Trench footings on cut and filled land

[liam1369]

Good Afternoon All,

I have a site that is sloped with the soil being medium change potential stiff clay with gravels and silts.

As part of the site works, cut and fill is being undertaken to better level the site.

In some areas, we are building up the site with compacted cut.

I was wondering, can we bear our trench footings onto the compacted fill (which is stiff clay with gravels and silts? Or will settlement be an issue?

NHBC guidance for the soil type and volume change potential recommend foundations be a minimum 1.0m below ground level. Therefore, in areas where we are filling in, we are considering putting the foundations at 2.0m + below proposed ground level owing to the top one metre being backfilled.

Hope this makes sense.

Any guidance on this matter is hugely appreciated.

Kind regards,

Liam

 

[geotechguy1]

Will the fill be compacted to an engineered specification during construction and tested? How long will there be between completing the earthworks and building the houses? What will the foundations look like?

If the houses are on Canadian-style basement walls and footings (

https://th.bing.com/th/id/R.3e6d6ff594c7d261fda5791d12bb05b3?rik=NaIrTcGeMFx3/w&pid=ImgRaw&r=0)

I wouldn't worry about it, but if the footings are isolated / can move independently it might be an issue

 

[EireChch]

Hi Liam,

Sounds like you are in the UK.

Yes you can found your foundations in stiff CLAY fill, provided it is well compacted and tested etc. There should be a earthworks spec that provides minimum number of tests etc. Its also dependent on whether the CLAY is suitable to use as structural fill, the primary reason it wouldnt be suitable is moisture content. You should have a Maximum dry density / optimum moisture content (MDD/OMC) curves available for the material you are thinking of using. This shows you the moisture content required to achieve 95% compaction. If its too wet, which is usually the case in the UK, then you will never reach 95% MDD compaction (without spreading it and letting it dry, which it may not in the middle of winter).

If moisture content is ok, and fill is tested and QA/QCd etc, then you can found at 1m depth. NO need to go to 2m depth.

However, if the moisture content is too wet, which will mean you dont meet your 95% MDD, then ideally you should found at 2m depth, i.e. 1m into natural ground. IMO, the 95% MDD is somewhat arbitrary. If its 90% then it is likely still pretty good and probably stronger that your natural ground (well maybe). But it is very difficult to get others to deviate from the spec. In order to do so you would likely need to run triaxial UUs, odeometers etc to prove that the fill is good ground. You would likely need to wait a little before testing to let the increased pore pressure from compaction dissipate etc. All this takes time and effort and it may just be easer to go the extram 1m into natural ground.

A last note, the expansion potential is based on the clay fraction and type, which can be somewhat assessed by plasticity index. Just because you compact the CLAY somewhere else doesn't change its expansion potential. There likely is some change but its unquantifiable. I would still include the expansion protection measures (cordek boards etc) required for medium volume change material.