November 2006

Conceptually, waterproofing solutions should be kept as simple as possible. However, reinforced concrete structures built against the site’s boundary line can prove difficult and complex for traditional waterproofing systems.

So how can you simplify your waterproofing strategy for today’s concrete structures built on boundary lines, and what are the waterproofing strategies available to you?

For many reasons such as ground conditions, land restrictions and utilisation of space, many of today’s structures are built up to the boundary line generally formed using concrete or sheet piles. These forms of construction provide little or no working space which limits your waterproofing options to the following:

(i) Pre-applied waterproofing using the piled structure as the back shuttering, or
(ii) Waterproofing the structure internally

For the purpose of this article we will concentrate on pre-applied systems. For more information on waterproofing the structure internally, click here.

As outlined in BS 8102:1990 Code of Practice for the protection of structures against water from the ground, there are a number of important considerations that should be ascertained at this point.

• It is important to understand the client’s expectations, the proposed usage of the basement and any possible change of use in the future,
• What has the site investigation told you about the geology of the site? For instance, where is the water table and how free draining is the soil? Also,
• Is it possible to install effective sub-soil drainage throughout the site?

These considerations will influence the selection and design of your waterproofing strategy.

Pre-applied Waterproofing

Currently, there are three generic forms of pre-applied membranes:

• Sodium Bentonite Geo-composite membranes
• Pressure sensitive adhesive laminates and
• Mechanically bonded plastic laminates.

Each generic system has its attributes and limitations and it is important that these are considered and fully understood prior to selection.

Pre-applied waterproofing systems can be easily applied throughout the horizontal plain to either well compacted ground, sand or concrete blinding layer. It is important that the system bonds directly to the poured concrete to prevent water tracking between the concrete and the membrane and to ensure it remains integral with the structure, should the adjacent ground or building be removed. It is also important that the chosen system has self healing qualities as it will not be protected during the placement of the reinforcement or the concrete.

When applied vertically, consideration should be given to which substrate the membrane can be applied. Some systems, such as Sodium Bentonite membranes, can be applied directly to the concrete or sheet piles prior to pouring the concrete, whereas other systems may require some form of additional permanent shuttering.

The levels of substrate/surface preparation required for pre-applied membranes should be considered as this may affect the membrane’s overall performance. For instance, removal of irregular finishes to concrete piles, stability of infill between contiguous piles and effective sealing of cleats between sheet piles.

Another important consideration is how to continue the waterproofing system up to DPC level once the pre-applied system reaches ground level. The detailing of the pile capping beams is critical to the success of the waterproofing strategy and consideration should be given to using products from one manufacturer at this point to ensure one single point of responsibility.

The design team should accept that a basement structure will be subjected to hydrostatic water pressure at some point in its life therefore, to conform with BS8102:1990, consideration should be given to the provision of effective sub soil drainage, which can be difficult with boundary line construction.

However, the positive effect of adequate sub soil drainage cannot be under-estimated as it will significantly reduce the risk of water ingress and enhance the performance of the waterproofing strategy.

Finally, if the water table is categorised as permanently high and the long term effectiveness of sub soil drainage cannot be guaranteed or is unfeasible to install, the severity of the risk increases and it would be prudent to contact a specialist waterproofing manufacturer to assist in the design of the waterproofing strategy combining both internal and external solutions.

Further information about the subjects discussed in this article is available by contacting technical@riw.co.uk