Large-scale land subsidence often occurs after
large-scale land formation caused by dredger fill, which affects the sustainable
development of the region. In order to prevent and control land subsidence
in the area with dredger fill, the characteristics of land subsidence and
its main influencing factors need to be studied. A typical region was
examined using geological survey data, land-level monitoring and comparative
analysis, to provide insight regarding the variability of dredger-fill
characteristics and impacts on land subsidence. The geological survey
results provided the information about burial distribution characteristics
of dredger fill and its underlying soil layers. The land-level monitoring
results were analyzed to characterize the spatial–temporal distribution of
land subsidence. The comparative analysis of land subsidence with the
formation time, soil properties, thicknesses of dredger fill and the lower
soft soil layer provided information about the different impacts. The
monitoring results show that the land subsidence of dredger fill areas was
substantially larger than that of adjacent areas. The later the filling was
formed, the thicker the filling is, and the more clay-rich the soil property and
the thicker the soft soil layer is, the larger the land subsidence is.
Finally, the future trend of land subsidence in the study area are given and
some suggestions on the prevention and control of land subsidence are also given.
Introduction
Many coastal cities require large-scale urban construction and expansion of
urban development space, as the rapid development and the population
increases. But limited land resources seriously restrict the development.
Reclamation is an important measure to solve the shortage of land resources
in coastal cities. It is also a way to dredge silt and clean the coastal
water environment (Huabo et al., 2016). There is a Chinese coastal city with
substantial historical land subsidence (Shujun et al., 2016), needed to
expand urban development space and therefore formed a large area using
dredger filling distribution techniques (Linbo et al.,
2019).
The abovementioned city is situated on the west coast of the Pacific Ocean, the east coast
of the Asian continent, and the central point of China's north–south coast.
There is a large area of dredger fill in the coastal area in the abovementioned city (Chen et al., 2018; Linbo et al.,
2019). The area was rapidly formed and because it was underconsolidated, and it
settled. Moreover, the large area of dredger fill caused additional
subsidence from compaction of the lower soft soil layer. The research on
dredger fill has mainly focused on the engineering characteristics of
dredger fill, the deformation and strength of dredger fill, and the
engineering treatment technology of dredger fill foundations. Few studies
have been done on the characteristics of regional land subsidence in large
areas of dredger fill. In these new large land areas, InSAR is a good way to
study land subsidence (Qing et al., 2011). However, the accuracy and
reliability of InSAR monitoring results are not as good as that of
high-precision leveling monitoring.
This study's objectives are to understand the impact of dredger fill and
provide useful information for coastal city planning and construction. Based
on the variable distribution of dredger fill in a Chinese coastal city and the land-level
monitoring results over several years, this paper analyzes the spatial and
temporal characteristics of land subsidence induced by the distribution of
recent dredger fill over a large area and studies the influence of the
differences of dredger fill soil properties, thickness, formation time and
the underlying soil layer on land subsidence.
Analysis of influencing factors in the study area
There are five main confined aquifers in the study city. From shallow to deep,
they are called the first, second, third, fourth, and fifth confined
aquifers respectively. Groundwater exploitation in these aquifers is the
main cause of land subsidence in the study city (Yun et al., 2015).
With field investigation and collected groundwater exploitation data in
the study city, a new coastal land formation that did not undergo groundwater exploitation
from 2013 to 2017 was selected as the study area. In the study area, the
groundwater level of five confined aquifers steadily increased during that
period. Water level of the first confined aquifer increased by about 0.5 m.
Water levels of the second, third and fourth confined aquifer increased by
between 2 and 3 m. And water level of the fifth confined aquifer
increased by about 5.0 m. Therefore, the influence of groundwater on land
subsidence should not be the main factor in the study area. Additionally,
there were no large-scale construction activities from 2013 to 2017 in the
study area. We conclude that the large-scale distribution of dredger fill is
the main factor causing land subsidence in the study area.
To provide insight regarding the variability of dredger-fill characteristics
and impacts on land subsidence, 14 leveling points were selected for
comparison. Their relative positions and numbers are shown in Fig. 1. The
ocean lies to the east of these leveling points.
The relative positions and numbers of leveling points.
Influences of formation time of dredger fill
In the study area, the dredger fills nearer to the sea were formed later. The formation time has a great influence on the recent land
subsidence. In order to understand this effect, we assigned the same
properties and thicknesses but different formation times to make the
comparison.
In leveling points D004 and D005, dredger fill was formed earlier in the process. In D009 and D010, dredger fill was formed later. The
cumulative subsidence of D004, D005, D009 and D010 from 2013 to 2017 was
26.03, 28.42, 45.95, and 59.25 mm, respectively (Fig. 2). The newer the
fill is, the larger the accumulated subsidence was.
Land subsidence curves (D004 and D005 dredger fill formed earlier,
D009 and D010 formed later).
Since some leveling points providing comparative data were buried in
2012–2013, in order to reduce the impact of the stability of the monitoring
facilities, the data after 2013 was used in the comparative study.
Influences of soil properties of dredger fill
Dredger fill can be divided into silty soil and clay soil according to
filling composition in the study area. For this comparison, we assigned the
same formation time and thicknesses of dredger fill but different soil
properties to make the comparison.
Leveling point D001 is where there is silty dredger fill and D002 is where
there is clay dredger fill. The cumulative subsidence of D001 and D002 from
2013 to 2017 was 14.54 and 22.01 mm, respectively (Fig. 3). So when the
thickness, formation time and underlying soil layer are similar, the
presence of clay soil resulted in more subsidence than that of silty soil.
Furthermore, the subsidence rate decreases with time.
Land subsidence curves (silty dredger at D001 and clay dredger at
D002).
Influences of the thickness of dredger fill
To examine the effect on subsidence from the thickness of dredger fill, we
assigned the same formation time and soil properties but different
thicknesses of dredger fill to make the comparison.
Leveling points D014 and D013 are where the thickness of dredger fill is
about 3.5 m. D012 and D011 are where the thickness of dredger fill is about
2.5 m. The cumulative subsidence of D014, D013, D012 and D011 from 2013 to
2017 was 109.84, 91.79, 77.92, and 73.52 mm, respectively (Fig. 4).
Divided by their respective thicknesses, the computed results are 31.38,
26.23, 31.17, and 29.41 mm m-1. These values are very similar; therefore, the
subsidence was approximately proportional to the thickness of dredger fill.
Land subsidence curves (the thickness of dredger fill at D014 and
D013 is about 3.5 m; the thickness of dredger fill at D012 and D011 is about
2.5 m).
Influences of the thickness of lower soft soil layer
According to “Code for investigation of geotechnical engineering”
(DGJ08-37-2012), the engineering geological layers are unified in the study city.
There are 11 engineering geological layers and several sublayers 100 m
below the surface. The sequence numbers are (1)1, (1)2, (1)3,
(2)1, (2)2, (2)3, (3)1, (3)2, (3)3, (4), (5)1,
(5)2, (5)3, (5)4, (6)1, (6)2, (6)3, (6)4,
(7)1, (7)2, (8)1, (8)2, (9)1, (9)2, (10), and (11). The order
of the lower soil layer (within 40 m depth) from top to bottom below the
dredger fill in the study area are shallow sand layer (2)3, soft soil
layer (3) + (4), clayey soil layer (5), hard soil layer (6) and lower sand
layer (7). The sequence numbers of dredger fill layers are (1)3. The
distribution of dredger fill adds a load to the underlying layers, which
could cause compaction of those underlying soil layers. The soft soil layer
(3) + (4) is highly compressible and sensitivity.
We assigned the same formation time, soil properties, and thicknesses of
dredger fill but different thicknesses of the soft soil layer (3) + (4) to make
the comparison.
Leveling point number D008 is where the thickness of the soft soil layer (3) + (4)
is about 7 m; D007 and D006 are where the thickness of the soft soil layer (3) + (4)
is about 5 m; and D003 is where the thickness of the soft soil layer (3) + (4) is
about 3.5 m. The cumulative subsidence of D008, D007, D006 and D003 from 2013
to 2017 was 58.77, 43.48, 43.56, and 32.09 mm, respectively (Fig. 5).
Divided by their respective thicknesses, the computed results are 8.40,
8.70, 8.71, and 9.17 mm m-1. These values are very close; therefore the
subsidence is approximately proportional to the thickness of the soft soil layer
(3) + (4).
Land subsidence curves (the thickness of the lower soft soil layer
(3) + (4) at D008 is about 7 m, the thickness of layer (3) + (4) at D007 and D006 is
about 5 m, and the thickness of layer (3) + (4) at D003 is about 3.5 m).
Future trend analysis of land subsidence in the study area
According to the law “Regulations of Shanghai Municipality on the Administration of Prevention and Control of Ground Subsidence” issued by Shanghai government in 2013 and the “Shanghai Urban Master Plan (2017–2035)” issued by Shanghai government in 2018, the possibility of large-scale groundwater exploitation in the
study area is very small. Our analysis indicated the main factors
influencing subsidence have been, and are likely to be in the future, the
large-scale distribution of dredger fill and engineering construction
activities. The later the filling is formed, the thicker the filling and the
more clay-rich the soil will be; the thicker the soft soil layer, the larger the
land subsidence effect will be.
According to the definition of the average consolidation degree of soil layer,
Ut=sts∞,
where Ut is the average degree of consolidation of the soil
layer, s∞ is the final compaction of the soil layer,
and st is the compaction of the soil layer in time t.
The relationship between the degree of consolidation and time of large area
dredger fill due to self-weight pressure and its load on lower soil layer
can be written in the following unified form:
Ut=1-ae-bt,
where a, b are the parameters. Then,
st=(1-ae-bt)s∞.
Equation (3) was used to fit the multi-year leveling monitoring data. The
fitting results are shown in Fig. 6 and indicate that subsidence caused
from the impact of the dredger fill gradually diminishes.
Fitting and forecasting curves of land subsidence.
Conclusions and discussion
The distribution of dredger fills over a large area is the main influencing
factor of land subsidence in the study area. The differences of formation
time, soil properties, thickness of dredger fill, and the thickness of soft
soil layer result in differential land subsidence in the study area. The
analysis presented here indicates that land subsidence will continue but
subsidence rates will decrease and eventually stop.
This study needs further research and more data to support the research
conclusion more comprehensively, and to establish the quantitative
relationship between the various elements of dredger fill and land
subsidence. Based on the investigation and monitoring results, it is
necessary to study the prediction method and numerical calculation model of
land subsidence in the dredger fill area.
In order to provide more accurate land-subsidence information for government
decision-making in the large area of dredger fill, we should strengthen the
construction of monitoring networks to provide early warnings of subsidence
risk. Dredger fill treatment should be carried out before construction,
strict groundwater control should be implemented, and urban construction
should be planned rationally.
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time, as the data also forms part of an ongoing study.
Author contributions
JL provided the main research ideas and was responsible for writing the paper. HW and TY contributed to the comparative analysis of some influencing factors. XH was responsible for drawing the main figures.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “TISOLS: the Tenth International Symposium On Land Subsidence – living with subsidence”. It is a result of the Tenth International Symposium on Land Subsidence, Delft, the Netherlands, 17–21 May 2021.
Acknowledgements
The authors thank the geological environment monitoring department of the
Shanghai Institute of Geological Survey for the monitoring data.
Financial support
This research has been supported by the IGCP (grant no. IGCP-663), the Shanghai Science and Technology Commission (grant no. 18DZ1201100), and the Shanghai Municipal Bureau of Human Resources and Social Security (“Study on land subsidence mechanism and safety warning in new land reclamation area”).
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