Steps in laying of a building foundation in Kenya

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.

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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.

Begin  building 

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.

Foundation base

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. 

©copyright East Africa Little hardware ltd 2017.

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