Ground Motions and Structures - Earthquake Engineering for Structural Design - page 305

Ground Motions and Structures 305

The passage of seismic waves through soft soil during strong earthquakes may cause
significant strains developed in the soil and piles, causing, in some cases, the failure of
piles. The main examples refer to the 1964 Niigata, 1978 Off-Miyagi, 1989 Loma Prieta
and 1995 Kobe earthquakes, where much damage was suffered by the pile foundations.
Examining the causes of this kind of damage, producing the failure of piles, one can
conclude that it is the result of three main reasons:
- bad behavior of the pile to cap connections;
- reduced bearing capacity due to the discontinuity of layers, producing high
stress concentration in the discontinuity zone;
- liquefaction of soil under buildings, producing a loss of lateral support to the
piles and consequent free buckling of them.
Seismic behavior of pile-cap connection. The seismic performance of piles is directly
related to the earthquake response of the pile-cap connections. Potential inelastic
damage occurs at the interface between pile heads and pile cap as evidenced in recent
earthquakes . Some seismic design conceptions consider the possibility of the formation
of plastic hinges at the pile-cap contact, but it presents serious risks, because the repair
of damaged piles in high-rise building systems is impracticable, because of the elevated
cost and the difficulty associated with ground excavations (Teguh et al, 2006) . It should
be noted that, from the reason presented above, it is desirable to design the piles to
remain undamaged. So, the design concept should aim at dissipating seismic energy just
by the structure above the foundations. The pile foundations should be provided with
sufficient strength to ensure, as far as possible, that they remain in the elastic range.
Therefore, special attention must be paid to the pile-cap connection details.
Layer discontinuity effect, due to passage of seismic waves through two different
layer soils, generating bending moment in the piles. The research results show that
bending moments due to this effect, named kinematic moments , tend to be amplified in
the vicinity of interfaces between stiff and soft soil layers (Pappin, 1996, Mylonakis,
2002). This effect can produce the damage of piles, due to the fact that these moments
can exceed the moments resulting in the pile-cap connection. The same observation as
for the pile-cap connections is valuable in general: the pile must remain in elastic field,
because it is impossible to repair this kind of damage.
Liquefaction effect. Collapse of pile foundation in liquefiable areas has been
observed in the majority of the recent strong earthquakes. Two phenomena are
considered to be the cause of these collapses (Bhattacharya et al, 2003):
- The soil pushes the pile , so the failure is due to lateral spreading of soil crust, the
soil liquefies producing large lateral displacements and the soil crust drags the pile with
them, causing bending failure due to soil lateral pushing. The deformation of the ground
surface adjacent to the pile foundation is indeed indicative of this collapse mechanism.
- The pile pushes the soil , considering that failure is due to the pile buckling. This is
the case of slender precast piles. During earthquake-induced liquefaction, the soil
surrounding the pile loses its effective strength and it can no longer offer sufficient
support to the pile, which now acts as an unsupported column prone to axial instability.