UKTC Archive

Heave Risk Assessment

Subject: Heave Risk Assessment
From: Jon Heuch
Date: Jan 20 2021 12:28:40
It's easy to ask, but not see easy to answer. The assessment relies on
someone deciding where the appropriate location(s) is/are to retrieve soil
samples & at what depth they need to go. Do you need 1, 2 or more boreholes?
Not always straightforward. The calculations themselves could be undertaken
by a geotechnical engineer but someone has to interpret the results within
the context of the building. Can the building take 43 mm of uplift, for
example? Or 95 or 151? That is not a subject for a geotechnical engineer but
for a structural engineer...and as they will have PI insurance they may well
err on the side of caution. After all, if  you are told that part of the
building will go up by 7 cm who wants to take that risk. So you will be
steered towards underpinning with appropriate anti heave measures.

 

So you might think that if the number comes out quite small, let's say 0 or
10mm, you are entirely in the safe zone. Well, it depends on where the soil
samples were taken. Were you able to get down to 4 metres depth? Are your
soil samples representative of the soil conditions for the whole property?

 

It's one reason why people are steered towards an arboriculturist to advise
on heave risk by...surveyors & the like. They want you to say that it is OK
to remove the tree!

 

All I can say is that I wouldn't do it without having good insurance!

 

Where to go? I would attempt to find an engineer who is local to the site.
There are still a few regional small engineers that deal with subsidence who
haven't been swallowed up by nationwide companies. You could start here:
https://abbott-holliday.co.uk/

 

Jon 

 




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