Today’s foundation ideas aren’t all that different from the ones that the first home builders had to figure out. The main difference nowadays is that all foundations are built with a sufficient ‘factor of protection’ to account for unexpected events. The most important considerations are as follows:
- All expected building loads or “actions” must be safely transferred onto soil with sufficient load-bearing capacity by the foundations.
- The potential of settlement must be considered, including differential settlement where the soil under one section of the building differs from that under another, such as gravel and solid rock. It is often possible to bridge over an obstruction or weak point where the soil type changes across a relatively small area.
- The size and depth of the foundations must be sufficient for the building type; for example, a large structure would need larger and deeper foundations than a small light structure.
Alternative foundation types
Many different types of foundations can be identified depending on the site and its surroundings, but the following brief definitions are restricted to the most common types of foundations for domestic-scale buildings in the United Kingdom and Ireland.
Pad Foundation
When the load of the building is carried in a single column, the footings are normally circular, square, or rectangular concrete pads.
The total weight on the column is divided by the soil’s safe bearing capacity when building a pad base. The form of shape that is ideal for construction will be determined by the pressure. They are recommended when the building carries a separate column.
Pad foundation is the most available and does not necessarily require highly skilled labor. As compared to other types of foundations, it is less expensive and takes less time. When the columns are not closely spaced and the building load is low, this footing is appropriate.
Strip Foundation
They provide support for a wall’s weight. This footing is also known as spread footing because of its broader base. Its more extensive base, which spans the entire range, helps to provide more stability.
Strip footings are appropriate for walls and bridge piers with a layer thickness of 10 feet or less, and their size and thickness are determined by the type of soil on the job site. The earth must be strong enough to withstand the structure’s weight.
Strip foundations are suitable for structures with more load-bearing and boundary wall construction.
Raft or Mat Foundation
When constructing a basement, this form of foundation may be used. The foundation is the entire basement floor slab. Raft footings are ideal for weak soil. To minimize the stress on the ground per space, the building’s weight is evenly distributed on the ground. The pressure is calculated by dividing the weight by the area.
You can consider raft footing on compressed soil like soft clay where pad, strip, or pile cannot be possible without excessive excavation.
Pile Foundation
This is suitable when hard soil is located deep inside the earth and is difficult to access. When a raft or mat foundation is incredibly expensive. It’s also appropriate when the structure’s load is heavy and concentrated.
For compressible soil and marshy areas, piles are ideal. They’re suggested for bridges over large bodies of water and canals. And if irrigation is possible in the area.
Piles are driven vertically into the ground and have two different operating modes. Some piles are used to support weight, while others are used to reduce friction.
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Rubble trench
While this solution is not commonly used in UK 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.
Wrap Up
When choosing a suitable type of foundation, the soil type and load bearing capacity are important factors to consider. Regardless of the size of the structure, an engineer must be completely conscious of the environment in order to construct a long-lasting structure.
If you’re an engineer, there are a few steps you can take to choose the best structure.
You must first examine the soil on which you plan to build. Second, figure out how much live and dead weight the foundation would support. This will help to determine which type of foundation to use. After that, create structural specifications such as size, weight, and depth.
