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/
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
D. Choudhury, IIT Bombay, India
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
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Ground ShakingFrequency 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.
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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)
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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)
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Foundation and column of a dwelling at the long-bean-shaped hill(Kashmir October 8, 2005)
Failure of Bridge Abutment
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Suspension Bridge in Balakot (Kashmir October 8, 2005) Right Abutment Moved Downstream
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Building design: Buildings that are not designed for earthquake loads suffer more
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Nishinomia Bridge 1995 Kobe earthquake, Japan
Flow failures of structures - caused by loss of strength of underlying soil
Earthquake Destruction: Liquefaction
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Sand blow in mud flats used for salt production southwest of Kandla Port, Gujarat
Sand Boil: Ground water rushing to the surface due to liquefaction
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Upslope portion of lateral spread at Budharmora, Gujarat
Lateral Spreading: Liquefaction related phenomenon
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Cracked Highway, Alaska, 1964
Lateral spreading in the soil beneath embankment causes the embankment to be pulled apart, producing the large crack down the
center of the road.
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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
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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
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• 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
Youd Empirical Approach
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Sloping ground modelLog 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 modelLog 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
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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
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Liquefied soil exerts higher pressure on retaining walls,which can cause them to tilt or slide.
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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 worseeg. 1989 Loma Prieta
1906 San Francisco
Earthquake Destruction: Fire
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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.
Earthquake Destruction: Tsunamis
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Tsunami Movement: ~600 mph in deep water
~250 mph in medium depth water
~35 mph in shallow water
Tsunami
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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.
D. Choudhury, IIT Bombay, India
•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.
Geomorphological Changes
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“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.”
Ground Improvement in IS Code
IS 13094 : 1992 (Reaffirmed 1997)
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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
Ground Improvement in IS Code
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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
Types of Ground Improvement by Function
Note some method serve multiple functions
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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
Vibrating probe/vibroflotation→ Vibrations of probe cause grain structure to collapse densifying soil;
raised and lowered in grid pattern
Densification through vibration and compaction
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Most Suitable Soil Type Cohesionless soil with less than 20% fines
Max effective treatment depth
30 m
Special materials required Granular Backfill
Special equipment required Vibrofolt equipment, steel casing, hopper for backfill
Properties of treated material Can obtain high relative density
Special advantages and limitations
+ Rapid, useful for a wide range of soil types– May require a large volume of backfill, noisy
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
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Most Suitable Soil Type Cohesionless soil, waste fills, partly saturated soils, soils with fines
Max effective treatment depth
30 m, less at the surface, degree of improvement usually decreases with depth
Special materials required None
Special equipment required Tamper and crane
Properties of treated material
Good improvement and reasonable uniformity
Special advantages and 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
Dynamic Densification (heavy tamping)•A heavy weight is dropped in a grid pattern, for several passes
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Other methods
→Displacement piles: densification by displacement of pile volume, usually precast concrete or timber piles
→Compaction grouting: densification by displacement of grout volume
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Most Suitable Soil Type Saturated medium to coarse sand
Max effective treatment depth > 30m
Special materials required Grout
Special equipment required Mixers, tanks, pumps, hoses, monitoring equipment
Properties of treated material
Impervious, high strength where completely mixed
Special advantages and 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
Permeation or penetrating grouting: High permeability grout is injected into the ground at numerous points, results in solidified soil mass
Stiffening through cement or chemical addition
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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
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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.
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• 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
Guideline as per Indian Code