Assessment of Risk of Liquefaction - A Case Study

Transcription

1 Assessment of Risk of Liquefaction - A Case Study ASHWAI JAI Deptt. of Civil Engineering ational Institute Technology Kurukshetra IDIA ashwani.jain66@yahoo.com Abstract: - Catastrophic failures in recent earthquakes have provided a sobering reminder that liquefaction of sandy soils as a result of earthquake ground shaking poses a major threat to the safety of Civil Engineering Structures. Major landslides, lateral movements of bridge supports, settling and tilting of buildings, and failure of waterfront retaining structures have all been observed in recent years as a result of this phenomenon and efforts have been increasingly directed to the development of methods of evaluating the liquefaction potential of soil deposits. In the present study, an attempt has been made to evaluate the liquefaction potential of some selected sites in Haryana (India) using standard penetration resistance value. These sites have been selected keeping in mind their increased susceptibility to liquefaction due to favourable soil & water table conditions. According to IS: 1893 (Part I): 2002, Haryana lies in the Seismic Zone IV, which is likely to be subjected to an earthquake of maximum moment magnitude of 7.5 and the same magnitude value has been adopted to evaluate the liquefaction susceptibility of the sites. Key-Words: - earthquake, liquefaction, standard penetration resistance, fines content, cyclic stress ratio, cyclic resistance ratio, stress reduction factor. 1 Introduction One of the major causes of earthquake-induced damages to structures has been liquefaction of saturated loose cohesionless soil deposits. It is manifested either in the form of sand boils and mud spouts at the ground surface formed by seepage of water, or in some cases by the development of quicksand like condition where the liquefaction occurs. Building may sink substantially into the ground or tilt excessively; lightweight structures and foundations may get displaced laterally causing structural failures. Recognition of liquefaction as a significant problem in Geotechnical Engineering was led primarily by major failures observed during iigata and Alaska earthquakes of 1964, but undoubtedly, liquefaction is best illustrated in iigata earthquake [1]. In the present study, an attempt has been made to evaluate the liquefaction potential of some selected sites in Haryana (India) using standard penetration resistance value. The standard penetration test value and the other relevant data have been collected from the field studies conducted at these sites for consultancy purpose by Geotechnical Engineering Division of IT Kurukshetra. Idriss & Boulanger approach [2, 3] has been used for evaluation of liquefaction potential of sites. 2 Soil Liquefaction When a suitably intense earthquake shakes a deposit of loose, saturated, coarse-grained soil, the grain structure is triggered towards more compact packing. It is unable to do so because pore water does not have sufficient time during the earthquake to drain off. The pore water pressure then shoots up as the incompressible water takes up the applied stress and the effective stress approaches zero - the deposit liquefies with zero effective stress and therefore with no shear strength, disastrous consequences follow liquefaction. The soil deposit that is susceptible to liquefaction must be loose, saturated and coarse grained. A loose state is essential since soil in that state suffers decrease in volume during drained shear and during undrained loading this contracting nature increases the pore water pressure. The soil should be fully saturated, i.e. the soil should be below ground water table. If the soil deposit is above the water table, then the vibrations from the earthquake will succeed in increasing the soil density because there is no time lag for air to escape, and finally the soil must be coarse grained, and this can be interpreted as implying that clays do not liquefy because vibrations are not effective in bringing clay particles together into closer packing and the shear strength of clays does not become zero when the effective stress becomes zero. Liquefaction only occurs in saturated ISS: ISB:

2 soils; its effect is most commonly observed in lowlying area near bodies of water such as rivers, lakes, bays and oceans. Sand most susceptible to liquefaction has coefficient of permeability in the range of to m/s. Sand deposited with rounded and sub-rounded grains are more susceptible to liquefaction than those with angular grains. Liquefaction is more likely for clean sand having a relative density lesser than about 40%, the susceptibility with in the range of % depends on the confining pressure and sands with relative density more than 60 % are less likely to liquefy. 3 Standard Penetration Test One of the oldest and most common in-situ tests is the standard penetration test or SPT. It was developed in the late 1920s and has been used extensively in orth and South America, the United Kingdom, Japan, India and elsewhere. Because of this long record of experience, the SPT is well established in engineering practice. It is performed inside exploratory boring using inexpensive and readily available equipment, and thus adds little cost to a site characterization programme. Although the SPT also is plagued by many problems that affect its accuracy and reproducibility, it probably will continue to be used for the foreseeable future, primarily because of its low cost. However, it is partially being replaced by other test methods, especially on larger and more critical projects. Seed et al. [4] recommended the following additional criteria for conducting standard penetration test: Use the rotary wash method to create a boring that has a diameter between 100 and 125 mm. The drill bit should provide an upward deflection of the drilling mud. If the sampler is made to accommodate liners, then these liners should be used so that the inside diameter is 35 mm. Use A or AW size drill rods for depths less than 15 m, and or W size for greater depths. Use a hammer that has an efficiency of 60 percent. Apply the hammer blows at a rate of 30 to 40 per minute. Inspite of certain disadvantages, the SPT does have at least three important advantages over other in-situ test methods. First, it obtains a sample of the soil being tested. This permits direct soil classification. Most of the other methods do not include sample recovery, so soil classification must be based on conventional sampling from nearby borings and on correlations between the test results and soil type. Second, it is very fast and inexpensive because it is performed in borings that would have been drilled anyway. Finally, nearly all drill rigs used for soil exploration are equipped to perform this test, whereas other in-situ tests require specialized equipment that may not be readily available. 4 Evaluation of Liquefaction Potential 4.1 General A number of methods have been developed to help design engineer in evaluation of liquefaction potential of cohesionless soil deposits based on data obtained from the laboratory test results [5] or by the guidance derived from field performance of sand deposits [6]. Both laboratory investigations and observations of field performances have shown that the liquefaction potential of a soil deposit to earthquake motions depends on properties of the soil and characteristics of the earthquake involved. In the present study, Idriss & Boulanger approach [2, 3] based on SPT-value has been used for evaluation of liquefaction potential at the sites. 4.2 Criteria for Soils Loose fine sandy soils below water table are susceptible to the earthquake-induced liquefaction, a state where excess pore water pressure generated in the soil results in a temporary or complete loss of strength of the soil. However, recent experiences with ground failure in low plasticity silts and clays during strong earthquakes have highlighted the fact that earthquake loading can trigger the development of significant strains and strength loss in a broad range of saturated soils, from sand to clay, even though earthquake-induced ground failure is observed less frequently in clays than in sands. Monotonic and cyclic undrained loading test data for silts and clays show that they transition, over a fairly narrow range of plasticity indices (PI), from soils that behave more fundamentally like sands (sandlike behaviour) to soils that behave more fundamentally like clays (clay-like behaviour). It is recommended that the term liquefaction be reserved for describing the development of significant strains or strength loss in fine-grained soils exhibiting sand-like behaviour, whereas the term cyclic softening failure be used to describe similar phenomena in fine-grained soils exhibiting clay-like behaviour. For practical purposes, clay-like ISS: ISB:

3 behaviour can be expected for fine-grained soils (ML) that have PI 7. If a soil plots as CL-ML, the PI criterion may be reduced to PI 5. For lowplasticity silts and clays, there is insufficient guidance on the engineering procedures that are most appropriate for estimating potential strains and strength loss during earthquake loading. Consequently, it is common practice to assume that soils classify as liquefiable by current liquefaction susceptibility criteria should be evaluated using procedures developed primarily for sands, even though current liquefaction susceptibility criteria were not necessarily developed for that purpose [7]. 4.3 Evaluation Appropriate soil type: Determine if the soil has the ability to liquefy during an earthquake. Groundwater table: The soil must be below GWT. The liquefaction analysis could also be performed if it is anticipated that the groundwater table will rise in future, and thus the soil will eventually be below the groundwater table. Cyclic stress ratio (CSR): Determine CSR that will be induced by the earthquake. Cyclic resistance ratio (CRR): By using the standard penetration resistance test data, the CRR of the in-situ soil is determined. Factor of safety (FOS): If the CSR induced by the earthquake is greater than the CRR determined from the standard penetration test, it is likely that liquefaction will occur during the earthquake, and vice versa. FOS = CRR/CSR Cyclic Stress Ratio (CSR) CSR σvo amax rd (CSR) M= 7.5 = = 0.65 ' MSF σ vo g MSF Where, ( CSR) M = 7.5 = adjusted value of CSR for equivalent uniform shear stress induced by earthquake ground motions having moment magnitude, M = 7.5. MSF = magnitude scaling factor. σ vo = total vertical stress = z. = total unit weight of soil. z = depth below ground surface. ' σ vo = effective vertical stress. a max = maximum horizontal acceleration at the ground surface induced byearthquake (peak ground acceleration) = 0.24g [8]. g = acceleration due to gravity. r d = stress reduction coefficient to account for deformability of soil Stress Reduction Coefficient The following relation was derived: Ln (r d ) = α (z) + β(z) M, z α( z) = sin , z β( z) = sin , Where, z is in meters and M is moment magnitude. These equations are applicable for z 34m. For z > 34m, use following expression. r d = 0.12 exp(0.22m) Magnitude Scaling Factor (MSF) It is used to adjust the induced CSR during earthquake magnitude M to an equivalent CSR for an earthquake magnitude, M = 7.5. MSF = CSR M /CSR M=7.5 M MSF = 6.9 exp 0.058, 4 MSF ormalization of Penetration Resistance SPT penetration resistances are routinely normalized to an equivalent σ ' vo = 1 atm to obtain quantities that more uniquely relate to the relative density, D R, of sand (i.e., they no longer depend upon σ ' vo). ( 1 ) 60 = corrected SPT blow count. = C () = C m, m being the measured SPT value. C = overburden correction factor to normalize ' SPT value to an equivalent σ vo of one atmosphere or 101 kpa or 1 kg/cm 2 or 1 tsf or 2000 lb/ft = SPT blow count after correction to an equivalent 60% hammer efficiency. α P a C = 1.7 ' σ vo ( ) α= , 1 60 ISS: ISB:

4 With ( 1 ) 60 limited to a maximum value of 46. Solving for C requires iteration because ( 1 ) 60 depends on C and C depends upon ( 1 ) 60. The SPT values for sands with FC > 5% have to be corrected to get equivalent clean sand SPT values [( 1 ) 60CS ] in order to use the above relations which are valid for clean sands. = +Δ ( ) ( ) ( ) 1 60CS Δ ( 1 ) = exp FC FC FC = m C R C S C B C BR E m /0.60 Where, C R = rod length correction C S = sampling method correction C B = bore hole diameter correction The above three corrections have been adapted from CEER/SF Report (1996, 1998) as reported by Youd et al. [9]. E m = hammer efficiency = 0.55 (donut hammerhand drop release) adapted from Clayton [10] C BR, the correction for rate of blow application, has been introduced by the author to account for the deviation from the standardized procedure, which states that blows per minute are to be applied. In case of donut hammer with hand dropped hammer release mechanism, the practice adopted for consultancy purposes by IT Kurukshetra, it is only possible to apply a maximum of 5 blows per minute. A lower frequency of blow application will permit positive excess pore water pressures developing in loose sands ( < 20) to dissipate between blows than a higher frequency of blow application, resulting in a higher effective stress condition and a correspondingly higher resistance to penetration. Based upon the studies conducted by Seed et al. [4], a correction factor (C BR ) of is suggested to standardize the SPT values < Cyclic Resistance Ratio (CRR) The values of CRR for a moment magnitude M = 7.5 earthquake and an effective vertical stress σ ' vo = 1 atm can be calculated based on ( 1 ) 60CS using the following expression. The use of this equation provides a convenient means for evaluating the cyclic stress ratio to cause liquefaction for a cohesionless soil with any fines content. ( ) ( ) ( ) CS 60CS 1 60CS CRR = exp 4 ( 1 ) 60CS Interpretation of Data The borehole data, soil properties and results of liquefaction analysis for the five sample sites under study have been reported in tabular form. In the tables following abbreviations have been used. IS = Indian Standard Soil Classification Symbol LL = Liquid Limit PL = Plastic Limit FC = Fines Content = SPT-value = Unit weight of soil CSR = Cyclic Stress Ratio CRR = Cyclic Resistance Ratio FOS = Factor of Safety 6 Conclusion Most of the sites under study show high susceptibility to liquefaction. Although the results have been presented for 7.5-moment magnitude earthquake, these could be easily extended to locations of other earthquake magnitudes. It should be noted that the standard penetration test cannot be performed at all depths, greater than 30.5 m or through large depths of water and in soils containing a significant proportion of gravel particles. Thus, it is desirable that it be supplemented by other in-situ test methods, which can also be correlated with soil liquefaction potential. In many cases, the static cone test, which can be performed more rapidly and more continuously, may provide a good means for evaluating liquefaction potential, especially if it is correlated on a site dependent basis with SPT results. However, this procedure is limited also to sands and silty sands. In dealing with soils containing large particles, or in difficult environments, other in-situ characteristics, such as the shear wave velocity or the electrical characteristics of the soil may provide a more suitable means for assessment of liquefaction potential. ISS: ISB:

5 SITE 1. RD OF DELHI CARRIER CHAEL (PAIPAT DIVISIO) 1.5 ML P ML P ML P CL-ML 28/ ML P SITE 2. RD AT BISHAGARH VILLAGE (AMBALA DIVISIO) 0.75 CL-ML 27/ CL-ML 28/ ML P ML P ML P ML P CL-ML P Depth (m) IS Symbol SITE 3. JALMAA KURLA ROAD (ASSADH DIVISIO) LL/PL % FC % 0.75 ML P ML P ML P ML P ML P ML P ML P SITE 4. RD 3477 LIK DRAI CROSSIG SIRSA BRACH (KAITHAL DIVISIO) 1.5 ML P ML P ML 28/ ML P ML P ML P ML P ISS: ISB:

6 SITE 5. ABUBSHEHAR (SIRSA DIVISIO) 0.75 ML P ML P ML P ML P ML P ML P ML P References: 1. Seed, H.B. and Idriss, I.M. (1967). Analysis of Soil Liquefaction: iigata Earthquake, Journal of the Soil Mechanics and Foundation Engineering Division, ASCE, Vol. 93, o. SM3, pp Idriss, I.M. and Boulanger, R.W. (2004). Semiempirical Procedures for Evaluating Liquefaction Potential during Earthquakes, 11 th International Conference on Soil Dynamics and Earthquake Engineering and The 3 rd International Conference on Earthquake Geotechnical Engineering, Berkeley, California, USA, pp Idriss, I.M. and Boulanger, R.W. (2006). Semiempirical Procedures for Evaluating Liquefaction Potential during Earthquakes, Soil Dynamics and Earthquake Engineering, Vol. 26, pp Seed, H.B., Tokimatsu, K., Harder, L. F. and Chung, R.M. (1985). Influence of SPT Procedures in Soil Liquefaction Resistance Evaluation, Journal of the Geotechnical Engineering Division, ASCE, Vol. 111, o. 12, pp Seed, H.B. and Idriss, I.M. (1971). Simplified Procedure for Evaluating Soil Liquefaction Potential, Journal of the Soil Mechanics and Foundation Engineering Division, ASCE, Vol. 97, o. SM9, pp Seed, H.B., Idriss, I.M. and Arango, I. (1983). Evaluation of Liquefaction Potential using Field Performance Data, Journal of the Geotechnical Engineering Division, ASCE, Vol. 109, o. 3, pp Boulanger, R.W. and Idriss, I.M. (2006). Liquefaction Susceptibility Criteria for Silts and Clays, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 132, o. 11, pp IS: 1893 (Part I) (2002). Criteria for Earthquake Resistant Design of Structures - Part 1: General Provisions and Buildings, Bureau of Indian Standards, ew Delhi. 9. Youd, T.L. et al. (2001). Liquefaction Resistance of Soils: Summary Report from the 1996 CEER and 1998 CEER/SF Workshops on Evaluation of Liquefaction Resistance of Soils, Journal of Geotechnical and Geoenvironmental Engineering, ASCE, Vol. 127, o. 10, pp Clayton, C.R.I. (1990). SPT Energy Transmission: Theory, Measurement and Significance, Ground Engineering, Vol. 23, o. 10, pp ISS: ISB: