MortarThePoint

Risk of leaving trenches unpoured

Recommended Posts

I'd like to acknowledge that there are people with bigger issues, but hoped I could ask people's advice. The groundworkers are onsite as planned and progressing with prelims prior to digging foundations. I am worried that they may get in to the dig and it turns out that, due to Coronavirus restrictions, either they have to stop or their supply base stops. It strikes me as a disaster if the trenches are dug (1.5m to 1.75m depth with claymaster) and then can't be filled for a number of months. I'd really appreciate your thoughts. Should I call a halt to things now as the situation is changing rapidly and one day everything may be possible and the next nothing possible.

Share this post


Link to post
Share on other sites

You could pour in stages and do only what is possible in a day. Would be more expensive and a bit more complicated though. Who is responsible for the risk?

Share this post


Link to post
Share on other sites
2 minutes ago, Oz07 said:

Who is responsible for the risk?

 

Good question. It's a fixed price job but they are a small independent company and this sort of situation hasn't arisen before so it's anyone's guess really. I have to assume that it would be my risk in the end.

Share this post


Link to post
Share on other sites

Is it worth suggesting the idea and sharing the increased cost?

Share this post


Link to post
Share on other sites

They are very open to discussion so I we will be talking things through. I expect from their side they will want to carry on and may not be able to weigh up the risks dispassionately as it's their livelihood.

 

I need to find out how long it will take them to dig the trenches, but they are not going to be able to do the full dig and pour on the same day so there is a real risk that the government announces some form of shutdown that affects concrete deliveries an leaves us with an unstable hole to fill.

Share this post


Link to post
Share on other sites
Posted (edited)

My trenches were left open for about 3 months with no major issues (clay). I had the odd corner fall in but nothing I was too bothered about

Edited by Vijay

Share this post


Link to post
Share on other sites
3 minutes ago, Vijay said:

My trenches were left open for about 3 months with no major issues (clay). I had the odd corner fall in but nothing I was too bothered about

 

That's comforting to hear. I'm just not really aware of what could go wrong as people normally don't have a delay at this point.

Share this post


Link to post
Share on other sites

What is your soil? Does it drain?

 

One risk is they are filled with water and it stays.

 

Dad's client once had that happen to a huge outdoor lake before they got the liner in 🙂 .

Share this post


Link to post
Share on other sites
12 minutes ago, Ferdinand said:

What is your soil? Does it drain?

 

One risk is they are filled with water and it stays.

 

Dad's client once had that happen to a huge outdoor lake before they got the liner in 🙂 .

 

It doesn't drain well and I think the bottom of the trenches will be within the water table so it will be flooded one way or the other

Share this post


Link to post
Share on other sites
1 hour ago, MortarThePoint said:

I'm just not really aware of what could go wrong as people normally don't have a delay at this point.

 

 

How good is your site insurance? I am thinking personal accident should someone fall in.

Share this post


Link to post
Share on other sites
8 minutes ago, MortarThePoint said:

Good thought about the insurance, they may have specific conditions. I'd take precautions anyway.

 

 

Things should move swiftly from dig to pour for conventional strip foundations, so I think it unlikely a pandemic crunch point will arrive during those few days. You could consider opting for a maximum trench fill with fewer footing block courses, this is a relatively cheap trade off between cost and advancing your build quickly. Once the first course of footing/trench blocks is in you can relax and no longer worry about 6" of water pooling on top of your concrete fill.

Share this post


Link to post
Share on other sites

The Structural Engineer has specified a maximum of 750mm of blockwork below external ground level and maximum 450mm below internal ground level (i.e. under void). Looking at these two pictures highlights that I should make sure it all works as the Structural Engineer has finished floor level way above external ground level whereas it will be 150mm above external ground level. The floor beams and screed are 375+75=450mm thick and the void 300mm so that makes internal ground level 450+300-150=600mm below external finished ground level.

 

Based on this I'd expect the pour would come up to level with the ground under the void. Ground level outside has been reduced during demolition and needs to be built back up by 600mm.

 

image.png.3284749e7e6e0f17e04f482b4d177e4a.png

 

image.png.71598acec53d4c800473f3fb797c1176.png

Share this post


Link to post
Share on other sites

Due to the nature of the soil the ground work team don't think they can dig the whole foundation and pour in a day so they are proposing to pour in stages anyway. They should be able to confirm that cement is deliverable first thing and then dig and pour same day. Any shutdown that might be announced is unlikely affect the day of announcement so at least that day's work can be concluded. There is the risk that the second day gets called off which wouldn't be good as the concrete is more likely to 'bond' the two halves together if it is fresher I imagine.

 

Does anyone know about any downsides of having the foundations done in two pours like this?

 

The Structural Engineer has recommended "to dowel foundations at the construction joints, use 6no. H16 dowels, 600mm long encased 300mm deep in concrete on each side of joint; install dowels in three rows evenly spaced and providing minimum 150mm edge distance." I don't know how many would be on each row and will ask. The ground works team were talking about starter bars. What do people think?

Share this post


Link to post
Share on other sites

The rebar dowels are fairly straightforward.  I think you can use a bit of ply with holes drilled and locate it where the joint is.  Push the rebar halfway so it is part in the excavation and part in the unexcavated soil.  Concrete one bit, dig the rest of the trench and bingo.  Never done it or googled it but I think that is the basic plan.

 

Unless the plot is huge, you should be able to get all the concrete in in a day.

Share this post


Link to post
Share on other sites

The subsoil is very sloppy and he foundation quite long with a number of corners. They are suggesting doing the blue shaded bit on the first day and the orange shade bit on the second day:

 

image.thumb.png.bac299612e0cc68afa686c25c655f36f.png

Share this post


Link to post
Share on other sites

I think your supposed to step it so it keys but perhaps the rebar negates this. Just go with what SE says. It won't be the first time it's been done in stages will be fine. 

Share this post


Link to post
Share on other sites
On 18/03/2020 at 22:13, MortarThePoint said:

think the bottom of the trenches will be within the water table

This bit concerns me. Is the SE aware of that possibility?

Share this post


Link to post
Share on other sites

If your soil has a  concentration on sand and gravel then the water will eventually wash this out which will make the banks unstable. That's the biggest risk in to leaving them open for to long.

Do it in two halfs as you have proposed and you should be fine.

Share this post


Link to post
Share on other sites
Posted (edited)

I did my extension foundations in 2 or 3 stages a few years ago the same way as your structural engineer has proposed, mine was done following advice from the local Building Inspector.

The rebar dowels as said can probably use a bit of ply with holes drilled and locate it where the joint is.  Push the rebar halfway so it is part in the excavation and part in the a short excavated section of the next trench but make sure the ply is secure enough to hold back the full weight of concrete when poured. One mistake I did make was to hammer the rebar half way into the adjoining clay before the next trench was dug at a later date, rather than using a sheet of ply to support the rebar in an already open section.........hand digging the clay out 1m deep around several half projecting buried rebars was not an easy task in practice! I presume using the ply method the ply board can be left in place when the next section is poured?

Where the new foundations butted up to the existing house foundations the Building Inspector also asked me to insert a sheet of polystyrene a couple of inches thick before the concrete went in.

Edited by MAB

Share this post


Link to post
Share on other sites
On 20/03/2020 at 07:46, Brickie said:

This bit concerns me. Is the SE aware of that possibility?

 

Hi I'm probably being a bit vague here. The clay is stiff/firm but above it there is sloppy wet cay that is I guess the water table. So I guess that means we are below the water table.

Share this post


Link to post
Share on other sites
22 hours ago, MAB said:

Where the new foundations butted up to the existing house foundations the Building Inspector also asked me to insert a sheet of polystyrene a couple of inches thick before the concrete went in.

 

No keen to have a polystyrene separator, feels like an expansion joint. Probably made sense for your setup but I'd like the foundations to behave as one.

Share this post


Link to post
Share on other sites
21 hours ago, MortarThePoint said:

 

No keen to have a polystyrene separator, feels like an expansion joint. Probably made sense for your setup but I'd like the foundations to behave as one.

 

I agree in your case.....I think mine was for expansion joint and to allow some movement between the new and existing foundations as existing house foundation were 1930's and not as deep as the new extension @ 1m in clay.

Share this post


Link to post
Share on other sites

I notice the spec is for A393 mesh top reinforcement.  Is there anything specified to stop it sinking to the bottom of the trench?

Share this post


Link to post
Share on other sites
5 hours ago, Mr Punter said:

I notice the spec is for A393 mesh top reinforcement.  Is there anything specified to stop it sinking to the bottom of the trench?

 

I'm sure there would be, not quite sure what but I have a vision in my head of the mesh hanging from timbers laid across the trench.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now