You can do many nice things with sand, but never build a house foundation on it. Upon my word! it’s not a great idea, because the weakness of that foundation will prove too much temptation for bad weather to leave it alone;before year is out the house will be reduced to a pile of rubble . For consistently good outcomes, a building foundation must be built on a rock or compact soils and must follow the step below:
Firstly the plot must be professionally surveyed to distinctly mark it’s physical boundary with beacons . Then the site must be cleared of tuft and vegetation matter, – to make walking and working easier .
Then with input from the contractor , the mason should with mathematical accuracy, determine and physically mark out the foundation corners , using four pegs or spikes. The pegs should be driven into the ground using a hammer and in such a way that they describes a square, to the beholder.
Next, the mason has to determine probable trenches widths and with the help of a measuring line, plumb bob, a square and the building drawings, string two lines from one corner to the next, till the whole building is covered. And he must ensure that the widths between the lines is slightly greater than the width of the wall of the proposed building.
Using these two lines as a guide ,the masons and the unskilled laborers should begin excavating the trenches.Embed from Getty Images
The foundation trenches excavation must be done with an eye on every detail to ensure that the proposed building is in the right place on the plot and that all load bearing walls have been identified and set out accurately on ground. The result is correct orientation of the proposed building with say the drive way, garage, stairs and balcony set to located in right places.
According to the Kenya building code of 1997 , “the foundation should designed and constructed in such manner as to sustain the dead load of the building imposed vertically and laterally and to transmit these loads to the ground in such manner that the pressure may not impair the stability of the building or adjoining structures. In addition foundation laying should also factor into the equation the effect of wind on the building. Also it should be taken down to such a depth that will safeguard the building from swelling, shrinking or erosion of the subsoil.”.
Foundation trench depth.
The depth of the trenches will depend on the type of soil and slope of the construction site. Black cotton soil, clay, alluvial, sand and red soil are considered weak and will require deeper trenches that go past its substrata-possibly more than 7 feet deep. Also a sloping landscape will necessitate deeper foundation trenches to be dug out.
Good soils with hard surface that require standard trench depth of less than 6 feet, are compact murrum soil, compact gravel, compact magadi soil, brown shale, coral, and Nairobi stone soil.
As has already been mentioned before, the building foundation must always rest on a rock solid base beneath the ground.
Thirdly and what follows the foundation trench excavation, depends on whether you are building a standard house or storey building. To begin with, a concrete blinding on the trench floor is laid, making it ready for the laying of the first course of bricks, stones or whatever you have planned to use.
For Storey buildings, the mason must then install ribbed steel bar squares in all the corners of the building and then the concrete is poured and spread in and around them. Also the mason may lay two Y 16 ribbed steel bars in the foundation trenches. All these is done to add strength to the foundation and make it one compact unit.
Fourthly, once the concrete blinding has cured, the masons must begin building the foundation wall also known as the stem wall.
For storey building foundation,the building of the stem wall must be done concurrently with the wall columns that must reinforced with appropriate iron or steel bars.
Fifthly, after the foregoing, backfill foundation base with soil that came out of the foundation trenches to make it flat and level. Then add on top hard-core stones and level them. Finally cover all these with Murrum to create an even and smooth surface . The murram is used to minimize the amount of concrete to be used, since it seals all cracks and spaces within hard core stones that would allow concrete group to slip by .
Sixthly and last when the foundation based is at the desired level and flat, prepare to pour the concrete slab on it. To improve the durability of the building apply anti termite solution like renowned gladiator brand and a damp proof membrane(DPM) to make the foundation resistant to rising damp.
This work must go hand in hand with plumbing and electrical piping work. While the plumber will install water and sewerage piping the electrician will lay necessary electrical wiring conduits before the floor concrete slab hardens.
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