ارزیابی ظرفیت باربری و نشست پی‌های سطحی واقع بر ماسه با قابلیت روانگرایی با استفاده از مدل‌سازی فیزیکی

نویسندگان
یاسر جعفریان 1
عبدالحسین حداد 2
بهروز مهرزاد سلاکجانی 3
مجتبی وارسته 3
1 عضو هیئت علمی پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله
2 دانشکده مهندسی عمران - دانشگاه سمنان
3 دانشگاه سمنان
چکیده
روانگرایی یکی از خسارت بارترین عوامل خرابی ناشی از زلزله است. بالا رفتن فشار آب و کاهش تنش موثر ناشی از تحریک موجب کاهش سختی و ظرفیت باربری پی‌ها می‌‌شود. حتی اگر خرابی‌های ناشی از کمبود ظرفیت باربری اتفاق نیفتد، به‌ هر حال امکان ایجاد نشست وجود دارد که می‌تواند سازه را از حالت سرویس‌دهی خارج نماید. در کاربردهای مهندسی، نشست‌های ناشی از روانگرایی با استفاده از کرنش‌های حجمی ناشی از بازتحکیم ماسه روانگرا تخمین زده می‌شود، حال آنکه مطالعات اخیر نشان‌دهنده اهمیت سهم کرنش‌های برشی ناشی از وجود سازه در کنار کرنش‌های حجمی است. مطالعه حاضر اثر تولید فشار آب بر ظرفیت باربری و نشست‌های برشی پی‌های سطحی را با استفاده از مدل‌سازی فیزیکی مورد بررسی قرار داده است. نتایج حاکی از کاهش ظرفیت باربری ناشی از تولید فشار آب بوده اما حتی در حالت روانگرایی کامل ظرفیت باربری قابل توجهی وجود دارد که می‌تواند در مسائل مهندسی مورد توجه قرار گیرد. فشار اضافی آب زیر پی همواره مقداری کمتر از فشار اطراف پی بوده و در هیچ‌یک از آزمایش‌ها، روانگرایی کامل دقیقاً در زیر پی مشاهده نشده است. همچنین اثر ضریب اطمینان طراحی بر نشست‌ها مورد بررسی قرار گرفته است.

کلیدواژه‌ها

عنوان مقاله English

Evaluating Bearing capacity and settlement of shallow footings rested on liquefiable sand using physical modelling

نویسندگان English

Yaser Jafarian 1
A. Haddad 2
چکیده English

Bearing capacity failure and seismically induced settlement of buildings with shallow foundations rested on liquefied soils have resulted in significant damage in recent earthquakes. Engineers still largely estimate seismic building settlement using procedures developed to calculate post-liquefaction reconsolidation settlement in free-field. They commonly disregard residual strength of liquefied soil in their design procedures. Previous studies of this problem have identified important factors involving shaking intensity, the liquefied soil’s relative density and thickness and the building weight and width. Newly studies have also showed that shear deformation combined with localized volumetric strains during partially drained loading are dominant mechanisms. Bearing capacity degradation due to high excess pore pressure development also has been reported in previous studies. Two series of physical modelling experiments involving shallow footing rested on liquefied sand have been performed to identify the mechanisms involved in liquefaction-induced building settlements and bearing capacity degradation. Experiments have performed on Babolsar sand with moderate relative density in a box with two plexiglass sides to observe sand deformations. Earthquake waves can cause pore pressure build up in saturated sands but complete liquefaction always do not occur. Anyhow excess pore pressure generation can cause bearing capacity degradation and excessive settlements. Various pore pressure ratios have been generated by static seepage through box base to assess bearing capacity degradation and excessive settlements before and in complete liquefaction conditions. First series of experiments are consisted of 8 tests related to bearing capacity measurements of square and spread foundation in pore pressure development conditions. Results show bearing capacity reduction due to excess pore pressure development, but there is remarkable strength even in complete liquefaction that is related to post-liquefaction strength of liquefied sand. Square foundations are more affected than spread footings by excess pore pressure ratios. Foundation’s loading behaviour and Shape factors are not affected by excess pore pressure ratios. Pore pressure decreased with loading increscent under the middle of foundation because of particle rearrangement and always was below the induced excess pore pressure. Complete liquefaction has never observed under footing. Safety factor selection is a challenging step in shallow foundation designs for engineers because of its economical view. Recent studies show the important role of shear deformations in shallow foundations as discussed before. In second test series of this experimental study, foundations firstly loaded to some safety factors and then its settlements due to pore pressure build up has measured, loading then increased to complete bearing capacity failure. This series are consisted of 12 tests for two types of foundations and various excess pore pressures, only shear deformations are assessed in this series because there is no volumetric deformation as excess pore pressure is constant during the tests. Results show increase of foundation settlements with safety reduction progressively, settlements for safety factors below 2 are negligible.

کلیدواژه‌ها English

Shallow foundation
bearing capacity
Settlement
Liquefaction
Physical Modelling
         
[1] Kawakami, F., and Asada, A.“Damage to the ground and earth structures by Niigata earthquake of June 16, 1964.”; J. soils and found.; 1966, 6(1), 14-30.
[2] Kishida, H. “Damage to reinforced concrete buildings in Niigata City with special reference to foundation engineering”; J. Soils and Found.;1966, 6(1), 71-88.
[3] Ohsaki, Y. “Niigata earthquakes, 1964 building damage and soil condition.” J. of  Soils and Found.; 1966, 6(2), 14-37.
[4] Yoshimi, Y., and Tokimatsu, K.“Settlement of buildings on saturated sand during earthquakes.”SoilsFound.; 1977, 17(1), 23–38.
[5] Nagase, H., and Ishihara, K. “Liquefaction-induced compactionand settlement of sand during earthquakes.”; J. of  Soils and Found.; 1988, 28(1), 65–76.
[6] Seed, H. B., and Idriss, I. M.“Analysis of soil liquefaction: Niigata earthquake.”; J. Soil Mech. And Found. Divission; ASCE; 1967, 93(3), 83-108.
[7] Adachi, T., Iwai, S., Yasui, M., and Sato, Y. “Settlement and inclination of reinforced concrete buildings in Dagupan City due toliquefaction during the 1990 Philippine earthquake.”; Proc., 10thWorld Conf. on Earthquake Engineering; International Association forEarthquake Engineering (IAEE); Madrid, Spain; 1992, 147–152.
[8] Ishihara, K., Acacio, A., and Towhata, I. “Liquefaction-inducedground damage in Dagupan in the July 16, 1990 Luzon earthquake.”; Soils Found.; 1993, 33(1), 133–154.
[9] Tokimatsu, K., Kojima, J., Kuwayama, A. A., and Midorikawa, S “Liquefaction-induced damage to buildings I 1990 Luzon Earth-quake.”; J. Geotech. Engrg.; 1994, 120(2), 290–307.
[10] Acacio, A. A., Kobayashi, Y., Towhata, I., Bautista, R. T., and Ishihara, K.“Subsidence of buildingfoundation resting upon liquefiedsubsoil case studies and assessment”; Soils and Found.; 2001, 41(6), 111–128.
[11] Yoshida, N., Tokimatsu, K., Yasuda, S., Kokusho, T., and Okimura, T. “Geotechnical aspects of damage in Adapazari city during 1999 Kocaeli, Turkey earthquake”; Soils Found.; 2001, 41(4), 25–45.
[12] Bray, J., Sancio, R., Durgunoglu, T., Onalp, A., Youd, T., Stewart, J., Seed, R., Cetin, O., Bol, E., Baturay, M., Christensen, C., andKaradayilar, T. “Subsurface characterization at ground failure sites in Adapazari, Turkey”; J. Geotech. Geoenviron. Eng.; 2004, 130(7), 673–685.
[13] Sancio, R., Bray, J. D., Durgunoglu, T., and Onalp, A. “Perfor-mance of buildings over liquefiableground in Adapazari, Turkey.”Proc., 13th World Conf. on Earthquake Engineering, St. Louis, Mo.,Canadian Association for Earthquake Engineering, Vancouver,Canada, Paper No. 935; 2004.
[14] Liu, L., and Dobry, R.“Seismic response of shallow foundationon liquefiable sand.”; J. Geotech. Geoenviron. Eng.; 1997, 123(6(, 557–567.
[15] Liu, L. “Centrifuge earthquake modelling of liquefaction and its effect on shallow foundations,”; PhD thesis, Dept. Of Civ.AndEnvir.Engrg.; Rensselaer Polytechnic Institute, Troy, N.Y.; 1992.
[16] Liu, L. And Dobry, R. “Centrifuge study of shallow foundation on saturated sand during earthquakes.”; Proc. 4th Japan-U.S. workshop on earthquake Resistant Design of Lifeline Facilities and Countermeasures against Soil Liquefaction, Nat. Ctr. For Earthquake Engrg.Res., State Univ.Of New York at Buffalo, Buffalo, N.Y.;  1992, 493-508.
[17] Dobry, R., and Liu, L. “Centrifuge modelling of soil liquefaction.”; Proc., 10th World Conf. on Earthquake Engineering, International Association for Earthquake Engineering(IAEE); Madrid, Spain; 1994, 7801–6809.
[18] Coelho P., Haigh S. K., Madabhushi S. P and O'brien T. “Centrifuge modeling of the use of densification as a liquefaction resistance measure for bridge foundations”; 13 the World Conference on Earthquake Engineering; 2004.
[19] Coelho, P.A.L.F., Haigh, S.K., Madabhushi, S.P.G.,“Centrifugemodeling of liquefaction of saturated sand under cyclic loading”; Intern.Conf. CBS04, Bochum, Germany; 2004.
[20] Ueng, T.S., Wu, C.W., Cheng, H.W., Chen, C.H., “Settlement of saturated clean sand deposits in shaking table tests”; Soil Dynamics and Earthquake Engineering; 2010, 30(1); 50-60.
[21] Dashti, S., Bray, J. D., Pestana, J. M., Riemerm M. and Wilson D.,“Mechanisms of Seismically Induced Settlement of Buildings with Shallow Foundations on Liquefiable Soil”; Journal of Geotechnical and Geoenvironmental Engineering; 2010, 136(1).
[22] Dashti, S., Bray, J.D., Pestana, J.M., Riemer,M.R., andWilson,D.“Centrifuge testing to evaluate and mitigate liquefaction-induced building settlement mechanisms.”; J. Geotech. Geoenviron. Eng.; 2010, 136(7), 918–929.
[23] Adalier, K., Elgamal, A., Meneses, J., Baez, J.I. “Stone columns as liquefaction countermeasure in non-plastic silty soils”; Soil Dynamics and Earthquake Engineering; 2003, 23(7), 571–584.
[24] Adalier, K., Elgamal, “Mitigation of liquefaction and associated ground deformations by stone columns”; Soil Dynamics and Earthquake Engineering; 2004, 72(3), 275–291.
[25] Dashti, S., Bray, J.D. “Numerical simulation of building response on liquefiable sand.”; J. Geotech. Geoenviron. Eng.; 2013, 139(8), 1235-1249.
[26] Silver, N.L. and Seed, H.B. "Volume changes in sands during cyclic loading"; Journal of the Soil Mechanics and Foundations Division; ASCE; 1971, Vol 97, No. SM9,pp. 1171-1180.
 [27] Tokimatsu, K. and Seed, H.B. "Evaluation of settlements in sand due to earthquake shaking,"; Journal of Geotechnical Engineering, ASCE; 1987, Vol. 113, No. 8, pp. 861-878.
[28] Ishihara, K. and Yoshimine, M. "Evaluation of settlements in sand deposits following liquefaction during earthquakes"; Soils and Foundations; 1992, 32(1), 173-188.
[29] Shamoto, Y., Zhang, J.-M., and Tokimatsu,K. “Methods for evaluating residual post-liquefaction ground settlement  and horizontal displacement”;  Soils and Foundations; Special Issue, 1998, No. 2, 69-83.
[30] Wu, J. and Seed, R.B. “Estimation of liquefaction-induced ground settlement (case studies)”; Proceedings, Fifth International Conference on Case Histories in Geotechnical Engineering, New York; 2004, 1-8.
[31] Calvetti, F., Prisco, C. And Nova, R. “Experimental and Numerical Analysis of Soil-Pipe Interaction.”; J. Geotech. Geoenvron. Eng.; 2004, 130(12), 1292-1299.
[32] Vargas-Monge, W. “Ring shear tests on large deformation of sand”; Ph.D. Thesis, University of Tokyo; 1998.
[33] Iai, S. “Similitude for shaking tabletestsonsoil-structure-fluid model in 1g gravitational field”; Soils and Foundations; 1989, 29(1), 105-118.
[34] Kagawa, T. “On the similitude in model vibration tests of earth-structures.”; Proc. JSCE; 1978, 275, (in Japanese).
[35] Kokusho, T. and Iwatate, T. “Scaled model tests and numerical analyses on nonlinear dynamicresponse of soft grounds”, Proc. JSCE; 1979, 285, 57-67 (in Japanese).
 [36] Marcuson, W.F. and Hynes, M.E. “Stability of slopes and embankments during earthquakes”; Proceedings, ASCEIPennsylvania Department of Transportation Geotechnical Seminar, Hershey, Pennsylvania; 1990.
[37] Marcuson, W.F. and Hynes, M.E, and Franklin, A.G. (1990). “Evaluation and use of residual strength in seismic safety analysis of embankments”; Earthquake Spectra; 1990, 6(3), 529-572.
مهندسی عمران مدرس
دوره 16، شماره 3
مهر و آبان 1395
صفحه 27-36