Salisbury Square Development

The scheme

Salisbury Square is the City of London Corporation’s Justice Quarter development on Fleet Street: the new City of London Law Courts (18 courtrooms, with custody cells in the basement), the new City of London Police headquarters and a commercial building, all sitting on one common three-storey basement around 16 m deep, with public realm over it. A listed building on the same plot was retained and also had to be waterproofed. Eric Parry Architects designed the scheme; Mace won the construction tender; Keller carried out the piling. The design life is 125 years.

Two things made the waterproofing unusually consequential. The basement houses court and custody functions, so the dry areas have to meet BS 8102 Grade 3 and stay there. And the building is security-vetted: if a leak appeared after handover, simply getting a competent person on site to look at it could take two months. A leak here is not a maintenance item; it is an operational problem for a court.

Why an independent designer was appointed

Three reasons, which Ben set out in a talk on the project at Ground Engineering’s Basements and Underground Structures conference:

• Architects, engineers and their insurers increasingly do not want to carry basement waterproofing risk. It is a niche discipline, and competent generalists know it is not theirs.
• The project was design and build, but by the time a scheme reaches RIBA Stage 4 and goes out to tender, the waterproofing design has in effect already been done, even if by accident. The structure, the piling and the ground-floor interfaces have constrained what any contractor can do. Deferring the design to the contractor does not transfer the risk; it just leaves the design undone. See design and build and waterproofing risk transfer.
• The consequences of failure were high enough that the client wanted the design produced, peer-reviewed and specified by people who do this every day.

What CLW did

Stage 2: risk assessment and design philosophy. We reviewed the ground investigation and asked for ground-gas monitoring that had not been planned: the site sits on river deposits, and if methane or carbon dioxide had been present the whole waterproofing approach would have changed. We then wrote the waterproofing design philosophy: what each area of the basement is for, how dry it needs to be, and broadly how that would be achieved.

Stages 3 and 4: design development. Roughly 2,000 hours of design development went into the basement waterproofing, which is far beyond the norm. The scale of the basement, the fire compartmentation, the security requirements and the number of interfaces meant that a generic “watertight concrete plus cavity drain” answer would not have been maintainable.

Tender and construction. We produced a comprehensive performance specification that went out with the tender. The specification was adhered to and a specialist waterproofing contractor was appointed. CLW now sits on the client monitoring team, checking that what is built matches what was designed, and dealing with the inevitable “equal and approved” substitutions. Our position on those is simple: an alternative is approved when the designer approves it, not when the supplier says so.

The design

Watertight concrete, with Eurocode 2 Part 3 used intelligently. The bulk of the waterproofing is Type B watertight concrete. We referenced BS EN 1992-3 but deliberately did not comply with its crack-width limits in full. At 16 m depth the required crack widths become very small, the reinforcement becomes congested, concrete cannot be placed and compacted properly, and the result is voids and leaks: the standard, followed literally, would have made the structure less watertight. We reduced the design crack width and added a waterproofing admixture instead. (See our article on Eurocode 2 Part 3 and watertight concrete.)

A zoned cavity drain system that can be maintained. The Type C cavity drainage is what makes this design unusual. On a basement this size, a sump pump working hard tells you only that water is getting in somewhere across three storeys of perimeter. So the perimeter drainage channels are cast to falls and routed to outlets by zone, with twin drop-downs at each zone and wall access panels, so that in 60 years’ time a leak can be traced to “basement 2, zone 3, left-hand drop” and repaired through a removable panel. The complication was never the hydraulics; it was keeping the zoning coherent as fire compartments and room layouts changed through design. (We set out the general approach in cavity drain zoning and maintainability.)

No hydrostatic head at B3. The secant-pile wall forms a cut-off into the London Clay and the lowest slab has sub-slab drainage to a sump, so the base slab is designed without hydrostatic pressure and is not watertight concrete. Where the risk is genuinely removed by the ground and the structure, the waterproofing is not duplicated.

Sequence. A pre-applied membrane had to be installed before the capping beam was cast. This is the detail that most often arrives too late: when an enquiry says “piling starts next week, can we have a design”, the honest answer is usually no.

What the client said

“I’ve worked closely with CLW on the Salisbury Square Development scheme, and they have shown outstanding value in picking up on issues early, guiding the design team and ensuring work on site complies with the best practice. Working with CLW has given me confidence in our development and the basement waterproofing. I would highly recommend engaging with them on any development with a basement.”

Jianxi Cheng, Director, Avison Young

“I know the CMT role is not a competition between disciplines, but I have felt that CLW as its own discipline has really excelled and ensured that the requirements and best interests of the Employer have continuously been upheld by Ben and his team.”

Kiran Ragupathy, Associate, AECOM

What this project illustrates

• Appoint the waterproofing designer at Stage 2, before piling and the capping beam fix the options. See when to appoint a waterproofing consultant.
• A performance specification that is actually enforced is worth more than a branded detail. See structural waterproofing design.
• On long-life, high-consequence basements, design the repair strategy, not just the barrier.

Images on the related episode page courtesy of Eric Parry Architects and dbox. Project credit: City of London Corporation.