Global journal 40

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014

ISSN:- 2349–283X

A REVIEW ON LIQUEFACTION HAZARD ASSESSMENT FOR A REGION 1Abhishek

Kumar Tiwari Department of Civil Engineering, Madan Mohan Malviya University of Technology, Gorakhpur, India Email: abhicivilengg07@gmail.com

2Dr.S.M

Ali Jawaid, 3 Deepti Singh, & Tarik Salman Department of Civil Engineering, Madan Mohan Malviya University of Technology, Gorakhpur, India Email: 2smaj@rediffmail.com, 3deeptisingh6791@gmail.com 4salmantarik10@gmail.com

ABSTRACT: Earthquake is most dangerous natural disaster because of is unpredictable nature. And liquefaction is one of seismic event where soil can fail with devasting effect such as land sliding, lateral spreading or large ground displacement. The phenomenon of liquefaction has been seen after Nigata (1964) and Alaska (1964) earthquake. Earthquake reduces the strength and stiffness of soil by shaking and other rapid loading.

Liquefaction is one of the most interesting but complex and controversial topics in earthquake engineering. It’s devastating effects in Alaska (USA) and Niigata (Japan) as a result of the 1964 earthquake of magnitudes 9.2 and 7.5 Richter scale drew the attention of the geological engineers of the world and compelled them to think about it. Both earthquakes produced spectacular examples of liquefactioninduced damage including ground failures, bridge and building foundation failures and floatation of buried structures (Kramer, 1996). Since then hundreds of researchers around the world has been studying this hazard. The term liquefaction was originally coined by Mogami and Kubo, (1953). The generation of excess pore pressure under undrained loading condition is a hallmark of all liquefaction phenomena. When cohesionless soils are saturated with water and rapid loading occurs under undrained condition, the tendency for densification causes excess pore pressure to increase and effective stress to decrease hence liquefaction results.

Keywords: Earthquake, liquefaction, microzonation, seismic hazard, Standard penetration test.

1. INTRODUCTION Liquefaction is a soil behaviour phenomenon in which a saturated soil losses a substantial amount of strength due to high excess porewater pressure generated by and accumulated during strong earthquake ground shaking.

Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless, liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them. Some of those earthquakes are Hyogo-ken Nanbu Earthquake around Kobe in 1985, Loma Prieta Earthquake, California in 1989, Mexico Earthquake in 1985, Nigata Earthquake in 1964 (Sakai, et al. 2001

Liquefaction is caused by earthquake shaking.

Before going to the liquefaction hazard assessment, it is important to know that: 

What strength of Earthquake motion and under what conditions of the ground surface causes liquefaction?

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What are the effects of liquefaction and how it can be mitigated?

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What are the guidelines for the preparation of Liquefaction hazard map?

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What are the applications of liquefaction hazard map?

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014

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LITERATURE REVIEW Different experts have various ways of defining liquefaction. Some definitions are given below.

Liquefaction is defined as:

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A phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking or other rapid loading (ABAG's Report, "Real dirt on Liquefaction”, 2001).

The capacity of relatively loose saturated sandy soil to lose a large proportion of its strength under seismic shaking (UNDP / UNCHS/HABITAT, 1994).

Processes by which sediments below the water table temporarily loose strength and behave as a viscous liquid rather than solid mainly caused by seismic waves (Hazus99SR2Technical Manual).

Liquefaction has been observed in earthquakes for many years. In fact, written records dating back hundreds and even thousands of years describe earthquake effects that are now known to be associated with liquefaction. Nevertheless, liquefaction has been so widespread in a number of recent earthquakes that it is often associated with them. Some of those earthquakes are listed below.

Alaska earthquake, USA (1964) as a part of the Pacific Ring, the southern coast area of Alaska experiences many earthquakes. On Good Friday, March 27, 1964, a great earthquake of magnitude 9.2 struck Prince William Sound and caused severe damage in the form of landslides and liquefaction as seen in fig.1 This seismic event is not only the second largest ever to have been recorded but it lasted for over 3 minutes and was felt over an area of 500,000 square miles Liquefaction in sand layers, and in sand and silt seams in the clayey soils beneath Anchorage, caused many of the destructive landslides that occurred during the earthquake (Seed,1973). The liquefied seams and lenses disturbed the sensitive clays, and caused their strengths to drop below the levels needed for stability.

Fig .1 Alaska Earthquakes, USA (courtesy:http://www.ce.washington.edu/~li quefaction/html/quakes) Niigata earthquake, Japan (1964) as the Niigata earthquake of June 16, 1964 had a magnitude of 7.5 and caused severe damage to many structures in Niigata. The destruction was observed to be largely limited to buildings that were founded on top of loose, saturated soil deposits. Even though about 2000 houses were totally destroyed, only 28 lives were lost. The Niigata earthquake, together with the Alaska earthquake also of 1964, brought liquefaction phenomena and their devastating effects to the attention of engineers and seismologists. A remarkable ground failure occurred near the Shinano river bank where the Kawagishi-cho apartment buildings suffered bearing capacity failures and tilted severely as shown in fig. 2.

Despite the extreme tilting, the buildings themselves suffered remarkably little structural damage. Sand boils and ground fissures were observed at various sites in Niigata. Lateral spreading caused the foundations of the Showa bridge to move laterally so much that the simply supported spans became unseated and collapsed.

Fig .2 Niigata earthquakes, Japan (courtesy:http://www.ce.washington.edu/~li quefaction/html/quakes) Loma Prieta, earthquake (1989) as the October 17 1989 Loma Prieta earthquake (M=7.1) caused severe damage not only in the vicinity of the epicentre near Santa Cruz, but

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 also in more distant areas to the north around San Francisco and Oakland. Soil liquefaction caused major damage to waterfront facilities, structures, and buried pipelines at locations in the Bay Area where loose saturated, sandy soils were susceptible to liquefaction. The numerous sand boils that were observed (fig.3) provided indisputable evidence of the occurrence of liquefaction. Liquefaction was observed at a number of sites, including the Oakland airport, sites along the Salinas River, and Moss Landing Marine Station.

Fig .3 Loma Prieta earthquake (courtesy:http://www.ce.washington.edu/~li quefaction/html/quakes) Kope earthquake, Japan (1965) as the 1995 Great Hanshin Earthquake (M=6.9), commonly referred to as the Kobe earthquake, was one of the most devastating earthquakes ever to hit Japan; more than 5,500 were killed and over 26,000 injured. The proximity of the epicentre, and the propagation of rupture directly beneath the highly populated region, help explain the great loss of life and the high level of destruction. The spectacular collapse of the Hanshin expressway illustrates (fig .4) the effects of the high loads that were imposed on structures in the area. The strong ground motions that led to collapse of the Hanshin Express way also caused severe liquefaction damage to port and wharf facilities as can be seen to the left and below.

Fig .4 Kope earthquakes, Japan (courtesy:http://www.ce.washington.edu/~li quefaction/html/quakes)

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SOIL LIQUEFACTION Soil liquefaction has been a major cause of damage to soil structure, lifelines and building foundation. Zoning for liquefaction, therefore, has been an important goal for seismic hazard mitigation. Soil liquefaction occurs in loose, saturated cohesionless soil units (sands and silts) and sensitive clays when a sudden loss of strength and loss of stiffness is experienced, sometimes resulting in large, permanent displacements of the ground. Even thin lenses of loose saturated silts and sands may cause an overlying sloping soil mass to slide laterally along the liquefied layer during earthquakes.

Liquefaction beneath and in the vicinity of a waste containment unit can result in localized bearing capacity failures, lateral spreading, and excessive settlement that can have severe consequences upon the integrity of waste containment systems. Liquefaction-associated lateral spreading and flow failures can also affect the global stability of a waste containment facility.

Mechanism of soil liquefaction There is need to understand the mechanism of soil liquefaction, where it occurs and why it occurs so often during earthquake. Liquefaction of soil is a process by which sediments below the water table lose their shear strength and behave more as viscous liquid than as a solid.

The water in the soil voids exerts pressure upon the soil particles. If the pressure is low enough, the soil stays stable. However, once the water pressure exceeds a certain level, it forces the soil particles to move relative to each other, thus causing the strength of the soil to decrease and failure of the soil. So when the earthquake occurs the shear waves passes through saturated soil layers and causes the granular soil structure to deform and weak part of soil begins to collapse.

Then collapse soil starts filling the lower layer and forces the pore water pressure in this to increase layer. If increased water pressure cannot be released, it will continue to build up and after a certain limit effective stress of the soil becomes zero .If situation aroused then the soil layer losses its shear strength and it cannot

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 certain the total weight of the soil layer above, thus the upper layer soils are ready to move down and behaves as a viscous liquid, if then it is said to be soil liquefaction occurred.

Fig .5 Mechanism of Liquefaction (courtesy: http://www.cee.ehime-u.ac.jp) Liquefaction occurs in saturated soils, in which the space between individual particles is completely filled with water. Its effects are most commonly observed in low-lying areas near bodies of water such as rivers, lakes, bays, and oceans (ABAG's Report, "Real dirt on Liquefaction”, 2001).

According to Kramer (1996), two types of liquefaction exist.

Flow Liquefaction: It occurs 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. When liquefaction occurs in such case the strength of the soil decreases and the ability of soil deposit to support for the structure is reduced.

Cyclic mobility: It occurs when the static shear stress is less than the shear strength of the liquefied soil. It produces unacceptably large permanent deformation during earthquake shaking, which is also known as lateral spreading. It can occur on very gently sloping ground or on virtually flat ground adjacent to bodies of water.

Flow liquefaction occurs much less frequently than cyclic mobility but its effects are usually far more severe. Besides these two types, Ground oscillation, loss of bearing strength and sand boils are common phenomena of Liquefaction.

Factors affecting the liquefaction susceptibility

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Liquefaction susceptibility is a function of the geotechnical properties and topographic position of the unit and is independent of the region’s expected seismicity. Factors affecting liquefaction susceptibility include sedimentation process, age of deposit, water table depth, geologic history, grain size distribution, depth of burial, density state, proximity to a free face and ground slope (Youd and Perkins, 1978). They are briefly described below.

Grain Size Both field and laboratory data show that loosely packed cohesionless sands that are clean without clay or silt can readily liquefy and form great number of clastic dykes, sills and sand blows. According to Valera et al., (1994), the threshold magnitude for sand to liquefy is 5.5 whereas for gravel deposit is 7. Laboratory and insitu observation in recent years showed that non-plastic silts might also liquefy (Ishihara, 1985). Coarse silts with bulky particle shape, which are non-plastic and cohesionless, are fully susceptible to liquefaction. Whereas finer silts with flaky or plate like particles generally exhibit sufficient cohesion to inhibit liquefaction (Ishihara, 1993). Clays remain non susceptible to liquefaction, although sensitive clays exhibit deformation-softening behaviour similar to that of liquefied soil (Kramer 1996). Fine-grained soils that satisfy each of the following four Chinese criteria (Wang, 1979) may be considered susceptible to significant strength loss: · Fraction finer than 0.005 mm = 15% · Liquid Limit (LL) = 35% · Natural water content = 0.9LL (Liquid Limit). · Liquidity index = 0.75.

When clay is contained in the sand and the cohesion is high, liquefaction does not usually occur, and if it does, the displacement is small.

Therefore, there is little settlement of ground, and damage is slight. The presence of even a small amount of cohesion from clay or silt sized material can impede particle rearrangement during cyclic straining and greatly increases resistance to cyclic loading.

As little as 5% clay or silt sized material in a sand deposit can make liquefaction significantly

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 less likely than for the same sand deposit lacking fine grains research. Generally, coarse-grained material such as coarse sand and gravel do not go liquefaction but under very strong seismic shaking and close to the source of the quake, gravel also liquefies. For example during the 1983 Borah Peak, Idaho earthquake M-7.3 extensive liquefaction occurred. During this earthquake large amount of gravel bearing sand were vented on to the ground surface.

Significant lateral spreading also occurred (Andrus et al., 1991). In general, gravel rich deposits are more densely packed than sands and high gravel content increases internal friction resistance making liquefaction more difficult. Liquefaction susceptibility is also influenced by gradation. Well-graded soils are generally less susceptible to liquefaction than poorly graded soil. The filling of voids between larger particles by smaller particles in a wellgraded soil results in lower volume change potential under drained conditions and consequently lower excess pore pressure under undrained condition. Field evidence indicates that most liquefaction failures occur in uniformly graded soils. Similarly, particle shape also influences the liquefaction susceptibility.

Soils with rounded particle shapes are known to densify more easily than soils with angular grains. Since particles rounding frequently occur in fluvial and alluvial environment, liquefaction susceptibility is often high in those areas.

Relative density The arrangement or packing of sand grains has a profound effect on sediment liquefaction susceptibility. Susceptibility of the same sand can be changed from very high to nonsusceptible simply because of a change in packing. The state of looseness or denseness of cohesion less sediment is generally measured in place by the Standard penetration test (SPT).

SPT blow-count values in granular deposits principally reflect states of relative density and static vertical effective stress, which are the factors mainly controlling liquefaction susceptibility.

Table 2.1 shows the relation between blow count and relative density in terms ranging from very loose to very dense. Sands that are

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moderately dense or looser liquefy in many field situations. Dense sands require exceptionally strong shaking and very dense sands probably rarely generate residual pore pressure large enough to form fluidisation features (Seed et al., 1983).

Table .1: Relation between blow count and relative density (Source Craig, 1986)

Depth to water table Liquefaction susceptibility decrease strongly with increasing depth to the water table (Obermeier, 1996). The extent of the control is indicated in the Table 3.2. According to this table, the lowering of water table as little as several meters can change the susceptibility from high to low. Normally liquefaction is not expected at a place where water table is greater than 10 m.

Table 2: Influence of age deposit and depth to water table on liquefaction susceptibility (Obermeier, 1996)

Depth to thickness of Strata Liquefaction during earthquake shaking frequently originate as shallow as 1.5 to 2 m below the ground surface (Obermeier et al, 1986). According to Seed (1979), the source zone for liquefaction can exceed 20 m. The most common depth however is a range between a few meters and about 10 m. optimal depth is about 2-4 m in many field settings where the water table is within a few meters of the surface.

Lacustrine sediment seems to be most favourable for the development of liquefaction effects in thin strata because of the large aerial extent to the strata. According to Ishihara (1985) and Obermeier (1996), a cap thickness exceeding 10 m prevents ground rupture even in the most favourable circumstances for liquefaction.

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014

Previous Liquefaction A great deal of information on liquefaction behaviour has come from post-earthquake field investigations, which have shown that liquefaction often recurs at the same location when soil and groundwater conditions have remained unchanged (Youd, 1984a). Thus, liquefaction case histories can be used to identify specific sites or more general site conditions, that may be susceptible to liquefaction in future earthquakes.

Human made deposits (Fill) those placed without compaction are also very likely to be susceptible to liquefaction (Kramer, 1986).

Conditions for soil liquefaction For the liquefaction to occur in any place the following conditions should be met.  The soil must be susceptible to liquefaction (i.e. the soil should be loose, water-saturated, sandy soil typically between 0 and 10 meters below the ground surface).  Ground shaking must be strong enough to cause susceptible soils to liquefy.  Ground water should lie within 15 meter deep inside the surface Criteria for identifying and mapping Liquefaction areas A number of guidelines are being proposed for developing liquefaction hazard map all over the world. The geotechnical earthquake engineer can systematically evaluate potential liquefaction hazards by addressing the following questions:   

Is the soil susceptible to liquefaction?

If the soil is susceptible, will liquefaction be triggered?

If liquefaction is triggered, will damage occur?

If the answer to the first question is no, the liquefaction hazard evaluation can be terminated with the conclusion that liquefaction hazard do not exist. If the answer is yes, the next questions must be addressed. If answer to all questions is yes then one should be ready for the mapping of liquefaction areas.

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The basic criteria for identifying and mapping the liquefaction areas are:  Areas known to have experienced liquefaction during historic earthquakes.  Areas where uncompacted fills containing liquefaction susceptible materials are present.  Areas underlain by saturated geologically young material mostly Holocene deposit.  Areas where soil testing (geotechnical data) indicates probability of liquefaction.

To understand more about the liquefaction why it occurs and when, it is important to recognize the conditions that exist in a soil deposit before an earthquake. A soil deposit consists of an assemblage of individual soil particles as shown in Fig 5. If we look closely at these particles, we can see that each particle is in contact with a number of neighbouring particles. The weight of the overlying soil particles produce contact forces between the particles. These forces hold individual particles in place and give the soil its strength. Liquefaction occurs when the structure of loose, saturated sand breaks down due to some rapidly applied loading. As the structure breaks down, the loosely packed individual soil particles attempt to move into a denser configuration. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is "trapped" and prevents the soil particles from moving closer together. This is accompanied by an increase in water pressure, which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil deposit. In an extreme case, the pore water pressure may become so high that many of the soil particles lose contact with each other. In such cases, the soil will have very little strength, and will behave more like a liquid than a solid hence; the name "liquefaction" is given to this phenomenon.

Recommended Screening Liquefaction Potential

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Criteria

for

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 The following five screening criteria, from the above reference, are recommended by Ohio EPA for completing a liquefaction evaluation: 

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Geologic age and origin. If a soil layer is fluvial, lacustrine or aeolian deposit of Holocene age, a greater potential for liquefaction exists than for till, residual deposits, or older deposits.

Fines content and plasticity index.

Liquefaction potential in a soil layer increases with decreasing fines content and plasticity of the soil. Cohesionless soils having less than 15 percent (by weight) of particles smaller than 0.005 mm, a liquid limit less than 35 percent, and an in situ water content greater than 0.9 times the liquid limit may be susceptible to liquefaction (Seed and Idriss, 1982).

Saturation. Although low water content soils have been reported to liquefy, at least 80 to 85 percent saturation is generally deemed to be a necessary condition for soil liquefaction. The highest anticipated temporal phreatic surface elevations should be considered when evaluating saturation.

Depth below ground surface. If a soil layer is within 50 feet of the ground surface, it is more likely to liquefy than deeper layers.

Soil Penetration Resistance. Seed et al, (1985), state that soil layers with a normalized SPT blow count [(N1)60] less than 22 have been known to liquefy. Marcuson et al, 1990, suggest an SPT value of [(N1)60] less than 30 as the threshold to use for suspecting liquefaction potential.

Liquefaction has also been shown to occur if the normalized CPT cone resistance (qc) is less than 157 tsf (15 MPa) (Shibata and Taparaska, 1988).

Liquefaction effects Liquefaction-related ground failure historically has caused extensive structural and lifeline damage in urbanized areas around the world.

The damage caused by liquefaction leaves very painful effects to the people. A lot of property and lives are lost due to the effects of Liquefaction. When the ground liquefies in an earthquake, sandy or silty materials saturated with water behave like a liquid, causing pipes to

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leak, roads and airport runways to buckle, and building foundations to be displaced.

Liquefaction potential Analysis If potential exists for liquefaction at a facility, additional subsurface investigation may be necessary. Once all testing is complete, a factor of safety against liquefaction is then calculated for each critical layer that may liquefy.

A liquefaction analysis should, at a minimum, address the following: 

Developing a detailed understanding of site conditions, the soil stratigraphy, material properties and their variability, and the areal extent of potential critical layers.

Developing simplified cross-sections amenable to analysis. SPT procedures are widely used in practice to characterize the soil (field data are easier to obtain on loose cohesion less soils than trying to obtain and test undisturbed samples). The data needs to be corrected as necessary, for example, using the normalized SPT blow count [(N1)60] . The total vertical stress (σvo) and effective vertical stress (σvo') in each stratum also need to be evaluated. This should take into account the changes in overburden stress across the lateral extent of each critical layer, and the temporal high phreatic and piezometric surfaces,

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Calculation of the force required to liquefy the critical zones, based on the characteristics of the critical zone(s) (e.g., fines content, normalized standardized blow count, overburden stresses, level of saturation),

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Calculation of the design earthquake’s effect on each potentially liquefiable layer should be performed using the site-specific in situ soil data and an understanding of the earthquake magnitude potential for the facility, and

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Computing the factor of safety against liquefaction for each liquefaction susceptible critical

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 FIELD METHODS The use of insitu testing is the dominant approach in common engineering practice for quantitative assessment of liquefaction potential. Calculation of two variables is required for evaluation of liquefaction resistance of soils (Seed et al, 2001;Youd et al,2001).They are as follows 1. The seismic demand on a soil layer expressed in terms of CSR and 2. The capacity of the soil to resist liquefaction expressed in terms of CRR. The models proposed by Seed & Idriss methods are extensively used for predicting liquefaction potential using field data.

Seed & Idriss method After the disastrous earthquakes in Alaska and in Niigata, Japan in 1964, Professors Seed and Idriss (1971), developed and published a methodology termed the ‘‘simplified procedure’’ for evaluating liquefaction resistance of soils.

Seed and Idriss (1971) developed and published the basic ‘‘simplified procedure.’’ That procedure has been modified and improved periodically since that time, primarily through landmark papers by Seed (1979), Seed and Idriss (1982), and Seed et al. (1985). In 1985, Whitman (1985) convened a workshop on behalf of the National Research Council (NRC), USA in which 36 experts and observers thoroughly reviewed the state-of-knowledge and the state-of-the-art for assessing liquefaction hazard. That workshop produced a report (NRC 1985) that has become a widely used standard and reference for liquefaction hazard assessment. In January 1996, T. L. Youd and I. M. Idriss convened a workshop of 20 experts to update the simplified procedure and incorporate research find in from the previous decade.

This paper summarizes recommendations from that workshop (Youd and Idriss 1997).

Liquefaction is defined as the transformation of a granular material from a solid to a liquefied state as a consequence of increased pore-water pressure and reduced effective stress (Marcuson 1978). Increased pore-water pressure is induced by the tendency of granular materials to compact when subjected to cyclic shear deformations. The change of state occurs most

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readily in loose to moderately dense granular soils with poor drainage, such as silty sands or sands and gravels capped by or containing seams of impermeable sediment. As liquefaction occurs, the soil stratum softens, allowing large cyclic deformations to occur. In loose materials, the softening is also accompanied by a loss of shear strength that may lead to large shear deformations or even flow failure under moderate to high shear stresses, such as beneath a foundation or sloping ground. In moderately dense to dense materials, liquefaction leads to transient softening and increased cyclic shear strains, but a tendency to dilate during shear inhibits major strength loss and large ground deformations. A condition of cyclic mobility or cyclic liquefaction may develop following liquefaction of moderately dense granular materials. Beneath gently sloping to flat ground, liquefaction may lead to ground oscillation or lateral spread as a consequence of either flow deformation or cyclic mobility. Loose soils also compact during liquefaction and reconsolidation, leading to ground settlement.

Sand boils may also erupt as excess pore water pressures dissipate.

CYCLIC STRESS RATIO (CSR) AND CYCLICRESISTANCE RATIO (CRR) Calculation, or estimation, of two variables is required for evaluation of liquefaction resistance of soils: (1) the seismic demand on a soil layer, expressed in terms of CSR; and (2) the capacity of the soil to resist liquefaction, expressed in terms of CRR. EVALUATION OF CSR Seed and Idriss (1971) formulated the following equation for calculation of the cyclic stress ratio: CSR = (τav /σvo) = 0.65(amax /g) (σvo /σ'vo)rd (1) Where amax = peak horizontal acceleration at the ground surface generated by the earthquake; g = acceleration of gravity; σvo and σ'vo are total and effective vertical overburden stresses, respectively; and rd= stress reduction coefficient. The latter coefficient accounts for flexibility of the soil profile. For routine practice and noncritical projects, the following equations

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 may be used to estimate average values of rd (Liao and Whitman 1986b):

<z ≤23 m

rd= 1.0 -0.00765z for z ≤ 9.15 m (2a) rd= 1.174 -0.0267z for 9.15 m (2b)

Where z= depth below ground surface in meters.

Some investigators have suggested additional equations for estimating rd at greater depths (Robertson and Wride 1998), but evaluation of liquefaction at these greater depths is beyond the depths where the simplified procedure is verified and where routine applications should be applied. Mean values of rd calculated from (2) are plotted in Fig. 2.6, along with the mean and range of values proposed by Seed and Idriss (1971). With past practice, rd values determined from (2) are suitable for use in routine engineering practice. Factor rd calculated from (2) are the mean of a wide range of possible rd, and that the range of rd increases with depth (Golesorkhi 1989).For ease of computation, Blake (1996) approximated the mean curve plotted in Fig. 2.6 by the following equation:

(3) Where z= depth beneath ground surface in meters. Eq. (3) yields essentially the same values for rd as (2), but is easier to program and may be used in routine engineering practice.

FIG.6. rd versus Depth Curves Developed by Seed and Idriss(1971) with Added MeanValue Lines Plotted from Eq. (2)

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EVALUATION OF LIQUEFACTION RESISTANCE (CRR) A plausible method for evaluating CRR is to retrieve and test undisturbed soil specimens in the laboratory. Unfortunately, in situ stress states generally cannot be reestablished in the laboratory, and specimens of granular soils retrieved with typical drilling and sampling techniques are too disturbed to yield meaningful results. Only through specialized sampling techniques, such as ground freezing, can sufficiently undisturbed specimens be obtained.Several field tests have gained common usage for evaluation of liquefaction resistance, including the standard penetration test (SPT), the cone penetration test (CPT), shear-wave velocity measurements (Vs), and the Becker penetration test (BPT).

SPTs are generally preferred because of the more extensive databases and past experience, but the other tests may be applied at sites underlain by gravelly sediment or where access by large equipment is limited.

Criteria for evaluation of liquefaction resistance based on the SPT Criteria for evaluation of liquefaction resistance based on the SPT have been rather robust over the years. Those criteria are largely embodied in the CSR versus (N1)60 plots reproduced in Fig. 2.7 (N1)60 is the SPT blow count normalized to an overburden pressure of approximately 100 kPa (1 ton/sqft) and a hammer energy ratio or hammer efficiency of 60%. Fig. 2.7 is a graph of calculated CSR and corresponding (N1)60 data from sites where liquefaction effects were or were not observed following past earthquakes with magnitudes of approximately 7.5. CRR curves on this graph were conservatively positioned to separate regions with data indicative of liquefaction from regions with data indicative of nonliquefaction. Curves were developed for granular soils with the fines contents of 5% or less,15%, and 35% as shown on the plot. The CRR curve for fines contents<5% is the basic penetration criterion for the simplified procedure and is referred to hereafter as the ‘‘SPT clean sand base curve.’’

The CRR curves in Fig. 2 are valid only for magnitude 7.5 earthquakes

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 Rauch (1998), approximated the clean-sand base curve plotted in Fig.7 by the following equation:

(4) This equation is valid for (N1)60< 30. For (N1)60≥30, clean granular soils are too dense to liquefy and are classed as nonliquefiable. This equation may be used in spreadsheets and other analytical techniques to approximate the cleansand base curve for routine engineering calculations.

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α= exp [1.76 2 (190/FC)] for 5% < FC < 35% (6b) α= 5.0 for FC ≥35% (6c) β= 1.0 for FC ≤5% (7a) 1.5 β= [0.99 1 (FC /1,000)] for 5% < FC < 35% (7b) β= 1.2 for FC ≥35% (7c) Where α and β= coefficients determined from the following relationships: These equations may be used for routine liquefaction resistance calculations. A backcalculated curve for a fines content of 35% is essentially congruent with the 35% curve plotted in Fig. 2.7.The back-calculated curve for fines contents of 15%plots to the right of the original 15% curve.

Other Corrections Several factors in addition to fines content and grain characteristics influence SPT results, as noted in Table 2. Eq. (8) incorporates these corrections

FIG.7 SPT Clean-Sand Base Curve for Magnitude 7.5 Earthquakes with Data from Liquefaction Case Histories (Modified from Seed et al. 1985) Influence of Fines Content In the original development, Seed et al. (1985) noted an apparent increase of CRR with increased fines content. Whether this increase is caused by an increase of liquefaction resistance or a decrease of penetration resistance is not clear. Based on the empirical data available, Seed et al. developed CRR curves for various fines contents reproduced in Fig. 2.7 The following equations were developed by Seed and Idriss (1971) with the assistance of R. B. Seed for correction of (N1)60 to an equivalent clean sand value, (N1)60cs: (N1)60cs= α+β (N1)60 (5) α= 0 for FC ≤5% (6a)

(N1)60 = Nm CN CE CB CR CS (8) Where:Nm= measured standard penetration resistance; CN=factor to normalize Nm to a common reference effective overburden stress; CE= correction for hammer energy ratio (ER); CB= correction factor for borehole diameter; CR= correction factor for rod length; and CS= correction for samplers with or without liners.

Because SPT N-values increase with increasing effective overburden stress, an overburden stress correction factor is applied (Seed and Idriss 1982). This factor is commonly calculated from the following equation (Liao and Whitman 1986a): CN= (P /σ'VO) (9)

Where:-

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 CN normalizes Nm to an effective overburden pressure of σ'vo approximately 100 kPa (1 atm) P a.

CN should not exceed a value of 1.7 The effective overburden pressure σ'vo applied in equation (8) should be the overburden pressure at the time of drilling and testing.

Although a higher ground-water level might be used for conservatism in the liquefaction resistance calculations, the CN factor must be based on the stresses present at the time of the testing.

The CN correction factor was derived from SPT performed in test bins with large sand specimens subjected to various confining pressures (Gibbs and Holtz 1957; Marcus on and Bieganousky 1997). The results of several of these tests are reproduced in Fig. 2.8 in the form of CN curves versus effective overburden stress (Castro 1995).

These curves indicate considerable scatter of results with no apparent correlation of CN with soil type or gradation. The curves from looser sands, however, lie in the lower part of the CN range and are reasonably approximated by (8) and (9) for low effective overburden pressures [200 kPa (<2 tsf)].

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energy ratio for various types of hammer sand anvils are listed in Table 2.3 Because of variations in drilling and testing equipment and differences in testing procedures, a rather wide range in the energy correction factor CE has been observed as noted in the table. Even when procedures are carefully monitored to conform to established standards, such as ASTM D 158699, some variation in CE may occur because of minor variations in testing procedures.

Measured energies at a single site indicate that variations in energy ratio between blows or between tests in a single borehole typically vary by as much as 10%. Where measurements cannot be made, careful observation and notation of the equipment and procedures are required to estimate a CE value for use in liquefaction resistance calculations. Use of goodquality testing equipment and carefully controlled testing procedures conforming to ASTM D 1586-99 will generally yield more consistent energy ratios and CE with values from the upper parts of the ranges listed in Table 2.3.

TABLE.3 Corrections to SPT (Modified from Skempton 1986) as Listed by Robertson and Wride (1998)

Another important factor is the energy transferred from the falling hammer to the SPT sampler. An ER (Energy Ratio) of 60% is generally accepted as the approximate average for U.S. testing practice and as a reference value for energy corrections. The ER delivered to the sampler depends on the type of hammer, anvil, lifting mechanism, and the method of hammer release. Approximate correction factors (CE = ER/60) to modify the SPT results to a 60%

FIG. .8.CN Curves for Various Sands Based on Field and Laboratory Test Data along with Suggested CN Curve Determined from Eqs. (9) and (10) (Modified from Castro 1995) Skempton (1986) suggested and Robertson and Wride (1998) updated correction factors for rod lengths <10 m, borehole diameters outside the recommended interval (65–125mm), and sampling tubes without liners. Range for these correction factors is listed in Table 2.3 For liquefaction resistance calculations and rod lengths <3 m, a CR of 0.75 should be applied as was done by Seed et al. (1985) in formulating the simplified procedure. Although application of rod-length correction factors listed in Table 2.3 will give more precise (N1)60values, these corrections may be neglected for liquefaction resistance calculations for rod lengths between

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 3 and 10 m because rod-length corrections were not applied to SPT test data from these depths in compiling the original liquefaction case history databases. Thus rod-length corrections are implicitly incorporated into the empirical SPT procedure.

STANDARD PENETRATION TEST (SPT) The standard penetration test (SPT) is an empirical dynamic penetration test developed in USA in the 1920’s and was usually carried out in 50 to 100 mm diameter wash borings. The test is most commonly used in situ test especially for cohesionless soils, which cannot be easily sampled. The test is extremely used for determining the relative density, bearing capacity and the angle of shearing resistance of the cohesionless soil. The test is performed using a split barrel sample tube of 50 mm external diameter, 35 mm internal diameter and about 65 mm in length as specified in BS 1377.

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the settlement of cohesionless soil and are described by a number of authors (Meyerhof, 1956; Terzaghi and Peck, 1967). It has wide application in determining the liquefaction susceptibility of a place.

There are a number of factors involved in the SPT, which can affect the blow count, mainly related to poor testing practice. The standard penetration number is corrected for dilatancy effect and overburden effect (Craig, 1986) (i) Dilatancy correction Silty fine sands and fine sands below the water table develop pore pressure, which is not easily dissipated. The pore pressure affects the resistance of the soil and hence the penetration number (N). Terzaghi and peck (1967) recommend the following correction in the case of silty fine sands when the observed value of N exceeds 15.

1

There are different methods of releasing the hammer in different countries. However, the borehole must be cleaned out to the required depth before the hammering is taken place and care must be taken to ensure that the material to be tested is not disturbed.

In this process, a hammer of 63.5 kg weight with a free fall height of 750 mm is used to drive the sampler. Initially the sampler is driven 150 mm into the sand to seat the sampler and by-pass any disturbed sand at the bottom of the borehole. The number of blows required to drive the sampler a further 300 mm is then recorded. This number is called the Standard penetration resistance (N) value. If 50 blows are reached before a penetration of 30 cm no further blows should be recorded. If the test is to be carried out in gravelly soils, the driving shoe is replaced by the 600 cone.

SPT test is very specific tool for the liquefaction hazard analysis. It also has the unique feature of supplying samples for soil classification purpose. Usually SPT is conducted at every 1 m depth or at the change of stratum.

The N-values are extensively used in determining the bearing capacity and predicting

N'= 15 + (N -15) 2

(10) (Craig, 1988) Where:N’ – Corrected SPT value N – Observed SPT value.

If NR ≤ 15, Nc =NR (ii)Overburden Pressure Skempton summarized the evidence regarding the influence of test procedure on the value of standard penetration resistance (Craig, 1986).

Measured N value should be corrected to allow for the different methods of releasing the hammer, the type of anvil and the total length of boring rods. Only the energy delivered to the sampler is applied in penetrating the sand, the ratio of the delivered energy to the free fall energy of the hammer being referred to as the rod energy ratio, which varies between 45% and 78% for the operating procedures used in several countries. It has been recommended that a standard rod energy ratio of 60% should be adopted and that all measured N values should be normalized by simple proportion of energy ratios, to this standard: the normalized values are denoted (N1)60. If a short length of boring rods (<10m) is used in a test, a reflection of energy occurs and a further loss in delivered energy results. A further correction should

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 therefore be applied to the measured N values if the total length of rods is less than 10m.

Standard penetration resistance depends not only on relative density but also on the effective stresses at the depth of measurements. Effective stresses can be represented to a first approximation by effective overburden pressure.

This dependence was first demonstrated in the laboratory by Gibbs and Holtz and was later confirmed in the field. Sand at the same relative density would thus give different value of standard penetration resistance at different depths. Several proposals have been made for the correction of measured N values. The corrected value (N1) is related to the measured value (N) by the factor CN, where N1 = CN N (11) CN = Correction factor and can be obtained from the graph prepared by Liao and Whitman, (1986). Also CN=â&#x2C6;&#x161;

1 đ?&#x153;&#x17D;

´

(12) SPT correction factor, CN

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in the Table 3.1 and are considered to apply to normally consolidated sands. The relative density of sand was described by Terzaghi and Peck (1967) in general terms on the basis of standard penetration resistance, numerical values of relative density as shown in column 3 were subsequently added by Gibbs and Holtz (Craig, 1986) MAGNITUDE SCALING FACTORS (MSFs) The clean-sand base or CRR curves in Figs. 2.7, only to magnitude 7.5 earthquakes. To adjust the clean-sand curves to magnitudes smaller or larger than 7.5, Seed and Idriss (1982) introduced correction factors termed â&#x20AC;&#x2DC;â&#x20AC;&#x2DC;magnitude scaling factors (MSFs).â&#x20AC;&#x2122;â&#x20AC;&#x2122; These factors are used to scale the CRR base curves upward or downward on CRR versus (N1)60, qc1N, or Vs1 plots. Conversely, magnitude weighting factors, which are the inverse of magnitude scaling factors, may be applied to correct CSR for magnitude. Either correcting CRR via magnitude scaling factors, or correcting CSR via magnitude weighting factors, leads to the same final result.

To illustrate the influence of magnitude scaling factors on calculated hazard, the equation for factor of safety (FS) against liquefaction is written in terms of CRR, CSR, and MSF as follows: FS = (CRR7.5/CSR) MSF (13) Where CSR = calculated cyclic stress ratio generated by the earthquake shaking; and CRR7.5 = cyclic resistance ratio for magnitude 7.5 earthquakes. CRR7.5 is determined from Fig. 2 7 for SPT data

Fig .9: SPT overburden correction factor after Liao and Whiteman (1986)

(N1)60 is the standard penetration resistance normalized to a rod energy ratio of 60% and an effective overburden pressure of 100 kN/m2.

Appropriate values of (N1)60 were added to the Terzaghi and Peck (1967) classification of relative density by Skempton (1986) as shown

Seed and Idriss (1982) Scaling Factors Because of the limited amount of field liquefaction data available in the 1970s, Seed and Idriss (1982) were unable to adequately constrain bounds between liquefaction and nonliquefaction regions on CRR plots for magnitudes other than 7.5. Consequently, they developed a set of MSF from average numbers of loading cycles for various earthquake magnitudes and laboratory test results. A representative curve developed by these

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investigators, showing the number of loading cycles required to generate liquefaction for given CSR, is reproduced in Fig. 2.10. The average number of loading cycles for various magnitudes of earthquakes is also noted on the plot. The initial set of magnitude scaling factors was derived by dividing CSR values on the representative curve for the number of loading cycles corresponding to a given earthquake magnitude by the CSR for 15 loading cycles (equivalent to a magnitude 7.5 earthquake).

These scaling factors are shown in Fig. 2.10.

These MSFs have been routinely applied in engineering practice since their introduction in 1982.

The workshop participants recommend these revised scaling factors as a lower bound for MSF values. The revised scaling factors are significantly higher than the original scaling factors for magnitudes <7.5 and somewhat lower than the original factors for magnitudes >7.5. Relative to the original scaling factors, the revised factors lead to a reduced calculated liquefaction hazard for magnitudes <7.5, but increase calculated hazard for magnitudes >7.5.

Revised Idriss Scaling Factors In preparing his H. B. Seed Memorial Lecture, I. M. Idriss re-evaluated the data that he and the late Professor Seed used to calculate the original (1982) magnitude scaling factors.

1) By Avoiding Liquefaction Susceptible Soils Construction on liquefaction susceptible soils is to be avoided. It is required to characterize the soil at a particular building site according to the various criteria’s available to determine the liquefaction potential of the soil in a site.

Remedial measures The soil liquefaction hazard cannot be prevented but can be reduced by taking the following steps:

2) Build Liquefaction Resistant Structures The structure constructed should be liquefaction resistant i.e., designing the foundation elements to resist the effects of liquefaction if at all it is necessary to construct the structure on liquefiable soil because of favourable location, space restriction and other reasons.

FIG. 10: Representative Relationship between CSR and Number of Cycles to Cause Liquefaction (Reproduced from Seed and Idriss 1982) In so doing, Idriss replotted the data on a log-log plot and suggested that the data should plot as a straight line. He noted, however, that one outlying point had strongly influenced the original analysis, causing the original plot to be nonlinear and characterized by unduly low MSF values for magnitudes <7.5. Based on this reevaluation, Idriss defined a revised set of magnitude scaling factors shown in Fig. 2.10.

The revised MSFs are defined by the following equation: MSF = 102.24/M w 2.56 (14)

3) Improve the Soil This involves mitigation of the liquefaction hazards by improving the strength, density and drainage characteristics of the soil. This can be done using variety of soil improvement techniques.

These are normally applied when designing and constructing new buildings or other structures as bridges, tunnels, and roads etc. The liquefiable soil can be improved by increasing the drainage capability of soil or by applying drainage techniques for example using gravel sand or synthetic material (ABAG's Report, 2001) Conclusion Determination of liquefaction potential due to earthquake is complex geotechnical problem.

Many factors including soil parameters and seismic characteristics influence this

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GJESR REVIEW PAPER VOL. 1 [ISSUE 10] NOVEMBER, 2014 phenomenon. The devastating damage of liquefaction induced ground failures in the Alaska 1964 and Niigata 1964 earthquakes serve as a clear reminder of such events.

Liquefaction is one of the ground failures in potential earth science hazard. Since area with high seismic probability, there is need for the assessment of liquefaction potential to recognize the conditions that exist in a soil deposit before an earthquake in order to identify liquefaction.

The simplified procedure suggested by seed & Idriss (1971). The method (on the basis of field performance data) suggested by seed & Idriss (1971) is used for determination of liquefaction potential and also to present a liquefaction hazard map using SPT data collected from the various sites at different locations of an study area. From the analysis it is observed that the areas with river channel deposit are the most hazardous area for liquefaction. From the study it is also concluded that if acceleration level is increased then more area will be affected due to liquefaction. In this study we concluded that if earthquake more than or equal to 6.5 ritcher scale occurs in region, it will be extensively damaged due to liquefaction. The percentage of silt and poorly graded sand is high in the area under “Ramgarh Tal Pariyojna” indicating that there is a great chance of soil liquefaction. Here liquefaction potential analysis is done to determine the factor of safety at different depth.

The structure constructed should be liquefaction resistant i.e., designing the foundation elements to resist the effects of liquefaction if at all it is necessary to construct the structure on liquefiable soil because of favourable location, space restriction and other reasons.

REFERENCES 1. Rao, K. S. and Mohanty, W. K., Microzonation of Delhi region: An approach. J. Indian Build. Congr.,2001, 8, 102–114.

2. Rao, K. S., Liquefaction studies for seismic microzonation of Delhi region.

In Indian Geotechnical Conference, 2001, vol. 2, pp. 44–51.

3. IS: 1893-2002, Criteria for earthquake resistant design of structures.

4. Rao, K. S., Stability and rehabilitation aspects of earth dams dam- aged during

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7.

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9.

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the Bhuj earthquake, India.

Proc.Forensic Geotech. Engg. 2003, 1, 151–158.

Youd, T. L. and Perkins, D. M., Mapping liquefaction induced ground failure potential. J. Geotech. Engg. Div. ASCE, 1978, 104, 433–446.

DST Report, Geo-Scientific studies in and around Delhi, 2004, p. 74.

TC4, Committee of ISSMFE Manual for Zonation on Seismic Geotechnical Hazards, Japan, 1993, p. 149.

Seed, H. B. and Peacock, W. H., Test procedures for measuring soil liquefaction characteristics. J. Soil Mech.

Found. Div., 1971, 97, 1099–1199.

Seed, H. B., Soil liquefaction and cyclic mobility evaluation for level ground during earthquakes. J. Geotech. Eng. Div., 1979, 105, 210–255.

Iwasaki, T., Tokida, K., Tatsuoka, F., Watanabe, S., Yasuda, S. and Sato, H., Microzonation for soil liquefaction potential using simplified methods. In Proceedings of Third International Conference on Microzonation, Seattle, 1982, vol. 3, pp. 1310–1330.

Seed H.B., Tokimatsu, K., L.F.and Chung, R.M. (1985). “The Influence of SPT Procedures in soil Liquefaction Resistance Evaluations”, J. of Geotech Engg Div., ASCE, 111(20), pp. 14251445.

12. Whitman, R.V., 1971. Resistance of Soil to Liquefaction and Settlement. Soils Found 11 (4), 59–68.

13. Youd, T. L. and Perkins, D. M, (2004) Mapping Liquefaction Induced Ground Failure Potential. J. Geotech. Eng. Div. ASCE, 1978, 104, 433–446. DST Report, Geo-Scientific Studies in and around Delhi, , p. 74.

14. Youd,T.L.and Idriss, (2001).Liquefaction Resistance of Soils: Summary Report from 1996 NCEER and 1998 NCEER/NSF Workshop on Evaluation of Liquefaction Resistance of Soils”, J. of

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Geotech & Geo-env., ASCE 127(4) pp.297 15. Youd, T. L., 1991. Mapping of Earthquake Induced Liquefaction for Seismic Zonation. Proc. Fourth Int. Int. Conf. Seismic Zonation 1, pp. 111-147 16. Youd, T. L., (1984a). Recurrence of Liquefaction at the same site, Proceedings, 8th World Conference on Earthquake Engineering, Vol. 3, pp.

2313-238.

17. Youd, T. L, and Perkins, D.M. (1978).

Mapping Liquefaction-Induced Ground Failure Potential, Proc. ASCE Civil Eng., v.104, n0. GT4, p. 433-446.

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