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HS2: Overcoming geotechnical challenges on phase one – Ground Engineering

30 May, 2022 By Thames Menteth
The geology and scale of earthworks are among the geotechnical challenges that HS2 Ltd have overcome along the high speed 2 (HS2) phase one route, delegates heard at GE‘s Transport Geotechnics conference.
At the event earlier this month HS2 Ltd lead geotechnical engineer Sarah Trinder told the audience that the company and its contractors had dealt with various geotechnical construction and design challenges along the phase one route. These ranged from ground movements, earthworks, and variable geology.
Over half of the phase one route which runs from London to Birmingham will be in a tunnel or a cutting to reduce visual and noise impacts arising from the scheme.
About three quarters of the route at ground level will have features such as noise barriers and landscape earthworks alongside the line to help the railway blend into the landscape.
As a result, HS2 Ltd’s tunnelling and earthworks campaigns for the scheme are vast and complex.
In her keynote speech Trinder said that the geotechnical design challenges on HS2 phase one arise from three main drivers. These include tight tolerances for ground movement due to the high speed of the trains, ensuring reliability of the earthworks over a 120-year design life, and sensitivity to the surrounding environment which means material movements must be planned and managed with care.
The geology itself brings challenges, she noted.
HS2 phase one traverses a whole range of strata. Starting in London the route passes through London clay, chalk, gault clay, Kimmeridge clays, and Kellaways and Oxford clay. Heading into the West Midlands it passes through strata of the great oolite group, strata of the lias group and ends in the Mercia mudstone group in Birmingham.
While the high and medium plasticity clays are suitable for low embankment fill in high speed earthworks if treated, Trinder said that “care needs to be taken with sulphates in these materials.”
Because of this, a series of stabilization trials have been undertaken on how these materials may be used to best advantage on a project.
The chalk, meanwhile, can be problematic because it contains dissolution features. To overcome this HS2 Ltd’s contractors have used ground improvement measures which have included “grouting and treatment of those features.”
In the northern half of phase one teams have encountered the full sequence of the two principal mudstone strata of the lias group – the whitby mudstone and the charmouth mudstone. “Two challenges in these materials are deep cutting heave and time dependent slope behaviour”, said Trinder.
She added: “We’re also dealing with lesser studied geologies, for example, tunnelling through Mercia mudstone and meeting heave tolerances in deep cuttings in high plasticity clay.”
Another geotechnical challenge for HS2 Ltd is the sheer size of the scheme and the earthworks necessary to create it.
HS2 is the biggest earthworks project in the UK. According to Tinder, “2022 will see us move three times the volume [of earthworks] excavated last year.”
HS2’s main works contractor EKFB – made up of Eiffage, Kier, Ferrovial Construction and Bam Nuttall – will move 15M.m3 on its 80km section. In the north of the route BBVA – made up Balfour Beatty and Vinci – will move about 8M.m3, double last year’s amount.
Give its enormous scope, HS2 phase one has presented opportunities for geotechnical learnings.
As an example, HS2 Ltd is working with the Royal Academy of Engineering Kevin Briggs from the University of Bath who is studying the weathering of the charmouth mudstone in detail.
As part of Briggs’ study at HS2’s Boddington site in Warwickshire a full-scale trial to investigate heave is underway.
The trial cuttings are 300m long and 14m deep through charmouth mudstone. The ground investigation programme involves drilling 65m deep rotary holes, downhole geophysics, lab testing and instrumentation installed to pick up the early parts of that heave movement.
On top of these trials, “there is so much work going on being recorded”, noted Trinder.
All data collected from ground investigations across the HS2’s various phases are recorded in one database, which Trinder said will be shared with the BGS in due course.
HS2 Ltd are also undertaking an ASGI trial, an AGS piling trial and it has at least another three data workstreams at present to increase its earthworks productivity. This includes the development of a digital twin for geotechnical engineering and identifying potential collaboration with a connected and autonomous plant (CAP) project.
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