Foundation Materials

From page 190 one of the functions of a foundation can be seen to be the ability to spread its load evenly over the ground on which it rests. It must of course be constructed of a durable material of adequate strength. Experience has shown that the most suitable material is concrete.

Concrete is a mixture of cement - aggregates - water in controlled proportions.

Foundations - Cracking in Walls

Cracks are caused by applied forces which exceed those that the building can withstand. Most cracking is superficial, occurring as materials dry out and subsequently shrink to reveal minor surface fractures of < 2mm. These insignificant cracks can be made good with proprietary fillers. Severe cracking in walls may result from foundation failure, due to inadequate design or physical damage. Further problems could include: A survey should be undertaken to determine:

1. The cause of cracking, i.e.

* Loads applied externally (tree roots, subsoil movement).
* Climate/temperature changes (thermal movement).
* Moisture content change (faulty dpc, building leakage).
* Vibration (adjacent work, traffic).
* Changes in physical composition (salt or ice formation).
* Chemical change (corrosion, sulphate attack).
* Biological change (timber decay).

2. The effect on a building's performance (structural and environmental).

3. The nature of movement † completed, ongoing or intermittent (seasonal).

Observations over a period of several months, preferably over a full year, will determine whether the cracking is new or established and whether it is progressing. Simple method for monitoring cracks.

Foundations - Subsoil Movements

These are due primarily to changes in volume when the subsoil becomes wet or dry and occurs near the upper surface of the soil. Compact granular soils such as gravel suffer very little movement whereas cohesive soils such as clay do suffer volume changes near the upper surface. Similar volume changes can occur due to water held in the subsoil freezing and expanding † this is called Frost Heave.