SOLUTIONS TO PROBLEMS - Geotechnical Earthquake Engineering - page 640

Step 2: Check site class E, which must have more than 10 feet of soil having the fol-
lowing characteristics: plasticity index PI

20, moisture content w 40%, and undrained

shear strength s u 500 psf. All three layers exceed s u of 500 psf (24 kPa), so this condition

is not met.
Step 3: Site class will be C, D, or E, depending on the undrained shear strength. Using
Eq. (14.1):

s u d c [ ( d i s ui )] (20) (5 150 10 100 5 60) 92 kPa or 1900 psf
From Table 14.11 the answer is site class D—stiff soil profile.

14.5 The upper three layers remain the same and hence based on the clay layers, it is
still site class D.
Check the sand layer as follows:

( N 1 ) 60 d s [ ( d i ( N 1 ) 60 )] (10) (10 55) 55 blows/ft
From Table 14.11 the sand layer would be site class C. However, for different soils, use
the weaker soil and hence the answer is site class D—stiff soil profile.
14.6 The upper two layers remain the same and hence for the clay layers:

s u d c [ ( d i s ui )] (15) (5 150 10 100) 110 kPa or 2300 psf
From Table 14.11, for the clay layers, site class C.
For the two lower sand layers:

( N 1 ) 60 d s [ ( d i ( N 1 ) 60 )] (15) (5 12 10 8) 9 blows/ft
From Table 14.11, the sand layers would be site class E. For different soils, use the
weaker soil and hence the answer is site class E—soft soil profile.

14.7 The upper layer consists of very soft organic silt, very loose organic silty sand,
and very soft peat. The standard penetration tests in this layer indicate uncorrected N values
from 1 to 4. Given these low SPT values and the expected strong shaking from the earth-
quake, it is likely that the silt and silty sand layers will liquefy during the design earthquake.
Hence, per Table 14.11, the answer is site class F.
14.8 Since the structure will be built with piers that penetrate the potentially liquefi-
able layer and are embedded in solid rock, the dynamic forces exerted on the structure
during the earthquake would probably be closer to those values based on site class A rather
than site class F.
14.9 Since the structural engineer has determined that the proposed structure will
be Seismic Design Category D, from Section 1803.5.12 of the International Building
Code (2012), the maximum considered earthquake geometric mean (MCE G ) PGA must be
determined. ASCE Standard 7-10 (2010), that is titled “Maximum Considered Earthquake
Geometric Mean (MCE G ) Peak Ground Acceleration,” states:
“21.5.1 Probabilistic MCE G Peak Ground Acceleration: The probabilistic geometric
mean peak ground acceleration shall be taken as the geometric mean peak ground accel-
eration with a 2 percent probability of exceedance within a 50-year period.”
This value has been determined by the engineering geologist and is 0.55 g .
“21.5.2 Deterministic MCE G Peak Ground Acceleration: The deterministic geometric
mean peak ground acceleration shall be calculated as the largest 84 th percentile geometric