Having to dig deeper, who checks if changes are needed to foundation
Anonymous user 09/03/2024 - 3.18 PM
Hi, we are having a 3.3 x 4.5m single storey rear extension built. There is a tree nearby (not sure what type of tree exactly, sorry!) and this was factored in to the structural drawings that were initially submitted to building control. The foundations had to be dug to a depth of 1.8m
My builder dug the trenches and called the Building Control Inspector to check everything. the inspector has said that instead lf 1.8m we need to dig to 2.5m.
From what I have read, having to dig deeper may change the type of foundation required or some changes made to the design of what was in the original plans. My question is, is this something the building control inspector will advise on and check and change if necessary, or do I need to go back to a structural engineer to check and redesign if necessary? The inspector did not mention anything about changes to the foundation, just that we need to dig deeper. My builder has said that he will stick to the plans, just dig deeper.
Any advice would be appreciated.
Thanks!
Are you a tradesperson and able to answer this question?
Any changes should be put straight back to the engineer/architect for them to calculate and agree to, this is why they are employed at stage 1
I/we/you are not qualified to say if the foundations are suitable for the new project
Building near trees could be very tricky. The tree specie, distance and soil type all have a part to play in getting it right. Normally your founding depth is between 1m and 1.25m. For you to dig down to 1.8m, most likely the tree must be of high water demand in highly shrinkable clay. The NHBC Guidelines specify that any foundation deeper 2.5m due to nearby trees should be Engineered i.e. other foundation types to be considered and to be designed by a Structural Engineer. For the size of the extension being built, digging down to 2.5m has its own risks. The safety of personnel and the existing building is at risk. As a Structural Engineer, I feel any excavation up to 2.3m is becoming less cost effective. A reinforced concrete raft or piles will be an economic alternative.The raft is normally shallow and the piles are driven into the ground with very little spoil so the accidental risks are minimal. The Building Control Officer will not change the design of any structural element, that is the responsibility of the Structural Engineer. The Building Control Officer is to ensure that the extension complies with the Building Regulations, how that is achieved (albeit in an uneconomic manner) is not his problem. Bottom line is that the Structural Engineer should take responsibility for the re-design of a safe and cost effective foundation solution.
In this case it is always good to have a structural engineer tell you what is the best way as they are trained for these problems.
Answered7 January 2021
1
Anonymous user
If the Building Control inspector has told you to dig footings deeper ... due to moisture in the ground consumed by the near by plant growth ... and given you a depth to dig to you should follow that advice. Once dug to the specified depth the inspector will return to determine if that is sufficient.
Deeper footings may also mean either temporary or permanent support to the sides of the footing for both safety and resistance to ground movement once complete.
If the Building Control inspector asks you to supply calculations that would be a structural engineer.
