Water and salt migration and erosion characteristics of gravel sulfate salty soil subgrade under effect of pavement covering
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摘要: 为明确中国西部季节性冻土地区机场道面覆盖作用下含硫酸盐砾质混合料的水盐迁移特征, 探明由于水盐迁移导致的道基底部侵蚀危害, 依托西北某机场改扩建工程, 采用自主设计的试验设备, 针对含硫酸盐砾质混合料道基, 首次开展了温度范围为—30 ℃~25 ℃冻融循环工况中道面覆盖作用下道基上、下部不同含盐量组合模型的水盐迁移及变形试验, 并通过电镜扫描仪和X射线衍射仪从微观角度分析了冻融循环及水盐迁移共同作用下的道面底部侵蚀特征。试验结果表明: 当道基上部含盐量由0增至0.3%时, 工况间含水率的变化幅度为0.37%, 与下部含盐量由0.3%增至0.7%相比增加83.8%;9次冻融循环期间, 各工况砾质混合料的含盐量均在道基深度35 cm范围内变化明显, 且该范围内盐分向上的迁移量大于自下方迁移而来的补充量; 当含盐砾质混合料道基上部含盐量由0增至0.3%时, 试样的最终累计变形量增加47.8%, 而道基下部含盐量由0.3%增至0.7%时, 其最终累计变形量仅增加7.4%;在含盐量和循环周期改变条件下含盐砾质混合料道基累计变形量的预测模型中, 道基上部含盐量是驱动含硫酸盐砾质混合料道基变形的主导因素; 在水盐相变及硫酸盐侵蚀的耦合作用下, 道基下部盐分由0.3%增至0.7%时, 盐分无法及时补充至基底来维持钙矾石稳定所需的高碱环境, 基层底部表面钙矾石的数目反而有所减小。研究成果为揭示季冻区道面覆盖作用下含硫酸盐砾质混合料道基的水盐迁移及其对水泥基层的侵蚀规律提供了基础数据。Abstract: To clarify the water and salt migration characteristics of sulfate-containing gravel mix under the effect of airport pavement covering in the seasonal frozen area of Western China and to identify the erosion hazards at the bottom of the subgrade due to water and salt migration, based on the reconstruction and expansion project of an airport in Northwest China, independently-designed experimental equipment was used to conduct experiments on the water and salt migration and deformation of models with different combinations of salt content in upper and lower parts of the subgrade under the effect of pavement covering and the working condition of freeze-thaw cycles in the temperature range of —30 ℃ - 25 ℃ for the first time. The pavement bottom erosion characteristics under the joint action of freeze-thaw cycles and water and salt migration were analyzed by a scanning electron microscope and an X-ray diffractometer. Experimental results show that the change in water content is 0.37% between working conditions when the salt content in the upper part of the subgrade increases from 0 to 0.3%, which is an increase of 83.8% compared with the situation when the salt content in the lower part of the subgrade increases from 0.3% to 0.7%. During the nine freeze-thaw cycles, the salt content of the gravel mix in each working condition changes significantly within the range of 35 cm below the subgrade, and the migration of the salts upward in the range is greater than the replenishment of the salts migrating from the lower part. When the salt content of the upper part of the salt-containing gravel mix subgrade increases from 0 to 0.3%, the final cumulative deformation of the sample increases by 47.8%, whereas when the salt content of the lower part of the subgrade increases from 0.3% to 0.7%, the final cumulative deformation of the sample increases by only 7.4%. In the prediction model of cumulative deformation of a salt-containing gravel mix subgrade under conditions of varying salt content and cycle time, salt content in the upper part of the subgrade is the dominant factor driving deformation of the sulfate-containing gravel mix subgrade. Under the coupling effect of the water-salt phase change and sulfate erosion, when the salt content of the lower part of the subgrade increases from 0.3% to 0.7%, the salts fail to be replenished to the bottom of the subgrade to maintain the high alkaline environment required for the stability of ettringite. Instead, the amount of ettringite on the bottom surface of the subgrade decreases. The research results provide basic data for revealing the water and salt migration of sulfate-containing gravel mix subgrade under the effect of pavement covering and its erosion law on cement base in the seasonal frozen area.
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图 4 地区年平均温度及单个周期的温度设置
Figure 4. Regional annual average temperatures and temperature settings for a single cycle
图 8 工况1、3、5试样的温度分布曲线
Figure 8. Temperature distribution curves of specimens under working conditions 1, 3 and 5
图 9 9次冻融循环结束后基底下方不同深度处
Figure 9. Temperature changes from initial values at various depths below the subgrade after 9 freeze-thaw cycles
图 10 工况1、3、5试样各深度处含水率和含盐量整体分布
Figure 10. Distributions of water and salt content at each depth under working conditions 1, 3 and 5
图 11 各试样不同深度处含水率与含盐量变化特征
Figure 11. Variations of water and salt content at various depths of each specimen
图 12 各工况砾质混合料变形特征
Figure 12. Deformation characteristics of gravelly mixture under each working condition
图 13 含盐量和循环周期改变条件下拟合累计变形量
Figure 13. Fitted cumulative deformation under conditions of varying salt content and cycle period
图 14 冻融循环结束后各工况水泥基层底部的XRD图像
Figure 14. XRD images of cement subgrade bottom after the freeze-thaw cycle under each working condition
图 15 各工况最大裂缝宽度和道基深度5 cm处盐分迁移量
Figure 15. Maximum crack width and salt migration at 5 cm depth of subgrade under each working condition
图 16 冻融循环结束后各工况下水泥基层底部的SEM图像
Figure 16. SEM images of cement subgrade bottom after the freeze-thaw cycle under each working condition
表 1 洗盐前后试样各粒径占比
Table 1. Proportions of each grain size of samples before and after salt washing
状态 不同孔径(mm)下小于某粒径百分比/% 80 60 40 20 10 5 2 1 0.5 0.25 0.075 洗盐前 100.00 95.50 91.60 76.90 51.30 29.80 12.80 9.35 4.62 1.51 0.30 洗盐后 100.00 95.40 91.50 76.70 50.20 29.10 11.90 8.70 4.20 1.27 0.32 
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表 2 试样基本特性
Table 2. Basic properties of samples
不均匀系数 曲率系数 最大干密度/ (g·cm3) 最佳含水率/% 相对体积质量 7.65 1.13 2.20 5.07 2.75 (≤5 mm) 2.69 (＞5 mm)
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表 3 物理模拟试验工况设计
Table 3. Condition design of physical simulation test
试验工况 1 2 3 4 5 基底下方0.00~ 93.75 cm试样含盐量/% 0.00 0.15 0.30 0.30 0.30 基底下方93.75~125.00 cm试样含盐量/% 0.30 0.30 0.30 0.50 0.70
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