DHARMCAISSONS AND WELL FOUNDATIONS 799and R = B nD BnD(^22) B
1 4
2
2
22
2
F
HG
I
KJ
+=+()
The above equation for Fn may be written as
Fn =
W nD
B
B
nD
BnD
R
u
2
1 4
2 2
22
2
1
++ 2
F
HG
I
KJ
tan− ()
or Fn =
W nD
B
B
nD
nBD
BnD
u
2
1
4
2
2
4
22
2
1
++ (^222) +
F
HG
I
KJ
tan−
()
...(Eq. 19.53)
The moment of resistance of the base about the point of rotation, Mb, is
Mb = R(Fn tan φ) ...(Eq. 19.54)
For circular wells, the right hand side of Eq. 19.54 is multiplied by a shape factor of 0.6.
Assuming the point of rotation to be at a height of 0.2 D above the base, the moment of
resistance about the base is
Mb = C.W.B.tan φ ...(Eq. 19.55)
where B = Width parallel to the direction of forces or the diameter for circular wells,
φ = angle of shearing resistance of the soil,
and C = A Coefficient (given in Table 19.6).
Table 19.6 Value of coefficient c in Eq. 19.55
D/B 0.5 1.0 1.5 2.0 2.5
Rectangular 0.41 0.45 0.50 0.56 0.64
Well
Circular 0.25 0.27 0.30 0.34 0.38
Well
Resisting Moment from Sides
Figure 19.20 shows the ultimate soil pressure distribution at the front and back faces of the
well on its sides.
D 1
D
0.2 D
gD(K –K )pa gD(K –K )pa
Fig. 19.20 Pressure distribution at the sides of a well (IRC)