Case Study: Salisbury Square – Ep 11

Hi there,

Watch the episode on YouTube

I am afraid I’ve been very inconsistent with my ‘Waterproofing Wisdom’ emails – apologies.

Nevertheless, I decided that an easy way to get back on the horse was to share a presentation I’ve already delivered.

Last October, I had the chance to share a presentation on the Salisbury Square Development at the GE Basements and Underground Structures conference.

We have loved working on this massive basement project and if this sort of thing is your cup of tea you might enjoy it (although it’s 13 minutes long).

The talk covers:

• What the Salisbury Square Development is: three buildings on one common basement in the City of London.
• Why the client appointed an independent waterproofing designer.
• What we did from RIBA Stage 2 through to the construction monitoring team.
• How the watertight concrete was designed against Eurocode 2 Part 3, and why we did not comply with it in full.
• The zoned cavity drain system, designed so a leak can be found and repaired decades from now.
• Why the waterproofing has to be in before the capping beam.

The written case study is on the Salisbury Square project page. We got some nice feedback from the project manager on this scheme:

“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

Please feel free to reach out if you have any questions or would like to discuss this further.

Thanks,

Ben Hickman Technical Director 0207 315 4152

Transcript

Lightly edited for readability. Images referred to in the talk were provided by Eric Parry Architects and dbox.

Good afternoon. I’m pretty sure they’ve saved the best till last — either that, or waterproofing is an afterthought, which is normal for me as well. I’ve got 15 minutes to talk to you about a case study.

My name is Ben. I’m a waterproofing consultant. I lead a team of four technical staff and we design basement waterproofing, podium decks, blue roofs, that sort of thing. I do a lot of expert witness work. That’s a little intro to me. I need to thank Eric Parry and dbox, because they gave me the images I’m using. I should also say: you can only trust what I say about waterproofing. Everything else about this project, I might be talking complete nonsense — and there’s a whole bunch of security stuff, so I hope I’m not sharing something I shouldn’t.

The project

This is a new project in the City of London, currently ongoing. Piling is happening, excavation is happening, right now. It is providing three buildings on one common basement.

Building number one is the City of London Law Court. That’s going to provide 18 courtrooms, with a whole bunch of associated things that I didn’t know you need in a law court — if you go to trial, the defendant and the accuser can’t meet in the corridor, and things like that cause a lot of complication. There are some cells in the basement. Building number two is the new City of London Police headquarters. Building number three is a commercial facility. All of that sits on one common basement, and it’s the basement that I care about.

Here is the project — that little blob there. You can see St Paul’s; that’s the Thames; that’s the City of London. That whole area is supposed to be the Justice Quarter. One thing I didn’t know is that UK justice is apparently quite a big export: in an international case, a lot of people would rather have the case tried in the UK because of the justice system. So this is a new piece of infrastructure built around creating the Justice Quarter — there are a lot of chambers in that area, a lot of lawyers — and this combined police and law court facility is what that’s about.

Here’s a CGI of what will be on Fleet Street; that is St Paul’s Cathedral. It’s a really cool-looking building. Eric Parry Architects — phenomenal, by far the best architects I’ve ever worked with. Another angle: there you can see the back of the law court, that is the police headquarters, that’s the commercial building. That public realm is over the basement, so there’s a big podium there. And that is the ugly stepsister: it’s a listed building, so they couldn’t knock it down. It’s on the same plot, so we’ve also had to waterproof that.

Why were we appointed?

I think there is an increasing awareness among architects and engineers, and their insurers, that they really don’t want the risk of basement waterproofing. It’s quite a risky construction element, and so there is a push for architects and engineers to say “not my problem”.

At the same time, normally we’d want to push the risk onto the contractor — it’s going to be design and build, so they can design it. The reality is that if you get to RIBA Stage 4 and you’re tendering for waterproofing, you’ve already done design, even if it’s by accident. You’ve constrained what a main contractor could do, even if they were a brilliant contractor.

There’s an increasing realisation that basement waterproofing is a niche profession in its own right. If you’re doing something complex like this, you want somebody who’s trained, who’s competent, who’s checked by peer review, who does this day in, day out. Many are recognising that, as competent as architects and engineers and other people are, if you don’t do waterproofing every day it’s probably not your niche area.

And finally, this is a fairly unique building. With the police and the law court there’s blast protection, there are prefabricated prison cells in the basement, there’s security vetting. If there were to be a leak, to get a competent professional to come out and do a survey you’d be waiting two months for their security vetting. So any leak would be a pretty big deal and something to be avoided.

Because of all that, we got an appointment.

What did we do?

We did a desktop analysis. We looked at a soil investigation, scratched our heads a little bit, asked quite a few questions. One of the things we needed was some gas monitoring. Nobody seemed to have thought about it — there were some river deposits; is there any methane? From a waterproofing perspective, if there were ground gas contamination our whole approach to waterproofing would be very different, so it’s quite a big issue. So we did that.

Then we did what we call a waterproofing design philosophy, which is the big-picture overview: what’s going to be in the basement, how dry does it need to be in all the different areas, and, broadly speaking, how are we going to do that. That was all at RIBA Stage 2.

Then we did a lot of design development throughout RIBA Stages 3 and 4. We probably did about 2,000 man-hours of design development for the basement waterproofing. I think that’s unheard of — we really went to town on it, and I’ll try to explain a little more about why in a moment.

Then we did a very comprehensive performance specification that went out to tender. Mace won the job, and we are now on the client monitoring team, trying to make sure that everything is coherent and going as it should.

Watertight concrete — Eurocode 2 Part 3

The bulk of the waterproofing on the job is coming from watertight concrete. While we referenced Eurocode 2 Part 3, which I’m sure some of you in the room know well, we didn’t really comply with it. Eurocode 2 Part 3 is about liquid-retaining and containing structures and it’s all about crack width: the further underground you are and the thinner the concrete section, the thinner the maximum crack you can have. The problem is that if you get down 16 metres your crack widths get very, very small, and then you’ve got loads of rebar, and then you’ve got congestion, and then you don’t get the concrete in the right place, and then you don’t get compaction, you get voids, and you get leaks. So we didn’t quite comply with Eurocode 2 Part 3, but we did reduce the crack width and we used a waterproofing additive.

All the waterproof elements are shown in blue on our drawings. That area there is the podium, the public realm; then you’ve got blue waterproof concrete down here and up the left-hand side of the building as well. There’s a big section through the building — I’ll drop through all of those sections and pick up some pieces along the way.

Ground level

This is pavement level. From a waterproofing perspective, often the most complicated part is at ground level — which may be surprising — but it’s very difficult to get waterproofing from below ground to above ground, and you need to finish above ground. So that’s quite a funky detail we came up with there, and it’s got some drainage on this little shoulder just below ground.

The cavity drain system

This is for a cavity drain system. It’s not really shown there, but if any water were to come through this mass of capping beam it would collect here and run down, and then it would run down to the next level.

Part of what’s unusual about this project from a waterproofing perspective is the cavity drain system we developed. A cavity drain system means that should there be any water coming through at any point in the next 125 years, which is the design life, it should collect and be drained down and eventually get to a sump where it’s pumped out. But on a building of this scale, if your pumps are working very hard, where do you go to do a repair? It could be anywhere within the full building perimeter, across three different storeys.

So what we did is develop a routing system where all the perimeter drainage channels are cast to falls, and they all go to outlets, and those outlets are in zones. Should there be any major water ingress, we should be able to trace it to “second-storey basement, zone 3”. It’s not that complicated in itself; it becomes complicated when you’ve also got to factor in fire compartmentation and the other things like that — and then the client changes a room layout and the fire compartment changes. That sort of thing is what makes it complicated.

Cavity drain systems need maintenance access, so that’s a wall access panel: in 60 years’ time somebody can poke their head and shoulders through there, look down, see the line and do the repair or the maintenance if necessary. We came up with pairs of drop-downs at each zone. That would be the low point, and further up the wall you’d have a high point and a perimeter drainage channel, so water would come down here and drop down there. The benefit of these twin drop-downs is that you can see “more water is coming out of the left, so the defect is probably up there on the left rather than the right”. That was the strategy. And this is a panel of cavity drain membrane that you could remove and put back.

B3 — sub-slab drainage, no hydrostatic head

Finally you get down to B3 level. Water would collect there and go to a sump and a pump. Interesting to note: there is sub-slab drainage, so we don’t have waterproof concrete throughout this basement floor slab. We shouldn’t have any hydrostatic head: the secant piles should form a cut-off into the London Clay, and any water that does come into that zone should be pumped out. We’re not anticipating any hydrostatic pressure in this area, hence it’s not waterproof concrete.

Where are we now?

That’s a picture that’s probably two weeks old. Fleet Street is up there. They’ve excavated down one storey in this area. You might see a little bit of white there — that would be Preprufe, a waterproofing membrane. The waterproofing membrane has to go on before your capping beam. That’s one of the things people often don’t think about: you’ve actually got to have your waterproofing in before your capping beam, a lot of the time. So when we get an enquiry that says “piling starts next week, can we have a design?”, our answer is normally no — it’s too late. But that’s quite a common occurrence.

Keller are on site doing the piling. There are massive geothermal piles going in, and massive piles elsewhere. Some really awkward temporary works, with these king posts to hold up the pavement.

We’ve got a great specialist waterproofing contractor in, which is a win for us — our performance specification was actually adhered to, which is nice. We’re having some challenges with contractors having certain supplier preferences: they’re very keen to tell me “this is an equal and approved alternative to what you specified”, to which I’m a little confused — who’s approved it? I’m pretty sure that’s my job. So that goes on, and some contractual challenges, but generally I think we are progressing well.

I’ve raced through that so there’s more time for questions. Apologies if I’ve gone a little quick. Thank you.