David Keffen CSSW
As an active member of the Maclennan design team, I see numerous waterproofing challenges in the course of a week. We have an advantage over Engineers and Architects in that we specialise in waterproofing, and therefore the challenges faced by the Basmenteer are not unfamiliar to us.
This is no negative reflection on the engineer or Architect as they are advised by the British Standard BS8102:2009 to call us in.
A waterproofing specialist should be included as part of the design team so that an integrated waterproofing solution is created. The waterproofing specialist should:
A: Be suitably experienced.
B: Be capable of devising solutions that accommodate the various project constraints and needs.
C: Provide the design team with information and guidance that assists with and influences the design, installation and future maintenance of the waterproofed structure.
- British Standard BS8102:2009
Here's a typical scenario highlighting a fairly common problem, I wonder if you can identify with it?
The main contractor is starting to see light at the end of the tunnel. The end of the project in site (only a month over Programme dates but that has all been accounted for now) ... The basement M & E is nearing completion (finally!). Then they get a phone call from the clients representative "The clients have suddenly decided that they require an extra sauna, plunge pool and shower facility on Basement level 3...Oh and 'A Multi Media and Technology Command Centre'.." that necessitates a further bundle of fibre optic cables. (Are you imagining Bond Villain now? Interestingly they aren't as rare as one might imagine... Multi Media and Technology Command Centres...I mean, not Bond Villains).
Is that doable? Well, yes apparently 'it is' according to the engineer calling in from a Beach Barbados having thought her work was over, but only with two more support columns and another 5 pipe penetrations through the retaining wall on the top basement level.
Hmm that's a nuisance; even the 3 additional 150mm service pipe penetration sleeves installed in case of emergencies are not going to be enough.
Familiar tale? What is the best approach to these problems?
Well, best of all (as I have already alluded to) would be to ask a waterproofing specialist for some design input after all they have already made sure the basement is completely dry so far. That's where we come in.
Question: Where are the services in relation to the basement?
Top Answer: That they can all enter the basement via a dry riser from a dry space in the Ground Floor building, but as we know that is not always practical. So what is best practice from there on?
Next Best: Well certainly keep them as near ground level as possible. The lower they sit, the higher the chance of hydrostatic pressure causing a leak, especially if they are hot services. If they are just below ground level, then it might be practical to provide a dry space such as a manhole or service trench that can also be waterproofed, lessening the risk. Unfortunately it's too late for that/ there is not enough room. What next?
Not as Good, but Workable: No chance of any dry space externally, but they will be brought in near ground level. Quite a common problem alas but there is always something worse lurking:
Dire: No dry space externally, base of the wall. High Water table. Maximum risk.
OK, so let's just cut out the flimflam and get down to brass tacks. How do we approach this worst-case scenario?
I’m pushing the boundaries slightly here for the purpose of this article as it is highly unlikely that this would be necessary, but we have seen the odd one here and there.
It is tempting to tell the client's design team in no uncertain terms to 'go back to the drawing board' (remember them?), but as we know, that may not be the option and we are here to help our clients in the best way we can.
At the base of a wall (yes we have seen this, and been told categorically that this 'has to be like this') the only truly safe approach is to use mechanical gasket systems such as that produced by a rising number of engineering companies. We might use other flexible waterproofing products when the risk is low, but these rely on service not being able to move as the products remain flexible, and as we know, pipes and cables get pulled and pushed in plant rooms so even with extreme clipping; even when the flexible material is capped with a Natural cement repair mortar plug, there is always a chance the seal will be broken.
First of all the core. Certainly, an oversized and safe access trench (either battered back or properly shuttered temporarily) would need to be dug in order to pump out the groundwater and provide a safe, dry space externally while the core drilling takes place. Next there are a number of options with sleeves that can be retro-fitted (too many to mention here). Some are designed with crystalline coatings that merge with the concrete in order to alleviate the need for Hydrophilic strips, others rely entirely on mechanical fixing plates attached to the face of the concrete as a flange. Sleeves should protrude to allow attachment to the cavity membrane system (you would be fitting this if we are involved as it is the most reliable form of waterproofing).
Next the plug. Mechanical gasket systems are generally well-engineered products that are robust and reliable. They should be the 'go to' product in risky (and perhaps not so risky) situations. The mechanical plug has a rubber EPDM press seal core that is pre-drilled to the size of each of the services running through the duct, and then a metal plate is compressed using bolts and a torque wrench to squeeze the joints tight. It is important, as with all service ducts that services are not bundled together. This means that our fibre optic cables would need to be separated and the gasket would be drilled for each cable individually.
It is imperative that these are sized exactly and then fitted by trained Technicians who understand how to fit them with correctly, or inevitably trouble will follow.
Maclennan are expert waterproofing specialists, and are not tied to any one manufacturer’s system. We are approved installers of all the leading manufacturer’s products and will always specify the best product for each individual situation. Our main objective is a ‘Leak Free’ basement.
So the morals of our story are that if you have a basement to waterproof involve a design specialist at an early stage so we can do full solution for you and if you must drill holes in your lovely water-tight structure after we have protected for you, and you need your penetrations sealing, give us a call for a full design and installation service that is second to none.