM) earthquake actuator - phase I. Cambridge University Engineering Department Report CUED/D-SOILS/TR.298 (1996).
F8. Madabhushi, S.P.G. Recent developments in centrifuge modelling of earthquake events. Proceedings, UK-Japan Seismic Risk Forum, London, 21-22 (April 1996).
F9. Madabhushi, S.P.G. Modelling of deformations in dynamic soil-structure interaction problems. Cambridge University Engineering Department Report CUED/D-SOILS/TR.277 (1996).
F10. Madabhushi, S.P.G., Usmani, A., Fairbairn, D.D, Brewster, N., Howe, C. The influence of preparation (frozen vs defrosted) of allograft bone for impaction grafting. Proceedings, Society Internationale Chirugie Orthopaedic and Surgery (SICOT) Conference, Amsterdam, The Netherlands (March 1996).
F11. Madabhushi, S.P.G., Usmani, A., Fairbairn, D.D, Brewster, N., Howe, C. The influence of preparation using different bone mills on allograft bone for impaction grafting. Proceedings, Society Internationale Chirugie Orthopaedics and Trauma (SICOT) Conference, Amsterdam, The Netherlands, 51-61 (March 1996).
F12. Madabhushi, S.P.G., Usmani, A., Fairbairn, D.D, Brewster, N., Howe, C. Some mechanical properties of articulated grafts. Proceedings, Society Internationale for Research in Orthopaedics and Trauma (SIROT), Amsterdam, The Netherlands (1996).
F13. Madabhushi, S.P.G., Usmani, A., Rajalakshmi, M., Fairbairn, D.D. Finite element analysis of additional stressing during (light) sporting events following total hip arthoplasty. Proceedings, International Conference on the Engineering of Sport, Sheffield (July 1996); Edited by S. Haake, 51-61 (Balkema, 1996).
F14. McKINLEY, J.D. Extracting pattern from scattered data - applicability of artificial neural networks to the interpretation of bearing capacity data. Cambridge University Engineering Department Report CUED/D-SOILS/TR.299 (1996).
F15. McKINLEY, J.D., PRICE, B.A., SCHOFIELD, A.N. Modelling the transport of a pulse of two contaminants through a clay layer, Cambridge University Engineering Department Report CUED/D-SOILS/TR.301 (1996).
F16. Nakagawa, K., Soga, K., Mitchell, J.K. Pulse transmission system for measuring wave propagation in soils. Journal of Geotechnical Engineering, American Society of Civil Engineers, 122, (4), 302-308 (1996).
F17. SAVVIDOU, C., Penn, M. Study of pollutant spillage and electrokinetic soil remediation; final report on EPSRC grant GR/JO 5644. Cambridge University Engineering Department Report CUED/D-SOILS/TR.302 (1996).
F18. SHARMA, J.S., Bolton, M.D. Centrifuge modelling of an embankment on soft clay reinforced with a geogrid. Geotextiles and Geomembranes, 14, 1-17 (1996).
F19. SHARMA, J.S., Bolton, M.D. Finite element analysis of centrifuge tests on reinforced embankments on soft clay. Computers and Geotechnics, 19, (1), 1-22 (1996).
F20. Soga, K., Bransby, M.F. Dynamic soil properties at small strains. Ground Engineering, 29, (2), 25 (1996).
F21. Soga, K., Nakagawa, K., Mitchell, J.K. Measurement of stiffness degradation characteristics of clays using a torsional shear device. Proceedings, IS-TOKYO'95, 1st International Conference on Earthquake Geotechnical Engineering, Toyko, Japan (November 1995); Edited by K. Ishihara, 107-112 (Balkema, 1995).
F22. SPRINGMAN, S.M. Discussion on `Design of piled bridge abutments on soft clay for loading from lateral soil movements' by D.P. Stewart, R.J. Jewell, M.F. Randolph. Geotechnique , 46, (1), 165-173 (1996).
F23. SPRINGMAN, S.M., NORRISH, A., NG, C.W.W. Cyclic loading of sand behind integral bridge abutments, Transport Research Laboratory Project Report 146 (1996).