Civil Engineering Reference
In-Depth Information
The methods for the calculation of volumetric compression as presented in Sec. 7.4 can-
not be used for the following cases:
●
Heavy buildings bearing on loose soil:
Do not use the methods when the foundation
applies a large net load onto the loose soil. In this case, the heavy foundation will punch
downward into the loose soil during the earthquake. It is usually very difficult to deter-
mine the settlement for these conditions, and the best engineering solution is to provide
a sufficiently high static factor of safety so that there is ample resistance against a bear-
ing capacity failure. This is further discussed in Chap. 8.
●
Sloping ground condition:
These methods will underestimate the settlement for a slop-
ing ground condition. The loose sand may deform laterally during the earthquake, and the
settlement of the building could be well in excess of the calculated values.
7.5 SETTLEMENT DUE TO DYNAMIC LOADS
CAUSED BY ROCKING
Details on this type of settlement are as follows:
●
Settlement mechanism:
This type of settlement is caused by dynamic structural loads
that momentarily increase the foundation pressure acting on the soil, such as illustrated
in Fig. 7.10. The soil will deform in response to the dynamic structural load, resulting in
settlement of the building. This settlement due to dynamic loads is often a result of the
structure rocking back and forth.
●
Vulnerable soil types:
Both cohesionless soil and cohesive soil are susceptible to rock-
ing settlement. For cohesionless soils, loose sands and gravels are prone to rocking set-
tlement. In addition, rocking settlement and volumetric compression (Sec. 7.4) often
work in combination to cause settlement of the structure.
Cohesive soils can also be susceptible to rocking settlement. The types of cohesive
soils most vulnerable are normally consolidated soils (OCR
1.0), such as soft clays and
organic soils. There can be significant settlement of foundations on soft saturated clays
and organic soils because of undrained plastic flow when the foundations are overloaded
during the seismic shaking. Large settlement can also occur if the existing vertical effec-
tive stress
v
0
plus the dynamic load
v
exceeds the maximum past pressure
vm
of the
v
vm
.
Another type of cohesive soil that can be especially vulnerable to rocking settlement
is sensitive clays. These soils can lose a portion of their shear strength during the cyclic
loading. The higher the sensitivity, the greater the loss of shear strength for a given shear
strain.
●
Susceptible structures:
Lightly loaded structures would be least susceptible to rocking
settlement. On the other hand, tall and heavy buildings that have shallow foundations
bearing on vulnerable soils would be most susceptible to this type of settlement.
●
Example:
Figure 7.11 presents an example of damage caused by rocking settlement.
The rocking settlement occurred to a tall building located in Mexico City. The rocking
settlement was caused by the September 19, 1985 Michoacan earthquake, which is
described in Sec. 4.6.1.
cohesive soil, or
v
0
