3m deep trenches - how to protect

[BartW]

Hi everyone,

I received a foundation design for our self build, and that stipulates 3m deep trenches with 2.5m concrete fill.

 

The amount of trenching is (in my opinion) very extensive. I tried to reason with the party responsible for the design of the above, but they are not willing to change this. 

 

I could have a separate dig at why it has been designed to be so deep and over the top, but it will not change anything. 

 

So here is my asking, how one would protect the trenches from cave-ins, and work the process of foundation pouring in view of the below:

 

 

 

 

 

 

 

[Conor]

You'll need to use trench boxes. But they are difficult to setup in narrow trenches, and you need people that know what they are doing. There are many practical challenges to digging that deep in a confined footprint. You'd likely need to dig, shutter, pour, temp backfill a section at a time.

 

A huge factor is the soil conditions and specific modulus. Your engineer/designer should be designing something that is safe and practical to construct - fundamental part of CDM. How are they proposing it should be constructed safely?

 

 

I would absolutely not be digging trenches that deep and narrow for purely practical and safety reasons. I would be pushing for a different foundation design - eg a ground bearing raft with micro piles, if needed. This may mean a different designer/engineer. 

 

 

 

Edited November 4, 2021 by Conor

[ProDave]

If this is at Graven Hill, it is not your problem to make it work.  But I would have thought piles might make more sense, it is obviously poor or made up ground.

[Moonshine]

those are deep, you must have crap ground!

[Thorfun]

I agree with the comments above and suggest speaking to a different SE. we had a 3.5m-ish dig for our basement and it's scary how deep that is! my mind couldn't comprehend it until I was actually there to witness it. I definitely wouldn't be wanting narrow 3m deep trenches if there was another solution. and there is always another solution!

[tonyshouse]

I have dug trenches that deep several times by booking inspector for lunch time and concreting them in the afternoon. 
 

DO NOT GO DOWN IN THE TRENCHES 

 

 

[BartW]

Hi,

Yes it is at GH, and it will be my problem, as I managed to get them to agree to use my own contractor for the golden brick.

 

I have an alternative, which would be mini piles, but they want £3k + VAT at this stage to redesign the foundation system altogether. So I am in a slight pickle. 3m trenches are simply not what I expected to be dealing with...

 

[Moonshine]

2 minutes ago, Thorfun said:

we had a 3.5m-ish dig for our basement and it's scary how deep that is! my mind couldn't comprehend it until I was actually there to witness it. I definitely wouldn't be wanting narrow 3m deep trenches if there was another solution. and there is always another solution!

 

From the over sight to the ground level of the shed on the left was about 4m, it was a worrying time, but thankfully our earth it fantastic. I couldn't imagine digging 3m down with 600mm wide our how you would do it, surely the digger straddling the trench would cause cave ins?

 

[Moonshine]

5 minutes ago, tonyshouse said:

DO NOT GO DOWN IN THE TRENCHES 

 

not even for an inspection? ???

 

 

 

 

[ProDave]

Did either of them come back up?  Alive?  Madness.

[BartW]

33 minutes ago, ProDave said:

If this is at Graven Hill, it is not your problem to make it work.  But I would have thought piles might make more sense, it is obviously poor or made up ground.

 

This is what I am proposing now. Plus, we are sitting in-between two plots so the space is quite narrow with no ability to remove / temporarily store spoil on the adjacent land.

 

I am curious to understand how the other contractor (their preferred subbie, i.e. RM) would do it. It almost feels that whatever Waterman have designed makes no sense from a practical standpoint.

 

I have got a competitive price for 27no 8m piles (just under £11k), but not only would I be paying £1k to the piling contractor for the design, but would also need to pay £3k to Waterman for them to do the same (on their paper)...

 

 

[Russell griffiths]

Dont do it, I did an extension at 2.6 m deep, it is the most ridiculous and dangerous thing going. 

Plus the cost of removing all the spoil. 

Get it piled, minimum mess, just right the cheque out and move on to the next bit. 

Now is not the time to be quibbling over 3 grand. 

 

You will spend £35-40 grand by the time you are up to dpc. It’s just what it is. Don’t even consider doing it in strip foundation. 

[BartW]

16 minutes ago, Russell griffiths said:

Dont do it, I did an extension at 2.6 m deep, it is the most ridiculous and dangerous thing going. 

Plus the cost of removing all the spoil. 

Get it piled, minimum mess, just right the cheque out and move on to the next bit. 

Now is not the time to be quibbling over 3 grand. 

 

You will spend £35-40 grand by the time you are up to dpc. It’s just what it is. Don’t even consider doing it in strip foundation. 

I worked out £43k + vat for the trenched with all spoil removal and drains, but this is flawed in the way that it does not address the deep trenches. 
 

I am around £34k for piles and ring beam, then got to allow for Jetfloor and drains on top + the £3k redesign fee, so very likely £47k - £50k. 
 

I suppose my only concern is that Waterman will come back with some ridiculous piles design that will again push things beyond feasible. 

[Cpd]

12 minutes ago, Russell griffiths said:

Don’t even consider doing it in strip foundation. 

Absolutely, it will turn out to be far more work than you might imagine, I have just finished digging a pipe in at 2.5 meters over 50m in length and the bloody sides were falling in by the time I got to the other end…… track back with the digger and dug it out again….. and again as the digger constantly threatened to subside into the trench….. what a nightmare. My ground conditions were fairly good…. The pile of soil was alarmingly big. 

[Temp]

Might be worth asking Building Control what they expect/accept on neighbouring plots? Ask if any houses went to 3m or if they went for piles how deep did they go.

 

Our BCO allowed slightly shallower trenches than the engineer specified. House down the road went for piles and they joke there is an Australian using the other end to hold up his car port. 

 

[BartW]

35 minutes ago, Temp said:

Might be worth asking Building Control what they expect/accept on neighbouring plots? Ask if any houses went to 3m or if they went for piles how deep did they go.

 

Our BCO allowed slightly shallower trenches than the engineer specified. House down the road went for piles and they joke there is an Australian using the other end to hold up his car port. 

 

Trouble with Graven Hill is that:

- they are responsible for the foundation design and indemnity, or their counterpart being Waterman Ltd

- no deviation is therefore allowed from what’s drawn

- neighbouring plots are dug @ 2.5m deep, but only two storey. Ours is three

- ground is indeed made up

 

Plots down the road are built on piles, but I would imagine much deeper than the 27no @ 8m as proposed by the independent structural engineer based on the soil survey on site

 

[Iceverge]

It seems like hella trouble for strip foundations. It dawned on me that you're half way to making a basement. Is this an option? 

[Gordo]

Looking at the design with heave protection to inner face. I suspect there are or were trees in close proximity for this? Installing this heave protection is not a safe operation. I would refuse to install this heave protection and observe others do same. Unless heave is very likely

 

Deep trenchfill foundation are quite common. I have seen some even deeper but I understand around 4m is point where piles become economical. Just dig and pour dry lean mix concrete as you go. Pack down lean mix with digger and it will shore up excavation. Cast a reinforced concrete foundation on top. Be sure to excavate plumb and best widen trench out to at least 900mm to allow for inaccuracies.

 

NO ONE IS EXPECTED TO ENTER TRENCH WITH THIS TECHNIQUE FOR FEAR OF COLLAPSE.

 

Engineers assume a level knowledge and competence in design. As this is a well established practice they obviously didn’t feel the need to provide a risk assessment.

 

no not do it like this lolol use LEAN MIX DIG AND POUR

 

Edited November 5, 2021 by Gordo

[Moonshine]

20 minutes ago, Gordo said:

Looking at the design with heave protection to inner face. I suspect there are or were trees in close proximity for this? Installing this heave protection is not a safe operation. I would refuse to install this and observe others do same. Unless heave is very likely

 

NO ONE IS EXPECTED TO ENTER TRENCH WITH THIS TECHNIQUE FOR FEAR OF COLLAPSE.

 

Engineers assume a level knowledge and competence in design. As this is a well established practice they obviously didn’t feel the need to provide a risk assessment

 

That is a very good point, @BartW i think that you should refuse this design as under CDM regs the design is not designing out risks, how do they expect heave protection to be suitably installed without getting into the trench.

 

See this thread with some dicey imagery of a guy in a deep trench and heave protection.

 

 

[Conor]

When I woke up at 3am worrying about my own build, this popped in to my head as well. I echo what @Moonshine said about CDM. I'm a designer and engineer by trade. Safe constructability and maintenance of works is the number one consideration when preparing a design. 90% of my experience with laying pipelines in deep trenches. I know all about how things can go wrong. I'm really worried that there is a reputable firm proposing a design that is so blatantly dangerous and cannot be undertaken safely (or at least they have not shown how it should be undertaken). I would be asking them directly how they have come to this design and how it incorporates CDM requirements. I could never propose a design like this. It requires somebody to enter the excavation. There's no room for proper shuttering/trench support and they've not shown how the sides of the protection are to be secured and how machinery and personnel can operate around the open excavations. It's their responsibility to design this. If there was an accident, I'd be expecting the designers to be prosecuted. You, as the client also have responsibilities, so do not accept this design.

 

I'd speak to the neighbours and see what they did. But sounds like you are tied to this firm?

[Iceverge]

A man was killed near us not too long ago in a trench.

 

Can you reverse the decision in using your own contractor. On such a large project the vendors must have established an efficient and safe method to do it themselves. 

[BartW]

Hi,

thanks everyone for advice. 
 

I managed to fix up an appointment with GH on Tuesday, and will air all my concerns then. 
 

From all of the comments I figure that the mini piles is the only, and quickest way. 
 

One thing that would have added to the list of challenges is that we are sitting amongst other plots, whose house are both being built up to a metre from the boundary line. That would make things even more complicated. 
 

Let’s see what I manage to establish. 

[scottishjohn]

maybe me,could be totally wrong -- but why not a concrete raft if ground is so bad .

a house was built here next to the river nith  as in 30ms away from the tidal nith -- very soft as it was really just sand +clay -which will never dry out-

 the solution dig down and insert 1m square blocks of polystyrene with rebar tying them together --then concrete on top of that --been there for 20 years and nearly gets flooded sometimes -- but it ain,t moved -

I reckon the Buoyancy of the blocks is floating the house  

 

 

[saveasteading]

Piling is favourite. Don't hesitate to contact several companies as they have different kit, processes and overheads.

I would phone and ask for their Estimator or Sales Engineer, who might be the same person. 

If it really doesn't suit them they will likely give you another suggestion.

 

Also there are driven precast, driven in-situ, bored in-situ, and gravel piles to consider. 

 

If other neighbours wanted the same at the same time, then it could help the cost a great deal, in the arriving ('mobilising')  and tidying up costs.

 

Watch for the exclusions. They will want and need good access, hard standings, water, and someone else to clear up the leftover concrete.

then you have to cut back the heads of the concrete too.

 

If I had to guess I would say the best value will be from either a small, local piling contractor, or a driven pre-cast specialist.

Precast can include tidying the tops off and installing beams to your plan, and I would suggest getting a price for that.

[LSB]

On 04/11/2021 at 17:08, Moonshine said:

 

not even for an inspection? ???

 

 

 

 

that gives me the heeby jeebies