Basement excavation - safely retaining earth banks during works

[Ed_]

I am excavating a basement and due to the confined plot there is not enough room to batter back at what I believe to be a safer battering angle, which seems to be 45 degrees at best from any seeminly scientific source I can find, therefore I had imagined I had to create a retaining wall.

Said retaining wall, in piles, is going to cost in the region of £30k.

 

What is giving me pause is that almost every picture I see on industry website, e.g. waterproofing specialists, shows bare earth battered at 60+ degress and seemingly without any safety measure, for example:

 

And then the neighbouring plot to mine have done almost the identical excavation to me, and left it like this:

 

I see how this could be safe whilst excavating with a digger, as no one is in the collapse zone, but how about doing any work under it?

To me, it seems unlikely the bank would suddenly collapse, it will probably be fine, but thats not good enough when the consequences could be death?

 

Just looking for views and experience on whether there are ways for a contractor either to safely build a retaining wall behind this or to erect shuttering for a basement, or whether my gut feel that it has to be a proper retaining wall or similar is correct.

 

Thanks!

 

[Mr Punter]

I have done a basement on a tight site and used a combination of semi contiguous auger piles and king posts with sleepers for the temporary works.  A lot of this is down to soil type.  Can you get your SE to design this, or suggest someone who is experienced at this?

[Ed_]

I've looked into kingpost, but at first study it looks like being no cheaper.

Still have to mobilise the piling machine, piling mat etc. Additionally probably need a crane because how else do you get an 8m long steel into a narrow hole, then need to pay for concrete for the panels and probably some sort of gravel to back fill the reverse side...

 

Seemingly everywhere i look people (professionals even) are doing this sort of thing without a retaining structure, are they just risking it or am I missing something? I don't intend to just risk it.

[markc]

As above, soil type is everything, then you look at the depth of excavation and what works will be done and proximity to an unprotected face which will all be included in your risk assessment.

[Mr Punter]

The piling firm we used a rubber duck excavator with an auger.  No piling mat or crane.  Someone welded bent rebar to the steels so they could be picked up and lowered by an excavator.

[Thorfun]

our lot just angled the banks back to a suitable angle. not sure how much science was involved in the angle but we didn't have any bank collapses. they did put plastic sheeting over it though to reduce the risk of rain causing soil slippage.

[Ed_]

4 hours ago, Mr Punter said:

The piling firm we used a rubber duck excavator with an auger.  No piling mat or crane.  Someone welded bent rebar to the steels so they could be picked up and lowered by an excavator.

Learn something new everyday, rubber duck excavator! 

 

Would you mind sharing details of the piling firm you used with me? I don't think I'm too far away.

[Pendicle]

We have a similar issue when digging for basement, the house we demolished had a basement but was not deep enough and only used for garden storage. I did but didn’t really appreciate how tight our build is until it came to sloping the ground to accommodate the basement build. But we are in a slop, several slops in all directions, just to make things fun. 
we are heavy clay although we found some building sand had be used on the original fill in, which gave us some issues and had to dig wider to remove as this could have easily have fallen on us when working in our hole. We are still in our hole! 

Edited yesterday at 09:48 by Pendicle
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[Ed_]

@Pendicle How did you assess the slope angle?