Question

Ultimate bearing capacity (in kN/sq.m) of a square footing on surface of a saturated clay having unconfined compressive strength of 60 kN/sq.m, using Skempton equation will be
1. 150
2. 120  
3. 100
4. 125
5. 180

Answer 1

Correct Answer - Option 5 : 180

Concepts:

The net ultimate bearing capacity of pure clay soils as per Skempton’s Method is given as:

Q_nu = C × N_c

Where, C = cohesion

N_{c =} Skempton’s bearing capacity factor which depends on D_f/B ratio and type of footing. It is given as

Case 1: If footing is on ground surface i.e. D_f/B = 0.

For strip footing, N_c = 5

For Square or Circular or rectangular footing, N_c = 6.

Case 2: If D_f/B ≥  2.5.

For strip footing, N_c = 7.5

For Square or Circular or rectangular footing, N_c = 9.

Case 3: If 0 < D_f/B < 2.5

For strip footing, N_c = 5(1 + 0.2 × D_f/B)

For Square or circular footing, N_c = 6(1 + 0.2 × D_f/B)

For rectangular footing, N_c = 5(1 + 0.2 × D_f/B) × (1 + 0.2×B/L)

Further, the relation between unconfined compressive strength (UCS) and Cohesion (C) of pure clay is given as:

 UCS = 2 × C

Calculation:

UCS = 60 kN/m² ⇒ C = 60/2   30 kN/m²

In our case footing is square and it is on ground surface i.e. D_f/B = 0

Therefore, N_c = 6.

Q_nu = C × N_c = 30 × 6 = 180 kN/m²