What happened to the riverwall?

[studiot]

Would anyone care to hazard a guess and say why?

 

 

[pavelcherepan]

Ground/surface waters washing away the foundation?

[studiot]

24 minutes ago, pavelcherepan said:

Ground/surface waters washing away the foundation?

Thank you for the reply.

I don't know the answer, hopefully we will find out when they pump out the cofferdam.

However the movement of the wall suggests one thing but the stability of the ground behind it suggests something else.

[Ghideon]

Is the water frozen in winter?

(If the river is near your location in your profile then I guess not, according to a quick googling)

 

[studiot]

24 minutes ago, Ghideon said:

Is the water frozen in winter?

(If the river is near your location in your profile then I guess not, according to a quick googling)

 

Not often, perhaps once in 25 years or so.

The wall has stood several hundred years.

 

The failed section of wall appears to have gone down and moved to the left, parallel to the line of the riverwall, - downriver - both about 75mm.
It doesn't seem to have rotated in any direction or displaced into the river at right angles to the line of the riverwall.
The crack appears sensibly of constant width.

One thing to note is that this side of the river is very shallow and slow running.
Looking at the downstream picture you can see the gravel islands that collect on this side.
This is surprising since the river scour will be least on this side.

There is nothing to suggest the earth behind the wall has become active. If active earth pressure developed, it would be greatest at the bottom so the wall would have rotated outwards at the bottom.

 

[mistermack]

Could be plate tectonics in miniature. You might have the ground settling in chunks, rather than grains, and underground, you could have one layer sliding on another. (over a very long time span)

Pure guesswork of course.

[Ghideon]

 

49 minutes ago, studiot said:

The wall has stood several hundred years.

Was the wall built on top of parts of an older structure? An old wooden construction could possibly have supported the wall for a long time. Here is an example; oxygen-poor and humid environment slows down decomposition of organic material. 

Quote

Houses were built on top of old layers

Approximately every hundred years since Queen Kristina’s sluice in the 1600s, Slussen har been remodeled with new traffic solutions. When the subway and the traffic roundabout was built in the 1930s, it was excavated mostly manually with shovel. Old layers of human activity are therefore left relatively unaffected. New houses and streets were simply built on top of earlier remains.

Garbage with preserved fragrances

With the city’s founding in the 1200s came the garbage. It was thrown not only out on the streets but emptied in the outskirts. Planned ground fillings with old boats, piles and soil were mixed with household waste and sewage. The oxygen-poor and humid environment makes objects from these laysers often surprisingly well preserved. Thanks to the preserved garbage we learn a lot about how people lived in earlier times and also how it could smell!

https://medeltidsmuseet.stockholm.se/in-english/exhibitions/slussen-below-the-surface/

 

[Sensei]

Existing and old abandoned mines, or natural caves, can cause damages like cracks on buildings existing on the surface, when their tunnels are collapsing.

 

[studiot]

1 hour ago, Sensei said:

Existing and old abandoned mines, or natural caves, can cause damages like cracks on buildings existing on the surface, when their tunnels are collapsing.

 

 

2 hours ago, Ghideon said:

 

Was the wall built on top of parts of an older structure? An old wooden construction could possibly have supported the wall for a long time. Here is an example; oxygen-poor and humid environment slows down decomposition of organic material. 

 

Yes some old human relic may be uncovered. I'm sure there are no natural cavities in the gravel and marl of the river bed, which is in the middle of its flood plain extending at least half a kilometre on both sides.

Ghideon,

I remember the refurbishment of the old (13 century) packhorse bridge in Trowbridge, which I underpinned in the 1970s.

A surprise then was the old Roman oak foundations (piles and cross beams) that were in better condition that the stones of the walls (Bath stone).
It was extremely difficult to remove these oak foundations, jack hammers etc just bounded off, unlike with stones.

Edited August 8, 2018 by studiot

[Ghideon]

 

3 hours ago, studiot said:

The failed section of wall appears to have gone down and moved to the left, parallel to the line of the riverwall

Guesswork from what we have so far: If there was an old wooden wall built before the current one, is it possible that the stone wall was build at exactly the same location? Since the wood under water may have been well preserved it was used as a foundation. Now, after hundreds of years, the old wood is decomposed and cannot support the stone wall. This caused the stone wall section to move almost straight down.

[Ghideon]

On 2018-08-08 at 11:01 AM, studiot said:

hopefully we will find out when they pump out the cofferdam

I am curious; did you find out what happened?

[studiot]

2 hours ago, Ghideon said:

I am curious; did you find out what happened?

You may have noticed I have been away on holiday.I went down last evening to observe progress and took some more photos.

The investigation has not yet revealed the cause.
During the last couple of weeks, they have completed the cofferdam and knocked a small hole into the crack.

What can be seen is that the weir and the sound section of wall is sat on a concrete bench, now dry in the first new picture, taken across the river.
The failed section is at the back and left of the picture.
No movement is evident in this part of the structure.

 

The end of this bench is shown in the new closeup of the failure.
This also shows that coursing lines of the failed section suggest it has rotated anticlockwise relative to the section on the bench, which has horizontal coursing.
However the coursing in the triangular counterfort section of wall is still horizontal.
The wall is constructed from blue lias blocks.
A few of these that have been broken out are lying in the bench, not shown in the picture.
The lias appears sound.
Removal of the blocks makes the crack appear wider (perhaps it has also been cleaned out).
The removed blocks came from the upper part of the wall, so I'm not sure what information was gained there as there is no subsidence behind the wall.
There is no evidence of investigation in the river bed yet.

[Ghideon]

Thanks for the update! +1

[Endy0816]

What's the metal(?) plate alongside the crack there?

Edited August 23, 2018 by Endy0816

[studiot]

6 hours ago, Endy0816 said:

What's the metal(?) plate alongside the crack there?

It's a nameplate screwed on the wall

I can't get there to read it as it is old and corroded, but it says that some Lord Muck built/opened the installation etc.

[DrP]

Any nearby fracking sites?

There have been some minor earth quakes in the UK over the past couple of decades also. I have felt 2 in the last 15 years. Last one I was quite close too and it woke me at 3am. I'm not saying this is the cause but just enquiring as they could have contributed possibly.

 

[studiot]

Good questions but this is not in the (former) mining, earthquake or fracking part of Somerset.

This is a near flat river flood plain at the lower end of its course.
Without the system of weirs this river would be tidal.
The ground is Keuper Marl on top of a layer of river gravel.

[Klaynos]

Where in Somerset is this? (I didn't realise you were that close)

I'm wondering if the vertical velocity from the Weir might have caused a very gradual undercut. Which possibly together with the recent very dry warm spell causing some movement above the low water line has caused the drop. The fault in the wall following some weak point, possibly a repair or showing how it was built in sections?

 

[DrP]

The possibility of undercut or water seeping under the thing and softening the earth below the foundation was going to be my next question Maybe, finally after a few hundred years.  That wall was built before the USA probably.  I have seen 500 year old bridges.  

Edited August 23, 2018 by DrP

[Ghideon]

Some thoughts: The bottom of the crack is very close to what looks like a solid construction not covered by water. Is the part of the wall (that has not failed) resting on top of this structure? See blue arrow in picture. The red arrow shows some sort of structure under water. Does that continue in under the failing part of the wall? If so, is the gap at the green arrow a crack? Does it look like the thing underwater at the red and green arrow has moved down?

[studiot]

Thanks for the continued interest folks.

The quick sketches may help.

The river channel generally varies from about 15 metres to 25 metres in this area.
At the weir it turns through nearly a right angle (about 75 - 80 degrees) and impounds a total of about 3metres of water.
So the water flows from right to left behind the weir at the top of the diagram and from top to bottom once over the weir.

There is a further offtake, through a sluice, to feed an old mill stream at the top left corner, from the pool behind the weir.

The failed flank wall is at the left hand end of the sketch.
As far as I can tell there is no concrete retaining wall behind it ie it is not a facing wall.
The failure is on the outside of the 75 degree bend and the water flows much more slowly here, and shoals on this side,
As previously noted in the first pictures.

The inside of the bend sees the main flow and a boxed spillway takes this beyond the weir.

 

Section AA shows the long bench previously referred to that supports the main weir wall throughout its length and terminates in flank walls at each end, the left hand of these having failed as detailed with a nearly vertical diagonal crack at the drop off from the bench.

Yes, Ghideon, that is shown under your blue arrow and has currently been stanked off in the dry but is normally under water.

I'm not sure if the object referred to by your orange and green arrows is original or something put there by the workforce temporarily.
It is certainly in the position you might expect to see a scour pool from the water tumbling over the bench in the photos.
But I also note that the water is at its most sluggish here and would expect to see scour pools more towards the other end (but don''t)

As noted hopefully riverbed investigations are now proceeding and I will report when something is known.

Klaynos this is the River Tone in Taunton.

 

 

[dimreepr]

All we can know for sure is, either: the right side was thrust up at that specific point, or the left was undermined at that specific point. I'm sorry Studiot but I got my pedantic head on ATM. 

[Ghideon]

5 minutes ago, dimreepr said:

All we can know for sure is, either: the right side was thrust up at that specific point, or the left was undermined at that specific point

If I put my pedantic head on for a while then your comment dimreepr actually seems like scientifically sound reasoning and not a joke  One reason why I asked about freezing in my first post. In the area where I grew up it was not uncommon for subsurface ice to push stones upwards: 

 

ref https://www.skogskunskap.se/vagar-i-skogen/drift-och-underhall/slitage-nedbrytning-och-skador-pa-vagen/skadekatalogen---vad-kan-vi-gora-at-vagskadorna/

 

[dimreepr]

42 minutes ago, Ghideon said:

If I put my pedantic head on for a while then your comment dimreepr actually seems like scientifically sound reasoning and not a joke

The pedant never really jokes. 

[studiot]

1 hour ago, dimreepr said:

All we can know for sure is, either: the right side was thrust up at that specific point, or the left was undermined at that specific point. I'm sorry Studiot but I got my pedantic head on ATM. 

A true pedant would include the third possibility of both.

1 hour ago, Ghideon said:

If I put my pedantic head on for a while then your comment dimreepr actually seems like scientifically sound reasoning and not a joke  One reason why I asked about freezing in my first post. In the area where I grew up it was not uncommon for subsurface ice to push stones upwards: 

 

ref https://www.skogskunskap.se/vagar-i-skogen/drift-och-underhall/slitage-nedbrytning-och-skador-pa-vagen/skadekatalogen---vad-kan-vi-gora-at-vagskadorna/

 

 

Thanks for the pic but Somerset has not been periglacial since the last ice age.

https://www.google.co.uk/search?source=hp&ei=Y9x-W-OnLOTIgAab47uoBg&q=periglacial+environment&oq=periglacial&gs_l=psy-ab.1.4.0l10.932.3458.0.7670.11.8.0.3.3.0.446.1052.0j3j1j0j1.5.0....0...1c.1.64.psy-ab..3.8.1138...0i131k1j0i10k1.0.0qSFFbDNCZU

We sometimes get clay heave due to clay expansion on taking up water after a (hot) dry spell or subsidence due to clay shrinkage in the dry spell.

And Keuper Marl has a high clay content/ is a form of clay.