I am designing a G+2 factory building founded on dense sandy soil as per our geotechnical investigation report.
Load considered: (As Client was not sure about the purpose of use)
Live Load on Slab= 3.0 kN /m2
Dead Load on Slab= 1.5 kN/m2 (other then Self weight)
Grid Spacing: 27'9"x21'3"
Plot Dimensions: 56'x165'
Characteristic Load on Columns: (decisive case)
Dead Load: 1900 kN
Live Load: 950 kN
The report recommended bearing capacits:
Isolated Footing:
160 kPa at 2.0m depth
Settelment within 25 mm
Raft foundation
300 kPa at 2.0 m below existing ground level (EGL),
With the column loads of around 2800 kN and grid spacing isolated footings would require very large foundation sizes leaving little or no clearance between footings and cause overlap issues. Therefore i decided to go for raft foundation.
However, the site condition is such that:
The EGL (existing ground level) in the report corresponds to be the site’s current level +0.0 m, while the outer road level is about +0.9 m higher.
Our finished factory floor will be at +1.5 m.
Now i am evaluating two options:
Option A – Construct the raft foundation at the recommended -2.0 m below EGL (per the GI report), then build up columns, cast a plinth beam at the raised level, backfill up to the required ground/floor level, and finally prepare a floor over the compacted backfill.
Option B – Raise the site to match the road level using engineered, well-compacted backfill, and then construct the raft foundation directly at the new raised ground level, allowing the raft slab itself to serve as the factory floor.
I want to understand:
Which option is more technically sound and economical considering bearing capacity, settlement control, and long-term performance?
Is placing the raft on engineered backfill (Option B) acceptable practice if compaction and quality control are ensured,
or is it safer to strictly follow the geotechnical recommendation (Option A)?
Any insights or experience-based advice on this choice would be highly appreciated.
Furthermore:
Are the load assumed too conservative?
Kind Regards
Abdul Malik
