Search

The necessity of accounting for small-strain behaviour of soils in numerical analyses of the serviceability limit states of geotechnical structures is well established both nationally and internationally. This has led to further development of appropriate soil constitutive models, as well as to further advances in accurate laboratory measurements of small-strain stiffness. The current paper considers recent laboratory research into the behaviour of London Clay, performed at Imperial College in conjunction with the major ground works for Heathrow Terminal 5 in London, UK. This research has shown the small-strain response of London Clay to be different to that assumed previously and in particular to that determined from good-quality commercial experiments. Both sets of small-strain stiffness data are applied in this paper in finite-element analyses of a deep excavation and a tunnel construction in London Clay, with the objective of investigating their effect on predicted ground movements. [ABSTRACT FROM AUTHOR]

The front-shunt tunnel was the first tunnel of the Terminal 5 project at Heathrow to be constructed, and was the first section of sprayed-concrete-lined (SCL) tunnel to be constructed using the method known as LaserShell. This innovation represented a significant deviation from the methods previously used in SCL construction. Therefore it was subjected to a careful examination before and during construction using sophisticated 3D numerical modelling and monitoring during construction. The paper presents typical results from surface settlement levelling, inclinometers and extensometers, pressure cells and tunnel lining displacement measurements, and comments on the performance of the methods and instruments used. The paper then presents the methodology and typical results of the numerical modelling, and shows that the predictions of displacements and stresses compared well with the field measurements. In terms of the control of ground deformations and structural safety the tunnel performed well. [ABSTRACT FROM AUTHOR]

This paper describes the nature of ground movement observed during construction of the Jubilee Line Extension at St James's Park and beneath Elizabeth House for different stages of tunnelling in London Clay, using open-face shield construction and sprayed concrete linings respectively. Volume loss is defined as the volume of ground loss as a proportion of the final tunnel volume and is measured in the plane of the tunnel. Volume losses corresponding to different phases of tunnelling at St James's Park and beneath Elizabeth House are identified in the paper. A companion paper presents a new approach to estimating volume losses for each phase of open-face tunnelling in London Clay. [ABSTRACT FROM AUTHOR]

This paper describes how a series of large-diameter auger-bored test piles were constructed for the Terminal Five development at London's Heathrow Airport. The piles were tested over a period of one year, prior to the start of the permanent works, to investigate how the shaft resistance within the London Clay varies with time. The test procedure is described, and some of the results are presented and briefly discussed. A further two piles were constructed to investigate the base resistance in the London Clay at depths of up to 47 m. Osterberg Cells were used to generate the testing force, and the construction and testing of these piles is also discussed in the paper. [ABSTRACT FROM AUTHOR]

Ironstone extraction around Corby in Northamptonshire, UK involved opencast excavation of ore beneath 20 m thickness of overburden soils. The overburden was randomly tipped back into the quarry behind the advancing face, leaving a legacy of large tracts of land with an extensive thickness of compressible soils effectively sterilised for development in the absence of significant engineering. The Priors Hall site includes over 134 ha of backfilled quarry abandoned in the 1970s, on which 1724 new dwellings and associated retail and educational facilities are proposed. The characteristics of the fills are documented from research commencing in the 1960s, when experimental ground treatments were undertaken on the partially saturated fills. Recent investigation has indicated significant change in soil properties compared with the early research, notably in the development of distinct post-depositional stratification. The paper describes the geotechnical characteristics of the soil, highlighting apparent changes in properties that have occurred through weathering processes, most significantly leading to an increase in the degree of saturation in the shallow soils, and densification below a critical depth. A theoretical basis is proposed for the stratification in the soils, and how this has influenced the development of the ground treatment design. The case study concludes with a discussion of the findings from large-scale ground treatment trials. [ABSTRACT FROM AUTHOR]

In the UK, the High Speed 1 project formally pioneered on a large scale the earthworks design philosophy of maximising reuse of excavated materials and minimising waste. Now, in 2011, it is considered best practice for sustainable earthworks on all major infrastructure projects. This philosophy was a requirement of the Channel Tunnel Rail Link Act 1996, and a waste minimisation hierarchy was therefore included within construction specifications. This paper presents two different solutions found on one contract to enable two difficult silty soils, which would historically have gone to mitigation or taken off site, to be successfully retained and used as engineered fill. The first soil was quarry overburden material of Thanet Sand with considerably varying optimum moisture contents. A 'family of curves' approach was developed to enable rapid single-point compaction testing to predict optimum moisture contents. The second was an artificial soil called cement kiln dust, generated as a by-product of cement manufacture. This material had a high silt proportion, and very high natural and optimum moisture contents, and was in a condition much wetter than optimum. A strength-based end-product acceptance criterion was developed using a CBR MEXE probe. [ABSTRACT FROM AUTHOR]

A detailed site characterisation was undertaken in order to assess methods that could be used to determine the depth and lateral extent of a potentially collapsible loessic brickearth deposit at a site in Kent. The investigation presented in this paper covers both geotechnical and geophysical evaluations, correlated to detailed lithological logs of trial pits at the site. This enabled cone penetrometer profiles to be compared directly with a suite of rapid geophysical survey techniques, which included electromagnetic (EM31 and EM34), electrical resistivity surveys and shear wave profiles, calibrated against the same soil profile. From this it was possible to determine the variability and extent of a collapsible loessic soil across the entire test site, thus demonstrating that appropriately calibrated geophysical techniques can provide a rapid indication of the presence at depth of a potentially unstable soil. [ABSTRACT FROM AUTHOR]

A modification to the load factor approach to estimating undrained volume loss for open-face tunnelling in London Clay is proposed. The modified approach is based on the nature of ground movement observed during construction of the Jubilee Line Extension at St James's Park and Elizabeth House for different stages of open-face shield tunnel construction and tunnelling using sprayed concrete linings, described in a companion paper. The new approach is shown to provide good estimates of the volume loss observed for open-face shield tunnelling in London Clay at Green Park and Regents Park. [ABSTRACT FROM AUTHOR]

The Thames Water ring main extension is a 4·5 km long tunnel from Stoke Newington, in the London borough of Hackney, to New River Head in Finsbury, in the London borough of Islington. The 2·85 m i.d. tunnel was excavated by an earth pressure balance tunnel-boring machine (TBM) at depths between 40 and 60 m below the surface. Surface settlements along the route were measured by precise levelling, and were found to be small. It was therefore even more important to measure these settlements as accurately as possible, in order to provide informed estimates of subsurface movements induced in third-party underground structures much closer to the tunnel horizon. Because of the relatively large magnitude of the background movements measured, when compared with the small tunnel-induced settlements, it was necessary to adopt a rigorous statistical method to fit a Gaussian curve to the data. This exploited the analogy of the 'error function' to define the Gaussian curve parameters and . In all, 13 tunnels were underpassed successfully by the TBM, all within the 'conservative expected value' predictions, and without incident. The predictions and structural monitoring schemes undertaken for the High Speed 1 tunnels near Corsica Street and the Northern line tunnels near Angel station are described in the paper. It was found that the surface and subsurface trough width parameter did not vary with depth as predicted: therefore a new relationship is proposed. [ABSTRACT FROM AUTHOR]

An 8·2 m diameter, 37·5 m deep shaft has been successfully constructed from within the basement of a new development, Moorhouse, near Moorgate in the City of London. Programme constraints led to the shaft being constructed after completion of the foundations, basements and most of the superstructure for the development. At its closest point the shaft is less than 2 m from the large-diameter piles that support Moorhouse, and the presence of these foundations placed tight constraints on acceptable ground movements associated with construction of the shaft. The depth of the shaft is such that it penetrates through stiff London Clay and is founded at the bottom of the Lambeth Group. The paper describes the contingency measures to deal with potentially difficult ground conditions, including the water-bearing layers of the Lambeth Group. The construction processes included a complex temporary works dewatering system around the shaft, with the option to carry out additional dewatering from within the shaft during excavation. Provision was also made for radial grouting to 'restress' the ground, to prevent long-term settlement of the Moorhouse piles, should the need arise. The success of the project was due, in no small part, to the detailed planning and consideration of contingency measures to deal with perceived risk. [ABSTRACT FROM AUTHOR]

This paper describes the development of the design for an innovative retaining structure built to support an existing highway on the Isle of Wight. At Afton Down on the south-west coast of the island the A3055 Military Road passes above high chalk cliffs. Continuing erosion at the toe of the cliffs led to the cliff edge receding to within 15 m of the highway. As a result options to maintain the road were investigated. A public inquiry ruled out realignment inland onto the adjacent downland, whilst measures to prevent erosion at the toe of the cliffs were unacceptable for environmental whilstcoastal management reasons. Consequently the road was kept open, with instrumentation installed to warn against failure of the cliff, and work began on the design of a scheme to maintain the road along its existing alignment. An environmental assessment was carried out to address the potential impacts of schemes on the adjacent downland, with the resulting arrangement of a tied retaining wall aiming to minimise these. Furthermore, outline methods for decommissioning the structure when it reaches the end of its serviceable life were identified. Planning permission was granted in 2002, and following a series of trials the scheme was constructed in 2003 and the road reopened to the public. [ABSTRACT FROM AUTHOR]

This paper discusses the relationship between climate, in the form of soil moisture deficit, the presence of trees and pore water pressures in railway embankments. The pore water pressure criteria used in the assessment of slope stability by London Underground Ltd are presented and examined in the light of data gathered during the particularly wet winter of 2000-2001. This unusually persistent wet period caused the pore water pressures in some embankments on the London Underground network to increase to unprecedented levels. The results indicated that in some instances the reduction in pore water pressure that tall deciduous trees were assumed to provide could not be relied upon. [ABSTRACT FROM AUTHOR]

An anchorage test programme carried out on the site of a deep basement construction in Norwich, UK, allowed the determination of anchorage capacities and creep characteristic data, and the exhumation of several anchor bodies in chalk. This paper reports on the performance of the chalk-based anchorages, the observation of exhumed grout body shapes and sizes, the extent of grout penetration into dissolution cavities, pressure-grouted fissures, and the formation of grout cake around the grout body periphery. Knowledge of construction techniques utilised for drilling, flushing, pre-grouting, redrilling, pressure grouting and the general methods of control implemented to prevent dissolution cavity collapse has been related to the characteristics of the exhumed anchor bodies. Analysis of these grout bodies has allowed the proposal of a failure mechanism, comprising shaft resistance and shear of grouted fissures. Tests were carried out on both conventional and single-bore multiple anchors (SBMAs). Using short, fixed-length capacities with recently developed efficiency factors allowed accurate evaluation of the ultimate capacity of a typical 10 m fixed-length conventional anchorage. This design approach confirms that the SBMA system mobilised double the capacity of a conventional 10 m fixed anchor. Advancements in SMBA technology have enabled designs of 20 m fixed lengths, safely supporting working loads of 3000 kN. [ABSTRACT FROM AUTHOR]

Between 1999 and 2001, the Channel Tunnel Rail Link (CTRL) and the A2/M2 widening scheme between Cobham and Junction 4 of the existing M2 motorway were both under construction in central Kent. The activity was particularly intense immediately to the east of the River Medway, where the earthworks for the two projects were immediately adjacent. Nearly 300 000 m³ of surplus chalk was exported from the CTRL works to the A2/M2 widening project in a fill transfer agreement that represented the successful culmination of about three years of negotiations between the two project teams and the local planning authorities. It was a good example of waste minimisation, sustainable design and construction, and cooperative working, and involved the resolution of several planning, technical and construction management issues. The ways in which these were successfully overcome are addressed in this paper. [ABSTRACT FROM AUTHOR]

The article presents a case study to assess the short and long-term settlements and bearing capacity of a 13 meter high chalk fill platform for a housing development in Medway Gate in Kent, England. Findings from a roped-access inspection followed by an intrusive investigation provided evidence that the quality is improved in situ chalk with depth. It also indicates that there was sufficient structured chalk on site for the construction of the platform.

Low-profile push-in pressure cells (spade cells) are commonly used to measure total horizontal stresses in both normally and over consolidated clays. It is known that spade cells over-read the in situ stress in such deposits as a result of the complex localised stresses created during installation. Previous investigations into the magnitude of over-read are limited in number and have produced scattered results. During construction of the Channel Tunnel Rail Link at Ashford, Kent, a spade cell was installed horizontally in an overconsolidated clay, aligned to measure initial vertical stress, and the change in vertical stress due to excavation of material above the cell. The results of this experiment are presented and interpreted with reference to a finite element analysis and a comparison of spade cell and selfboring pressuremeter test data of horizontal stresses at the same site, to assess the performance of this type of spade cell in the Atherfield Clay. [ABSTRACT FROM AUTHOR]

The creation of the Olympic Park, commenced with the formation of the development platform, on which the venues and infrastructure was built. This was part of the enabling works, and included 2 million m of excavation, and the remediation of contamination. To enable the contaminated soils to be reused, a number of soil treatment techniques were adopted: soil washing, bioremediation, chemical stabilisation and material sorting. Soil washing was the main technique, treating around 700 000 m of soil by mixing it with water. This removed the fine grained fractions on which the contaminants were predominantly based, creating a filter cake, typically around 20% of the total volume of soil washed. Bioremediation and chemical stabilisation together treated less than 100 000 m of soil. A regime of chemical testing was established to test the materials at the excavation stage, manage the treatment and finally validate the final material. With the exception of the final validation, testing was carried out off site in MCERTS accredited laboratories, the bulk of the testing was undertaken in the site laboratory, which also handled geotechnical testing. [ABSTRACT FROM AUTHOR]

A detailed borehole investigation has recently been undertaken for an extension to the Thames Water ring main in north London. Results of this were presented in a preconstruction geological model. This formed the basis for tunnel designers to assess ground conditions prior to construction and implement mitigation design and/or contingency plans in the event of adverse geological conditions being encountered. Previous sections of the ring main were constructed using open-faced tunnelling techniques. These allowed for inspection and continuous record of ground conditions at the tunnel face, enabling an identification of the geology and prediction of potentially adverse ground ahead. In facilitating such close geological supervision, tunnelling benefited from significant programme savings - in particular, a reduction of risk to the health and safety of underground personnel. In constructing the tunnel extension, a closed-face tunnel boring machine was opted for; this largely concealed the exposure of geology in situ during the excavation cycle. Apart from a limited number of tunnel face inspections, an indication of the geology was formed from observation and logging the spoil arising from the tunnelling machine, aided by an assessment of the performance of the machine itself in response to changes in geology over the alignment. These activities were necessary in validating the preconstruction geological model and in providing assurances to the on-site design and construction teams. [ABSTRACT FROM AUTHOR]

A site in central London called One New Change is currently being redeveloped. The new development has a basement that extends to approximately 14 m below ground level. Demolition of the building that formerly occupied the site, excavation of the new basement and construction of the new development have caused unloading and subsequent reloading of the ground. This change of loading has resulted in displacements occurring to the nearby London Underground Central Line tunnels that run parallel to the northern boundary of the site. The crowns of the tunnels vary from approximately 20 to 25 m below ground level. The piles and retaining wall for the proposed works are located at least 7 m clear distance to the south of the tunnel closer to the site boundary. The results of analyses carried out to assess the effects on the tunnels indicated that the imposed distortions caused by the proposed works were within tolerable limits. Monitoring was proposed to check that ground and structural movements were within the range of calculated values, and contingencies were identified in the event of agreed trigger levels being exceeded. Displacements measured during the demolition, excavation and construction phase of the project are presented. Conclusions are drawn from comparing the measured displacements with the tunnel deformation calculations. [ABSTRACT FROM AUTHOR]

A trial strain-monitoring system using Brillouin optical time-domain reflectometry (BOTDR) technology was set up to monitor joint movements in the concrete tunnel lining in an existing London Underground tunnel. The BOTDR strain sensor system allows the measurement of strain distribution along an optical fibre using the reflective technique, requiring access to only one end of the fibre. Measurements were obtained by a strain-sensing optical fibre installed along the tunnel lining. The joint movements were captured by measuring the strain along the fibre across the segment joints. The results show that there is good agreement between the joint movements evaluated by the BOTDR strain sensor system and those by conventional vibrating-wire strain gauges. Whereas conventional strain measurement gauges monitor the strain variations at discrete locations, a BOTDR strain sensor can provide a continuous strain distribution of the tunnel lining. The results demonstrate the practicality of using the BOTDR strain-sensing system to monitor the movement of tunnel linings. [ABSTRACT FROM AUTHOR]

A hybrid wireless sensor network has been developed for structural monitoring of transport tunnels. The system combines the advantages of wired and wireless sensing to simplify and speed up installation. The wireless modules are based on low-cost sensors for inclination or displacement, and temperature and humidity. These are combined with a commercial radio module to eliminate the need to install cables around the tunnel. The modules report readings back to a gateway installed on the wall of the tunnel, automatically relaying the readings from other modules as required. The readings are transmitted to the surface along cables that can be quickly and easily installed in the cable trays running along the walls of the tunnel and up the closest vent shaft. From the surface the readings are sent to a server via the mobile phone network and made available in real time on a web page. The results of a trial installation on the London Underground Jubilee line are reported. [ABSTRACT FROM AUTHOR]

The design, construction and monitoring of a 12 m deep basement for the Chapelfield Shopping Centre, Norwich, UK are reviewed. The extent to which data and reports from similar nearby construction projects allowed cost savings and reduced risks, particularly regarding solution features in the chalk, has been assessed. By having robust engineering solutions and contingency measures agreed at the start of construction, geotechnical works for the project were completed without delays or significant additional costs. Back-analysis has shown a good fit between actual performance and that predicted using the soil parameters chosen. Using hindsight, an assessment is made of whether all potential benefits from the prior knowledge of the performance of similar excavations in the area were realised. [ABSTRACT FROM AUTHOR]

The design and construction of civil and geotechnical work in a marine environment presents unique challenges to the engineering team charged with the delivery of such works. The work is particularly challenging when carried out close to the sea, where the combined effects of a fluctuating tide level and the presence of an aggressive, erosive environment have significant effects on how the structure is designed and constructed. Against this background, a foundation for a new linkspan structure was built at Heysham Port in Lancashire, UK. The design needed to consider the presence of potentially unstable rock at the site, and the significant vertical and horizontal loads imposed by the new structure. The chosen design solution required piles and ground anchorages to be installed. Construction took place within a working port, adjacent to an existing live linkspan, which had to remain operational during most of the construction period. There was limited space available to carry out the works, and the client required the site works to be completed within a tight programme. [ABSTRACT FROM AUTHOR]

The redevelopment of King's Cross Station in central London includes the construction of a new Northern Ticket Hall and tunnelled connections to existing infrastructure. These works are to be carried out beneath a congested site including Grade I and II listed buildings, and heavy rail and road traffic both above and below ground (three London Underground lines). Arup and Soldata were required to design and implement a monitoring solution that would assist with the management of the risks. A direct and unique contract from the client to the instrumentation specialist was set up in order to keep a tight control on the data quality, and to ensure an optimal distribution of the results. A real-time system, based on an automated total station network combined with standard monitoring methods (inclinometers, precise levelling, etc.), was chosen. The detailed design and optimisation of such a system involves estimating the precision based on site-specific information. Since installation on site, the site monitoring data have shown the adequacy of the design, with the estimated field precision easily attained, and in some cases well exceeded. The main conclusion is that, with adequate care, the design and implementation of a large monitoring system can lead to fruitful results that are reliable, rapid and accurate. [ABSTRACT FROM AUTHOR]

An actively pumped groundwater control system has been installed to protect the Channel Tunnel Rail Link (CTRL) Stratford box against uplift. This structure is critical to the operation of the railway, and its integrity is dependent upon the effectiveness of the dewatering system. The reliability of the scheme is provided by a combination of the use of simple, proven, water industry technology and a robust power supply and control system. Quantitative risk assessment techniques were used to guide the design process and demonstrate the security of the system. Stratford is a location where groundwater levels have been rising, and the CTRL scheme will contribute to General Aquifer Research Development and Investigation Team (GARDIT) objectives of controlling groundwater level rise in central London. [ABSTRACT FROM AUTHOR]