Civil Engineering Reference
In-Depth Information
and distance to the seismic source
3 mi, the value of
N
v
1.6. Therefore, the value of
the seismic coefficient
C
v
0.56
N
v
0.56(1.6)
0.90.
5.
Values of T
s
and T
0
[Eqs. (11.5) and (11.6)]:
The values of
T
s
and
T
0
can be calculated
as follows:
C
v
0.90
_____
_________
T
s
2.5
C
a
2.5 (0.48)
0.75 s
T
0
0.2
T
s
0.20 (0.75)
0.15 s
By using Fig. 11.10 and the values of
C
a
0.48,
C
v
0.90,
T
s
0.75 s, and
T
0
0.15 s,
the response spectrum can be developed such as shown in Fig. 11.11.
11.6
FOUNDATIONS ON ROCK
This section presents an introduction to foundations on rock. In areas of high seismic
activity and weak surface soils, it is always desirable to have the foundations supported
by rock. If the depth to rock is shallow, then strip footings and/or pad footings bearing on
the rock are often constructed. For deeper rock layers, driven piles or piers can be used to
provide the needed foundation support.
11.6.1
Lightly Loaded Foundations on Rock
For lightly loaded foundations supported by hard and sound rock, the total settlement
max
and maximum differential settlement
may be essentially zero. In some cases, the rock
may actually be stronger than the foundation concrete. When dealing with lightly loaded
foundations to be supported by hard and sound rock, an extensive investigation is usually
not justified and the allowable bearing pressure from the building code is often recom-
mended (e.g., see Tables 8.2 and 14.6). For hard and sound rock, usually the allowable
bearing pressure from the building code will have an ample factor of safety. Exceptions
include the following:
1.
Weathered or fractured rock:
The rock may be so weathered that its behavior is
closer to a soil than intact rock. The allowable bearing capacity from the building code,
such as the values listed in Tables 8.2 and 14.6, may be too high for foliated, friable,
weakly cemented, highly jointed, or other conditions that result in weak rock.
2.
Expansive rock:
Some types of rock, such as claystone and shale, may be expansive.
Instead of settlement or bearing capacity problems, these types of rock may expand upon
moisture intrusion.
3.
Unstable ground:
Lightly loaded foundations on sound rock can experience
excessive settlement during the earthquake if there is collapse of underground limestone
cavities, sinkholes, and abandoned mines and tunnels.
4.
Cut-fill transition:
A cut-fill transition occurs when a building pad has some rock
removed (the cut portion), with a level building pad created by filling in with soil the re-
maining portion. If the cut side of the building pad contains non-expansive rock that is hard
