Foundation Issues (trench collapse, piling options)

[carson321]

Hi All, 

I have been reading a lot of information on these forums, all incredibly helpful, but this is my first post. 

I am just starting the process of building a single storey rear extension (~7.5 x 3m) on my house. Ground type is gault clay and there is a cherry tree about 7m from the foundation location. 

 

SE drawings called for 1.65m deep foundation. We began excavation last week and had just finished it all at depth, including oversite, when one small corner within the oversite area collapsed. No biggie, plan was to dig it out and pump out any water ready for BCO inspection. Overnight, significantly more of the trench collapsed. the following day, when clearing the bottom of the trench, it was also noted that along the short side, which is close to a party wall, the ground was beginning to slip closer to the building. This was immediately shored up with ply and the decision was taken to begin backfilling the trenches before any further collapse happened, particularly ahead of the storm that rolled through yesterday. 

 

Half of the excavation was perfectly stable, and was going through the clay ground, the areas that were collapsing appeared to be a pit of soil! And no tree roots have been encountered making the whole process rather frustrating. 

 

I am at the point now where my SE has recommended we contact some piling companies and look at a piled foundation with a suspended floor. I have had one quote back so far, and they are coming in at over £10k for the piling, with much of the gorundwork to be undertaken by someone else, and then they quoted over 10K for a reinforced concrete slab on top of that, despite the recommendation for a suspended floor. 

 

Sorry for the saga, now for the questions! 

First of, anyone have any suggestions for ways that I could proceed with the original concrete fill foundation plan but mitigate against the collapsing ground conditions? Groundworks guys said that even with shoring or shuttering, they think the amount of wet ground and collapse that was happening before, it would be difficult to keep the trench sides stable and the bottom clear to full depth.

Secondly, is the quote for piling I have received so far what I should be expecting or is it a bit OTT? It was for design and installation of 8 SFA piles and cage forming over them, no additional concrete included in that, only what was needed to form the piles themselves. They also stipulated things like I would need to arrange a 1 tonne excavator and operator to be present the whole time they are there and provide a 250mm hardcore compacted base for them to work from among other things.

Lastly, if I am going down the route of piled foundations, what do i need to do about the fact that I have already had huge trenches dug and backfilled? This ground isn't exactly compacted and will continue to sink as it does so under it's own weight. Do I need to get in there with a whacker plate and start compacting and refilling the trenches as far as I can?

 

Any help, advice or pointers much appreciated at this point as currently feeling a little lost!

 

[Russell griffiths]

I would carry on with the trench footing, you are so close to getting finished. 
you will need 25 sheets of plywood and a bundle of 4x2

you need building control involved to get a feeling for finished depth as you can only dig this once. 
 

start by the house and pull the trench in to max depth for aprox 2 m, get it all propped properly

that means full sheets all the way to the bottom, progress along to the corner and keep propping, your groundwork guy should not have a problem with this. 
if he’s not happy I think he lacks experience 

that is a very small extension and should not pose any problems 

you need concrete pre ordered and a concrete pump ordered, as the building inspector looks at it get it poured an hour afterwards 

it will only take an hour to unload 3-4 trucks of concrete into that

get it dug one day and poured the next. 

[saveasteading]

I suspect the collapsed area is loose fill from previous works.

The virgin ground underneath will be strong and support the foundations.

Just ensure that all the loose stuff is removed. 

 

If you phone the bco and ask for permission to pour , I reckon they will be sympathetic.

An option is to only pour the bit you are worried about And/or pour part depth.

 

As to the loose ground supporting your floor slab, we can look into that later.

 

Either replace the loose area or support the slab on the footings. 3m span is easy.

 

The absence of roots should not annoy you. The design is for the tree when mature, and it may have a way to grow yet.  It isn't an optional requirement  anyway.