Current projects:

• THE BEHAVIOUR OF SEA-BED SOILS UNDER GENERAL STRESS CONDITIONS

• THE STRESS-STRAIN & STRENGTH BEHAVIOUR OF RECONSTITUTED UNSATURATED SOILS

• SWELLING PRESSURE AND PORE WATER SUCTION IN COMPACTED BENTONITE AS USED IN ENGINEERED BARRIERS

• DEFORMATION CHARACTERISTICS OF SANDY SOILS  FROM LABORATORY TESTING

• VEGETATION AND EMBANKMENT MOVEMENTS

• THE MEASUREMENT OF GROUND MOVEMENTS DUE TO BORED TUNNELS

• THE DIRECT MEASUREMENT OF SOIL SUCTION

• INVESTIGATIONS INTO THE BEHAVIOUR OF DISPLACEMENT PILES UNDER CYCLIC AND SEISMIC LOADS

The first commercial gas reserve was discovered in the North Sea in 1966. During the last 30 years over 100 commercial discoveries have been made resulting in the installation of over 175 fixed structures, 90 per cent of which are based on pile-supported steel jacket structures, the remaining 10 per cent of platforms being gravity based structures. The future is likely to bring new discoveries, particularly from the deep water fields located offshore north-west Scotland and north of latitude 62^oN in the Norwegian Sea.

The total cost of the investment in existing platforms can be in excess of 1.5 billion pounds with those planned in the future as high as 3 billion pounds. Because of the high cost of developing offshore hydrocarbon reserves it is essential that production platforms be designed and built in the most economical way. Foundation design is one of the most important factors in the cost of a production platform, however, the procedures used in offshore foundation design are mostly of an empirical or semi-empirical nature. Significant improvements in both empirical design procedures, and the theoretical understanding of foundation behaviour can be obtained by large scale field tests. The cost of carrying out such tests offshore is largely prohibitive. Therefore, there is a requirement for onshore geotechnical test sites having soils similar to the North Sea.

In 1985 the large diameter pile test program was undertaken, the purpose of the work being to improve the design method of offshore piled foundations and gain more insight regarding the fundamental behaviour of long piles in cohesive soils. A thorough literature review of UK Quaternary deposits was undertaken to identify potential onshore sites similar to the North Sea sediments. From this survey two sites were chosen, Tilbrook Grange (OC silty clay) in the Bedford and Huntington region and Pentre (NC silty clay) in the Shrewsbury region in Shropshire. Extensive site investigations were carried out at both sites using techniques adopted in offshore investigations followed by compressive and tensile load tests on two 0.762m diameter tubular steel piles of length's 40m and 60m respectively. The Norwegian Geotechnical Institute (NGI) also conducted two pile tests on 0.219m diameter piles at the Pentre site.

As part of the ongoing research on the fundamental behaviour of displacement piles Imperial College has performed research using an instrumented model pile at several sites namely Canons Park (London Clay), Labenne (loose sand), Cowden (glacial till), Bothkennar (NC silty clay), and more recently at Dunkerque (dense sand) and Pentre (NC silty clay / clayey silt).

The purpose of this research project is to perform a comprehensive study on the behaviour of sea bed soils through similar onshore geotechnical tests sites, in particular the material found in Pentre in Shropshire, and to compare its behaviour with other "benchmark" materials. A smaller site investigation was commissioned by Imperial College at Pentre with thin walled piston samples retrieved for further laboratory testing.

The initial testing program on the Pentre clay-silt has just been completed with a wide range of tests performed including sample descriptions, index tests, grading analyses, one dimensional compression tests, filter paper tests, SHANSEP stress path triaxial tests with small strain measurements, resonant column/hollow cylinder tests and bender element tests. An important conclusion from this initial study has been the identification of a range of soil structures over the depth of the sampled profile - laminated, massive and highly non-uniform structures including small and large scale slumping - a direct consequence of the freshwater glacio-lacustrine environment in which they were deposited. It has been shown that these fabrics can effect the fundamental behaviour of the material. In general, it has been shown that the sedimentation processes that occurred in the glacio-lacustrine environment around Pentre are part of a larger picture involving lacustrine, estuarine, deltaic and marine environments. Indeed, the sedimentation processes for the glacio-lacustrine environment at Pentre are similar in many respects to all the other aforementioned environments and as such should be considered in the more general picture.

The next stage of this research project is to try quantify and systematically relate the effects of these sedimentary fabrics on the geotechnical soil properties and link the individual material behaviour to the in-situ tests performed to date and those planned in the near future.

Until fairly recently the investigation of soils has concentrated on those which are fully saturated (ie composed of solids and liquid). This is mainly because of the inherent problems of testing soils in the unsaturated state (ie composed of solids, liquid and air). in which the principal of effective stress no longer applies.

The last 10 years has seen a significant increase in the amount of research into unsaturated soil behaviour. The first generalised model of stress-strain-strength behaviour has been put forward by Alonso, Gens and Josa (1990), and new laboratory testing techniques have been developed. However there is still a shortage of high quality laboratory data to substantiate the model.

Traditionally one of the main problems associated with the testing of unsaturated soils has been the inability to directly measure the negative pore-water pressures (suctions) present within them. This has been largely overcome by the development of the Imperial College tensiometer (Ridley,1993). This small device allows suctions up to 1500kPa to be directly measured in a few minutes.

More recently, a suction controlled oedometer has been constructed in which the suctions in a soil are measured using the tensiometer and can be controlled using a osmotic system developed by Dineen (in prep.). This allows a known suction to be applied to a sample by circulating a solution of Polyethylene Glycol (PEG) under a semi-permeable membrane which is in contact with the sample. The PEG draws moisture from the sample through the membrane but, because of the large diameter of the PEG molecules, is unable to transgress the membrane itself. The suction generated within the sample is dependent on the concentration of the PEG solution.

The aim of this project is to extend the range of testing of unsaturated soils into a triaxial stress-path apparatus. This will include a modified version of the osmotic suction control system which will be included in one of the loading platens. Measurement of the suctions within the sample will be via mid-height suction probes and also by a probe included within the sample top cap.

Because the cell fluid in an unsaturated soil is compressible, it is not possible to assess volume change by measuring the volume of water flowing in or out of the sample and therefore direct measurement will be needed. It is intended to use local strain measurement devices to determine the sample volume change in the form of strain belts (for radial changes) and inclinometers (for axial strain measurement).

Most of the high quality testing which has previously been carried out on unsaturated soils has been on compacted samples. For this research it has been decided to use an artificial reconstituted soil in order to determine the intrinsic behaviour of the soil without any affects of structure which are induced by compaction.

An artificial soil mix has been created, comprising a silt/clay mix, which is suitable for testing within the equipment. The samples will be reconstituted from a slurry and should display the full range of unsaturated soil behaviour (ie from fully saturated to largely de-saturated) within the measurement range of the suction probe. In addition, the soil is mildly expansive to allow investigation of its volumetric properties.

It is hoped that a suite of tests will be carried out using the new apparatus which will enable a constitutive model to be developed in the context of the Barcelona framework for use in the Imperial College Finite Element Program (ICFEP).

Alonso E.E., Gens, A. and Josa, A (1990) A constitutive model for partially saturated soils. Geotechnique 40, No.3, pp 405-430

Dineen, K. and Burland J.B. (1995) A new approach to osmotically controlled oedometer testing. Proc. 1st International Conference on Unsaturated Soils, Vol X, pp xxx-xxx

Dineen, K. (1997) The influence of suction on compressibility and swelling. Ph.D Thesis, University of London

Ridley A.M. (1993) The measurement of soil suction, Ph.D Thesis, University of London

Researchers : Dr. Kieran Dineen

Mr. Julio E. Colmenares Montañez

Supervisors : Dr. Andrew M. Ridley

Prof. John B. Burland

Sponsors : Engineering and Physical Sciences Research Council

Universidad Nacional de Colombia, Bogotá.

Classical Soil Mechanics has developed in temperate areas of the world. As a result, it is mainly concerned with the behaviour of fully saturated soils. Compacted soils are a special case in which the pore water is at a pressure less than the ambient atmospheric pressure and the pores are not fully saturated. Engineered barriers used in the containment of waste products are frequently constructed from compacted expansive clays. Gens and Alonso (1992) introduced a constitutive model that describes the behaviour of highly expansive compacted clay. The authors acknowledged that, with only limited data available, many facets of the proposed model were uncertain and required further verification.

Over the past forty years the research team within the Soil Mechanics Section at Imperial College has undertaken laboratory work to investigate the behaviour of partly saturated soils. Recent work has resulted in the development of an oedometer system that includes radial stress measurement (Schreiner and Burland, 1992), a miniature tensiometer capable of measuring absolute pore water suctions up to 1500 kPa (Ridley and Burland, 1993) and a system for controlling suction (and hence the wetting/drying of the soil) under atmospheric conditions, over the same range as the tensiometer (Dineen and Burland, 1995). A recent interpretation of stress path oedometer tests on a compacted expansive African clay which incorporated radial stress measurement has produced fresh evidence with which to examine the Gens and Alonso model (Burland and Ridley, 1996).

In the present work, it is proposed to undertake a new testing programme developing and using equipment that measures the appropriate stresses. The data will be used to enhance the Gens and Alonso model for predicting the behaviour of engineered fills used in the containment of waste products.

Burland, J.B. and Ridley, A.M. (1996) The importance of suction in soil mechanics. Keynote address, Proc. 12th South-East Asian Geotechnical Conference. Kuala Lumpur.

Dineen, K. and Burland, J.B. (1995) A new approach to osmotically controlled oedometer testing. Unsaturated Soils (Ed. by Alonso, E.E. and Delage, P.), Balkema, Rotterdam.

Gens, A. and Alonso, E.E. (1992) A framework for the behaviour of unsaturated expansive clays. Canadian Geotechnical Journal, 33, 11-22.

Ridley, A.M. and Burland, J.B. (1993) A new instrument for the measurement of soil moisture suction. Geotechnique 43, No. 2, 321-324.

Schreiner, H.D. and Burland, J.B. (1992) A comparison of three swell test procedures.Geotechnics in the African Environment (Ed. by Blight, G.E. et al.), Balkema, Rotterdam.

DEFORMATION CHARACTERISTICS OF SANDY SOILS

In many geotechnical engineering problems, accurate evaluation of the deformation properties of materials is essential for the prediction of ground behaviour and displacement of structures. Several methods for the measurement of soil stiffness are currently used in the laboratory and field for this purpose.

For this research, the deformation characteristics of sandy soils at relatively low strain levels are being investigated with particular attention to anisotropic behaviour. This involves triaxial testing, and hollow cylinder work will be supplemented for the independent control of the intermediate stress or for the application of torsional shear. Two sands, Ham River Sand (clean quartz sand) and Dunkerque Sand (fine dense sand with calcareous) are being currently used. A preliminary study involving resonant column, torsional shear, and bender element oedmeter tests on these materials have just been completed, with a larger testing program in progress. This work will reinforce our understanding of the anisotropic behaviour of these materials.

Triaxial testing involving 100mm diameter specimens is being currently performed utilising axial and radial local strain measurements. Specimens are reconstituted by air pluviation followed by the application of a vacuum and water flushing to obtain quick saturation. Lubricated ends are being used to achieve a deformation as uniform as possible throughout the test. The initial program involves undrained shearing in compression and extension and will be extended further using drained tests allowing different effective stress path directions from a common stress state to be investigated.

Separately, the measurement of the shear and compression wave velocity in the triaxial apparatus using piezoceramic elements is being planned. These non-destructive tests will provide us with information on the shear modulus and bulk modulus at very small strain levels, and will bridge the gap between laboratory and field measurement.

In the south east of England much of the railway network is founded on embankments made from high plasticity London Clay fill. Over the past 70 years these embankments have become quite densely vegetated and are now an integral part of the city environment. Recent studies have shown that these embankments exhibit seasonal movements, resulting in high maintenance costs through the need for regular monitoring and remedial works. If the mechanism behind the movements could be identified, a more cost effective maintenance schedule could be designed.

It is widely known that vegetation helps to stabilise slopes through several mechanisms. One such mechanism is the reduction of the pore water pressure through evaporation and transpiration, resulting in an increase of effective stress within the embankment. During periods of wet weather the pore water pressures within the embankment can be quite substantial, however during periods of prolonged dry weather the vegetation may exert a pore water suction. Tensiometers are currently the only method for the direct measurement of soil suction. A better understanding, of the theory behind the use of tensiometers in engineering, is being developed.

Past research, carried out principally by arboriculturists, indicates that trees have a variety of responses to changes in the soil moisture. Deep rooting trees can rapidly produce a second root system in times of drought. Intermediate rooting trees seem to require a higher degree of drought stress to initiate the growth of a second root system. Whilst shallow rooting trees will be the first to show signs of drought distress as they have limited genetic capabilities to pursue moisture at depth.

If vegetation types respond uniquely to changes in the soil moisture, they could also be capable of generating different magnitudes of pore water suction, and hence different seasonal movements. One of the principal aims of the present research is to correlate vegetation type with seasonal embankment movements. Through analysing the data from simple vegetation surveys, precise levelling of embankments and measured pore water suctions, it is hoped to produce more detailed guidelines for vegetation management.

In addition, the compressibility characteristics and permeability of the fill material is being defined by filter paper drying, oedometer and dissipation tests.

THE MEASUREMENT OF GROUND MOVEMENTS DUE TO BORED TUNNELS

EPSRC and LUL have funded a project to measure the response of the ground due to bored tunnelling at two greenfield "control" sites along the route of the Jubilee Line Extension Project. The first site is located in St. James's Park where twin running tunnels are constructed in London Clay overlain with Terrace Gravels and Made Ground. The second site is situated south of the river Thames in Southwark Park; the tunnels beneath this site are constructed in Woolwich and Reading beds and Thanet Sands. At both sites state-of-the-art instrumentation is used to measure the three-dimensional surface and subsurface movements of the ground above and adjacent to the tunnels along with associated piezometric and earth pressure changes.

The project compliments a LINK/CMR program on building subsidence, damage and repair which is also based at Imperial College.

Monitoring consists of both traditional and novel techniques to capture a complete three-dimensional picture of the ground response. The table below summarises the monitoring techniques and equipment being employed.

St. James's Park. Instrumentation was installed in January - March 1995. The westbound running tunnel constructed from Waterloo to Green Park passed beneath the site in April 1995. The eastbound tunnel was constructed in January 1996.

Southwark Park. Instrumentation installation was completed in October 1995. The westbound running tunnel passed beneath the site in January 1996. The eastbound tunnel passed beneath the site in June 1996.

Precision levelling results shown in Figure 1 for St. James's Park show the subsidence troughs measured above both tunnels. The volume losses, represented as the volume of the surface settlement trough divided by the theoretical volume of soil excavated at the tunnel face per metre advance, varied from 3.3% for the westbound tunnel (axis 31m bgl) to 2.9% for the eastbound tunnel (axis level 20.5m bgl). The inset plot of Figure 1 showing the logarithm of settlement, s, divided by maximum settlement, s_max versus the squared offset distance from the tunnel axis, y², suggests that the displacement profiles do not follow exactly a Gaussian distribution. It is also apparent that the eastbound subsidence profile is asymmetrical about the tunnel axis with a significantly wider trough exhibited nearer the previously constructed westbound tunnel.

Burland, J.B., Mair, R.J., Linney, L.F., Jardine, F.M. and Standing, J.R. (1996). A collaborative research programme on subsidence damage to buildings: prediction, protection and repair. Proc. Int. Symp. on Geotechnical Aspects of Underground Construction in Soft Ground, City University, London.

Standing, J.S., Nyren, R.J., Longworth, T.I. and Burland, J.B. (1996). The measurement of ground movements due to tunnelling at two control sites along the Jubilee Line Extension. Proc. Int. Symp. on Geotechnical Aspects of Underground Construction in Soft Ground, City University, London.

THE DIRECT MEASUREMENT OF SOIL SUCTION.

[This research is a continuation of work formerly sponsored by TRL and EPSRC]

In many ground engineering situations the pore water pressures (both in situ and in laboratory tests) are positive. However, even in temperate climates, there are instances when the soil has a net attraction for water (eg. in compacted fills, where desiccation may have occurred, to quantify the disturbance caused during sampling and in the testing of natural unsaturated soils).

Until recently the attractive stress (or soil suction) was measured in the laboratory using either the axis translation method (eg. the triaxial suction test) or the filter paper method. However these techniques are relatively slow to reach equilibrium; several hours in the case of the axis translation and one week for the filter paper method. Furthermore in situ measurements were limited to values of less than 100 kPa using a tensiometer and the interpretation of such data was hampered by the hysteretic nature of the suction-water content characteristics for soil.

Research in the soil mechanics section of Imperial College has resulted in a new form of tensiometer that is for the first time capable of measuring absolute pore water suctions in excess of 100kPa (Ridley and Burland, 1993). The "suction probe" consists of a small pressure sensing diaphragm which is strain gauged on one side and fitted with a fine porous ceramic filter on the other (figure 1). Between the filter and the active face of the transducer is a small reservoir of water which is inundated using a high positive water pressure.

The instrument can measure pore water suctions up to 1500 kPa and will also react in response to positive pore water pressures. Equilibrium is typically reached in a few minutes and the small amount of moisture exchange means that the measurements are very accurate.

The suction probe has been adapted for measuring in situ suction (Ridley and Burland, 1995). A 4 inch diameter borehole is augered and fitted with a plastic lining tube. A secondary borehole is carefully augered at the bottom of the initial hole using a very sharp flat faced cutting tool and the soil taken from this depth can be used to determine the water content. The suction probe is then located flush with the soil at the bottom of the secondary borehole.

Recent work with the suction probe in a residual soil profile of southern Brazil (Ridley et al. 1997) has demonstrated the usefulness of making simultaneous in situ suction and water content measurements.

Ridley A.M. and Burland J.B. (1993). A new instrument for the measurement of soil moisture suction. Géotechnique 43, No. 2, 321-324.

Ridley A.M. and Burland J.B. (1995). A pore pressure probe for the in situ measurement of soil suction. Advances in Site Investigation Practice. Thomas Telford. London. pp 510-520.

Ridley A.M., Schnaid F., da Silva G.F. and Bica A.V.D. (1997). In situ suction measurements in a residual soil of southern Brazil. NSAT'97 - 3^o Simpósio Brasileiro sobre Solos Não Saturados. Rio de Janerio, Brazil.

DISPLACEMENT PILES UNDER CYCLIC AND SEISMIC LOADS

Most common design methods for the shaft capacity of piles in clay use simple empirical correlations relating shear stress with soil parameters such as undrained shear strength, S_u or effective overburden pressure, s’_vo. Research into the effective stress conditions affecting shaft capacity has shown that shaft resistance is sensitive to factors such as pile length, pile type, method of installation, soil overconsolidation, clay sensitivity and direction and rate of loading which the traditional design methods are unable to account for.

In the last few years Imperial College has developed a research focus on the need for a more fundamental approach to pile design using the principle of effective stress, which enable the variations caused by different soil conditions and pile types to be analysed in a more rational manner. This research, that has led to the development of the Imperial College Design Method, has consisted mainly of three stages:

a) The first stage involved developing the Imperial College Pile (ICP) instruments and experimental procedures, and performing multiple ICP tests at the Canons Park test site (Bond, 1989). The Imperial College pile (ICP) consists of a heavily instrumented, 100 mm diameter, closed-ended steel pipe. The instruments are able to measure radial total stresses, local shear stresses, axial pile loads and pore pressures at various levels along the pile.

b) In the second stage, experiments allowed new design approaches to be proposed for closed-ended piles in sands and clays (Lehane, 1992). The results shown that pile behaviour is heavily dependent on the soil consistency, interface friction angle, overconsolidation ratio and sensitivity. Pile characteristics also influence the load capacity, particularly the distance of a soil element from the pile tip (h/R effect).

c) The third phase (Chow 1997) involved performing advanced site investigations and multiple ICP tests in silty clay and dense sand. Chow's work include performing and interpretating tests on full-scale driven open-ended piles at the ICP sites to assess the effects of scale, installation methods and pile-end conditions, including experiments to assess pile group and ageing effects in dense sand. Using those results the design method for closed-ended piles was refined and extended to cover open-ended driven piles. The new design method was calibrated and validated with an up-to-date database of over 120 full-scale pile tests.

In the proposed design method shaft failure is governed by Coulomb effective stress interface sliding saw, in which the basic mechanism is: