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Dealing with Complicated Foundations

Being informed about the site and local problems will make you more prepared when tackling the foundations on your self build

When you break ground to dig foundations for a house you can never be 100% sure about what you are going to find. If you are lucky, you will come across nothing but good load bearing sub-soil and your foundations may proceed with great speed and little cost. But there are many things that can conspire against you. Principal amongst these are difficult ground conditions that require something more elaborate than simple trenched foundations. Other possible problems include archeological remains or sewers, or electricity cables.

You need to assess the site properly

There are two approaches to this risk. One is the cautious one, that involves making as many investigations as possible beforehand — which often involves considerable expense in itself. Cautious builders dig trial holes and have them inspected by structural engineers who write reports recommending certain courses of action.

The other approach is more cavalier — don't worry about the foundations until you open up the ground. In theory, the cavalier approach shouldn't cost you any more, and in fact it will save you the investigation costs. But in practice, if you are unlucky and you meet problems, it is likely to delay progress substantially and in an unplanned way. This is a cost in itself.

Use local knowledge

Tap whatever sources of information are available to you about your plot. If people have built nearby recently, ask them about what foundations they put in. If there is a local builder or groundworker, get in touch with them and find out what they know. Don't be afraid to ask your local building inspector, who should have as good an idea as anyone.

Building inspectors work for district councils (ask the switchboard for Building Control) and their workload is normally divided up into wards or parishes so there is likely to be a particular inspector for your patch.

Any advice a building inspector can give over the phone cannot be binding. Should your excavations reveal something unexpected, the building inspector can insist on whatever they think is required to provide adequate foundations, even if this runs counter to what they have indicated to you already.

Any problem can be solved

If you do encounter problem ground conditions there are several solutions. The simplest and commonest is to dig the foundations a little deeper; sometimes this turns out to be much deeper, as deep as 2.5m, enough for a basement. At that point, digging down becomes prohibitively expensive and dangerous and it becomes cheaper to go for one of the engineered solutions, principally concrete rafts or concrete piles.

Rafts, such as the Housedeck system by Abbey Pynford, are essentially thick, reinforced floor slabs, strong enough to build the house walls off. In contrast, piles involve sinking a series of concrete stilts (or piles) down as far as necessary to find good load bearing ground or gaining enough friction (friction piles) to be stable and then laying a large beam around the top of the piles. This ring beam, as it is known, takes the place of the traditional foundation trenches.

These foundations are the province of the specialist. You must either hire an independent structural engineer or go directly to a piling contractor.

Don't over-engineer

Many cautious builders choose to hire in professional help at the earliest stage to avoid uncertainty over foundation costs. The person you want is a structural engineer and they will undertake a site survey for a few hundred pounds.

You need to dig two or three trial pits so that the engineer can see what lies beneath the ground. The trial pits need to be as deep as possible - typically around three metres - and you have to be careful to keep them just outside the house footprint. The problem with this arrangement lies in the fact that by hiring a structural engineer you are transferring the risk of future foundation failure onto his (or her) shoulders. A qualified structural engineer will carry professional indemnity insurance but, even so, they don't want to have to make a claim on it, so they tend to be very cautious in their assessment of the situation. Their foundation designs will tend to be extremely durable and often rather expensive to build.

There are many instances where an engineer will specify a raft or piling when the alternative specifier (i.e. your building inspector, who incidentally you can't sue for negligence) would have been happy with regular foundation trenches. If you hire a structural engineer, their recommendation overrules the observations of the building inspector, so you may end up having to build engineered foundations whether you really need to or not, and that may turn out to be expensive.

Have a contingency fund

House foundations can be surprisingly cheap to construct. The normal way of comparing costs is to present them as a cost per m² of footprint - i.e. the area taken up by the ground floor of the house. A ballpark figure of £60/m² footprint is standard to excavate the trenches, pour the concrete, build up the footings and install the ground floor. Thus a small three bedroom house (footprint = 50m²) might only cost £3,000 for the foundations and a five bedroom house (footprint = 125m²) perhaps £7,500. However, if you to have to construct much more elaborate foundations or use engineered concrete rafts or piles, your costs will balloon. In these instances, it is not unusual to see unit area costs of between £100 and £150/m² footprint, enough to blow a huge hole in your budget.

The best advice is to place a contingency sum in your budget equal to double your anticipated foundation costs so that if disaster strikes on this front it won't compromise the project. If all is well below ground, you can throw the money at the finishes instead.

Slopes will cost you more

Unlike foundations, there are no hidden costs with slopes. But it is still easy to underestimate the costs of building on them. Extra excavation, stepped sites, retaining walls, added landscaping costs all contribute significantly to your costs.

If you are building on a steeply sloping site, it may make sense to fit a semi-submerged basement into the sloping ground. Basements are normally an expensive way of gaining extra space but sometimes they can be an excellent solution for a sloping site.

Account for trees

The presence of mature trees can do wonders for a building plot visually. But their roots can play havoc with your foundations. Because trees draw water in from their roots, the ground surrounding their root systems is prone to drying out and this creates instability. The bigger the tree and the closer it is to your foundations, the worse the problem tends to be. The problem is most acute in the clay soils common in southern England.

The structural warranty providers NHBC and Zurich publish tables on the effect of tree roots on foundation design. The larger, hardwood species such as elm and oak have the most significant effect, together with thirsty trees such as weeping willow and poplar. If these species are within 10m of your proposed foundations, expect to have to dig much deeper and pour much more concrete. You may also be required to line your foundations with some compressible board material that can absorb movement, such as Clayboard or Claymaster.

Even if you cut down the trees before you start work, you may still have to make provision for them in your foundation design - the roots continue to live for years afterwards and the impact on the ground by felling a tree can be far more dramatic than leaving it.

Know the Party Wall Act

If your foundations are to be deeper than your neighbours buildings, you must observe the Party Wall Act provisions if you are within three metres of those buildings - note not boundary, but the buildings. In addition to this, there is a provision that states that if you are building deeper than the neighbouring houses foundations, you may still be affected by the provisions of the Act at a distance of six metres. You draw an imaginary line at 45° down from the bottom of your neighbours foundations and if it strikes your projected foundation works within six metres, then your work is covered by the Act. This is likely to be pointed out to you when you put a full planning application in.

If the Party Wall Act does come into effect, you are obliged to notify your neighbour and to assuage their concerns that your building work may damage their property. Often this can be done with an informal exchange of letters but, if the neighbour is worried about what you are up to, then he or she is entitled to hire, at your expense, a qualified structural engineer or surveyor to verify your plans. You must state whether you propose to strengthen or safeguard the foundations of the building or structure belonging to the adjoining owner. Such aspects need to be considered at the design stage of the project to avoid unexpected costs arising. This can become a long and involved process, as the two surveyors have to reach agreement about just how the work should be undertaken. Your neighbour cannot prevent you undertaking this work but they can delay works. If the next door building is split into flats, then you have to entreat with each occupier individually.

You can obtain a copy of the Party Wall Act for free at

Decide between footings or trenchfill

On a standard foundation job, you excavate the foundation trenches to a depth of about a metre. The actual depth is something the building inspector will rule on but if there are no unforeseen problems, a metre is generally accepted as a good depth.

You are then faced with a choice of how you fill this metre deep trench. There are basically two methods, often referred to as traditional footings and trenchfill.

The traditional method involves pouring concrete into the bottom of the trench to a depth of around 300mm and then building up to floor level using bricks or blocks. In contrast, with trenchfill, as its name suggests, you just fill up the trench with readymixed concrete.

Trenchfill is quick and easy and its by far the commonest method now used. But it is expensive on materials using, as it does, around three times as much readymix as traditional footings do.

Whichever method you use, one thing to note is that the level you lay your concrete to should relate to your DPC (damp proof course) level in whole brick or block courses. If you get it right (and you get the foundations level to boot) it saves an enormous amount of work cutting bricks and blocks.

Don't over order

Familiarise yourself with the basic laws of geometry before ordering your concrete. The volume of a rectangle is the length times the width times the height. Think of your foundation trenches as a series of rectangles and you should be able to derive a pretty accurate figure for the total volume required to fill them to your predetermined level. Though note that sometimes in soft ground, the trenchwalls may collapse or may not be straight and your calculations may be subject to some guesswork. Work in metres because you will be ordering the concrete in cubic metres.

If you over order you can't return what you don't need. Not only do you have to pay for it but you also have to find somewhere on site to dispose of it; think about this beforehand there may be the odd spot which could usefully use some concrete.

Specify the right concrete

Readymix lorries mostly carry 6m³, though some are 5m³ and a few are 8m³. Much of the cost involved with readymixed concrete is transport so the amount you pay depends very much on how far the truck has to travel, so it pays to buy local. Generally, readymix charges vary from around £40/m³ up to £60/m³ depending on which part of the country you are in. A detached four bedroom house with straightforward foundations might use as much as £25m³ in total, giving a readymix bill of well over £1,000.

Many people make the mistake of phoning up their local readymix company and asking for some concrete not realising that concrete comes in many different guises. Your plans or your notes should specify a particular mix of concrete and this is what you should be asking for. Don't expect the readymix company to advise you on this - technically they are not even allowed to do this, though many will help out as best they can. Your mix is likely to have a specific name: GEN 1 for foundations is very common but far from universal.

Prices for basic mixes like GEN 1 currently vary from around £45/m³ up to over £60/m³ in and around London. But be aware that on your first two truckloads you will be charged something for the empty carrying space on each lorry. So if you were to order 8m³ of GEN 1, for instance, you'd pay for 8m³ of concrete plus 4m³ of unused capacity on the second truck - the charge for this unused capacity can be as much as a third of the price of the concrete itself. If you order over 12m³ (i.e. more than two loads) this charge is usually waived, but be sure to find out beforehand how it is to be applied.

Hire a concrete pump

If its hard to get at all or even part of your foundations, you can make things a lot easier by hiring a concrete pump. This is basically an enormous hosepipe that comes on its own dedicated lorry. The readymix truck empties its contents in one end of the hosepipe and the pump operator can move the other end of the hosepipe around your foundations, thus doing away with the procession of wheelbarrows that usually accompanies a concrete delivery.

To hire a concrete pump, allow around £150-200 per session. They pump a full load (6m³) in 20 minutes, three times quicker than three men barrowing might do. If using a pump be sure to let the readymix supplier know, because the mix design is wetter and the throughput of lorries is much faster than on a normal job. Its also a little more expensive. Some lorries have pumps on board, ask.

Consider self-placing concrete

RMC have introduced a self-placing foundation concrete called Foundation Flow. It saves on having to barrow, dump or pump concrete around your foundation trenches as it finds its own level around your trenches with a minimum of assistance. It sells for 10% premium but they reckon you can claw back the costs through labour and/or plant savings.

Think ahead

You will probably be planning to run services and drains through the top of the foundations. Most people do. If you were filling the trenches with concrete, you would do well to leave a gap for them to pass through. Foundations don't have to be continuous and you can bridge the gap later using a concrete lintel or two. Use some offcuts of plywood or OSB to shutter off a section of trench and brace between them with some timbers long enough to give you adequate space to run your pipes. Alternatively, place some ducting through the trench so that you can feed your pipes and cables through afterwards.

It is possible to solve the problem of collapsing trenches

One of the nightmares facing groundworkers is what to do if the trenches collapse before the concrete is poured. It happens, often as a result of prolonged rain combined with particularly heavy ground. This is not usually a major problem on shallow trenches but it can be very dangerous if you are working down below waist height. You have to be very careful about such risks; if you have to dig down below one metre, you may have to shore up the trench walls to ensure everyone is safe.

Water in the bottom of the trench is another perennial problem. Usually you can get rid of most of it using a mechanical pump but sometimes a building inspector will ask for the trench bottom to be flattened off again because, when wet, they can get very chewed up.

All prices and estimates quoted correct as of September 2003.