Groundworks & Foundations: Excavations

The process of excavating a site before building commences is often very simple and straightforward, but on awkward sites it can develop into a major task.

Simple Sites

The site is stripped of vegetation and the topsoil is removed (and stored for later use). Then the process of setting out takes place. This involves marking out the site so that the foundation trenches are dug in the correct place. It is obviously important that this is done as accurately as possible, so ensure that you have someone who knows what they are doing at this point.

The next stage is the digging of the foundation trenches. This is normally done with a digger and a lorry to take the ‘muck away’. On easy sites, the trench depth is no more than one metre and the foundations for a whole house can be dug out in a couple of days at most. Ideally, the trench bottoms should be flat and level, ready to accept a ready-mixed concrete foundation pour the next day.

Difficult Sites
In addition to the basic work needed to construct foundations, many sites have extra work needed to make them viable, such as bad ground, trees and slopes.

Building on a Slope

ABOVE: Sandra Metcalfe and her late husband Dick built a new oak framed house on a perilously steep riverside site using reclaimed materials and bags of ingenuity. Read more about this project.

Building work has to take place off level ground and, if there are slopes, then the amount to be excavated will expand dramatically. Instead of two or three muckaway lorries being required, it could easily be 30 or 40 and, at £200 a time, the cost soon escalates.

Don’t be caught out by this. A site survey will reveal what the slope is and how much ground you will have to shift to get a level site. Plan for where the spoil should go — you may be able to make a landscaping feature out of it.

Bad Ground and Trees

The depth of the foundation trenches is dependent on ground conditions. Whilst some soils are good to build on, many are not. Clay is a particular problem because it swells when wet; tree roots also bring problems because they cause ground movement. The standard solution to difficult ground conditions is to dig deeper trenches, but there comes a point (around 2.5m deep) when it’s no longer safe nor practical to go any deeper.

Many builders used to just open up the ground, see what was there and then get the building inspector to decide how to proceed. Now most undertake pre-build ground surveys to ascertain just what risk there is of excessive overspend.

The problem with doing this is that you hand the initiative to a surveyor who will tend to err towards caution as, if anything goes wrong, the liability will fall on him. Surveyors will tend to steer the project towards engineered foundations, which are never cheap.

The alternative is to carry out a few discreet enquiries and to ascertain just what the ground conditions are likely to be. A phone call to the local build - ing inspector is always a good place to start. Also check with neighbours who may have had building work done.

However you do it, have a contingency sum to hand to deal unforeseen problems.

Further reading:

• Groundworks & Foundations: Foundations
• Groundworks & Foundations: Ground Floors
• Groundworks & Foundations: Drainage
• Groundworks & Foundations: Rainwater Management

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