Thursday, June 21, 2018

Basement Construction - Part 2 – Waterproofing

Nobody will want to deal with water ingress into a basement, especially when construction is well advanced, or even worse when the basement is occupied and in use. It is therefore necessary to carefully select an appropriate water proofing system, as failure to carry out thorough investigations and careful design can prove disastrous and particularly expensive!

In my previous article I discussed the growing popularity of basement construction and highlighted a number of factors that require consideration during their design. Undoubtedly one of the most significant issues in relation basement construction is how to keep the internal environment dry and therefore exclude sub-surface water. The impact of water and particularly hydrostatic pressure was highlighted: ‘Water in the ground has the ability to exert a lot of force onto the structure of the basement depending on the head or height of the water. This is something known as hydrostatic pressure. This is better defined as ‘the pressure at a point in a fluid at rest due to the weight of the fluid above it’. Basement design therefore needs to take into account the height of the water table because that will influence the amount of hydrostatic pressure that a basement structure will be exposed to. The method of waterproofing will also need to be designed to consider hydrostatic pressure’.

Nobody will want to deal with water ingress into a basement, especially when construction is well advanced, or even worse when the basement is occupied and in use. It is therefore necessary to carefully select an appropriate water proofing system, as failure to carry out thorough investigations and careful design can prove disastrous and particularly expensive! There are many specialist companies and waterproofing products on the market who offer a variety of different solutions for dealing with water ingress into basements however for the purposes of this article I will provide examples of a number of well established methods of basement waterproofing. Selection will vary depending on factors, such as ground conditions, the height of the water table, the method of basement construction, the proposed use of a basement and as ever, cost.

When considering an appropriate way of waterproofing a new basement it is advisable to review the recommendations within BS8102:2009 ‘Code of Practice for Protection of Below Ground Structures Against Water from the Ground’. The standard advises on the types of waterproofing available and confirms the performance grade to be achieved:

Type of Waterproofing:

Type A (Barrier) protection - A barrier to water ingress is applied to the inner or outer surface of the structure

Type B (Structurally Integral) Protection - The structure is formed as a watertight construction and requires no additional protection

Type C (Drained) Protection - Water entering the structure is received by planned cavities or voids and safely removed

Grades of Waterproofing Protection:

Grade 1 - Some water seepage and damp is tolerable depending on the intended use. Car parking, plant rooms etc.

Grade 2 - No water penetration is acceptable. Damp areas are tolerable depending on the end use. Plant rooms, workshops etc.

Grade 3 - No dampness or water penetration is acceptable - Ventilated residential and commercial

Type A (Barrier) Protection relies totally on a waterproofing membrane to keep water permanently out of the internal basement environment. Concrete and blockwork are typical examples of materials used in basement construction, however these materials are highly porous, particularly in concealed enclosed environments such as below ground. Masonry materials have the ability to absorb high volumes of water, which once saturated will seep through to the internal environment. Barrier protection, often referred to as tanking is a method which prevents water saturating through the basement wall with the provision/application of an impervious membrane to the internal or external face of the wall. Tanking can also be provided within the structure, something referred to as sandwich tanking, although this method is less commonly used.

In my early years working as a labourer for a ground works Contractor, I remember a particularly project where I was required to paint the external face of a number of in-situ concrete constructed lift shafts, at their bases, with a liquid bitumen paint, which was referred to as ‘black jack’. At the time, I never really understood why it was necessary to paint concrete walls that were going to be buried in the ground, until someone explained that what I was doing was providing waterproofing protection.

Nowadays there are many products on the market in the form of brush applied surface coverings, trowel applied renders and rolled sheet applied materials such as elastomeric which are used for tanking solutions for basements. The success of a tanking method will be determined by the selection of the correct method as well as the quality of the installation. Many tanking solutions require installation by approved contractors and although these systems may seem expensive, it is worth considering the likely disruption and excessive cost of trying to rectify water ingress to a basement when it is occupied!

Type B (Structurally Integral) Protection relies on the basement structure itself to be robust enough to resist water ingress. In most cases the external basement structure will be constructed with concrete which must be designed to minimise joints as well as being cast with plenty of reinforcement to reduce the risk of cracking. It is not uncommon for concrete basements designed to achieve structural integral protection to include additional waterproofing measures to provide a barrier against water and water vapour. This may include the introduction of waterproofing admixtures into the concrete mix in order to help reduce porosity and drying shrinkage.

Structural integral protection will nearly always have a cooler internal surface temperature compared to other forms of waterproofing and such will be more prone to the effects of condensation. It is therefore necessary to additionally consider control of atmospheric moisture with the possible installation of controlled ventilation fans and de-humidifiers.  Clearly the design solution will depend upon grade and proposed use of the basement and additional measures may not be required in all situations

Type C (Drained) Protection takes the view that some water will be allowed through the external basement structure, however it will be dealt with or controlled when it arrives.  Drained protection may be a possibility in heavily waterlogged ground, possibly with a high water table or where for other reasons it will prove difficult to prevent moisture entering into an internal basement environment.  Any water that enters into the basement is gathered and disposed of in an appropriate way.

Drained protection usually takes the form of a raised floor and an additional membrane or wall installed/constructed in front of the main basement structure with a small cavity in between. Any water that finds its way through the main basement wall will seep behind the cavity (wall and floor), where through design the water will be channelled to a sump, which is basically a low point that will collect water, which is then usually pumped away from the basement.  Internally, there may be water entering the basement but this is concealed within the cavity. Therefore the internal basement environment remains dry.

As you would expect there are a number of disadvantages with the use of drained protection: Due to the installation of as wall and floor cavity there could be a loss in floor to ceiling height and useable space and pumps will need regular maintenance. There is also a possibility that high hydrostatic pressure will result in excessive amounts of water through the basement structure, which may not be able to be effectively drained. This will however be avoided with suitable design.

It is clear that waterproofing of a basement takes careful consideration, where the method of waterproofing should be determined by the range of different factors discussed above. Failure to understand ground conditions, including the impact of water in the ground, together other site conditions/restraints may result in the selection of a waterproofing system that is not fit for purpose. It is therefore always worth seeking specialist advice as remedial works will often prove to be very expensive.

Author: Gary O’Neill

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