1、第 9 章 地面沉降主要危害 (1).沿海地区沉降使地面低于海面，受海水侵袭；(2)一些港口城市，由于码头、堤岸的沉降而丧失或降低了港湾设施的能力;(3)桥墩下沉，桥梁净空减小，影响水上交通.(4)在一些地面沉降强烈的地区，伴随地面垂直沉陷而发生的较大水平位移，往往会对许多地面和地下构筑物造成巨大危害;(5)在地面沉降区还有一些较为常见的现象，如深井管上升、井台破坏，高摆脱空，桥墩的不均匀下沉等，这些现象虽然不致于造成大的危害，但也会给市政建设的各方面带来一定影响。位于未固结或半固结疏松沉积层地区内的大城市，因为潜水易于污染往往开发深层的承压水作为工业及生活用水的水源。在孔隙承压含水层中，抽汲
2、地下水所引起的承压水位的降低，必然要使含水层本身和其上、下相对含水层中的孔隙水压力随之而减小。根据有效应力原理可知，土中由复盖层荷载引起的总应力是由孔隙中的水和土颗粒骨架共同承担的。由水承担的部分称为孔隙水压力，它不能引起土层的压密，故又称为中性压力，而由土骨架承担的部分则能直接造成土层的压密，故称为有效应力；二者之和等于总应力。定抽水过程中土层内的总应力不变，那么孔隙水压力的减小必然导致土中有效应力的等量增大，结果就会引起土层成比例的固结。由于区域性地面沉降范围较广阔，压缩层厚度与沉降范围相比较，又相对较小，因此无论从理论或实际应用上，即可以把这类由于抽水引起的地面沉降问题按一维固结问题处理
3、。在天然应力条件下，由于地质结构的不同，抽水所引起的土层中孔隙水压力及有效应力的变化，可有不同模式，图9-8 以三层结构条件下单层抽水的情况为例，对抽水过程中土层中应力的转变及土层的固结问题进行具体分析。在三层结构及天然状态下，如果下部承压含水层中的承压水位与上部潜水含水层的水位相一致，【图9-9】由于透水性能的显著差异，上述孔隙水压力减小，有效应力相应增大的过程，在砂层和粘土层中的表现是截然不同的。在砂层中这一过程基本上可志着固结进展程度的应力转换线逐渐地向最终边界线坝推进如图99(b)，而达到AB线(与降低后的承压水位相平按的孔隙水压力线)所需的时间，正如模型试验(图910)所表明的，拄往
4、需要几个月、几年甚至几十年(取决于土层厚度和透水性)。这样，在承压水位降低后，直到应力转变过程(也就是固结过程)最终完成之前的相当长的一段时间里，粘土层中始终不同程度地存在有高于和新的承压水位相平衡的孔隙水压力，这部分孔隙水压力通常被称为剩余孔隙水压力或超孔隙水压力。土层内现有的剩余孔隙水压力的大小，是衡量该土层在现存的应力条件下可能最终产生的固结、压密的强烈程度的重要标志，通常可以通过实测加以查明。以上通过一种较简单的三层结构、单层抽水模式，地面沉降的机制。其它多层结构(甚至多层抽水)类型的沉降，但基本机制仍然是相同的。土体压缩是土颗粒距离重排，孔隙减小以及土颗粒重排粘性土的微结构：因此，粘
5、土层的压密和非弹性永久变形在地面沉降中起主导作用。粘性土的弹性变形和砂层的膨胀回弹构成地面总的回弹量 此外，粘性土层孔隙水压力的调整需要较长的时间，即有效应力增长与粘性土相应的压密变形过程之间存在着时间滞后。预测所需要的地质资料预测性图件 各种基础性地质图件，反映砂层、粘性土层厚度变化，地下水位或液压变化及人类活动诱发因素指标变化的等值线图预测性曲线意大利威尼斯的预测工作根据上述五种抽水计划所推出的地面沉降发展趋势美国德克萨斯州休斯顿地区的预测工作，对该区两个主要含水层承压水位下降及地面沉降的观测资料进行对比具体措施是进行地下水资源管理，方法有：压缩地下水开采量，减少水位降深幅度向含水层进行人
6、工回灌，注意水质，防治污染调整地下水开采层次，合理开采，适当开采更深层地下水或以地面水源代替在沿海低平原地带修筑或加高挡潮堤，防洪堤等改造低洼地形，人工填土加高地面改建城市给排水系统等，使之适应地卖弄沉降后的情况修改城市建设规划，调整城市功能分区及总体布局对可能发生的地区，硬预测地面沉降的可能性和危害程度：估算沉降量，预测其发展趋势结合水资源评价，研究确定地下水资源的合理开采方案采取适当的建筑措施，预先对可能发生地面沉降量作充分考虑图中的G-G线将代表由土层自重所造成的总应力线 H-C 线则将代表土层中的天然孔隙水压力线持续开采，导致地下水继续降低，如持续开采，导致地下水继续降低，如1 1部分
7、停止开采，水位缓慢上升，如部分停止开采，水位缓慢上升，如2、3 3、4 4完全停止开采，恢复到最初状态，如完全停止开采，恢复到最初状态，如5 5Land subsidence is the lowering of the land-surface elevation from changes that take place underground.Common causes of land subsidence from human activity are pumping water,oil,and gas from underground reservoirs;dissolution of
8、 limestone aquifers(sinkholes);collapse of underground mines;drainage of organic soils;and initial wetting of dry soils(hydrocompaction).Land subsidence occurs in nearly every state of the United States.(figure 1).Overdrafting of aquifers is the major cause of subsidence in the southwestern United S
9、tates,and as ground-water pumping increases,land subsidence also will increase.In many aquifers,ground water is pumped from pore spaces between grains of sand and gravel.If an aquifer has beds of clay or silt within or next to it(figure 2),the lowered water pressure in the sand and gravel causes slo
10、w drainage of water from the clay and silt beds.The reduced water pressure is a loss of support for the clay and silt beds.Because these beds are compressible,they compact(become thinner),and the effects are seen as a lowering of the land surface.The lowering of land surface elevation from this proc
11、ess is permanent.For example,if lowered ground-water levels caused land subsidence,recharging the aquifer until ground water returned to the original levels would not result in an appreciable recovery of the land-surface elevation.Land subsidence causes many problems including:changes in elevation a
12、nd slope of streams,canals,and drains;damage to bridges,roads,railroads,storm drains,sanitary sewers,canals,and levees;damage to private and public buildings;failure of well casings from forces generated by compaction of fine-grained materials in aquifer systems.In some coastal areas,subsidence has
13、resulted in tides moving into low-lying areas that were previously above high-tide levels.An example of damage caused by land subsidence can be seen in figure 3.The concrete base at the top of the well is above ground level because the land surface has lowered and the rigid well casing has not sunk.
14、lIn many areas of the arid Southwest,earth fissures are associated with land subsidence.Earth fissures can be more than 100 feet deep and several hundred feet in length.One extraordinary fissure in central Arizona is 10 miles long.These features start out as narrow cracks,an inch or less in width.Th
15、ey intercept surface drainage and can erode to widths of tens of feet at the surface.Examples of earth cracks are shown in figures 4,5,6,and 7.Earth fissures are caused by horizontal movement of sediments that occurs when ground-water is pumped.In areas where climate change results in less precipita
16、tion and reduced surface-water supplies,communities will pump more ground water.In the southern part of the United States from states on the Gulf Coast and westward including states of New Mexico,Colorado,Arizona,Utah,Nevada and California,major aquifers include compressible clay and silt that can c
17、ompact when ground-water is pumped.Also,increased population in the Southwest will increase demands on ground-water supplies,causing more land subsidence in areas already subsiding and new subsidence in areas where subsidence has not yet occurred.In the past,major subsidence areas have been in agric
18、ultural settings where ground-water has been pumped for irrigation.In the future,however,increasing population may result in subsidence problems in metropolitan areas where damage from subsidence will be great.Several methods are available to monitor land subsidence.The most basic approaches use rep
19、eated surveys with conventional or GPS leveling.Another approach is to use permanent compaction recorders,or vertical extensometers(figure 8).These devices use a pipe or a cable inside a well casing.The pipe inside the casing extends from land surface to some depth through compressible sediments.A t
20、able at land surface holds instruments that monitor change in distance between the top of the pipe and the table.If the inner pipe and casing go through the entire thickness of compressible sediments,then the device measures actual land subsidence.If both ground-water levels and compaction of sedime
21、nts are measured,then the data can be analyzed to determine properties that can be used to predict future subsidence.About 19 of these installations are operated in Southern Arizona and additional stations are operated in California,Nevada,New Mexico,and Texas.Another subsidence monitoring method under development and testing uses Interferometric Synthetic Aperture Radar(INSAR).With this method,individual radar images from satellites are compared and interferograms are produced.Under the best conditions,land-surface elevation changes on the order of 1 inch or less can be determined.THE END
