Soil Sampling - Is it worth it?

Mr_Mrs_MoneySaver

Hi All!

I've read a few posts here about extensions, foundation depths and builders quotes etc. Can't quite find what I'm looking for so was after some thoughts/recommendations please for anyone with expertise in the area.

We've had professional single storey rear extension drawings done, planning permission completed, buildings regs drawings and structural engineers calculations all done. We're just waiting on our contractors availability now. We are unsure of the depths of foundations, I believe this is normal. Our garden is surrounded by conifers, there is also a willow tree perhaps 6 meters away. In the drawings the foundation depths are staggered, with 2.05m deep being the max (unless roots are found then we have do keep going deeper) and 1.2m being the least. Our contractor has advised a soil sample may be preferable to get an idea of if we would have to end up going any deeper than this but seen they can cost at least £1k. Otherwise another option would be to look at piling and get a cost for that then we know from the outset how deep we're going and how much it is going to cost.

Is this all normal dilemmas and could it be very possible to find tree roots where we need to build? It's about 7m away from the closest conifers. Also removing them isn't an option as they aren't ours.

I've seen I could possibly get in touch with building control to see if they have any information about the soil but does anyone have any other tips or recommendations on who to use for soil investigation?

Thanks in advance!

Ace_Fontaine

Soil testing would tell you the volume change potential of your soils (assuming you are on cohesive (clay) rather than granular (sand/gravel) soil).  The foundation depths could then be calculated based on this information along with accurate height, distance and type of your trees. 

NHBC Standards Chapter 4.2 'Building near Trees' contains useful charts to determine these depths.    

The BGS have a useful freely available online geology viewer which also contains historical borehole data.  You could search for your area and if you're lucky there may be some ground investigation nearby, from which you could get a rough idea of what your ground conditions could be like.

If you decided to go down the route of taking soil samples, you could search for local geotechnical laboratories and do it all yourself, although then the onus would be on you if the foundation depths are wrong.  Other than that look for a small ground investigation company.  

naf123

Instead of wasting money on soil samples , can one overcompensate and build extremely deep foundations ?

FreeBear

naf123 said: Instead of wasting money on soil samples , can one overcompensate and build extremely deep foundations ?

Indeed. Getting a company in to do soil sampling is not going to be cheap. So keep the money to one side as a contingency to pay for deeper foundations if it becomes necessary when the footings are dug.

A geotechnical survey is only warranted for large developments and/or tall structures where the costs can be absorbed & spread.

stuart45

I paid £40 for a soil test about 10 years ago when I built my extension. 
If you are on high shrinkable clay the NHBC calculator states you need engineered designed if a Willow(depends on the type of Willow) is 6M away.
On low shrinkable it changes to a 2M strip foundation which would be cheaper.
LABC have their own calculator now which is similar but a bit more flexible.

Rosa_Damascena

naf123 said:

Instead of wasting money on soil samples , can one overcompensate and build extremely deep foundations ?

The cost of deeper foundations will vastly outweigh the cost of a soil sample!!