Tight Spaces and Tanking
In Dublin’s older housing stock, space can be at an absolute premium. This is one reason why basements are such a great solution as they make space available at a price far lower than the market rate. But for the specialist basement contractors the lack of space can be a real challenge.
We recently completed a project in the East Wall Area of Dublin on a very tight site. It is a terraced house with no rear access. There was an existing basement so we did not have to excavate that much soil. But the basement has been poorly constructed and contained about half a meter of water.
There was so much water and silt flowing into the basement that water management was not a suitable long term solution. To make the basement habitable we had to retrofit a tanking waterproofing system.
Since the basement had so much standing water our first step was to strip-out all the fittings, plaster board, doors etc. The best way to 
waterproof this basement involved replacing the concrete floor and walls which also needed to be broken up and removed. Also the owners requested as much additional head room as possible, so we dug down to the level of the existing underpinning. During this time the owners were still in residence as we removed 40 tonnes of waste and brought in 20 tonnes of material to construct a new waterproof shell. Every iota of waste and material needed to pass along the front hall and out the front door.
We knew from the first inspection when the water level was 500 mm (a
foot and a half) in the basement that this basement was below the level of the water table. If no action was taken the basement would have been underwater and impossible to work. This required us to dewater the site to below the target level and keep the water away for some weeks. To dewater we used a pump in a specially constructed well to artificially drop the water table underneath the house. Space being such an issue the only space for a dewatering well was inside the basement. Naturally the well could not be left there indefinitely so the final phase of the project involved sealing the well.
To provide an economical long term solution, one that will last for many generations, we decided to use a concrete and bentonite based solution to seal the well. Had we simply turned off the pumps and poured concrete, the hydrostatic pressure of the ground water would have quickly pushed the soft concrete aside. This is why 
we re-flooded the basement until the water pressure inside the basement matched the pressure of the ground water. This permitted us to use a method first identified by the ancient Romans and use concrete which hardens under water to seal the well once and for all.
When the plug had hardened we simply pumped the water out for the final time, revealing a fully waterproof basement.