75 London Wall

The scheme

75 London Wall is a major commercial redevelopment on the London Wall corridor in the City, designed by ORMS for Castleforge Partners with Opera as project manager. The defining below-ground decision was to reuse the existing basement rather than demolish and dig again. That is the right instinct on carbon, programme and cost, but it moves the risk: a new basement is designed to be dry; an old one has to be made dry, and you inherit every construction joint, crack and penetration the previous owners left behind.

CLW was brought in early in the design to carry that risk properly, from the first look at the historic record through to the performance specification and then monitoring on site.

What CLW did

Found what the structure actually was. Before anyone could decide what grade was achievable where, the existing basement had to be understood. Working through the historic as-built information, the team identified construction details that had not previously been picked up by the project, and that changed how the retained walls and slabs could be treated. The project director’s words, not ours: CLW “identified important as-built information from the historic data which had not been previously identified”.

Zoned the performance. Rather than specifying a single blanket grade, the specification sets out separate BS 8102 Grade 3 and Grade 1b performance requirements, so each area of the basement is designed to the dryness its use actually needs and no further. Matching grade to use is where most of the money is saved or wasted on a basement; see what grade of waterproofing do I need.

Wrote a performance specification the contractor could be held to. The 75 London Wall waterproofing performance specification covers remedial repair of the existing basement walls, a Type C cavity-drained system (membranes, perimeter and floor channels, load-bearing insulation, sump chambers, pumps, power supplies, controls and discharge), the hot-melt podium membrane, protection and repair, and a benchmark and mock-up schedule. Its appendices set the employer’s requirements for the specialist waterproofing contractor: CSSW/WDS-qualified staff, in-house design capability, NVQ-qualified directly employed installers, and the rule that sourcing the waterproofing design from a supplier is unacceptable. The specification also requires the same specialist to carry the waterproofing across every works package it touches, so there is no gap in responsibility between the basement, the podium and the remedial works.

Stayed through the PCSA and construction. CLW worked with the main contractor through the pre-construction services period to bring the risk register down to specific, priced mitigations, and continues on construction monitoring so that what is installed matches what was specified. Operation and maintenance requirements for the cavity drainage and pumps, including alarm testing and power back-up integrated with the BMS, were written into the specification rather than left for handover.

What the client said

“Derisking basement reuse required specialist waterproofing early in our design. CLW were a great addition to the design team and immediately showed their worth. They quickly understood the project and identified important as-built information from the historic data which had not been previously identified. The team have continued to show great value through collaboration with the main contractor during the PCSA; ultimately bringing clarity to the risk and enabling robust mitigation.”

Will Jarrett-Kerr, Project Director, Opera

What this project illustrates

• Basement reuse is a waterproofing problem before it is a structural one. Appoint the specialist before the retention decision is locked, not after. See waterproofing design for basement refurbishment and retrofit.
• Zoning Grade 3 and Grade 1b by use is how a specification stays affordable without becoming a compromise.
• A performance specification with contractor competence requirements and a mock-up schedule is what turns “specialist contractor” from a hope into a contract term. See waterproofing performance specification.

Project credit: ORMS · Castleforge Partners.