Ng
=21 (entering Fig. 6.5 with f′=32°, and intersecting the Ng
curve, the value of Ngfrom
the vertical axis is equal to 21)
B=1.5 m for the strip footings and 2.5 m for the spread footings
r
u
=0.20 (entering Fig. 4.25 with a factor of safety against liquefaction =1.3, the value of
r
u
for sand varies from 0.05 to 0.35. Using an average value, r
u
=0.20)
Inserting the above values into Eqs. 6.37 and 6.38, or:
For the strip footings:
q
ult
=1
/
2
(1 −r
u
) g
bBN
g
=1
/
2
(1 −0.20)(9.7 kN/m3
)(1.5 m)(21) =120 kPa
And using a factor of safety =5.0
q
all
=q
ult
/F=(120 kPa)/5.0 =24 kPa
For the spread footings:
q
ult
=0.4 (1 −r
u
) g
bBN
g
=0.4 (1 −0.20)(9.7 kN/m3
)(2.5 m)(21) =160 kPa
And using a factor of safety =5.0
q
all=qult/F=(160 kPa)/5.0 =32 kPa