Case Study of CPT Application to Evaluate Seismic Settlement in Dry Sand

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1 Case Study of CPT Appliation to Evaluate Seismi Settlement in Dry Sand Fred (Feng) Yi Senior Geotehnial Engineer, C.H.J. Inorporated, Colton, CA, USA ABSTACT: Interpretations of geotehnial parameters based on Cone Penetration Test (CPT) data were performed and ompared with Standard Penetration Test (SPT) data and laboratory testing results olleted from various sites in California. Speifi attention was paid to the estimation of fines ontent and onversion of CPT data to SPT (N ) blowounts sine they are often needed in seismi settlement evaluation of dry sand for the use of Pradel s (998) method. A new relationship between volumetri strain, yli shear strain, and normalized tip resistane was derived based on the laboratory test data of Silver and Seed (97) for dry lean sands. An example of the proposed CPT-based method is presented with a omparison to the results alulated using Pradel s original method as well as with the results based on SPT data from adjaent borings. INTODUCTION Sine its development in the 95 s, the CPT has beome one of the most used and aepted in-situ testing methods for geotehnial investigation due to advantages suh as ontinuum of sampling, repeatability, and eonomial effiieny. Sine atual soil samples are not reovered during CPT, no laboratory soil testing is performed. Interpretation of CPT data with regard to soil parameters beomes important in the appliation of CPT results to various designs. obertson & Campanella (983a, 983b) published two major papers in 983 on the interpretation of CPT data. Sine then, various papers have been published by researhers in this field (Mayne et al ; Mayne 7; obertson 9). CPT interpretations are widely applied in geotehnial engineering. Various methods have been established for the appliation of CPT results, suh as evaluating shallow and deep foundation bearing apaities, liquefation potential, as well as liquefation-indued settlement and lateral spreading deformation (Lunne et al. 997; obertson & Wride 998; Idriss & Boulanger 8). However, there seems to be no work on the predition of seismi settlement of dry sand diretly based on CPT data. The intention of the present work is to ompare various interpretations with measured data and propose a new method to estimate seismi settlement of dry sand diretly based on CPT data. The validity of the proposed method has been verified by om-

2 paring the settlement analysis results from adjaent SPT borings and the analyses performed using traditional methods for dry sand settlement using SPT data. CPT DATA INTEPETATION. Estimation of Fines Content Fines ontent is an important parameter used in the evaluation of liquefation potential as it relates to the orretion to lean sand resistane. Several orrelations have been proposed in reent years (obertson & Wride 998; Suzuki et al. 998; Idriss & Boulanger 8; Cetin & Ozan 9). obertson & Wride (998) use the term, apparent fines ontent (referred to as FC hereafter), and suggest the following relationship orrelated to soil behavior type (referred to as SBT hereafter) index ( I ). If I <.6, FC = % (a) 3.5 If I is between.6 and 3.5, FC(%) =.75I 3. 7 (b) If I > 3.5, FC = % () If I is between.64 and.36, and F <.5%, FC = 5% (d) The expression for was derived by obertson & Wride (998) as I.5 I = [( 3.47 logqtn ) + (log F +.) ] (a) where Qtn is the normalized CPT penetration resistane and F is the normalized frition ratio. Q tn n [( q σ ) p ]( p / ' = ) (b) v / a a σ v ( q ) % F f / σ () = s v where σ v is the total overburden pressure, σ v ' is the effetive overburden pressure, q is the measured tip resistane, f s is the measure sleeve frition, pa is atmospheri pressure, and omponent n varies from.5 in sands to. in lays (obertson & Wride 998). Based on the data from Suzuki et al. (998), Idriss & Boulanger (8) derived a orrelation between FC and as.6 FC =.8 (%) (3) I I Cetin & Ozan (9) proposed another approah based on a probabilisti method. ( 38.5)/.75 ±.93 (%) FC (4a) = FC where. is a parameter similar to I. FC [ log( F ) ] + [ log( q ) 33.5] (4b) FC = t,, net where F is as defined in Equation and q t,, net is the normalized net one tip resistane and is defined as ( q σ )/( ' p q = / ) (4) t,, net t v σ v a where is a power law stress normalization exponent with a value between.5 and.. Iterations are needed to alulate and. q t,, net

3 The author olleted 44 measured fines ontent results from projet sites. This data and another 44 data points from Suzuki et al. (998) were plotted on an I versus FC hart in Figure as originally onstruted by obertson & Wride (998). The solid irles in Figure show the data olleted from southern California sites. The empty irles represent the data from 4 sites in Moss Landing obtained by Boulanger et al. (995) in the investigation after the 989 Loma Pieta earthquake. The diamonds illustrate data from Suzuki et al. (998). Equations and 3 as well as the SBT zones defined by obertson & Wride (998) are also shown in the figure. It an be seen that both equations underestimate the fines ontent, espeially when I is larger than approximately.3. Moreover, by examining the relationship between FC and the SBT zone, it is lear that the relationship is inonsistent with that based on the Unified Soil Classifiation System (USCS) in whih the fines ontent is defined as less than 5% for lean sand, between 5 and % for sand with silt, between and 5% for silty sand, and higher than 5% for silt or lay. Although, obertson & Wride (998) did not diretly utilize the Apparent Fine Content to orret the equivalent lean sand resistane, it is antiipated that this kind of orretion may be performed by readers erroneously. 4 Zone : organi soils - peats Soil Behavior Type Index, I 3 Clean sand Zone 3: silty lay to lay Zone 4: layey silt to silty lay Sand with silt Silty sand Zone 5: silty sand to sandy silt Zone 6: lean sand to silty sand Zone 7: gravelly sand to dense sand obertson & Wride (998) Idriss and Boulanger (8) eommended Silt to Clay This study Data from Suzuki et al.(998) Fine Contents, FC(%) Figure. elationships of Soil Behavior Type Index, Fines Content and Soil Classifiation Based on the measured FC data as shown in Figure, it is suggested that the following relationship ould be utilized to predit the values of FC for a given value of I. I <.3, FC (%) = (5a) I.35.3 I <.35, FC (%) = 43.67I sin π (5b).5.35 I < 3., FC (%) = 63.6I (5) I 3., FC (%) = (5d).3 < I.36 and F <.6%, FC(%) = 5. F (5e) 3

4 The orrelation between Equation 5 and the measured FC data as well as the soil types based on USCS lassifiation are also illustrated in Figure. Based on this relationship, the boundaries of soil behavior type proposed by obertson (99) ould be refined as shown in Table to obtain onsisteny with respet to the USCS. The omparison of the measured and alulated fines ontent is presented in Figure. Fines ontent predited by Equation 3 is less than that predited by Equation when I >.47 and was not inluded in the omparison. It an be seen that obertson & Wride s method seems to underestimate the fines ontent while Cetin & Ozan s method may overestimate the fines ontent when fines ontent is less than approximately perent, although the data satter of Cetin & Ozan s and reommended methods seems similar for fines ontent higher than perent. Overall, it an be seen that the proposed relationship (Eq. 5) generally provides better orrelations with measured data. Table. Boundaries of soil behavior type (refined from obertson 99) Soil behavior type index, I Zone USCS Classifiation Fines ontent (%) I <.3 7 Gravelly sand to dense sand.3 I <.6 6a Clean sand ~ 5..6 I <.8.8 I <.5 6b 6 Sand with silt Silty sand 5. ~.. ~ I <.4 5a Silty sand 4.8 ~ 5..4 I <. 5b Sandy silt 5. ~ 6.8. I < I < a Silt mixture: layey silt to silty lay Silty lay 6.8 ~ ~ 3. I < 3. 3b Clay I > 3. Organi soils: peats Calulated FC(%) from CPT Data 8 4 obertson & Wride (998) Cetin & Ozan (9) This study Calulated (N ) from CPT Data obertson et al. (986) Idriss & Boulanger (4) Lunne et al (997) 4 8 Measured FC (%) Measured SPT (N ) Figure. Comparison of measured and alulated Figure 3. Comparison of measured and alulated fines ontent (N ). Conversion to SPT Blowounts The onversion of CPT resistane to equivalent SPT (N ) blowounts may not be as important as the predition of fines ontent. However, if seismi-indued dry sand settlement is an important issue for a site, the onversion between CPT data and (N ) beomes neessary due to the absene of a method to diretly alulate of seismi settlement of dry sand based on CPT data. Several methods have been pro- 4

5 posed by individuals in past deades. obertson et al. (986) suggested ( q / pa ) / N ratios for eah non-normalized soil behavior type lassifiation zone. Jefferies and Davies (993) proposed a relation of q / N and I to provide a ontinuous variation with soil type. Lunne et al. (997) revised Jefferies and Davies relationships by utilizing the dimensionless variable ( q / pa ) and a modified I to give the following equation. q / p )/ N = 8.5( I / 4. 6) (6) ( a To evaluate liquefation based on both SPT and CPT data, Idriss & Boulanger (4) reevaluated the orrelation between (N), normalized tip resistane ( q N ), and relative density ( ) and reommended the following expressions. D q = + (7a) N /( N) (.9D.4) / 46( D ) For lean sand, Idriss & Boulanger (4) suggested.64 D.478( q ).63, ( q ) (7b) = N N > The equivalent (N ) alulated based on the above methods is ompared in Figure 3 with the measured (N ). The data were olleted from sites inluding 6 sites from southern California (solid symbols) and 4 sites from Moss Landing, California (other symbols), for a total number of 4. Figure 3 indiates that, although a large satter exists, the relationship by obertson et al. (986) tends to overestimate and Idriss & Boulanger s method (Eq. 7a) tends to underestimate (N ). The relationship shown in Equation 6 gives a more balaned distribution and is suggested by this author to be used when onverting to (N ). 3 EVALUATION OF SEISMIC SETTLEMENT OF DY SANDS 3. elative Density Silver & Seed (97) indiated that one of the important parameter affeting the settlement of dry sand under yli loading is the relative density of the soil. Several relations between relative density and tip resistane have been proposed in the past. Tatsuoka et al. (99) suggested a orrelation as shown in the following equation. D = log( q N) (%) (8) Based on hamber testing results for lean sands, Jamiolkowski et al () found a mean relationship as expressed elative density, D (%) 8 4 Tatsuoka et al. (99) Jamiolkowski et al () Idriss & Boulanger (4) eommended Normalized tip resistane, q ns Figure 4. elations between relative density and equivalent normalized lean sand resistane in Equation 9. The original equation was slightly modified by using a onsistent symbol for. q N D = 6.8ln( q N) 67.5 (%) (9) 5

6 The most reent work performed by Idriss & Boulanger (4) is shown in Equation 7b. These relationships are plotted in Figure 4. Beause the proposed relationships are based on test results for lean sand, an equivalent normalized lean sand tip resistane, instead of N, was adopted for the absissa in Figure 4. It an be seen that q N s q D above relationships generally give a range of the estimated where the differene varies from approximately to %. As suh, an average value as expressed in Equation is reommended. D = 77.9log( qn ) (%), ( qn 5) () 3. elationship between volumetri strain and shear strain of dry lean sand Silver & Seed (97) onduted a series of one-diretional yli shear tests on dry sand with relative densities of 45,, and 8%, and obtained relationships between volumetri and shear strains as shown in Figure 5. The relationship is obtained under 5 equivalent uniform strain yles, equivalent to a magnitude of 7.5 earthquake. Vol. Strain due to Compation, εv,m=7.5 (%).. D =45% D =% D =8% D =3% D =45% D =% D =8% D =9%.... Cyli Shear Strain, γ (%) Figure 5. elationship between volumetri strain and shear strain for dry lean sands (after Silver & Seed 97) Vol. Strain due to Compation, εv,m=7.5 (%). D =45%, q Ns 64 D =%, q Ns 99 D =8%, q Ns 8 q Ns =4 q Ns =5. q Ns = q Ns =5 q Ns = q Ns =5.... Cyli Shear Strain, γ (%) Figure 6. elationship between volumetri strain, shear strain and normalized CPT tip resistane for dry lean sands By adding Equation into the relationships shown in Figure 5, the equivalent volumetri strain due to ompation ould be expressed as a funtion of q N s and yli shear strain as in Equation and as shown in Figure 6. n.6 ε = γ, n = 8.4 ( q ) () v, M = 7.5 Ns where γ is the yli shear strain and is alulated using Pradel s method. 3.3 Corretions for earthquake magnitude and multidiretional shaking The relationships shown in Figures 5 and 6 are for 5 equivalent uniform strain yles, equivalent to a magnitude 7.5 earthquake. By reviewing previous studies, Tokimatsu & Seed (987) summarized a sale fator for earthquake magnitudes 6

7 between 5.5 and 8.5. The original numerial data is expressed by Equation. K v, M v, M / v, M 7. 5 = ε ε = =.6M.96 () where M is the magnitude of an earthquake. Pyke et al. (975) suggested that the volumetri strain should be doubled to aount for the multidiretional effets. As suh, the volumetri strain for any magnitude ould be alulated using following equation., M = v, M v, M = 7.5 ε v K ε (3) 3.4 Case study of proposed method An example of the proposed modified CPT-based method is shown in Figure 8 for Site A loated in southern California. The proedures adopted by Tokimatsu & Seed (987) were followed in the alulation. The shear strain was alulated based on the Pradel s (998) equation and the maximum shear modulus was estimated using the equation reently proposed by obertson (9). Figure 7 presents the measured tip resistane and the alulated shear strain, volumetri strain, and settlement. The volumetri strain and settlement alulated based on Pradel s method utilizing the onverted (N ) (Equation 6) are also shown in the figure. It an be seen that the alulated settlements generally agree with eah other. For omparison, the results from data obtained from a SPT boring approximately 5 feet away from the CPT sounding are also illustrated in Figure 7. These results indiate that the new method provides good agreement with SPT results. Tip resistane, q (MPa) Shear strain (%) Volumetri strain (%) Settlement (m) Depth (m) This study (CPT) SPT SPT Pradel (CPT) N Figure 7. Calulated shear strain, volumetri strain, and settlement of dry sand for Site A 4 CONCLUSION Interpretations of geotehnial parameters were performed based on CPT data obtained from sites. A set of equations have been proposed based on data olleted in this study and previous studies for alulating fines ontent, relative density, and the volumetri strain under yli loading of dry sand. By inorporating these equations 7

8 into the proedures adopted by Tokimatsu & Seed (987) and Pradel (998), seismi settlement of dry sand was omputed and ompared with the results alulated using Pradel s method as well the results from adjaent SPT data. The results indiate good agreements between these results and suggest that the proposed method ould be used in the predition of seismi-indued settlement in dry sand based diretly on CPT data. However, due to the absene of measured data, further verifiation of the proposed method will be neessary. EFEENCE: Boulanger,.W., Idriss, I.M., and Mejia, L.H Investigation and Evaluation of Liquefation elated Ground Displaements at Moss Landing during the 989 Loma Prieta Earthquake, eport No. UCD/CGM-95/, University of California, Davis, May. Cetin, K.O. and Ozan C. 9. CPT-Based Probabilisti Soil Charaterization and Classifiation, Journal of Geotehnial and Geoenvironmental Engineering, Vol 35, No.. Idriss, I.M., and Boulanger,.W. 4. Semi-empirial proedures for evaluating liquefation potential during earthquakes. thicsdee/3rdicegeproeedings, D. Doolin et al., eds., Stallion Press, Vol., Idriss, I. M., and Boulanger,. W. 8. Soil Liquefation During Earthquake, Earthquake Engineering esearh Institute, EEI Publiation MNO-. Jamiolkowski, M., LoPresti, D.C.F., and Manassero, M., Evaluation of elative Density and Shear Strength of Sands from Cone Penetration Test and Flat Dilatometer Test, Soil Behavior and Soft Ground Constrution (GSP9), ASCE, eston, Va.,, pp. 38. Jefferies, M.G., and Davies, M.P Use of CPTu to estimate equivalent SPT N, ASTM Geotehnial Testing Journal, 6(4), Lunne, T., obertson, P.K. and Powell, J. J. M Cone Penetration Testing in Geotehnial Pratie, Blakie Aademi and Professional. Mayne, P.W., Christopher, B.., and DeJong, J.,. Manual on Subsurfae Investigation, National Highway Institute, FHWA NHI--3, Federal Highway Administration, Washington, DC Mayne, P.W. 7. Cone Penetration Testing, National Cooperative Highway esearh Program Synthesis 368, Transportation esearh Board, Washington, DC Pradel, D. 998, Proedure to Evaluate Earthquake-Indued Settlement in Dry Sand Soils, Journal of Geotehnial and Geoenvironmental Engineering, Vol 4, No Pyke., Seed H.B., Chan C.K Settlement of sands under multidiretional shaking, Journal. Geotehnial Engineering, ASCE, (4), obertson, P.K. and Campanella,.G. 983a. Interpretation of one penetration tests: Part I (sands), Canadian Geotehnial Journal, Vol. (4), obertson, P.K. and Campanella,.G. 983b. Interpretation of one penetration tests: Part II (lays), Canadian Geotehnial Journal, Vol. (4), obertson, P.K., 99, Soil Classifiation Using the Cone Penetration Test, Canadian Geotehnial Journal, Volume 7. pp obertson, P.K. and Wride, C.E Evaluating yli liquefation potential using the one penetration test, Canadian Geotehnial Journal, 35: obertson, P.K., 9, Interpretation of Cone Penetration Tests a unified approah, submitted to Canadian Geotehnial Journal, May, 9 Aepted for Publiation Seed, H.B. and Silver, M.L. 97. Settlement of dry sands during earthquakes, J. Soil Mehanis and Foundations Div., ASCE, 98 (4), Silver, M.L., and Seed, H. B., 97. Volume hanges in sand during yli loading, J. Soil Mehanis and Foundations Div., ASCE 97(SM9), 7 8. Suzuki, Y., Sanematsu, T., and Tokimatsu, K Correlation between SPT and seismi CPT. Proeedings, Conferene on Geotehnial Site Charaterization, Balkema, otterdam, pp Tatsuoka, F., Zhou, S., Sato, T., and Shibuya, S. 99. Method of evaluating liquefation potential and its appliation. ep. on Seismi hazards in the soil deposits in urban areas, Ministry of Eduation of Japan, 75 9 (in Japanese). Tokimatsu, K. and Seed, H.B Evaluation of Settlements in Sands Due to Earthquake Shaking, Journal of Geotehnial Engineering, Vol 3, No. 8 8

IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE

IGC. 50 th INDIAN GEOTECHNICAL CONFERENCE ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE 5 th 5 th INDIAN GEOTECHNICAL CONFERENCE 17 th 19 th DECEMBER 215, Pune, Maharashtra, India ESTIMATION OF FINES CONTENT FROM CPT PARAMETERS FOR CALCAREOUS SOILS OF WESTERN INDIAN OFFSHORE Alankar Srivastava