We have just embarked on a two storey side and rear extension. There is a public sewer 5m from the house about 4m deep. We needed to obtain a build over agreement which we did by following their recommendation to build a pad either side of the line of the sewer with a beam spanning it. We had the load calculations worked out by a structural engineer which were submitted and approved by both the water board and local planning office building regs. The plan was to make each pad 1x1x1m 1m from the line of the sewer and as we are on clay soil the foundations on the rest of the build would need to be nearer 2m.
Today the building inspector turned up and insisted that the pad areas of the foundations be 4m deep and as a result about 4x3m wide. The rest of the foundations had to be 2.5m with those polystyrene boards (can't remember the name). As the 4m deep foundations are so large the builders added some form of wooden box and wooden boards presumably for support. Them filled with concrete.
Our builders have been very efficient and just got on with it all but I've read a couple of websites where it says that trench foundations over 2.5m are dangerous, impractical and prohibitively expensive. Impractical and expensive aren't good but are not a major problem (she says having not seen the bill yet) my concern however is that they could be dangerous. From what I can work out with the ever trusty googling is that the weight of such large foundations puts unpredictable loads on the surrounding soil.
I cannot find enough information on this so was wondering, is 4m too deep? What are the risks? Has anyone had trench foundations of this size or did you opt for piles? My concern is mostly down to my lack of control and lack of understanding and having prepared for the type of soil and having provided all the load calculations and preparations for the build over, is randomly throwing more concrete into a larger hole the safest way to go? How do building inspectors make such rapid decisions on what to do in place of a calculated and prepared plan? Feel free to give me a virtual kick up the butt if i'm worrying over nothing.
Today the building inspector turned up and insisted that the pad areas of the foundations be 4m deep and as a result about 4x3m wide. The rest of the foundations had to be 2.5m with those polystyrene boards (can't remember the name). As the 4m deep foundations are so large the builders added some form of wooden box and wooden boards presumably for support. Them filled with concrete.
Our builders have been very efficient and just got on with it all but I've read a couple of websites where it says that trench foundations over 2.5m are dangerous, impractical and prohibitively expensive. Impractical and expensive aren't good but are not a major problem (she says having not seen the bill yet) my concern however is that they could be dangerous. From what I can work out with the ever trusty googling is that the weight of such large foundations puts unpredictable loads on the surrounding soil.
I cannot find enough information on this so was wondering, is 4m too deep? What are the risks? Has anyone had trench foundations of this size or did you opt for piles? My concern is mostly down to my lack of control and lack of understanding and having prepared for the type of soil and having provided all the load calculations and preparations for the build over, is randomly throwing more concrete into a larger hole the safest way to go? How do building inspectors make such rapid decisions on what to do in place of a calculated and prepared plan? Feel free to give me a virtual kick up the butt if i'm worrying over nothing.
