- 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).

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- 239 -

*&

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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

\ \ / \ \ / \ \ /

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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.