Liquefaction
Li
f ti and
d Seismic
S i i Slope
Sl
Stability
y
Prof. Ellen M. Rathje, Ph.D., P.E.
Department of Civil,
Civil Architectural
Architectural, and
Environmental Engineering
University of Texas at Austin
19 November 2010
Seismic Design Framework
Source Characterization
Ground Motion
Characterization
Locations of sources (faults)
Magnitude (Mw)
Recurrence
Closest distance fault to site (Rcl)
Local site conditions
Ground Motion Level
Liquefaction?
Landslide?
Rrup
Soil conditions
Topographic conditions
Liquefaction
Occurs in loose sand below the water table
Strong shaking:
– Increases pore water pressures
– Decreases
D
effective
ff ti stress
t
– Decreases strength
Effects
– Soil and water shoot out of ground
– Buildings tilt and settle
– Ground spreading on slopes (e.g., river bank)
Liquefaction Video
1964 Niigata (Mw = 7.5) Earthquake in Japan
Niigata Airport
Foundation Failures
Lateral Spreading
Liquefaction Assessment
• Determine if soil type is liquefiable
− Sand,
Sand non-plastic
non plastic silt
• Characterize the cyclic resistance of soil
− Cyclic stress ratio (CSR) =  / v
− Cyclic resistance ratio (CRR) = CSR to cause
li
liquefaction
f ti
− Estimated from SPT blowcount
• Characterize seismic loading (CSREQ)
− Estimated from PGA and earthquake M
• Factor of Safety = CRR / CSREQ
Cyclic Resistance Ratio
C
CRR
• (N1)60 = SPT blowcount
corrected to 60%
theoretical energy and
v = 1 atm (100 kPa)
From Youd et al. (2001)
Standard Penetration Test (SPT)
• 63.5 kg mass dropped 0.75 m on
top of drill rod
• 5 cm diameter split spoon sampler
• Count blows to advance sampler
3 15 cm intervals
3,
N = blowcount = # blows / 30 cm
from the 2nd and 3rd 15 cm intervals
SPT Procedures
• Standard procedures apply 60% of
theoretical drop energy
− Safety hammer with rope/pulley release (60%)
( 40%)
− Donut hammer with rope/pulley (~40%)
− Automatic trip hammer (~70%)
• Smaller energy  increases N
• Larger energy  decreases N
Safety Hammer
SPT Analyzer
SPT Corrections
• Overburden stress (i.e., depth) also affects
blowcount
• Correct blow count to 60% theoretical
energy and v = 1 atm
((N1)60 = N · ((Energy
gy Ratio / 60)) · ((CN)
Energy Ratio = % of theoretical energy imparted
CN = 1 /  v with v in units of atm
O h corrections
Other
i
for
f rod
d llength,
h b
bore h
hole
l
diameter, etc.
CRR
CRR
CRR = CRRM=7.5, =1 atm · MSF · K
M = 7.5,
v= 1 atm
CSREQ
• Related to PGA and earthquake magnitude
CSREQ = 0.65 · PGA · (v / v) · rd
FS = CRR / CSREQ
FS > 1.2 ok!
FS  1.2
Liquefaction!
Liquefaction Assessment
• Port at Port-au-Prince
N60 (blows/30cm)
0
0
5
Depth (m)
10
15
20
25
30
10
20
30
40
Dealing with Liquefaction
• Soil Improvement
− Stone columns (densify and reinforce soil)
− Deep dynamic compaction (densify)
− Grouting
G ti (ground
(
d modification)
difi ti )
• Foundation Improvement
− Deep foundations (driven piles, drilled piers)
that extend beyond the liquefaction layer
− Must be stiff enough to resist lateral forces
Earthquake-Induced Landslides
Yield Acceleration (ky)
tan    w  m  tan  
c
FS 


  t  sin  tan 
  tan 
ky 
( FS  1)  g
tan    (1 / tan  )
When acceleration = ky, FS = 1.0
Stability Assessment
• If PGA  ky
Acceleration (g)
A
− FS < 1.0 (but only at
peak in time history)
− Performance
P f
still
till may
be ok
− Often movements
mo ements not
large until PGA > 2· ky
(i e ky / PGA < 0.5)
(i.e.,
0 5)
0.2
k y = 0.1 g
0
0
2
4
6
8
0
2
4
6
8
-0.2
Time (s) 10
-0.4
Sliding Vel. (cm/s)
• If PGA > ky
04
0.4
30
20
10
0
10
Time (s)
Slidin
ng Displ. (cm)
− FS  1.0
1 0 (no problem!)
If accel exceeds ky:
8
6
4
2
0
0
2
4
6
8
10
Time (s)