Building on Difficult Ground

Of all the things that can hike up your build costs, spending extra money on foundations is the most common. Frequently, in the rush to plan a home, the foundation design is overlooked. A new foundation system promises to make coping with difficult ground conditions a lot less stressful.

Pencilled in on the plans will be the sentence Foundation: to satisfaction of Building Inspector. This means that the groundworkers will excavate the foundation trenches down to one metre below ground level and then await the building control surveyors approval before pouring. If you are lucky, the surveyor will nod and within a couple of hours the foundation trenches are filled, ready for the footings to go on. But increasingly, building control surveyors are not happy. There are a number of situations where they will require far more extensive foundation work.

The presence of large tree roots, previously dug ground or the absence of any firm material at the trench bottom are all bad news. Clay soils in particular are a common cause of concern. Clay shrinks and heaves, depending on how wet it is, and it is quite capable of damaging concrete foundations, thereby causing serious subsidence.

Foundation work on trouble-free sites is generally relatively cheap. On a detached house with a footprint of around 100m2 you might anticipate spending between £5,000 and £10,000 to complete the substructure, including the ground floor. But if your site has problems it is very easy for these costs to spiral upwards and it is not unusual to hear of people having to spend three or four times as much.

There are various solutions to the problems posed by difficult ground conditions. Sadly, the best and most cost effective solutions are often the ones that are difficult to implement once a start has been made on traditional strip footings. Often the most cost effective way of dealing with difficult ground is not to use foundation trenches at all but to switch to one of the alternative techniques such as ground bearing rafts or piles supporting beams or rafts: work usually undertaken by specialists.

It is a field of construction which is evolving rapidly, fuelled in no small part by increasing demand. The best known firm in this arena is Roger Bullivant, who have pioneered a system using pre-cast concrete for piles and ring beams. However, their key market remains the major developers doing multi-home sites. Of more interest to the self-builder is a new system called Housedeck, pioneered by specialists Abbey Pynford. Housedeck combines the two techniques, piles and rafts, to create a suspended floor slab. Unlike Bullivants systems, Housedeck uses readymixed concrete delivered to site making it more flexible on small sites.

When Housedeck is specified, the groundworkers reduce the level over the house footprint level and place a 50-75mm capping layer of concrete. The ground is then piled to agreed specifications, dependent on ground conditions. Once the piles are in place, a cast for the concrete slab is created using shuttering set on temporary supports which are later removed.

The result is a 225mm reinforced concrete floor slab suspended on the concrete piles. The slab needs a damp proof membrane, but no insulation as the latest designs use expanded polystyrene for the casts supporting platform, thus providing ultra-low U-values in the floors. The slab has a stepped edge which is designed to take a brick outer skin and either blockwork or timber frame on the inside.

The cost typically between £10-15,000 for a 100m2 footprint is dependent partly on the depth to which the piles need to be installed, but the system will cope with almost all difficult ground conditions.

Abbey Pynfords Phil Jones comments: “Our unit area rate gets lower as the house footprint increases, up to about 250m2. We are never going to be competitive with simple strip foundations, but we find that Housedeck starts to prove economic if you have to dig down more than 1.5m. When self-builders call us, our first question is always Where is the site? that gives us a pretty good clue. The follow-up question is What do you know about the ground conditions? Often this can be gleaned from neighbours, builders or building inspectors. From this we can usually tell if Housedeck is appropriate. If so, we then get a site investigation company to report on the sites ground conditions and design an appropriate system. We aim to provide a one-stop shop. “

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