Lambeth Group Research Articles

Jet grouting to new depths in the Lambeth Group and Thanet Formation beneath London

A jet grouting trial was undertaken within the Thames Tideway Tunnel shaft at Kirtling Street in Battersea, SW London, to assess the viability of the technique within the Lambeth Group and Thanet Formation sediments up to 60 m below ground level. There was no precedent industry experience of jet grouting in the UK at this depth and in these soil types. Four, separate, upper and lower grout columns were constructed respectively within upper and lower granular zones inside the shaft footprint, and then exhumed for inspection and measurement during ensuing excavation within the diaphragm wall lining. Target diameters for the upper columns were achieved within granular channel sand and Laminated Bed material, with minimal vertical deviation. These were not so successfully achieved, however, within stiff cohesive Laminated Bed and Lower Shelly Clay materials. Formation of the lower columns was severely restricted in dense gravel belonging to the ‘Pebble Bed’ and stiff gravelly clay, at the top of the Upnor Formation, but target diameters were being achieved in the lower portion of the formation, comprising Upnor sand. At their base, the column diameters were, again, restricted in the very dense sand at the top of the Thanet Formation.

Effects of changing geology on the performance of a tunnel boring machine for the Thames Tideway Tunnel, London, UK

An earth pressure balance tunnel boring machine (TBM), 8.8 m in diameter, was used to construct a 7.6 km long section of the Thames Tideway Tunnel through central London, UK, between Battersea and Bermondsey. Two major geological structures – the Lambeth Anticline and the London Bridge fault zone – have imposed significant changes in the tunnel face geology. The TBM was continually monitored for operating parameters such as cutterhead torque, thrust and injection of spoil-conditioning additives. This led to recognition of the structures, in addition to a high frequency of sand channels within the Upper Lambeth Group strata. The monitoring led to validation of the pre-construction geological model and, in conjunction with inspection of the tunnel face during scheduled interventions and geological logging of several interception shafts, provided valuable assurances for the predicted ground ahead of tunnelling operations.

Observations on the geology of the country around Chalfont St Giles, Buckinghamshire

This paper presents new field observations from the area around Chalfont St Giles and relates them to existing information from the wider Chiltern Hills region on lithostratigraphy, structural geology, geomorphological and karstic features, and landscape development. In this area the NW-SE aligned valley of the River Misbourne, an internationally rare chalk stream, and flanking interfluve plateaus form a transition zone on the chalk dipslope between the lowest elevations of outcropping chalk around Denham and the characteristic chalk valleys and clay-with-flints capped upland present from Amersham northwards. The area straddles the northernmost extent of both the Paleogene strata, represented by Lambeth Group outliers, and SW–NE aligned pre-Anglian palaeo-Thames River Terrace deposits, and contains an anomalous Lambeth Group, Quarrendon Gap, mid-slope platform. Ground investigations carried out for HS2 have helped substantiate other evidence for significant faulting across the area, with intermittent movement episodes dating from the Mid- to Late Cretaceous onwards, which have affected both the Chalk and Paleogene strata and had a major influence on shaping the valley network and wider landscape. Karstic features are widespread, including contemporary interfluve plateau level streams sinks around the Lambeth Group outliers forming active dolines, together with many concealed chalk dissolution related features. There are numerous, generally small scale, clay, chalk, sand and gravel workings most of which are long abandoned but still provide useful insights into the near surface geology.

Clay stratigraphy of the Lambeth Group of borehole IOD8R

The clay mineralogy of the Lambeth Group is shown to be highly variable, demonstrating a strong correlation with palaeoenvironment. The principal causes of this variability are changes in intensity of contemporaneous weathering (pedogenesis) and possible input of volcanic tephra. Previous studies have suggested that smectite is the dominant clay throughout the Lambeth Group in the London Basin, but we show that this is not the case in borehole IOD8R. Absolute clay mineral abundances are also highly variable, reflecting changes in sediment grain size and environment of deposition. The engineering properties of the Lambeth Group are largely controlled by the distribution of clay minerals, and in particular, the abundance of smectite. The data we present for borehole IOD8R illustrate the importance of assessing the scale of variability of such factors as swelling clay abundance when planning and supervising a site investigation.

Construction geological logging of the Thames Tideway Tunnel beneath central London: unearthing the ground truth

Large-scale underground structures excavated during the construction of the Thames Tideway Tunnel have facilitated observations of the in situ geology and revealed sedimentary characteristics that were not fully evident in disturbed samples from ground investigation boreholes or laboratory tests. These have led to significant improvements in the understanding of groundmass behaviour of several strata within the London Basin geological sequence that are not easily studied by intrusive ground investigations, especially those involving sand channels and the ‘Pebble Bed’ within the Lambeth Group deposits. Further validation of the ground model and geology projected over the scheme during the preconstruction design has been gained from logging the geology within deep shafts, open-face connection tunnels and the main 8.8 m-outer-diameter tunnel from behind the tunnel-boring machine cutterhead. It is recommended that an archive of field logs, as obtained during construction, be established to augment archives of borehole logs. Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

50 Years of Safeguarding – 950 Years of Clergy Discipline: Where do we go from here?

This article is based on an Ecclesiastical Law Society London Lecture delivered on 17 November 2021. It builds on a previous lecture entitled ‘Safeguarding in church and state over the last 50 years: “from Ball and Banks to Beech via Bell”’, which also formed the basis of an article published in this Journal in 2020. That article, among other things, identified a number of significant cases of sexual abuse by clergy which were/are the subject of ‘lessons learned’ reviews and concluded by suggesting how investigation and fact-finding might take place in the future, independent of the bishops, but under the supervision of a ‘judge’, and argued that effective risk assessments could only be based on findings of fact. The author was subsequently asked to chair a working party for the Ecclesiastical Law Society aimed at addressing how the Clergy Disciplinary Measure 2003 (‘CDM’) should be reformed. This article now deals with events that have occurred since, from the publication of the Independent Inquiry into Child Sex Abuse (IICSA)'s investigative reports into the Anglican Church (inter alia), the Lambeth group's proposals for reform of the CDM, through to General Synod's responses to both sets of recommendations. It also surveys in some detail the approach the church has taken to the assessment of risk within its safeguarding policy in recent years, as well as its historical approach to clergy discipline. The article concludes by drawing some threads together as a result of the author's own work and research into these two subjects over the past two years, and finishes by suggesting some possible directions in which the church ought now to move.

Enigmatic rocks and sarsen stones of the West Sussex Coastal Plain, southern Britain

The coastal plain of West Sussex and south-east Hampshire preserves a succession of Pleistocene raised beaches, periglacial deposits, and estuarine channels. These incorporate numerous ‘erratics’ or ‘exotic’ rocks, here described as enigmatic rocks, and sarsen stones. The enigmatic rocks include large boulders of non-local igneous, metamorphic, and sedimentary rocks, which have attracted interest since the early 19th century. Many suggestions have been made regarding their origins, transport, and date of emplacement. More recently, it has been realised that these rocks may have important implications for understanding the Quaternary history of southern Britain and the palaeogeography of the English Channel region during the Pleistocene. This paper presents a literature review, supported by details of historic and extant boulders, distribution, and dimensions. In agreement with previous authors, it is concluded that the enigmatic rocks were likely transported within ice floes or icebergs, but further studies are required to identify possible sources. Suggestions have included northern Britain, Scotland, Scandinavia, Greenland and Laurentia. There are doubts concerning an adequate build-up of periglacial sea-ice to accept the historic consensus for a source in Brittany, the Cotentin Peninsula, and the Channel Islands. The date of emplacement is suggested to be MIS 13, but multiple phases of emplacement cannot be excluded. In contrast, the sarsen stones are considered to be relic of pre-MIS 7 periglacial erosion of weathered Lambeth Group in a regolith of Clay-with-flints formerly capping the local Chalk outcrop. Many questions remain unanswered, particularly regarding the enigmatic rocks, so aspects for future study are identified.

Logging the Lambeth Group Upper Shelly Beds for the Thames Tideway Tunnel in London, UK: More than just “dark grey clay with shells”

Despite recent advances in the procedures for logging rotary borehole core, the Lambeth Group Upper Shelly Beds are often inadequately characterised during ground investigations for underground construction. With no recognised industry guidance, there has been a lack of clarity and detail in many logging records, other than a generic description of “dark grey clay with many shells and (local) hard bands.” Improvements in describing these deposits have been hindered by the relatively limited extent of their subcrop and rarity of surface exposure, allied to a lack of precedent intersection during underground construction excavation.Geological logging during construction of the Thames Tideway Tunnel has revealed a laterally continuous sequence of strata within the Upper Shelly Beds through central London. This ranges in lithology from strongly indurated shell ‘coquinas’, mudstones and limestones to organic-rich laminated clays, silts and fine sands. Abrupt changes in lithology and fossil fauna are displayed across sharp contacts between beds, indicative of rapid fluctuations in sea level (and so potentially of climate change) during the period of deposition.Notwithstanding their comparatively limited thickness of between 1.4 and 1.6 m within the area of investigation, the Upper Shelly Beds represent significant challenges for underground construction, most notably a reduction in the rate of excavation through the stronger layers of material. Early identification of the sequence has enabled modification and improvement to the on-site construction cycle and provided valuable assurance for the existing engineering design.This paper is intended to provide guidance for recognising the sequence of strata within the Upper Shelly Beds during future ground investigations in central London, with similar benefits for early recognition of the stronger layers during engineering design.

Development of the ground model for the Clay-with-flints Formation: adventures in geological boundaries

The Clay-with-flints Formation crops out on the high chalk plateaux in southern England. Changes in terminology are summarized and distinctions from adjacent similar deposits (e.g. Valley Gravel) are detailed. A threefold division of the transition zone between the two deposits is proposed based on degree of disturbance. Relationships with parent materials are discussed through structural and spatial relationships, as shown on published maps and in memoirs. Dominant pedological processes involved in the formation of the Clay-with-flints Formation from the Lambeth Group are described. A twofold division between the two is proposed based on physical changes. In addition to geological and pedological information, the deposit was studied through ground investigation data from 60 km of motorway network, which covers the variation in the Paleocene deposits (Lambeth Group facies (Reading Formation to Woolwich Formation) and the increasing presence of Thanet Sand Formation along the North Downs, east of approximately Leatherhead). The resulting ground model requires detailed knowledge of the microstructures of the chalk, the facies variation of Paleocene and Pliocene deposits, and temperate climate pedological processes in the vadose zone. Future work to study the engineering properties reflecting the variation of both the Paleocene facies and the development of the vadose zone is summarized. Thematic collection: This article is part of the Ground models in engineering geology and hydrogeology collection available at: https://www.lyellcollection.org/cc/Ground-models-in-engineering-geology-and-hydrogeology

Rates of oxygen-depletion in granular Lambeth Group strata and evidence for barometric pumping

Laboratory experiments have been performed to demonstrate significant levels of oxygen depletion within samples of Upnor Formation of the Lambeth Group deposits. These have been used to explain the reasons for several incidences of confined space hypoxia during underground construction within the stratum beneath London. Further investigation, using a relatively small-scale field pump-out test, revealed the rapid effects of dewatering on the generation of hypoxic gas (within the ground) and that the amount of oxygen falls to fatal levels within a very short time after commencement of pumping. Monitoring of boreholes during ground investigation for the Thames Tideway Tunnel has indicated barometric control on the release of hypoxic gas from installations within granular deposits of the Lambeth Group, namely the Upnor Formation, as well as channel sand deposits within the Laminated Beds. In these, reduced levels of oxygen coincide with low and/or falling barometric pressure. Continuous 24-hour monitoring demonstrates that the reduction is almost instantaneous and has serious implications for the Health and Safety of underground construction personnel.

Determining the source of silcrete sarsen stones

Materials used in the construction of man-made structures can reveal information about societies, their environment, culture and mobility. Cultures throughout human history have used natural stone to construct funerary monuments and tool technologies. Stone materials from igneous rocks can be provenanced by petrographic examination and geochemical methods that provide a direct link between the artefact and their source. For sedimentary sarsen stones, provenance is often complicated by diagenetic processes acting on source rocks. Here I use an example of stones used to build 6000-year-old earth and sarsen megalith structures from the River Medway valley (Kent, UK). These megalith structures lie on the strategic and pre-Neolithic North Downs migration route into Britain, but the provenance of the sarsens used in their construction has not been established. The highly competent sarsens are diagenetic groundwater silcretes, resulting from acid leaching of glauconitic clay components, and addition of a quartz cement. Heavy rare earth and high field strength element abundances and ratios are resistant to diagenetic effects and show a geochemical association with Paleogene sandstones from the Upnor Formation of the Lambeth Group at the 95% confidence limit. Detrital zircon and titanite grain compositions confirm this relationship. Preferential erosion and removal of the less competent sandstone around silcretes left large (>2 m), durable and isolated sarsen blocks on top of the chalk downs that were subsequently exploited by Neolithic peoples. Use of local stone to construct the Medway megaliths is consistent with their distinct morphological characteristics that have been used to imply independence from continental European cultures at that time. Combined with careful field collection of possible source rocks and sarsen materials, provenance of stones of sedimentary origin that have been affected by diagenesis can be accurately assessed through a combination of petrography, bulk-rock heavy rare earth element and high field strength element abundances, and detrital mineral compositions.

The role of risk and assumption in the engineering geology of Crossrail

The Crossrail project is a new underground railway running east–west through the heart of London. It will connect with 110 km of new or upgraded sections of surface rail to Reading, Shenfield and Abbey Wood. Crossrail co-ordinated ground investigations for the central tunnelled sections began during the 1990s and continued to 2011 according to a strategy that was based on a review of other major tunnelling projects and best practice. The level of detail in the investigations varied according to local geological complexity and design requirements. However, the state of knowledge on the geological structure is hampered by London's long history, leading to assumptions on the level of complexity or simplicity. Drift-filled hollows and Lambeth Group sand channels were known features presenting risks to excavations but relationships with faulting were unknown or unproved. The emerging geological knowledge was developed through multiphase investigations and collation of third-party data to allow far-field considerations. The risk profile for tunnel boring machine tunnels is lower than that for sprayed concrete lined tunnels but the risk for both increases with increasing proximity to pre-existing infrastructure. Ground movement assessments become critical and mitigation of movements can be significantly affected by unexpected variations in ground conditions.

Characterisation of hydraulic head changes and aquifer properties in the London Basin using Persistent Scatterer Interferometry ground motion data

In this paper, Persistent Scatterer Interferometry was applied to ERS-1/2 and ENVISAT satellite data covering 1992–2000 and 2002–2010 respectively, to analyse the relationship between ground motion and hydraulic head changes in the London Basin, United Kingdom. The integration of observed groundwater levels provided by the Environment Agency and satellite-derived displacement time series allowed the estimation of the spatio-temporal variations of the Chalk aquifer storage coefficient and compressibility over an area of ∼1360km2. The average storage coefficient of the aquifer reaches values of 1×10−3 and the estimated average aquifer compressibility is 7.7×10−10Pa−1 and 1.2×10−9Pa−1 for the periods 1992–2000 and 2002–2010, respectively. Derived storage coefficient values appear to be correlated with the hydrogeological setting, where confined by the London Clay the storage coefficient is typically an order of magnitude lower than where the chalk is overlain by the Lambeth Group. PSI-derived storage coefficient estimates agree with the values obtained from pumping tests in the same area. A simplified one-dimensional model is applied to simulate the ground motion response to hydraulic heads changes at nine piezometers. The comparison between simulated and satellite-observed ground motion changes reveals good agreement, with errors ranging between 1.4 and 6.9mm, and being 3.2mm on average.

On the nature and significance of two mega Chalk dissolution pipe infills, Goring Gap, Chiltern Hills, south Oxfordshire, UK

Two partially exposed infilled pipes hosted by chalk were revealed in excavations into a hillside some 50m above the modern River Thames. The pipes are interpreted as collapse features induced by voids originating at depth rather than classical stream fed sinkholes. Two kinds of infill could be distinguished, sands tentatively assigned to the early Eocene Reading Formation (Lambeth Group) and gravels associated with the mid-Pleistocene Beaconsfield Terrace. The age of development can be constrained by the Thames terrace chronology and this implies major landscape changes due to fluvial erosion since initiation of the pipes some 2Ma and a reactivation event about 1Ma.

Pile tests to justify higher adhesion factors in London Clay

The London District Surveyors Association guidance notes for the design of straight shafted bored piles in London Clay recommend an adhesion factor of 0·5 for shaft friction pile design. This value, based on the back-analysis of many maintained load pile tests to failure in London Clay, was selected to cover all design scenarios irrespective of site location, pile construction method, pile geometry and whether or not pile load testing is carried out. This ‘one size fits all’ approach to pile design is potentially over conservative. For sites where maintained load tests (MLTs) to failure on representative preliminary test piles are carried out, the results can be back-analysed to determine the appropriate adhesion factor for the site, which may then be used to improve the efficiency of the pile design. This paper discusses the pile design philosophy, pile load testing and construction sequence for a large development in Whitechapel, London, where MLT results were used to justify an increased adhesion factor of 0·6. This increased adhesion factor ensured that piles were ‘dry’ bored and founded above the water-bearing strata of the Lambeth Group, leading to lower project costs and risks.

Innovative geotechnical risk management for SCL tunnels

Following the construction of the Crossrail Project, Farringdon will become one of Britain's busiest train stations connecting three major networks – Thameslink line, Crossrail and London Underground. The station tunnels are constructed using sprayed concrete lining techniques. The geotechnical conditions were the principal challenge, governing both design and construction processes. The tunnels were excavated mainly within the Lambeth Group, a formation comprising various units, mainly stiff to very stiff overconsolidated clays with interbedded sand lenses of unknown size, hydraulic properties and continuity. Moreover, four major geological faults cross the footprint of the station, effectively changing the position of the risk-imposing sand lenses in relation to the tunnelling works. A robust and innovative geotechnical risk management approach was adopted from the design to the construction phase, utilising additional surface investigation, in-tunnel investigation and a ‘live' three-dimensional ground model. This provided a framework for effective on-site decision making and a cycle of risk reduction related mainly to the presence of multiple faults and potentially high water pressures within sand layers at the tunnel face. This approach is demonstrated in various stages of the project. This ‘best practice' model could be applied to other projects with challenging geotechnical conditions.

Depressurisation for the excavation of Stepney Green cavern

At over 50 m long, 17 m wide and 15 m high the Stepney Green Cavern is one of the largest caverns ever built using sprayed concrete lining techniques in central London. The east- and westbound caverns lie approximately 20 m below the existing ground level in a constrained urban site and are located within the London Clay and underlying Lambeth Group, which is a variable series of clays, silts and sands; the higher permeability sands contain high pore pressures that are hazardous to sprayed concrete lining construction without depressurisation. Successful implementation of a practical depressurisation scheme was therefore crucial to safe construction. The overall strategy adopted for investigating these water-bearing sands within the Lambeth Group is outlined and the ground investigations and pumping tests required to determine typical material permeabilities and the response of the sands to pumping are described. The paper also reviews how that information was developed into the outline depressurisation scheme and how this was subsequently developed into the final contractor-designed depressurisation scheme that was implemented. Changes to the scheme during construction owing to ground conditions are discussed, together with the response of the ground during pumping.

An observational approach to railway tunnel voids grouting

Water and fine sand ingressed into a section of running tunnels on the Jubilee line of the UK's London Underground railway in 2010. Concern arose that voids around the tunnel would lead to tunnel instability due to a reduction of lateral confining pressure from the soil. Ground investigation works identified that the running tunnel passed through a very large buried sand channel in the Lambeth Group. Initial attempts to arrest the ingress using an expanding grout were unsuccessful, and manual monitoring showed a slow but continuing deformation of the tunnel. A detailed tunnel investigation revealed that intermittent voiding was present over a wide area. A more comprehensive tunnel investigation and grouting solution was procured and carried out successfully by a group of specialists working together with London Underground. An important aspect of the design was the use of an observational approach, a key component of which was an innovative wireless monitoring system with trigger levels linked to the grouting injection system.

Fibre optic monitoring of a deep circular excavation

This paper describes part of the monitoring undertaken at Abbey Mills shaft F, one of the main shafts of Thames Water's Lee tunnel project in London, UK. This shaft, with an external diameter of 30 m and 73 m deep, is one of the largest ever constructed in the UK and consequently penetrates layered and challenging ground conditions (Terrace Gravel, London Clay, Lambeth Group, Thanet Sand Formation, Chalk Formation). Three out of the twenty 1·2 m thick and 84 m deep diaphragm wall panels were equipped with fibre optic instrumentation. Bending and circumferential hoop strains were measured using Brillouin optical time-domain reflectometry and analysis technologies. These measurements showed that the overall radial movement of the wall was very small. Prior to excavation during a dewatering trial, the shaft may have experienced three-dimensional deformation due to differential water pressures. During excavation, the measured hoop and bending strains of the wall in the chalk exceeded the predictions. This appears to be related to the verticality tolerances of the diaphragm wall and lower circumferential hoop stiffness of the diaphragm walls at deep depths. The findings from this case study provide valuable information for future deep shafts in London.

Results of monitoring at the British Library excavation

The main phase of excavation for the basements of the British Library at St Pancras, London, was completed in 1987. The project included basements extending up to 25 m deep, through the London Clay and into the Lambeth Group. The excavations were formed using both the top-down method and open excavation with ground anchors. Existing major buildings lie within 25 m of the site and London Underground tunnels lie below and adjacent to the site. The purpose of this paper is to present the results of displacement monitoring; they are summarised in the paper and presented in more detail in online supplementary data files. The retaining walls advanced towards the site by up to about 32 mm and the clays expanded rapidly on unloading beneath the excavations, causing the Victoria Line tunnels to heave by up to 22 mm. The slow progress of the project provided an unusual opportunity to monitor ground and structure movements in the surroundings before site activity began. Ironically, it was found that the largest settlements of adjacent buildings were caused by the installation of equipment intended to measure the settlements. Extensive condition surveys were carried out, but no damage to adjacent structures or tunnels has been recorded.