Heavy plant- ground pressures?

[richy_B]

Hello,

wasn't sure the best place to post this.

 

In autumn I have a job working on a pond/vegetation project. All straightforward - to assist with reach we are going to use 8-10t excavator. An item has come up : there is a very large Victoria sewer that we have to cross. 

 

This sewer is said to be 9ft/2.5m diameter, brick built and 100 plus years old. It's part of the mains infrastructure and managed by Thames Water. The sewer is 12 metres below where we need to cross it. That seems huge and I'd imagine you'd need hundreds of tons to be a risk- but I'm not an engineer. Thames Water are reluctant to make an suggestion to its loading bearing for crossing it. 

 

I think we are all in agreement it seems highly unlikely but everyone is bit reluctant to be the one to make the call and put their name on it. If it did collapsed it would be a huge issue though as I'm told it serves 5000+ homes. And again its 12 metres below ground...

 

I was wondering about making some sort of basic bridge. Some trakmats a couple of metres either side of the sewer and maybe some beams or something - so basically at no point are we putting direct downward pressure on it. Hoping this might alleviate clients concerns. 

 

Any thoughts? Am I massively overthinking it? (feel like i am, but not a bad thing).

 

 

Edited June 27, 2020 by richy_B
edited for clarity.

[Haironyourchest]

12 meter below ground sounds very deep. Are you sure you didn't mean to write 12 foot?

What's on top of the sewer? Compacted clay, gravel? Cities are riddled with old sewers and 30 tonne lorries drive around on top of them all day long.

[richy_B]

1 hour ago, Haironyourchest said:

12 meter below ground sounds very deep. Are you sure you didn't mean to write 12 foot?

What's on top of the sewer? Compacted clay, gravel? Cities are riddled with old sewers and 30 tonne lorries drive around on top of them all day long.

Yep, 12 metres deep and 100 metres up the track it is 22 metres deep to take into consideration the local terrain. It hasn't been dug since they built it in late 19th century so no idea on what's there but I'm assume it's mainly London clay.

 

It is really deep and as you say cities are full of sewers - no where near as deep. I don't want to be the exception to collapse a massive sewer!

Edited June 27, 2020 by richy_B

[monkeybusiness]

There is no way in the world that a 10 tonne tracked machine   will have any effect on anything 12m underground. Any forces will be dissipated through the soil in the first couple of metres max. 

[topchippyles]

Christ what are you worrying about ?? 12 metres is a massive depth and as you say its never been disturbed. 

[richy_B]

1 hour ago, topchippyles said:

Christ what are you worrying about ?? 12 metres is a massive depth and as you say its never been disturbed. 

I'm not really worried about it but the ultimate concern is collapsing a sewer that could cost vast amounts of money in emergency works should anything go wrong. 

 

Clearly it's very deep and as monkeybusiness says - forces will be dissipated quickly. 12 metres is 4 storeys so huge. I am just fishing for information to properly risk assess and mitigate.  Anecdotal information although generally obvious/common sense is hard to use for large national clients. Trying to find some info I can reference. 

Edited June 27, 2020 by richy_B

[topchippyles]

6 minutes ago, richy_B said:

I'm worried about collapsing a sewer that could cost vast amounts of money in emergency works should anything go wrong. 

 

Clearly it's very deep and as monkeybusiness says - forces will be dissipated quickly. I am just fishing for information to properly risk assess and mitigate.  Anecdotal information although generally obvious/common sense is hard to use for large national clients. 

If your worried that much and you can locate the actual sewer then just bridge it with some heave duty ramps. 

[richy_B]

3 minutes ago, topchippyles said:

If your worried that much and you can locate the actual sewer then just bridge it with some heave duty ramps. 

That could be a solution, if there is indeed a problem. I'm just trying to find something to refer to at this stage. Can't find much via Google in terms of weight displacement on soils etc. Althought not entirely sure I'm phrasing my searches correctly. 

Edited June 27, 2020 by richy_B

[monkeybusiness]

Knock yourself out in here!...

 

Just consider the immense weight of a 12m tower of soil, and how insignificant a little digger on long, wide tracks would be on top of all of that! 

Distribution of Stress in the Soil Study Notes for Civil Engineering : Civil Engineering

GRADEUP.CO

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Get the complete details about the stress distribution topic in soil

 

[topchippyles]

1 minute ago, monkeybusiness said:

Knock yourself out in here!...

 

Just consider the immense weight of a 12m tower of soil, and how insignificant a little digger on long, wide tracks would be on top of all of that! 

Distribution of Stress in the Soil Study Notes for Civil Engineering : Civil Engineering

GRADEUP.CO

---

Get the complete details about the stress distribution topic in soil

 

Like you say, How many tons of soil and clay 12 metres high already on top of it.