Civil Engineering Reference
In-Depth Information
Fig. 11.14, a common cause of downdrag loads
Q
D
on piles is the recent placement of
a fill layer atop a clay layer. Other conditions that could cause downdrag loads are as
follows:
●
Above the end-bearing strata, there is a soil layer that is in the process of settling due to
consolidation or collapse.
●
Above the end-bearing strata, the original groundwater table is permanently lowered
causing an increase in effective stress and resulting in settlement of the soil strata.
●
An earthquake causes liquefaction of a sand layer, and it settles as the water flows out
of the soil due to the dissipation of excess pore water pressures. This settlement of the
liquefied sand layer causes downdrag loads on the pile or pier. Those soil layers located
above the liquefied soil layer will also induce downdrag loads on the deep foundation.
The analyses presented in Sec.7.2.2 can be used to estimate the amount of settlement of
liquefied sand layers.
●
An earthquake causes weakening of cohesive soil, and it then settles, resulting in down-
drag loads on the pile or pier.
●
An earthquake causes volumetric compression of dry and loose sand and gravel, result-
ing in downdrag loads on the pile or pier. The analyses presented in Sec. 7.4 can be used
to estimate the amount of settlement of dry and loose sand and gravel.
Downdrag Loads on a Single Pile or Pier.
The following equations can be used to
calculate the downdrag load
Q
D
for a single isolated round pile or pier.
For a pile or pier in cohesionless soil subjected to downdrag:
Q
D
2
RL
1
v
k
tan
w
(11.14)
For a pile or pier in cohesive soil subjected to downdrag:
Q
D
2
RL
1
c
A
(11.15)
where:
Q
D
downdrag load acting on the pile or pier (lb or kN).
R
radius of the pile or pier (ft or m).
L
1
vertical distance over which the pile or pier is subjected to the downdrag load
(ft or m).
v
average vertical effective stress over the portion of the pile or pier subjected
to the downdrag load (psf or kPa).
k
dimensionless parameter equal to
h
divided by
v
. Because of the
densification of the cohesionless soil associated with driven displacement
piles, values of
k
between 1 and 2 are often assumed. For bored piles or
excavated piers, use
k
k
0
which is the coefficient of earth pressure at
rest. Commonly used equations are
k
0
1
sin
for loose sands and
k
0
0.5(OCR)
0.5
for preloaded sands where OCR
overconsolidation
ratio.
w
friction angle between the cohesionless soil and the perimeter of the pile or
pier (degrees). Commonly used friction angles are
w
3
4
for wood and
concrete piles and
w
20
for steel piles.
c
A
adhesion between the cohesive soil and pile or pier perimeter (psf or kPa).
Figure 11.15 can be used to determine the value of the adhesion
c
A
for
different types of piles and cohesive soil conditions.
