Introduction to Earthquake Geotechnical Engineering and It’s Practices by Dr. Deepankar Choudhury...

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

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 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 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 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)

http://earthquake.usgs.gov/bytopic/photos.html








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


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


Earthquake Destruction: Landslides


Nishinomia Bridge 1995 Kobe earthquake, Japan

Flow failures of structures - caused by loss of strength of underlying soil

Earthquake Destruction: Liquefaction


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


Lateral Deformation and Spreading


Upslope portion of lateral spread at Budharmora, Gujarat

Lateral Spreading: Liquefaction related phenomenon


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.


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


• 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, 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


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


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 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


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 Code

IS 13094 : 1992 (Reaffirmed 1997)


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


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


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


Most Suitable Soil Type Cohesionless soil with less than 20% fines


30 m


Granular Backfill


Vibrofolt equipment, steel casing, hopper for backfill


Can obtain high relative density


+ 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


Most Suitable Soil TypeCohesionless soil, waste fills, partly saturated soils, soils with fines


30 m, less at the surface, degree of improvement usually decreases with depth


None


Tamper and crane


Good improvement and reasonable uniformity


+ 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


Other methods

→Displacement piles: densification by displacement of pile volume, usually precast concrete or timber piles

→Compaction grouting: densification by displacement of grout volume


Most Suitable Soil Type Saturated medium to coarse sand


> 30m


Grout


Mixers, tanks, pumps, hoses, monitoring equipment


Impervious, high strength where completely mixed


+ 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


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.


• 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


Questions?

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Introduction to Earthquake Geotechnical Engineering and It’s Practices by Dr. Deepankar Choudhury...

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