Civil Engineering Reference
In-Depth Information
where:
Q
D
downdrag load acting on the pile group (lb or kN).
B
width of the pile group measured to the outside edge of the perimeter piles
(ft or m).
L
1
vertical distance over which the pile group is subjected to the downdrag load
(ft or m).
v
average vertical effective stress at the portion of the pile group subjected to
the downdrag load (psf or kPa).
k
0
coefficient of earth pressure at rest (dimensionless). Commonly used equa-
tions are
k
0
1
sin
for loose sands and
k
0
0.5(OCR)
0.5
for preloaded
sands where OCR
overconsolidation ratio.
effective friction angle of the cohesionless soil (degrees).
s
u
average undrained shear strength along the portion of the pile group subjected
to the downdrag load (psf or kPa).
As a conservative approach, the downdrag load should be calculated for both cases
outlined above and the maximum value used for design.
Example Problem.
Consider the same situation as the previous example, except that
there is a 2 by 2 pile group. The piles are spaced 3 pile diameters (centerline to centerline).
Determine the downdrag load on the pile group.
Solution.
Since there are 4 piles in the 2 by 2 group and assuming each pile is separately
subjected to a downdrag load, or:
Q
D
(4)(118)
470 kN
The second downdrag condition is to use Eq. (11.16), or:
B
3 times the diameter
2
r
(3)(0.3)
(2)(0.15)
1.2 m
Using
k
0
1
sin
1
sin 30
0.5
v
(at
z
1.5 m)
(1.5 m)(19 kN/m
3
)
29 kPa
v
(at
z
4.5 m)
(3 m)(19 kN/m
3
)
(1.5 m)(9.9 kN/m
3
)
72 kPa
Substituting values into Eq. (11.16): For
z
0 to 3 m,
L
3 m and therefore
Q
D
equals:
Q
D
4
BL
1
v
k
0
tan
(4)(1.2 m)(3 m)(29 kPa)(0.5)(tan 30
)
120 kN
For
z
3 m to 6 m,
L
3 m and therefore
Q
D
equals:
Q
D
4
BL
1
v
k
0
tan
(4)(1.2 m)(3 m)(72 kPa)(0.5)(tan 30
)
300 kN
Adding together both values of
Q
D
, the total downdrag load
120 kN
300 kN
420 kN.
Hence the governing condition would be assuming individual downdrag for each of the
four piles in the group, or
Q
D
470 kN.
11.7.4 Eccentric Loads
Earthquakes will often induce eccentric loads and moments onto the foundation. For example,
Fig. 7.10 illustrates the load distribution on the foundation for the static case and the
