UKTC Archive

Re: Subsidence in a riparean area

Subject: Re: Subsidence in a riparean area
From: Vicki Harding
Date: Oct 05 2021 11:54:00
Tom,

My limited experience is local to London and I’m not a qualified
engineering geologist(!) but while I’d agree that the canal is unlikely to
be leaking, there is/was a river here too.

The soil all over the river flood plain has the potential to be silty (and
erodible) and could be laminated and patchily permeable.  Some groundwater
usually follows old river and tributary pathways.  It is likely to have
passed through slowly but with storm surges, even if the ground is mainly
clay, for many centuries - particularly if this river bend was formed by
silt deposition and erosion.  Outside the canal, groundwater may behave
like that following the outside of conduits or service pipes laid in silty
clays or superficial deposits like the London quaternary ‘Head’.  Following
‘outside' courses, over time such water erodes wider channels for more to
pass and for silt to be washed into before being washed out and replaced
again.  Land drains of course are also a route for silt wash-out and a
space for silt to wash into.  Where there’s silt and groundwater – even if
intermittent – then subsidence could potentially continue, slowly or
intermittently.

If the movement is serious, sand partings may be found to be present and
ground investigation of the silt composition could be appropriate.  Being a
riparian site too, (as well as assessing flood risk) assessment of the
groundwater flow through the site with continuous monitoring across high
rainfall would assess the ground response to storm surges for an expert to
evaluate its likely effect.  Standard augered boreholes break through the
ground layers and are unable to assess if there is groundwater or where
it’s coming from, and occasional monitoring can miss the more erosive storm
surges.  If it's dug during a period of high rainfall, even a deepish Trial
Pit may tell you water can be here, but this may not be sufficient
evidence: proper groundwater testing is probably required.  There may be
other geological factors and faults here that require investigation too.
As you say, they seem to be going through a process of elimination rather
than using an appropriate expert who knows how to assess the whole
site/environment and focus on the most likely factors.  Doing the
easy/cheap ones first can hold things up for ages, and it puts you in a
difficult position.

I don’t know how old the house is, but if there are signs of general
settlement of the ground, it’s reasonable to suspect that something like
silt erosion is involved.  Trees may have been planted in the past to bind
the ground and attempt to slow this process down, but they won’t be
sufficient unless the property owner is prepared to put up with
‘characterful wonkiness’ and do regular crack repairs.  If I was a
purchaser I think I’d want a professional to decide if it's bad enough to
support me in pushing to deduct the cost of further expert investigation
and a sufficient extent of ground stabilisation from the sale price.

Vicki

On Tue, 5 Oct 2021 at 08:39, Jim Quaife <jq@xxxxxxxxxxxxxxxxx.co.uk> wrote:

If a canal, the water would be contained.  Dug by "navigators" (navvies)
the lining was clay mixed with straw and puddled with flocks of sheep
(their feet having the perfect size/weight ratio - modern machines are
called "sheeps' foot rollers")
Accordingly the canal will have no influence on local soil moisture
content.
Jim

-----Original Message-----
From: uktc-request@xxxxxx.tree-care.info [mailto:
uktc-request@xxxxxx.tree-care.info] On Behalf Of Tom Thompson
Sent: 05 October 2021 05:48
To: UK Tree Care
Subject: Re: Subsidence in a riparean area

Sorry for my slow response to this one.

I wasn't a river despite being labelled as a river on the plan I had.

It was a canal cutting out a long bend in the river.

The trees were black thorn bushes, growing on top of a bund.  Previously
topped
On the adjacent land.

It was for a house purchase.

Building looked like settlement but I can't know or comment on that so just
talked about the trees.

I have had a few weird ones lately
Another site where my large clients oak was implicated.

The movement was away from the oak side of the building. This was put down
to 0revious underpinning so couldn't prove otherwise.

It just seemed that they had selected the biggest remaing tree and were
going through a process of elimination.

Regards Tom Thompson





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