BUILDING FOUNDATION. A foundation is a critical stage of building construction. Its the first step taken when building a structure. Normally a foundation serves the following purposes-adequately transfers the load of the structure to suitable ground and to spread building loads over a sufficient area of soil to avoid undue settlements. Foundations also d istribute weight from load bearing walls to the soil beneath . According to the Kenya building code ,foundations for the load bearing structure of a single storey - building were constructed as strip foundations of plain concrete situated centrally under the walls of piers . They were constructed in a way such that there isn’t a wide variation to the type of subsoil over the loaded area and no weaker type of soil exists below that on which the foundations are raised at such a depth that may impear stability of the structure. When building a foundation, the concrete should be composed of cement well graded in the proportion of 50kg of cement in not more than 0.35cm3 of well graded aggregate. Concrete thickness should not be less than its projection from the base of the wall or footings. Where foundations are laid at more than one level at each change of level, the higher foundations extend over an unite with the lower foundations for a distance not less than the thickness of foundations and in no case less than 300mm. There are five common types of foundations normally considered shallow as they do not exeed 3m in depth. They are- strip foundation -stepped foundation -raft foundation -pad foundation -pile foundation [1].STRIP FOUNDATION This kind of foundation is commonly used in domestic structures where a continuous strip of concrete provides a continuous support under load bearing walls. This kind of foundation is composed of plain concrete usually to a mix of 1:3:6 in volume. The thickness of the foundation must not be less than 150mm.Reinforcement bars are sometimes used in small domestic buildings to strengthen the structure and make it less vulnerable to earth tremors . [2].STEPPED FOUNDATION Normally applied where the natural surface of the ground is sloped. As an economical solution in this case, the foundation will take the form of a series of concrete horizontal steps following the slope of the ground. This helps minimize the amount of excavation below ground wall construction that will otherwise be required. Stepped foundations can also be used to transmit from deep foundations to shallow foundations and at corners and intersections. Stepping foundations also avoid abrupt and excessive changes in level that could cause a weakness resulting in movement.The factors to consider when designing a stepped foundation are that ;each step in the foundation should not be higher than the thickness of the foundation ;foundation at the higher level should overlap the lower foundation typically by at least twice the height of the step. [3].RAFT FOUNDATION Covers the entire area of the building and usually extends beyond it . Its usually a thick concrete slab resting on a large area of soil. It is reinforced with steel, supporting columns and transfer loads from structure to the soil . Usually raft foundation is spread over the entire area of structure it is meant to support .It is generally used to support structures like residential or commercial buildings where soil condition is poor .In raft foundation the contact area of foundation with soil is much more than any other type of foundation hence the load is distributed over a large area and thus stress on the soil is lesser. Raft foundation is used when soil has a low bearing capacity; when the load of structure has to be distributed over a large area. [4].PAD FOUNDATION Generally shallow foundation but can be deep depending on the ground conditions. They are a form of spread foundation formed by a rectangular , square or circular concrete that supports localized single point loads such as structural columns. The load is then spread by pad in the bearing layer of soil or rock below. [5].PILE FOUNDATION Basically a long cylinder of a strong material set as concrete that is pushed into the ground so that structures can be supported on top of it. It is used where there is a layer of weak soil at the surface which cannot support the weight of the building therefore the loads of the building will have to bypass this layer and to be transferred to the layer of stronger soil or rock below weak layer. In conclusion, a foundation is the root and pillar of any structure or building therefore the materials used in laying it should be of high quality and correct quantity as directed in the bill of quantity.