Catch the Crossrail Lecture here
Review by Peter Elliott, NVentures
The Engineering Geology of Crossrail
With John Davis, Chief Engineers Group, Geotechnical Engineering Team, Crossrail
It is not surprising to see an excellent turn out at the PESGB London Evening Lecture meetings these days, it being the premier networking event and unofficial industry social club during low prices. Without a hint of hydrocarbons on the agenda, Tuesday was no exception. The audience were noticeably intrigued by the subject being presented by the vary capable John Davis, a story of boring through rocks at an immense scale. Mr Davis, of the Geotechnical Engineering Team at Crossrail, delivered the goods, giving an excellent talk on this impressive engineering project.
The presentation went from the concept of Geotechnical Engineering, through the scope of the Crossrail project and the techniques and compensation mechanisms employed. John also used some case studies to illustrate the complexity and ingenuity behind the tunnel linings we all take for granted. If this modern railway project dwarves our own industry’s efforts to create impressive holes in the ground, it wins hands down in the acronym stakes as well! The TLAs were flying as the speaker explained the various tools and techniques of the tunnelling method. TBMs (tunnel boring machines) carve out the majority of the new Elizabeth Line, using Earth Pressure Balance (EPB) in firm London Clay or Slurry Shield (SS) in the soft toothpaste of the Chalk. It was fascinating to the engineers in the audience to recognise the use of bentonite to stabilise trench and pile construction, as well as the EPB technique (keeping pressure equilibrium with the rock face at the cutting edge) carefully managing the integrity of the bore and its surroundings. Other areas are excavated with open face machines and finished with sprayed concrete lining (SCL).
One of the main roles of the geotechnical engineers were the compensation techniques used to manage settlement and subsidence and other risks, using injection grouting and piling. Whilst settlement is unavoidable to some extent, and can be minimised, it is the variation in settlement that can cause damage, warping the land and buildings thereon. The engineering team consider the stiffness or strength of the sediments, permeability and water content. The engineers can achieve great efficiencies in the London Clay where water content is low and stiffness is high, but struggle through alluvium and the Thanet sands where rock can literally flow under the pressures involved. The team can actually manage the water table quite accurately, locally and over large areas, to alter the stiffness of the sediments so it can be bored or excavated. Most familiar to the terrestrial pedestrian are the access shafts and station boxes that make commuting so interesting. The bucket shaped excavators and tall piling machines are relatively familiar to the harried road users, creating walls and linings down to 50 or so metres, which are then excavated from the inside to crate the space for stations and utilities.
Mr Davis finished with a good series of examples from Crossrail, some of which are being excavated as we speak. Impressive pictures of 7.1m tunnels in 17m “halls” illustrated well the scale of the project. In one example to the east of the new railway the team are challenged with boring and creating a station through mixed geology, amongst a very busy subsurface. The audience fairly held its breath as John described one case where the tunnel and station meet, at an angle, cutting though layers of clay, sands and loose alluvium and peat, with sewers and utilities just feet above, and the underground line tunnels just feet below. A good deal of grouting (via tube a manchettes, TAMs) and surface strengthening was required. One could imagine a capricious god in some fictional future, lifting London up by its “hair” of high rise sky scrapers to reveal a coagulated inorganic concretion that is the vast subterranean construction of the capital’s underworld.
Q&A included questions on the spoil (most of which has gone to enhance an RSPB site in Essex) and whether the geotechnical team had used or acquired any seismic. They had used some geophysics, as well as 1100 boreholes, to assess the geology and soil mechanics
The VP Ricki Charles gave the vote of thanks and led the wide-eyed audience into the hallowed geolsoc library to recover with a cold beer.