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Case studies
Published in David Chapman, Nicole Metje, Alfred Stärk, Introduction to Tunnel Construction, 2017
David Chapman, Nicole Metje, Alfred Stärk
The excavation at both stations was mainly within the London Clay. The underlying stratum, the Lambeth Group, was close below the invert in the Whitechapel Station tunnels and was encountered in the invert of the deeper excavations at the Vallance Road Crossover and at Liverpool Street Station. The Lambeth Group is dominated by clay, which with respect to tunnel excavation and support can be considered the same as for the London Clay. The biggest concerns were related to the sand layers within the Lambeth Group as the sand was likely to contain pressurised water, which could generate a local collapse of the ground if not depressurised; a scenario which had to be avoided. Depressurisation means reducing the ground water pressure by local pumping to a level suitable for safe excavation. However, the sand layers were virtually unpredictable in terms of location, lateral extent, thickness, water content, water pressure and recharge. With the exception of the Vallance Road Crossover, all the areas were too congested with buildings and existing underground structures to sufficiently investigate the ground from the surface. Therefore, an in-tunnel probing and depressurisation scheme had to be developed in order to identify these sand layers and get the water depressurised before the excavation reached that location.
Heathrow Express Cofferdam (1994–1995)
Published in Alan Powderham, Tony O’Brien, The Observational Method in Civil Engineering, 2020
A site investigation, to evaluate the changed conditions, was initiated immediately after the collapse. The ground conditions in the CTA, prior to collapse, were relatively uniform with approximately 6 m of Terrace Gravels overlying the London Clay which has a thickness of around 60 m at this location. The London Clay overlies the Lambeth Group, which generally consists of heavily over-consolidated clays and sands. This in turn overlies the Chalk which is present at a depth of approximately 90 m below ground level.
Investigation of tunnel movement of the Thameslink tunnels below site S3 of King’s cross zone development
Published in Geomechanics and Geoengineering, 2020
Oli O’Shea, Chrysothemis Paraskevopoulou, Richard Miller
The London Basin, basin is the phrase used when describing the sedimentary geology of London (Sheppard 1917) with outcropping Chalk in the North and South forming the original limits of the basin (Royse et al. 2012). The basin itself represents a shallow, generally east-west synclinal fold, formed during the Variscan Orogeny roughly 290 Ma. A Palaeozoic basement has been established at ~300 m beneath London during oil and gas exploration drilling (Sumbler 1996) that has been overlain by a widespread layer of Gault which is topped by a ~ 200 m thick group of Chalk, (Figure 4). Above the Chalk are the later marine deposits that begin with the fine-grained sand with clay called the Thanet Sands at the base of the succession. The Lambeth Group succession follows, consisting of clay mottled with fine-grained sand and flint pebble beds, which then grade into the Harwich Formation as the lower Member of the Thames Group. In this group is the Claygate Member with interbedded sand with clay at the top, the London Clay Member, the focus of this paper, with clay and silt, fine sand clay at the base and the Bagshot Formation at the surface. In areas of central London, these Palaeogene deposits are overlain by river terraces formed by the River Thames.
Investigation of the interaction of the construction of building S1 on underlying Thameslink
Published in Geomechanics and Geoengineering, 2021
Jonathan Foster, Chrysothemis Paraskevopoulou, Richard Miller
The predicted displacement for both tunnels from the excavation stage are shown in Figure 8a. The immediate predicted displacements for the HSS model show significant heave of the tunnel of ~4.53 mm of total displacement. The displacement is predominantly vertical with negligible transverse displacement of 0.07 mm. The crown shows a 0.3 mm greater displacement than the invert Figure 8b. This is an expected response for tunnel heave (Wang et al. 2013). This difference will result in a small vertical elongation of the tunnel cross section but is not significant. The displacements predicted using the MC constitutive model are contrasted in Figure 8b. The use of the HSS model predicts ~1.48mm less in the tunnel displacements. The total displacement field for this stage is shown Figure 8c. There is a concentration of higher displacements (5–8.5 mm in heave) in the upper meter of the Made Ground. The displacement field extends down to the London Clay – Lambeth Group boundary and an additional 23 m into the Lambeth Group before it becomes negligible. The lateral extent of the displacement field is 37 m in each direction until displacement is negligible. The ground displacements within the London Clay directly adjacent to the tunnel are similar to that of the tunnel displacements. The actual tunnel displacements, discussed in Section 6, are lower than both the HSS and MC models Figure 8b. As expected the HSS shows a better representation of reality but still a relatively significant difference of ~2.5 mm for the maximum displacements. Although this level of displacements predicted is not surpassing an SLS limit of the tunnel, this level of displacement can result in damage as exemplified by Chang et al. (2001) where 10 mm can be enough to begin to see crack initiation and propagation on the concrete slab. The high heave predicted for Made Ground is considered to be an aspect which in reality would be prevented by compaction and loading from site activities which have not been accounted for in this model. Based on the TRTS case example which covered induced heave of shallow metro tunnels in soft ground (discussed in Section 2), the movement of the NB and SB tunnels relative to the ground was expected to be much larger than this prediction. Instead, there is no evidence of soil pressures being resisted by the ‘stiff elastic beam’ of the tunnel which goes against analytical reviews such as by Zhang et al. (2013).