Thames Tideway geology has influenced construction design, says project engineer – Ground Engineering

18 March, 2022 By
Geological conditions along the Thames Tideway Tunnel have had a significant influence on the design of its tunnels and shafts, Jacobs geotechnical engineer Tim Newman has said.
At an online event organised by the British Geotechnical Association and the Engineering Group of the Geological Society, Newman said that the London Basin geological sequence which the tunnel passes through had determined elements of its construction and design.
The 25km long stormwater storage tunnel is constructed up to 65m deep beneath central London and incorporates twenty shafts and ten smaller diameter connection tunnels.
Known as London’s “super sewer”, Tideway passes through many geological sequences such as the Thames Group, Lambeth Group, Thanet Formation and White Chalk subgroup.
Newman, a chartered geologist and engineer, worked on the initial ground investigations for the Thames Tideway Tunnel.
He said that geological conditions had influenced the engineering design of Tideway’s connection tunnels, which are constructed via open faced techniques by personnel working in potentially “hazardous” confined spaces.
By using a construction geological model, the project team was able to identify areas of potentially unstable ground. As a result, certain connection tunnels were made as short as possible to ensure that personnel were not exposed to potentially dangerous unstable ground conditions for any length of time.
Newman added: “Where the ground conditions are better and more stable, we can relax these conditions and the main tunnel can follow its more preferred route or hydraulic route along the centre of the river.”
The geology has also had an impact on shaft construction through London Clay at Tideway’s west London sites.
On these sites “all the shafts have been constructed or initiated using bored concrete piles in a secant pile arrangement”, said Newman. These piles are interlocking to form a cut off of the groundwater in the upper aquifer.
However, as the shaft construction progresses deeper “the ground becomes stiffer, so the bored piles have a limit of about 20m to 25m depth beyond which point they start to deviate off the vertical”. Because of these changing ground conditions, the team has installed a sprayed concrete lining (SCL) below the secant piled wall.
Where conditions do not require a cut off of the upper aquifer in London Clay some of the shafts have been constructed using concrete segments (jacked caisson) which are built from the top.
“The jacked caisson is also restricted by the increasing stiffness of the London Clay to again about 25m depth,” explained Newman.
In the east of the scheme where shaft construction is through the Seaford Chalk Formation, Tideway has used a heavy-duty rig to cut trenches into the ground. The rig has been aided by rotating cutting teeth at the end of its boom.
The “considerable indurated layers of rock” in the geological sequence that Tideway passes through were also a major consideration for Tideway’s tunnel boring machines (TBM) designers.
For this reason, the cutterhead designs or arrangements for TBMs Ursula and Rachel were “purely dictated by the geology”, said Newman.
“[Rachel] has a much more reinforced cutterhead arrangement, a lot more steel reinforcement, and very small windows to deal with the potentially more unstable ground conditions to be encountered in the Lambeth Group deposits. And a lot more cutting teeth mounted on that cutterhead arrangement.”
Newman added that the excavations for the project have enabled observations of the site geology. These have revealed sedimentary characteristics that could not be fully evident in disturbed samples from ground investigation boreholes and laboratory tests.
He said: “In the case of all the shafts, the ground has been dug out from the core of the structure as a bulk excavation. Doing so has revealed some fantastic 3D exposure of geology.”
The observations have led to significant improvements in understanding the in-situ groundmass behaviour of several strata within the sequence and their impact on the performance of different construction techniques.
At the GE Basements and Underground Structures 2021 conference, Barhale regional manager Ovi Frunza highlighted other construction challenges at Victoria Embankment on the central section of the Tideway scheme.
In its latest interim report from November, Tideway, the company building the project, confirmed that the project remains on track to be completed in 2025.
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