Simple Foundations and Bases
The foundation for a building or wall is the part of the structure that is in direct contact with the ground. A structure will not stand without an adequate foundation. Its purpose is to transfer loads safely into the ground without excessive movement, settlement, or damage to the structure, throughout its life and during the initial construction process. The term ‘load’ has a number of meanings:
• The ‘dead load’ is the constant weight of the structure itself and all its component parts.
• ‘Imposed loads’ or ‘live loads’ are those loads that are created by the weight of people, vehicles, furniture and other objects placed in or on the structure. Inevitably, this type of loading can vary greatly.
• Other loads include the forces and stresses produced by wind and snow, which must be allowed for in the design of the foundation.
Foundation design depends largely on the type of sub-soil on which the structure is to be built. The soil must be capable of resisting the downward loads from the structure (including the foundations themselves) by way of equal and opposite upward forces. The soil, therefore, needs to have
sufficient compressive strength or bearing capacity to prevent settlement and contraction.
Bearing capacity, which is a measure of how much pressure can be supported, varies from one soil type to another. Since Unit Pressure equals Force divided by Area, enlarging the plan size of the foundation reduces the pressure on the soil. A soil with a low bearing capacity will need foundations with a greater surface area. Alternatively, it may be necessary to excavate down to significant depth until a sub-soil of sufficient bearing capacity is found.
All soils have a tendency for movement due to changes in temperature and water content. In particular, cohesive, shrinkable soils such as clay can swell and shrink quite violently as their water content increases or decreases, and it has been known for the sub-structure of a building to be crushed. Non-cohesive soils, such as sandy ones, can be affected by surface water eroding particles or by freezing (‘frost heave’) in winter.
The depth of ground cover must be sufficient to prevent the foundation being subjected to ‘frost heave’, so the foundation must be below the point at which frost can penetrate and expand, causing damage. Current Building Regulations require simple strip foundations to be located 1000mm below finished ground level.
Generally, the ground level will rise and fall due to normal seasonal changes in water content but the presence of trees will exacerbate any problems, as they can draw a considerable amount of water out of the soil. It is an issue especially in clay sub-soils, which have the greatest tendency to shrink and swell with changes in water content. During dry summer months, trees will continue to draw increasing amounts of water from the ground causing a clay sub-soil to shrink further. Around a tree it is possible for the ground level to rise and/or fall by up to 40mm between winter and summer.
One option is to chop down the tree so that its roots are no longer draining water from the ground, but this can result in a clay sub-soil swelling by as much as 150mm. For this reason, the ground must be given an extended period of time to stabilize following the removal of trees. Due to the ground instability that results from the presence (or removal) of trees, and its potentially negative effects on foundations, it is generally accepted that foundations, and the structure thereon, should be kept well away from trees. As a ‘rule of thumb’ the distance should be at least equal to the height of an individual mature tree, or its anticipated height at maturity. Where several trees are located nearby, this distance could be increased to the height of the tallest of the group of trees.
The presence of dead trees near to proposed building work cannot be ignored as eventually they will rot below ground. This results in hollows and depressions forming in the ground which can have a negative effect of the bearing capacity of the ground near by.
Fig. 39 Concrete strip foundation.