GEOTECHNICAL CENTRIFUGE CENTRE
Geotechnical Centrifuge Centre
Offshore Structures Foundation Fixity Earthquake Centrifuge Modelling
ENVIRONMENTAL GEOTECHNICS LABORATORY
Environmental Geotechnics Laboratory
Environmental Ground Engineering
Soil Characterisation and Element Testing
Numerical Analysis of Porous and Granular Media
Numerical Analysis of Soil-Foundation Interaction
BIO-MECHANICS OF CELLULAR AND GRANULAR MEDIA
Dr M.D. Bolton
Dr S.P.G. Madabhushi
Professor A.N. Schofield
The Centre provided teaching to 16 fourth year students, and the focus for 5 4th year projects. We also welcomed 4 overseas visitors on sabbatical. We continued to have links with the other European Centrifuge Centres via the NECER TMR network funded by the EC. Our international links include cooperation with the US Army Corps of Engineers in the development of their Centrifuge Centre at Vicksburg, collaboration on soft ground engineering with National University of Singapore, and discussions with many other groups on the promising future of drum centrifuges.
Industry-sponsored work has continued in relation to a broad range of topics. Work on submarine pipelines has included ploughing and backfilling, and pipe uplift resistance, in clays. Work has been carried out on the driving of piles; fine, saturated soil layers have been seen greatly to reduce the resistance of approaching piles, as pore pressures are induced which do not have time to dissipate. Work has also continued on tunnelling, including the influence of grouting on tunnel lining stresses, and the influence of building stiffness and weight on the creation of subsidence troughs due to ground loss, and the incidence of corresponding structural deformations.
Work has continued on the upgrade of facilities on the balanced beam, the 2m drum and the mini-drum centrifuges.
Professor A.N. Schofield
Professor Schofield lectured in Cornell University, gave keynote lectures at the Muryama memorial symposium in Kyoto and at Professor Randolph's Perth UWA workshop on drum centrifuges, and lectured in the University of Sydney. He gave lectures in a course in Hanoi and also lectured twice in Bangkok. He continued his cooperation with the US Army Engineer Waterways Experiment Station in Vicksburg Mississippi, and worked with Butler, his research student from WES, on the new earthquake actuator for the new Army Centrifuge. He also continued his co-operation with a number of other drum centrifuge workers, including Professor Kusakabe of Tokyo Institute of Technology and Thomas Broadbent and Sons Ltd of Huddersfield. Professor Schofield participated in the International Conference on Soil Mechanics and Foundation Engineering in Hamburg. He is working to secure that the advances in centrifuge technology that he has helped to achieve in other Geotechnical Centrifuge Centres are also achieved in Cambridge before his retirement at the end of the academic year 1997-98.
Past work on offshore jack-up foundation fixity and on earthquake modelling has led to continued publication with former research students and colleagues. Work on the error in the Mohr Coulomb equation has continued
Dr Gopal Madabhushi
Research in soil dynamics and earthquake engineering continues with the ongoing development of new earthquake actuators. The Stored Angular Momentum (SAM) earthquake actuator was used to study the seismic performance of embankments founded on deep, saturated soil deposits(F21). Implication of realistic earthquakes like the Kobe earthquake of 1995 in the seismic design of soil reinforced structures were investigated(F18). Future research needs were identified and further modifications to the SAM actuator to deliver these research needs are underway. Funding for these modifications was obtained from an EPSRC research grant. Research is also underway to further develop the Equivalent Shear Beam (ESB) model container which simulates the semi-infinite extent of a soil layer in the field. Wavelet analysis techniques are currently being used to further investigate the behaviour of the ESB model container. The Electro Magnetic Mini Earthquake (EMME) actuator was developed to subject small payloads in-flight to earthquake vibrations with required spectral content(F20). The EMME actuator is being used in various MEng projects including evaluation of seismic response of rock-fill dams with clay cores and response of tower structures on liquefiable sand deposits.
Dr R.J. Lynch
Dr K. Soga
Dr M.D. Bolton
The Environmental Geotechnics group organised the EPSRC Symposium on Contaminated Land Research in February 1997, held in the Moller Centre at Churchill College. A Workshop for NECER (EU funded Network: Network for Environmental Centrifuge Research) was also organised in April. A presentation was also made on monitoring of contaminants in soil, to the NICOLE (Network for Industrially Contaminated Land in Europe) Workshop on contaminated land, held in Brighton in May.
Dr R.J. Lynch
The fibre-optic photometric system was used to investigate the shape of plumes of dye tracer and copper sulphate moving through sand. The design of the sensors has been refined. The data was used(F34) to calculate the coefficient of hydrodynamic dispersion and the Peclet number which indicates which mechanism (diffusion or dispersion) is dominant in this flow regime. Preliminary steps have been made towards the use of this system in both Beam and Minidrum Centrifuges.
A fourth year project has investigated the detection of non-aqueous phase liquid pollutants (NAPLs) in soils using a simple optical technique. The system uses the principle that these NAPLs have very different refractive indices compared to water. A trial probe using fibre-optics was made and tested in laboratory trials.
Dr K. Soga
The disposal of mineral wastes, which by their nature contain soluble contaminants, presents major technical and environmental problems. These wastes are commonly distributed hydraulically in retention ponds. During the design of these ponds, the process of consolidation of the mineral waste is often considered with little or no regard for its effect on the movement of contaminants. A centrifuge test, simulating the one-dimensional consolidation of, and contaminant migration from, a mineral waste site was conducted. The result from the test was compared with theoretical predictions made using a newly developed finite element code(F28).
The use of the frequency dependent dielectric properties of clay soils is of interest as a potential method of contaminant detection. The study of
dielectric properties at low frequencies is of particular importance as this has been theoretically argued to be the range in which the interfacial
polarisation and double layer polarisation occurs. With a support from Nishimatsu Construction, a new consolidation cell has been developed to
measure the dielectric properties of soils from kHz up to the GHz range at various stress condition. The cell contains three
probes to cover this large range of frequency. One of the probes is a commercial probe that is normally used to measure the biomass of biological suspensions. It was tested for its suitability to make dielectric measurements in clays(F10).
The study on migration of non-aqueous phase liquid (NAPL) in soils started with a support from a new European Commission Training and Mobility of Researchers Grant and the existing European NECER network of centrifuges in environmental research. The effect of soil heterogeneity on NAPL migration in unsaturated soils was examined using a 20 cm diameter, 1 m long soil column with resistivity and time domain reflectometry measurements. The investigation of the fingering process of dense NAPL in a fractured plane of a clay has also started using a triaxial permeameter device. This experimental set-up allows us to monitor the changes in the migration pattern under different effective stress conditions. Furthermore, tank experiment of DNAPL migration in inclined soil layers was conducted in collaboration with the University of Colorado at Boulder.
Dr A. Al-Tabbaa
A research and development contract funded by the Department of the Environment and carried out in collaboration with industry was successfully completed. The work involved the development and application of an in situ treatment methodology on a contaminated site using physical and chemical immobilisation of the contaminants applied using a soil mixing technique with a specially developed auger(F14). Aspects of the research work will appear in the Proceedings of the ICE in 1998. The collaborating companies have just completed their first commercial job using the developed methodology, this being the first project of its kind in the UK. Current work is investigating the medium and long term behaviour of treated ground(F2), applications of soil mixing to in situ bioremediation using biofilm technology in order to combine immobilisation with eventual treatment(F9), simulation of in situ testing is being carried using laboratory scale model augers(F3), investigation of the effect of heterogeneous site conditions on the effectiveness of the treatment method and the application of the technology to active containment. Poster and platform presentations were made at the XIV International Conference on Soil Mechanics and Foundation Engineering in Hamburg and the Geo-environmental Engineering Conference in Cardiff respectively both in September 1997(F2,F14).
The work on the reuse of waste materials in soil-waste mixtures for geotechnical applications such as light weight fill materials and landfill liners continues concentrating on shredded tyre and waste compost(F1).
Dr K. Soga
Measurement of elastic wave velocities in a triaxial cell continues(F26). The compression wave of the second kind which was theoretically predicted by Biot (1956) was observed in granular soils(F27). The measured Biot-wave velocities agreed well with the theoretical values computed using properties and parameters obtained primarily from laboratory tests. Theoretical studies of the effect of frequency on wave velocities and the influences of other parameters were also investigated.
Most soil mechanics analyses are for the evaluation of limit equilibrium or for time-independent deformations. Nonetheless, in practice, time-dependent deformations and stress changes that result from the time-dependent or viscous rearrangement of the soil structure may be a significant part of the total ground response. A state-of-the-art report on time effects on the stress-deformation behaviour of soils was presented at the Professor Sakuro Murayama Symposium in Kyoto(F25). Structured natural soils exhibit large time-dependency with respect to deformation as a result of softening or destruction of the metastable structure(F33). Further experimental investigation on time effects (creep/ageing) of soils on construction activities is currently conducted. The fundamental time dependent behaviour is also examined from a micro-mechanical perspective.
Work continues on the study of cyclic behaviour of soils. A series of resonant column tests was performed to investigate the dynamic behaviour of sands containing varied pore fluids, in association with Shimizu Construction. The investigation showed that shear stiffness at a given shear strain was unaffected by the pore fluid viscosity for sands at the frequency tested. Damping ratio is increased by the presence of high viscosity pore fluid. This additional viscous damping increases both with shear strain amplitude and decreasing particle size. By introducing the roughness feature of the particle as a characteristic length, a semi-empirical equation to describe viscous damping was proposed(F28).
Clastic mechanics(F7) refers to the quasi-continuum deformation of an aggregate of elastic but brittle grains which fracture and slide (Chambers:
clastic - fragmented, especially applied to a rock composed of fragments of pre-existing rocks). A statistical description of the successive fracturing
of particles shows promise for the understanding of self-similarity in a wide range of materials from tectonic mélanges to sedimented clays.
Any supposed relationship between voids ratio e and the effective stress s' is incomplete (i.e. dimensionally inconsistent). The normalisation of stress with respect to the crushing strength of grains and their asperities should be pivotal in the formulation of simple but meaningful constitutive laws for soils.
There are two influences on grain crushing. Smallness of grains leads to some relative strengthening: small brittle components are always stronger on a Griffith analysis of fracture since the flaws within them must be smaller. Smallness of grains leads to relative vulnerability, on the other hand, since the smallest grains have the fewest contacting neighbours (smallest coordination number) and the tensile stresses induced inside particles are strongly reduced if a particle is well-supported by many neighbours. For a given macroscopic compressive stress, it emerges that the co-ordination effect of reduced tensile stress outweighs the Griffith effect of increased fracture strength. The smallest particles are the ones which tend to break. Griffith simply provides for some hardening as the broken fragments themselves continue to break, generation by generation. Novel algorithms have been developed for the efficient tracking of fragments, and the computation of particle size distributions.
We have shown(F24) that the "normal consolidation line" may be viewed as an evolution towards a fractal distribution of grain sizes through the successive fracture of grains. From this we offer an understanding of the existence, for brittle granular materials, of a straight line on a plot of e versus log s' for which fractal compression might be a better term than "normal compression". "Plastic yielding" on what Schofield and Wroth call the "wet side" of critical states may then be seen as clastic yielding with the voids ratio reducing as the breaking fragments fit more neatly into the voids between the pre-existing grains(F29). The origins of plastic hardening are then seen to be linked with the greater fracture strength of smaller grains, and clastic hardening might be a better term than "isotropic plastic hardening" to describe the observed expansion of yield surfaces as soils compress under monotonically increasing stresses.
The effects of the dispersion of particle sizes, due to crushing, can inform other facets of soil behaviour. Void filling should obviously link with changes in the soil's hydraulic conductivity. Disparity of particle sizes following crushing has also been shown(F23) to relate to the strong hysteresis observed in unloading and reloading tests on soils. The contrast in stiffness between relatively large inclusions comprising original grains, and the fractal matrix which comes to surround them, leads to progressive slippage of the smaller grains. This is presumably the origin of so-called "kinematic hardening" observed in over-consolidated soils, though kinematic yielding might now be thought to be a more accurate description.
Work has also been carried out at and near a state of zero effective stress, for which particle crushing may be negligible. The behaviour of fresh cement grout was modelled(F8) as a two-stage process - pressure filtration to form a contacting aggregate at zero effective stress, followed by consolidation of the aggregate. This general approach also seems to provide the best treatment for hydraulic fills. An analagous compaction-front approach was used for the re-consolidation of fluidised clays placed as back-fill in submarine trenches.
At the other extreme of particle behaviour, fast debris flows have been modelled. Here, the rate of expulsion of fluid from the slip zone was included as a parameter, together with the kinematics of particles which could glide on the fluid, impact on each other, slide and over-ride. The model was highly idealised, but intended mainly to inspect the character of its rate-dependent predictions.
Dr K. Soga
A two-dimensional finite element program to simulate electric osmosis consolidation was developed with support from Sunkyong Construction. The program was used to assess the electro-osmosis consolidation field tests conducted in Korea.
A finite element program which simulates coupled consolidation and the contaminant transport problem continues to be developed. The program was used to model the capping of compressible mineral wastes(F15). Numerical implementation of the finite deformation Cam-clay model is in progress.
Numerical modelling of pipeline-soil interaction has begun with a support from Tokyo Gas. The effect of lateral spreading of soils in landslides and earthquakes on pipeline integrity is currently investigated.
Dr S.P.G. Madabhushi
Research is underway to investigate the response of the saturated porous media subjected to earthquake loading. The VErification of Liquefaction Analysis by Centrifuge Studies (VELACS) Extension Project compared the performance of the dynamic FE codes with data from the centrifuge studies. Class A predictions were made for two model embankments built from loose and dense sand deposits respectively(F21). Modelling of deformations in various dynamic soil-structure interaction problems was also investigated(F19).
Dr A. Al-Tabbaa
Numerical analyses, in parallel with experimental work, are being carried out on the problem of the design of piled foundations in swelling soils in order to minimise the use of reinforcement and hence costs. The analyses incorporate the use of a realistic soil model which predicts the behaviour of clay in overconsolidated stress states by modelling it as an elasto-plastic material rather than a purely elastic material which is the common practice. Initial results have already shown considerable reduction in the tensile forces developed along the pile shaft.
Dr S.P.G. Madabhushi
Collaboration with Princess Margaret Rose Orthopaedic hospital in Edinburgh continues. Finite element analysis were carried out to simulate the stresses induced in the bone graft following a revision Total Hip Arthoplasty and settlements in the bone graft following application of the load were studied(F22). Efforts are underway to determine the mean pore size of compacted bone graft to ensure growth of blood vessels following revision THA.
Dr M.D. Bolton
Collaboration continues with the Department of Neurosurgery and the Addenbrookes Hospital Brain Imaging Unit. Numerical simulations are under way of a variety of brain deformations arising from hydrocephalus, tumour growth and associated surgery, stroke, and contusion.
Dr K. Soga
The report on seismicity and geotechnical aspects of the Kobe earthquake of 1995 was finally published by Earthquake Engineering Field Investigation Team of the UK(F30,F31).
Dr K. Soga
Dr M.D. Bolton
The work continues on EC funded COSMUS project, in collaboration with Soletanche-Bachy, Tractabel, Glotzl, CEA-LETI and EPFL. The project
aims to develop a tunnel construction system using improved
monitoring techniques, a soil characterisation system and an advanced computer simulation model. The contributions from Cambridge are (i) to develop methods to help characterising in-situ soil condition from drilling parameters and (ii) to model compensation grouting and tunnel operation by the finite element method. Soletanch-Bachy's ENPASOL drilling method has been standardised to improve the reliability of recorded data and to allow for comparisons between data obtained from various sites and soil types. Work continues to correlate drilling parameters to soil properties. Modelling of hydraulic fracturing in relation to compensation grouting has also begun.
F1. Al-Tabbaa, A., Blackwell, O., Porter, S.A. An investigation into the geotechnical properties of soil-tyre mixtures. Environmental Technology, 18, (8), 855-860 (1997).
F2. Al-Tabbaa, A., Evans, C.W. Medium-term performance of stabilised/ solidified contaminated soil-grout material. Proceedings, 14th International Conference on Soil Mechanics and Foundation Engineering, Hamburg, Germany, 3, 1941-1946 (September 1997).
F3. Al-Tabbaa, A., Lander, S.A., Evans, C.W. The performance of a model auger in the stabilisation/solidification of a contaminated sand. Environmental Technology, 18, (9), 913-920 (1997).
F4. Barker, H.R., Sartain, N., Schofield, A.N., Soga, K. Modelling of embankment construction on soft clay in Mk II mini-drum centrifuge. Cambridge University Engineering Department Technical Report CUED/D-SOILS/TR.303 (1997).
F5. Belloti, D., Clematis, A., Fernandes, P., Madabhushi, S.P.G. Parallelisation of a finite difference code for modelling ground water flow. Proceedings, International Conference on High Performance Computing and Networking Europe `97, Vienna, Austria (April 1997).
F6. Bolton, M.D., Barefoot, A.J. The variation of critical pipeline trench back-fill properties. Proceedings, IBC Conference on Risk-Based and Limit State Design and Operation of Pipelines, Aberdeen (May 1997).
F7. Bolton, M.D., McDowell, G.R. Clastic mechanics. Proceedings, IUTAM Symposium on Mechanics of Granular and Porous Materials, Cambridge (July 1996); Edited by N.A. Fleck, A.C.F. Cocks, 35-46 (Kluwer Academic, 1997).
F8. Bolton, M.D., McKinley, J. Geotechnical properties of fresh cement grout - pressure filtration and consolidation tests. Géotechnique, 47, (2), 347-352 (1997).
F9. Brough, M.J., Martin, R.J., Al-Tabbaa, A. Laboratory investigation of biofilm development in a clayey sand. Proceedings, 2nd International Conference on Microorganisms in Activated Sludge and Biofilm Processes, Berkeley, CA, USA (July 1997).
F10. Carrier, M., Soga, K. A four terminal measurement system for the investigation of the dielectric properties of clay at low frequency. Geoenvironmental Engineering, Contaminated Ground: Fate of Pollutants and Remediation, Conference Proceedings, Cardiff (September 1997); Edited by R.N. Yong, H.R. Thomas, 3-10 (Thomas Telford, 1997).
F11. Chan, A.H.C., Madabhushi, S.P.G. Class A numerical prediction and back analyses for model no. 2 and no. 3 in VELACS extension project. Numerical Models in Geomechanics, Proceedings, 6th International Symposium, NUMOG VI, Montreal, Canada (July 1997); Edited by S. Pietruszczak, G.N. Pande (Balkema, 1997).
F12. Chan, A.H.C., Madabhushi, S.P.G. Finite element method and centrifuge modelling of dynamic soil behaviour. Proceedings, CMEM 97, Computational Mechanics and Engineering Methods, Thessaloniki, Greece (August 1997).
F13. Chan, A.H.C., Madabhushi, S.P.G. Numerical predictions for model no.2 and no.3 in VELACS extension programme. Invited paper. Proceedings, VELACS (Verification of Liquefaction Analysis by Centrifuge Studies) Extension Conference, Davis, CA, USA, 35-36 (October 1996).
F14. Evans, C.W., Al-Tabbaa, A. Treatability study on the solidification of two soils. Geoenvironmental Engineering, Contaminated Ground: Fate of Pollutants and Remediation, Conference Proceedings, Cardiff (September 1997); Edited by R.N. Yong, H.R. Thomas, 293-298 (Thomas Telford, 1997).
F15. Loroy, J.J.C., Soga, K., Savvidou, C., Britto, A.M. Finite element analysis of consolidation and contaminant transport in porous media. Environmental Geotechnics, Proceedings, 2nd International Congress on Environmental Geotechnics, Osaka, Japan (November 1996); Edited by M. Kamon, 263-268 (Balkema, 1996).
F16. Lynch, R.J. Advances in monitoring and in-situ measurement of contaminants in soil. NICOLE (Network for Industrially Contaminated Land in Europe) Workshop on Contaminated Land Monitoring and Measurement, Brighton (May 1997).
F17. Lynch, R.J. Detecting non-aqueous phase pollutants in soils using a probe based on refractive index changes. Seminar/Workshop on Contaminants in Freezing Ground, Cambridge (July 1997).
F18. Madabhushi, S.P.G. Ground accelerations in the aseismic design of soil reinforcements. Proceedings, International Symposium on Earth Reinforcements, IS-Kyushu 96, Fukuoka, Japan (November 1996).
F19. Madabhushi, S.P.G. Modelling of deformations in dynamic soil-structure interaction problems. Proceedings, VELACS (Verification of Liquefaction Analysis by Centrifuge Studies) Extension Conference, Davis, CA, USA, 3738 (October 1996).
F20. Madabhushi, S.P.G., Collison, C.H., Wilmshurst, T. Development of a mini electro-magnetic earthquake actuator. Proceedings, 1st International Symposium on Structures and Foundations in Civil Engineering, Hong Kong, 231-236 (January 1997).
F21. Madabhushi, S.P.G., Peiris, L.M.N., Schofield, A.N. Seismic modelling of embankments on saturated soil deposits. Proceedings, 1st
International Conference on Earthquake Resistant Engineering Structures, Thessaloniki,
Greece (October/November 1996).
F22. Madabhushi, S.P.G., Rajalakshmi, M., Usmani, A., Fairbairn, D.D. Finite element analyses of impacted allografts for total hip arthoplasty (THA). Proceedings, BIOSIM 96, 1st International Conference on Simulation Modelling in Bioengineering, Merida, Venezuela, 164-174 (October 1996).
F23. McDowell, G.R., Bolton, M.D. A micro-mechanical model for over-consolidated behaviour in soils. Powders and Grains 97, Proceedings, 3rd International Conference on Powders and Grains, Durham, NC, USA (May 1997); Edited by R.P. Behringer, J.T. Jenkins, 203-206 (Balkema, 1997).
F24. McDowell, G.R., Bolton, M.D., Robertson, D. The fractal crushing of granular materials. Journal of the Mechanics and Physics of Solids, 44, (12), 2079-2102 (1996).
F25. Mitchell, J.K., Baxter, C.D.P., Soga, K. Time effects on the stress-deformation behavior of soils. Invited paper. Proceedings, Professor Sakuro Murayama Memorial Symposium, Kyoto, Japan, 1-64 (April 1997).
F26. Nakagawa, K., Soga, K., Mitchell, J.K. Closure: pulse transmission system for measuring wave propagation in soils. Journal of Geotechnical and Geoenvironmental Engineering (ASCE), 123, (9), 884 (1997).
F27. Nakagawa, K., Soga, K., Mitchell, J.K. Observation of the Biot compression wave of the second kind in granular soils. Géotechnique, 47, (1), 133-147 (1997).
F28. Potter, L.J., Loroy, J.J.C., Soga, K., Savvidou, C., Gibson, R.E. Mineral waste disposal - numerical and centrifuge modelling. Proceedings, 14th International Conference on Soil Mechanics and Foundation Engineering, Hamburg, Germany, 1847-1852 (September 1997).
F29. Robertson, D., Bolton, M.D. Densification by successive crushing of grains. Numerical Models in Geomechanics, Proceedings, 6th International Symposium, NUMOG VI, Montreal, Canada (July 1997); Edited by S. Pietruszczak, G.N. Pande, 439-444 (Balkema, 1997).
F30. Soga, K. Geotechnical aspects. In: Hyogoken Nanbu (Kobe) of 17 January 1995, a Field Report, Institution of Structural Engineers, Earthquake Engineering Field Investigation Team (EEFIT), chapter 8 (Institution of Structural Engineers, 1997).
F31. Soga, K. Seismological aspects. In: Hyogoken Nanbu (Kobe) of 17 January 1995, a Field Report, Institution of Structural Engineers, Earthquake Engineering Field Investigation Team (EEFIT), chapter 3 (Institution of Structural Engineers, 1997).
F32. Soga, K., Bransby, M.F., Sato, M. Experimental investigation of stiffness degradation and damping behaviour of sands with different viscosity pore fluids. Proceedings, SDEE'97, 8th International Conference on Soil Dynamics and Earthquake Engineering, Istanbul, Turkey (July 1997).
F33. Soga, K., Mitchell, J.K. Rate-dependent deformation of structured natural clays. Proceedings, American Society of Civil Engineers National Convention, Washington, DC, USA, 243-257. Geotechnical Special Publication 61 (November 1996).
F34. Treadaway, A.C.J., Lynch, R.J., Bolton, M.D. Pollution transport studies using an in-situ fibre-optic photometric sensor. Geoenvironmental Engineering, Contaminated Ground: Fate of Pollutants and Remediation, Conference Proceedings, Cardiff (September 1997); Edited by R.N. Yong, H.R. Thomas, 151-60 (Thomas Telford, 1997).
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