Black Bridge, Nuneham: southern abutment failure

[whole diary]

[Electric train]

So maybe they do intend to did a big trench and have a look at exactly what is there, and if it's not what they like replace it.

Site investigations into ground conditions involve a specialist geotechnical company drilling down and collecting core samples.  Trench digging is not required, which is just as well because you could be going down 25 metres or more.  The cores are laid out and the materials are identified and logged to record the depths at which they occur.  The strength and other attributes of the materials found are analysed and noted. 

This information, along with the load to be supported, is used to design the piles.  It is not especially complicated and the conditions at Nuneham are unlikely to be abnormal for the Thames Valley.     

I suspect NR^» (Network Rail - home page) has sufficient geology data to base their "permanent" fix on otherwise they would be going down the route of a short term fix.  BR^▸ (British Rail(ways)) had its own soil mechanics that sampled such places, as this was a known failing structure NR Structures and Geotech Route teams will have had it surveyed and bored in the past.

The point I took from the video on Twitter was they attempted a fix with a polymer that did not work in this case.

There will be and possibly already is a review of where the process has fallen down

[eightonedee]

I was also puzzled by the comment about "heavy clay" making the work difficult. I'd have though that was the best sort of clay! Clay rather than gravels might be a bad thing, of course. So maybe they do intend to did a big trench and have a look at exactly what is there, and if it's not what they like replace it.

It's not just the subsoil that the piles have to be laid in. There's also heavy construction equipment to be got on site, and wet heavy clay is probably the last thing you want to be moving a large piling rig across. We have just had an exceptionally wet March, with much of the Thames between Abingdon and Reading on extended flood alert, and although February was dry, September through to January were wet too.

This site is probably the furthest from a metalled road that any Thames railway bridge is situated (unless any of our knowledgeable forum know any different), so the logistical challenges will be significant.

[CyclingSid]

For those with interest in the geotechnical side, and specifically boreholes I had a rummage round the BGS site

https://mapapps2.bgs.ac.uk/geoindex/home.html?layer=BGSBoreholes&_ga=2.172622368.2043912742.1681400817-2822824.1681400817

and then search for Culham. Lots of boreholes in the area, mostly less than 10m deep and to do with the RADLEY ASH DISPOSAL SCHEME for Didcot power station (?) probably confirmed by a "Confidential" borehole either end of the bridge. I would imagine there is other data, but not deposited with BGS.

There are some deeper boreholes/wells in Abingdon itself but appear to be connected with domestic water supply. To my untutored eye they suggest that you possibly have to go down 100ft to solid rock

[paul7575]

Another Network Rail brief on Twitter. Mentions they’re going to build a structure in the river to support the spans, and they expect to pile to a depth of 25 metres:
https://twitter.com/networkrailwest/status/1646540219959721987

[eightonedee]

Lots of boreholes in the area, mostly less than 10m deep and to do with the RADLEY ASH DISPOSAL SCHEME for Didcot power station (?)

The disused aggregate pits on the west side of the railway were partly back-filled with ash from the power station - next time you are passing on the train (if you remember by then!) you will see, depending on the water level, that there has been some fine material deposited. I think some of the pits have also been back-filled with landfill from London, which used to arrive by train, and that part of the old Abingdon branch might have been left in place for a while for one or other of these (but I might be mistaken!).

[stuving]

Another Network Rail brief on Twitter. Mentions they’re going to build a structure in the river to support the spans, and they expect to pile to a depth of 25 metres:
https://twitter.com/networkrailwest/status/1646540219959721987

If you compare the state of the abutment in that video (00:50-1:00) with last summer (from LinkedIn, Post #75) you get a striking impression of how much the abutment has dropped since then.

[ellendune]

I was also puzzled by the comment about "heavy clay" making the work difficult. I'd have though that was the best sort of clay! Clay rather than gravels might be a bad thing, of course.

From a bridge foundations point of view clay is fine as long as you are away from a slope and you get get well below the groundwater level so there is no risk of it drying out (Thames valley clay it shrinks if dries out then you have a problem). 

The problem is a wet March and getting equipment across a field.  However, the land immediately by the river often has sand and gravel overlying the clay which would be ok for moving heavy equipment.

They might of course be able to move a piling rig on a barge - especially if the are piling in the river!

[ChrisB]

Wasn’t that mentioned as the plan in the EA river closure notice was hich was referenced here earlier in the thread?

[stuving]

Wasn’t that mentioned as the plan in the EA river closure notice was hich was referenced here earlier in the thread?

The EA notice referred to a "pontoon", i.e. something floating. NR^» (Network Rail - home page)'s video says they will build "a pile-foundation and a temporary trestle so we can support the steel structure and hold that level" "in the middle of the Thames". I assume "middle" is not to be taken literally, but just as meaning away from the bank. That in itself is a bit odd, in that if you just want to substitute for the failed support while not allowing the span to move you don't want to support it near mid-span. Keeping its weight on the far bearing (mid-river) is what stops that and moving.

The other question (alluded to by ellendune) is how to pile into the river bed. Certainly marine floating piling rigs exist, but they could not get into what is in effect a narrow canal. "Piling" on canals almost always means sheet piling of banks, and the rigs to do that are usually small, and  mounted on little diggers. I guess the method chosen will be whatever is quickest, in this case, even if it involves exotic machinery.

And what about piling for the abutment "probably 25 metres deep"? The mid-river pier has foundations on gravel eleven feet below the river bed, dug inside a coffer dam ("of the ordinary type"). I guess the river is no more than 11 feet deep, so that's less than 8 metres from ground level. So would this really mean reaching the same gravel bed and sinking the piles more than 15 metres into it?

[CyclingSid]

Piling to 25 m, as ellendune says fine if they are happy to rely on clay, as suggested in my previous posts boreholes in the area suggest 30+ m to solid rock. I hope they have had access to the Confodential borehole data (probably means you have to pay for it) at either end of the bridge.

I am amused by them closing half the river, means the "floating gin palaces" will need to improve their steering.

I hope they will do a really thorough investigation of the central pier and the northern side of the bridge, would be embarrasing to have to close the railway again if problems appeared there.

[Electric train]

Another Network Rail brief on Twitter. Mentions they’re going to build a structure in the river to support the spans, and they expect to pile to a depth of 25 metres:
https://twitter.com/networkrailwest/status/1646540219959721987

Re watching this video, he states that a temporary "piled structure" will but in the "middle of the river" this will mean closing one of the channels the EA will agree to this the adverse PR^▸ (Public Relations) would not be good.

In the video he does mention the permeant piles down to around 25 meters, that's not to far off of the 100 ft mentioned in other posts

[ellendune]

The question in my mind - and it affects the nature of the temporary structure in the river and the equipment needed to build it - is whether it is merely to support the dead weight of the bridge while the abutment is rebuilt (my assumption) or whether it has to allow the bridge to reopen while the permanent abutment is built.   

Small piling rigs are widely available that can get inside buildings but I am not sure how suitable they would be for this application. 

Another option to move plant to site is to bring it as near as possible by rail and building a short haul road along the line of the railway. 

[ChrisB]

Another Network Rail brief on Twitter. Mentions they’re going to build a structure in the river to support the spans, and they expect to pile to a depth of 25 metres:
https://twitter.com/networkrailwest/status/1646540219959721987

Re watching this video, he states that a temporary "piled structure" will but in the "middle of the river" this will mean closing one of the channels the EA will agree to this the adverse PR^▸ (Public Relations) would not be good.

In the video he does mention the permeant piles down to around 25 meters, that's not to far off of the 100 ft mentioned in other posts

The EA Have already agreed to close one side of the river, initially for a monrg - see the closure notice referredced in previous post

[Electric train]

The question in my mind - and it affects the nature of the temporary structure in the river and the equipment needed to build it - is whether it is merely to support the dead weight of the bridge while the abutment is rebuilt (my assumption) or whether it has to allow the bridge to reopen while the permanent abutment is built.   

Small piling rigs are widely available that can get inside buildings but I am not sure how suitable they would be for this application. 

Another option to move plant to site is to bring it as near as possible by rail and building a short haul road along the line of the railway. 

In the Twitter video Stuart Calvert says the temporary structure in the Thames is just to support the bridge while the new abutment is constructed.  I am guessing they will demo the old abutment dig out part of the existing embankment to give them space to work, I'm not Civil Eng but what I've seen of piling rigs for 25 meter piles they are big

[stuving]

.. Yesterday's BBC^» (British Broadcasting Corporation - home page) piece showed showed some aggregate corraled in sheet piles as where the temporary support would go. So maybe piles are not involved there, unless it's just something for the piling engine to stand on...

That "new ground" within the piles is in fact some way downstream from the bridge. So it looks more like exactly what Paul Clifton called it - a jetty - for access onto a pontoon, presumably.