Piled foundation cost

[Bonner]

10 hours ago, Gus Potter said:

The soils are just above the plasticity A line, not much to worry about. 

 

Lots of rafts get designed with an L shaped building, we design joints for this and make sure the super structure can cope with anticipated movement. 

 

The allowable bearing pressure is good at 115 kPa, a standard benchmark of 100kPa is a good comfortable reference point. 

 

Where does the water table sit?

 

You probably need to post the full SI for the best feedback, there will other stuff buried in it that you may not have the experience to be able recognise it's importance. 

 

 

It would a lot easier to share the whole report and I prefer in principle, to be open and transparent but in this case need to be discrete. I will try to redact it later.

If I am not mistaken, It seems to be mainly a concern about trees rather than soil bearing capacity.

The report states (as previous post, site investigations done in January)

Groundwater seepage was generally seen at 1.00-1.10m, thought to be mainly a

perched water in the granular Head Deposits but also locally out of fissures in the DF

where granular Head Deposits were absent.

 

[Oz07]

1 hour ago, Bonner said:

Thanks for all the comments. The site is a former garden which has been cleared of said former (smallish) trees. It is still bordered on one side by tall conifers within 10m of the nearest foundation. Site of the house is virtually level however the back of the site slopes gradually up from 5-10 degrees.

No signs of standing water but ground infiltration test was nil, presumably due to underlying clays (consent has been granted to discharge storm water to sewer). Note tests were done in January.

Did you get consent for storm to sewer easy enough? Our next place has clay ground. Last time I built near there they let me send storm into the combined due to ground conditions. Our site currently has a garage with an existing connection which I'm assuming is into combined. 

 

21 hours ago, saveasteading said:

Me too, but language should be clearer and I was being flippant.

 

But how does anyone know there was tree there last year  or whether 2 or  10 years ago matters.

In my opinion a normal year's weather cycle will likely allow the clay ground to come to stability. 

 

I was chatting to a guy at a firm i did some work for the other week. I think these engineers err on the side of caution now they're not happy with 1 years cycle to restore the ground. 

 

This thread is one to watch for me. Our new plot is similar. We have clay ground. There are 5 apple trees I've cut down. 2 on the building line or within it, 3 just outside. I'm pretty confident we're going to go with trench fill foundations. I've gone as deep as 2.2 previously. I will get a plasticity test done on some samples when I have the site cleared. My understanding is 2 holes. 1 near trees with 3 samples at 1m,2m and 3m. Then one away from the trees. Give these results to engineer to see if medium or high plasticity. 

[Bonner]

28 minutes ago, Oz07 said:

Did you get consent for storm to sewer easy enough? Our next place has clay ground. Last time I built near there they let me send storm into the combined due to ground conditions. Our site currently has a garage with an existing connection which I'm assuming is into combined.

The vendor got consent (with S106). It’s on a road with the other back land developments so precedent has been set.

30 minutes ago, Oz07 said:

I'm pretty confident we're going to go with trench fill foundations. I've gone as deep as 2.2 previously

Presumably not full fill trenches otherwise doesn’t concrete cost eventually overtake piling? Maybe I am worrying too much about the trench depth.

[Oz07]

35 minutes ago, Bonner said:

The vendor got consent (with S106). It’s on a road with the other back land developments so precedent has been set.

Presumably not full fill trenches otherwise doesn’t concrete cost eventually overtake piling? Maybe I am worrying too much about the trench depth.

Ive never had 106 before just an email from local BCO saying in his experience local clay ground unsuitable for soakaway. Severn Trent accepted this. 

 

I think my conc was about 900 below ffl. Then 3 course block and 3 brick. For drainage and block and beam flooring etc 

[saveasteading]

Conifers at 10m are unlikely to be an issue but your designer will confirm that formally for the bco.

 

[saveasteading]

54 minutes ago, Bonner said:

doesn’t concrete cost eventually overtake piling

I'd be surprised. Clay should dig neatly so the concrete goes in efficiently.

Also there are usually nasty extras with piling: mobilisation cost, hardstanding, muck arising from some methods, pile caps.

 

Small fruit trees and pines don't need 2m deep trenches.  Plus the closest distance does not apply to the whole building so can all be different depths.

 

[Gus Potter]

On 04/09/2026 at 08:31, Bonner said:

It would a lot easier to share the whole report and I prefer in principle, to be open and transparent but in this case need to be discrete. I will try to redact it later.

If I am not mistaken, It seems to be mainly a concern about trees rather than soil bearing capacity.

The report states (as previous post, site investigations done in January)

I appreciate what you are saying. 

 

A common complaint on BH is about SE's over engineering founds or say BC coming out and saying they need to be much deeper. 

 

For all. SE's don't deliberately over engineer your founds, if they did it would be a lack of duty of care not least. They have to balance what information is available against the risk. Often the reference point is say the NHBC regs, maybe BC regs. These in lots of cases can be very conservative. Often a Client has spent little time or money on even understanding the ground, the trees (species and height not least), looked at the local and wider topography, possible perched water table,  the topography and where ground water may be flowing form and to. 

 

@Bonner if you are serious about buying a site that may have "issues" then you need to find a good SE that can help you. I have commercial Client's that keep me on a "retainer". They can phone me up for a 30 min chat for free, it usually ends up much more as we touch base, ask about family and so on, then get back to business, it's good fun. If I need to chip in and commit in writing then I charge a fee, which they are happy to pay as they appreciate the value I bring. Over time I teach them and they can progress under their own steam.. it might seem to the cynical I'm doing myself out of work.. but the more successful they are, the other parts of the design come my way. 

 

Now in terms of trees. The key thing is to learn about how they grow, different root depths, types of roots and how they influence the soil depending on soil type. Next is to understand ground water and typical soil profiles in the areas where you may wish to buy a plot. 

 

@Bonner Here is a play. Sometime it's best not to be discrete when pitching for a plot! If the plot is a bit blighted you figure out you own solution, don't tell anyone. Put you own value on it but publicise potential issues so your competition backs off! It also brings the seller to the table to spell out what challenges the plot presents.  If they are that nosey and raking about about on BH then it's fair game? 

 

OK how does that apply to a first time self builder wanting to buy a plot. In the current market there are fewer plots with "clean hands", the ones available often command a premium. The ones with mucky hands are often avoided by developers, but they present a great opportunity to self build. They go a a lower price, but if you don't want to loose you shirt then you need to find say an SE like me or really know your onions. Your best bet is probably not me !, try and find someone local first. 

 

You need to find a local SE or a designer that really knows their stuff. Now the fee is going to be more, but a good designer will make sure they wash their face fee wise and will steer you towards the right price to pay for the plot.