Minimum depth of foundation

[pnmkapoor]

Could anybody please provide me the guidelines of minimum depth of foundation? I have a situation where foundation is to be rested directly on the ground. The ground consists of 250mm compacted fill underlain by sound partially weathered sandstone. The size of foundation is 6mx0.5m.

I am asking for minimum embedment of 0.5m below ground. However civil guys want to rest foundation directly on the ground. Their argument is that foundation pressure is less than allowable bearing pressure of 1000kPa and it satifies all the stabilty requirments of sliding and overturning.
However, I am not very comfortable with resting foundation directly on the ground. Could you please provide your valuable feedback. Thanks & regards

 

[Ron]

Most building codes require the foundation to be at least 12 inches (~300 mm) below the ground surface to protect from undermining by surface erosion. Provided the engineering considerations of bearing capacity, sliding, and overturning are met, the minimal code embedment is usually satisfactory. I would caution you that it also depends on the local soil conditions, potential for runoff, and the specific site design for runoff (for instance, if you have downspouts that deposit roof runoff directly adjacent to the foundation...not good).

Putting the foundation directly on top of the ground with no embedment is not a good idea. Even wind scour can undermine a foundation.

 

[Focht3]

Ron is right - the allowable bearing pressure is only one issue. In my view, putting the foundation directly on the ground surface is generally a bad idea.

I have seen this done for temporary footings - such as a pad footing beneath a tower crane. But your circumstance probably wouldn't meet the constraints that makes this practice acceptable, namely 1) it is temporary and 2) it is inspected ("monitored&quot daily.

A strip footing for a permanent structure that is only 0.5 meter wide - without any embedment? Sounds foolish to me. I wouldn't sign off on that design.

 

[VoyageofDiscovery]

Hi pnmkapoor,

In countries subject to winter effects, the base of the foundation is placed below the frost line to prevent frost heaving.

Regards

VOD

 

[RockEngineer]

I agree that using a slab with no thickened edge is not normally the preferred option. There are places that this is the only option. I have engineered foundations for buildings that were on leased US Forest Service Land (Ski Resorts) where they were only allowed a slab with nothing more than 12" below grade (well above frost depth). In those cases you have to design the slab strong enough to make the building move with the soil. Pnmkapoor needs to check and see if the slab foundation is due to some local restriction.