ieq-05 ground response to seismic wave-notes

8/10/2019 IEQ-05 Ground Response to Seismic Wave-notes

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Course Title

IEQ-05 Earthquake Geology and Geoinformatics(Dept. of Earthquake Engineering, IIT Roorkee)

Ground Response to Seismic Waves

When a fault ruptures below the earth’s surface, body waves travel away from the

source in all directions. As they reach boundaries between different geologic materials,

they are reflected and refracted. Since the wave propagation velocities of shallowermaterials are generally lower than the materials beneath them, inclined rays that strike

horizontal layer boundaries are usually reflected to a more vertical direction. By the time

the rays reach the ground surface, multiple refractions have often bent them to nearlyvertical direction.

Figure 1: Refraction process that produces nearly vertical wave propagation near the

ground surface.

Soil Properties those play role in effecting the behavior of seismic waves

Density, Resistivity, Cohesive strength , Porosity, Permeability, Compressibility, Bulk

modulus, Modulus of rigidity, Shear strength, Tensile strength

Terms that are commonly used to describe ground motion are as follows. The

motion at the surface of a soil deposit is the free surface motion. The motion at the base

of the soil deposit (also the top of bedrock) is called a bedrock motion. The motion at alocation where bedrock is exposed at the ground surface is called a rock outcroppingmotion. If the soil deposit was not present, the motion at the top of bedrock would be the

bedrock outcropping motion.

Figure 2 Ground response nomenclature: (a) soil overlying bedrock; (b) no soil overlying

bedrock.

Local Site Effect



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to produce amplification factors about 50% higher than those associated with the softer

rock conditions.

Figure 4: Amplification functions for sites A and B. Note that the softer soil at site A will

amplify low-frequency input motions much more strongly than will the stiffer soils of site

B. At higher frequencies, the opposite behavior would be expected.

Soil Amplification

Impedance (ρ.vs) decreases in soil.Seismic wave amplitude increases although velocity of wave decreases.

Rock impedance is high – velocity is high, wavelength is longer.

In soil – wavelength reduces but energy i.e. amplitude increases.

Evidence from Measured Surface Motions

Recordings of ground motion at several locations at a site during a nearbyearthquake indicated variations in ground motion along a section. Ground surface

motions at the rock outcrops were quite similar, but the amplitude and frequency contentof the motions at sites underlain by thick soil deposits were markedly different.

During the Mexico (Michoacan) earthquake of 19 September 1985 (Mag.= 8.0)significant pattern of local site effects have been observed. This earthquake caused only

moderate damage in the vicinity of its epicenter (near the Pacific coast of Mexico) but

caused extensive damage some 350 km away in Mexico City. Studies of ground motions

recorded at different sites in Mexico City illustrated the significant relationship betweenlocal soil conditions and damaging ground motions. Shallow, compact deposits of mostly

granular soil, basalt, or volcanic tuff are found in the Foothills Zone, located west ofdowntown. In the Lake Zone, thick deposits of very soft soils formed from the pluviationof airborne silt, clay, and ash from nearby volcanoes through the waters of ancient lake

extend to considerable depths, as shown in the figure below. These soft soils generally

consist of two soft clay layers separated by a 0- to 6-m thick compact sandy layer.Groundwater is generally found at a depth of about 2 m over most of the lake zone.

Between the Foothill and Lake Zones lies the transition zone, where the soft soil deposits

are thin and interspersed erratically with alluvial deposits.



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Figure 5: Strong motion instruments and geotechnical conditions in Mexico City.

(a) locations of strong motion instruments relative to Foothills, Transition, and Lakezones; (b) contours of soft soil thickness.

The Mexico earthquake produced acceleration at the UNAM (rock) site of only0.03g to 0.04g. In the Transition Zone, peak accelerations at the VIV site were slightly

greater than those at UNAM but still quite low. In the Lake Zone, however, peak

accelerations at the CDA and SCT sites were up to five times greater than those at

UNAM. The frequency contents of the SCT and CDA motions were also much different

than that of the UNAM motion; the predominant period was about 2 sec at SCT andslightly longer at CDA. Strong levels of shaking persisted over a very long duration at the

SCT and CDA sites. The SCT site was underlain by 35 to 40m of soft clay with anaverage s-wave velocity of about 75 m/sec. Structural damage in Mexico City was highly

selective; large parts of the city experienced no damage while other areas suffered

pronounced damage. Damage was negligible in the Foothill Zone and minimal in theTransition Zone. The greatest damage occurred in those portions of the Lake Zone

underlain by 38 to 50 m of soft soil, where characteristic site periods were estimated at

1.9 to 2.8 sec. Even within this area, damage to buildings of less than five stories and

modern buildings greater than 30 stories was slight. Most buildings in the five-to-20-sotry range, however, either collapsed or were badly damaged. Using the crude rule of

thumb that the fundamental period of an N-story building is approximately N/10 sec,most of the damaged building had fundamental periods equal to or somewhat less thanthe characteristic site period. According for the period-lengthening effect of soil-structure

interaction and the tendency for the fundamental period of a structure to increase during a

strong earthquake (due to the reduction in stiffness caused by cumulative architecturaland structural damage), it seems likely that the damaged structures were subjected to

many cycles of large dynamic forces at periods near their fundamental periods. This

“double-resonance” condition (amplification of bedrock motion by the soil deposit and



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amplification of the soil motion by the structure) combined with structural design and

construction deficiencies to cause locally devastating damage.

Evidence from Observed Surface Effects

Shillong Plateau earthquake of 12 June 1897 (M > 8) in northeast India is the

largest intraplate event in the last two centuries to have occurred in the Indiansubcontinent. This earthquake holds a prominent place among the great earthquakes of

the world, not only because of its large magnitude but also because of the large area overwhich it caused damage, liquefaction, and landslides. Destruction was widespread on the

Shillong Plateau and in surrounding areas. The epicentral area of the Assam earthquake

consists of large areas with different geologic site conditions: from predominantly soft

rock on the Shillong plateau to loose saturated sands and silts in the Brahmaputra valley,and even morass in the Sylhet Plains areas which under normal conditions are covered by

extensive and very shallow bodies of water.

Short-period ground motions on the plateau and vicinity were violent and of large

amplitude. Long-period ground motions were reported from large distances and were

apparently responsible for damage sustained in isolated cases by flexible structures suchas timber-frame houses, minarets, and dwellings build on thick alluvial saturated

deposits. Slow oscillatory movements persisted long enough to cause water to splashfrom ponds, suspended objects to sway, and people to experience nausea.

Many free-standing monoliths on the hills were overthrown and the shock broke

down most of the piers of the ancient bridge. Report of projected stone from the ground

on the Shillong Plateau indicates high accelerations, exceeding gravity in verticaldirection, in the epicentral region.

Bhuj (Gujarat) earthquake of 26 Jan 2001 (Mw – 7.6, Ms – 7.6; Depth – 24 km).

The quake has caused total destruction of many small villages close to epicenter and has

caused extensive damages to many dams, embankments, ports, bridges and buildings.This earthquake was characterized by wide spread liquefaction that caused sand

volcanoes, ground cracking, lateral spreading of ground and embankments and water

spouts. Eastward-trending faults and folds characterize Mesozoic, Tertiary, andQuaternary geologic units, the most significant of this study being the Kachchh Mainland

fault. Anticlines and salt domes from the folding of Quaternary fluvial structures form the

northern portion of the region, and folded and thrusted Quaternary units make up the

southern Kachchh geology.

The Rann of Kutch is of completely marshy land with soils at 100% saturation. In

Bhuj, Gandhidam and Anjar area, loose deposits extending to greater depths resulted in

liquefaction thereby causing full or partial subsidence of the structures. Soils containing a

large percentage of medium and fine sand with appreciable amounts of non-plastic finesthat are more prone to liquefaction during ground shaking. There are many cases reported

in the literature where soils containing 10–50% non-plastic fines liquefied under

undrained cyclic loading conditions as the liquefaction potential is to a large extent afunction of its plasticity. In south Gujarat, earthquake tremors have triggered heaving of

expansive clay in to vibro-viscous mass, which has helped the seismic waves to shake

strip-foundations of load bearing walls.



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Liquefaction

Liquefaction is one of the most important, interesting, complex, and controversial.

The liquefaction involves soil deformations caused by disturbance of saturated

cohesionless soils under undrained conditions. The generation of excess pore-pressureunder undrained loading conditions is a hall mark of all liquefaction phenomena.

Liquefaction phenomena that result from this process can be divided into two maingroups: flow liquefaction and cyclic mobility.

Flow Liquefaction : Flow liquefaction can occur when the shear stress required

for static equilibrium of a soil mass (the static shear stress) is greater than the shear

strength of the soil in its liquefied state. Once triggered the large deformations produced by flow liquefaction are actually driven by static shear stresses. Flow liquefaction failures

are characterized by the sudden nature of their origin, the speed with which they develop,

and the large distance over which the liquefied materials often move.

Cyclic Mobility : Cyclic mobility can also produce unacceptably large permanent

deformations during earthquake shaking. Cyclic mobility occurs when the static shearstress is less than the shear strength of the liquefied soil. The deformations produced bycyclic mobility failures develop incrementally during earthquake shaking.

Geologic Criteria

Soil deposits that are susceptible to liquefaction are formed within a relatively

narrow range of geological environments. The depositional environment, hydrologicalenvironment, and age of a soil deposit all contribute to its liquefaction susceptibility.

Geologic processes that sort soils into uniform grain size distributions and deposit them

in loose states produce soil deposits with high liquefaction susceptibility. Consequently,

fluvial deposits, and colluvial and aeolian deposits when saturated, are likely to besusceptible to liquefaction. Liquefaction has also been observed in alluvial-fan, alluvial

plain, beach, terrace, playa, and estuarine deposits, but not as consistently as in those

listed previously. The susceptibility of older soil deposits to liquefaction is generallylower than that of newer deposits. Soils of Holocene age are more susceptible than soils

of Pleistocene age, although susceptibility decreases with age within the Holocene.

Liquefaction occurs only in saturated soils, so the depth to groundwater influences

liquefaction susceptibility. Liquefaction susceptibility decreases with increasing

groundwater depth; the effects of liquefaction are most commonly observed at siteswhere groundwater is within a few meters of the ground surface. At sites where

groundwater levels fluctuate significantly, liquefaction hazards may also fluctuate.

The fact that a soil deposit is susceptible to liquefaction does not mean thatliquefaction will necessarily occur in a given earthquakes. Its occurrence requires a

disturbance that is strong enough to initiate, or trigger, liquefaction.



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Effects of Liquefaction

Liquefaction phenomena can affect buildings, bridges, buried pipelines, and otherconstructed facilities in many different ways. Liquefaction can also influence the nature

of ground surface motions. Flow liquefaction can produce massive flow slides and

contribute to the sinking or tilting of heavy structures, the floating of light buried

structures, and to the failure of retaining structures. Cyclic mobility can cause slumpingof slopes, settlement of buildings, lateral spreading, and retaining wall failure. Substantial

ground oscillation, ground oscillation ground surface settlement, sand boils, and post-earthquake stability failures can develop at level-ground sites.

Liquefaction due to the Assam earthquake of 12 June 1897 was severe andwidespread throughout the Brahmaputra valley and the Sylhet Plains. In the plains the

groundwater gradually rose to the surface and flooded large areas. In many cases, houses

of all types, levees, embankments, and bridge piers sank bodily, their tops aloneremaining above grounds, and liquefied sands filled river channels, water tanks, and

wells. Ground failures caused by lateral spreading, loss of bearing capacity and excessive

ground settlement, eruption of san boils and mud volcanoes, and flows on slopingground, where the earthquake shaking apparently acted as a trigger for the initiation of

retrogressive failures.

With a magnitude of 7.5, the Niigata Earthquake in Japan on June 16, 1964caused considerable damage to Niigata City and the surrounding area because of the

liquefaction phenomenon. About 310 of the 1500 reinforced concrete buildings in NiigataCity were damaged, but of those, 200 settled without significant structural damage as this

photograph illustrates. The building in the center of the picture is at a 70 degree angle to

its upright position. These apartment buildings were built on shallow foundations or on

friction piles in loose soil, whereas many of the same type of buildings that were built onfirm strata at 20 meters deep did not suffer damage.

Widespread liquefaction was observed during Bhuj earthquake. Liquefaction was

sufficient in some cases to activate desert rivers that have been dry for more than a

century. Many mud-volcanoes in the Rann of Kachchh have dimensions of hundreds ofmeters: one covers a 5 km diameter stretch of the southern Rann with dark sand and mud.

Numerous ancient river channels have been illuminated by a pock mark pattern of sand

vents, and some have clearly flowed, and breached their old channels.

ieq-05 ground response to seismic wave-notes

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