Ce152 dc

Introduction to Earthquake Geotechnical
Engineering and It’s Practices
by
Dr. Deepankar Choudhury
Assistant Professor, Department of Civil Engineering,
IIT Bombay, Powai, Mumbai 400 076, India.
URL: http://www.civil.iitb.ac.in/~dc/

Earthquake Hazards related to
Geotechnical Engineering
D. Choudhury, IIT Bombay, India

• Ground Shaking: Shakes structures constructed on
ground causing them to collapse
• Liquefaction: Conversion of formally stable
cohesionless soils to a fluid mass, causing damage to
the structures
• Landslides: Triggered by the vibrations
• Retaining structure failure: Damage of anchored
wall, sheet pile, other retaining walls and sea walls
• Fire: Indirect result of earthquakes triggered by broken
gas and power lines
• Tsunamis: large waves created by the instantaneous
displacement of the sea floor during submarine faulting

Damage due to Earthquakes
Earthquakes have varied effects, including changes in
geologic features, damage to man-made structures and
impact on human and animal life.
Earthquake Damage depends on many factors:
 The size of the Earthquake
 The distance from the focus of the earthquake
 The properties of the materials at the site
 The nature of the structures in the area

Ground Shaking
Frequency of shaking differs for different seismic waves.
High frequency body waves shake low buildings more.
Low frequency surface waves shake high buildings more.
Intensity of shaking also depends on type of subsurface
material.
Unconsolidated materials amplify shaking more than rocks do.
Buildings respond differently to shaking depending on
construction styles, materials
Wood -- more flexible, holds up well
Earthen materials, unreinforced concrete -- very vulnerable to
shaking.

Collapse of Buildings

Soft first story
Loma Prieta earthquake damage in San Francisco. The soft first story is due to construction of garages in the first story and resultant
reduction in shear strength. (Photo from: http://earthquake.usgs.gov/bytopic/photos.html)
On October 17, 1989, at 5:04:15 p.m. (P.d.t.), a magnitude 6.9 (moment magnitude; surface-wave magnitude, 7.1)

Inadequate attachment of building to foundation
House shifted off its foundation, Northridge earthquake.
(Photo from: Dewey, J.W., Intensities and isoseismals, Earthquakes and Volcanoes, Vol. 25, No. 2, 85-93, 1994)

Image of Bachau in Kutch region of Gujarat after earthquake

Failure of Bridge Abutment
Foundation and column of a dwelling at the long-bean-shaped hill
(Kashmir October 8, 2005)

Suspension Bridge in Balakot (Kashmir October 8, 2005)
Right Abutment Moved Downstream

Building design: Buildings that are not
designed for earthquake loads suffer more

Causes failure of lifelines

Earthquake Destruction: Landslides

Earthquake Destruction: Liquefaction
Flow failures of structures - caused by loss of strength of underlying soil
Nishinomia Bridge 1995 Kobe earthquake, Japan

Sand Boil: Ground water rushing to the surface due to liquefaction
Sand blow in mud flats used for salt production southwest of Kandla Port, Gujarat

Lateral Deformation and Spreading

Lateral Spreading: Liquefaction related phenomenon
Upslope portion of lateral spread at Budharmora, Gujarat

Lateral spreading in the soil beneath embankment causes the
embankment to be pulled apart, producing the large crack down the
Cracked Highway, Alaska, 1964
center of the road.

Lateral Deformation and Spreading
Down slope movement of soil, when loose sandy
(liquefiable) soil is present, at slopes as gentle as 0.50
In situations where strengths (near or post
liquefaction) are less than the driving static shear
stresses, deformations can be large, and global
instability often results

Estimation of Lateral Deformation
Estimates of “large” deformations are usually accurate
within a factor of +/- 2; it has been argued that accuracy
is not an issue, because “large” demands mitigation,
regardless of the exact figure
Approaches for estimating lateral displacements:
Statically-derived empirical methods based on back-analysis
of field case histories (Youd et al. 2002,
Hamade et al. 1986)
Simple static limit equilibrium analysis, Newmark sliding
block (with engineering judgement)
Fully non linear, time-domain finite element or finite
difference analyses

Youd Empirical Approach
• Based on earthquake case histories in U.S. and
Japan
• Accurate within a factor 2, generally, least accurate
in the small displacement range
• Two models; sloping ground model and free face
model

 Sloping ground model
Log Du = -16.713 + 1.532 M – 1.406 log R* - 0.012 R + 0.592 log W
+ 0.540 log T15 + 3.413 log (100 – F15) – 0.795 log (D5015 + 0.1 mm)
 Free face model
Log Du = -16.213 + 1.532 M – 1.406 log R* - 0.012 R + 0.338 log S
+ 0.540 log T15 + 3.413 log (100 – F15) – 0.795 log (D5015 + 0.1 mm)
Where Du = estimated lateral ground displacement, m
M = moment magnitude of earthquake
R = nearest horizontal or map distance from the site to the seismic energy source, km
R0 = distance factor that is a function of magnitude, M; R0 = 10(0.89M-5.64)
R* = modified source distance, R* = R + R0
T15 = cumulative thickness of saturated granular layers with corrected below counts (N1)60 < 15, m
F15 = average fines content (fraction passing no. 200 sieves), %, for granular materials within T15
D5015 = average mean grain size for granular materials within T15
S = ground slope, %
W = free face ratio defined as the height (H) of the free face divided by the distance (L) from the
base of the free face to the point in question
Youd Empirical Approach

Other Methods for Lateral Displacement
®Newmark sliding block analysis, which
assumes failure on well defined failure plane,
sliding mass is a rigid block, and so on
®Dynamic finite element programs with
effective stress based soil constitutive models

Newmark’s Sliding block analysis

Liquefied soil exerts higher pressure on retaining
walls,which can cause them to tilt or slide.

Increased water pressure causes collapse of dams

Earthquake Destruction: Fire
Earthquakes
sometimes cause fire
due to broken gas lines,
contributing to the loss
of life and economy.
The destruction of lifelines
and utilities make
impossible for firefighters to
reach fires started and
make the situation worse
eg. 1989 Loma Prieta
1906 San Francisco

Earthquake Destruction: Tsunamis
Tsunamis can be generated when the sea floor abruptly deforms and
vertically displaces the overlying water.
The water above the deformed area is displaced from its equilibrium
position. Waves are formed as the displaced water mass, which acts
under the influence of gravity, attempts to regain its equilibrium.
Tsunami travels at a speed that is related to the water depth - hence,
as the water depth decreases, the tsunami slows.
The tsunami's energy flux, which is dependent on both its wave speed
and wave height, remains nearly constant.
Consequently, as the tsunami's speed diminishes as it travels into
shallower water, its height grows. Because of this effect, a tsunami,
imperceptible at sea, may grow to be several meters or more in height
near the coast and can flood a vast area.

Tsunami Movement: ~600 mph in deep water
~250 mph in medium depth water
~35 mph in shallow water
Tsunami

The tsunami of 3m height at Shikotan, Kuril Islands, 1994 carried
this vessel 70 m on-shore. The waves have eroded the soil and
deposited debris.

Foundation failure in Kerala during Tsunami (December 26th, 2004)

Geomorphological Changes
•Geomorphological changes are often caused by an
earthquake: e.g., movements--either vertical or horizontal--
along geological fault traces; the raising, lowering, and
tilting of the ground surface with related effects on the flow
of groundwater;
•An earthquake produces a permanent displacement across
the fault.
•Once a fault has been produced, it is a weakness within
the rock, and is the likely location for future earthquakes.
•After many earthquakes, the total displacement on a large
fault may build up to many kilometers, and the length of the
fault may propagate for hundreds of kilometers.

Ground Improvement for Liquefaction
Hazard Mitigation

Ground Improvement in IS Code
“In poor and weak subsoils, the design of conventional shallow
foundation for structures and equipment may present problems
with respect to both sizing of foundations as well as control of
foundation settlements. Traditionally, pile foundations have been
employed often at enormous costs. A more viable alternative in
certain solutions, developed over the recent years, is to improve the
subsoil itself to an extent such that the subsoil improvement would
have resultant settlements within acceptable limits. The techniques
for ground improvement has developed rapidly and has found large
scale application in industrial projects.”
IS 13094 : 1992 (Reaffirmed 1997)

Ground Improvement in IS Code
Ground improvement is indicated if
→Net loading intensity of the foundation
exceeds the allowable bearing pressure as per
IS 6403:1981
→Resultant settlement or differential settlement
(per IS 8009 Part 1 or 2) exceeds acceptable
limits for the structure
→The subsoil is prone to liquefaction in
seismic event

Types of Ground Improvement by Function
1. Excavation, fill placement, groundwater table
lowering
2. Densification through vibration or compaction
3. Drainage through dissipation of excess pore water
pressure
4. Resistant through inclusions
5. Stiffening through cement or chemical addition
Note some method serve multiple functions

Densification through vibration and compaction
Vibrating probe/vibroflotation
→ Vibrations of probe cause grain structure to collapse densifying soil;
raised and lowered in grid pattern
Most Suitable Soil Type Saturated or dry clean sand
Max effective treatment
depth 20 m, ineffective in upper 3-4 m.
Special materials
required None
Special equipment
required Vibratory pile driver or vibroflot equipment
Properties of treated
material Can obtain up to Dr = 80%
Special advantages and
limitations
+ Rapid, simple, cheaper than VR stone columns, compaction
piles – less effective than methods that employ compaction as
well as vibration, difficult to penetrate stiff overlayers, may be
ineffective for layered systems
Relative Cost Moderate

Vibro-compaction/replacement stone/sand columns
→Steel casing is driven in to the soil, gravel or sand is filled from the top
and tamped with a drop hammer as the steel casing is successfully
withdrawn, displacing the soil
Most Suitable Soil Type Cohesionless soil with less than 20% fines
depth 30 m
Special materials
required Granular Backfill
Special equipment
required Vibrofolt equipment, steel casing, hopper for backfill
material Can obtain high relative density
limitations
+ Rapid, useful for a wide range of soil types
– May require a large volume of backfill, noisy
Relative Cost Moderate

Dynamic Densification (heavy tamping)
•A heavy weight is dropped in a grid pattern, for several passes
Most Suitable Soil Type Cohesionless soil, waste fills, partly saturated soils, soils with
fines
depth
30 m, less at the surface, degree of improvement usually
decreases with depth
Special materials
required None
Special equipment
required Tamper and crane
material Good improvement and reasonable uniformity
limitations
+ Rapid, simple, may be suitable for soils with fines
– lack of uniformity with depth, not possible near existing
structures, may granular backfill surface layer
Relative Cost low

Other methods
→Displacement piles: densification by displacement of
pile volume, usually precast concrete or timber piles
→Compaction grouting: densification by displacement
of grout volume

Stiffening through cement or chemical addition
Permeation or penetrating grouting: High permeability grout is injected into the ground at
numerous points, results in solidified soil mass
Most Suitable Soil Type Saturated medium to coarse sand
depth > 30m
Special materials
required Grout
Special equipment
required Mixers, tanks, pumps, hoses, monitoring equipment
material Impervious, high strength where completely mixed
limitations
+ Produces a hard, stiff mass of soil, useful for existing
structures as it causes little or no settlement or disturbance, low
noise
– Area of permeation can vary, can be blocked by pockets of
soil with fines, difficult to determine the improved area, requires
curing time
Relative Cost Least expensive of grout systems, but moderately expensive
compared to vibro methods

Earthquake resistant design of geotechnical structures
Geotechnical structures like,
Retaining wall/Sheet pile
Slope
Shallow foundations
Deep foundations
Must be designed to withstand the earthquake loading

Seismic Design of Retaining Wall

Mononobe-Okabe (1926, 1929) Method

Seismic Slope Stability

Wedge Method of Analysis by Terzaghi (1950)

Seismic Bearing Capacity of Shallow Foundations

Seismic Bearing Capacity of Shallow Strip Footings
Choudhury, D. and Subba Rao, K. S. (2005), “Seismic bearing capacity of shallow strip footings”,
Geotechnical and Geological Engineering, An International Journal, Springer, Netharlands, 23(4): 403-418.

Guideline as per Indian Code
• According to IS 1893, isolated RCC footing without tie
beams, or unreinforced strip foundation shall not be
permitted in soft soils
• Shallow foundation elements should be tied together so that
they move uniformly, bridge over areas of local settlements,
resist soil movements which ultimately reduces the level of
shear forces induced in the elements resting on the
foundation
• Buried utilities, such as sewage and water pipes, should
have ductile connections to the structure to accommodate
the large movements and settlements that can occur under
seismic loading

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