Land Subsidence Problems in Taipei Basin - IAHS by wuyunyi

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									                                                   Land subsidence problematic in Taipei Basin


   taciones:Universidad Nacional Autonoma de Mexico,Contribucion del institutode Ingenieria
    a Primer Congreso Panamericano de Mecanica de Suelos y Cimentaciones,614 p.
     l
MEADE, R.H. (1967): Petrology of sediments underlying areas of land subsidence in central
                  .
   California,U S. Geol. Suruey Prof. Paper 497-C,83 p.
MILLER,R.E. (1961): Compaction of an aquifer system computed from consolidation tests and
   decline in artesian head, in Geological Survey Research 1961, U.S.Geol. Suruey Prof. Paper
   424-B,pp. B54-B58.
MIYABE, (1962): Studies in the ground sinking in Tokyo, Tokyo Inst. C v l Eng., Rept.,
         Naomi                                                                 ii
   no. 40, 38 p.
_____-          (1967): Study of partial compaction of si layer -In reference t the land subsi-
                                                        ol                     o
   dence in Tokyo, Tokyo Inst. C v l Eng., Rept., no. 44,7 p.
                                   ii.
NEUMAN, S.P. and WITHERSPOON, Theory of f o in aquicludes adjacent t slightly
                                    P.A.  (1968):          lw                         o
   leaky aquifers, Am. Geophys. Union, Water Resources Research, vol. 4,no. 1, pp. 103-112.
OLSEN,H. . (1966):Darcy’s law i saturated kaolinite, Am. Geophys. Union, Water Resources
          W                          n
   Research, vol. 2,no. 2,pp. 287-295.
POLAND, J. F. (1968): Compressibility and clay minerals of sediments i subsiding ground-water
                                                                      n
   basins,southwestern United States, Geol. Soc. America Ann. Mtg., 8Ist, Mexico City, 1968,
   Program,p. 241.
                           G .H.
POLAND, J.F. and DAVIS, (1969):Land subsidence due t withdrawal of fluids, Geol. Soc.
                                                              o
   America, Rev. Eng. Geol., vol. 2.
RILEY,S.(1970):Analysis ofboreholeextensometerdata from central California,Internat Sym-
       F.
   posium on Land Subsidence, Tokyo, 1969,Proc.pp.
SANO,S.-I. and KANAYA, H. ( 9 6 : Observation of partial shrinkage of strata,in Radioisotope
                               16)
   instruments i industry and geophysics, Internat. Atomic Energy Agency, Vienna, vol. 2,
                  n
   pp. 279-291.
TANG,  MIN    (1967): Groundwater decline and land subsidence in the Taipei Basin, Taiwan,
   Republic of China,Internat.Assoc. Scientific Hydrology Cong.,Istanbul 1967,Mém., pp. 376-
   382.
TERZAGHI, (1925): Principles of soil mechanics: IV,Settlement and consolidation of clay,
            Karl
   Eng. News-Ree.,November 26,pp. 874-878.
                             R.
TERZAGHI, and PECK, B.(1948): Soil mechanics in engineering practice, N e w York, John
            Karl
   Wiley and Sons, 566 p.
                   W.                     A.                                     n
VAN DER KNAAP,and VAN DER VLIS, C.(1967): O n the cause of subsidence i oil-producing
   areas, i Drilling and Production: World Petroleum Cong., 7th, Mexico, 1967, Proc. ,vol.3,
           n
   pp. 85-95.
WITHERSPOON,    P.A.and NEUMAN, S.P. (1967): Evaluating a slightlypermeablecaprock in aquifer
   gas storage,I. Caprock of infinite thickness,Jour. Petroleum Technology, p. 949.
ZEEVAERT,Leonardo (1957): Consolidation of Mexico City volcanic clay, i Conf. on s i s for
                                                                           n            ol
   engineering purposes,Am. Soc. Testing Materials Spec. Tech. Pub. 232,pp. 18-32.




            LANIDSUBSIDENCE PROBLEMS IN TAIPEI BASIN
                         hi-Ming HWANGI and Chian-Min WU2


ABSTRACT
                               n
   Releveling o bench marks i 1968 and 1969 indicates that subsidence o the land
               f                                                          f
surfacr i the Taipei Basin has now exceeded 1.3 meters. In the sharp subsidence area,
         n
subsidence which was as much as 0.6meter in 1966 now has about doubled.The maximum

         r
1. Vice D .Maintenance Div., Public Works Bureau, Taipei Municipal Government,Rep. of
   China.
2 Chief,Hyd-Lab.Water Resources Planning Commission,M O E A . ,Republic of China.
 .


                                                                                           21
Jui-Ming Hwang and Chian-Min Wu


                     n
rate of subsidence i recent years has been about 25 cm a year. Plots of subsidence
against decline in artesian pressure suggest that pressure decline is a major cause of
the subsidence. Consolidation data are used to verify the relationship between the
pressure drop and the subsidence. However, other causes such as compaction o soil,f
tectonic adjustment and loading at land surface m a y have contributed to the subsidence.

RESUME
   Le renivellement de repères en 1968 et 1969 indique que l’affaissement de la surface
du sol dans le Bassin de Taipei dépasse maintenant 1,30m. Dans la zone des forts affais-
sements, la descente du sol qui était de 0,6 m.en 1966, a maintenant sensiblement doublé.
Le taux maximum de l’affaissement dans les dernières années a été d’environ 25 cm
                                                                      a
par an. D e s diagrammes de l’affaissement en fonction du déclin de l pression artésienne
suggèrent que la diminution de pression est une cause majeure de l’affaissement. Der
données sur la consolidation sont utilisées pour vérifier la relation entre la chute de
pression et l’affaissement. Cependant d’autres causes c o m m e la compaction du sol, des
ajustements tectoniques et la surcharge de la surface du sol peuvent avoir contribué à la
production des affaissements.


INTRODUCTION
For many years substantial subsidence of the land surface has been occurring at several
places in Taipei Basin, figure 1. However, l t l attention was received until 1964 when
                                              ite
the large scale groundwater [developmentresulted in rapid decline of ground water level.
                                    f
    Information on the magnitude o the subsidencei available chiefly through first-order
                                                     s
leveling and releveling of the Provincial Water Conservancy Bureau, with supplementary
leveling available from several other agencies. The subsidence area is about 125 square
kilometers,nearly half of the total basin area. Subsidence was as much as 30 centimeters
in 1961 and is n o w tripled. The maximum rate of subsidence is recent years has been
about 25 centimeters per year.
    This paper presents information on the status of subsidence in the Taipei Basin,
describes what is being done at the present time to determine the causes and to predict
the extent,magnitude, and rate of future subsidence,and suggests additional work needed
as a basis for making pians to alleviate or minimize the various problems caused by land
subsidence.


GENERAL FEATURES
The Taipei Basin is located in the northern corner of Taiwan, Republic of China, bordered
by Hseuling on Tahan Creek, Hsintien on Hsintien Creek and Sungshan on Keelung River,
having an area of 243 square kilometers and with a general elevation of less than 9 meters
above sea level (fig. 1. This basin is roughly triangular in shape, defined by the surface
                       )
          t                                                                t
geology a the foothill line of the surrounding hills and in the southwest,a the line formed
by the uplifted older sediments.
            iy
    Taipei Ct is located a the eastern edge of the basin. With a population of 1,200,000
                           t
(1969), most of the basin area is extensively developed. Tamshui River drains an area of
2,726 square kilometers along a length of 195 kilometers and is the main stream system
in the basin.
    Wells have been used for domestic water supply in Taipei Basin since about 1895. In
a field examination in 1894-1896,W. . Bardon and others found that the area of flowing
                                       K
                                                            l                    t
artesian wells extended south from Chinmei to almost a l of the basin area. I was also
known that an extensive area of free water underlaid the whole basin. However, as a result
of the accelerated urbanization of the Basin, most of the modern ground water develop-
ment has occurred since 1957, with pumpage tripling from 1957 to 1964. In 1964 it was
about 329.1 x 106cubic meters.


22
                                                    Land subsidence problematic in Taipei Basin


                                                                                    l
    The strata exposed and the deposits containing fresh water in this area are a l of
unconsolidated Quaternary sediments, extending to depths from less than 1 meter to a     t
least 240 meters below the land surface.They can be divided into three main formations:
(I) an upper unit of Recent alluvium and soil, chiefly consisting of sand and gravei, that
extends from the land surface to depths of about 3 meters; disseminated peat lentils are



                         \ I   (P      BM.T 1
                                         O 8
                                                                             KEY MAP




FIGURE Location m a p and land subsidence in the Taipei City, 1963-1967(linesof equalsubsidence
     i.
in meter)


                                                                 2
found in its lower part in the southeastern part of the basin, ( ) a middle unit, named
Sungshan formation, composed mainly of gray m u d and sandy m u d with intercalated
sand and gravel, commonly 40 to 60 meters thick, and (3) a lower unit, named Linkou
formation, composed essentially of gravei and subordinately of sand and clay, with or
without impervious reddish lateritic cover, 90 meters to 130 meters thick. The Linkou
formation unconformably overlies the Tertiary formations. With the exception of the
western part of the basin, this formation often pinches out toward the margin forming the
intake area of the groundwater basin.
   A body of semiconfined to free water occupies most of the middle unit (Sungshan
                                            0
formation); the water table is 1 meter to 1 meters below the land surface. The ground-
water in the lower unit (Linkou formation), the lower water-bearing zone, is confined in


                                                                                           23
Jui-Ming Hwang and Chian-Min Wu
              W
              (3
24
                                                 Land subsidence problematic in Taipei Basin


most of the area by the Sungshan formation and the impervious reddish lateritic cover.
Most of the pumping draft,probably 99 percent o more, is from the lower unit. The
                                                      r
general position o the middle and lower water-bearingzones and the impervious lateritic
                  f
clay that separates them,as encountered along line A-A'f g i, are shown in figure 2.
                                                            (i. )
   The water table in the middle water bearing zone is controlled by the fluctuation of
the stream flow,but has not changed much from its i i i l position. O n the contrary,the
                                                       nta
heavy pumping draft has decreased the pressure head in the lower water-bearingzone as
much as 32 meters i the past 1 1 years. The decline has ranged from 25 meters near
                     n
Shungshan on the northeast to roughly 32 meters near Taipei City and 5 meters on the
south. Since 1964 the average rate of yearly decline has been 1 to 2 meters i the northern
                                                                            n
part and 2 t 3 meters i the southern part of the area. With this increasing rate,it is
             o           n
               n
expected that i 1972 the piezometric head would probably drop below the top of the
pressure aquifer.

SUBSIDENCE
Subsidence of the land surface in the Taipei City area was noted as early as 1961, when
releveling o the first order bench marks established in 1950 indicated changes in altitude
              f
of several centimeters t 3 decimeters i some areas. Subsequentlevelings in 1963, 1966,
                          o               n
1967, and 1969 indicate the maximum subsidence along the line has been 1.35 meters
since 1950 a bench mark 9536,figure 1, indicatingan averagerate of about 7.4 centimeter
              t
per year.
    Levelings by the Provincial Water Conservancy Bureau (PWCB)in 1963, 1966, and
1967 have supplied the data for the map showing subsidence from 1963 t 1967 ( i . 1.
                                                                            o       fg )
Approximately 125 sq. km is enclosed within the subsidence line. The sharp subsidence
area, in which the decline exceeded 40 centimeters during 4-yearperiod of 1963 t 1967
                                                                                    o
 fg
( i . i), is located in the central part of the Taipei City where sharp decline in artesian
head also is noted ( i . 3.
                      fg )
                f
    Profiles o land subsidence and decline in artesian head of the lower water-bearing
zone along the first-orderlevelingline offigure 1, presented on figures4and 5,respectively,
show the relationship of subsidence t decline in head in the years 1961 t 1969.Although
                                       o                                 o
the ratio of subsidence to decline change along the profile, the profiles along this line
indicate a rude correlation.
    Figure 5 shows changes in altitude a bench marks 9536 and 9537 and change in pres-
                                          t
sure head in well 2770/349NW 1, as w l as the maximum and minimum pressure heads
                                         el
 n
i nearby wells. Both the hydrograph and the maximum and minimum pressure heads
show seasonal fluctuations and the long-termwater-leveltrend since 1957 in typical wells
tapping the lower confined aquifers. They show a continuing decline from 1957 to date,
evidently a result of overdraft on local supplies o groundwater.
                                                    f
                            n
    The plot of change i altitude of bench marks 9536 and 9537 ( i . 5 was included
                                                                       fg )
merely t show conditions in the area of greatest 18-yearsubsidence,but leveling to other
          o
bench marks in the vicinity suggests that the rate has been increasing in recent years,
especially in the south-easternpart o the basin ( i . 1) where the development of industry
                                       f           fg
has accelerated in recent years. The ratio o subsidence t pressure decline as shown for
                                              f             o
the bench marks i figure 5 for the 5-year 1962 to 1967 was 1/27.However, the corres-
                     n
ponding ratio in the vicinity of the bench marks was 1/20t 1/30.
                                                              o

CAUSES OF SUBSIDENCE
Subsidence of the land surface has been noted in many areas and ascribed to various
causes.However,as was stated above,comparison o change in artesian head in confined
                                                   f
                                                  ar
aquifersand subsidence of the land surface shows f i correlation,suggesting that decline


                                                                                         25
Jui-Ming Hwang and Chian-Min Wu


                                             f
in artesian head has been a primary cause, i not the major cause, of subsidence in the
Taipei Basin. There m a y be other causes, however, such as tectonic adjustment,loading
 t
a the land surface,and compaction due to irrigation. N o specific measurements of subsi-
dence of this nature are yet available to assist in evaluating its relative importance. In
an effort to evaluate its importance, samples from each stratum were obtained and the
rate of consolidation was computed to verify the result.




FIGURE Piezometric head in Taipei Basin, 1968
     3.


S T R A T I G R A P H Y AND SOIL C H A R A C T E R I S T I C S

According to geohistorical study, Taipei Basin is a tectonic basin formed by the sinking
of huge crust blocks by faulting [i]. The sea water and its marine organismes transgressed
into the basin probably through the original water gap at Kuantu, resulting in two cycles
of deposition of the Sungshan formation. As the deposition of the Sungshan formation
was under a lacustrine environment characterized by shallow depth, weak current and
quite water, and took place in less than 350 years, most of the subgeology is still muddy in
nature. Thus,the stratigraphic study of the subgeology plays a great roll in the evaluation
of the potential subsidence in this area. As the depositional environment of the lower unit,
Linkou gravei, was interpreted as a transitional coastal environment, relatively consoli-


26
                                                        Land subsidence problematic in Taipei Basin


       n                                n
dated i nature and comparatively deep i stratum,detailed stratigraphicstudy was made
                                                                                el
on the Sungshan Formation.The large amount of subsurfacedata and adequate wl logs
from more than 50 drill holes serve as a basis for stratigraphicstudy and subdivision of
the Sungshan formation-which has two cycles of deposition and can be divided into s x i
main members. In ascending order, the six members and their s i characteristics are
                                                                   ol
           n
described i table 1.

                                              RM.        No.
                   29




                                    \
                                     \
                                      \
                                      \




                                          2   O     2      4KM



          .
FiGuRe 4 Profiles showing land subsidence and change in artesian head along 1st order leveling
line, Taipei Basin



PREDICTION OF LAND SUBSIDENCE
 t s el
I i wl known that lowering of the water table within or above a stratum ultimately
                                                            n
increases the intergranularpressure accompanied by strains i accordancewith the stress-
                                       n
strain relationships for the material i question. The resulting displacements produce
a settlement of the ground surface.The analytical methods for dealing with settlement due
to this pressure must be chosen i accordance with the properties of the subsoil and the
                                  n
nature of stratification.The intergranular pressure,p, i a typical section bounded by a
                                                        n
free and confined aquifer ( i . 6 )i computed as shown i table 2
                           fg , s                         n       .
    With the rate of change of piezometric head as shown on figure 4,   increases in inter-
                     n
granular pressures i each clay stratum are computed.

                                                                                                27
Jui-Ming H w a n g and Chiau-Min Wu

    Displacements due to those intergranular pressure changes are computed by using the
Terzaghi theory on the one-dimensional consolidation of clay as a first approximation.
Then, Mikasa’s modified consolidation theory was applied for further sirnuitation [ ]  2.
As the characteristics of clay strata in Taipei Basin are comparatively harder than those
of described by Mikasa, no significant difference was noted.




             --
FIGURE Change in altitude at BM9536,-37 and change in artesian head in nearby wells
      5.



   Figure 7 shows the results of settlement computed by Terzaghi theory using subsurface
log data at Ambassador Hotel, about 1 km north of BM 9536, as well as actual changes
i altitude at BM 9536. The graphs of computed subsidence is about 75 pct less. This is
 n
expected because subsidences in the cohesiveless strata are not taken into consideration.
Again, other causes such as loading at land surface, vibrations at or near land surface,
compacting due to seepage water and tectonic movement might contribute appreciably
to the overall land-subsidence. Computation results show that the second clay layer
contributes the largest part of the soil settlement, and also show that even if the drawn-
down of the confined aquifer were to cease, the land surface would continue to settle for
several years. Hence, on the basis of presently available data it is concluded that the
extensive drawdown of the confined water and the softness of the muddy strata have been
                                n
the major cause of subsidence i the Taipei Basin. Application of the laboratory physical
property and field drawdown data might be used to estimate with fair accuracy the
location, rate, and magnitude of future subsidence of the land surface.


28
Land subsidence in the Shiroishi Plain Kyushu, Japan
   e-rn      Nci
   wp'e      I--e
   N N N     N N
   2 2 2
   o e e
              2 1 +
             -e-
   ???
   N N N
             ???
             N N N
   O
   xipi 171
        io
                II
   T I T 171
   N
   O    N
        0
                O
                hl
    I I I     I I I
                                                 29
hi-Ming H w a n g and Chian-Min Wu
30
Land subsidence problematic in Taipei Basin
           4
       L
                                       31
hi-Ming H w a n g and Chian-Min Wu


TABLE Computation of intergranular pressure
    2.

                                 Total vertical                                  Porewater           Intergranular
Position                          pressure, p                                   pressure, uw          pressure, p




where :
ru) = unit weight of water, r ó
                                     Gs
                                  = -I     - Gw     W
                                          fo
G8 = specific gravity of dry particle,
Gw = specificgravity of water,
fo = 1 + e.    e = void ratio




                                                    PERMEABLE 'ZONE
                                                     ..    . .
                                                                 .
                                                                    .
                                                                                     .
                                                                                              . .
                                                                                               ..'

                                                  _ .   .   3
                                                                .       .
                                                                            .             .    .

                                               zo
FIGURE Definition sketch
      6.


                                                                YEAR
                            I
                            O


                           10


                           20


                       5   30

                           40

                      W
                      u    50
                      2
                      W
                      0
                      v,
                           60
                      m
                      3
                      u)   70


                           00


                           9c


                           1oc

FIGURE Computed subsidence, Ambassador Hotel, Taipei Basin
      I.


32
                                                Land subsidance problematic in Taipei Basin

PROBLEM CAUSED BY THE SUBSIDENCE

The subsidence of the land surface in Taipei Basin poses serious problems in connection
                   f
with maintenance o limited flood controlworks and construction of proposed additional
levee works,as well as in construction and maintenance of other engineering structures
such as pipeline,drainage,sewerage,power,and water-supplysystems,    highways,railroads
                n
buildings,and i various aspects of land development and use. Again,because the defor-
mation and failure of water-well casings and the resulting loss of wells are a common
occurrencein the Taipei Basin,continued ground water withdrawal o expanded develop-
                                                                   r
ment becomes criticai in the region.


SOLUTION AND STUDIES NOW B E I N G MADE

As an immediate action to alleviate the problems caused by land subsidenceand ground-
water over-draft,a groundwater development management Code has been adopted.
However,detailed knowledge concerning the extent,magnitude,rate,and causes of land
                      s
surface subsidence i essential t proper planning for repair,maintenance, and construc-
                                 o
tion of structures,as well as for continued ground water withdrawal and development,
and formost economicalland use. This can be done only by adequate investigationo the  f
area in which subsidence has occurred to date.
    Basic data are necessary for appraisal of a groundwater basin. One of the most d f i
                                                                                       if-
                                                    il
cult problems in basin appraisal in Taipei Basin wl be the establishment of a criteria for
safe yield.T w o particular dangers exist for most Taiwan groundwater basins: (1) intrusion
 f at
o s l water and ( ) consolidation due to depletion of hydrostatic pressures in both free
                    2
and confined aquifers. Both of these factors required a minimum f o through and
                                                                          lw
disposal of water t sea.Enough data are not availableon the magnitude o this required
                    o                                                        f
minimum flow. Studies o the mean hydraulic gradient of the ground water and the
                            f
amount of electrical energy consumed during the year lead t an estimate of safe yield.
                                                               o
Figure 8 shows the result of the safe yield study.As can be seen,after a period of signifi-
cant drawdown,the linear relation between average annual draft and average annual
change in ground water level was no longer in existence even though the moving average
method was adopted. The safe yield thus obtained is 150 x 1 ' u i0 c b c meters per year.
Likewise, data on recharge are meager but they may be estimated by the hydrologic
equation. The mean annual rainfall in this area is 2810 mm,equivalent t 7530 x lo6 m3
                                                                           o
                                                                              o
per year. Analysis of base flow data shows abundant water is available f r recharging
the ground water basin. The estimated base f o amounted t 1,410 lo6 m3per year.
                                                 lw             o       x
Though 170x lo6 m 3 is considered to be contributed to ground water, is contribution
                                                                            t
t the free aquifer and confined aquifer is not well known. Again, other times items
 o
           n                                          ifcl
included i the hydrologic equation are often more d f i u t to evaluate,even qualitatively,
          ul                                            for
than the f l hydrologic equation.Evapotranspiration, example,is not measured.The
average rate o evapotranspiration as computed by the Thornthwaiteformula is 587 mm
               f
( , 0 x lo6 m3);
 160               whereas by the Blaney-Criddlemethod it rangesfrom 350 mm to 400mm
per year (940-1,080 lo6 m3per y a ) Hence,the probable error of the computed ground
                     x            er.
water inflow and outflow is affected by the evapotranspiration.A continuing inventory
program is now being carried on, so as to bridge the gap and to achieve an optimum
development o the ground water resources in this basin.
               f
   Recently,stabilization o the subsoil by quicklime piles has been one of the most
                              f
successful measures in the Taipei Basin. Test results showed that use o quicklime piles
                                                                          f
induced reduction in water content and void ratio of the soil,and consequently increased
the apparent angles o shearing resistence and improved the bearing capacity of the soft
                       f
          3.
material []


                                                                                       33
Jui-Mtng H w a n g and Chian-Min Wu


A D D I T I O N A L WORK NEEDED

Though some study has been made of the land subsidence in Taipei Basin, the results are
not extensive enough to be conclusive.Additional work, such as (1) the establishment and
maintenance of adequate verticals, both at surface and at selected depth intervalsin wells,
 2
( )adequate programs of water level measurement and inventory of groundwater draft,
and (3) proper sampling and testing of subsoil, are badly needed.
    Recently, land subsidence has been of widespread interest in the field of engineering,
agriculture, geology, soil science, meteorology, hydrology and economics. Cooperation
of the various professional fields is badly needed for the further investigation of the
problem.




REFERENCES
1.   HUANG, ( 9 2 : “The Sungshan Formationin the Taipei Basin”, Memoir of The Geologi-
          T.C. 1 6 )
     cal Society of China, No. 1, July 1962,pp. 133-152
           M. 1 6 )
2 MIKASA ( 9 3 : “The Consolidation of Soft Clay”, Institute of Kasima, Japan.
 .
3. CHIU, K.Y.( 9 3 : “The Study of Improving Bearing Capacity of Taipei S l by Using
                 16)                                                            it
   Quicklime Pile” Proc. 2nd Asian Regional Conference on Soil Mechanics and Foundation
   Engrg; International Soc. of Soil Mechanics and Foundation Engrg; pp. 387-389.




34

								
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