Foundations, Soil Surveys and Structural Engineers

[MikeFleming]

I am doing a new build, and the architect has asked for quotes for soil surveys and engineers. The quotes are coming back at around 3k and 4k.

The site has an existing building which will be demolished. The soil is aluvium (clay, silt, sand and gravel) to 10-15 meters, then chalk. There is a river close to the site, about 2 meters away, and the highest water is 1.5 meters below site ground level. I think ground water is at the same depth.

The construction will be timber frame with brick cladding.

I dont know how much the nearby river complicates things, but the soil isn't the worst thing to build on, so I was hoping that the foundations wouldn't be too big a job, was hoping for around 10K?

Given this, is all that consultancy necessary? Could we just put a bigger margin of safety on the foundations and save money?

I was hoping to dig a trial pit and get a structural engineers opinion, and then run this past BCO, is this a good approach? Or am I being too frugal and naive?

 

[^woody^]

An Architect is not qualified to comment on ground conditions nor the need for ground testing, nor design anything other than basic strip foundations. An structural engineer is.

 

[Footsoldier888]

Wow - sounds really exiting!

Don't mean to sound a Jonah but whenever I've seen Grand Designs, sites by rivers have involved many piles to anchor the foundations to the bedrock. They talk about a tendency for houses to want to slide very very slowly towards rivers. Mind you they have been proper rivers like the Thames and also at Lewes.

Do you know the history of the land? Any industrial usage in the past say? There is a lot more to sites than soil because for example you could hit rock or the site may have been previously used and backfilled.

Did you have a land survey done? Reading the self build books is a good starting point.

Good luck with it anyway.

 

[wessex101]

River aluvium can be very soft and irregular. My money would be on piling.

However you could engage a structural engineer and get some trial holes dug for inspection, probably about £1k or get a dynamic probe test £500 - £1k

 

[newbuildnoob]

Mike, if this is new build and you plan on selling it in the next 10 years odd, you will need a warranty (BLP, NHBC etc). No mortgage provider will let you off without one.

That company will insist on structural engineer calcs/drawings for foundations as there's an indemnity behind it. The engineer will then insist on soil/site investigation.

I don't think you have anywhere to go here, unless you're doing it for yourself in cash and plan on living there yourself for many years to come.

Would be interested if anyone had a different view on this?

 

[bernardgreen]

In 1980 when we self built a timber framed house on 22 pad foundations next to a large stream it was the Building Control Officer who approved the depth of the foundations.

We dug one test hole ( 2 foot by 2 foot ) and 4 foot deep and, after removing ground water, he inspected it and was happy that we were down to a layer of blue clay. I seem to recall digging into the blue clay to ensure it had some thickness to it. He had the opinion that provided all the foundations were dug down to that layer of blue clay the house would be OK. The blue clay would not dry out and shrink due to the water table being above the blue clay.

The house is still standing straight.

 

[charliegolf]

In 1980 when we self built a timber framed house on 22 pad foundations next to a large stream it was the Building Control Officer who approved the depth of the foundations.

We dug one test hole ( 2 foot by 2 foot ) and 4 foot deep and, after removing ground water, he inspected it and was happy that we were down to a layer of blue clay. I seem to recall digging into the blue clay to ensure it had some thickness to it. He had the opinion that provided all the foundations were dug down to that layer of blue clay the house would be OK. The blue clay would not dry out and shrink due to the water table being above the blue clay.

The house is still standing straight.

1995: same deal, timber frame 5m from a statutory main river. Groundworkers dug a hole, BCO said "Keep going" and repeated it until we were digging pebbles off the river bed at 2m deep. "That'll do". Deep strip founds followed by trench fill. Warranty by Zurich, not an engineer in sight. Clay and pockets of peaty soil on the plot. I suggest asking your warranty company's surveyor to look at a trial hole. He/she's gonna sign it of at the end of the day.

 

[Footsoldier888]

Good point Bernard.

Guess it's a case of hoping for the best but preparing for the worst as regards budget.