Astrid Madsen
The most important part of any build is the foundations. Putting things right if there is a problem when the structure is completed is a difficult and expensive process. You only get one chance with foundations.
The design and performance of foundations is dependant on two factors; the nature of the ground on which they are placed, and the load placed on them.
Ground movement or land instability under or close by a building, may cause the foundation to move and as a result the building above to crack. This process is called settlement, and while some is perhaps unavoidable, the foundations should be designed to minimise this.
The load is usually the building itself, but in some cases the contents of the building can have a huge impact on design considerations, for example, a swimming pool. In a domestic situation, it is usually the weight of the house itself that is of most concern to the designers of the foundations.
Site Examination
If possible, examine the site thoroughly before you buy it, and certainly ahead of drawing up plans. A structural engineer or chartered civil engineer should carry out the inspection with you. A thorough visual examination of the surrounding area often gives an indication of ground conditions, and how any problems were dealt with by neighbours. Look carefully to see if any of the neighbouring houses are showing cracks, or rooflines sagged – both may be an indication of subsidence caused by ground movement. Try to find out what else was on site before. Nature also offers some clues. Oak trees tend to flourish in heavy soils, conifers in peat, willow and alder in wet loam, beech light clay and lime trees in limestone.
Ground Conditions
Ground types can be broadly summarised under five headings: Rock, Sand & Gravel, Clays, Peat and Filled Ground. All five are common in Ireland. Sloping sites and sites close to waterways present challenges of their own, in addition to the geology of the ground itself. The proximity of the site to trees or from where trees have recently been removed can raise issues too, all of which have to be factored-in during design.
Rock is very common in coastal regions in Ireland and appears sporadically throughout the rest of the country. Foundations placed on rock must have the weathered (exposed) rock removed and the foundation should get an even bearing. Many people believe that there is no need at all for foundations where rock is consistent, and while they may well be right in this assumption, planners usually require some foundation design before approval.
If the site contains rock it could add substantially to the costs. Rock breaking is an expensive process involving pneumatic or hydraulic hammers, and can be time consuming as well. Examine closely the tender price from the groundworks sub-contractor as rock breaking may be excluded from it, and, as a result, consume your contingency fund before any actual building takes place!
Sand and gravel can be problematic as they can move or settle during heavy rain or long dry spells. The usual solution is for a raft foundation, but the opinion of a structural engineer should be sought to advise and design the most appropriate type.
Clay can shrink and also change in volume due to the absence or presence of water. It tends to compress gradually and houses built on this type of ground may settle gently over time. Strip foundations are commonly used on clays, but again a structural engineer should be consulted to confirm suitability of the ground for the proposed foundation type.
Peat is mostly found along the western seaboard and midland areas of Ireland. It is unsuitable for building on but can be dug out and filled, unless the costs of doing so are prohibitive due to the depth involved. Peaty ground can usually be piled successfully.
Filled Ground
Sometimes low sites are filled with builder’s rubble or other unusable material and levelled off before the site is placed on the market. Filled sites are fine if the filling process was planned and carried out properly. Unfortunately the filling process is usually sporadic and compaction only occurs when the fill is piled too high to accommodate anymore. Then the top is levelled and a ramp constructed to allow the lorries to tip more debris on the site. If the fill is of a suitable material and was compacted adequately in layers during filling, it can make a very good base for foundations. Normally, however, filled ground will usually require a raft or piled foundation.
Sloping sites
These can offer architectural opportunities if the slope is sufficient for the inclusion of a basement in the house design. If this is not an option, perhaps part of the roof of the house can be designed into the slope. The alternative is to either fill or level the site, both expensive operations. If part of the site is to be lowered, or filled, a retaining wall – usually made from reinforced concrete – may be required to hold the ground. Alternatives include prefabricated walling which arrives in panels and is much quicker to construct. Remember to drain water away from the ground immediately behind as this could undermine the wall.
Trees and Roots
Mature trees are an aesthetically pleasing feature of a site, but can be a major problem too. The roots of a growing tree can push against a local foundation and damage it, whilst decaying roots from one that’s been removed can leave voids under foundations, resulting in loss of support. Mature trees also consume vast quantities of water, and the removal of one can cause the ground to rise in the area, due to an increase in the presence of water. Sometimes, trees may have been cleared prior to the site being put up for sale and so the change in water levels may not be immediately evident. Additional or specially designed drains are normally required as a result. If a tree is to be retained or a new one planted, a barrier below ground may be advisable to restrict the growth of roots in the direction of the house.
Water
Sites beside rivers and streams are always desirable, but not only do they pose a danger to children, there are potential problems for selfbuilders as well. If the site is fairly flat, the water level in the stream or river should be monitored in winter and summer as well as during and immediately after heavy rainfall. If this is not possible, try getting some local knowledge. Is the site liable to flooding? What affect has the river on the ground in the area? Where is the source of the river or stream? What is it draining and to where? With the aid of this information, it should be possible to design suitable foundations and site drainage to prevent future problems.
If the water table rises to within 250mm of the ground floor or there are any other concerns regarding groundwater, it may be necessary to drain the site. Drainage should be designed by an engineer and installed in strict accordance with his instructions. The likely effects on the later landscaping of the site should also be taken into account.
Tanking might be an alternative to drainage in some situations. This is a process whereby the surfaces of the foundations, floors and/or walls are treated with a special waterproof membrane and a drainage channel. Above all it is essential that the house doesn’t flood in heavy rain, irrespective of the prevention method involved.
Trial pits give a good indication of ground conditions and provide a snapshot of soil types in a site. A trial pit is essentially a hole dug in the ground usually one to two metres deep to show the types of soil at different depths and reveal the presence of water and the water table. It might not be possible to excavate trial pits prior to purchase, but it’s worthwhile asking for permission anyway The excavation should be done prior to designing the foundations, the pits fenced off securely while open to prevent accidents to people and pets, and backfilled as quickly as possible. Mark the location on a drawing so that they can be filled properly to prevent weak spots later, should they encroach upon the actual excavations. A sufficient number of trial pits should be dug to give a clear indication of ground conditions, normally about four.
Mini bore holes are a very useful first step in establishing ground conditions and may provide sufficient information without having to go to a trial pit.
Excavations will be required in most situations and ideally should be filled as soon as possible. With strip footings, concrete should be poured immediately to prevent the sides of the opening from falling in. This is vital in wet weather. Open excavations should be fenced off to prevent accidents, even to trespassers. Plan where to place temporary storage heaps of topsoil or other excavated material so that it will not be in the way of delivery trucks or interfere with the storage of other materials.
In NI Building Control will want to inspect the ground prior to any concrete being poured; this is not a requirement in ROI.
Types Of Foundations
Foundation design, matching house type to ground type, is a job for a structural engineer. It is likely that the engineer will specify from one of the following foundation types.
If you are building a timber frame, foundations should be level to within 20mm and square to within 12mm. This tolerance is required because the panels are manufactured off site and, whilst a small amount of tweaking is possible during erection, don’t rely on this. Most manufacturers supply a base plan showing where all the load bearing walls need to be placed, a copy of which should be given to the builder. The timber frame panels can be attached directly to either raft or strip foundations.
Strip Foundations
Sometimes called footings, these are the most popular type of foundations in Ireland. The theory behind their design is simple; the wall acts as a point load and the ground pressure applies tension on the bottom of the foundation. The normal rules for strip foundations are that the width of the foundation should be three times that of the wall it is supporting. Typical strip foundation depths are 300mm – that’s the concrete depth not the excavation depth. The base of the foundation must be below the frost line, that is, the depth to which ground will freeze during the winter.
Depending upon the ground conditions, sometimes strip footings are reinforced. Builders occasionally put this in as good practice in the absence of an engineer’s detail. The only person really qualified to decide on reinforcement requirements is a structural engineer or a chartered civil engineer.
The bottom of the excavation should be cleaned immediately before the concrete is poured. This is very important where the side of the open trench may have fallen in. Any water collecting in the open trench should be removed by means of a pump if necessary. The source of the water should be investigated and anything other than recent rainfall may need further investigation. Even if it all looks good, it is advisable to have the trench checked by an engineer immediately prior to pouring concrete Remember if you live in NI that Building Control will want to inspect at the pre concrete stage also.
Where the ground level varies, steps may be needed. These should coincide with the coursing of the blockwork in the rising wall, and the distance between them should be no less than one metre. It is important to ensure an adequate overlap in the steps; as a rule the overlap should be twice the height of the step or 300mm, whichever is the greater.
Rising walls – sometimes called dead blockwork – are built on top of the foundations and brought up to damp proof course level. The walls of the house are then constructed on top of these.
Filled trenches
Some trenches are partially filled with a weaker mix of concrete to bring them up to the level of the rest of the foundation. This is common where there are soft spots or where the formation level is low relative to the rest of the excavation. The regular foundation is then placed on this “lean mix.”
Raft
This type of foundation spreads the load over a bigger area of ground; their design is best handled by a structural engineer. A raft is more expensive to form than strip footings because it is a slab of reinforced concrete the size of the footprint of the building. The slab is deepest immediately under the outer walls, the edge beam, and load bearing internal walls, cross beam. The steel reinforcement for the edge beams and cross beams is pre-fabricated on site and lifted into position.
The first step in the construction of a raft is the excavation to formation level. Next, broken rock, (hardcore), is placed in layers, and each layer must be compacted before the next layer is placed. The level of the hardcore is lower in the areas where the beams are to be constructed. A final layer of fine quarry dust or blinding is then placed and compacted. Formwork is placed around the outer edges to keep the concrete in place, and the pre-fabricated steel beams put in position. Steel mats are formed, placed and tied to the steel beams and the concrete is then poured. This concrete is usually of a stronger grade than that used for strip footings.
Radon pipes and sump should be located under the concrete in the raft and the outlet pipe must be taken outside the raft area and turned upwards where it will be extended to ground level later. Alternatively, the standing pipe can protrude through the raft where it will be continued later into the attic and eventually outwards. An extractor fan is required to ensure that the radon is drawn out.
Piles
These are concrete columns sunk deep into the ground to support the foundations. Specialist contractors drive them into the ground in sections, or they may use steel tubes later filled with concrete as an alternative. Either way the piles are located to an engineered design, and are very common in filled or soft ground. A mini-piling service is normally sufficient for houses and extensions with some companies offering a complete sub-floor system. ‘T’ shaped concrete beams support the sub floor slab with the beams in turn being supported by the piles. Concrete is poured between the beams forming a raft-like slab, and the sub-floor is ready for the follow-on trades. The system is engineered to suit the specific house and is ideal for filled ground.
A third system is similar to that above and involves pre-cast concrete planks placed on the beams to form a sub-floor. This is quicker to install but may be more expensive.
An incorrectly installed pile is more likely to cause downward movement or settlement and cracking rather than upward movement, but whichever is the case, the consequences are extremely serious, resulting in destabilisation.
The thought of a piling rig may produce visions of huge machinery but this is not necessary. There are even systems that use air driven hammers, specifically for conservatories and sunrooms, and others that are small enough to get through gateways or down narrow lanes.
Pads and Beams
Pads are sometimes used to support the ground beams that are the foundation of the house. They are tied together by steel reinforcing to prevent movement. They are made on site from concrete poured into temporary forms. The quantity, location, and design of the pads and beams is a matter for a structural engineer.
Health and Safety
Before any excavation, including trial pits, commences, it’s very important that you locate any services running through the site. Damage to water, sewer, and gas pipes as well as telecoms cables is not only expensive, and inconvenient for the builder, but will also seriously affect your neighbours – not a very good start! The most critical are electricity cables as damage to these could cause loss of life. Your local council may have plans showing the location of services, as will the telecoms and gas companies. Neighbours may also be able to help. It should never be assumed that the site is free from services.
Protect open trenches and have the site well signed warning of the dangers, not just the entrance but also around it and on access roads. All machinery should be properly parked and locked when not in use.
Radon
Radon is a naturally occurring radioactive gas found throughout the country, but to varying levels. Unfortunately new research findings published in the British Medical Journal (December 04) confirmed the link between radon and lung cancer and highlighted the number of deaths occurring in areas of low concentration. Radon exposure in the home is now a bigger risk than previously thought. Additionally, a survey in ROI reported in June of this year that radon reducing measures are not guaranteeing safe levels. In NI monitoring is carried out by the Environment & Heritage Service (E&HS), in ROI this is the function of the Radiological Protection Institute of Ireland (RPII). Under Building Regulations, all homes are required to have some form of radon prevention, but the degree depends upon radon levels in the area – contact the bodies above for advice. A simple test kit is available to test for radon post construction, action that is recommended by the RPII in high radon areas. The kit costs about €45.
Radon protection normally takes the form of a fully sealed membrane covering the footprint of the house. It is vitally important that the membrane is not damaged during or after installation as this would compromise its effectiveness. Special sealing collars are available for placing around sewer pipes etc. protruding through the barrier. The membrane may be placed either above or below the concrete floor slab. Below the membrane (usually in the hardcore), a collection chamber is placed, normally in the centre of the house, and perforated collection pipes gather any radon and direct it to the chamber. A pipe from the chamber leads to the exterior where the gas will disperse. If the house is very large, more than one chamber may be required.
Other gas risks to be aware of are methane and methane mixtures which carry an explosion risk, sometimes a good distance away from landfill areas by migration through pipes etc.
Insulation
Insulation can’t be placed under the foundations, as it would compress under the weight of the house and cause foundation failure. Instead it is placed between the finished ground floor and on top of the sub-floor for heat retention. Avoid placing insulation immediately under cross-walls because it would compress under the weight of the wall and could cause cracking. This is especially true with outer walls and load bearing cross walls as any movement could have serious implications for the structural integrity of the house.
Services
Water and central heating pipes, ducting for telecoms etc. are normally placed on top of the insulation. Waste pipes are usually located under the sub-floor and taken outside the building through the rising walls above the foundation. For this reason it is important to get their location correct and ensure that any bends or joints are correctly fitted as it will be extremely difficult to reach them once the slab is poured. Taking photographs of all of these or drawing a plan to scale provides a very useful record.
Inspections and compliance
In both NI and ROI it is the self builder’s responsibility to comply with both Planning and Building regulations. Normally an engineer is engaged to sign off on compliance, providing certification for a mortgage if this is necessary. Inspections are random, not mandatory in ROI, but in NI the local council will carry out a number of inspections at various stages of building. Building Control surveyors are called out by the builder at the appropriate stage and failure to do so may result in covered up work being either exposed again or removed.
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