Dollis Hill Replacement Retaining Wall
London Underground
Delivered for London Underground under the London Rail Infrastructure Improvement Framework (LRIIF), this project involved replacing a failing retaining wall adjacent to the operational Metropolitan and Jubilee lines. Enable Infrastructure acted as Main Contractor from March to November 2024 to stabilise the cutting slope. The project replaced dilapidated masonry with a new King Post Wall (KPW) while protecting nearby structures and maintaining rail operations, finishing on budget and schedule with zero safety incidents.
The project
The primary objective was to replace a severely deteriorated brick retaining wall and return wall located 200m north of Dollis Hill station. The scope involved the design and installation of a new King Post Wall (KPW) consisting of 24 x 600mm diameter augured piles installed to depths of 12 meters. Following piling, pre-cast concrete panels were installed between H-piles, and a 1.6m high cast in-situ retaining wall was constructed to connect with existing wing walls.
The works required extensive enabling measures, including vegetation clearance, the creation of a 3.0m wide pile mat/haul road, and the removal of the life-expired masonry structure. The site was regraded, and a concrete platform with Key Klamp edge protection was installed for future maintenance access.
Read the full technical study here: Enable Case Study – Dollis Hill
The challenge
The project faced significant constraints due to its location adjacent to the live southbound Metropolitan ‘fast’ track, requiring strict exclusion zones and multiple Tall Plant Approvals (TPAs). The existing return wall was dilapidated, displaying large fractures, while a neighbouring residential property exhibited extensive cracking, posing stability risks during construction.
Logistically, the site was highly confined, with access only possible via a private garden gate, necessitating a remote compound strategy. Ground conditions were hazardous, containing chrysotile asbestos fragments and high zinc concentrations that required specialist remediation. Additionally, unforeseen obstructions, such as undocumented existing piles and poor ground interfaces, threatened to disrupt the piling programme.
The solution
Enable implemented a “Just-in-Time” logistics strategy and utilized a specialist remote compound to maximise the limited site footprint. A comprehensive structural monitoring regime with daily readings and agreed trigger levels was established to protect adjacent buildings. To manage rail interface risks, Enable secured TPAs and physically marked plant exclusion zones with sleepers.
A proprietary “Layher Allround Scaffolding system” was used to protect fragile outbuildings during haul road construction. Subsurface risks were mitigated through pre-probing for services and employing specialist piling contractors (Van Elle) using low-vibration auger methods. Enable also utilised SymTerra, a real-time digital reporting platform, to provide LUL with immediate visibility of site progress.
The deliverables
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Installation of a King Post Wall using 24 x 600mm diameter piles at 12m depth.
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Construction of pre-cast concrete wall panels and a 1.6m cast in-situ retaining wall.
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Safe removal and disposal of hazardous asbestos and zinc-contaminated waste.
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Creation of a 3m wide pile mat and haul road to facilitate piling rig access.
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Repairs to existing wing walls using a HeliBar Crack Repair System.
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Real-time reporting and data capture via the SymTerra digital platform.
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Final site regrading, landscaping, and installation of maintenance platforms.
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Successful project delivery with zero accidents or service disruptions.
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