Soil investigation/borehole for new ground floor extension - advice needed!

Kate0408

Good afternoon,

We are in the process of getting plans together for a proposed ground floor extension to our home.

We currently have a garage with a utility room behind the garage, joined into the side of our house.

We are looking to rebuild the above, bringing the garage further forward and creating a downstairs bedroom.

We have spoken to a structural engineer, who has agreed to design the foundations etc, but he says we need to get one borehole dug to check the soil type, before he can decide on which foundations will be required.

We have a fairly limited budget for the whole project and we are uncertain if our engineer is being overly cautious. We don't want to throw money away if we can avoid it.

Do we really need to get this done? Can't we just dig a test pit to check?

The cost for boreholes look to be very expensive (£1k upwards).

We previously had planning passed (inc building regs) for a double height extension and the planning approval did mention the following:

"The applicant is advised that the site to which this planning permission relates is recorded by The Local Borough Council as being on or in the vicinity of land of potentially contaminative use, that was a works and gravel pit.

Prior to the commencement of the permitted development, the applicant is advised to undertake a suitable and sufficient site investigation and any necessary risk assessment to ensure the land is free from significant levels of contamination."

Our previous architect said that we did not to do anything about the above, unless we found anything once the build commenced.

The road we live on is full of 1960's built homes along with much older houses. There is an old gravel pit which is being redeveloped at present on the very edge of our riverside village, but there is nothing to make me think there would be a problem with the soil where we are!

Any advice or information would be much appreciated.

TIA







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System

You have a suspensvie condition requiring that a site investigation and associated risk assessment are carried out "Prior to commencement" i.e. before you start, so not sure how that would mean you don't have to do anything about it until after you start? Normally you write to the Local authority and get them to confirm that all conditions have been satisfied to allow you to start work.
I don't understand how you managed to get building regs passed without an engineer confirming the suitability and design of foundations?
What is his reasoning for asking for a borehole? perhaps the ground conditions would mean that this is most suitable way to assess the ground for construction? Or is the borehole for the Site investigation in regards to contamination etc?

Kate0408

This is the whole section in full (taken from planning approval).

4. In the event that historic land contamination is found at any time
when carrying out works in relation to the development, it must be reported in writing immediately to the Local Planning Authority and
all development shall cease immediately.

Development shall not re- commence until such times as an investigation and risk assessment has been submitted to and approved in writing by the Local Planning Authority, and where remediation is necessary, a remediation scheme has been submitted to and approved in writing by the Local Planning Authority.

Development shall only re-commence thereafter following completion of measures identified in the approved remediation scheme, and the submission to and approval in writing of a verification report.

This must be conducted in accordance with DEFRA and the Environment Agency’s ‘Model Procedures for the Management of Land Contamination, CLR 11’ and the Essex Contaminated Land Consortium’s ‘Land Affected by Contamination: Technical Guidance for Applicants and Developers’.

Reason – The site lies on or in the vicinity of a former works and gravel pit where there is the possibility of contamination.


Informatives


The applicant is advised that the site to which this planning permission relates is recorded by The Local Borough Council as being on or in the vicinity of land of potentially contaminative use, that was a works and gravel pit.

Prior to the commencement of the permitted development, the applicant is advised to undertake a suitable and sufficient site investigation and any necessary risk assessment to ensure the land is free from significant levels of contamination.

REASON

The site lies adjacent to land with potentially contaminative uses and Environmental Protection wish to ensure that development only proceeds if it is safe to do so.

This informative should not be read as indicating that there is any known danger from the former use of land in this locality.

Kate0408

We had nothing but errors and mis-information from our previous architect so are now going with a structural engineer to design foundations etc (my husband has worked with him previously, so knows he is good.)

My worry is that he may be too thorough as very old school (not a bad thing).

We have just wasted quite a bit of money through our naivety over the last year and don't want to keep throwing money down the drain .

The SE says he needs the one borehole to test the soil for foundation design. He has not seen the advisory notes on the previous planning approval.

Kate0408

I should add that on our previous application, the architect had a colleague who did all the engineering calculations and designed the foundations. Building Regs were sent all this relevant info and they passed as follows:

"THE COUNCIL NOW GIVES YOU NOTICE, in pursuance of Section 16 of the Building Act 1984, that the Notice is accepted, for the proposal detailed above.

**No Conditions**"

tonyh66

have you asked the SE if a trial pit would be sufficient?

System

so you have planning permission and building regs passed for the project, but you are asking a new structural engineer to redesign the foundations?
Has the new structural engineer said that the original design is not sufficient for some reason?

Kate0408

We will be asking him in due course. I just wondered if anyone else had any experience of this and costs involved.

I'm assuming a trial pit would be cheaper but we are built to our boundary so it would have to be at the front of our house, through the driveway.

Kate0408

Our original design was for a two-storey side extension, however we have had to shelve those plans as it's just too much money (more than we anticipated).

The SE looked over the previous foundation design but would want to redesign them based on the revised design (only one-storey).

teneighty

Is this in England or Scotland?

If it is England getting a structural engineer to design the foundations for a single storey extension is massive overkill. Unless you have been told to expect difficult ground conditions it will probably be a simple standard trench fill foundation 1 - 1.2 metres deep and 450 - 600 mm wide. The local Building Control Officer at the Council will probably be able to advise you as they spend most of their time looking at muddy holes for foundations and are a wealth of knowledge for the local ground conditions.

As for the contaminated land, your house is already sitting on it so how bad can it be? That condition only really needs taking seriously if it is a new build on a former industrial site etc.

tonyh66

why not just keep the found design for the 2 storey build incase you want to do it in the future?