Construction foundations - Top tips for a successful build

Foundations are the most important element of a building’s structural design. They stabilise unpredictable natural ground, offer an engineered flat surface for a structure to be built on, and they ensure the structure doesn’t sink or shift, even in the face of earthquakes or total flooding.

So how it is possible for engineers to build 100% reliable sub-structures successfully? The truth is they apply thousands of years of learning to the problem in well-defined parameters. The hard part is understanding the requirements and restrictions for the foundation, such as every possible load, challenge, or variable that might apply to the structure over the course it’s life. To do this an engineer needs to know as much as possible about the project, ground, purpose, and regulations involved, which can be daunting. To help, our team of experts share their top considerations:

1. Proposed building design (including required services)

It’s essential to thoroughly understand the purpose and function of a structure before constructing the foundation. To calculate the loads that will be imposed on a foundation the so called “static loading” must be determined by assessing the proposed materials to be used in the above ground structure and how much of them there is. For example, a square meter of 200mm thick brick wall will weigh a lot more than a square meter of timber stud wall with lightweight cladding.

Once the static load is calculated (the weight of the building itself when nothing is happening to it), the “dynamic” or “imposed” load can be assessed. Imposed or dynamic loads are the weight and forces of furniture and fixtures, installations, plant, machinery or transport, humans or animals, wind force, the weight of precipitation and extreme events such as earthquakes or hurricanes.

Add these loads up and you get a factor of safety that is then applied to the plans. This allows for any unforeseen loadings or damage/wear that may reduce the strength of the structure. This safety factor is often three times the calculated loading.

With this information a structural engineer or civil engineer can build a foundation that will support the proposed structure in an ideal situation. That said, buildings are almost never constructed in ideal scenarios, so more information needs to be sought by the engineer.

2. Topographical survey

A topographical survey is, put simply, a very accurate map of the site in which you wish to build. The survey will very accurately plot important features, other buildings, and services on a scale drawing. Crucially, the survey will also record the heights of key features, contours, and items of note to advise the levels of things in relation to each other. These levels are usually marked down in relation to Sea Level to make the levels consistently relevant between designs (this may be written as Above Ordinance Datum or AOD on the plan and will refer to meters above sea level or an agreed datum height for that site as set by the surveying engineer). All works will then be set out to this “topo” survey, usually forming the basis for the engineer and architect’s designs and plans.

3. Existing building plans

When building a structure next to another one, it is important to understand how that building interacts with the ground on which it is sited. If a structure has pre-existing structural issues such as subsidence or erosion, then this must be considered and rectified before a new foundation is laid nearby. Similarly, some footings extend far outside the building perimeter, so this may have an impact on how close the new foundation can be placed. It’s generally not recommended for old and new foundations to directly interact or share the same ground unless specifically designed for that purpose.

4. Existing services plan/survey

When an engineer is considering underground construction, it is critical to understand as much as possible about what is in the ground already. This information can be found by either consulting the plans of the local buildings (note: these are notoriously unreliable!) or by getting a services survey carried out. A services survey will use surveying equipment such as CAT scanners, CCTV, and transponders to accurately plot the route of the services in the ground. If all else fails an exploratory excavation can be carried out in very sensitive areas to expose the services and understand their extent (for instance, when examining a Victorian-era brick culvert).

There may be any number of things under the ground from services that might simply need rerouted, through to major obstructions that could require massive redesigns of the building’s foundations. Generally, services such as power, water, gas, and telecoms can be rerouted if needed, or encased by the concrete of the foundations (using specified techniques). However, things like gravity drainage are much less flexible, and larger installations such as manholes, septic tanks, attenuation tanks and large bore sewers are much more of a concern and are typically worked around if they are known about.

5. Existing covenants or planning requirements

Property has a very old and developed set of laws that governs it, and these laws can even affect the plans of a new building in some cases. Two key “local” level laws that are of particular interest are covenants and planning requirements.

Covenants are legally binding promises to do, or not do something. These promises are usually made when a piece of land is sold to another person or company, and usually binds all subsequent owners of that land forever. Some covenants can be negotiated out of or overturned, but generally they are upheld if reasonable. For example, you’ve purchased land that is being used as part of the neighbour’s surface water soakaway and the covenant says you may not interfere with the drainage. As a result, any foundation you build near this might affect the permeability of the land and the foundation would need to be designed to ensure it does not affect the requirements of the covenant.

Planning requirements are also promises, but this time to the local Planning Authority to abide by their restrictions put in place when you’ve been granted permission to build. These restrictions can run from purely aesthetic to essential structural concerns. For example, the Planning Authority may make it clear that no structure is to be built closer to a road or pavement within a certain number of meters. This would then have to be incorporated into any foundation plans, including whether the structure will be in violation of those restrictions once it’s built (ie. no structural overhang).

6. CBR test

A Californian Bearing Ratio (CBR) test is conducted to determine the structural and mechanical values of a given piece of ground. If the ground that you wish to build on is not achieving the CBS value you require to make the design work, excavating the ground and importing in graded materials will greatly improve not only the strength of the ground, but also the consistency of the ground capabilities (as it is a known quantity laid in a consistent build-up). This engineering of the sub-base before laying foundations can give the engineer more flexibility with their design as the foundation can do less work in stabilising the structure.

7. Soil report

A soil report is usually a combination of a desk study and on-site testing (usually through boreholes) and is often tied in with the CBR test. The desk study will collate evidence on the proposed site such as historical uses (useful for determining contamination risks), flood risk surveys and other public domain information. This is backed up by onsite soil sampling which will confirm the presence of specified contaminants. It provides a visual breakdown of the ground soil strata and often includes a Waste Acceptance Criteria (WAC) test, which is required if spoil is to be taken away by waste carriers to landfill sites. The soil report will also note where the water table is at the time of testing and often offer remediation solutions for any issues discovered.

8. Arboricultural survey

On sites where construction is planned in areas adjacent to trees it may be necessary to consult an arboriculturist’s survey of the existing trees. The main issue is that tree roots can be very wide-reaching, potentially running into a planned foundation excavation area, and if these roots are severed the tree can be harmed. If the tree is protected by a Tree Protection Order (TPO) then you could be facing legal ramifications if the design does not account for the existing tree roots. Similarly, building next to a growing tree can mean that the tree roots will grow around the foundations, potentially leading to undermining issues later. For instance, if the tree dies, and the root rots away, there will be a cavity where the root was which can lead to damage of the foundation.

While this is a thorough list, there’s so much to know when it comes to construction foundations. It’s always advisable to work with a civil engineer to ensure all your basis are covered.