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 September 5 by Pendicle
Spelling

[Ed_]

@Pendicle How did you assess the slope angle?

[Pendicle]

The first photo shows the front south facing trees behind security fence, they have TPOs which is why the fence is there to protect them. The second shows the north back garden trees, which also hides a 45 degree slope of 9m which then goes down to a flat area of 10m, it then goes down another 45 degrees for 6m and then flat for 5m. Our farmer digger operator tracked a path for us for his digger and us after the photos were taken to the bottom of our hole, he also created a secret path behind the trees which leads to the flat garden but also able to access what will be the entrance to our basement. The skill of his digging and navigate his digger in and out the hole and moving the soil was amazing to watch. 

[Gus Potter]

On 04/09/2025 at 14:28, Ed_ said:

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

This is a great question. 

 

If you search about on BH you'll find posts by Geotech Engineers, but someone who is much active is @saveasteading 

 

But here is a rough rundown on things, I'll put myself in the shoe of acting for a Contractor designing their temporary works.. we want to dig a big hole, build something in it and back fill without anyone getting hurt. 

 

We refer to an old addage called "stand up time". Which means how long can we keep the excavation open for before the sides etc start falling in. 

 

Now in say chalk the stand up time is often good. Chalk is graded a bit like a rock where we look at fractures and where water might be getting in that loosens the chalk. This is important as we dont want big slabs falling into the excavation. @saveasteading knows a lot about this so I'll stop there. In summary though, we can often dig a big hole in a Chalk soil, do what we need to do and go home, provided we control the ground water. 

 

Clay soils are a different animal. Lots of soils are a mixture of things but let's just stick to a pure Clay soil. Now we know from gardening that clay is "sticky". But if you dry it out it can turn to dust, almost like a sand and it fractures, starts to fall apart.. like your garden soil cracking. 

 

When we dig a hole in a sticky clay soil you'll often see that the sides don't fall in right away. But after a few days/ months the sides do fall in. As Engineers we might call this the "undrained soil strength" (right after digging the hole) and the "drained strength" once the clay starts to dry and loses its stickyness, called the cohesion. 

 

In summary what the Contractor Engineers do is assess all of this and find a way of executing the work safely at minimal cost.. 

 

That is why you see batters at different angles and weird looking stuff. Each site is different. 

 

[Ed_]

Fabulous @Gus Potter, been scratching my head for weeks over this one and don't think I'd have ever worked it out myself.

 

Looks like a temporary works engineer will be my next call.

[Gus Potter]

47 minutes ago, Ed_ said:

Looks like a temporary works engineer will be my next call

Hang fire for now before you incurr professional cost until more folk from BH chip in.

 

You can do a lot to help yourself by providing a bit more info if you feel able. Main thing is BH is not a competition, there is no such thing as a daft question! Don't worry about abbreviations, the lingo or that. 

 

You can look at it this way.. you provide info, some folk on BH can chip in for free and have fun sharing what they know. I like to teach so enjoy trying to turn what can be  very dry SE stuff into something that folk can benefit from. When I did my self build the internet did not exist! Can you imagine, it was onl;y 30 years ago roughly. 

 

BH members get to learn from your journey, then you are more informed and can ask you SE / Goetech folk the right questions.. everyone is a winner. 

 

Now I post on and off on BH but there are some clever Geotec guys lurcking and a good few retired CE's SEs and Qs's .. if you spark their interest it would be great! 

 

Then @ETC can chip in with 20- 30 years experience of Architectural input... FFS you have one a watch!

 

Now the seasoned self builders chip in and contractors.. you literally have access to thousands of years of man  hours experience on BH.

 

So gather some thoughts and then see if your SE / Achitect can take all this in their stride, you are just digging a hole after all.

 

That said you are doing a basement @Alan Ambrose is a good thread to follow. 

Edited Sunday at 23:35 by Gus Potter

[Conor]

We batted back to about 60degrees. This was based on an assessment of the soil by the SE following a trial hole and a determination of soil modulus. A key thing to remember, is that weather and time are enemies of a stable slope. To help mitigate this you can employ various methods to stabilise the slopes. I chose to use woven membrane, overlapped and staked in to the ground, lapped under compacted stone at the top. At the bottom we had a sturdy catch fence to stop any material to spill in to the working area. No issues. through a wet winter. One corner was particularly wet with a constant flow of water the pulled out a bit of clay. had to be cleared out a few times.

 

Also, critically, an exclusion area at the top of the slope so vehicle or material weight aren't too close to the top edge. 

 

 

Edited Monday at 07:51 by Conor

[saveasteading]

9 hours ago, Ed_ said:

temporary works engineer will be my next call.

I don't what one of them is. 

A Civil Engineer can assess the ground and the stability of an excavation.

But then there are the options and costs involved. That's a specialist contractor with the Engineering and cost knowledge ... and there may be several options. 

1. Dig a big hole to work in. Build  basement and fill the surrounds back in.

2. Piling on the perimeter. Temporary that comes out again. Or permanent and it becomes part of the construction. 

 

1. Is simplest IF you have space and time. You can use a general contractor.

 

2. Is generally specialist and more ' big time'.

 

Question 1. What is your ground? As in the photo referring to neighbours? Is that actually it?

 

[Ed_]

3 hours ago, saveasteading said:

What is your ground? As in the photo referring to neighbours? Is that actually it?

 

No, this actually is the neighbour's, not me trying to cover up! 1m made ground over stiff clay (site of a former brickworks, 100 years ago), but with a severely sloped section that probably needs to be levelled for a piling rig to work, just out of shot to the left on my picture.

 

3 hours ago, saveasteading said:

Or permanent and it becomes part of the construction. 

 

This is my new focus, seems to me if I need to pile why not weld them up (or similar) and that is my boundary. Ring beam on top to put the rest of the house on and insulate inside. I'd been avoiding this as it seems like a retrofit basement method and i'd thought that implied high cost, but it could be the best solution for my constraints. 

[saveasteading]

How close did the neighbour come to your boundary?

How close to the boundary do you intend to build?

 

Why do you want a basement? It will cost about double £/m2, maybe more, be dark and carry more risk.

During construction it will hold all the rain like a tank.

[Ed_]

I'm 1m to the boundary. The current neighbour building is 2.5m and new ones will be 1-1.5m.

 

Many reasons for a basement:

More space - urban plot so footprint limited.

Ground slopes- front is 2.5-3m above back, basement spans this slope so GF is ground level at front and basement is ground level at back. 

I have made ground with possibility of a slightly mobile slope so would likely have needed deep foundations anyway.

 

As it is walkout it both becomes cheaper to build and more functional than a traditional basement. Hopefully.

[Ed_]

On 07/09/2025 at 23:21, Gus Potter said:

 

When we dig a hole in a sticky clay soil you'll often see that the sides don't fall in right away. But after a few days/ months the sides do fall in.

 

About 2.5 weeks is the stand-up time it appears, it has now started to collapse!

 

Nice to know I wont be spending money for no reason.

[Gus Potter]

On 05/09/2025 at 10:31, Pendicle said:

We are still in our hole! 

Very impressive! 

 

Look forward to the point when you start installing the structure.. going up instead of down. 

 

Fascinating looking project. 

 

 

[saveasteading]

7 hours ago, Ed_ said:

About 2.5 weeks is the stand-up time it ap

I've just been cutting back a clay bank by spade. Dry and hard on the surface but wet just 150mm back. After a week of warmth and wind, the new surface was crumbling, and cracks appearing. For some reason it seemed to be forming bricklike shapes and coming loose in big  lumps.

[Gus Potter]

28 minutes ago, saveasteading said:

I've just been cutting back a clay bank by spade. Dry and hard on the surface but wet just 150mm back. After a week of warmth and wind, the new surface was crumbling, and cracks appearing. For some reason it seemed to be forming bricklike shapes and coming loose in big  lumps.

Clay soils are probably one of most unpredictable materials that we encounter on self builds.  The soil mechanics books we commonly use as designers refer to undrained and drained shear strengths, the drained shear strength is commonly used by CE's like @saveasteading for earth works design when say designing a dam. 

 

However, I have a pal that did his PHD and he developed, and I think got a patent, on how you measure the "latent cohesion" of clays when drained. In my earlier post I tried to explain the difference between drained and undrained strength of clay.  Although my pal was an Engineer he clocked that the ceramic manufacture's that make toilet bowls/ sinks etc were having a high failure rate in manufacture.. and thus he got his research funding on partly this basis. 

 

Basically (I think) it works like this. Even if you dry out a clay it still binds together due to the behavoir of the minerals and the molecular interaction. 

 

@saveasteading the brick like shape I think is driven by the way the clay is deposited in layers coupled with the above. 

 

Incedentally my pal now runs a specialist Geotech Company and gets most of his work from the major UK developers designing ground improvement and lime stabalisation schemes. I can see his work as on the outskits of where I live, he has desinged the groundworks for a few thousand houses in tricky ground. 

Edited Wednesday at 23:13 by Gus Potter

[Gus Potter]

On 04/09/2025 at 14:28, Ed_ said:

I am excavating a basement

Do you fancy sharing your ground investigation results. Over lay that with your site boundary and then give us some section details so we can see what you want to build in the excavation. A few good and clear drawings are worth thousands of works. 

 

Once we know a bit more then you'll get more targetet advice and suggestions.

On 04/09/2025 at 15:28, Ed_ said:

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.

You'll probably get the most impartial advice on BH. Yes, it may be that there is a way of executing your design with limited support but we can't comment properly unless we have access to all the information.