Woking - Basingstoke (near Hook) landslip (15/01/23)

[fgwrich]

It's much closer to houses than I realised. Was much (noisy) work done overnight?

Not sure regarding when the work is carried out, but yes. On the other side (where the 1960s slip was) is the Hook Holt Estate, one of the many new large estates springing up across the countryside around Hook (and A30) and just down the road from me.

 

[DelW]

Looking at the video, I imagine the areas either side are at far lower risk, having far less height from the base and being vegetated (with the binding benefit of root systems).

While some vegetation may help (though even that's not universally accepted), I don't know of a method for quantifying any effect it has. It certainly wasn't an input into the software I used to use for stability calculations, nor was it used in the hand calculations I carried out in pre-computer days.

 

[507020]

Following the recent infrastructure failure at Nuneham Viaduct, where are we with the last infrastructure failure on the SWML?

 

[swt_passenger]

Following the recent infrastructure failure at Nuneham Viaduct, where are we with the last infrastructure failure on the SWML?

I saw a YouTube update a few days ago. Work continues in either direction in addition to the original landslip area:

 

[Meerkat]

I watch that and think how expensive its going to get if age/weather/usage/better monitoring lead to significant mileages of embankment needing that kind of upgrade!
Could kill an uncommercial line or two.

 

[alf]

I remember a retired & justifiably venerated Western Region chief engineer writing that proper regularly checked drainage will ensure there are no land slips.
The wide flat south north valley near Hook has a suvstantial water flow which might be bottled up by the hefty east west rail embankment.

Incidentally the delays & costs at Hook would have been far greater if RSSB had won the day & the simple Southern third rail had been replaced with elephantine quad track steel gantries.

 

[PG]

I watch that and think how expensive its going to get if age/weather/usage/better monitoring lead to significant mileages of embankment needing that kind of upgrade!
Could kill an uncommercial line or two.

Could also kill any contractor that gets their sums wrong!

 

[Bigfoot]

I watch that and think how expensive its going to get if age/weather/usage/better monitoring lead to significant mileages of embankment needing that kind of upgrade!
Could kill an uncommercial line or two.

There are already a significant number embankments all over the network with remote geological movement sensors monitoring for movement. There are fairly regular alarms that trigger a visit from a geo engineer to check what is actually happening to the structure.

 

[swt_passenger]

Theres’s been another updated video today, it can immediately be seen they’ve removed yet another hundred yards of trees and shrubs from the embankment at the London end of the existing worksite. The culvert under the embankment carrying the River Whitewater is now visible, with a temporary bridge installed for contractors vehicles. I wonder how much further east they’ll eventually go?

 

[Meerkat]

I guess it is ‘cheap’ to do the whole embankment whilst they have the access and worksites set up.
You wouldn’t bet on the rest of it being built more securely than the failed bit.

 

[swt_passenger]

I guess it is ‘cheap’ to do the whole embankment whilst they have the access and worksites set up.
You wouldn’t bet on the rest of it being built more securely than the failed bit.

It also strikes me that although there’s never been a definitive answer, leaving existing trees in-situ doesn’t seem part of the solution here…

 

[zwk500]

It also strikes me that although there’s never been a definitive answer, leaving existing trees in-situ doesn’t seem part of the solution here…

I think the answer is hinted at given that new embankments and cuttings are not covered with trees...

 

[swt_passenger]

I think the answer is hinted at given that new embankments and cuttings are not covered with trees...

That’s what I’ve always thought but a few people have suggested earlier, both in this and a few other discussions, that trees help…

 

[zwk500]

That’s what I’ve always thought but a few people have suggested earlier, both in this and a few other discussions, that trees help…

Somebody pointed on another thread that origin of the idea that trees help comes from one section of one paper, where one type of very specific tree was found to have a positive impact in local cases.

Somebody else will like find the post first, but I'll link to it if I remember which thread it was on.

 

[Chris M]

Somebody pointed on another thread that origin of the idea that trees help comes from one section of one paper, where one type of very specific tree was found to have a positive impact in local cases.

Somebody else will like find the post first, but I'll link to it if I remember which thread it was on.

I too can't immediately find the post, but from what I remember reading here and elsewhere specific types of tree are beneficial in combination with specific types of geology and construction. Not on this forum, but I do recall reading somewhere that low-growing bushes are beneficial more often than trees are, but again not every combination of species and geology.

 

[zwk500]

I too can't immediately find the post, but from what I remember reading here and elsewhere specific types of tree are beneficial in combination with specific types of geology and construction. Not on this forum, but I do recall reading somewhere that low-growing bushes are beneficial more often than trees are, but again not every combination of species and geology.

Found it: https://www.railforums.co.uk/thread...leaves-on-the-line.246960/page-2#post-6186882

There was a very old engineer (I think pre-Victorian) who established that only a very small number of species of tree or bush (one I'm sure being a juniper) that helped stabilise embankments, but that was very quickly turned into "trees stabilise embankments" even though he'd also said any other tree makes it worse, and on a properly constructed embankment even the stabilising ones had no beneficial effect. That one bit, taken out of context was repeated time and time again. Now it's a different matter if you're cutting down trees leaving a substantial dead stump and root, which can rot to leave a void but I suppose that might be why these days you often see stumps allowed to resprout and stay alive.

 

[DelW]

I've carried out many slope stability analyses, both by hand and using software, and none of the methods or programs I used contained an input variable dependent on the presence or absence of surface vegetation. It's possible that there are now more sophisticated methods that do take account of it, but I was never aware of one that did.

 

[zwk500]

I've carried out many slope stability analyses, both by hand and using software, and none of the methods or programs I used contained an input variable dependent on the presence or absence of surface vegetation. It's possible that there are now more sophisticated methods that do take account of it, but I was never aware of one that did.

This doesn't mean it's a negligible or zero impact though, just that the analysis doesn't take it into account.

 

[DelW]

This doesn't mean it's a negligible or zero impact though, just that the analysis doesn't take it into account.

That's very true, indeed the whole analysis is really a gross simplification of the real world situation, which is why quite a large load factor (or factor of safety) has to be applied to the calculations .

 

[Deepgreen]

From the video it looks as though 'astroturf' is being installed along a huge section - is that the case?

 

[zwk500]

From the video it looks as though 'astroturf' is being installed along a huge section - is that the case?

It's difficult to tell but I think it's actually a porous or perforated geotextile material, which vegetation will be able to grow through.

 

[Taunton]

which is why quite a large load factor (or factor of safety) has to be applied to the calculations .

Is this a euphemism for "bits of the arithmetic which, despite our large fee, we have left out" ?

 

[WesternBiker]

That's very true, indeed the whole analysis is really a gross simplification of the real world situation, which is why quite a large load factor (or factor of safety) has to be applied to the calculations .

There's an interesting US study referred to in the Landslide Blog here - aimed at preventing landslides on natural as well as man-made slopes, so not quite the same situation, but it is nevertheless interesting. In particular, the author states that:

"...The results are interesting and perhaps surprising. For lower gradient slopes (20º and 35º in the model), tree planting increased slope stability. It should be noted that in the real world a 35º slope would be considered to be very steep. However, on steeper slopes (50º and 60º), tree planting reduced stability."

What slope is normal on embankments and cuttings?

 

[zwk500]

What slope is normal on embankments and cuttings?

Depends when they were built.

 

[Deepgreen]

I think the answer is hinted at given that new embankments and cuttings are not covered with trees...

Possibly but I wonder if that is to avoid deliberately having trees for autumn leaf fall issues rather than stability per se. Of course, after a few years trees will no doubt appear from natural seeding. Incidentally, are piles driven into the ground at various points within these rebuilt embankments/cuttings to provide anchor points, or is it just rubble and soil?

 

[Hellzapoppin]

I think that the remaining part of the embankment, once the slip material has been removed, is benched (stepped) and then compacted material then used to build the embankment back up again, then topsoil to finish things off.
The sheet piles change the angle of the embankment which again stabilises things. Looking at the vid it looks like soil nails have been installed either side of the slip site to stabilise the ground in those areas.

 

[DelW]

Is this a euphemism for "bits of the arithmetic which, despite our large fee, we have left out" ?

No, it's an acceptance that real-world soils don't always behave in the way calculations based on lab samples might lead you to expect. When I was being taught geotechnics, example questions typically started something like "In a clean sand with phi = 35° ..." or "In a firm clay with C = 50kPa ...". Then you go out on site, are told to check the stability of an excavation, and the only basis to estimate those crucial phi and C values is a borehole log that records something like "clayey silty SAND with some to much gravel and cobbles". So you take a semi-educated guess at the parameters and put a cautious factor on at the end to cover the substantial uncertainty. As long as it stands up, no-one can tell how close to the stability limit the slope was anyway .

There's an interesting US study referred to in the Landslide Blog here - aimed at preventing landslides on natural as well as man-made slopes, so not quite the same situation, but it is nevertheless interesting. In particular, the author states that:

What slope is normal on embankments and cuttings?

For modern roads I worked on, other than in chalk or rock, anything between 1:2 at best, 1:3 typical, 1:6 at worst (the latter in Gault Clay a.k.a. blue slipper, M25 in Kent), (26.6°, 18.4°, 9.5°). Railway slopes (other than new alignments) are generally much steeper.

 

[Deepgreen]

For modern roads I worked on, other than in chalk or rock, anything between 1:2 at best, 1:3 typical, 1:6 at worst (the latter in Gault Clay a.k.a. blue slipper, M25 in Kent), (26.6°, 18.4°, 9.5°). Railway slopes (other than new alignments) are generally much steeper.

Yes, it's much quicker and cheaper (in Victorian times with 'navvies') to dig out a narrow, steep-sided cutting. Not far from me are a couple of superb examples - the deep cuttings between Coulsdon and Merstham, which have cliff-like chalk sides (and which have needed huge amounts of stabilisation works over the decades).

 

[swt_passenger]

A fortnight on since the video I linked in post #579 and there’s another drone video update:

No major changes that stood out, still working away on the embankment reinforcement at the London end.

 

[swt_passenger]

The latest video update on the embankment repairs has been uploaded a few days ago. I wonder what their target date is to leave the worksite, that’s 6 months now and I’m guessing there’s a few months work still to do?