Environmental and Engineering Geoscience; August 2007; v. 13; no. 3;
p. 217-228; DOI: 10.2113/gseegeosci.13.3.217
© 2007 Association of Engineering Geologists
Stress-strain Measurements of Deforming Aquifer Systems That Underlie Shanghai, China
Yun Zhang1,
Yu-Qun Xue1,
Ji-Chun Wu1,
Shu-Jun Ye1 and
Qin-Fen Li2
1 Department of Earth Sciences, Nanjing University, Nanjing, China
2 Shanghai Institute of Geological Survey, Shanghai, China
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ABSTRACT
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This article describes aquifer system deformation in Shanghai, where land subsidence, caused by excessive groundwater withdrawal, has caused considerable economic loss during the past 40 years. The article investigates the relationship between the amount of groundwater pumped and land subsidence and the relationship between layers with the most significant deformation and the main exploited aquifers. Based on monitoring data, including compaction of individual strata documented by extensometer groups and the groundwater levels from observation wells, the characteristics of aquifer system deformation are studied. When the groundwater level fluctuates within a certain range or the average declines slowly above the previous lowest value, sand layers exhibit elastic or elasto-plastic behavior, whereas soft clay layers exhibit visco-elastic-plastic behavior. The softer the clay, the more the viscous deformation. When the average groundwater level decreases below the previous lowest value, both sand and clay layers exhibit visco-elasto-plastic behavior. Although the sand layers have less plasticity and creep than the clay layers, sand deformation can be significant, and a sand layer may become a primary compaction layer if the sand is thick. In the period 1981–1990, soft clay layers one, two, and three were the major compaction layers, but in the period 1991–2000, sand layer five was the major compaction layer that contributed more than 50 percent of the total subsidence.
Key Words: Land Subsidence • Groundwater Withdrawal • Aquifer System Deformation • Stress and Strain • Shanghai
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Introduction
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Land subsidence caused by excessive withdrawal of groundwater is a geological hazard. Many countries and areas in the world, such as Thailand (SIG, 1978), Venice (Lewis et al., 1978), San Joaquin Valley, California (Lofgren, 1969), Houston-Galveston region, Texas (Gabrysch et al., 2000), Mexico City (Adrian et al., 1999), Shanghai (Qian et al., 1981), and Tianjin (Shearer, 1998), have encountered this problem. Land subsidence can cause significant damage, including (1) the loss of ground surface elevation and the loss of the ability of flood-control and drainage projects to resist floods; (2) the cracking of building foundations and serious effects on the use and life spans of buildings and an adverse impact on the safety of railways, highways, subways, tunnels, bridges, and underground pipelines, especially where land subsidence is differential; and (3) the loss of clearance under bridges and passing capacity of inland rivers. This damage causes considerable economic loss. For example, the total loss caused by land subsidence in Shanghai has exceeded 35 billion US dollars during the past 40 years.
Land subsidence caused by excessive groundwater withdrawal can be explained through the principal of effective stress (Lofgren, 1969; Poland et al., 1975; and Galloway et al., 1999). The variation of pore water pressure resulting from groundwater withdrawal is the external cause for land subsidence and the compressibility of soil strata is the internal cause. Land subsidence is always associated with these two elements.
The characteristics of aquifer system deformation are important in the study of land subsidence. Field measurements of compaction and correlative change in water level were used to investigate aquifer system deformation. From these measurements, the stress-strain curves of aquifer systems were constructed for measuring sites (Lofgren, 1969; Riley, 1969; Poland et al., 1975; Helm, 1976; Ireland et al., 1984; and Riley, 1984). Poland and others (1969) and Ireland and others (1984) analyzed the stress-compaction and stress-strain relations based on field data of the San Joaquin Valley, California. They found that the aquifer system had different responses. At some sites, the compaction continued and there was no net expansion of the measured interval at any time, even during the period of rapid increase in artesian head. At other sites, however, aquifer-system response was in an elastic range and expansion occurred when the stress decreased. Liu and others (2004) investigated the stress-strain relations from the measured data of the Choshui River alluvial fan of Taiwan. They recognized that the sand layers exhibited obvious elasto-plastic behavior and there were notable irrecoverable strains.
Features of aquifer system deformation vary from region to region because of complicated stratigraphic and pumping conditions. Shanghai is typical in that land subsidence has been induced by excessive groundwater withdrawal, whereas the bedrock is nearly stable and tectonic movement has been negligible during the historical span of measuring (Qian et al., 1981; Su, 1979; and Zhang et al., 2005). There are 27 extensometer groups and more than 1,400 observation wells in Shanghai. Some of them were built in the 1960s and have been monitored continuously. Extensometer groups can monitor compaction of individual strata. Observation wells were installed in aquifers to observe the groundwater levels in each aquifer. Together, the extensometer groups and observation wells provide an invaluable historical record of compaction and pore water pressure changes and make it convenient to investigate the details of aquifer system compaction when the groundwater level changes because of pumping. Monitoring data show that the land subsidence patterns in Shanghai are complicated and the aquifer system compaction has some new and unique characteristics. Besides elasticity and elasto-plasticity, sand layers may exhibit visco-elasto-plasticity and become primary compaction layers, which has not been reported before. This article aims to describe the empirical deformation of the aquifer system in Shanghai, which will provide important constraints for constructing mathematic models of land subsidence and for proposing reasonable measures to prevent land subsidence from continuing.
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Aquifer System Stratigraphy and Previous Studies of Land Subsidence in Shanghai
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Shanghai is located in the front of Yangtse delta and occupies an area of nearly 6,340 km2, as shown in Figure 1. The thickness of the Quaternary deposits ranges from 100 m in the southwest part to 400 m in the east part of Shanghai and averages 280 m in the city and its outskirts. The Quaternary deposits are composed of a phreatic aquifer, five confined aquifers, and six aquitards. Figure 2 presents the conceptual hydrogeologic model of Shanghai. The aquifers consist of silty sand, fine sand, medium sand, coarse sand, and gravel. In some areas these aquifers contain clay lens structures or thin clay interlayers. Except for confined aquifer five, which is mainly in the north and east part of Shanghai, the other aquifers are continuous beneath Shanghai. The aquitards consist of clay and silty clay and contain several layers of silty sand or fine sand in some areas. From the point of engineering geology, the six aquifers, from top to bottom, are also called sand layers one, two, three, four, five, and six, respectively, as illustrated in Table 1. The aquitard between the phreatic aquifer and confined aquifer one consists primarily of two soft clay layers: soft clay layer one and soft clay layer two. Soft clay layer one is more compressible than soft clay layer two.
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Figure 1. Location map and administrative divisions of Shanghai, and the locations of extensometer groups
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Studies of land subsidence in Shanghai began in 1981. Qian and others (1981) used Terzaghi's one-dimensional consolidation theory to simulate the cumulative compaction of two aquitards above confined aquifer two at one site during the period 1965–1978. In their study, the aquitards were treated as bi-linear materials. When the groundwater level was higher than the previous lowest values, the reloading coefficient of consolidation was adopted in the consolidation equation. When the groundwater level dropped below the previous lowest values, the standard coefficient of consolidation was adopted. On the basis of the previous study, LEHGGS and others (1989) investigated land subsidence in the urban area of Shanghai and proposed a predictive model to calculate land subsidence. In their model, the sand layers (aquifers) were assumed to behave as linear elastic materials, and the clay layers (aquitards) were treated as bi-linear materials. The depth of this study was limited to 0 m to 70 m below the ground surface (above sand layer three). Men (1999a, 1999b) and Gu and others (2000) studied the creep deformation of shallow soft clay layers one and two in laboratory tests. They concluded that soft clay can deform by creep and that this should be considered in the calculation of land subsidence. Before 2000, the studies of land subsidence in Shanghai were focused on the upper 70 m and the emphasis was on the compaction of soft clay layers one, two, and three. Accompanying a change in the exploited aquifers and the increase of groundwater pumping, the compaction of deep soil layers became important (Li et al., 2000; Wei, 2002). Li and others (2000) proposed a mathematical model to predict land subsidence. Their study considered all Quaternary deposits beneath urban and suburban areas of Shanghai. As far as the features of compaction of clayey and sandy layers were concerned, they made the same assumptions as LEHGGS and others (1989). Wei (2002) analyzed the relations of stress and strain of the deep sand layer (sand layer five) on the basis of monitoring data. He proposed that this sand deformed plastically, which had not been recognized in the previous studies (Lewis et al., 1978; Su, 1979; LEHGGS et al., 1989; Shearer, 1998; and Gu et al., 2000).
As described above, aquifer system deformation as a consequence of excessive groundwater pumping in Shanghai is complicated and is not yet fully understood. Deformation is strongly correlated with the changes in effective stress, i.e., the changes in groundwater level. A specific stratum may exhibit elastic, elasto-plastic, or visco-elasto-plastic behavior for different changes in groundwater levels. As such, previous studies underestimate the complexity of deformation.
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Groundwater Pumping and Land Subsidence
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The Relationship Between The Amount of Groundwater Pumping and Cumulative Land Subsidence
Extraction of groundwater from deep wells in Shanghai began in 1860 and land subsidence was first reported in 1921. From 1921 to 2001, the average amount of total cumulative subsidence in the city center was approximately 1.93 m and the maximum was 2.63 m (Sun, 2002). Records dating back to 1921 indicate that groundwater withdrawal changed with time. Figure 3 depicts the variation of the annual volume of groundwater pumped, recharged, and net groundwater pumped since 1961. The amount and rate of land subsidence in Shanghai changed correspondingly and can be separated into seven stages (Figure 4): increasing (1921–1948), strongly increasing (1949–1956), very strongly increasing (1957–1961), decreasing (1962–1965), rebounding (1966–1971), slightly increasing (1972–1989), and slowly increasing (1990–2001). The average yearly rates of subsidence during these seven periods are approximately 24, 40, 100, 60, –3, 3.5, and 16 mm/a, respectively (SGEAEB 2002; Chai et al., 2004). The corresponding net groundwater pumping rates are 0.09, 1.40, 2.00, 1.56, 0.62, 0.75, and 1.13 x 108 m3/a, respectively. The change in the subsidence rate was strongly correlated with the amount of groundwater pumping. After very strongly increasing from 1957 to 1961, some measurements were taken, showing a decrease in groundwater pumping, an increase in artificial recharge, and a gradual change of exploited aquifers. As a result of these measurements, the subsidence rate decreased rapidly and some recovery took place (the cumulative rebound was 18.1 mm at the center of the city from 1966 to 1971). During the 1990s, because of the increase in the exploitation of groundwater in Shanghai and its vicinity, the subsidence accelerated again and rose to about 16 mm per year on average (Zhang, 2002).
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Figure 4. The developing history of land subsidence in Shanghai. (a) Average cumulative subsidence of Shanghai. (b) Cumulative subsidence at Labor Park, Shanghai
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Main Exploited Aquifers and Primary Compaction Layers
With the change of exploited aquifers, the primary compaction layer also changed. Before 1963, groundwater was extracted mainly from confined aquifers two and three in the urban district (accounting for 86 percent of the total), which resulted in significant compaction of the upper 70 m. To mitigate land subsidence in the urban area, pumping of groundwater from confined aquifers two and three was decreased significantly, whereas the amount of groundwater pumped from confined aquifers four and five was gradually increased after 1968. They currently account for 70 percent and 15 percent, respectively, of the total exploitation of Shanghai aquifers. Figure 5 shows the net amount of groundwater pumped from individual aquifers from 1961 to 2000 (SGEAEB, 2002).
Changes in the amount of groundwater pumped and the exploited aquifers changed groundwater levels in those aquifers. Monitoring of groundwater levels in observation wells revealed the changed patterns of groundwater levels within individual aquifers. Figure 6 shows the typical groundwater level variations with time in five confined aquifers. The phreatic aquifer was not exploited and is recharged by rainfall and surface water. The groundwater level in the phreatic aquifer does not change much so it is not considered here. Confined aquifer one was not pumped directly, but was affected by the groundwater pumping from confined aquifer two. Therefore, the groundwater level in confined aquifer one changed in a similar pattern to confined aquifer two. For confined aquifers one to three, the groundwater levels reached their lowest values at the beginning of the 1960s, then rose quickly because of strict pumping limits and increased recharge. They increased to their highest values at the beginning of the 1970s. Then the groundwater levels fluctuated within a narrow range because of the seasonal pumping (in the summer) and recharging (in the winter), and the average was nearly steady. After the middle of the 1980s, the average groundwater levels began to decrease slowly as a result of slightly increasing groundwater pumping from these confined aquifers, but they were always much higher than their previous lowest values. For confined aquifer four, the initial groundwater level before pumping was 2.0 m (Wei, 2002). It fell to approximately –8 m at the end of the 1960s. The groundwater level then rose slightly and fluctuated until the middle of the 1970s. After that, the average groundwater level began to decline slowly. It fell significantly and was lower than its previous lowest value from the end of the 1980s to 1999 because of greatly increased groundwater withdrawal from this confined aquifer. After 1999, the amount of groundwater exploited from confined aquifer four decreased, thus the average groundwater level remained steady to slightly increased. The groundwater level in confined aquifer five changed in a similar pattern to that in confined aquifer four.
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Figure 6. Variation of groundwater level with time. (a) Confined aquifer one, two, and four. (b) Confined aquifer three and five
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Accompanying the change in exploited aquifers, the contribution of individual strata to the total subsidence changed. Based on the available data, the compaction of individual strata from extensometer groups in two periods is considered: from January 1981 to December 1990 and from January 1991 to December 2000. The percentage of compaction in different strata is illustrated in Table 2 (positive values indicate compaction, negative indicate expansion). Because of length limitations, only three extensometer groups are presented in Table 2. The data for other extensometer groups are similar. It is noted that the shallow strata (above sand layer three) contributed more to total subsidence in the period 1981–1990, whereas the deep strata, especially sand layer five, contributed more to total subsidence in the period 1991–2000. This indicates that the primary compaction layers shifted from the shallow strata (three soft clay layers) in the period 1981–1990 to the deep strata (sand layer five) in the period 1991–2000. During these two periods, the contribution of sand layers three and four was small.
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Table 2. Percentage of compaction for individual strata at different depths below land surface (positive values indicate compaction and negative values indicate expansion)
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The compaction of a specific stratum depends on its compressibility, the change in pore water pressure it has experienced, and its thickness. Previous studies (Qian et al., 1981; LEHGGS et al., 1989; Men, 1999a, 1999b; Gu et al., 2000; and Li et al., 2000) focused on the stratum compressibility but gave little attention to the other two factors when considering the primary compaction layers. Here we take extensometer group four as an example. Figure 7 illustrates the total subsidence and compaction of individual strata at extensometer group four, including soft clays one and two, soft clay three, and sand layer five, because the major compaction took place in these strata either in the period 1981–1990 or in the period 1991–2000. In the period 1981–1990, the groundwater level in each confined aquifer declined slowly and the total subsidence and the compaction of each stratum were small. Three soft clay layers, especially soft clay layers one and two, had much higher compressibility than other strata and their thickness accounted for approximately 17 percent of the system. Hence, the ratio of compaction in total subsidence was prominent. In the period 1991–2000, the groundwater level in each confined aquifer declined more rapidly than in the former period and the total subsidence rate increased. The groundwater levels in confined aquifers one to three fell slightly, and they were much higher than their previous lowest values, whereas the groundwater level in confined aquifer four fell much more and was lower than its previous lowest value. Furthermore, sand layer five is low to medium density and has medium to high compressibility. Its thickness accounted for approximately 20 percent of the system. Thus, the compaction rate of sand layer five exceeded that of the shallow strata. It contributed up to 64.5 percent of the total compaction and played an important role in land subsidence.
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Features of Aquifer System Compaction
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The compressibility of unlithified strata is related to their composition and state of stress. Under a specific stress condition, the soft clay presents higher compressibility, greater plasticity, and greater creep than the sand and the hard clay. Thus, the amount of compaction of soft clay is greater than that of sand or hard clay with the same thickness. On the other hand, the compressibility of strata is also a function of the stress conditions it has experienced. A given stratum, e.g., sand, behaves elastically when it is subjected to a cyclic loading-unloading-reloading stress pattern but, when it is subjected to a monotonic loading stress, it shows remarkable plasticity and even some creep. The variation in groundwater levels result in changes in effective stress. Therefore strata exhibit different kinds of deformation in different periods of groundwater level changes, and the deformation becomes more complicated. We again take extensometer group four as an example to discuss the features of individual strata compaction in Shanghai. The three soft clay layers and sand layer five contributed more than 10 percent of the total subsidence in the period 1981–1990 and/or in the period 1991–2000. The deformation features of these strata are discussed below.
Soft clay layers one and two are composed of mucky clay, muck, and silty clay. They have very high compressibility. The compaction of the soft clay layers is related to the adjacent confined aquifer one. Figure 8a shows the variation of the compaction of these two soft clay layers and the groundwater level change in confined aquifer one during 1981–2003. It is noted that the strata compacted continuously, even when the groundwater level increased in yearly cycles. Little recovery was observed. Figure 8b plots the compaction strain of these two strata versus the effective stress change (transferred from the change of groundwater level in confined aquifer one in which there was no loop in the yearly stress cycles). One reason for this phenomenon is that the soft clays have low permeability. Dissipation of pore water pressure within the strata is slow and lags behind the groundwater level change of confined aquifer one. When the groundwater level rose and pore water pressure increased in the adjacent confined aquifer one, the clay continued to compact because of previous groundwater level and pore water pressure decreased. When the groundwater level decreased again, the compaction rate of soft clays accelerated again. In other words, a short-term minor increase of groundwater level in confined aquifer one reduced the compaction rate of soft clay layers one and two only slightly and temporarily. Therefore, it showed continuous compaction when the groundwater level rose yearly. The other reason is that the deformation of soft clays is primarily plastic and creep. Gu and others (2000) and Men (1999a, 1999b) studied the creep properties of soft clay layers one and two in Shanghai and documented the remarkable creep of those two clay layers. The creep deformation of soft clay layers may counteract some rebound when the groundwater level increases yearly. Thus, the strata presented obvious compaction and almost no rebound even though the groundwater level fluctuated with an average of approximately –1 m from 1981 to 1990 after rising from the previous lowest value. After 1990, the deformation of the strata accelerated and presented more plasticity and creep because the average groundwater level declined slowly (although it was much higher than its previous lowest value). Soft clay layers one and two are therefore labeled visco-elasto-plastic in the empirical description presented here.
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Figure 8. (a) Compaction variation of soft clay layers one and two and groundwater level change in confined aquifer one. (b) The relation of compaction strain and effective stress change from January 1981 to December 2002
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Soft clay layer three consists of clay, silt, and silty sand. It has lower compressibility than soft clay layers one and two, and is subjected to the influence of groundwater levels in confined aquifer two. Figure 9a shows that its deformation characteristics are different from soft clay layers one and two. When the groundwater level rose yearly, the stratum apparently rebounded. When the groundwater level fell, the stratum compacted. Furthermore, the variation of the deformation almost synchronized with groundwater level changes. There was irrecoverable deformation, however, in each yearly stress cycle, and there were loops in the stress-strain plot (Figure 9b). This indicates that the stratum is plastic but undergoes little creep when the groundwater level fluctuated during 1981–1987 or decreased slowly but was higher than the lowest value during 1988–2002. In other words, the compaction can be considered primarily elasto-plastic in its in situ empirical description. The coefficient of volume compressibility computed by the graphic method is approximately 9.09 x 10–3 MPa–1 for loading and 1.63 x 10–3 MPa–1 for reloading.
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Figure 9. (a) Compaction variation of soft clay three and groundwater level change in confined aquifer two. (b) Effective stress change versus strain of soft clay three from January 1981 to December 2002
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At the site of extensometer group four, sand layer three and sand layer four are connected and contribute only slightly to the total land subsidence. We discuss them here to understand comprehensively the characteristics of sand deformation as a consequence of groundwater pumping. Figure 10 shows that these two strata compacted and rebounded almost in synchronization with the falling and rising groundwater levels in each yearly stress cycle from 1981 to 2002. In these two strata, the groundwater levels decreased to the lowest values at the end of the 1950s, then increased to high values and oscillated at those levels during 1970–1986. In the period 1981–1986, therefore, sand layers three and four exhibited primarily elastic behavior, and the coefficient of volume compressibility is approximately 9.03 x 10–4 MPa–1. After that, the average groundwater levels fell slowly again. Although they were much higher than the previous lowest values, the strata exhibited elasto-plastic behavior. The coefficient of volume compressibility is around 1.94 x 10–3 MPa–1. In each yearly cycle, the coefficient of volume compressibility when reloading is approximately 9.03 x 10–4 MPa–1.
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Figure 10. (a) Compaction variation of sand layer three and four and groundwater level change in confined aquifer three. (b) Effective stress change versus strain of sand layers three and four from January 1981 to December 2002
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Sand layer five consists primarily of fine sand and medium to coarse sand. Figure 11a shows the history of groundwater level and compaction from 1981 to 2002. Before 1986, the groundwater level fluctuated within a certain range after increasing from a lower level, and the sand layer presented apparent compaction and rebound in synchronization with the falling and rising groundwater levels. In each cycle of groundwater level, there was little irrecoverable deformation. The sand layer exhibited elastic behavior. During the period 1986–1990, the average groundwater level fell but was higher than its previous lowest value. Although the sand stratum still compacted and rebounded in synchronization with the falling and rising groundwater level, there was obvious irrecoverable deformation for each yearly cycle. It exhibited elasto-plastic behavior. The coefficient of volume compressibility is 6.22 x 10–4 MPa–1 for elastic deformation and 2.17 x 10–3 MPa–1 for elasto-plastic deformation. After 1990, the average groundwater level fell below the previous lowest value, and the sand layer presented mainly plastic deformation and creep. When the groundwater level increased, the sand layer had little or no rebound. Its compaction continued. There are two reasons for this. One is that there are some clay interlayers or lens structures in sand layer five. For example, there is an interlayer of silty clay that is 5.3-m thick in sand layer five at the site of extensometer group four. The pore water pressure in the clay interlayer dissipates more slowly than the change of the groundwater level in the sand. Furthermore, the deformation of clay occurs by plasticity and creep. The second reason is that the sand creeps. The creep of clay is well known, but sand may also creep. Several researchers (Prisco, 2000; Lade, 1998) have studied and described the creep of sands. Figure 12 shows the creep curves of the clay sample from soft clay layer two and the sand sample from sand layer five of Shanghai under the pressure 400 kPa in oedometer tests. The creep of sand is much less than that of clay, but when the elapsed time is long enough and the thickness of the sand layer is great, the sand layer will present noticeable creep deformation. In yearly cycling of the groundwater level, the rebound from the short-term minor increasing of groundwater level may be offset by the creep compaction. Thus, there was no rebound observed when the groundwater level increased, but the compaction rate of the sand layer decreased temporarily. This is not consistent with the previous conclusions that the deformation of the sand layer is elastic or elasto-plastic (Su, 1979; Wei, 2002).
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Figure 11. (a) Compaction variation of sand layer five and groundwater level change in confined aquifer four. (b) Effective stress change versus strain of sand layer five from January 1981 to December 2002
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From the preceding discussion, it appears that the aquifer system deformation in Shanghai is strongly related to stratigraphy and groundwater level changes. If the average groundwater level rises from a low to a high value and fluctuates within a certain range, the deformation of sand is primarily elastic. If the average groundwater level declines slowly but is higher than the previous lowest value, the deformation of sand is elasto-plastic. Soft clay layers, however, especially soft clay layers one and two, are characterized by plastic and creep deformation. The softer a stratum is, the more significant the plastic and creep deformation becomes. When the average groundwater level falls rapidly and is lower than its previous lowest value, both clay and sand are characterized by plastic and creep deformation. Although sand has less plastic and creep deformation than clay, its compaction is not negligible, especially when the sand is thick.
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Conclusions
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Land subsidence induced by excessive groundwater withdrawal has resulted in environmental hazards and economic losses in Shanghai. Based on the analysis of extensometer groups and observation wells data in Shanghai, the following conclusions can be drawn: (1) There is a strong relationship between the net groundwater pumped and land subsidence. This indicates that groundwater pumping is the main reason for land subsidence in Shanghai. (2) The main compaction layer is correlated with the compressibility of strata and the main pumped aquifers. In some periods (for example, from 1991 to 2000 in Shanghai), sandy strata can become the primary compaction layers. (3) It is usually thought that the deformation of clay strata is elasto-plastic and the deformation of sand strata is elastic when the average groundwater level falls and is lower than the previous lowest value and that the deformation of both clay strata and sand strata is elastic when the average groundwater level rises or declines while being higher than the previous lowest value. However, monitoring data from Shanghai reveal that the deformation is much more complicated than previously realized. In summary, soft clay layers one and two exhibit visco-elasto-plastic behavior even when the groundwater level fluctuates after an increase of the average level from a lower value or the average level falls slowly but is higher than the previous lowest value. Soft clay layer three, in contrast, primarily exhibits elasto-plastic behavior in response to these patterns of groundwater level change. Sand exhibits elastic behavior when the groundwater level fluctuates after rising above the average level from a lower value, and exhibits elasto-plastic deformation when the average groundwater level falls, but is still higher than the previous lowest value. In addition, sand deforms plastically and by creep when the average groundwater level in it is lower than the previous lowest value. (4) The radical way to control land subsidence is to limit the groundwater level changes. This means to limit the groundwater pumping and to keep groundwater levels from descending, especially from falling below the previous lowest groundwater levels.
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ACKNOWLEDGMENTS
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This paper is financially supported by the National Nature Science Foundation of China grant 40335045.
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ABSTRACT
Introduction
