The quick witted amongst you will be thinking what medical ailment you are suffering from this week. I however will be referring to construction techniques, which hopefully are substantially less painful on the body, but have much more serious consequences on the wallet. So how did our involvement with piles come about? Well we are looking at a site which is having three terraced houses constructed on it. Normally a builder would turn up with a digger, pull strip foundations out, and back fill with concrete, after the trench had been inspected by a building inspector who would verify that the trench was deep enough to support the weight of the proposed house to be constructed on top of the foundation.
In most instances a depth of 1 metre is sufficient, however if there are problems encountered as you excavate, the building inspector can insist that you go deeper, in fact as deep as is needed to find a sub soil strong enough to support the weight of the proposed house. The type of problem you can encounter would be tree roots, sand, or soft soil. Running sand can also be a nightmare. In this instance the builder hit soft soil with little mechanical strength. There were also some tree roots present, the trees happily growing on the neighbouring property.
The building inspector inspected the excavated trench and requested that its depth be increased to 3 metres. The machine operator duly complied, and carried on digging to the required depth. Unfortunately, you may remember that we have had a very wet summer. In fact the wet weather continued into the Autumn and is still continuing as I write. The end result is that we have a very high water table, and within several hours, horizontal migration of ground water started to fill the trench almost as quickly as the trench was excavated.
To add even more problems to the job, the sides of the trench also started to collapse in, resulting is a rather large muddy hole. Unfortunately these houses did not have cellars planned into their design, so the hole would have to be filled in, with appropriate material to support the weight of the house. Given the cost of pre mix concrete, the profit from constructing these houses was literally disappearing down a black hole. The hole was quickly filled in and the planned work stopped. Time for a rethink!
Returning back to basics, the main function of a foundation design is to allow the weight loadings of the property to be transferred back to ground level in a uniform manner across the whole of the building. There cannot be any differential movement allowed to take place, otherwise the end result will be settlement and a serious amount of cracking developing in the walls of the house. Clearly this would be an unacceptable result, which must be avoided if at all possible to prevent the occurrence of insurance claims. The next step was to consult with a structural engineer, who could design a different scheme capable of supporting the weight of the finished building.
After a short deliberation, the proposed scheme was to use short bore piles, in fact a total of 34 piles were proposed, each capable of supporting up to 10 tonnes in dead weight. Each of the piles would then be connected via a collar beam comprising reinforcing steels set into a very strong concrete mix. The beam is created within temporary wooden shuttering, and each pile top is connected directly into the beam using reinforcing rods. If one of the piles fails for any reason, there should be adequate strength in the collar beam to support the weight of the house above, thereby avoiding any chance of settlement taking place in the house. Bring on the specialist contractor, the one chosen coming down from Manchester.
I was expecting to see a big piling rig, but instead when I arrived on site, all I saw was a compressor and a few lengths of pipes with points on the end. The pipes were about 5metres in length. To create the pile, the operators just carry the pipe to the appointed place inserts a pneumatic compressed air hammer into the pipe, and pull the pipe up until it is vertical. Once vertical, the operator then opens the value and uses compressed air to operate the hammer. The pipe them gets forced into the ground, the depth of each pile will depend on the resistance the operator meets when running the hammer. Once there is sufficient pressure, the pile is deemed to be sufficiently deep, the air supply is cut off, and the air hammer withdrawn. Any excess steel is then cut off at ground level, and the steel reinforcing bars inserted. In our case, a total of 34 piles were driven into the ground, to an average depth of 3.5 metres.
The builder then creates the wooden shuttering around the load bearing walls, supported by pins at the appropriate place. The reinforcing rods are also supported with plastic clips so that they are evenly distributed throughout the profile of the beam. Concrete is then poured into the shuttering, allowing the piles to fill as they go. The concrete is then vibrated using a poker to ensure it is dense, making sure all the air pockets are forced out of the concrete mix. Great care is also taken to ensure each pile is vibrated, making sure that it is completely filled from top to bottom. Once poured, the foundation left to set.
Because they use a strong concrete mix the concrete sets relatively quickly, allowing the specialist contractor to hand the site back to the builder in a condition which will allow the builder to continue with the construction program. Because there is a link between the top of each pile and the collar beam, the whole structure looks rather like an oil rig, although most of it is below ground level. As each pile is connected to the collar beam, and the beam has a lot of reinforcing rods inserted into it at various depths, the beam will be more than capable of supporting the weight of the finished house. The contractor even gave a 20 year guarantee for his work. Let’s hope we are still around in 20 years to be able to check the work
This is an Original Blog by Tony Rowland, The Property Doctor of Timothy Lea and Griffiths.
Member since: 10th July 2012
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