Options for difficult sites, from piled foundations to earth bags.
Foundation solutions today are not so very different from those which the earliest house-builders had to work out. The big change nowadays is that all foundations are designed with an adequate ‘factor of safety’ to allow for unforeseen circumstances. The main considerations are simply that:
- The foundations must safely transfer all anticipated building loads or ‘actions’ onto soil which possesses an adequate load-bearing capacity.
- Account must be taken of the potential for settlement, including differential settlement where soil under one part of the building may differ from that under another, for example gravel alongside solid rock. Where the soil type changes over a relatively small area, it can sometimes be feasible to bridge across an obstruction or weak point.
- Foundations must be appropriate to the building type, in other words, a large building will need larger and deeper foundations than a small light structure.
Unusual situations are those where the soil load-bearing capacity does not fall within the parameters of the local building control guidance for ‘standard’ strip foundations. These instances will require the services of a structural engineer to design the foundation and he or she can engage a geotechnical engineer to ascertain the soil characteristics.
The big questions when faced with tricky soil conditions are how to design the foundations, drainage and any retaining walls. This is frequently left unplanned until construction begins. A lazy approach would be to just show standard strip foundations on the working drawings with the proviso that they may be ‘subject to alteration’ when works commence, but this leaves you with an unknown element in your building budget, in addition to the potential delay in getting a proper foundation design into the local building control offices.
The cost difference between simple continuous strip foundations on ‘good soil’ and deep piled foundations on ‘bad soil’ can be significant, so a good conscientious designer will get a structural engineer to carry out preliminary site investigations at the earliest possible stage.
Many foundation types can be encountered depending on the site and its environment, but the following brief descriptions are limited to the most common types of foundations for domestic-scale buildings in the UK and Ireland. The last three are seldom used, but should be considered as viable ‘green’ alternatives.
Traditional types: pad and strip
As with strip foundations (see below), pads (sometimes described as ‘bases’) will not usually be taken down to depths of much more than 3m for reasons of economy, and together with rafts, are described as ‘shallow’ foundations. Pad foundations are typically used to support columns or standalone structures; for example, a blockwork chimney stack in a timber frame house. Where the load-bearing soil is too far below the floor level, column bases are usually carried on reinforced concrete pile caps which are cast on top of a pile or group of piles. Foundation depths to prevent damage from frost and trees must be correctly specified and pads may be reinforced concrete or unreinforced ‘mass concrete’, depending on loads and soil composition.
Typically in NI and ROI, the most common type of foundations for domestic scale structures are concrete strip foundations which consist of exactly what the name implies, i.e. a strip of concrete laid in a shallow trench forms a stable base for each wall element.
Strip foundations are suited to ‘low-rise’ buildings and can be used for both lightweight and heavyweight buildings where the soil is uniformly firm and deep, but can also be used for lightweight buildings on soils which have a firm layer over a soft layer.
The width of strip foundations must be designed so that the subsoil adequately carries the weight of the building and also that the foundation does not shear under the load. Their depth will depend on ground conditions but the minimum in NI for example is 450mm, required to avoid the adverse effects of frost; but other regional requirements may specify deeper levels as will the presence of trees or deep, soft soil.
While there is no reference to a minimum depth in ROI’s Building Regulations (TGD A) 600mm depth is commonly adopted as ‘standard’ both in NI and ROI.
A light structure on good solid ground could use strip foundations which are just equal in width to the thickness of the wall, whereas a heavier structure on softer soil will require wide strip foundations.
For standard houses, the general rule of thumb is that the width of strip foundations is three times the wall thickness. While this provides an estimate for preliminary design purposes, it can lead to unnecessarily wide* (or in some cases, too narrow) foundations. That’s why it’s important to accurately design the foundations at an early stage according to the ground conditions and the anticipated loads imposed by the proposed structure.
Although strip foundations are typically formed at shallow depths, they can also be ‘trench filled’ whereby a deep trench excavated down to load-bearing soil is filled with a lean mix concrete and the strip foundation created closer to ground level. This also avoids the creation of unnecessary hazards such as trenches collapsing or the lifting of concrete blocks down to bricklayers working in a deep trench. Strip foundations can be stepped to allow for small variations in level.
* Nowadays external walls are often designed with wide cavities, so for example a cavity wall with 100mm inner and outer leaves and a cavity of 200mm would require, under the rule of thumb, a foundation width of 1200mm (and a minimum thickness – if unreinforced – of 400mm). This would be an unusual situation.
Raft
A floating or buoyant raft is designed to float on a soft base and a surface raft relies on adequate soil bearing capacity for support. Raft foundations are unusual in many parts of our countryside, but they do have their uses where the ground conditions are suitable.
For example, you would consider using ‘surface’ or ‘floating’ rafts where the soil type is uniform across the site and consists of either deep, soft soil or filled soil which is holding water. Floating rafts (not to be confused with simple surface rafts) are used where the building needs to be buoyed up by the weight of the earth displaced in creating the foundation – so they are also considered appropriate where the soil consists of a firm layer over a soft layer.
Where very wide strip foundations would be required, it can sometimes be more economical to use a raft instead.
Piles
These have been around longer than you might think. Herodotus recorded in the 4th Century BC how the Paeonians lived in dwellings on piles driven into a lake bed; and the Campanile Tower in Venice was rebuilt in 1902 on timber piles which were still in excellent condition, even after supporting the original structure for 1,000 years.
Nowadays, piled foundations are described as deep foundations and can vary enormously in dimension, from 5m to 100m in length and 75mm to 2,000mm in diameter. Small piles will usually be bored or drilled and the concrete poured into the hole, medium piles are usually made of reinforced pre-cast concrete whilst larger piles are typically steel. Medium and large piles are usually rammed into the ground.
There is a bewildering variety of types of pile foundations, but the main principles are that they rely for their stability either by bearing directly onto or into the underlying soil (bearing piles) or by the degree of friction between the pile and the surrounding soil (friction piles).
Where dwellings are concerned, piles will be typically used in ground where the load-bearing subsoil is too deep for strip foundations to be economical. For a typical dwelling, a general rule of thumb is that this cut-off depth is around the 2.5 to 3 metre mark.
Most (but not all) piles are essentially posts or poles in the ground and can be made of timber, steel, concrete, composite or chemical materials. They can be driven or screwed directly into the soil or formed inside drilled holes.
Screwpiles are a type of steel pile (early min 19th century ones were iron) with helical fins which are screwed into the ground. In a similar way to driven piles, these have the advantage that the piling machine can easily monitor the degree of resistance experienced during the process and so the vertical load which each pile will safely carry can be determined with good accuracy. As well as carrying vertical loads from above, piles can be designed to resist potential uplift.
Vibrated stone columns are a type of ‘speciality’ pile which are used to improve the load-bearing capacity or the drainage characteristics of the soil. Pier foundations are a form of pile which are built up or poured into prepared holes and can be used on most building types where the soil is a soft layer over firm soil or rock.
It would be appropriate to consider friction piles where the building is lightweight and the soil is uniformly soft and deep. They may also be used for heavier structures on firm soil over soft soil layers. Bearing piles will be used where there is a top layer of deep, soft soil above firm soil.
Rubble trench
While this solution is not commonly used in Ireland today it’s what our old stone-walled houses are built on. Some flex was acceptable with vernacular stonework as a certain amount of cracking could be safely accommodated (and pass unnoticed) in the lime mortar joints – but with ‘cut-stone’ or ashlar walls, settling would cause troublesome cracks. The use of rubble trenches therefore faded into obscurity when brick and block construction became widespread.
The construction was usually of a shallow trench, filled with rubble which may or may not have been cemented with a basic mortar such as a clay and lime mix.
More sophisticated foundations would use a rubble fill between built stone facings, similar to a dry-stone wall, but in a trench. Nowadays, these types of foundations are increasingly being used where a light eco-structure is to be built, such as rammed earth walls or straw bale and timber walls. Rubble trench was also popularised by architects such as Frank Lloyd Wright.
The rubble trenches can also be drained out to a soakaway to improve the site drainage and also maintain a dry void under suspended floors. Since the rubble is essentially flexible to some degree, some care must be taken to ensure that either the structure can absorb a bit of movement or that the foundation is suitably compacted and load-tested for the avoidance of differential settlement. The technique is generally regarded as unsuitable for bearing on soft or expansive soils.
Earthbag
Architect Nader Khalilie originally considered earthbag construction as a solution for NASA to construct habitats on the moon and Mars. It is becoming more popular in the US, where Cal Earth (California Institute of Earth Art and Architecture, Hesperia, California) projects are constantly developing the tecnique.
This isn’t an option you’d commonly find in Ireland as it’s of particular value in countries where concrete is too expensive to transport to remote areas, but where spare fill material is plentiful.
This method involves filling a trench with bags or tubes which are filled with compacted soil. Often, barbed wire is used between layers to help anchor them together.
If there is a scarcity of materials for building more traditional types of walls and roof, the layers can be brought up into a dome shape to form the walls and roof of the structure. The bags are then plastered to keep out the weather. If the soil is not free-draining, the bottom layers can consist of gravel-filled bags and incorporate a drain out to a soakaway.
Padstone
This is perhaps the simplest and most environmentally-friendly of all foundation types. A similar form of foundation is the ‘staddle-stone’ (also known by various other names), often similar in appearance to a mushroom – consisting of a domed top stone on top of a columnar one, which were typically used to carry the building above ground to prevent vermin infestation of foodstores. These types of surface-level foundation – or variations on the theme, will easily allow structures to be deconstructed at the end of their designed lifetime.
Trial pits and trenches
Any qualms about the site conditions must be answered at the earliest possible stage, ideally even before the site is purchased. Even greenfield sites can conceal underlying pockets of deep peat, so preliminary inspection trenches or trial pits near the intended foundation positions should be dug out to determine what lies below the surface.
This is something that engineers develop a feel for over many years so you’ll need to rely on their expertise regarding the position and depth of the test excavations, most of which will need to be carried out using a mechanical excavator.
Before deciding on an appropriate form of site investigation, the engineer will have established the position of the building and the foundation loads. If it is deemed apt to begin with trial excavations, they must be taken down to the proper depth.
For shallow foundations, the correct depth is at least 1.5 times the width of the loaded area. The loaded area is taken to mean either the width of a footing, the width of a raft or the width of the building if the foundation spacing is less than three times its width.
On most sites for the average dwelling, the simple rule is to keep digging until a suitable base is found, and if this isn’t achieved within the reach limit of the excavator from existing ground level or if still digging at a depth of much more than 3 metres, then a piled or raft foundation must be considered. When greater depths are needed, simple poured concrete strip or trench foundations will become uneconomic compared to piles.
If the soil conditions are found to be very unusual, a geotechnical engineer can be called in to run tests to provide data on the load-bearing capacity of the soil which can then be used to ‘prove’ the adequacy of the structural engineer’s foundation design. These tests can require the use of boreholes, driving pegs or test piles, and additional trial pits.
Apart from the soil itself, other factors which play an important part in foundation design include:
- The water table, which is defined as the level below which the ground is saturated with water. This level may be seasonal and can be affected by existing or proposed drainage, land use within or outside the site and tree growth or tree removal.
- The presence of adjacent structures
- The growth of trees or the removal of trees, particularly species such as oak, elm, willow and poplar.
Foundations near trees
Any qualms about the site conditions must be answered at the earliest possible stage, ideally even before the site is purchased. Even greenfield sites can conceal underlying pockets of deep peat, so preliminary inspection trenches or trial pits near the intended foundation positions should be dug out to determine what lies below the surface.
This is something that engineers develop a feel for over many years so you’ll need to rely on their expertise regarding the position and depth of the test excavations, most of which will need to be carried out using a mechanical excavator.
Before deciding on an appropriate form of site investigation, the engineer will have established the position of the building and the foundation loads. If it is deemed apt to begin with trial excavations, they must be taken down to the proper depth.
For shallow foundations, the correct depth is at least 1.5 times the width of the loaded area. The loaded area is taken to mean either the width of a footing, the width of a raft or the width of the building if the foundation spacing is less than three times its width.
On most sites for the average dwelling, the simple rule is to keep digging until a suitable base is found, and if this isn’t achieved within the reach limit of the excavator from existing ground level or if still digging at a depth of much more than 3 metres, then a piled or raft foundation must be considered. When greater depths are needed, simple poured concrete strip or trench foundations will become uneconomic compared to piles.
If the soil conditions are found to be very unusual, a geotechnical engineer can be called in to run tests to provide data on the load-bearing capacity of the soil which can then be used to ‘prove’ the adequacy of the structural engineer’s foundation design. These tests can require the use of boreholes, driving pegs or test piles, and additional trial pits.
Apart from the soil itself, other factors which play an important part in foundation design include:
- The water table, which is defined as the level below which the ground is saturated with water. This level may be seasonal and can be affected by existing or proposed drainage, land use within or outside the site and tree growth or tree removal.
- The presence of adjacent structures
- The growth of trees or the removal of trees, particularly species such as oak, elm, willow and poplar.
Foundations near other buildings
More commonly a problem on urban sites, the proximity of nearby structures can create particular issues with the installation of foundations for the new building.
One common problem is when the new foundations have been specified as strip or pad foundations, but after excavations commence, it is found that the proposed foundations are going to need to be too wide to be properly centred under the walls or columns as they should be. This usually calls for the design of ‘balanced’ foundations which are offset to avoid the adjacent buildings and the structural engineer’s calculations must prove that an effective counterbalance will exist against the overturning effects of the weight being placed towards the outside edge of the new foundation.
Another example would be where the new foundation excavations are taken down to a suitable load-bearing stratum which is found to be well below the level of the bottom of the adjacent foundations. This calls for propping the adjacent walls and underpinning the old foundations in alternating sections – a process which requires careful, skilled workmanship.
In both of the above cases, a diligent site investigation should highlight potential problems well before works commence.
Foundations on clay soils
Foundation design solutions where building is to be carried out on clay soils will require a prediction of the likely amount of movement, based upon expert knowledge of the potential ‘shrinkability’ of the soil, the ability of nearby trees to dry the soil, drainage (existing and proposed) and weather patterns.
Raft or other shallow foundations should only be used where the superstructure can absorb movement (thereby ruling out most forms of brick or block construction). The answer is generally to take foundation depth down to a level where little or no soil movement is anticipated and the depth will depend on the soil characteristics, drainage and tree species involved. Useful foundation types might therefore involve the construction of basements or the use of piles, deep bases or pads which support slabs or beams.
Foundations on rocky soil
Most rocky soil is fine to build on and can reduce the amount of excavation and concrete needed if it lies close to the intended subfloor level. On the other hand, a site where the footprint of the dwelling covers both rock and softer soils poses a challenge to the foundation designer. This is because the different soils will carry dissimilar loads, but concrete strip foundations and the brick or blockwork walls they commonly carry are inflexible and will crack in areas where there is a sudden change in soil quality, thus leading the problems associated with subsidence.
If rocks or boulders are found when digging out a site which consists otherwise of clay, the best thing to do is to remove the boulders. If voids are discovered they should be left to be filled with a weak mix concrete as opposed to filling with loose gravel; in the event of rainwater filling it up, hire a pump to get it out before pouring concrete.
In a situation where only part of your house foundations rest on solid rock and the rock drops steeply away under soft soil in other parts, the foundations will need to be designed so that there will be no differential movement. Another possible design choice would be to deliberately introduce a break in the building where movement can safely occur between two sections.
It is interesting to note that we actually have a diverse range of rock types throughout our countryside, with Karst regions probably presenting the toughest challenges to construction. These regions of limestone, dolomite or chalk can contain sink-holes, (some of which may well be filled with peat or silt), underground voids and caves, cracks, crevasses and important aquifers. They are therefore sensitive to pollution and will require detailed geotechnical surveys before building.
Underground sloping rock can be found during foundation excavations, where the foundation base drops away sharply and a decision has to be quickly made as to whether it is a good idea to keep digging down the slope (thus needing a lot of concrete and blockwork) or to consider another solution. Thinly stratified rock (where rock is layered over softer soil) can also lead to nasty surprises if it is not discovered in time, but a thorough site survey will ensure that trial pits are taken down far enough to identify these situations.
TOP TIPS
All self-builders can avoid the many mistakes which others have made. Here are some of the lessons learned from many years in the construction industry:
- Never assume that you know what lies under the surface of a site.
- Foundation design must be done as early as possible and based upon a site investigation by a suitably qualified professional.
- If they are unavoidable, the costs of more expensive foundation types are more easily budgeted for when they are decided upon at an early stage.
- Don’t be afraid to explore non-standard solutions. While this will in all likelihood require that your structural engineer carry out detailed calculations based on a geotechnical analysis, it could be worth your while.
- On poor ground, lighter structures can help to reduce foundation costs.
- All foundation types will require careful consideration of how thermal insulation can be incorporated.
- A cheap site on poor ground might not turn out to be any less expensive than one on good ground when cost differences in groundworks are calculated.
- Trees are not the enemy and with proper consideration, can often be easily integrated into a site.
- Don’t forget that it is not just your dwelling or extension that requires foundations; on softer soils especially, you might need to have foundations designed for drainage, fuel storage tanks, pillars, paths, walls, fences and driveways.
- You will always require a proper structural design for any retaining walls.
- Plan ahead for future additions to the structure as it will usually be much cheaper to provide the foundation and footings now for that sun lounge which you intend to build, as opposed to getting machinery and materials into your site in five years’ time after you have everything else built.
- And finally, enjoy the experience of your building project!
Disclaimer: The descriptions of soil and foundation types are given here for general information and should under no circumstances be used to design foundations. Always use a professional structural engineer and adhere to official health and safety guidance.
Additional information:
Damien McKay BEng.,CEng.,MIEI,Dip.LA.of HD McKay Consulting Engineers, Unit 5 The Starlite Business Centre, Derry Road, Letterkenny, Co Donegal, tel. 074 9129243, consultingengineers.ie
Ian Bogie of Hamilton Bogie The Mini Pilling Company, offices in Co Antrim and Co Meath.
Further reading:
Walter Segal; Architect; 1907-1985: www.segalselfbuild.co.uk/home.html
BS 5837: BSI; Trees In relation to Construction.
BS 8004: BSI; Code Of Practice For Foundations.
BS 8103-1: BSI; Structural Design Of Low-Rise Buildings: Code Of Practice For Stability, Site Investigations, Foundations and Ground Floor Slabs For Housing
Health and Safety Measures: www.hsa.ie (ROI) www.hseni.gov.uk (NI).
Institute of Chartered Foresters: www.charteredforesters.org
Arboricultural Association (Ireland): www.trees.org.uk/aa/branches/Ireland-IR.html
BRE Digests 240, 241 & 242: Best Practice Guides on Foundation Design and Construction on Shrinkable Soils.
