125 results found
Wan MSP, Standing JR, Potts DM, et al., 2021, Discussion of pore water pressure and total horizontal stress response to EPBM tunnelling in London Clay, Geotechnique: international journal of soil mechanics, Vol: 71, Pages: 368-372, ISSN: 0016-8505
Shire T, Standing J, 2021, Strength and stiffness properties of an unsaturated clayey silt: experimental study at high degrees of saturation, International Journal of Geomechanics (ASCE), ISSN: 1532-3641
Unsaturated constant water content triaxial compression tests with suction measurement using an Imperial College Tensiometer, and saturated consolidated undrained tests were carried out on reconstituted Brickearth, a naturally unsaturated clayey silt from London. The results show that the saturated effective stress can be applied to the critical state line (CSL) and normalised stiffness for unsaturated Brickearth but that Bishop’s effective stress variable gives a slightly improved CSL. The stiffness derived from local instrumentation demonstrates that Bishop’s effective stress is also beneficial for normalising the stiffness 16modulus over the small strain range (up to axial strainsofabout3%).
Standing JR, 2020, Identification and implications of the London Clay Formation divisions from an engineering perspective, Proceedings of the Geologists' Association, Vol: 131, Pages: 486-499, ISSN: 0016-7878
Historically, engineers frequently viewed the London Clay Formation (LC) as uniform, homogeneous and rather uninteresting. Chris King's seminal work provided a much deeper insight into the characteristics of the LC and how it can be split into divisions, based on the depositional history of the formation and the microfauna present. LC water content profiles were compiled from continuous cores as part of an investigation into variations in tunnelling-induced settlements across St James's Park, allowing distinct zones to be identified. These were found to coincide exactly with the divisions identified by Chris King almost twenty years earlier. Water content profiles can be developed as part of a ground investigation and used to help establish the boundaries between King's divisions. Based on two further water content profiles from Hyde Park and St John's Wood a new methodology for locating the boundaries of the divisions, involving a trend-line for sub-Division A3, is proposed and tested, relevant to conditions in central London. In developing the method, significant differences in the elevation of the divisions between the three sites is observed, suggesting geological processes such as folding or faulting have influenced the LC along the ∼5-km length of the section.Once the boundaries of the LC divisions are known, geotechnical engineers have a greatly improved overall understanding of the ground conditions and how the ground will respond to engineering works such as tunnelling and deep excavations. Broad engineering implications of the divisions are described and discussed, citing case histories where possible.
Standing JR, Buss SR, 2020, Measurement and monitoring, Quarterly Journal of Engineering Geology and Hydrogeology, Vol: 53, Pages: 349-351, ISSN: 1470-9236
Standing J, vaughan P, charles-jones S, et al., 2020, Observed behaviour of old railway embankments formed of ash and dumped clay fill, Geotechnique: international journal of soil mechanics, ISSN: 0016-8505
Many old railway embankments were originally formed from loose dumped clay fill on which ash fill was subsequently placed to maintain the track level. These have required considerable maintenance, primarily because of embankment movements. They are mostly covered by trees, and tree roots are present in both fills. As part of a London Underground Limited programme of stabilisation works in the 1990s, two embankments were instrumented to investigate the mechanisms and causes of movement. Lateral deformations, settlements and pore pressures were measured. This paper describes the instrumentation and monitoring techniques that were adopted and presents the findings from the study. It was found that non-recoverable seasonal movements occur in both the ash fill and the clay fill. The former occur in dry weather, particularly in the slopes of the embankments crests, due to ash particle mobility under train loading when the ash is dry. Clay fill deformations are exacerbated by the presence of tree roots. Movements correlate well with climate, as quantified by the soil moisture deficit determined from meteorological data. Establishing the mechanisms of movement within these ash–clay fill embankments helped to guide the design of stabilising measures.
Wong LX, Shire T, Standing J, 2020, Effect of depositional water content on the collapsibility of a reconstituted loess, Quarterly Journal of Engineering Geology and Hydrogeology, Vol: 53, Pages: 283-289, ISSN: 1470-9236
Loess, a wind-blown silty soil, can be deposited under a variety of moisture conditions, including dry deposition, wet deposition and gravitational settling of aggregations formed in moist air by capillary forces at grain contacts. This experimental study uses single and double oedometer tests to assess the effect of depositional water content on the collapse potential of reconstituted samples of the Langley Silt Member, known as Brickearth, a natural loessic soil. A freefall sample preparation technique was used to mimic loess formation and environmental scanning electron microscopy was used to relate the observed behaviour to sample fabric. The results show that loess deposited at higher water contents has a greater collapse potential, which is shown to be related to its looser, more granular fabric.
Standing J, 2020, John Burland’s deep-excavation- and tunnelling-related research and industry involvement, Goetechnical Engineering Journal of SEAGS and AGSSEA, ISSN: 0046-5828
During the time that Professor John Burland was an expert witnessforthe Parliamentary hearings for the JubileeLine Extension Project(JLEP), he realised that although tunnels had been constructed in London for more than a century, there were very few well documented case studies describing the response of buildings to tunnelling-induced settlement. Professor Burland had extensive knowledge of the effects of ground movement on buildings, having studied and published his seminal work with Professor Peter Wroth in the 1970swhich he with others developed into a staged process for assessing potential structural damage from excavation-induced ground movements. Construction of the JLEP provided an ideal opportunity to compile a set of exemplary case studies (involvingdifferent structural forms, foundation types,tunnelling methods and geological conditions)and he harnessed this to its full extent. At the start,anumber of ‘gaps in knowledge’ were identified and these were addressed over the following years of monitoring and data analysis. The research culminated in a two-volume book that is still widely referenced almost twenty years later.In this paper a background to ProfessorBurland’s tunnelling-and deep-excavation-related research is given and the gaps in knowledge are summarised along with how they were answered through the JLEP researchfindings. They are reinforced with other more recent tunnelling projects in London that hehas been involved with, in particular the Crossrail project, thus furthering the understanding of ground and structural response to tunnelling, benefittingboth industry and academia.
Avgerinos V, Potts DM, Standing JR, et al., 2019, Predicting tunnelling-induced ground movements and interpreting field measurements using numerical analysis: Crossrail case study at Hyde Park DISCUSSION, Géotechnique, Vol: 69, Pages: 936-939, ISSN: 0016-8505
Ghail R, Standing J, 2019, Development of an engineering geology field trip for civil engineering students, Quarterly Journal of Engineering Geology and Hydrogeology, Vol: 53, Pages: 74-87, ISSN: 1470-9236
This paper describes and discusses the various elements of a one-week Engineering Geology field trip that has been developed for second-year undergraduate students studying Civil Engineering at Imperial College London. It is an essential component of the education of civil engineers and, as such, is a requirement of the accreditation defined by the ICE JBM. The trip is structured to develop the students’ awareness of geological features and their ability to record and sketch key observations in the field. Having described the geological features, the students are prompted to think about consequent potential engineering hazards relating to them and also the influence of human activity, past and present, on the ground and environment. During the course of the week the students develop their observational and logging skills, with constant staff feedback both outdoors and during summary student presentation sessions in the evenings. A marked progressive improvement has been noted as a consequence of this approach. On the final day of the week the students have to map a coastal section, observing and recording the stratigraphy and significant features such as bedding, discontinuities and faulting, with the latter quantified by measuring quantities such as dip, strike and plunge, as appropriate. The students’ work, assessed as part of the field trip, is completed by them and handed in just before final departure at the end of the week, most of it being completed in the field.
Wan MSP, Standing JR, Potts DM, et al., 2019, Pore water pressure and total horizontal stress response to EPBM tunnelling in London Clay, Géotechnique, Vol: 69, Pages: 434-457, ISSN: 0016-8505
The ground response, in terms of surface and subsurface displacements, to twin-bore Crossrail tunnel construction beneath a research monitoring site in Hyde Park, London, using earth-pressure-balance machines (EPBMs) in London Clay, has recently been reported in two companion papers by the authors. This third paper presents and discusses corresponding changes in pore water pressure and total horizontal stress measured using multi-level piezometers and pushed-in spade cells. The three papers together provide a comprehensive and completely unique field monitoring case history of the short-term ground response to EPBM tunnelling in London Clay, making them invaluable for validating future numerical analyses. The fully grouted vibrating-wire piezometers were able to measure the rapid pore water pressure changes around the tunnels as they were constructed. Five distinct immediate pore water pressure responses are identified, induced by different stages of the tunnel drives as the EPBMs approached and passed the instruments. The responses are correlated with tunnel-boring machine operation variables and a postulated arching mechanism, identified for the first time through field measurements. The sense and magnitude of changes in horizontal total stress were reasonable and are correlated with overall pore water pressure changes. Both responses are linked where possible with measured subsurface displacements and generally correlate well, at least qualitatively. Limitations to the measurements and influencing factors are also discussed.
Avgerinos V, Potts DM, Standing JR, 2018, Numerical investigation of the effects of tunnelling on existing tunnels., Géotechnique, Vol: 67, Pages: 808-822, ISSN: 0016-8505
Construction of the Crossrail tunnels just beneath the existing Central line tunnels at the northern side of Hyde Park provided the impetus for this paper. A basic three-dimensional (3D) finite-element (FE) model was developed to study a general case of a new tunnel (NT) crossing perpendicularly below an existing tunnel (ET). A series of 3D FE analyses was carried out and the results presented in this paper reveal some of the interaction effects. Changes in hoop forces, bending moments and lining deformations of the ET due to excavation of the NT are discussed. Conclusions are drawn about how the relative position of the excavation face of the NT in relation to the ET's axis affects the latter's behaviour. Cross-sectional and longitudinal deformations of the ET are discussed, leading to recommendations for field monitoring of similar interaction cases. Two parametric studies were also carried out to quantify the effects of the magnitude of the earth pressure balance machine face pressure and the longitudinal stiffness of the ET on the predicted behaviour of the ET due to construction of the NT.
Mantikos V, Tsiampousi A, Standing JR, 2018, Swelling behaviour of an expansive clay at high suction, 7th International Conference on Unsaturated Soils, Publisher: UNSAT
Deep geological disposal designs for nuclear waste often include an engineered barrier to protect the waste canistersand prevent leakage. The long-term safety of the repository relies on studies of the buffermaterial.Oedometer tests provide values ofdesign parameters fornumerical simulations. Anewly-developed oedometer with automated suction control is presented to assist in the investigation of the coupled hydro-me-chanical-volumetric behaviour of an expansive clay, namely a natural sodium bentonite. The displacement-controlled device was developed to apply suctionover a range of10 MPa to 300 MPausing a divided-flow humidity-generator. The device allows the application of combined stress and suction states, and continuous stress paths of constant volume, stress or suction. The development of the new oedometer is described. Results obtained during the preliminary tests are evaluated through comparison with experimental data from similar tests found in the literature. The current method benefits from continuous control of suction with servo-control of relative humidity using calibrated capacitance hygrometers. The system self-compensates for minor temper-ature changes and therefore the requirement for thermal insulation is not as crucial as in vapour equilibrium methods.
Kimpritis T, Standing JR, Thurner R, 2018, Estimating column diamters in jet-grouting processes, Proceedings of the ICE - Ground Improvement, Vol: 171, Pages: 148-158, ISSN: 1755-0750
Jet grouting is widely used in geotechnical engineering for a variety of applications and is a well-proven technique. As with many techniques developed from a practical perspective, there is still scope for improvements both in construction practice and design. This paper focuses on one of the most crucial elements of quality control required with jet grouting operations, the diameter of the constructed column. First, the jet grouting method is explained and the main issues of the concept highlighted. A description follows of the techniques available for estimating column diameter, discussing their application and evaluation on site. There is a particular emphasis on two methods: inclined core drilling and a newly-developed thermic approach (Meinhard et al., 2010) both of which were implemented on two construction projects. Data from these case studies are reported and analysed extensively, in conjunction with influencing factors such as the ground conditions, to assess their effects on the achieved diameter. An empirical approach for evaluating the diameter of jet-grouted columns is developed based on various factors influencing their size such as monitor lifting speed and soil strength.
Selemetas D, Standing JR, 2018, Response of full-scale piles to EPBM tunnelling in London Clay, Geotechnique: international journal of soil mechanics, Vol: 67, Pages: 823-836, ISSN: 0016-8505
The installation and working test performance of four full-scale instrumented driven piles and their subsequent response to twin tunnels constructed below the pile bases are described. One pair was designed to be largely friction piles and the other pair end-bearing. Their locations relative to the new tunnels were carefully chosen to optimise understanding of pile responses at varying offsets from the centre-lines. The site conditions and the greenfield response to earth pressure balance machine tunnelling at the site were described in a companion paper that reported an expanding displacement field around the tunnels rather than the contracting fields usually observed. The field monitoring results indicated that, during construction, zones of influence existed around tunnels, where the ground and piles were subjected to different degrees and senses of relative vertical displacement. Redistributions of load along the pile lengths occurred as the tunnel boring machines approached, passed beneath and continued beyond the pile bases; lateral pile deflections and bending moments were also induced. Based on the results from this field study, implications for the capacity of existing piles (and design of new piles) subjected to tunnelling-induced movements are assessed for cases of expanding and contracting displacement fields.
Wan MSP, Standing JR, Potts DM, et al., 2018, Measured short-term subsurface ground displacements from EPBM tunnelling in London Clay, Geotechnique: international journal of soil mechanics, Vol: 67, Pages: 748-779, ISSN: 0016-8505
Subsurface ground displacements from the construction of the twin-bore Crossrail tunnels in London Clay by earth pressure balance machines (EPBMs) are presented and discussed, complementing a companion paper by the authors that focused on the surface response. Both papers report vertical and horizontal displacements, in this case measured using comprehensive arrays of instruments installed within boreholes in Hyde Park, London. The Crossrail tunnels are deeper than those cited in most UK case histories concerning tunnelling in stiff clay. Clear insights were gained into subsurface displacement mechanisms: an ‘inward’ displacement field was observed around the Crossrail tunnel construction, in contrast to the ‘outward’ displacement field that developed around the shallower Channel Tunnel Rail Link tunnels constructed east of London using similar EPBMs in London Clay. This has important implications when estimating subsurface displacements using currently available empirical methods. Appraisal of the EPBM operational variables suggests that the relative magnitude of face and tail grout pressures to overburden stress is the key factor contributing to the opposing senses of the observed displacement fields. Earlier tunnelling-induced strain softening of the London Clay is evident from greater subsurface incremental volume losses and settlement trough width parameters relating to subsequent tunnel construction.
Hendarto H, Standing JR, 2018, Ground response to tunnel construction for Jakarta mass rapid transit project, Pages: 335-342
The Jakarta Mass Rapid Transit (MRT) Project, currently under construction, to relieve traffic congestion within the city, has both elevated and underground sections with a transition between them immediately north of Sisingamangaraja station. The tunnel runs from there to Bunderan Hotel Indonesia for 4 km beneath major roads in central Jakarta. Four Earth-Pressure-Balance (EPB) machines with outer diameters of ∼6.7 m are being used to excavate the northbound and southbound tunnels. The tunnels are mostly located in dilluvial strata which comprise stiff to hard silts or clays. Surface settlement predictions have been performed and field measurements analysed to assess the ground response to tunnel construction around the Bunderan Hotel Indonesia. In the early stages of tunnelling, in the northern part of the project (CP106), small degrees of heave, up to about ∼6 mm, occurred directly above the centre-line of the southbound tunnel, reducing with transverse distance to small settlements. Once the Tunnel-Boring Machine (TBM) reached about 320 m southwards, surface movements changed from heave to settlement. The responses suggest that vertical displacements (heave or settlement) depend on TBM variables such as face and tail skin grouting pressures in conjunction with depth of overburden.
The structural testing and finite element (FE) analysis described in this paper were part of a major research project undertaken at Imperial College London to investigate the deformation of bolted segmental grey cast iron (GCI) tunnel linings. A key aim was to quantify how joints influence the behaviour of the lining, through a three-path approach comprising physical experiments, finite element modelling, and field instrumentation. The laboratory results have been used to assess the validity of the tunnel assessment methods used by industry.This study examined joint articulation under the serviceability limit state in the absence of hoop force focussing on factors such as applied bolt preload, the loading direction and the freedom of the circumferential flange to deflect. Two half-scale GCI lining segments were bolted together at the longitudinal flanges to form a bolted arch in a similar fashion to the tests performed by Thomas (1977). Modern instrumentation was implemented to gain detailed measurements quantifying changes in global displacements of the two segments, bolt forces and joint opening under applied loading. For the first time, the physical experiments were conducted contemporaneously with the development of a three-dimensional FE model of the joint. The experimental data and the results from the FE analysis indicate a reduction in joint stiffness as the joint articulates under applied load. It is shown that the presence of a joint has far greater influence on the behaviour of the ‘arch’ than the level of preload applied to the bolts in the joint. The FE analysis allowed the deformation behaviour of the joint under positive and negative bending to be investigated: its response under the two modes differs significantly.
Standing JR, Lau C, 2017, Small-scale model for investigating tunnel lining deformations, Tunnels and Underground Space Technology, Vol: 68, Pages: 130-141, ISSN: 0886-7798
Tunnel linings in situ are rarely truly circular in form. Shapes depend on factors such as the nature of the lining, method of build, connections between segments, self-weight deformations and ground stresses. A small-scale two-dimensional simplified tunnel lining model comprising six segments was developed where the lining was loaded using a reaction ring. The results provide insight into factors that control the lining response: e.g. the deformed shape depends on the nature of the imposed loading, tightness of the bolts, number of segments and joint flexibility. This small-scale model was developed as a precursor to a sophisticated large-scale structural model used to assess the effect of tunnelling on existing tunnels (Yu et al., 2017). In this respect it proved very instructive, especially for assessing the method of loading the ring. An important message from the paper is how an inexpensive, but carefully thought out, small-scale model can provide great insight to the development of large-scale models that will involve orders of magnitude more time and expense. Many of the findings from the small-scale study were verified by the more realistic large-scale model which was able to provide details on deformations and stress changes in the segments and bolts.
Avgerinos V, Potts DM, Standing JR, et al., 2017, Predicting tunnelling-induced ground movements and interpreting field measurements using numerical analysis: Crossrail case study at Hyde Park, Géotechnique, Vol: 68, Pages: 31-49, ISSN: 0016-8505
Ground response to the construction of the Crossrail tunnels in London Clay beneath Hyde Park has been modelled numerically using advanced finite-element analyses. The soil model used for modelling the London Clay was a kinematic hardening soil model (named M2-SKH). This model, when used for the St James's Park greenfield site, provided excellent predictions of tunnelling-induced ground movements. Comparison of the results from the analysis of the Hyde Park greenfield site with associated field monitoring data also suggests excellent predictions, even though in this case the tunnels were: of larger diameter; deeper in the London Clay; and constructed with earth-pressure-balance machines. The influence of lining permeability was found to influence significantly short- and longer-term predictions. Interpretation of the predicted surface and subsurface vertical and horizontal displacements due to the construction of the Crossrail tunnels exemplifies how numerical analysis can assist in explaining and identifying potential ambiguities in field measurements.
Wan MSP, Standing JR, Potts DM, et al., 2017, Measured short-term ground surface response to EPBM tunnelling in London Clay, Géotechnique, Vol: 67, Pages: 420-445, ISSN: 0016-8505
Earth-pressure-balance machines (EPBMs) were used for the construction of Crossrail tunnels in London, providing opportunities for field investigation of consequent ground response. Analysed results from an instrumented research site in Hyde Park with extensive surface and subsurface monitoring arrays are presented and discussed. The Crossrail tunnels at the site are 34·5 m below ground, deeper than those in most case histories of tunnelling in stiff clay in the UK. This paper characterises the tunnelling-induced ground response, both ‘greenfield’ and in the proximity of the existing Central Line tunnels, dealing with measurements at the ground surface. A companion paper covers the subsurface ground response. Vertical and horizontal ground surface displacements were obtained from manual precise levelling and micrometer stick measurements. Several key findings will benefit future tunnelling projects involving EPBMs. Volume loss values measured at the instrumented site were low, being less than 0·8% and 1·4% for the first and second tunnel drives respectively, higher values being associated with ground softening from the first tunnel construction. Smaller volume losses were recorded in the vicinity of the existing Central Line tunnels, compared with the greenfield location, suggesting that their presence inhibited the development of ground movements. Asymmetric settlement troughs developed due to either the nearby pre-existing tunnels or the construction of the first tunnel. Marginally smaller values of trough width parameter, Ky, were determined for these deeper tunnels compared with previous greenfield ground case histories. Resultant vectors of ground surface displacement were directed to well-defined point-sinks above the tunnel axis level.
Afshan S, Yu JBY, Standing JR, et al., 2017, Ultimate capacity of a segmental grey cast iron tunnel lining ring subjected to large deformations, Tunnelling and Underground Space Technology, Vol: 64, Pages: 74-84, ISSN: 0886-7798
Understanding the behaviour of existing tunnels subjected to in-service deformations, as a result of the construction of underground works (e.g. new tunnels) in their proximity, is of importance in order to safeguard infrastructure within the urban environment. The associated deformations that take place during tunnelling have to be carefully assessed and their impact on the existing tunnels needs to be considered. A half-scale segmental grey cast iron (GCI) tunnel lining ring was tested as part of an extensive research project investigating the impact of new tunnel excavations on existing tunnels conducted at Imperial College London. A sophisticated experimental arrangement was developed to deform the ring in a variety of modes under combined displacement and load control. This paper reports on experiments carried out to assess its structural response when subjected to large deformations. The tests reported are the first to be conducted on a realistic scale model under carefully controlled conditions, and provide valuable insight into the behaviour of a GCI segmental ring during distortions commonly observed in reality. Details of the experiments, including the adopted test set-up and the instrumentation employed, are presented. The measured bending moments around the ring, as a result of the applied deformations, are determined and compared with those predicted using the well-known equations given by Morgan (1961) and Muir Wood (1975), often used in industry, as well as those obtained assuming an elastic continuous ring.
Cassar J, Standing JR, 2017, Geomaterials: aggregates, building stone and earthworks: papers from 50 years of QJEGH, QUARTERLY JOURNAL OF ENGINEERING GEOLOGY AND HYDROGEOLOGY, Vol: 50, Pages: 95-105, ISSN: 1470-9236
Numerous papers included within the broad subject of geomaterials have been published in QJEGH over the past 50 years. These have been compiled and divided here into three main categories: aggregates, building stone and earthworks. The aim of this review is to provide a comprehensive summary of the relevant papers published in QJEGH with a view to identifying the main areas of interest historically, now and in the future. Some clear trends are evident from the survey and review; for example, there has been a steady interest in building stone and in particular its deterioration, a decline in papers on earthworks and an increase in those on ground improvement (also covered here). It is also noted that methods of characterizing geomaterials are becoming more sophisticated with advancing technology. The review makes relevant links with other Special Publications from the Geological Society, including also Engineering Geology Special Publications.
Ko FWY, Standing JR, 2017, New risk method to assess tree interaction with structures, Proceedings of the ICE - Municipal Engineer, Vol: 170, Pages: 38-59, ISSN: 1751-7699
Ground and structural responses to presence of trees can be detrimental if their probable interactions are not duly considered. For example, trees growing in swelling/shrinking clays can lead to damage of buildings that are in close proximity. Very often the problem is not identified until appreciable damage has occurred. Guidelines relating to acceptable distances of trees to buildings are available, but these usually require specialist knowledge of trees. Current practices concerning tree protection and the subject of assessing potential structural damage from trees are reviewed and shortcomings are identified. Following this, two new risk methods to assess likelihood of damage to structures are explained. One covers damage occurring from direct growth of tree roots and the other is associated with settlement/heave above shrink/swell clay. Both systems are intended to provide a basis for identifying situations where advice by tree specialists should be sought, ideally in advance of occurrence of damage. The former was tested by field surveys at several locations in and around London and the latter by well-documented case studies. Both demonstrated that the systems have broad applications. This led to the development of a set of recommendations, in the form of an index chart, for planting trees close to structures.
Yu J, Standing J, Vollum R, et al., 2017, Experimental investigations of bolted segmental grey cast iron lining behaviour, Tunnelling and Underground Space Technology, Vol: 61, Pages: 161-178, ISSN: 0886-7798
The need for the research reported in this paper was driven by the Crossrail project in London for which new tunnels were constructed close to numerous existing operational tunnels of the London Underground (LU) network.This research is based on experimental work conducted on half-scale grey cast iron (GCI) tunnel lining segments with chemical composition similar to the Victorian age GCI segments in the LU network. This paper discusses the deformation behaviour of the bolted segmental lining under the influence of factors such as overburden pressure, bolt preload and presence of grommets at small distortions. The measured behaviour of the segmental lining is compared against the calculated response of a continuous lining based on the assumption of elasticity.The industry practice for tunnel lining assessment is to calculate the induced bending moment in the tunnel lining using an elastic continuum model, while adopting a reduced lining stiffness to take into account the presence of the joints. Case studies have recorded that both loosening and tightening of lining bolts have been used as mitigation measures to reduce the impact of new tunnel excavations on existing GCI tunnels.The experimental work on the half-scale GCI lining has shown that a bolted segmental lining behaves as a continuous ring under small distortions imposed when subjected to hoop forces relevant to the depth of burial of LU tunnels. In the presence of hoop force, joint opening was minimal and the magnitude of preload in the bolts had little impact on the behaviour of the lining. It is therefore concluded that disturbance of the bolts in existing tunnels is not recommended as a mitigation measure as in addition to being ineffective it is both time consuming and introduces the risk of damaging the tunnel lining flanges.
Mantikos V, Ackerley S, Kirkham AD, et al., 2016, Investigating soil-water retention characteristics at high suctions using Relative Humidity control, 3rd European Conference on Unsaturated Soils, Publisher: EDP Sciences, Pages: 10007-10007
A technique for controlling relative humidity (RH) is presented, which involves supplying a sealed chamber with a continuous flow of air at a computer-regulated RH. The desired value of RH is achieved by mixing dry and wet air at appropriate volumes and is measured for servo-control at three locations in the chamber with capacitive RH sensors and checked with a sensitive VAISALA sensor. The setup is capable of controlling RH steadily and continuously with a deviation of less than 0.2% RH. The technique was adopted to determine wetting soil-water retention curves (SWRC) of statically compacted London Clay, under both free-swelling and constant volume conditions. The RH within the chamber was increased in a step-wise fashion, with each step maintained until vapour equilibrium between the chamber atmosphere and the soil samples was established. Independent filter paper measurements further validate the method, while the obtained retention curves complement those available in the literature for lower ranges of suction.
Ackerley SK, Standing JR, Kamal RH, 2016, A system for measuring local radial strains in triaxial apparatus, Géotechnique, Vol: 66, Pages: 515-522, ISSN: 1751-7656
The importance of local measurement of small strains is well recognised. Without such measurements, accurate stiffness determination is not possible and can result in overestimations of strains resulting from stress changes in the ground. Axial strain is often considered to be the primary strain to be measured under triaxial conditions and there has been a focus on its measurement. Radial strain is also important, for example for determining Poisson's ratio and bulk modulus, or maintaining K0 conditions during consolidation. There are a number of devices available that enable its determination, using both direct and indirect measurement approaches. This note describes a system for measuring radial strains directly using linear variable differential transformers. It is shown to be simple, reliable, accurate and robust.
Avgerinos V, Potts DM, Standing JR, 2016, The use of kinematic hardening models for predicting tunnelling-induced ground movements in London Clay, Geotechnique: international journal of soil mechanics, Vol: 66, Pages: 106-120, ISSN: 0016-8505
The use of a kinematic hardening soil model for predicting short- and long-term ground movements due to tunnelling in London Clay is investigated. The model is calibrated against oedometer and triaxial tests on intact samples from different units of the London Clay. The calibrated model is then used in finite-element analysis to simulate the field response at St James's Park during excavation of the Jubilee Line Extension tunnels. The finite-element predictions compare well with the available field monitoring data. The importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted. The stiffness response of different regions of the finite-element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short-term predictions significantly. The long-term predictions confirm that the compression characteristics of the soil control the magnitude of the consolidation settlements and its permeability the shape of the long-term settlement profiles.
Al Haj KMA, Standing JR, 2016, Soil water retention curves representing two tropical clay soils from Sudan, Geotechnique, Vol: 66, Pages: 71-84, ISSN: 1751-7656
Soil water retention curves (SWRCs) form an essential component of frameworks coupling the hydromechanical behaviour of unsaturated soils. The curves describe how suction changes with variables such as degree of saturation. SWRCs can be determined from incrementally drying initially saturated reconstituted samples to a final residual state, thus developing the primary drying curve (PDC). The primary wetting curve (PWC) is established from subsequent incremental wetting and is hysteretic compared with the PDC. SWRCs for two reconstituted, high-plasticity, tropical clays from Sudan have been determined using the filter paper technique, for which the maximum measurable suction is 30 MPa. At this suction the degree of saturation for both soils was greater than 40% and did not reduce much further on full drying under ambient laboratory conditions. Thus the primary SWRCs did not develop into the usual sigmoidal shape expected. The development of SWRCs under various subsequent cycles of wetting and drying are presented and discussed along with details concerning volumetric changes and cracking during drying. In order to investigate the uniqueness of the PDC and PWC and the effect of initial void ratio, SWRCs were determined for samples formed by dynamic and static compaction under different applied energy levels and also for intact samples. These were found to be located to the left of the PDC and often the PWC too.
Standing JR, Potts DM, Vollum R, et al., 2015, Investigating the effect of tunnelling on existing tunnels, Underground Design and Construction Conference, Publisher: IOM3, Pages: 301-312
A major research project investigating the effect of tunnelling on existing tunnels has beencompleted at Imperial College London. This subject is always of great concern during theplanning and execution of underground tunnelling works in the urban environment. Many citiesalready have extensive existing tunnel networks and so it is necessary to construct new tunnels ata level beneath them. The associated deformations that take place during tunnelling have to becarefully assessed and their impact on the existing tunnels estimated. Of particular concern is theserviceability of tunnels used for underground trains where the kinematic envelope must not beimpinged upon. The new Crossrail transport line under construction in London passes beneathnumerous tunnels including a number of those forming part of the London Underground network
Yu J, Standing J, Vollum R, et al., 2015, Stress and Strain monitoring at Tottenham Court Road Station, London, UK, Proceedings of the ICE - Structures and Buildings, Pages: 107-117, ISSN: 0965-0911
The redevelopment of Tottenham Court Road Underground Station started in 2011 as part of the Tube Upgrade Plan to improve and increase the capacity of the existing facility. The plan is to upgrade the station by 2016 to meet an estimated demand of more than 200,000 journeys per day once Crossrail is built. During April to November 2011, major structural work was carried out on the Northern Line platform tunnels as part of the station upgrade. This included removing grey cast iron tunnellining segments on the platform side to allow for construction of new cross passages to improve access to the platforms. The upgrade works provided an opportunity to trial in-tunnel instrumentation prior to implementation in other London Underground (LUL) tunnels which interface with the Crossrail project. Mechanical and electrical resistance strain gauges were installed on tunnel segments to make discrete measurements of changes in strain due to unloading as the segments were removed from the tunnel rings. Linear variable differential transformer type displacement transducers were installed to make continuous measurements of the opening and closing of circumferential and longitudinal joints on trackside segments which are left insitu and affected by adjacent excavations. This paper describes the installation process and highlights the lessons learnt for future applications. The insitu strain measurements are presented and compared to the expected response based on laboratory tests conducted on grey cast iron tunnel segments in the 1970s. The changes in strain measured by both types of strain gauges agreed well with the estimated changes assuming full overburden unloading.
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