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1.1 Foundations Buildings exert a load onto the ground and need foundations to support them and prevent settlement. Foundations can solve the problems in two ways. They can: Q spread the load of a building over a larger area Q transfer the load to a deeper, more suitable bearing strata 1 CONSTRUCTION AND THE BUILT ENVIRONMENT High Low Rocks Loose sands Granites Soft clays Chalk Silts Compact sand Peats Subsoils can be of many different types including rocks, sands, clays, silts and peats. Each has a different bearing capacity and needs a different type, size or depth of foundation. A lower bearing capacity generally means that foundations have to be wider or deeper, or must transfer loads to a suitable bearing strata. Clay is a special case, as it can be affected by seasonal weather changes to a depth of 1m – foundations in clay subsoils have to be a minimum of 1m deep. Normally. foundations have to be 750 mm deep – the depth to which frost can penetrate. Subsoils can contain water which expands when frozen, leading to ‘frost heave’. This is an upward pressure which can damage a building if the foundations are of insufficient depth. STRIP FOUNDATIONS This type of foundation is used in most types of low-rise housing up to three storeys. A concrete strip spreads the load of heavy walls. The width of the strip depends on the load of the building and the bearing capacity of the soil. If the width of the strip exceeds the thickness of the wall plus twice the thickness of the concrete strip, steel reinforcement is needed. Discuss what you see in the photo with a partner. How do you identify the different materials and components? DEEP STRIP FOUNDATIONS Deep strip foundations How does this diagram differ from the photo opposite? What do you think are the advantages and disadvantages of this type of foundation? Deep strip foundations, sometimes known as trench fill, are a more economical alternative when the foundation is relatively narrow (typically 450 mm). They reduce the amount of brickwork, blockwork and backfill needed in the substructures, thus cutting costs. 1 In pairs, discuss the dangers of trench excavations, then produce a safety poster for use on site (remember that workers from other countries may not be able to read English). Both of these foundations use concrete in their construction. Why do you think this is a suitable material? Discuss in groups, and use the Internet to research further. What would be the effect on the foundations of a building if it were designed to be five storeys high instead of two? Activities 1.1 CONSTRUCTION AND THE BUILT ENVIRONMENT The width of a strip foundation can be worked out using the following equation: Width = Load per metre run of wall Bearing capacity of subsoil Functional skills KEEPING US STANDING! Imagine walking through deep snow. You find yourself sinking in because the snow can’t support your weight – and that’s because it doesn’t have sufficient bearing capacity. Make a list of different types of soil you can think of, then sort them in order of bearing capacity. The bearing capacity of sub soils Foundation trench ‘Deep strip’ foundation Outer wall Damp-proof course Inner wall Load – the weight of the building that is supported by the ground Settlement – movement in the ground, usually caused by the building’s load compressing the soil below, or volume changes in the soil caused by seasonal weather changes Foundation – the base on which a building sits Bearing strata – a layer of soil or rock with greater bearing capacity Q Foundations support the building Q Foundations are also the building’s contact with the earth Summary Personal learning and thinking skills The sides of trenches can collapse if the ground is not supported, putting construction workers in great danger. It is important to use earthwork support, sometimes known as ‘timbering’ or ‘planking and strutting’. Safety tip
