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- 235 -
5.4 LAND SUBSIDENCE IN SHANGHAI, CHINA
5.4.1 Historical review
Land subsidence in Shanghai is first reported in 1921. From that time
to the eve of liberation, the city area sank at an average rate of
26 mm per year.
In 1949, with the rapid development of industrial production, ground
water extraction increased and subsidence continued. Up to 1965, the
maximum cumulative subsidence, as recorded by one of the bench marks
in the city area, was as high as 2.63 m (Figure 5.4.1), thus forming
two dish-shaped depressions in the urban and suburban districts.
Subsidence (mm)
BM No
0-282
0-301
0-283
0-293
0-264
1921-65
- 1933
- 2030
- 2170
- 2370
- 2630
1965
+ 15
+ 26
+ 14
+ 18
_
1920 30 40 SO 60 70 80 85
Figure 5.4.1 Cumulative deformation shown by typical bench marks in
Shanghai urban area
Land subsidence in Shanghai may be divided into five distinct periods
(Table 5.4.1).
"? „„
on
(m
i i
i/e d
efo
rmati
o c
(0 3 E O 2500
3000-
0-282 0-301
—) 0-283 0-293
- 236 -
Table 5.4.1 Variation of land subsidence over the years
Period 1921-1948 1949-1956 1957-1961 1962-1965 1966-1985
Mean annual
subsidence (mm) 24 40 110 59
Extent of subsidence
above 500 mm (km2) 19.3 7.4 66.1
From Table 5.4.1, it can be seen that before 1965 the urban area of
Shanghai had been subject to continuous subsidence. The greatest
subsidence occurred between 1957 and 1961, when the annual rate
attained 100 mm. In 1966, a series of countermeasures were taken, with
the consequence that land subsidence in the urban area of Shanghai
was brought under control.
5.4.2 Hydrogeological and engineering geological conditions
The Shanghai area lies on the coast of the East China Sea at the front
of the Yangtze Delta and the edge of the north Jiangsu segment. Loose
sediments, about 300 m thick, of alternating marine and continental
facies were deposited on the bed rock during the Quaternary period.
The upper portion of 150 m thickness is composed of clay and sand
layers of littoral and fluvial delta facies; the lower portion of
150 m thickness consists of alternating sand layers of fluvial facies
and variegated clays of lacustrine facies.
According to the hydrogeological characteristics of the overburden,
one phreatic water-bearing layer and five confined aquifers may be
distinguished (hereinafter called aquifers, Figure 5.4.2). The general
features of these aquifers are: horizontal, relatively thick, fine
grained, with small hydraulic gradient and low velocity of groundwater
flow. These aquifers demonstrate a distinct regularity of lithological
changes, finer grained with decreasing thickness as they go from
northeast to southwest.
- 237 -
WSW ENE
E3i E32 E233 E2« S s M3e ESI? QSJS freai9
Figure 5.4.2 Geological profile of Shanghai urban area
1 = surface soil; 2 = muddy clay; 3 = muddy clay loam;
4 = clayey loam with sand; 5 = stiff clay; 6 = sand; 7 = sand
with gravel; 8 = confined aquifer; 9 = compressible layer.
The main hydrogeological features of the various aquifers are listed
in Table 5.4.2.
The overburden may also be divided into 13 characteristic layers.
Among them are three stiff clay layers below the second aquifer with
fairly high compressive strength, their void ratio being less than
0.70, and their coefficient of compressibility less than 2 -1 0.025 cm .kg . Owing to the higher compressive strength, the extent
of compression of the layers has been comparatively small over the
years. Above the second aquifer are three compressible layers (soft
clay layers) with low compressive strength and one dark green stiff
clay layer with fairly high compressive strength. The principal
physical mechanical indices of these layers decrease as the depth of
the layers increases (Figure 5.4.3).
- 238
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- 239 -
*&
W/.
phreatic water
1st compressible
layer
confined phreatic water
2nd compressible
layer
1st aquifer
3rd compressible
layer
Void ratio Water content (%) Coef. of compres- Preconsolidation sibility (cm3kg -1) pressure(kgcm*2)
0 20 40 601»10"8 1»10"' 1«10"0 1«10' Clay (%) Coef. of permeability (cms"1)
%max [consoli-[dation
Figure 5.4.3 Variation of mechanical properties of soil-layers in
Shanghai urban area
5.4.3 The cause of land subsidence
In order to control land subsidence in the urban area of Shanghai, it
is necessary to determine the cause. Geological prospection was
initiated in the urban area in 1962 and a comprehensive investigation
of the factors affecting land subsidence was carried out. A synthesis
of the geological survey and the well data demonstrated a direct
relation between the withdrawal of groundwater and land subsidence as
regards time, region and depth.
- 240 -
1 Relationship between land subsidence and the volume of groundwater
withdrawals
The history of Shanghai's groundwater exploitation began in 1860, when
the first deep well was sunk. In those days groundwater was used
mainly to lower workshop temperature, to control humidity and to cool
and wash products.
In Shanghai land subsidence is directly related to the volume of water
extracted from the groundwater table, as shown in Figure 5.4.4, where
it can be seen that the greater is the volume of groundwater extracted
and the lower is the groundwater table, the greater is the area of
land affected by subsidence.
10i 0.1-. 25
Ê.104
o >-oa
30 -0.3 -75-
"5)-5T> Groundwater level
Subsidence
Figure 5.4.4 Variations of flows, water levels and surface subsidence
in Shanghai urban area
Since groundwater is more heavily exploited in summer, the rate of
land subsidence increases in summer and decreases in other seasons
(Figure 5.4.5) .
2 Relationship between the extent of subsidence and descending cone
of groundwater
The deep wells are mainly concentrated in the Eastern and Western
Shanghai industrial districts along the banks of the Suzhou Creek and
the Huangpu River, the volume of groundwater extracted in these two
industrial districts accounts for more than 80 percent of the total
amount of groundwater extracted in Shanghai, the two centres of land
subsidence coinciding with the cones of influence of the falling
groundwater table (Figure 5.4.6).
- 241 -
„ 20'
E O
c ,o "o 2 60 "x LU
Figure 5.4.5 Relationship between groundwater extraction, water
levels of the second aquifer and land subsidence in
Yangpu area
1 Top surface of 2nd aquifer, August 1964 (m)
2 Cumulated subsidence 1948-1963 (mm)
Figure 5.4.6 Relationship between drawdown cone and cone of
subsidence (1960)
3 Relationship between land subsidence and drawdown of groundwater
Regions of land subsidence coincide primarily with extraction of water
from the second and third aquifers and it is these which furnish the
largest volume of water, representing 85 percent of the total
(Table 5.4.3).
subsidence..
-10
ë-20 <D (U
S-30-CO Î
-10i
I E w - 2 0 O
<B •a .2-30 3
Quarter Year
\ \ / \ \ / \ \ /
~-~A\ /
1 | 2 | 3 | 4 1963
extraction I
/ \ \ '1 / \ \\ // \\v x
1 I 2 | 3 J 4 1964
A \\
^water level
1 | 2 | 3 | 4 1965
- 242 -
Table 5.4.3 Volume of groundwater extraction and deformation of soil
layers in Shanghai (1964-1965)
2nd 3rd 4th 5th Total aquifer aquifer aquifer aquifer
Volume of the 3 445 1 743 822 54 6 064 water exploited (Mm )
% of total 56.8 28.7 13.6 0.9 100
Depth of soil 0-150 150-334 layer (m)
Deformation of the -42.55 + 0.04 - 42.51 soil layer (cm)
Because of decrease of pressure in the rising head, as a result of the
exploitation of the second and third aquifers, the amount of
compression of the three soft compressible layers above the second
aquifer has been comparatively great and accounts for over 90 percent
of the total land subsidence (Table 5.4.3).
The circumstances described above clearly demonstrate that the main
cause of land subsidence in Shanghai has been the excessive withdrawal
of groundwater from the loose overburden of the Quaternary.
5.4.4 The conditions of land subsidence
1 Relationship between geological structure and land subsidence
To grasp internal factors causing land subsidence, a thorough study
was made of the different combinations of the three compressible
layers and one dark green stiff clay layer, from a depth of 70 m
upward, and the hydraulic interconnection between the first and second
aquifer.
The urban area of Shanghai may be divided into four distinct
geological structural zones of land subsidence (Figure 5.4.7,
Table 5.4.4).
- 243 -
1 = geological structure number and boundary line
2 = zone of slight subsidence
Figure 5.4.7 Relationship between geological structure and land
subsidence
Table 5.4.4 Description of various soil layers in the four
structural zones
No. of zone No. 1 No. 2 No. 3 No. 3
1st compressible present present present present layer
2nd compressible absent absent present present layer
Dark green stiff present present absent absent clay layer
3rd compressible absent present absent present layer
Hydraulic yes no yes no connection between 1st and 2nd aquifer
- 244 -
Based on geological surveys and analysis of levelling data, the
following tentative conclusions may be drawn:
- under the same condition of groundwater exploitation, subsidence in
zones of comparatively weak geological structure is greater than in
zone of stronger geological structure;
in a given geological structural zone with identical conditions of
groundwater exploitation, it is found that the greater the thick
ness and the compressibility of the compressible layers, the
greater is the subsidence volume;
- the amount of compression of the first compressible layer depends
upon whether the dark-green stiff clay layer is present in the
lower part or not. The amount of compression in zones 3 and 4,
where there is no dark-green stiff clay layer, is greater than in
zones 1 and 2 where the dark-green stiff clay layer is present;
the rate of compression of the first and second compressible layers
depends upon whether a hydraulic interconnection exists between the
first and second aquifers. Therefore the rate of compression in
zone 3, where there is a hydraulic interconnection, is greater than
in zone 2.
2 Basic characteristics of the deformation of soil layers subject to
exploitation and recharge of groundwater
The deformation of the soil in the urban areas is related to
fluctuations of the groundwater levels. Before counter-measures were
taken, these layers had been in a compressed state for years as a
result of low water levels in the past. Since the start of artificial
groundwater recharge along with the periodic fluctuations of ground
water levels during winter recharge and summer exploitation,
alternating expansion and compression has taken place (Figure 5.4.8).
- 245 -
start of remediahTieasures
1964 85 66 67 68 69 70 71 72 73 74
S = surface bench mark;
51 = 1st compressible layer;
52 = 2nd compressible layer;
53 = 3rd compressible layer;
54 = 2nd and 3rd aquifers;
55 = Aquitard between the 3rd and 4th aguifers;
56 = Soil layers beneath the top of the 4th aquifer;
Gl = Water table of the 1st aquifer;
G2 = Water table of the 2nd and 3rd aquifers;
G3 = Water table of the 4th aquifer.
Figure 5.4.8 Curves showing the variation of cumulative deformation
and water level fluctuation with time
3 Mechanism of deformation in soil layers
According to the theory of unidimensional consolidation, prior to
groundwater extraction, the sum of the pressure acting on the
particles of the soil layer (effective pressure or effective stress)
P , and of the pore-water pressure of the soil mass (neutral pressure)
P , is in equilibrium with the total pressure of the soil layer above w the aquifer (soil layer pressure) P, i.e.,
P = P + P . s w
When excessive groundwater is withdrawn from the aquifer, there is a
distinct drop of the water level in the aquifer. As a result, the
original state of equilibrium of pressure in the soil mass is
destroyed. A pressure gradient is introduced between the aquifer and
- 246 -
the clay layer. This causes the pore-water of the easily compressible
clay layer to flow out in large amounts, i.e., the pore-water pressure
P decreases whilst the effective pressure P increases. This results w s
in further consolidation and strong compression of the viscous soil
layer (Figure 5.4.9).
Further, because of the water level drop in the sand aquifer, there is
a decrease in the buoyancy force of the water in the sand aquifer,
with the result that compaction of the sand aquifer occurs. The
superposition of the compression of the clay layer and the compaction
of the sand aquifer gives rise to subsidence of the surface.
On the contrary, under artificial recharge, because of the rise of the
confined head in the aquifer, and the rebound of the sand aquifer,
there is an increase of the pore-water pressure of the clay layer and
swelling of the soil mass occurs.
Pressure (kgcm"2)
1 = Total stress line (P);
2 = Hydrostatic pressure line;
3 = Pore-water pressure line on 30 Sept., 1976;
4 = Mean preconsolidation pressure line;
5 = Pore-water pressure line on 30 Sept., 1966.
Figure 5.4.9 Distribution of pore-water pressure, hydrostatic
pressure and total stress of soil layers in the first
70 m below the surface in the Shanghai urban area
- 247 -
5.4.5 The prediction of land subsidence
1 Relationship between groundwater withdrawal and the head of the
confined aquifer.
It can be seen from Figure 5.4.10 that the volume of groundwater
withdrawal and the head of the confined aquifer show a steady annual
periodic variation, and therefore a linear steady model can be based
on a digital time-sequence analysis.
t+1 ao + al Qt+1 + a2Ht (1)
where:
H is the head level at time t;
Q .. is the volume of groundwater withdrawal between t and t+1;
t is time base.
Every calculation year (from the first ten days in October to the last
ten days in September of the following year) is divided into two
parts: period of rising head, period of falling head, according to the
consumption of water and head variations in Shanghai (Figure 5.4.10).
recharge
discharge
\ deforma-: — I tion
observed calculated
Figure 5.4.10 Calculated and observed deformations, water levels and
extraction volumes in the fourth aquifer
- 248 -
The parameters of Table 5.4.5 were found by introducing in Eq. (1)
head values of the fourth confined aquifer near Labour Park, Shanghai,
the consumption of water in the eastern part of the city and the mean
error of the posterior prediction. The parameter variations and errors
are small and meet the required precision for prediction
(Table 5.4.5).
Table 5.4.5 Model parameters for the head variation of the fourth
aquifer at Labour Park
Time interval Parameters
ao ai a2
Mean error checked every other year (m) Absolute Arithmetic value mean
From 2 March to 2 August (1969-1983)
From the last decad of August to the first decad*) of March (1969-1983)
-1.5139 -2.5047 -2.7994 -2.9803 -1.6897 -3.9197 -2.7017 -3.0575 -3.4606 -3.8095 -4.0802 -5.2372 -2.6747 -3.6903 -3.1366
-1.8724 -1.8335 -1.7062 -2.4160 -3.1178 -3.8279 -3.2427 -2.1939 -3.2125 -5.8468 -4.4804 -6.3760 -5.5165 -5.3279
-0.0492 -0.0726 -0.0685 -0.0696 -0.0824 -0.0780 -0.0410 -0.0617 -0.0770 -0.0741 -0.0991 -0.1348 -0.0521 -0.0512 -0.0712
-0.0452 -0.0429 -0.0352 -0.0481 -0.0555 -0.0592 -0.0557 -0.0426 -0.0505 -0.0693 -0.0658 -0.0782 -0.0714 -0.0701
0.7887 0.6626 0.6529 0.6494 0.8765 0.6271 0.7457 0.7214 0.6956 0.7093 0.6080 0.5220 0.8342 0.7471 0.7490
0.7571 0.7742 0.7837 0.7378 0.7019 0.6552 0.7074 0.7816 0.7382 0.6067 0.6524 0.6019 0.6318 0.6584
0.30 0.38 0.27 0.38 0.50 0.39 0.37 0.39 0.54 0.64 0.60 0.57 0.55 0.30
0.27 0.30 0.32 0.32 0.43 0.46 0.36 0.46 0.59 0.44 0;72 0.38 0.35
-0.29 -0.18 -0.16 -0.28 -0.14 -0.02 -0.20 -0.22 -0.49 -0.52 -0.56 -0.51 -0.01 -0.07
-0.13 -0.06 -0.11 -0.24 -0.18 -0.15 -0.12 -0.41 -0.55 -0.29 -0.71 -0.35 -0.11
*) 1 decad = 10 days
- 249 -
2 Relation between the head of the confined aquifer and deformation
of soil layers
The deformation of the soil layer which causes land subsidence is
mainly the result of variations of head. The aquifer in the Shanghai
area extends laterally over a distance much greater than its thickness
and therefore the theory of unidimensional consolidation can be used
to calculate the deformation of the soil layer.
For the clayey soil:
S t - - T - V Px Cl-Koe-ciKl(t-i)] (2) 1+e
o
and for the sand layer:
r .P. S = -JL-— h + S (3)
E
where :
S is the cumulative deformation of the soil layer in time t;
h is the thickness of the soil layer, half of which is taken when
drainage occurs on both sides;
e is the initial void ratio of the soil layer: o J
P. is the amplitude of the head variation during the time interval
between i-1 and i;
a. is the compressibility coefficient of the soil layer;
C. is the consolidation coefficient of the soil layer;
K = 8/h2 and K = 30 x 86 400/h2; o 1 t is the time interval;
r is the density of water; w E is the modulus of elasticity of the sand layer;
S is the pre-accumulated deformation. o
3 Forecasting the deformation of the soil strata and preparation of a
plan of rational groundwater development
According to the planned use of water, the deformations of the soil
strata of the different layers can be calculated using Eq. (1), (2),
(3), and after superposition, the amount of the land subsidence can be
predicted.
- 250 -
If the predicted value is large, the following measures must be taken:
1 the quantity of water supplied by the different aquifers must be
adjusted;
2 the total extraction volume must be reduced.
The land subsidence is then recalculated using the new extraction
values until acceptable results are obtained. The calculation
procedure is outlined in Figure 5.4.11.
(BEGIN)
' I '
-C S.P.ftWe. |
I Qml fH|o-A|
I Hlk
1
1 yes 1
Û P1k I
r v i Sjk
1 1 '
1 no i
* H 1 k
1 f Mj-E
[ s j k
— - , 1
I J*J»1
J=1 'K
yes
Hm'm*1l|Qm.H1k.Sjk.Sn
( END)
Figure 5.4.11 Forecasting land subsidence
- 251 -
5.4.6 Land subsidence control measures
As stated previously, land subsidence in Shanghai is induced primarily
by concentrated pumping of groundwater from the second and third
aquifer s and the corresponding drawdown of water levels. Therefore, to
control land subsidence and to rationalize the exploitation of ground
water, measures have been taken to raise water levels. In the specific
conditions of Shanghai, the principal remedial measures taken have
been the following.
1 Restricting the volume of groundwater extraction
Owing to the serious threat to industrial production and to city
buildings resulting from land subsidence, a resolution was passed in
1963 by the Shanghai Municipal Government to restrict groundwater use.
Some factories which relied upon energy from groundwater for cooling
have taken measures for using surface water and installed
refrigeration devices instead. From 1963 to 1965, after these measures
had been taken, land subsidence in the urban area decreased from year
to year with the decrease of the volume of groundwater extracted and
the rise of the water table (Figure 5.4.4)
2 Artificial groundwater of recharge
Since 1966, "recharging in winter for summer use and recharging in
summer for winter use" has been carried out Shanghai, the amount and
extent of recharge increased from year to year and the massive
artificial recharge of groundwater resulted in an extensive elevation
of the groundwater table. By 1970 water levels in the concentrated
recharge region were above the regional groundwater levels and to a
reverse funneling of the groundwater table. Recharge in Shanghai not
only controlled land subsidence but also provided new sources of heat
and cold for factories (Figure 5.4.12).
- 252 -
S14-1
Figure 5.4.12 Sectional chart of groundwater cone in the second
aquifer, Shanghai during high water
3 Adjustment of aquifer operation
Before the remedial measures had been introduced, groundwater was
mainly pumped from the second and third aquifers in the urban area.
Although the fourth aquifer contains abundant groundwater of fairly
good quality, the original temperature was high (24-25°C) and didn't
meet the requirements for cooling and lowering of temperature of
industrial processes. Hence withdrawals from the fourth and fifth
aquifers were very small in the past. After recharging these aquifers
with cool water in winter, the temperature of the groundwater
decreased from year to year (9-12°C), and measures were introduced for
increasing withdrawals from the fourth and fifth aquifers instead of
from the second and third aquifers. This will result in a decrease of
the rate of subsidence in the urban area.