Introduction to Earthquake Geotechnical Engineering and Its Practices
Earthquake Hazards related to Geotechnical Engineering
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 structuresLandslides: Triggered by the vibrationsRetaining structure failure: Damage of anchored wall, sheet pile, other retaining walls and sea wallsFire: Indirect result of earthquakes triggered by broken gas and power linesTsunamis: large waves created by the instantaneous displacement of the sea floor during submarine faulting
Damage due to EarthquakesEarthquakes 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 EarthquakeThe distance from the focus of the earthquakeThe properties of the materials at the siteThe nature of the structures in the area
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.
Collapse of Buildings
Soft first storyLoma 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 foundationHouse 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
Foundation and column of a dwelling at the long-bean-shaped hill(Kashmir October 8, 2005)Failure of Bridge Abutment
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
D. Choudhury, IIT Bombay, IndiaEarthquake Destruction: Landslides
Flow failures of structures - caused by loss of strength of underlying soilEarthquake Destruction: Liquefaction
Sand blow in mud flats used for salt production southwest of Kandla Port, GujaratSand Boil: Ground water rushing to the surface due to liquefaction
Lateral Deformation and Spreading
Upslope portion of lateral spread at Budharmora, GujaratLateral Spreading: Liquefaction related phenomenon
Lateral spreading in the soil beneath embankment causes the embankment to be pulled apart, producing the large crack down the center of the road.
Lateral Deformation and SpreadingDown 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 DeformationEstimates 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 figureApproaches 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
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
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, mM = moment magnitude of earthquakeR = nearest horizontal or map distance from the site to the seismic energy source, kmR0 = distance factor that is a function of magnitude, M; R0 = 10(0.89M-5.64)R* = modified source distance, R* = R + R0T15 = cumulative thickness of saturated granular layers with corrected below counts (N1)60 < 15, mF15 = average fines content (fraction passing no. 200 sieves), %, for granular materials within T15D5015 = average mean grain size for granular materials within T15S = 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 questionYoud 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
Newmarks 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
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
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
D. Choudhury, IIT Bombay, India
Tsunami Movement: ~600 mph in deep water ~250 mph in medium depth water ~35 mph in shallow waterTsunami
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 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
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 CodeIS 13094 : 1992 (Reaffirmed 1997)
Ground improvement is indicated ifNet loading intensity of the foundation exceeds the allowable bearing pressure as per IS 6403:1981Resultant settlement or differential settlement (per IS 8009 Part 1 or 2) exceeds acceptable limits for the structureThe subsoil is prone to liquefaction in seismic eventGround Improvement in IS Code
Excavation, fill placement, groundwater table loweringDensification through vibration or compactionDrainage through dissipation of excess pore water pressureResistant through inclusions Stiffening through cement or chemical additionTypes of Ground Improvement by FunctionNote some method serve multiple functions
Vibrating probe/vibroflotationVibrations of probe cause grain structure to collapse densifying soil; raised and lowered in grid patternDensification through vibration and compaction