Friday, June 15, 2018

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:Homebuilding & Renovating
An article in the London Evening Standard from 2013 (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.......

Although the news article identified above is a rather extreme example of a residential 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 and 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.

Source: Source:
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.

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.

Author: Gary O’Neill

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