Wednesday, 30 November 2016

Is it Structurally and Economically Efficient To Use Short Bored Piles For Perimeter Fencing In Swampy Areas? - Engr. Osaz’ ENOBAKHARE

Perimeter fence constructed on 
short-bored piles by Heavens 
at Isheri-North, Lagos, 2016.
It makes more economic sense to use short bored piles for perimeter fencing in swampy areas than to use raft or deep strip foundation which is in common use, it has been observed. However it is important to do it the right way. 

Generally, perimeter fence consists of non-load bearing walls or partially load-bearing walls depending on its use and configuration; hence the use of short bored piles is structurally permissible. By constructing reinforced concrete piles with adequate spacing, usually not more than 6m apart and capping them together longitudinally, fence walls of up to 4m can be built directly over the cap (or longitudinal bearing) saving the volume of concrete by up to 50%. Apart from being economical to use short bored piles capped together, it is equally structurally efficient.

Short bored piles are so called because they are constructed by boring the ground and then filling with reinforced concrete or mass concrete and they normally do not exceed 5-metres in depth.  They are relatively easier and quicker to construct unlike the longer ones. Once the depth of suitable soil strata is established, rather than constructing raft  or strip which consumes cost associated with trench excavation, formwork consisting of several boards lapped together with a lot of bracing as well as heavy reinforcement accompanying the concrete in foundation which is the common practice, it is faster and cheaper to use short bored piles.

Once the fencing layer is filled, prepared and the boring points are marked, the boring is done simultaneously with the insertion of the reinforcing steel piles (usually no casing is required) and concrete in an operation that may not usually do not last more than 30 minutes per point depending on the nature of the soil beneath. When the piles are set, a network of steel reinforcement is used to cap the starter bars of all the piles together. 

The set up is cast in concrete and is now set to receive the blocks or bricks for the fence. Presently, it cost an average of 10,000 NGN (depending on the area) to bore and fill a point. Hence for a 36m length which could be one face of a standard plot, only about 7 points is required and the volume of concrete is therefore minimal.  

With the cost per cubic metre of concrete still hovering around 30,000 NGN, it is more than logical to conclude that short bored pile foundation for perimeter fencing in swampy or water-logged areas is the way to go especially for large areas like residential estates, industrial complexes, etc

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