An example of Geotechnical analysis for drilled circular footings(bored-and-cast-in-place) for the interior columns footings to be used to support a four story proposed structure. I used the guide lines of the Canadian Foundation Engineering Manual 4th. Edition with additional references such as Soil mechanics by R.F. Craig and Soil mechanics in engineering practice by TERZAGHI, K. and PECK, R.B.
I considered these factors which govern the bearing capacity and settlement:
1. The depth to diameter ratio of 2.5 to 1.
2. The effect of drilling is to cause a relief of lateral pressure on the walls of the hole which results in swelling of the clay and there will be a migration of pore water towards the exposed clay face(unless bentonite slurry will be used to prevent this condition) ,which shall weaken the soil around the footing and eliminate the skin friction therefore, I considered them as end-bearing piles.
3. Experience of a very large number of loading tests taken to failure, made on piles of diameters up to about 1200mm and of many types both in clays and sands, has shown that if a safety factor of 2.5 is applied, then the settlement at the working load is unlikely to exceed 30mm.
4. Condition of the base of the drilled hole with respect to removal of drill cuttings and other loose debris to prevent uncontrolled settlement.
5. The Ultimate end-bearing pressure of drilled circular footing in clayey soil (using undrained shear strength obtained by Vane Shear tests (the term ru)) is calculated from the equation
* ru=5 µ τu (1+0.2D/B) (1+0.2B/L) + σo
where ru is the Ultimate bearing capacity, µ Vane strength reduction factor(equal to 0.9), τu is the undrained shear strength obtained by Vane Shear tests, σo is the overburden pressure(γD).
The minimum τu in native clayey silt subsoil( 2.5 m below the anticipated finish floor level) was 200 Kpa
and D/B =2.5
and B/L=1(circular footing)
and σo=γD=18*2.5=45 Kpa→ average γ for the 2.5 m below the anticipated finish floor level=18 KN/m³
ru=5*0.9*200*1.5*1.2+45= 1665 Kpa
Allowable bearing pressure= ru/2.5= 666 Kpa→ I used 650Kpa