Soft spot or subsidence?

[Teamdesign]

Dear all,

UK based project which baffles me the last few months (years of design experience in mainly residential works and this is the first with such issue)

A new build single rear storey extension of say 2.50m wide x 10m in length (build within the past 6months) has sudden minor sinking only at one outer corners.
Brick and block cavity wall say 12kN/m line load on foundation. Ground bearing rc floor slab.

15-20m away there is a row of mature horse chestnut trees.

New foundation is 0.60m wide x 1.0m deep concrete on stiff to firm clay. (100-150kN/m2 SBC) Plasticity index @ 25% ie. medium plasticity
I've considered latest design guidelines based on the trees location, plasticity index (I took higher percentage) to new foundation and design tables come back as 1.0m being OK!

Existing main 2 storey house foundation is shallower with no evidence of subsidence.

Builder did not tie new foundation to existing foundation with 16-20mm dowel bars as per good practices.

I am no geotechnical, but think it is a soft spot and I've recommended underpinning the corner increasing substantially to a wider and deeper footing and making good.

The client house insurance company hires subsidence surveyors (not geotechnical) who claims the foundation is not designed correctly
and the whole extension should be rebuild on 1.60 to 2.0m deep foundation.

Comments please

Thank you for your time!

 

[EireChch]

Dig a hand pit at each corner and undertake hand shear vanes to determine your undrained shear strength.

Could be down to any reason, soft spot of ground, badly compacted fill beneath foundation before pouring, leaky pipe hitting a sand layer. impossible to say without going to site and digging a hole.

and what do you define as 'major sinking'

 

[Teamdesign]

Hi EireChch,

Thank you very much for your guidance.

I would define "major sinking" as few cm drop and large cracks.

Below is part of the subsidence report where damage is classified as slight and I have few photos showing minor cracks and the wall ties been ripped off the existing wall.



I was send today in the late afternoon a borehole record made near the sinking corner.

I would really like an honest experienced opinion by a geotechnical engineer even if it means I am in the wrong
with my design so that I know how to proceed ie. contact our insurance company
or advise clients that I do not agree with their subsidence report etc..

For the record the trenches were inspected by a building control officer who accepted these before concrete was poured.

 

[HTURKAK]

Do RWP stands for Rain Water Pipe and RWG stands for Rain Water Gully ?

I would ask to excavate an observation pit to see if the RWP and gully are leaking. My gut says the gully and /pipe leaking and the soil is becoming submerged during rainy season .

 

[EireChch]

If you are still getting rootlets at 2m depth then you are still likely in desiccated ground, which is prone to shrink and swell. Your shear vanes show 140kPa plus, your buidling control said the foundations are on stiff ground. It would appear that its a water infiltration problem. You could be suffering from the below statement. Just because you followed BRE doesnt mean you might not see movement. You seem to have been unlucky

 

[Teamdesign]

Dear EireChch & HTURKAK,

Thank you both for the time spared to go through the paperwork and give your opinion on the matter.

Will keep you updated once we get to the bottom of the issue.

Wish you both all the best.