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EFFECTS OF TEMPERATURE FLUCTUATION ON STRUCTURAL EVOLU￣TION OF INITIAL LOESS DEPOSITS
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摘要: 现今的黄土结构是在黄土初始风成堆积及后期黄土化过程中逐步形成的。季节性冷暖更替和昼夜气温变化使得初始风积黄土不断经受升降温循环作用，由此引发的结构演变是黄土化过程不可或缺的一部分。而关于初始风积黄土在温度循环作用下的结构演化规律及机理目前尚不清楚。本文通过模拟风积环境，再造了初始风积黄土样品，开展了温度循环物理模拟试验。采用温度传感器、激光位移传感器和高清摄影系统对环境温度、土样内部温度、土样竖向变形及顶面结构进行了实时监测。试验结果发现：土样的温度和竖向变形均随环境温度的变化呈周期性波动，波动曲线相较于环境温度均呈现出一定滞后；且土样竖向变形和土样温度的波动具有同步性。随温度循环次数的增加，土样呈整体收缩趋势，土样也由弹塑性变形逐渐过渡为弹性变形。相比于湿-干循环和上覆荷载作用，温度循环导致的初始风积黄土竖向应变最小(约为0.25%)，结构扰动程度最弱。以上结果说明，虽然温度循环是风成黄土结构演化过程中的核心环境因素之一，但其在初期黄土结构演化中属次要角色。Abstract: The present-day loess structure is gradually formed with the process of initial accumulation of loess particles and the later lossification. The climate is typically dry in the period of loess accumulation and the initial loess deposit(ILD)suffers from daily and seasonal temperature fluctuations,which evidently leads to structural evolution of ILD. However,the mechanism behind is still unclear. In this study,ILD is reconstructed in the laboratory and physical simulation with temperature fluctuation is carried out. Ambient temperature,soil temperature,vertical deformation and structure of top surface of soil sample are monitored with thermocouple temperature sensors,laser displacement sensors and an industrial camera of high-resolution. The results show that both soil temperature and vertical deformation of soil sample fluctuate with the ambient temperature,but lag behind. The vertical deformation exhibits a synchronous fluctuation with the soil temperature. With temperature cycles,the soil sample keeps contraction,and the soil sample experiences elastic-plastic deformation and then elastic deformation. Compared with the effects of water and overlying load,the deformation and structure disturbance of ILD due to temperature fluctuation are minimum(about 0.25%). This indicates that temperature fluctuation plays a noncrucial role in the formation of present-day loess structure.
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Key words:
- Lossification /
- Initial loess deposit /
- Temperature fluctuation /
- Hysteresis /
- Contraction
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图 1 初始风积黄土样粒径级配曲线
Figure 1. Particle size accumulated curve of the initial loess deposit
图 2 温度循环试验装置与传感器布置
a. DHTH-500-40-P-SD环境试验箱；b. 土体结构监测系统；c. 土样竖向位移监测点布置
Figure 2. Setup of the experiment
图 4 不同循环次数后试样TEM-1顶面结构特征
a. 0次；b. 35次；c. 40次
Figure 4. Surface structure characteristics of sample TEM-1 after different cycles
图 5 环境温度(a)、土体温度(b)和试样TEM-2顶面代表点竖向变形(c)随时间变化趋势
T3-T12表示代表点正下方竖向深度分别为3 cm、6 cm、9 cm、12 cm处的土体温度。试验早期数据传输出现临时故障，导致第4~7次温度循环过程中的数据缺失
Figure 5. The variation of ambient temperature, soil temperature and vertical displacement of sample TEM-2 with time
图 6 代表性时间段(531 h至603 h)内的环境温度(a)、土体温度(b)和试样TEM-2代表点竖向变形(c)随时间变化趋势
and vertical displacement of sample TEM-2 with time in the representative period(from 531 h to 603 h)
Figure 6. The variation of ambient temperature, soil temperature
图 7 a. 土体温度随深度变化；b. 土样竖向变形理论计算值与实际监测值对比
注：变形为正值代表土样膨胀，负值代表土样收缩
Figure 7. a. Variation of soil temperature with depth;b. Comparison between the theoretical calculated value and the actual monitored value of vertical displacement
图 8 a. 土体导热系数随干密度变化趋势(图例中数字表示质量含水率)；b. 土体导热系数随含水率变化趋势(图例中数字表示土样干密度，单位g·cm-3)(数据来源：初始风积黄土样和原状黄土样相关数据由实验室实测获得，其余数据王铁行等(2007)；陈毅(2018))
Figure 8. a. Variation of soil thermal conductivity coefficient with dry density(the figures in the legend indicate mass moisture content); b. Variation of soil thermal conductivity coefficient with water content(the figures in the legend indicate the dry density of soil sample)(Data source: the data of initial loess deposit sample and undisturbed loess sample are obtained from laboratory measurement, and the rest data are from Wang et al. (2007) and Chen(2018))
图 9 温度循环作用下初始风积黄土结构演化机制模型
a. 微观水平(图中矿物颗粒颜色不同代表矿物种类不同)；b. 宏观水平
Figure 9. Structural evolution mechanism model of initial loess deposit under thermal cycles
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