The Specialists Verdict

Sorry - a delayed blog entry.

A representative from the specialist piling company came to see us last Friday. His recommendation is to install 6 inch diameter steel encased mini piles through the made-up-ground onto the bedrock (assuming we can find some). He anticipates these will need to be at 1.5m centres to meet the Ground Bearing Pressure requirements of the proposed extension. The key thing is that they will give the entire foundation the same structural integrity, so no opportunity for differential movement.

The piles are driven rather than drilled and I asked how disruptive this was likely to be (one of the neighbours has already expressed concern that our works are going to cause his house to slide down the hill). The answer is that this method is quiet and causes minimal disruption to the surrounding area. The steel encasement is initially hollow and the hammer that drives the pile moves inside the tube, banging it into the ground from the bottom rather than the top. The hammer is then withdrawn and the pipe filled with concrete. Steel mesh or rods (I am not sure which) are included in order to allow the top of the piles to be tied into the foundation structure.

In some instances it can also be necessary to drill a "socket" into the rock at the bottom of the pile. This is needed if the rock is likely to kick the pile sideways because of it's natural dip. This may be necessary in our case as the bedding planes dip at about 45 degrees. The drill is also lowered down the steel tube before it is filled with concrete.

In order that we know what we are dealing with, we are having some ground testing done this Thursday. A thin metal rod is to be knocked into the ground at various sites around the foundations to try and establish at what depth the bedrock is to be found. We should get a set of graphs detailing how long each mini-pile needs to be.

I think I mentioned in a previous blog that someone I know is having exactly the same works done at his house in a neighbouring village. I have already seen the output from the ground testing that was done there, and it looks like money well spent. As the major cost of getting these piles done is getting the kit on-site, the intention is that the piling specialists can do both jobs in one go. Of course, the savings made will be passed onto us both........ !