تأثیر حضور لای بر مدول برشی حداکثر شن ماسه دار با استفاده از آزمایش المان خمشی

نوع مقاله : پژوهشی اصیل (کامل)

نویسندگان
حمیدرضا رحمانی 1
سید ابوالحسن نائینی 2
1 دانشجوی دکتری
2 استاد، دانشگاه بین المللی امام خمینی
چکیده
ارزیابی سرعت موج برشی (Vs) در کرنش­های کوچک و مدول برشی حداکثر (Gmax) متناظر با آن، همواره بعنوان یک پارامتر مهم دینامیکی، مورد توجه پژوهشگران مهندسی ژئوتکنیک بوده است. هدف اصلی این تحقیق، بررسی تأثیر حضور ریزدانه غیرخمیری (لای) بر روی مدول برشی حداکثر خاک­های شنی می­باشد. با توجه به اینکه در طبیعت ترکیب شن و لای به تنهایی کمتر یافت شده و اغلب ترکیبات طبیعی بصورت مخلوط شن، ماسه و لای می­باشند، سه نوع مخلوط شن ماسه ­دار با نسبت­ های ماسه به شن متفاوت بعنوان خاک شنی پایه انتخاب شده و به آن­ها تا 45 درصد لای اضافه گردید. با استفاده از دستگاه المان خمشی، سرعت موج برشی در کرنش­های کوچک در همه نمونه ­ها و تحت سه فشار همه­ جانبه مختلف اندازه­ گیری شده و عوامل مختلف تأثیرگذار شناسایی گردیدند. نتایج آزمایشگاهی نشان می­ دهد که با افزایش فشار همه جانبه موثر، مدول برشی حداکثر در همه ترکیبات افزایش می ­یابد. همچنین میزان درصد ریزدانه غیرخمیری و نسبت ماسه به شن در خاک شنی پایه، بطور قابل ملاحظه­ ای بر روی پارامتر مدول برشی حداکثر خاک مخلوط، موثر می­ باشند، بدین صورت که با افزایش درصد لای و همچنین نسبت ماسه به شن در خاک پایه، مدول برشی حداکثر بطور پیوسته کاهش می­ یابد که این کاهش، از دیدگاه میکرومکانیک و نحوه تشکیل زنجیره­ های نیروی تماسی بین ذرات، قابل توجیه است. در نهایت بر اساس رابطه عمومی Hardin و تعیین بهترین ضرایب برازشی مربوطه با استفاده از تحلیل رگرسیون، برای هر سه نوع خاک شنی با نسبت ماسه به شن متفاوت، روابط تجربی جداگانه­ ای بر اساس تابعی از درصد ریزدانه غیرخمیری، نسبت تخلخل و فشار همه­ جانبه مخلوط، جهت تخمین مدول برشی حداکثر مخلوط شن، ماسه و لای ارائه گردید. با انطباق نتایج بدست آمده از این روابط با مقادیر واقعی اندازه­ گیری شده در آزمایشگاه و همچنین فراوانی این نوع از ترکیبات در طبیعت، قابلیت استفاده بسیار مفید از این معادلات در پروژه­ های مختلف ژئوتکنیکی مشخص گردید.

کلیدواژه‌ها

موضوعات

عنوان مقاله English

Influence of non-plastic fine on small strain shear modulus of sandy gravel using bender element test

نویسندگان English

Hamidreza Rahmani 1
Seyed Abolhasan Naeini 2
1 PhD Candidate
2 Professor, Imam Khomeini International University
چکیده English

Dynamic properties of soils at very small strain, i.e. small strain shear modulus (G0) and shear wave velocity (Vs), are used frequently in geotechnical applications. Many researchers have studied the effective factors on the small strain shear modulus in clean sands or sand-fine mixtures. However, less attention has been given to the dynamic properties of gravelly soils at small strain and they are poorly understood. Reviewing the technical literature, one may find it very interesting to study the impacts of non-plastic fine content on the small strain shear modulus of gravelly soils. A fundamental experimental study was designed to explore the influence of non-plastic fine content on the small strain shear modulus of gravel-sand-silt mixtures (common geomaterials in nature). Since gravel and silt mixtures with no sand particles are less common in nature, three types of sandy gravels with different sand-to-gravel ratios of 0.25, 0.43 and 0.67 were selected as base gravelly soils. In order to isolate the impacts of the fine content, sand-to-gravel ratio was kept constant in each soil type. Various percentages of silt (0~45%) were carefully added to the base soil. Eighteen mixtures of sandy gravels with different silt contents were prepared. Bender Element tests were carried out under saturated conditions to determine the small strain shear velocity. Samples were prepared by the moist tamping method due to its advantages for making loose and homogeneous samples without creating any segregation of grains. Following the saturation process, specimens were subjected to three isotropic confining pressure levels of 50, 100, 150 kPa. The relative densities of the samples were carefully kept constant to avoid the density effect on the responses. To keep the densities of samples constant while varying the fine content percentages, the initial relative density (Dr before consolidation) was selected in a way to ensure that the target relative density (Dr after consolidation) was approximately 35% (i.e., Dr=32% ~ 38%). The appropriate initial relative densities for each mixture and for various effective confining pressures were obtained using iterative efforts. Laboratory results show that the maximum shear modulus increases in all mixtures when the effective confining pressure increases. The small strain shear modulus is significantly impacted by the percentage of non-plastic fine content and sand-to-gravel ratio of base gravelly soils. Increasing the percentage of silt and also the sand-to-gravel ratio causes the shear modulus to decrease constantly. The reduction (rate of which is different for mixtures and effective confining pressures) can be explained from the micromechanical perspective and the formation of strong and weak force chains through interparticle contacts. Finally, Hardin's general equation is fitted to experimental data and fitting parameters are found using regression analysis. Empirical relationships are presented to estimate the small strain shear modulus in each of those three types of soil. These correlations are a function of non-plastic fine content, void ratio and effective confining pressure of mixtures. The comparison between the estimated and the measured small strain shear modulus clearly indicates that the purposed equations yield good agreement with experimental data. Therefore, these equations can be used to predict the small strain shear modulus for different mixtures of gravel-sand-silt in many geotechnical applications such as soil improvement, evaluation of liquefaction potential and the design of dynamic foundations.

کلیدواژه‌ها English

Shear wave velocity
Small strain shear modulus
Bender element
Sandy gravel
Silt
[1] Thomann, T.G. and Hryciw, R.D. 1990 Laboratory measurement of small strain shear modulus under K 0 conditions. Geotechnical Testing Journal. 13 (2), 97–105.
[2] Hardin, B. and Dmevich, V. 1972 Shear modulus and damping in soils: Design Equations and Curves, ASCE, Vol. 98, No. Journal of the Soil Mechanics and Foundations Division. 98 (7), 9006–9029.
[3] Youn, J.-U., Choo, Y.-W., and Kim, D.-S. 2008 Measurement of small-strain shear modulus G max of dry and saturated sands by bender element, resonant column, and torsional shear tests . Canadian Geotechnical Journal. 45 (10), 1426–1438.
[4] Wichtmann, T. and Triantafyllidis, T. 2009 Influence of the grain-size distribution curve of quartz sand on the small strain shear modulus Gmax. Journal of Geotechnical and Geoenvironmental Engineering. 135 (10), 1404–1418.
[5] Iwasaki, T. and Tatsuoka, F. 2011 Effects of grain size and grading on dynamic shear moduli of sands. Soils and Foundations. 17 (3), 19–35.
[6] Gu, X., Yang, J., Huang, M., and Gao, G. 2015. Bender element tests in dry and saturated sand: Signal interpretation and result comparison. Soils and Foundations, . Soils and Foundations. 55 (5), 951–962.
[7] Yang, J. and Liu, X. 2016 Shear wave velocity and stiffness of sand: the role of non-plastic fines. Géotechnique. 66 (6), 500–514.
[8] Wichtmann, T. and Triantafyllidis, T. 2009 On the influence of a non-cohesive content of fines on the small strain stiffness of quartz sand (submitted). Canadian Geotechnical Journal. 1 (2), 103–114.
[9] Sadeghzadegan, R., Naeini, S.A., and Mirzaii, A. 2018 Effect of clay content on the small and mid to large strain shear modulus of an unsaturated sand. European Journal of Environmental and Civil Engineering. 1–19.
[10] Salgado, R., Bandini, P., and Karim, A. 2002 Shear Strength and Stiffness of Silty Sand. Journal of Geotechnical and Geoenvironmental Engineering. 126 (5), 451–462.
[11] Chien, L.-K. and Oh, Y.-N. 2002 Influence of fines content and initial shear stress on dynamic properties of hydraulic reclaimed soil. Canadian Geotechnical Journal. 39 (1), 242–253.
[12] Choo, H. and Burns, S.E. 2015 Shear wave velocity of granular mixtures of silica particles as a function of finer fraction, size ratios and void ratios. Granular Matter. 17 (5), 567–578.
[13] Seed, H.B., Wong, R.T., Idriss, I.M., and Tokimatsu, K. 2008 Moduli and damping factors for dynamic analyses of cohesionless soils. Journal of Geotechnical Engineering. 112 (11), 1016–1032.

[14] Lin, S.-Y., Lin, P.S., Luo, H.-S., and Juang, C.H. 2000 Shear modulus and damping ratio characteristics of gravelly deposits. Canadian Geotechnical Journal. 37 (3), 638–651.

[15] Meidani, M., Shafiee, A., Habibagahi, G., Jafari, M.K., Mohri, Y., Ghahramai, A., et al. 2008 Granular shape effect on the shear modulud and damping ratio of mixed gravel and clay. Iranian Journal of Science &Technology,TransactionB,Engineering. 32 (B5), 501–518.

[16] Zou, D.G., Gong, T., Liu, J.M., and Kong, X.J. 2012 Shear modulus and damping ratio of gravel material. in: Appl. Mech. Mater., pp. 1426–1432.
[17] Zhu, S., Yang, G., Wen, Y., and Ou, L. 2014 Dynamic shear modulus reduction and damping under high confining pressures for gravels. Géotechnique Letters. 4 (3), 179–186.

[18] Senetakis, K. and Madhusudhan, B.N. 2015 Dynamics of potential fill-backfill material at very small strains. Soils and Foundations. 55 (5), 1196–1210.

[19] Zhou, W., Chen, Y., Ma, G., Yang, L., and Chang, X. 2017 A modified dynamic shear modulus model for rockfill materials under a wide range of shear strain amplitudes. Soil Dynamics and Earthquake Engineering. 92 229–238.
[20] DRNEVICH, V.P. 1978 Resonant-column testing: Problems and solutions. ASTM Special Technical Publication. (654), 384–398.
[21] Payan, M., Senetakis, K., Khoshghalb, A., and Khalili, N. 2017 Characterization of the small-strain dynamic behaviour of silty sands; contribution of silica non-plastic fines content. Soil Dynamics and Earthquake Engineering. 102 232–240.
[22] Hardin, B.O. and Kalinski, M.E. 2005 Estimating the Shear Modulus of Gravelly Soils. Journal of Geotechnical and Geoenvironmental Engineering. 131 (7), 867–875.
[23] Goudarzy, M., König, D., and Schanz, T. 2016 Small strain stiffness of granular materials containing fines. Soils and Foundations. 56 (5), 756–764.
[24] Lawrence Jr, F. V 1965 Ultrasonic shear wave velocities in sand and clay. The Response of Soils to Dynamic Loadings.
[25] Leong, E.C., Yeo, S.H., and Rahardjo, H. 2005 Measuring shear wave velocity using bender elements. Geotechnical Testing Journal. 28 (5), 488–498.
[26] Shirley, D.J. and Hampton, L.D. 1978 Shear-wave measurements in laboratory sediments. Acoustical Society of America Journal. 63 607–613.
[27] Bayat, M. and Ghalandarzadeh, A. 2018 Stiffness degradation and damping ratio of sand-gravel mixtures under saturated state. International Journal of Civil Engineering. 16 (10), 1261–1277.
[28] Da Fonseca, A.V., Ferreira, C., and Fahey, M. 2008 A framework interpreting bender element tests, combining time-domain and frequency-domain methods. Geotechnical Testing Journal. 32 (2), 91–107.
[29] Marjanovic, J. and Germaine, J.T. 2013 Experimental study investigating the effects of setup conditions on bender element velocity results. Geotechnical Testing Journal. 36 (2), 187–197.

[30] Brignoli, E.G.M., Gotti, M., and Stokoe, K.H. 1996 Measurement of shear waves in laboratory specimens by means of piezoelectric transducers. Geotechnical Testing Journal. 19 (4), 384–397.
[31] Lee, J.-S. and Santamarina, J.C. 2005 Bender Elements: Performance and Signal Interpretation. Journal of Geotechnical and Geoenvironmental Engineering. 131 (9), 1063–1070.
[32] Rio, J.F.M.E. 2006 Advances in laboratory geophysics using bender elements.
[33] ASTMD 2488-09a Standard Practice for Description and Identification of Soils ( Visual-Manual Procedure ). Annual Book of ASTM Standards.
[34] ASTM, D. 2003 Standard test methods for the determination of the modulus and damping properties of soils using the cyclic triaxial apparatus.
[35] ASTM D5311/D5311M-13 Standard test method for load controlled cyclic triaxial strength of soil. Astm D5311. 92 (Reapproved), 1–11.
[36] ASTM D4767-11 2011 Standard test method for consolidated undrained triaxial compression test for cohesive soils. American Society for Testing and Materials. 1–11.
[37] ASTM 2008 Testing materials, laboratory determination of pulse velocities and ultrasonic elastic constants of rock.

[38] Ishihara, K. 1993 Liquefaction and flow failure during earthquakes. Géotechnique. 43 (3), 351–451.
[39] YANG, J. and WEI, L.M. 2012 Collapse of loose sand with the addition of fines: the role of particle shape. Géotechnique. 62 (12), 1111–1125.
[40] Jafarian, Y., Ghorbani, A., Salamatpoor, S., and Salamatpoor, S. 2013 Monotonic triaxial experiments to evaluate steady-state and liquefaction susceptibility of Babolsar sand. Journal of Zhejiang University SCIENCE A. 14 (10), 739–750.
[41] Md. Mizanur, R. and Lo, S.R. 2012 Predicting the Onset of Static Liquefaction of Loose Sand with Fines. Journal of Geotechnical and Geoenvironmental Engineering. 138 (8), 1037–1041.
[42] Do, J., Heo, S.B., Yoon, Y.W., and Chang, I. 2017 Evaluating the liquefaction potential of gravel soils with static experiments and steady state approaches. KSCE Journal of Civil Engineering. 21 (3), 642–651.
[43] Wichtmann, T., Navarrete Hernandez, M.A., and Triantafyllidis, T. 2015 On the influence of a non-cohesive fines content on small strain stiffness, modulus degradation and damping of quartz sand. Soil Dynamics and Earthquake Engineering. 69 (i), 103–114.
[44] Radjai, F. and Wolf, D.E. 1998 Features of static pressure in dense granular media. Granular Matter. 1 (1), 3–8.
مهندسی عمران مدرس
دوره 19، شماره 4
مهر و آبان 1398
صفحه 43-57