ANALYSIS OF THE CAUSE OF LAND SUBSIDENCE IN , Zhang Qingzhi and Miu Xioujun Tianjin Municipal Bureau of Geology and Mineral Resources, Tianjin, China Aj3s_t£ac_t Based on the general geology of Tianjin area, the paper deals with land subsidence in the area, which may be attri­ buted in the main to the overextraction of groundwater. However, strong earthquake and compaction of less consoli­ dated are also considered non-negligible factors.

Tianjin, which is one of the important industrial cities of China, is located along the coast of the Bohai Gulf west of the Pacific Ocean. It is about 120km northwest of . The flat relief of the Tianjin area where many depressions and lakes can be found has an elevation of only 3-5 meters, and is drained by the Haihe River pouring into the . The loose sediments overlying the considerably frac­ tured bedrock have a great thickness and quite complex hydrogeological conditions. Since the year of 1959 when people became first aware of land subsidence, the maximum accumulative settlement of the ground surface reached 2.15m till 1982, the maximum subsiding rate was 261mm per year, and the subsided area was expanded to over 7300km2. There exist three settling centres in the urban area, with an accumulative amount of subsidence more than 2m. (Fig. 1) The effect of land subsidence imposed on the industry and agriculture and on people's life is an urgent problem to be solved.

G_e_o.i.o_2)! I. The city of Tianjin is situated in the settling zone of North China Plain, the 2nd subsiding zone of the Neo- Cathaysian tectonic system. The urban area spans -three structures of the system, i.e., the Jizhong depression, the Huanghua depression and the Cangxian uplift inbetween. Along the contact between the N-S uplift and the Jizhong depression on the western side-and the Huanghua depression on the eastern side are the respective Hangou fracture and Cangdong fracture. Within the Cangxian uplift zone three upwarps surrounding one downwarp can be observed, namely the Shuangyao, Xiaohanzhuang and Dadongzhuang upwarps, and the Baitangkou downwarp. All these structures consti­ tute the basic feature of the basement of Tianjin area. There exist two sets of fractures, one being compressional —compressional torsional fractures parallel to the NNE uplifts and depressions, and the other tensional—tensional torsional fractures in NNW direction. The former set in-

435 Fig. 1. Contour map of land subsidence in the urban area of Tianjin (1959-1982) 1- subsidence value (mm)/Bench mark eludes those in Hangu, Dacheng, northe Tianjin, west and east Baitangkou, madong, and east Cangxian; the latter in­ cludes the Haihe River fractures. (Fig. 2) The IMeoCathaysian tectonic system was basically formed since the middle-late Jurassic till the late Tertiary, being still active up to the present in local places. The North China Plain began to subside to a great extent in the Cenozoic, receiving loose sediments to a great thick­ ness. II. Geology and hydrogeology of the Quaternary system In the studied area, the loose cover in mainly composed of the Quaternary and upper Tertiary sediments, whereas the basement is formed by the Sinian, Cambrian, Ordovician and Carboniferous-Permian rock formations. The rocks at the axial part of the Cangxian uplift are relatively old, while those on either flanks are young. The sharp settlement occurring in the Yanshanian movement has provided an envi­ ronment for the Cenozoic sedimentation of great thickness, which varies in different structural positions, for in­ stance the axial part of the Cangxian uplift has a thick­ ness ranging from 800 to 1100 metres, whereas the depres- sional part up to several thousand metres. The Quaternary sediments, composed of clayey soil and sands, are about 600m thick. The top 50 metres are alternating marine and continental sediments of varying lithologie characters and poor engineering geological properties, mostly highly com­ pressions!. At the section of 50-200m are medium-low com-

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Fig. 2. Basement st ructure of Tianjin area I-Cangxian uplift; II-Jizho ng depression; III- Huanghua depression ; 1,2-fault ; 3-boundary between uplift and depression; 4-up warp; 5-downwarp; 6- Bouguer anomaly isa line; 7-boundary of urban area; A-Shuangyao upwarp; B-Dadon gzhuang upwarp; C-Xiao- hanzhuang upwarp; D -Baitank ou downuiarp; (1)-Hangou fracture; (2)-Dacheng fract ure; (3)-Morth lianjin fracture; (4)-West Baitangk ou fracture; (5)-East Baitangkou fracture ; (6)-|viafan g fracture; (7)- Cangdong fracture; (8)-Haih e Ri\/er fracture, pressional sediments mainly of fluvial facies. Below 200m, the sediments are of lacustrine facies, which change sligh- ly in lithologie character and are low compressional. Within the Quaternary system, one watertable aquifer and five confined aquifers can be identified. The depth of the bottoms of these aquifers are as follows: Watertable aquifer 5 - Bm Confined aquifer I 50 - 60m Confined aquifer II 160 - 170m Confined aquifer III 250 - 300m Confined aquifer IV 380 - 410m Confined aquifer V 500 - 600m

Historical background and present situation I. Groundwater development The city of Tianjin saw the development of shallow ground­ water early in the 19th century, and that of deep ground­ water in 1923. Before 1949 the .year of the founding of Mew China, there were only 51 deep wells,yielding 40,000m3/day

437 for water supply. After the Liberation of China, particu­ larly after 1958,the extraction of groundwater has been increasing with the.ever oeveloping industry and agricul­ ture and the improving living standard of the people in Tianjin. Up to the present, the number of deep wells counts more than a thousand. The confined aquifer II, being the main source of cooling water for-industry in the urban area of Tianjin, provides 50% of the annual pumpage. The exploitation of groundwater exhibits three pecu­ liarities: 1. Being concentrated in place. Most of the production water wells are concentrated in the industrial areas along both banks of the Hiahe River. Within an area of less than one kilometre there are 23 production wells, the intensity of the exploitation being as high as 3B9m3/hour.km2 in the peak season. 2. Being concentrated in specific layers. As Aquifer II has a shallow depth, great thickness and wide distribution, as well as fine water quality and lower temperature, it has been the main source that the pumping is concentrated upon for water supply. For instance in 1970, the production water wells reached a number of 263, making 52% of the total number of wells in Tianjin. The yielding of these wells occupied 59% of the total extraction. 3. Being concentrated in time. Take the Dazhigu industrial area as an example, the pumpage in the peak periods is five times that in the dull seasons. Because of the irrational development of ground­ water, the water level goes down year after year. The water level in the wells tapping the Aquifer II has gen­ erally drawn down 50-60m, occasionally even more, frdm its original height of plus 1.5m when the wells were completed in 1923. (Fig. 3) II. Land subsidence According to the history of groundwater development, the records of water table fluctuation and the rate of settle­ ment, land subsidence in the urban area during the several decades can be divided into four stages as follows: 1. 1923-1957, the starting period of groundwater deve­ lopment and land subsidence. The information obtained from the past 30 bench marks has shown that the surface subsided to a greater extent round a production well than remote from it. In the former case, the annual cumulative amount was about 7.1-12.3mm, whereas in the latter case, only a few milli­ metres. Generally speaking, the land subsidence has not brought about any obvious harms. 2. 1958-1966, the intensifying period of groundwater development and the preliminary formation of the settle­ ment center. The exploitation of groundwater was increasing dur­ ing this period, forming a cone of depression in Aquifer system II. Simultaneously seven centers of land subsid­ ence with an annually accumulative amount of 30-46mm appeared at Baimiao, Beizhan and several other places,

H38 -20 \ s ^-30- ,10^ ^?"~J 5 Fig. 3. Cone of depression in Aquifer system II 1-isoline of piezometric level of groundwater (m) coincidi ng r oughly with the cone of depression. The area of settl emen t expanded year after year, resulting in the phynomen a of cracking in the buildings, breaking of drain­ ing pipe lin es, and e,levating of well tubes. 3. 19 67-1 975, the period of sharp development of land subsiden ce. The set tling rate of the ground was accelerated. For exam pie, bench mark No. 195 gave an average annual settling amo unt of 54mm before 1971, but 163mm during the period f rom 1971 to 1975. The subsiding area enlarged greatly, hav ing an accumulative settling amount over 600mm an d ex panding from 1.15km2 in 1971 to 53km2 in 1974. The harm ful effect was considerably serious. Rain water was coll ecte d in the center of the settlement up to one metre de ep, and inverse flow occurred in the drains. 4. 19 76-1 980, the slowing-down period of land subsi- dence. The alleviation of land subsidence during this period was due to on the one hand the prohibition of drilling ne ID water wells issued by the municipal government and on the othe r hand the readjustment of the stress of the earth's crust.

Analysis of the cause of land subsidence I. Over with draw al of groundwater, particularly from the confined aqu ifer system II, being the main cause of land n Ti subsiden ce i anjin. This be proved by the occurrence, development and dist can tion of land subsidence in the urban area. 1. Th ribu nd s ubsidence versus time is closely related to the p e la d of heavy exploitation of the confined aquifer erio em I I„ For example, as indicated by the bench mark No. , the settling amounted to 106mm in a year's time fro syst vemb 284 er of 1977 to the same month of 1978. Of m No 439 C w

p. °1 10- Pumpage of aquifer system II 0 .(IS- 211- 0 .OH- so- 0..oa - 4(1- 0 iv- 50- 0° .IbJ No.296 ground water level (m) 50- 100 150 No. 284 subsidence 200 250 month 12 2 4 6 8 10 12 2 4 6 8 10 12 2 4 6 year 1977 1978 1979 Fig. 4. Correlation of water table of Aquifer System II with pumpage and land subsidence the total accumulative amount of 106n 97mm of settling occurred in the heavy pumping season from flay to Septem­ ber, making 93% of the total annual amount of subsidence. The land subsidence versus time forms a curve with regular depressions and recoveries in a duration of sev­ eral years. (Fig. 4) 2. Aquifer system II and its influenced zones con­ stitute the main compressional layer causing the land subsidence. In recent years the information obtained from the bench marks has proved that the deformation amount of the strata within the range of aquifer system II and its in­ fluenced zones occupies 50/S of the total land subsidence. 3. The center of land subsidence is related to the dis­ tribution of aquifer system II. The confined aquifer system II in the alluvium passes through the urban area in a NW-SE direction. The old chan­ nel is composed of alternating sand and clayey soil of similar thickness. Under pumping, the clayey soil layer drains toward the upper and lower sand layers, the pore- water pressure d sperses rapidly, resulting in consider­ able compaction of the clayey soil layer. Five settling centers of the urban area of Tianjin are all located in such zones. In contrast, the sand layers are relatively thin at the corresponding depths in the interfluvial land mass, which constrains the pumping of groundwater, such as in the SW and NE parts of the urban area where the least land subsidence occurred. II.The effect of strong earthquake on land subsidence In 1976, an earthquake registered 7.8 on the Richter scale took place in , Province. The epi­ center was pinpointed about 100km away from the studied

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.2 200 a 'a- îoo À LJJ h,.. iii..i .1 JJ k llh.ililllllml 69 70 71 72 73 74 75 76 77 78 79 (year) Fig. 5. monthly average water level in wells tapping saline aquifers in Tianjin area at the composite position of the NNE NeoCathaysian tectonic system and the W-E structure system. The authors consider that owing to the abnormal ;hanges of the stress of the regional structures, the

1. From the observation on the non-exploited saline aquifers by the Tianjin Municipal Bureau of Seismology, one can see the consequent water level variation of the confined aquifer due to the stress anomalies of the earth's crust before and after the strong quake. The ob­ servation borehole 64m deep is sited in the Hangu area, Tianjin, 55k m distant from the epicenter. The water level in the hole began to go down in January 1972, 5.44m per year till 1976 when reached its lowest level right before the shock. And after the shock the water level recovered immediately, even gushed out. (Fig. 5) Though the precipitation of 1975 in Tianjin in­ creased 141.1mm than that of 1974, the water level inthe observation well still declined somehow. The fluctuation of water level was not affected by the extraction of groundwater, nor was it controlled by rainfalls. This could only be explained by the abnormal changes of stress before and after the earthquake. 2. Similar to the fact that the lowering down of the water level in a confined aquifer which destroys the stress balance of the soil mass will lead to land subsi­ dence, the variation of water table during the pre- and post-quake periods will bring about the same effect. Land subsidence can not be explained only bythe intensification of groundwater extraction, according to laroe amounts of data obtained by accurate levelling measurements for several years before and after the earth­ quake in Tangshan. It is thus considered that:

441 2.1. During the pregnant period of the strong quake, the stress of the earth became concentrated round the epi­ center, where the .groundwater was forced to rise up and the land surface was uplifted slightly, as shown in Fig. 6, Remote from the epicenter area, reversal of stress oc­ curred in the earth's crust, which was under tension con­ •On) dition, the intrastratal pres­ 1.950 sure reduced, leading to a pressure drop of the pore wat­ er. We should say that the fluctuation of the confined ! aquifers in the area is the result of both groundwater 1969 70 71 72 75 76(year) extraction.and the stress of the tectonic structures. In Fig< 6. Elevation Change fact,the year 1975 before the at Nanbao strong earthquake saw the highest rate of annual subsidence of the ground surface in the urban area of Tianjin. (Fig. 7)

No.14 | jp io

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Fig. 7. Correlation between pumpage and land subsidence

in Tangshan, the earth's stress r in the vicinity of the epicente the maximum uplifting of 1.3m, wh from the epicenter the déformât with the attenuation of the stre the epicenter. For in- stance, own 1.3m before and af- ter the nt No.352, Shanjinj 0.93m at point 0.3m at the trigonomë- trie lev In contr res of subsidence, which was with error, was observed at the Zhan the epicenter. £. o *J e stress took place afte phenomena of elastic rebound ing obviously to the re­ ducing r quite in contradiction with the tion of groundwater was the only The pumpage of groundwater in 1980 showed that it surpasse d considerably that in 1975 , but the settlement shown by the levelling point did n ot increase with the in- creasing pumpage. On the contrary the land subsidence

442 slowed down under this circumstance as shown by the mea­ surements in the following table: Levelling point Settling amount (mm) No. 1975 1980 284 180 82 195 203 103 314 201 119 In Tanggu a rea, there was a noticeab le change i n land subside nee in 19 77 af ter th e Tang shan qua ke: the dat a from thirty levelling poin ts wer e then studied it was fo und that th e data fr om th e se ven poin ts among the thirty showed no subs idence at all, but u plift instead. The remain ing twenty-three poi nts s howed though slight subsidence appro- ximatel y 150mm 1es s t han th e annu al subsi dence befor e the quake, The avera ge va lue of subsi dence gi ven by the measure- merits a t the thi rty p oints was on ly 9.6mm From the curve of subs idence ve rsus time f or the levelli ng points a t San- baidun, Xinhe Bo at PI ant an d thre e other places, a s tair- case-li ke curve can b e seen (Fig . 8). Tihs phenomen on was the res ult of re adjus tment of the earth's stress aft er the Tangsha n earthqu ake. Th e effect of th e Tangshan earthquake on the studied area is to a cer tain extent related to the different structu- the , ral position s of area. In the duration of several years and after the shock, the land subsidence induced before ut from s orne t ens to one hundred millimetres. was abo is thus consi dered that a strong earthquake in its It period is a no nnegligible factor for land subsi- active ithin the rang e of its influence. dence w

1959 61 63 65 67 69 -71 73 75 77 79 (year)

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Fig. 8. Curve showing the subsidence at some of the bench marks in Tanggu

443 III. The influence of the compaction of less consolidated soil beds on land subsidence Within the depth ranging from 5 to 15m in the studied area are widely distributed grey—dark grey clayey soil and muddy clayey soil, which is the first layer of marine facies. Its water content is about 30-42%, porosity >1.0, liquidi­ ty index mostly >1.0, and compressibility coefficient appro­ ximately 0.05cm2/kg. It is a kind of moderately compacted soil in a state of soft and plastic flow, being medium- high compressional. The consolidation pressure test on the soil for the previous stage indicates that the soil bed is less consolidated, in which still exists residual por- water pressure. Under its selfweight, the soil bed trends to become normally consolidated^ causing land subsidence.

Conclusion Land subsidence of Tianjin can be attributed to two factors; man's activity such as overextraction of groundwater and crustal movement such as earthquake. In Tianjin, the former plays the main role and the latter appears to be a nonnegli- gible factor, since strong earthquakes display a specific form of crustal movement, resulting in apparent settlement in different stages. Therefore , in working out the control measures , identifying the inducing factors of land subsi­ dence is of practical importance.

444