Basement Construction - Part 2 – Waterproofing

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

Source: http://www.northernvirginiabasementwaterproofing.com/

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’.

Source: http://quality-waterproofing.com/

Nobody will want to deal with water ingress into a basement, especially when construction is well advanced, or maybe when the basement is in use. It is therefore necessary to carefully select an appropriate waterproofing system, as failure to carry out thorough investigation and undertake careful design consideration 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 know 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’. 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 actually 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/controlled 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 a 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.

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Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.

Basement Construction - Part 1 - Design Considerations

When considering whether to construct a basement it is first worth weighing up the advantages and disadvantages, and then also thinking about a number of design considerations which will undoubtedly impact on the construction method, waterproofing, safety, usability and ultimately, costs

Source: http://basementwaterproof.com/

A recent article in the London Evening Standard (link) highlighted the growing popularity of basement construction, particularly where land is at a premium or restricted above ground.  The scale of the proposed basement construction in the article was extensive to a point where it generated a section 106 contribution of £825,000!:

A millionaire hedge fund boss digging out a basement eight times the size of a typical London home has been ordered to pay £825,000 towards affordable housing in his area.

Kensington & Chelsea council planners said the two-storey, 9,160sq ft basement — complete with cinema room, swimming pool and whirlpool spa — is the biggest they have been asked to approve. The scale of the extension, below two large Notting Hill villas which have been turned into a single family home, means it has fallen foul of rules that normally apply only to major commercial developments.

The fashion for digging out super-size basements to create so-called “iceberg homes” in London, and the prospect of years of disturbance during excavation, has pitted residents against each other in some streets ......... neighbours are said to be horrified by the scale of the works which will involve scores of lorry loads of earth being removed from the site. One said: “It will certainly be one of the ‘iceberg houses’ and sadly, our house will probably be the Titanic.” The number of applications for subterranean spaces in Kensington & Chelsea has soared in recent years.......

Source: http://www.kitchentoday.net/

Although the news article identified above is a rather extreme example of a basement construction it does demonstrate an alternative way of providing valuable useable space when above ground construction may be restricted or unavailable. Basement construction is still considered a less conventional method of adding space compared to above ground construction and is often instigated by those who are prepared to challenge the conventional norm and think outside the box.  There are however many examples of residential buildings throughout the UK where basements were constructed as a normal part of the building process. Houses built during the Victorian period provide a typical example of where basements were commonly constructed.  Nowadays, these Victorian basements are often converted and refurbished to made them part of the useable habitable space within a dwelling.

When considering whether to construct a basement it is first worth weighing up the advantages and disadvantages, and then also thinking about a number of design considerations which will undoubtedly impact on the construction method, waterproofing, safety, usability and ultimately, costs. Clearly basements can add space and value to a property and it could also be argued that security can be less of an issue as there will be less accessible entry points into a basement, as by its very nature the structure in buried in the ground.  Also, as long as the basement is waterproofed appropriately (something I will be discussing in my next article), and insulated correctly, you could argue that a basement can be made energy efficient more readily that an above ground building. Conversely, the perceived disadvantages and the impact that these may have on costs will prevent a lot of people proceeded beyond the initial enquiry stage when considering basement construction.

Source: http://restorationeze.com/

One of the key things to consider is that by placing an enclosed structure such as a basement in the ground you are subjecting the structure to a number of different forces. The first is the presence of water in the ground.  Water is a naturally occurring element in the ground and the level of this water will vary from location to location.  Many will be familiar with the term ‘water table’ which can be understood as the layer below which the ground is completely filled up (or saturated) with water. Try to imagine a basement like a boat which is surrounded by water in the ground.  The problem is that boats leak, and so do basements!  A basement is unlikely to sink like a boat, but because the basement is an enclosed structure it has the ability to hold a lot of water if the basement is not adequately waterproofed.  Water in the ground also 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.

In order to design and construct a basement correctly it is first necessary to establish ground conditions. This will require a thorough ground investigation which although will have a cost attached to it, is essential at the very early stages of a project. This will also highlight the type of ground and any contaminants present together with information of water in the ground and importantly the height of the water table. Other design considerations will include; protection and stability to adjacent structures, basement depth, boundary issues including Party Wall etc. Act implications, method of excavation, temporary support, method of construction in addition to exclusion of ground water.  Of course all of this will have an impact on costs and there is no getting away from the fact that constructing a basement can be very expensive. 

In my next article I will consider a number of methods of waterproofing of basements and explain that the correct choice of which method to use is crucial to ensure that the internal environment within a basement remains dry.

Please feel free to share this article and other articles on this site with friends, family and colleagues who you think would be interested

Information/opinions posted on this site are the personal views of the author and should not be relied upon by any person or any third party without first seeking further professional advice. Also, please scroll down and read the copyright notice at the end of the blog.