29 Oct Fixing your foundations
Getting the first steps right and taking great care of what goes underneath your structure is crucial to the overall success of your project. By sight, it may seem that you have fairly solid and stable ground to build on, but it is the closer investigation which tends to reveal the difficulties and hidden challenges.
Below is a list of the types of problems which may arise, followed by a description of the types of foundations used. It is important to remember that problems often arise when the ground is broken, prior to the surveys are done so being prepared for these situations will save you a great deal of time and, if you have an appropriate contingency budget you won’t meet an unexpected financial burden.
Problems arising at ground-break:
- Pockets of weak soil
- May require stronger foundations- which would increase time and cost pressure, as well as redesign costs
- Past rotten organic matter
- Would need to be filled with concrete or special filler solution
- Unforeseen contamination, be that from sewers, oil spills, intensive farming, Agrochemicals, Landfill etc.
- On site/ off site treatment?
- Depending on the type of contamination, treatment could be done on site, or more likely, removed to a treatment facility.
- Some of the methods of cleaning are biochemical, washing, vapour extraction.
- If the soil cannot be treated on site, the disposal of contaminated soil incurs some costs as well (landfill tax, haulage, loading, duty of care).
- Softer ground
- Could need chemical alteration to improve its load bearing qualities, or even re-grading (Soil Stabilisation) to level the ground ready for the project
- Does the surrounding area give any clues? E.g. Quarries nearby, size of buildings in the vicinity
- Presence of trees
- Trees directly affect the soil, if the development is within 3 metres of vegetation, deeper foundations will be required
- Unknown voids, pipework or sewers
- They would have to be filled with reinforced concrete or special filler depending on the extent
- Ecological factors
- With certain species, usually things like newts or toads, special barriers have to put up around the site. The correct officials must visit the site also
- On site/ off site treatment?
In general, the deeper one digs, the stronger the load-bearing qualities of the soil.
The load-bearing capacity of the soil and the size of the development dictates the extent of the foundations required. One should make an informed decision based on the information at hand as to which foundations should be used. Below are a list of the most common types, beginning with the most basic, descending to the deepest and most comprehensive.
Strip (footings, or pads)
Basic foundations, for soils of good bearing capacity, or smaller projects with light loads.
Trench is filled with concrete up to frost line.
- Useful for loose/ weaker soil found in areas with a high water table
- Rubble trenches can also be used to save concrete.
- While costlier as more concrete is being used, often contractors swap to these early on as it gets them out the ground straight away, especially in bad conditions
Will be necessary if trees (3m) are near the site, or if the soil is weaker than expected.
Raft (or Mat) Foundations
Are usually used for basements or when there is exceptionally weak soil. A continuous slab of concrete resting on soil covering the entire footprint of the building.
Piling (Deep Foundations)
Pillars driven into the ground, made of wood/concrete/steel.
- Used for:
- Weaker ground and bypassing one layer of soil. Sometimes for basements/ underground parking/ on slopes/ more stories
- Special machinery, structural engineer’s fees, excavation, muck removal
- Often, as with Trench foundations, instead of doing extensive/expensive ground investigations, it is more time efficient to plan for piles in the possibly problematic soil.
This is the strengthening of existing foundations, may be required if:
- Original foundation isn’t strong enough
- Soil composition has changed
- Nearby structures have been built