Determination of Liquefaction Potential By Sub-Surface Exploration Using Standard Penetration Test

Transcription

1 Determination of Liquefaction Potential By Sub-Surface Exploration Using Standard Penetration Test 1 Sabih Ahmad, 2 M.Z.Khan, 3 Abdullah Anwar and 4 Syed Mohd. Ashraf Husain 1 Associate Professor and Head, Civil Engineering Department, Integral University, Lucknow, Uttar Pradesh (226022), India, 1 sabihahmed10@gmail.com 2 Professor and Head, Civil Engineering Department, I.E.T. Sitapur Road Lucknow, Uttar Pradesh (226022), India, 2 drkhan61@rediffmail.com 3 Assistant Prof., Civil Engineering Department, Integral University, Lucknow, Uttar Pradesh (226022), India, a.anwar14330@gmail.com 4 Assistant Prof., Civil Engineering Department, Integral University, Lucknow, Uttar Pradesh (226022), India, smah@iul.ac.in Abstract The Standard Penetration Test (SPT) is the most widely used in-situ test throughout the world for subsurface geotechnical investigation and this procedure have evolved over a period of 100 years. Estimation of the liquefaction potential of soils for earthquake design is often based on SPT test. Liquefaction is one of the critical problems in the field of Geotechnical engineering. It is the phenomena when there is loss of shear strength in saturated and cohesion-less soils because of increased pore water pressures and hence reduced effective stresses due to dynamic loading. Semi empirical field-based procedures for evaluating liquefaction potential during earthquakes uses experimental findings together with the theoretical considerations for establishing the framework of the analysis procedure. The major factors affecting the liquefactions potential of soils are the earthquake magnitude, the vertical effective overburden stress, SPT N-value, the peak acceleration at the ground surface, unit weight of soil above and below ground water table and the fine content of the soil. In this paper determination of liquefaction potential of soil is carried out using Semi-Empirical SPT based procedure. years through the efforts of countless researchers. Development of SPT-based correlations began in Japan (e.g., Kishida 1966) [1] and progressed through to the landmark work of Seed et al. (1984, 1985) [2]-[3] which set the standard in engineering practice for over two decades (Youd et al. 2001) [4]. Recent updates to SPT based procedures include those by Idriss and Boulanger (2008, 2010) [5]-[6]. The SPT-based procedures from Youd et al. (2001) [4] and Idriss and Boulanger (2010) [6] are compared in Figure 1.1 with the case history data. Keywords: Liquefaction, Earthquake, Semi-empirical, SPT 1. Introduction It is well recognized that structures located on the surface of liquefiable soil may severely damaged due liquefaction of supporting soil during earthquakes. Liquefaction of loose, cohesionless, saturated soil deposit is a subject of intensive research in the field of Geo-technical engineering over the past 40 years. The liquefaction characteristic of a soil depends on several factors, such as ground acceleration, grain size distribution, soil density, thickness of the deposits and especially the position of the ground-water table. Liquefaction and ground failures are commonly associated with large earthquakes. Standard Penetration Test (SPT) is the most widely used method for evaluating the liquefaction characteristics of soils. The development of SPT-based liquefaction triggering procedures has progressed over the Fig.1: Examples of SPT-based liquefaction triggering curves with a database of case histories processed with the Idriss-Boulanger (2008) procedure (from Idriss and Boulanger 2008) The liquefaction triggering databases provide an opportunity for researchers to re-evaluate liquefaction triggering procedures and updating them as per different soil conditions. The strength of semi-empirical procedure is the use of both 751

2 experimental findings together with the theoretical considerations for establishing the framework of the analysis procedure. 2. Standard Penetration Test (SPT) The Standard Penetration Test was introduced in 1947 and is now in widespread use because of its low cost, simplicity and versatility. In 1947, Karl Terzaghi described the Standard Penetration Test (SPT) in a presentation titled Recent Trends in Subsoil Exploration, which he gave at the 7th Conference on Soil Mechanics and Foundation Engineering at the University of Texas at Austin. The first published SPT correlations appeared in Terzaghi and Peck (1948) [7]. Estimation of the liquefaction potential of saturated granular soils for earthquake design is often based on SPT tests. The test consists of driving a standard 50-mm outside diameter thick walled sampler into soil at the bottom of a borehole, using repeated blows of a 63.5-kg hammer falling through 760 mm. The SPT N value is the number of blows required to achieve a penetration of 300 mm, after an initial seating drive of 150 mm. Correlations relating SPT blow counts for silts & clays and for Sands & Gravels, from Peck et al. (1953) [8] is depicted in Table 1. The SPT procedure and its simple correlations quickly became soil classification standards. Estimated values of Soil friction and cohesion based on uncorrected SPT blow counts from Karol (1960) [9] are presented in Table 2. S. No. 1. Cohesive Soil Soil Type SPT Blow Counts Undisturbed Soil Cohesion (psf) Friction Angle ( ) Very Soft < Soft Firm Stiff Very Stiff Hard >30 > Cohesionless Soil Loose < Medium Dense > Intermediate Soil Loose < Medium Dense > Table 1: Correlations relating SPT blow counts for silts & clays and for Sands & Gravels, from Peck et al. (1953) S. No. Blows/Ft (N SPT ) Sands and Gravels Blows/Ft (N SPT ) Silts and Clay Very Loose 0-2 Very Soft Loose 2-4 Soft Medium 4-8 Firm Dense 8-16 Stiff 5 Over 50 Very Dense Very Stiff 6. Over 32 Hard Table 2: Estimated values of Soil friction and cohesion based on uncorrected SPT blow counts, from Karol (1960) Fig. 2: Standard Penetration Test 752

3 b) Cyclic Softening: Cyclic softening is another phenomenon, triggered by cyclic loading, occurring in soil deposits with static shear stresses lower than the soil strength. Two main engineering terms i.e, Cyclic Mobility and Cyclic Liquefaction can be used to define the cyclic softening phenomenon. 4. Ground Failure Associated with Soil Liquefaction The National Research Council (Liquefaction ) [11] lists eight types of ground failure commonly associated with the soil liquefaction in earthquakes: a) Sand boils resulting in land subsidence accompanied by relatively minor change. b) Failure of retaining walls due to increased lateral loads from liquefied backfill or loss of support from the liquefied foundation soils. c) Ground settlement, generally linked with some other failure mechanism. d) Flow failures of slopes resulting in large down slope movements of a soil mass. e) Buoyant rise of buried structures such as tanks. f) Lateral spreads resulting from the lateral movements of gently sloping ground. g) Loss of bearing capacity resulting in foundation failures. h) Ground oscillation involving back and forth displacements of intact blocks of surface soil. Fig. 3: SPT conducted at the Site in Lucknow 3. MECHANISM OF LIQUEFACTION The phenomenon of liquefaction can be divided into two main categories [10]: a) Flow liquefaction: It is the phenomenon in which the static equilibrium is destroyed by static or dynamic loads in a soil deposit with low residual strength (strength of liquefied soil).it occurs when the static shear stress in the soil exceeds the shear strength of liquefied soil. This will cause large deformation in soils. Earthquakes, blasting and pile driving are all examples of dynamic loads that could trigger flow liquefaction. Once triggered the strength of the soil susceptible to flow liquefaction is no longer sufficient to withstand the static stresses that were acting on the soil before the disturbances. 5. DETERMINATION OF LIQUEFACTION POTENTIAL OF SOIL USING SPT SPT follows three main steps in evaluation of liquefaction assessment of an area i. Calculation of Cyclic Stress Ratio (CSR), induced at various depth within the soil by the earthquake. ii. Assessment of the capacity of soil to resist liquefaction using in-situ test data from SPT, expressed as Cyclic Resistance Ratio (CRR). iii. Evaluation of liquefaction potential by calculating the factor of safety (FS) against liquefaction, where; FS = CRR/CSR Semi-Empirical approach for determination of liquefaction of soil as suggested by Boulanger and Idriss using Standard Penetration Test (SPT) blow count (N-values) is summarized below: 753

4 5.1 Calculation of Cyclic Shear Stress Ratio (CSR): The cyclic shear stress ratios (CSR) induced by earthquake ground motions, at a depth z below the ground surface, using the following equation Eqn. 1 CSR = σ vo amax r σ d vo a max = maximum horizontal acceleration at the ground surface σ vo = total vertical stress σ`vo = effective vertical stress at depth z = depth (m) r d = stress reduction coefficient that accounts for the flexibility of the soil column Idriss (1999) [12] re-evaluated the MSF relation which is given by: MSF = 6. 9 exp M Eqn. 4 (b) 4 where; M= Magnitude of the earthquake The MSF should be less than equal to 1.8 Boulanger and Idriss (2004) [13] found that overburden stress effects on the Cyclic Resistance Ratio (CRR). The recommended K curves are expressed as follows: K σ = 1 C σ ln σ vo P a 1. 0 Eqn. 5 (a) The value of CSR is adjusted for the magnitude, M= 7.5. Accordingly, the value of CSR is given as: (CSR) M 7.5 = where, CSR MSF = σ vo amax σ vo MSF- Magnitude Scaling Factor r d MSF Stress reduction coefficient (r d ) is expressed as a function of depth (z) and earthquake magnitude (M): Ln (r d ) = α(z) + β(z)m α(z) = sin z β(z) = sin z where, z = Depth (m) M = Magnitude of earthquake r d = Stress reduction coefficient The above equations were appropriate for depth, z 34m. However, for depth, z > 34m; the following expression is used: r d = exp(0. 22 M) Eqn. 2 Eqn. 3(a) Eqn. 3(b) Eqn. 3(c) Eqn. 3(d) The coefficient C σ is expressed in terms of (N 1 ) 60 where, C σ = (N 1 ) 60 (N 1 ) 60 is limited to maximum value of 37 and 211 respectively (i.e., keeping C σ less than equal to 0.3) The evaluation of CSR on applying the K factor as described by (Boulanger and Idriss (2004) [13] is: (CSR) M=7.5 = σ vo amax σ vo r d 1 MSF K σ 5.2 Calculation of Cyclic Resistance Ratio (CRR): Boulanger and Idriss (2004) [13] adjusted the equation of CRR to an equivalent clean sand value as follows: CRR = exp (N 1) 60cs (N 1) 60cs (N 4 1) 60cs Eqn. 5 (b) Eqn. 6 (N 3 1) 60cs Eqn. 7 (a) Subsequent expressions describe the way parameters in the above equation are calculated as: The magnitude scaling factor, MSF, is used to adjust the induced CSR during earthquake magnitude M to an equivalent CSR for an earthquake magnitude, M = 7.5 (N 1 ) 60cs = (N 1 ) 60 + (N 1 ) 60 (N 1 ) 60 = exp FC 15.7 FC 2 Eqn. 7(b) Eqn. 7 (c) MSF = CSR M CSRM 7.5 Eqn. 4(a) (N 1 ) 60 = C N (N) 60 Eqn. 7 (d) 754

5 where, (N) 60 SPT N value after correction to an equivalent 60% hammer efficiency C N Overburden Correction Factor for Penetration resistance FC Fine contents 5.3 Calculation of factor of Safety (FS): If the cyclic stress ratio caused by an earthquake is greater than the Cyclic Resistance Ratio (CRR) of the in-situ soil, then liquefaction could occur during the earthquake and viceversa. The Factor of Safety against liquefaction is defined as : FS Liq = CRR Eqn. 8 CSR Liquefaction is predicted to occur when FS 1.0, and liquefaction predicted not to occur when FS > 1. The higher the factor of safety, the more resistant against liquefaction [14]. 5.4 Determination of Liquefaction: Experimental Methodology: Standard Penetration Test were conducted at a site in Lucknow to collect bore-hole datasets. The soil specimen was collected from these bore-holes up to depth of meters as well as SPT N-values were also determined at a regular interval of depth 1.5 m. The soil samples were used to determine liquid limit; plastic limit; angle of internal friction; particle size finer than 10mm, 4.75mm, 2 mm, 1mm, 600µ, 425µ, 212µ, 150µ, 75µ, natural water content, bulk unit weight. All experiments were conducted according to Bureau of Indian Standard s guidelines for soil testing Data Modification: To calculate liquefaction potential corrected SPT-N values are used. Value correction was adopted as given by IS: [15] Correction for overburden pressure: N-value obtained from SPT test is corrected as per following equation: (N 1 ) 60 = C N (N) 60 C N- Correction factor obtained directly from the graph given in Indian Standard Code (IS: ). (Fig. 6) Fig. 4: Correction due to overburden Pressure It can also be calculated using the relationship: C N = log σ z σ z = effective overburden pressure in kn/m 2 [16] Correction for Dilatancy correction: The values obtained in overburden pressure (N 1 ) shall be corrected for dilatancy if the stratum consist of fine sand and silt below water table for values of N 1 greater than 15 as under [17]: N c = (N 1 15) Eqn Results and Discussions: Eqn. 9 This study refers to the prediction of liquefaction potential of soil by conducting Standard Penetration Test at a site in Lucknow. To meet the objective four boreholes sets (BH-1, BH-2, BH-3 and BH-4) were analyzed, field and laboratory tests were conducted for the prediction of liquefaction potential. The water table at varying depth and earthquake magnitude of (M= 7.5) value were considered for assessing liquefaction potential The data sets were used to determine liquefaction parameters viz., Cyclic Resistance Ratio (CRR) and Cyclic Stress Ratio (CSR) by Idriss and Boulanger method to identify the liquefaction prone areas. 755

6 Table 3: Water Table and Earthquake Magnitude Parameter BH-1 BH-2 BH-3 BH-4 Depth of water table (m) Earthquake magnitude (rector scale) Table 4: IS Soil Classification (IS: ) Symbol SP SM ML CL CI CH Soil Description Poorly graded sand Silty sand Very fine sand Silty clay with low plasticity Sandy clay with medium plasticity Silty clay with high plasticity 1. Bore Hole (BH) 1: Table 5: Study about liquefaction potential for Water Table at 4.100m S.No Depth SPT-N CSR CRR FS Liq Status (Z) m value No No No No No No Yes Probability exist No No No No No No FS Liq Fig. 5: Bore Log Chart of Bore Hole (BH-1) Liquefaction Potential for Bore Hole (BH-1) Depth below Ground Surface (m) Depth below Ground Surface (m) vs FS Liq Fig. 6: Graph of FS Liq vs Depth (z) for Bore Hole (BH-1) 756

7 2. Bore Hole (BH) 2: IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 10, October Table 6: Study about liquefaction potential for water table at 4.600m 3.0 Liquefaction Potential for Bore Hole (BH-2) S.No. Depth SPT N CSR CRR FS Liq Status (Z) m value No No No No No No Probability exist No No No No No No No FS Liq Depth below Ground Surface (m) Depth below Ground Surface (m) vs FS Liq Fig. 8: Graph of FS Liq vs Depth (z) for Bore Hole (BH-2) 3. Bore Hole (BH) 3: Table 7: Study about liquefaction potential for water table at 4.600m S.No Depth SPT N CSR CRR FS Liq Status (Z) m value No No No No No No Yes Yes No No No No No No Fig. 7: Bore Log Chart of Bore Hole (BH-2) 757

8 4. Bore Hole (BH) 4: Table 8: Study about liquefaction potential for water table at 4.750m S.No Depth SPT N CSR CRR FS Liq Status (Z) m value No No No No No No Yes Yes No No No No No No Fig. 9: Bore Log Chart of Bore Hole (BH-3) Liquefaction Potential for Bore Hole (BH-3) FS Liq Depth below Ground Surface (m) Depth below Ground Surface (m) vs FS Liq Fig. 10: Graph of FS Liq vs Depth (z) for Bore Hole (BH-3) Fig. 11: Bore Log Chart of Bore Hole (BH-4) 758

9 FS Liq IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 2 Issue 10, October Liquefaction Potential for Bore Hole (BH-4) 7. Conclusion SPT- based liquefaction triggering procedure is presented in this study. The framework for liquefaction analysis based on SPT includes four key functional terms viz; (C N, K σ, MSF, and rd). Liquefaction is said to occur if the FS liq 1. However, some of the studies reveals that liquefaction have also occurred when FS Liq > 1 [18], uncertainties exist due to different soil conditions, validity of case history data and calculation method chosen. Further studies are required for assessment of liquefaction to obtain more accurate results. References Depth below Ground Surface (m) Depth below Ground Surface (m) vs FS Liq Fig. 12: Graph of FS Liq vs Depth (z) for Bore Hole (BH-4) Liquefaction Potential for Bore Hole (BH-1, 2, 3 & 4) 3.0 [1] Kishida, H. (1966). "Damage to reinforced concrete buildings in Niigata City with Special reference to foundation engineering." Soils and Foundations, Japanese Society of Soil Mechanics and Foundation Engineering, 6(1), [2] Seed, H. B., Tokimatsu, K., Harder, L. F. Jr., and Chung, R. (1984). The influence of SPT procedures in soil liquefaction resistance evaluations. Earthquake Engineering Research Center, University of California, Berkeley, Report No. UCB/EERC- 84/15, 50 pp. [3] Seed, H. B., Tokimatsu, K., Harder, L. F. Jr., and Chung, R. (1985). "Influence of SPT procedures in soil liquefaction resistance evaluations." Journal of Geotechnical Engineering, ASCE, 111(12), FS Liq [4] Youd, T. L., Idriss, I. M., Andrus, R. D., Arango, I., Castro, G., Christian, J. T., Dobry, R., Finn, W. D. L., Harder, L. F., Hynes, M. E., Ishihara, K., Koester, J. P., Liao, S. S. C., Marcuson, W. F., Martin, G. R., Mitchell, J. K., Moriwaki, Y., Power, M. S., Robertson, P. K., Seed, R. B., and Stokoe, K. H. (2001). [5] Idriss, I. M., and Boulanger, R. W. (2008). Soil liquefaction during earthquakes. Monograph MNO-12, Earthquake Engineering Research Institute, Oakland, CA, 261 pp Depth below Ground Surface (m) Depth below Ground Surface (m) vs FS Liq (BH-1) Depth below Ground Surface (m) vs FS Liq (BH-2) Depth below Ground Surface (m) vs FS Liq (BH-3) Depth below Ground Surface (m) vs FS Liq (BH-4) Fig. 13: Combined Graph of FS Liq vs Depth (z) for Bore Hole (BH-1, BH-2, BH-3, BH-4) [6] Idriss, I. M., and Boulanger, R. W. (2010). "SPT-based liquefaction triggering procedures." Report UCD/CGM-10/02, Department of Civil and Environmental Engineering, University of California, Davis, CA, 259 pp. [7] Terzaghi,K.and Peck, R. B., 1948, Soil Mechanics in Engineering Practice, 1st ed.: John Wiley & Sons, New York, 566 p. [8] Peck, R. B.; Hanson, W. E.; and Thornburn, T. H., 1953, Foundation Engineering: John Wiley & Sons, New York, 410 p. [9] Karol, R. H., 1960, Soils and Soil Engineering: Prentice Hall, Englewood Cliffs, NJ, 194 p. [10], Sabih A, Khan M. Z,, Abdullah A, Ashraf S.M., (2015), Assessment of Liquefaction Potential of Cohesionless Soil by Semi- Empirical: SPT- Based Procedure, International Journal of Recent Advances in Engineering & Technology (IJRAET), Vol.3, Issue 10, pp 53-59,

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