Laying Foundations

The type of foundation that will be required for your property will most usually be decided by the type of ground conditions that exist on site.

FoundationsThe most commonly used and basic foundation is the simple concrete strip, where concrete is poured into a trench of given size. The width and depth of foundations will depend on many factors including the number of stories, the floor construction (suspended wood or pre-cast concrete), whether a given wall is a gable end or not and, not least, the firmness of ground being built on and its susceptibility to frost. In areas with particularly poor subsoils, or on sites where landslip or subsidence may be a problem, a strip foundation may require steel reinforcement. BS8301 (Part 1:1995) is the code of practice for stability, site investigation, foundations and ground floor slabs for housing and contains calculation tables for foundation sizes based on all of the relevant criteria. See British Standards Online.

Other commonly used methods include piled foundations, where a precast concrete or metal pile is driven down into the soil until it reaches rock, or a solid load bearing soil which effectively stops the pile being driven significantly deeper. Reinforced beams are then laid on the piles to create a ring beam system on which the house can be constructed.

A variation of this system is to use caissons where deep holes are drilled into the ground, down to load bearing levels, and then filled with concrete often with steel reinforcement. Mat or raft foundations are a system in which an entire building is placed on a large continuous footing in locations where the soil is weak and the bedrock is extremely deep. Sometimes this type of foundation is used in conjunction with caissons or piles.

When deciding on the type of foundation, particularly in the case where the building is on land that has previously been built on, or where mining or geological features could present a risk of subsidence or landslip, a full site survey will need to be conducted to establish the necessary requirements. in the case of 'brown field' sites the factors to be investigated will include items like existing underground sewers and drainage structures, presence of chemicals or residues that might affect the integrity of concrete, the load bearing ability of the ground, etc.

The correct type of foundation, and proper attention to its construction is essential - costs for rectification of any faults once the house is built can be very high indeed so it is worthwhile paying special attention to getting it right at the outset. It is also important to involve the Building Control Officer (BCO) at this point in the project as, even for a simple concrete strip, he or she will want the trench inspected prior to the pouring of the concrete. Not calling the BCO in at the right time could make life harder later since their surveyor's will be overseeing work at regular intervals, so this first site visit should be as much about the foundations of a good working relationship as about concrete. The Building Regulations Approval document will list the various stages which require inspection.

Risk Assessments

It is possible to get risk assessments for land from organisations such as the Landmark Environmental Group - Landmark is Britain's leading supplier of land and property search information, providing digital mapping, planning and environmental risk information. Some basic searches are free of charge, more specific and detailed information can be obtained for a small fee. Visit their website on http://www.landmarkinfo.co.uk/corp/index.jsp for more information.

Concrete LorrySite Access

Getting the concrete to the foundations is a consideration which may need some attention - you will probably be using a ready mixed concrete solution, and provision will need to be made for a firm access and standing area for a heavy vehicle as close as possible to the area where you want the concrete poured.

The easier you make this the better from a cost and labour point of view. If it is not possible to get the delivery close to the pouring site you may need to use a concrete pump to handle the delivery.

TruckForethought and the preparations you make to facilitate delivery and product handling on site will pay dividends as large quantities of materials will be needed as the project develops.

On a well thought out site the distance heavy items need to be moved, to the places where they are needed, is minimised - this saves both time and labour.


 

 

 

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