In this article we will discuss with example on ‘How to calculate safe bearing capacity of soil on site ,when unit weight of soil , depth of foundation , width of foundation C & ɸ (Phi) value is given ? So what is bearing capacity of soil ? in simple way , bearing capacity is the load carrying capacity of the soil & bearing capacity after applying the factor of safety (FS) is called safe bearing capacity of soil. This vale is used for design of foundation. We can understand it with following example.
Q. A foundation in a sand is 5 metres wide & 1.5 metres deep. Considering factor of safety 2.50 what will be safe bearing capacity if the unit weight of sand is 1.9 gm/cc and angle of internal friction is 30 ˚. How does it compare with safe loading capacity for surface loading.
- Ans: From chart ɸ = 30 ˚ Nc =37.2 , Nq = 22.5 , N ү = 19.7 , C=0 , Y= 1.9 t/m³ , b= 5 m , d=1.5 m
- Safe bearing capacity =1 /F [ c Nc + ү. d (Nq – 1) + ½ ү b Nү] + ү .d
= 1/2.5 [ 0 x 37.2+1.9 x 1.5 (22.5-1) + 1/2 x 1.9 x 5 x 19.7] + 1.9 x 1.5
= 1/2.5 [ 0 + 1.9 x 1.5 x 21.5 + 0.5 x 1.9 x 5 x 19.7) + 2.85
= 1/2.5 [ 0 + 61.28 + 93.58] + 2.85
= 1/2.5 X 154.86 + 2.85
= 64.79 t/m²
If surface loading is there then d ( depth) will be zero then Safe Bearing Capacity
= 1/2.5 [½ үb Nү]
= 1/2.5 [ ½ x 1.9 x 5 x 19.7] = 1/2.5 x 92.63 = 37.052 t/m² hence we see that loading capacity of the foundation is 1.75 times more than surface loading capacity
