Information for a structural engineer

[Moonshine]

The house we are likely to get planning for is for is a split level house, with the basement half the footprint area of the floors above.

 

I want to get a idea of ground work and foundation costs, for which i need structural engineer design of the retaining walls.

 

What information do i need to give the structural engineer to carry out his design work.

 

Ground investigation? what sort of depth do pits / bored holes need to go?

[Bitpipe]

In our experience (full footprint basement) get your chosen SE to spec what they need from the GI report and then take that brief to a number of GI companies to get quotes.

 

You may want to wait until you have the PP in hand as there could be conditions on contamination, etc.. - we had these and folded in the necessary tests into the brief. Also needed soil WACS classification for the basement contractor so they could price muck away disposal.

 

We had a mix of dynamic probing (10m) and cores (5m) - our build up is chalk / river gravel and clay - typical for the Thames Valley. The big concern here are 'solution features' aka cavities in the chalk layer, so the survey was designed around that.

 

A few of the cores were sleeved and capped to monitor for gas and ground water. GI usually also does a historic survey to identify old workings or sources of contamination. 

 

Our GI was a bit of a car crash as the original supervising engineer mislocated some probes and drew the wrong conclusions (made ground = piling & £££). The crew had to come back (at a discounted rate) and do it again with a more positive outcome (no made ground, no piles).

 

I also found out, after the fact, that the GI should zero rate any disruptive or tool based parts of the work (digging, probing etc) whereas the walkover and desk work (reports etc) attract standard VAT.

 

You can access the national borehole database and see if there is anything near you  - this is really just for guidance as local conditions can always vary.

 

https://www.bgs.ac.uk/data/boreholescans/home.html

 

 

[Mr Punter]

You may be able to see the Environmental Health comments online.  If they suggest a contamination condition it will almost certainly be added to your consent.

 

If no contamination you will still need some soil information for soil type, bearing capacity and groundwater presence.

 

Basement is easier if you have plenty of space around so you have room to work when you build the walls.

[Moonshine]

@Bitpipe That borehole database is a good resource and there is one 200m from my site, and gives an indication of what I could be expecting, though a proper onsite GI will be done. 

Very good point about the WACS and something to get sorted.

 

@Mr Punter good point on contamination, i spoke with the EHO and they mentioned the potential contamination of a sub station next door, so may well be needed.

 

The site is restricted a bit on one side of the basement as the existing house is 1.5m away from where the retaining wall is, ?

[Bitpipe]

Dependant on the conditions obv, but you normally have a 1m working space outside the wall and then batter back to ground level which is usually another metre. So the mouth of the excavation is about 2m from the wall itself.

 

We put a land drain to a soakaway at the foot of the basement and backfilled the 1m gap with clean stone for good drainage. If you use spoil it needs compacted every 150mm.

 

If you don't have that space, you normally need to sheet pile on the restricted sides to give the crew safe working space. Piles get pulled out after build and go as scrap

 

Will need careful thought on how not to compromise the retaining wall that you're next to - all possible but the costs can start to rack up  - always best to have it 100% figured out before a spade hits the ground