When a below ground element is incorporated into a structure, it often has the feature of having earth bearing against the outer face.
The surrounding earth has the capability of acting as a conduit for water. Often the ground water table will intersect with the below ground element of the construction. As there is no water in the basement this creates a pressure difference, as there is a head of water bearing against the external side of the structure. If there are imperfections in the construction, water will enter through the construction material retaining the earth, filling the empty space. An alternative way of thinking about this is like putting a sieve into a sink full of water.
The type of protection shouldn’t be limited to dealing with the amount of ingress at the time of enquiry, as multiple geological changes can take place which alters the water table. Furthermore, it isn’t uncommon for events such as burst water mains to heavily saturate the ground causing excessive moisture to penetrate through the retaining element.
Careful consideration should be given to the waterproofing design and it should adhere to the recommendations set out by the BS8102:2009.
This will include identifying the intended use of the building and attributing the right level of protection, as well as the most appropriate systems to be used. The types of systems used are categorized by the following:
Type A – Barrier Protection
An example of a barrier system can be either a cementitious slurry, or an externally applied membrane system. This bonds to the structure and with an externally applied sheet membrane, prevents moisture from penetrating into the structure. With a cementitious slurry, (which is usually internally applied) holds back water pressure and creates a dry internal environment.
Type B – Integral Protection
Protection systems under this category look to integrate with the structure and make the structure inherently resistant to the passage of moisture through the retaining element. An example of this can be waterbars, which are usually placed at concrete construction joints. They are either hydrophilic (Active) and will create a swell pressure which is greater than the pressure created by the head of water inhibiting the passage of moisture. Alternatively, waterbars, which create a tight bond to the structure either by chemical or mechanical adhesion creating a torturous path for water ingress.
Type C – Drainage protection system
The cavity drain membrane system can be used on most substrates and requires little preparation, compared to liquid applied systems. The membrane creates a cavity which acts as a depressurization layer. The water trickles down to a channel void former which sits at the wall floor junction. Holes within the channel allow the water to drain out an open elevation, or into a sump and pump system.
The system can be installed with a battery back-up system to allow for continual pumping, even during a power outage. Manufacturers offer different systems for different levels of monitoring and back up protection.
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