Teng’s method – Teng (1962) based on the work of Terzaghi and Peck gave a relationship for allowable bearing capacity for a given permissible settlement.
The equation is:
qns = 35(N-3){(B+0.3)/2B}2RWRd ———- (1)
Where, qns = Net safe bearing pressure, kN/m2
N = SPT value corrected for overburden pressure
RW = Water table correction factor
Rd = Depth correction factor = 1 + 0.2Df/B ≤1.2
The above equation can also written in general form as:
qns = 1.4(N-3){(B+0.3)/2B}RwRdS ———- (2)
Where, S = Permissible settlement in mm.
Meyerhof’s method – Meyerhof (1965) suggested the following procedure to obtain allowable bearing pressure to give a settlement of 25 mm.
Procedure:
- N value should not be corrected with respect to overburden.
- qa is obtained from the chart shown in fig.4.11.
- qa should not be corrected against ground water table.
- Multiply qa by 150 %.
The equation proposed by Meyerhof (1974) is slightly different from Tongs equation. The equation proposed is:
qns = 12NRwRd for B ≤1.2m ———- (3)
qns = 8N{(B+0.3)/B}2RwRd for B>1.3 m ———- (4)
Where, qns = Net safe bearing pressure, kN/m2
N = SPT value corrected for overburden pressure
Rw = Water table corrected factor
Rd = Depth correction factor = 1 + 0.33Df/B ≤1.33
Bowles method – The equation proposed by Meyerhof and Tong were found to be very conservative and Bowles (1982) modified these equations. The equations proposed are:
Teng’s modified equation,
qns 53(N-3){(B+0.3)/2B}2RwRd ———- (5)
Meyerhof’s modified equation,
qns = 20NRWRd for ≤1.2 m and ———- (6)
qns = 12.5N{(B+0.3)/B}2RWRd for B>1.3 m ———- (7)
Where, qns = Net safe bearing pressure, kN/m2
N = SPT value corrected for overburden pressure
RW = Water table correction factor
Rd = Depth correction factor = 1+ 0.33 Df/B ≤ 1.33.
The value of bearing capacity factors are found out from the graph as shown in Fig.2 above. It is for mix shear condition. The values of Nγ and Nq are used to bearing capacity of granular soil on the basis of shear failure criteria.