Is raft foundation suitable

[optimalan]

Hey guys,
Is reinforced concrete raft foundation suitable for a two-storey residential building? The land has the following characteristics:

1. Loose silty sand with block rubbles and and fine to coarse gravel up to 1.5m depth
2. Loose silty, clayey sand with fine to coarse gravel up to 5 metre depth
3. Medium dense to dense silty sand with fine to coarse gravel up to 15 meters depth
4. Water table is 1.0 metre below suarface during the peak of raining season

Land is located in Nigeria with no history of earthquake.

Please help urgently.

Optimal.

 

[Lomarandil]

I would expect that a raft foundation is less optimal than a spread or continuous footing foundation (with some compaction or ground improvement), given a two story residence. A raft seems more complex/expensive than you would need.

What allowable beating pressure has been determined?

----
just call me Lo.

 

[GeoEnvGuy]

For a two storey residential house a reinforced concrete slab raft design is overkill. You would want to consider a continuous footing in poor soil conditions to lower the bearing pressure.

 

[optimalan]

Thanks for the being helpful.

1. "A Safe Bearing Capacity (SBC) of value of 40kN/m2 for square or circular footings placed on loose sand at a minimum depth of 1.5m below the existing groung level., with 3.0 as factor of safety".

2. Gross load of proposed building is estimated at 45kN/m2.

I personally have an issue with the depth of the footings (1.5m) used to calculate the SBC. The top soil (Loose silty sand with concrete rubbles and fine to coarse gravel) is better up to 1.5 metres than the soil from 1.5m to 5m (Loose silty, clayey sand with fine to coarse gravel). Placing the footings at 1.5m depth is putting them at the top of a weaker soil, and using that to calculate the SBC, will reduce the SBC.

Any further thoughts please?

Will be much appreciated.

Thanks.

Optimalan.

 

[GeoEnvGuy]

"4. Water table is 1.0 metre below suarface during the peak of raining season"

"square or circular footings placed on loose sand at a minimum depth of 1.5m below the existing groung level"

The plan to put the footings below the water table is unconventional. Also the land is in Nigeria, why bury the footings so deep I can't imagine a frost depth that deep in Nigeria. I do agree to not found your building on concrete rubble without first removing and placing compacted lifts of material back as the fill is likely not uniformly compacted.

 

[optimalan]

Thanks @ GeoEnvGuy.

The concrete rubbles are just scattered on the surface. The land was used as a block and paving stone making machine site previously, hence the concrete rubble. An escavation of 0.15m will take us to uniform land without the rubbles.