Civil Engineering Reference
In-Depth Information
pressure for the earthquake loading conditions is often recommended. This increase in
allowable end-bearing pressure for earthquake conditions would also be acceptable for
end-bearing in dense or hard soil that will not be weakened by the earthquake. However, a
higher pile load capacity should only be allowed if the end-bearing soil or rock supporting
the pile is not weakened during the earthquake and the pile or pier will not punch downward
into a weaker lower layer. If the soil is weakened by the earthquake, the load capacity of
the pile may actually be decreased. Examples are as follows:
1.
Liquefaction:
A friction pile in loose submerged sand will lose frictional resistance
if the sand liquefies during the earthquake. In these cases of weakened soil during the
earthquake, the recommended allowable pile load for earthquake design should be based
on the anticipated soil conditions that develop during the earthquake. For the friction pile
in liquefied soil, that part of the pile in the liquefied soil will not provide load bearing
capacity during the earthquake. Likewise, if the liquefied soil layer causes ground damage
(Sec. 7.3), such as ground cracks and sand boils, then the load-carrying capacity of this
layer could also be significantly decreased. Furthermore, as water flows out of the soil and
the soil layer settles, the pile will be subjected to downdrag loads, which is discussed in
the next section.
2.
Strain softening cohesive soil:
A second example would be a friction pile in
cohesive soil that is weakened during the earthquake. Once again, the load capacity of the
friction pile will be decreased during the earthquake.
Table 11.7 provides a summary of the geotechnical earthquake engineering for the
vertical load capacity of piles and piers.
Example Problem.
Use the prior example problem in this section of a 6 m long pile
embedded in a silty sand deposit, where the allowable tip resistance (60 kN) and the allowable
frictional capacity (40 kN) are added together to obtain the allowable combined end-bearing
and frictional resistance capacity of the pile of 100 kN. Assume that during the earth-
quake the sand will liquefy from a depth of 3 m to 7 m below ground surface. The upper
3 m of sand will not be subjected to liquefaction-induced ground damage. Determine the
maximum vertical load-carrying capacity of the pile during the earthquake. If the pile is
subjected to a vertical load of 100 kN during the earthquake, what will happen to the
pile during the earthquake?
Solution.
Since the sand will liquefy from a depth of 3 m to 7 m, only the upper 3 m of
the pile will be able to provide vertical support:
v
(at
z
(1.5 m) (19 kN/m
3
)
1.5 m)
29 kPa
For a concrete pile:
w
3
4
3
4
(30
)
22.5
Substituting values into Eq. (11.11): For
z
0 to 3 m,
L
3 m and therefore
Q
s
equals:
Q
s
(2
rL
)(
v
k
tan
w
)
(2
)(0.15 m)(3 m)(29 kPa)(1)(tan 22.5
)
34 kN
The maximum vertical load-carrying capacity (without a factor of safety) of the pile
during the earthquake is only 34 kN. This value should be reduced to account for the
downdrag load due to the liquefied soil (see Sec. 11.7.3). Because the vertical load on
the pile (100 kN) is much greater than the maximum load capacity of the pile during the
earthquake (34 kN), it is expected that the pile will punch downward into the liquefied
soil during the earthquake. The best option is to extend the pile through the zone of
liquefied soil.
