酸性环境干湿循环条件下膨胀土的膨胀特性及微观作用分析
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摘要
弄清酸雨及干湿循环共同作用下膨胀土的膨胀性能及其微结构与矿物成分的变化,对研究酸雨区膨胀土的基本性质劣化及工程问题意义重大。为此,以广西酸雨重灾区百色原状膨胀土为对象,模拟酸雨(pH=3,5,7)与干湿循环(n=1,2,3,4)两者共同作用的环境,开展了无、有荷膨胀率试验,并采用扫描电镜(SEM)、压汞仪(MIP)和X射线衍射仪(XRD)分析了该环境下试样的微结构及矿物成分的演变规律。研究结果表明:酸性环境使试样的膨胀率增大,溶液的pH值越小,膨胀率越大;随干湿循环作用次数的增加,不同溶液环境下试样的膨胀率均先增大后趋于稳定,且2次作用后的增幅最大;经酸性环境与干湿循环共同作用试样的膨胀率增大更多,溶液pH值为3和5,经2次干湿循环后其膨胀率比pH值为7的分别增长了24.7%和7.9%;上覆压力能明显抑制试样膨胀率的增长,设定测试压力越大,该值下降越显著。酸性环境与干湿循环共同作用下膨胀率增大的机理可通过微观结构分析作出解释:酸性环境作用下膨胀土中游离SiO_2,Al_2O_3,K_2O,MgO,CaO等胶结物出现不同程度的溶蚀和淋滤,削弱了叠聚体结构间的联结作用,使面面叠聚结构的排列趋于分散,微孔隙体积及数目不断增大,同时遭受干湿循环作用后,土中微孔隙加速发育,土颗粒与溶液水间化学反应更剧烈,致使其膨胀变形进一步增大。因此,酸雨重灾区的膨胀土工程建设,必须考虑酸性环境与干湿循环共同作用造成的膨胀土基本性质劣化的不利影响。
The expansion properties of expansive soils and their microstructure and mineral composition are significantly affected by the combined effects of acid rain and dry-wet cycles. It is, therefore, of utmost importance to study the deterioration of their basic properties and subsequent engineering problems in areas of acid rain. For this purpose, Baise expansive soil from the heavy acid rain area of Guangxi was selected as the target sample. The coupled effects of acid rain(pH=3, 5, 7) and dry-wet cycle(n=1, 2, 3, 4) on expansion properties of expansive soil were studied based on the load-free and loaded expansion rate tests. The evolution of the microstructure and mineral composition of the samples under the combined effects of acid rain and dry-wet cycles was analyzed using scanning electron microscopy(SEM), mercury porosimetry(MIP), and X-ray diffraction(XRD). The results show that the acid environment increases the swelling deformation of expansive soil, which is, in turn, inversely proportional to the pH value of the solution. With an increase in the dry-wet cycle times, regardless of the solution environment, the sample expansion rate first increases and is then almost stable. The largest increase occurs after two dry-wet cycles. Following the combined effects of acid environment and dry-wet cycle, the expansion rate of the sample increases more substantially. When the solution pH value decreases from 7 to 5 and 3, the sample expansion rate increases by 24.7% and 7.9%, respectively. Vertical pressure can significantly inhibit the growth in expansion rate of the sample; the greater the stress, the more significant the decrease in the expansion rate. The mechanism of increase in expansion under the combined effects of acid environment and dry-wet cycle can be explained by microstructural analysis. Under an acid environment, free colloidal minerals, such as SiO_2, Al_2O_3, K_2O, MgO, and CaO, present in expansive soil show different degrees of erosion and leaching. The erosion of colloidal minerals weakens the strength of the structural connection between the stacking structures and disperses the face-to-face stacked structures, resulting in continuous increase of the size and number of micro-voids. Moreover, the additional effect of dry-wet cycle promotes the chemical reaction between soil particles and water in the solution, resulting in further increase of micro-pores and soil swelling deformation. During engineering construction using expansive soil in the heavy acid rain area, the adverse effects of acid environment and dry-wet cycle on the deterioration of the basic properties of expansive soil should therefore be considered.
The expansion properties of expansive soils and their microstructure and mineral composition are significantly affected by the combined effects of acid rain and dry-wet cycles. It is, therefore, of utmost importance to study the deterioration of their basic properties and subsequent engineering problems in areas of acid rain. For this purpose, Baise expansive soil from the heavy acid rain area of Guangxi was selected as the target sample. The coupled effects of acid rain(pH=3, 5, 7) and dry-wet cycle(n=1, 2, 3, 4) on expansion properties of expansive soil were studied based on the load-free and loaded expansion rate tests. The evolution of the microstructure and mineral composition of the samples under the combined effects of acid rain and dry-wet cycles was analyzed using scanning electron microscopy(SEM), mercury porosimetry(MIP), and X-ray diffraction(XRD). The results show that the acid environment increases the swelling deformation of expansive soil, which is, in turn, inversely proportional to the pH value of the solution. With an increase in the dry-wet cycle times, regardless of the solution environment, the sample expansion rate first increases and is then almost stable. The largest increase occurs after two dry-wet cycles. Following the combined effects of acid environment and dry-wet cycle, the expansion rate of the sample increases more substantially. When the solution pH value decreases from 7 to 5 and 3, the sample expansion rate increases by 24.7% and 7.9%, respectively. Vertical pressure can significantly inhibit the growth in expansion rate of the sample; the greater the stress, the more significant the decrease in the expansion rate. The mechanism of increase in expansion under the combined effects of acid environment and dry-wet cycle can be explained by microstructural analysis. Under an acid environment, free colloidal minerals, such as SiO_2, Al_2O_3, K_2O, MgO, and CaO, present in expansive soil show different degrees of erosion and leaching. The erosion of colloidal minerals weakens the strength of the structural connection between the stacking structures and disperses the face-to-face stacked structures, resulting in continuous increase of the size and number of micro-voids. Moreover, the additional effect of dry-wet cycle promotes the chemical reaction between soil particles and water in the solution, resulting in further increase of micro-pores and soil swelling deformation. During engineering construction using expansive soil in the heavy acid rain area, the adverse effects of acid environment and dry-wet cycle on the deterioration of the basic properties of expansive soil should therefore be considered.
引文
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[2] BAKHSHIPOUR Z, ASADI A, SRIDHARAN A, et al. Acid Rain Intrusion Effects on the Compressibility Behaviour of Residual Soils [J]. Journal of Environmental Geotechnics, 2017,DOI:10.1680/jenge.15.00081.
[3] MOORE R, BRUNSDEN D. Physico-chemical Effects on the Behaviour of a Coastal Mudslide [J]. Geotechnique, 1996, 46 (2): 259-278.
[4] JABOYEDOFF M, BAILLIFARD F, BARDOU E, et al. The Effect of Weathering on Alpine Rock Instability [J]. Quarterly Journal of Engineering Geology and Hydrogeology, 2004, 37 (2): 95-103.
[5] 刘剑,崔鹏.水土化学作用对土体黏聚力的影响——以蒙脱石-石英砂重塑土为例[J].岩土力学,2017,38(2):419-427. LIU Jian, CUI Peng. Influence of Water-Soil Chemical Interaction on Cohesive Force—A Case Study of Montmorillonite-quartz Remolded Soil [J]. Geodynamics, 2017, 38 (2): 419-427.
[6] GRATCHEV I, TOWHATA I. Stress-strain Characteristics of Two Natural Soils Subjected to Long-term Acidic Contamination [J]. Soils & Foundations, 2013, 53 (3): 469-476.
[7] BENDOU S, AMRANI M. Effect of Hydrochloric Acid on the Structural of Sodic-bentonite Clay [J]. Journal of Minerals & Materials Characterization & Engineering, 2014, 2 (5): 404-413.
[8] 张信贵,易念平,吴恒.不同pH水环境下土变形特性的试验研究[J].高校地质学报,2006,12(2):242-248. ZHANG Xin-gui, YI Nian-ping, WU Heng. Experimental Study on Soil Deformation Under Different pH and Water Conditions [J]. Geological Geology of Universities, 2006, 12 (2): 242-248.
[9] 顾欢达,顾熙.酸雨环境对轻质土的工程性质的影响[J].环境科学与技术,2006,29(3):17-18 GU Huan-da, GU Xi. Effect of Acid Rain on Engineering Properties of Light-weight Soils [J].Environmental Science and Technology, 2006, 29 (3): 17-18.
[10] 朱春鹏,刘汉龙,沈扬.酸碱污染土强度特性的室内试验研究[J].岩土工程学报,2011,33(7):1146-1152. ZHU Chun-peng, LIU Han-long, SHEN Yang. Laboratory Tests on Shear Strength Properties of Soil Polluted by Acid and Alkali [J].Chinese Journal of Geotechnical Engineering, 2011, 33 (7): 1146-1152.
[11] 赵宇,崔鹏,胡良博.黏土抗剪强度演化与酸雨引发滑坡的关系——以三峡库区滑坡为例[J].岩石力学与工程学报,2009,28(3):576-582 ZHAO Yu, CUI Peng, HU Liang-bo. Relation Between Evolution of Clay Shear Strength and Landslide Induced by Acid Rain-taking Landslides in Three Gorges Reservoir Area for Example [J]. Chinese Journal of Rock Mechanics and Engineering, 2009, 28 (3): 576- 582.
[12] SHUZUI H. Process of Slip-surface Development and Formation of Slip-surface Clay in Landslides in Tertiary Volcanic Rocks, Japan [J]. Engineering Geology, 2001, 61 (4): 199-220.
[13] PRASAD C R V, REDDY P H P, MURTHY V R, et al. Swelling Characteristics of Soils Subjected to Acid Contamination [J]. Soils & Foundations, 2018, 58 (1): 110-121.
[14] HAIR P R P, CHAVALI R V P, PILLAI R. Swelling of Natural Soil Subjected to Acidic and Alkaline Contamination [J]. Periodica Polytechnica Civil Engineering, 2017, 61 (3): 611-620.
[15] 刘新荣,李栋梁,王震,等.酸性干湿循环对泥质砂岩强度特性劣化影响研究[J].岩石力学与工程学报,2016,35(8):1543-1554. LIU Xin-rong, LI Dong-liang, WANG Zhen, et al. The Effect of Dry-wet Cycles with Acidic Wetting Fluid on Strength Deterioration of Shaly Sandstone [J].Chinese Journal of Rock Mechanics and Engineering, 2016, 35 (8): 1543-1554.
[16] 刘新荣,袁文,傅晏,等.酸碱环境干湿循环作用下砂岩抗剪强度劣化规律研究[J].岩土工程学报,2017,39(12):2320-2326. LIU Xin-rong, YUAN Wen, FU Yan, et al. Deterioration Rules of Shear Strength in Sandstones Under Wetting and Drying Cycles in Acid and Alkali Environment [J]. Chinese Journal of Geotechnical Engineering, 2017, 39 (12): 2320-2326.
[17] 李相然,姚志祥,曹振斌.济南典型地区地基土污染腐蚀性质变异研究[J].岩土力学,2004,25(8):1229-1233. LI Xiang-ran, YAO Zhi-xiang, CAO Zhen-bin. Study on Physical and Mechanical Property Variation of Polluted Erosive Foundation Soils in Typical District of Jinan [J]. Rock and Soil Mechanics, 2004, 25 (8): 1229-1233.
[18] 孙平安,李秀存,于奭,等.酸雨溶蚀碳酸盐岩的源汇效应分析——以广西典型岩溶区为例[J].中国岩溶,2017,36(1):101-108. SUN Ping-an, LI Xiu-cun, YU Shi, et al. The Effect of Source and Sink in Shi, Dissolution of Carbonate Rocks of Acid Rain Analysis: Taking Guangxi as an Example [J]. Carsologica Sinica, 2017, 36 (1): 101-108.
[19] CHENG A, WEI H, TAN F. Analysis of the Temporal-spatial Distribution and Seasonal Variation of the Acid Rain in Guangxi Province [J]. Meteorological and Environmental Research, 2010, 1 (1): 62-65.
[20] 魏伟.酸性环境下重塑膨胀土的胀缩性和抗剪强度研究[D].桂林:桂林理工大学,2016. WEI Wei. Research on Swelling-shrinkage and Shear Strength of Disturbed Expansive Soil Under Acidic Environment [D]. Guilin: Guilin University of Technology, 2016.
[21] 杨和平,曲永新,郑健龙.宁明膨胀土研究的新进展[J].岩土工程学报,2005,27(9):981-987. YANG He-ping, QU Yong-xin, ZHENG Jian-long. New Development in Studies on Ningming Expansive soils [J]. Journal of Geotechnical Engineering, 2005, 27 (9): 981-987.
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