NHBC Guidance and good practice case study.
The following is based on an excerpt from issue 16 of the NHBC Technical extra booklet, available here.
Basement waterproofing design and construction
This good practice case study, on Basement waterproofing design and construction presented by the NHBC is a MacLennan project.
Who should read this:
Technical and construction directors and managers, architects, designers and site managers.
The new Standards Chapter, ‘Waterproofing of basements and other below ground structures’, covers a wide range of homes. Here, we discuss how waterproofing issues have been addressed on a prestigious new build property in Wentworth, which includes an impressive 1500m2 basement incorporating a swimming pool and underground parking. A sloping site and challenging ground conditions necessitated extensive ground works and a high-quality waterproofing system.
This case study considers a large property with extensive engineering works. Take note of the waterproofing principles adopted; they can apply equally to a range of situations and developments, both large and small.
Designing a waterproofing system for such a complex property requires a high degree of certainty.
Extensive site investigations were undertaken, not just for contamination and soil parameters but also for below ground waterproofing design and water risk management aspects.
The waterproofing design was undertaken by a waterproofing specialist, holding the PCA’s Certificated Surveyor in Structural Waterproofing (CSSW) qualification. With the knowledge of potential ground water above the basement formation level, the surveyor designed the following:
The car park and habitable areas of the basement were designed for BS 8102:2009 Grade 3.
‘No water penetration or dampness permitted.’
Type of waterproofing system
Below ground structure (basement) Combined System Type B & C - Structural water tight concrete and internal drained cavity.
Podium Slab Combined System Type A & B - Structural water tight concrete with a liquid applied membrane above.
Below ground structure waterproofing concept
The designed reinforced concrete structural waterproofing barrier will allow very little water ingress to occur.
With a combined system, the Type B barrier is backed up by the type C drained cavity system.
Any water ingress or condensation that does occur will be collected and drained away by the type C system, a relatively fail-safe solution.
Following demolition of the original building, extensive groundworks commenced with the installation of a secant piled retaining wall and reinforced concrete head beam at the rear of the basement.
Approximately 10,000m3 of soil was excavated to form the basement. During excavation, further temporary sheet piles were installed on the returns of the basement.
Type B perimeter barrier construction
On excavating to basement formation level, localised RC column pad foundations and required basement drainage was installed. A nominal blinding layer of unreinforced concrete was placed over the remaining area ready for the raft reinforcement and shuttering.
The in-situ RC structure was designed to BS EN 1992- 3:2006. Reinforcement was placed as per the design to restrict crack widths to less than 0.2mm.
The raft was poured, allowing for shrinkage, in approximately 60m2 sections with a monolithic elevated perimeter kicker. Active physical movement joint water bars were installed on all day work joints between pours. All services passing through the structure were physically sealed with a flange joint around the pipes.
Up to two storeys of formwork was constructed, keying off the monolithic kickers. Kickers were scabbled and cleaned; high-grade hydrophilic water bars were installed to seal gaps at kicker levels and other construction joints. On stripping the shutters, the concrete was visually checked for any blemishes.
Where required, blemishes were post-injected. Temporary tie bar locations were made good by filling with a hydrophilic polymer sealant.
Type C internal drained cavity construction
First, the concrete wall was treated with a lime inhibitor and then the cavity membrane was installed to all perimeter walls. Membrane studs were placed against the prepared substrate. Fixing holes were drilled through the base of the membrane studs at pre determined centres. Proprietary quick-seal plugs were then driven into the concrete substrate to fix the cavity drain membrane into position. Grommits provide an effective seal between the head of the plug and cavity membrane. At the bottom internal face of the membrane, a
condensation strip was fitted, collecting any potential internal condensation.
Drainage channels and floor membrane
The drainage membrane is not capable of resisting water pressure; therefore, to allow for direct drainage, a proprietary preformed drain channel was installed around the perimeter of the basement. The channel was connected to the sump area with flushing points at each change of direction. Closed cell insulation was fitted around the drainage
channels, bringing the overall floor level to the top of the channel level. The cavity drain membrane was placed on the floor, butted up against the perimeter wall
condensation strip. A corner strip tape was then used to seal the wall condensation membrane to the floor membrane. The floor membrane was covered with a mesh
reinforced screed. The wall membrane then had battens fixed within the quick-seal plugs, ready for plastering or other internal finishes.