To dig deep or shallow?

Choosing the right foundation design.

The home building process may be simple with the first step being to lay the foundation, but don’t underestimate the importance of following technical specifications in the plan, including concrete mix design with precision. When taking on a build project, the devil lies in the detail and it is these details in the drawings that prescribe which type of foundation design to apply.

Building contractors need to deliver on structural requirements and constructional requirements. By meeting structural requirements, safety is addressed as the foundation is built to carry the load of the building. Constructional requirements refer to project scheduling, ensuring that the right resources are in place and managing cost parameters.

Foundation design is the central enabler in delivering on structural requirements.

There are two types of foundation design, namely a shallow foundation or deep foundation. Shallow foundations, sometimes called 'spread footings' include strip footings, pads ('isolated footings') and rafts. Deep foundations include diaphragm walls, caissons, piles and pile walls. Shallow foundations are often used in structures such as homes and small buildings in which the floor height is limited to 10m, or where the founding depth is less than the width of the footing and less than 3m.

The most common type of shallow foundation is the strip foundation, which is used on non-waterlogged sites with a strong soil base. Buildings with just one floor also often have a strip foundation. On the opposite side of the design spectrum, a pile foundation would be laid in cases where the site is waterlogged, the foundation rests on poor subsoil, or the building is going to be a high rise. This foundation choice is therefore not only the strongest type of foundation, but the most expensive.

The pile foundation would be classified as a deep foundation. As indicated, this foundation design, which usually delves to depths greater than 3m below finished ground level, is used in the likes of skyscrapers and other high rise buildings. According to Wikipedia, deep foundations include piles, piers and caissons or compensated foundations using deep basements and also deep pad or strip foundations.

There are some key determinants when choosing the type of foundation design. These are ground and groundwater conditions; the site and its environment, including adjacent and nearby buildings; and the structure of the planned building.

Beamlock-Foundationsstrip-foundation

So, where do you begin to ensure that the right choice in foundation design is made?

First things first: soil tests need to be done to determine the quality of the soil on the building site. The bearing capacity of soil is the critical indicator of which type of foundation to use. A Geo-Technical Engineer does this testing and the results are given to either the appointed structural engineer or architect, who decides whether to lay a deep or shallow foundation.

A simplistic indicator of whether to opt for a deep foundation is that this approach needs to be followed when soil is weak or highly compressible, e.g. where there is poorly compacted fill, peat, recent lacustrine and alluvial deposits. One rule of thumb is that the safe bearing capacity of soil should be 180N/mm2 to 200N/mm2.

Other advice to keep in mind when it comes to foundation design is that when you are dealing with ‘yielding soil’, settlement must be reduced as much as possible. As far as possible, the settlement of all the footings in a foundation must be the same.

Once your foundation type is decided, the way forward becomes simple again:

• Before laying the foundation, there must be no debris or rubble on the site.
• Rebar needs to be placed in accordance with the drawing requirements.
• Concrete must be mixed according to the mix design, laid and cured.
• Concrete used in the foundation must be in strict accordance to the mix design, properly compacted and adequately cured

The choice of foundation design may not be rocket science, but it is just as revolutionary when considering the imperative for structural integrity of every building, albeit an office, home or hotel. Each building needs to be a safe haven for the people who will debate in its boardrooms, recharge in its bedrooms or walk its hallways.

In this light, every future structure must rest firmly on the right, robust foundation.

Written by Sephaku Cement.