Foundations and Footings.
Foundations are the material that a house rests on.
Footings are the base structure of the building that supports the house above the foundation material. If the foundations are not prepared appropriately, movement may occur which could be detrimental to the top structure.
It is a good idea, as part of the planning process, to try to determine the type of foundation material you will be building on. In fact, many councils will require some form of documentation from a structural engineer, of the foundation material before a permit is released. If the area is steep or has a history of movement, they may require a geotecnical report as well as an engineers report.
Excavations pose additional problems for footing construction.
Most deep excavations will have a dramatic effect on the immediate water table. The clay layer will be exposed to a much greater variation in moisture levels during wet and dry spells and this will create abnormal movement in the foundations.
Although an excavated site provides a nice flat surface to work on it exposes different types of foundation soil as the cut goes deeper. Typically, an excavation may have rock at the back or deepest part of the cut, then heavy clay changing to a loamy clay or topsoil at the front of the cut. Fig.(d). The spoil from the cut is usually pushed out in front to form more area that is level and the profile of this will generally be chaotic. If un-compacted fill of this nature is to be built over, the footings will have to be dug through the fill to the original surface. Alternatively, piers at 2 meter intervals may be bored deep enough to bear on a similar material to the rest of the house, usually rock. This can become extremely expensive and careful consideration of this should be made at the design stages of the house.
Care must be taken to ensure that the footings are based on an even and similar surface. Generally, undisturbed soils will not require compaction. If you are digging trenches for strip footings or thickened slab edges be sure to dig only as deep as the footing requires. It is generally unaccepted by engineers and councils to place compacted fill under strip footings. Footings should also be founded on similar soil where practicable. If a footing bears on rock in parts and reactive clay for the rest, the differential movement will almost certainly crack the footing, causing movement in the structure above. Fig (a). Up to a slope of 1 in 12, it is better to leave the base of the footing following a similar substrate and only step the top of the footing. Fig (b). Where the slope is greater and the footing needs to be stepped, the base of the concrete should, as much as practicable, rest on similar material. Fig (c)
Sand, as long it is contained, is the most stable foundation unless you have a monolithic rock surface to build on. If so, this rock can then be the footing and you can dispense with the cost of concrete.
Rock or shale is the next most desirable material to build on and will provide a very stable foundation and therefore minimise the size and strength of the footings required.
Clay or loam foundations are the most common and vary markedly from very stable soils to highly reactive clays. Large cracks in the ground after a long spell with no rain indicate a highly reactive soil. The engineer designing the footings should inspect the site and dig test holes or core sample the foundation soil to assess its reactivity. This will help them decide on the strength of the footing required. If the site is to be excavated, they should want to see the finished excavation to assess the foundation material.
If the area under a slab has to be filled, it is important to use a stable type fill and compact it well.
The best material for building up areas is rock, gravel or grit such as "crusher dust" or "grit fill". These materials compact well and remain stable once placed. Moisture changes will not effect them. Most quarries or soil suppliers will have suitable material available to deliver. Sand is suitable for compaction but it must be retained perfectly. Sand will escape from any holes in the ground or retaining walls. Water and rodents will move sand quite readily. Soil with up to 10% clay will be acceptable but any more and the fill will become difficult to compact and may move with moisture content changes. Even if fill is dry when a slab is placed over it, it will absorb moisture from the substrate and if there is any clay it may expand and cause problems.
Fill should be compacted in layers no more than 150mm deep and should be as even as possible over the entire area to prevent future differential settling. If the fill is deeper, the weight of the compactor or vibrations from a plate compactor will be too dispersed to be effective.
The easiest way to compact sand is to water it thoroughly if water is readily available. If not, suitable compaction can be attained by simply driving a vehicle back and forth as much as possible over the area. Plate compactors or whacker packers can be hired for a day rate if other methods fail.
If the fill is retained with a brick or concrete wall, care must be taken to ensure that the wall is strong enough the resist the compaction forces.
Most deterioration in houses is caused by water. Either leaking in through the roof and walls or rising from the ground. It is therefore important to keep moisture content under the house to a minimum. Good drainage should be built during initial excavations and maintained throughout the construction period and thereafter. If rain or ground water is allowed to run under the house the foundation soil moisture will increase and cause erosion or expansion of substrates. High soil moisture content under a timber floor will increase the likelihood of fungal decay and termite attack. A major open drain should be constructed to shed all surface water away from the site. If there is deep soil or clay, a trench may have to be dug deep enough to release ground water and carry it away. The trench will need an agricultural absorption pipe and clean gravel with hessian over it to prevent silt blocking the pipe. The same drain lines should be placed under the house where wet areas are likely and against the base of any retaining walls to prevent hydrostatic pressure deforming them. Fig (d).
The predominant method for footings for mass wall houses is to dig trenches under all masonry walls around the perimeter and internal walls. For this type of construction, council will require an engineer to specify the size of the footing, the strength of the concrete and the size and amount of steel required. Foundation materials are categorized into types and tables are available to work out what size footings are required for a particular type of construction on a particular foundation type. However these tables don’t come with an insurance policy to cover the mistakes and so you must employ an engineer who's mistakes will be insured, no doubt. Generally however the more reactive the footing material the deeper the trench and the greater the amount of steel. Where a stable foundation may require a strip footing 450mm wide and 450mm deep, a reactive foundation will require the footing to be deeper say 600mm, with extra or heavier steel bars. In some areas with known foundation movement problems such as mine subsidence or slip areas, the size and cost of footings can become astronomical.
On level ground, a strip footing is very simple. When setting out the house (another subject) profile boards are set out at the corners and junctions of all major walls. String lines are stretched between the profiles to show where one side of the vertical surface of the actual walls will be. For the perimeter, the strings will show the out side of the wall. Internal walls can be either side but you must make it clear on the profile board, which is the correct side or footings will end up not under the walls they should be supporting. If a 250 thick wall is centered on the footing, there should be 100mm of concrete showing on each side of the wall. You can mark one side of the footing on the ground using lime or white sand so the trench digger (Mechanical or person powered) can see where to dig. Make it very clear which side of the line is to be trenched. The trenches must be neat and straight. If the trench is too narrow, the steel wont fit in without touching the sides. If the trench is too wide, you will be wasting valuable concrete.
Footings on sloping ground present some minor problems. Although it is not imperative, it is easier for construction if the top of the footings are level. With a slope of less than 1 in 12 it is OK to have the base of the trench sloping and preferable if it follows the same foundation material. It is then simple to step the top of the footing to make it easier to lay bricks or blocks on. Level pins can be set into the side of the trenches before the concrete is poured so that the top of the footing can be stepped the right amount to suit brickwork or blocks. Fig (b) Once the slope is greater than 1 in 12 it is necessary to level the base of the footing and proceed down the slope stepping the top and base of the concrete. It now becomes more important to ensure that the base of the footing rests on similar substrate and steps should be made before the substrate changes too much. Fig (c)
Small steps in the concrete, up to 150mm, can be achieved simply by battering the concrete back and using a level and screed to smooth the top. Larger steps will need to be retained more carefully as the hydrostatic pressure of wet concrete is much greater than water. You can make shutters out of old roofing iron, plywood or any timber. (All timber formwork must be removed later. If left insitu it will rot and become a termite attractant). Shutters should be firmly supported, especially at the lower edge or the concrete will force them out. They should be strong enough to support your weight and you should not be able to move them. Avoid making steps higher than about 600mm if possible. If you need to have high steps, you can half fill them at first and give them a little time to go off (30minutes) before topping them up. This will reduce excessive pressure at the base of a high shutter.
Once the size of the footing is known it is simple to work out how much concrete and steel you will need. This can be done by the supplier but it is far better to do it yourself first and then have them check it for you if you are not certain. If the footing is 450mm x 600mm simply measure the length of the trench in meters and multiply it by 0.45 then by 0.6. If there are steps in the footing, don’t forget to add in the vertical length of footing before multiplying.
Steel should be on site before the trenches are dug. When ordering don’t forget to allow for minimum laps for joining, corners and steps. These will be specified by the engineer and will normally be about 600mm for joins and steps and 500mm for corners. Trench mesh usually comes as 6 meter lengths so at least 10% will go into laps.
Footing construction needs to be planned carefully. Once started, trenches should be completed, steel placed, council inspection called and concrete poured as soon as possible. With good organisation, the whole job for a normal house can be done in one day. The longer trenches are left open the more susceptible they are to damage and collapse. Try to avoid digging if the weather is inclement. If collapses happen due to rain or animals, the steel will have to be removed and the trenches cleaned out. Bad collapses should be retained with old roofing iron or plywood to prevent wasting concrete. Steel reinforcing should have at least 50mm of concrete cover at all places. So the steel should not touch soil at any place. This is to prevent rust and eventual concrete cancer. (Concrete bursting apart due to the expansion of rusting steel within). Trench mesh can be supported on plastic bar chairs (not pub) or suspended with tie wire from sticks across the trench.