Foundations for Bin Store structure

[EireChch]

Hi,as part of a larger development I am working on a bin store that comprises of a steel posts supporting a aluminium sheet roof with a poured concrete floor. Plan view of 4m * 7m. The ground conditions are made ground (non-engineered fill - gravelly silty clay with brick fragments - undrained shear strength typically 40-60kPa, SPT-N 34 and 27 at two different boreholes) to 2m depth, beneath that London clay (undrained shear strength 50-70kPA typically, SPT-N 13 and 11 at two different boreholes) is encountered to great depth.

The geotechs recommend that foundations should be conventional trench fill mass concrete spread foundations in London Clay with a net allowable bearing pressure of 80kPa. Given that made ground would extend to 2m depth this would involve excavation to 2.65 (made ground depth plus 0.15m embedment into london clay as recommended by geotech). This would then be filled with concrete with the posts bolted to concrete slab.

The contractor is looking to place concrete slab on 600mm of compacted aggregate from surface level. In someways I would think this is acceptable given that it is only a bin store however given that it is made ground there could be differential settlement of the concrete slab, however thinking about it again surely 600mm of compacted aggregate and say a 100mm thick reinforced slab would be capable of accommodating differential settlement so why worry. My boss is adamant that the they should excavate down to made ground and pour mass concrete strip footings. I believe that this involves a lot of excavation and concrete.

Would an alternative idea be to pile the down to 2.5m depth around the perimeter and construct a self supporting ring beam and pour a self supporting slab or run with the contractors idea or go with what my boss says!!

Any thoughts greatly appreciated.

Regards

 

[TigerGuy]

I generally try to accommodate the person that hands me my paycheck every month. What kind of slab and foundation loads are you facing? Is frost a concern in the project area?

While I love working with contractors, I have learned that their suggestions are usually in the interest of saving themselves time, effort and money. 600mm of compacted aggregate sounds a little thin to try and bridge over the material in question.

 

[EireChch]

Thanks for the response TG.

This problem is actually from a structural colleague in our office. I am working in the civil department at the minute but offered to post here to get other engineers thoughts.

He has informed me that frost is an issue given that it is in the UK. Generally foundations need to be located 1m BGL to mitigate the risk of frost heave. He has also stated that the contractor is discussing the possibility of placing the posts on isolated pads say 600mm x 600mm and supported on 600mm of compacted aggregate. Given the above these would have to be located 1m BGL we are nearly at London Clay anyways!

Loads are as follows, axial load per pad 10kN and an overturning moment due to wind loading of 8kN/m.

I think given that we are founding at 1m depth we may as well excavate the last few hundred millimeters and found on natural ground. The only issue would be excavating a strip footing around the perimeter and one central wall to 2.5m depth would be a lot of excavation given that we may have to batter (maybe 1:1 side profile) the excavation sides. Or we could possibly use a sacrificial form work which may be easier.

This makes me think that using a bored (cased) pile to 2.5/3m and a 500mm thick self supporting ring beam is a good option?

 

[TigerGuy]

Maybe my unit conversions are off, but this sounds like uplift resistance is your governing concern with this structure. I would look at the cost difference between piles and just increasing the size of foundation elements, using the weight of the foundation to counter the overturning moment. Is the cost to mobilize in additional equipment worth the benefit?

I'm not familiar with London Clay. Unless it is highly compressible, it sounds like your foundation loads won't have much effect on the clay.

 

[oldestguy]

Have you looked into a temporary surcharge? Even a rolling surcharge can do wonders for sites like this. Placing settlement platforms will show what happens. Even a week of such loadings will amaze you.