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Review article
Study on soil reinforcement param in deep foundation pit of marshland
metro station
Wei Wang
a
, Zhao Han
a
,
*
, Jun Deng
b
, Xinyuan Zhang
a
, Yanfei Zhang
a
a
School of Civil Engineering, Central South University, Hunan, 410075, China
b
Guangdong Province Communications Planning &Design Institute Co.,Ltd, Guangdong, 510507, China
ARTICLE INFO
Keywords:
Civil engineering
Geotechnical engineering
Soil engineering
Construction engineering
Foundation engineering
Structural analysis
Structural health monitoring
High water level
Subway station
Deep foundation pit
Soil reinforcement outside the excavation
Stability control
Parameter optimization
ABSTRACT
In this paper, the reinforcement method outside the deep foundation pit of a subway station in marshland under a
high water level was proposed, and the reinforcement parameters were studied. The influence of reinforcement
measures outside the deep foundation pit on deformation and the anti-overturning stability coefficient of the
diaphragm wall under high water levels was analyzed. According to the economic law of diminishing marginal
returns, the grouting reinforcement parameters were analyzed and optimized. The effects of different reinforce-
ment measures on the safety control of deep foundation pits was analyzed based on field-measured data. The
results show that the change in the water level has a significant impact on the stability of the foundation pit, and it
is necessary to adopt reinforcement measures for the foundation pit at 10 m below the surface; the change in the
reinforcement depth has a more significant impact on the stability of the foundation pit; and the optimized
reinforcement parameters can reduce the grouting volume by 45%.
1. Introduction
With large-scale construction of subways in major cities, increasingly
deeper foundation pit engineering will be required. The Juzizhou station
deep foundation pit of the Changsha Metro Line 2 is located in the
Xiangjiang River, which is currently the world's first subway deep
foundation pit on an island in a river. The large water level changes and
high water levels in the Xiangjiang River and the large excavation depth
and small rock socketed depth of the Juzizhou subway station often cause
deformation of the retaining structure under high water levels, which
greatly reduces the stability of the deep foundation pit structure. To
ensure the safety of foundation pit excavation under high water levels,
reinforcement measures are needed to strengthen the foundation pit
support system. At present, there are many measures to control the sta-
bility of the foundation pit [1,2,3], but there is no precedent for such
projects. In this paper, the method of reinforcement outside the pit is
proposed, and the reasonable design parameters of the method are
determined.
Many scholars worldwide have researched through theoretical anal-
ysis, experiment and numerical calculation the effect of soil
reinforcement in and out of foundation pits and the influence of rein-
forcement designs on the stability of the foundation pits and have
accumulated some experience and achievements. In soil reinforcement
inside the pit, some scholars [4,5,6,7,8,9,10] mainly studied the in-
fluence of reinforcement forms and parameters on the stability of the
foundation pit and surrounding structures. Someone optimized the pa-
rameters of grouting reinforcement. In the aspect of reinforcement
outside the foundation pit, Chengjun Hu and others [11,12,13] mainly
studied the control effect of reinforcement outside the foundation pit on
the stability of the foundation pit through theoretical analysis, field
measurement and numerical calculation. According to the existing
research results of foundation pit reinforcement, the current research
mainly focuses on in-pit reinforcement, and the results are relatively rich,
but there is little research on out-of-pit reinforcement. Moreover, due to
the lack of subway foundation pits built in marshland in the world, there
is little research on the reinforcement of deep foundation pits outside the
pit in marshland [14,15], and there is a lack of theoretical basis for the
design and construction of foundation pit reinforcement for subway
systems in marshland. Under the conditions of high water levels and low
embedding ratios, the effects of reinforcement measures outside deep
* Corresponding author.
E-mail address: hanzhaocsu@csu.edu.cn (Z. Han).
Contents lists available at ScienceDirect
Heliyon
journal homepage: www.heliyon.com
https://doi.org/10.1016/j.heliyon.2019.e02836
Received 4 May 2019; Received in revised form 4 June 2019; Accepted 11 November 2019
2405-8440/©2019 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-
nc-nd/4.0/).
Heliyon 5 (2019) e02836
foundation pits on the stability control of foundation pits and the quan-
titative design of grouting reinforcement parameters need to be studied.
Based on the background of the Juzizhou deep foundation pit project
in Changsha City, this paper uses FLAC3D to analyze the influence of
reinforcement measures and grouting parameters on the stability of the
deep foundation pit and the deformation of the diaphragm wall under the
high water level, which provides a reference for the design and con-
struction of similar projects in marshland.
2. Project overview
The Juzizhou subway station is located on an island in the Xiangjiang
River and is arranged parallel to the Juzizhou bridge from east to west.
The engineering position diagram is shown in Fig. 1.
The Station is an underground four layers and three crosses island
type station. The effective length of the platform is 118 m, its width is 12
m, the station total length is 138 m, and the standard width is 22.2 m. The
station is constructed by the open-cut and sequential construction
method. The depth of the foundation pit is approximately 30.8–31.6 m,
and the width is 22.2–25 m. The main enclosure structure of the station is
a 1000 mm-thick diaphragm wall and reinforced concrete internal sup-
port system with a 6 m-wide reinforcement zone and five vertical sup-
ports. The horizontal spacing of the first reinforced concrete support is 8
m, and the cross section is 600 mm 1000 mm. The horizontal spacings
of the second through fifth reinforced concrete supports are 4 m, and the
cross sections are 700 1200 mm. The design drawings of the retaining
structure of the foundation pit are shown in Fig. 2 (see Fig. 3).
The deep foundation pit of the Juzizhou subway station is located in
the Xiangjiang River. The eastern and western ends of the station are
close to the Xiangjiang River, and the minimum distance from the
Xiangjiang River is 13.5 m and 15 m, respectively. There are thick sand
layers and gravel layers in the surrounding area, which are highly
permeable. The groundwater changes the mechanical properties of the
soil, including the cohesion and the internal friction angle, by erosion
and increases the deformation capacity of the soil. In particular, the
water level in the river increases significantly during the rainy season
from July to September each year. According to the relevant recorded
information, the maximum water level difference of the Xiangjiang River
is up to 10 m, which greatly increases the risk of the instability of the
foundation pit. At the same time, the depth of the foundation pit buried in
a slate formation is only 6 m, the elastic resistance at the bottom of the
foundation pit is small, and the water pressure outside the foundation pit
is large, which seriously affects the stability of the deep foundation pit.
Therefore, the stability of the diaphragm wall under high water levels is
an important risk point for the deep foundation pit construction. To
reduce the impact of river water on foundation pit excavation, rein-
forcement of the east and west ends of the station must be strengthened.
The reinforcement zone in the original plan is that the standard section
on the north and south sides has a reinforcement width of 8 m and a
depth of 24 m. The expansion section on the east and west sides has a
reinforcement width of 9 m and a depth of 31.5 m.
3. Modeling and materials
3.1. The grid model of calculation
The standard section of foundation pit is selected using FLAC3D
software to establish a three-dimensional foundation pit model for
researching the rock, soil and diaphragm wall using solid element
simulation, the purlin, and support using Beam3 element simulation.
Because the model, load, and mesh materials are symmetrical, half of the
models are built to meet the computational needs. The type of interface is
glued, and the contact between different materials adopts the common
node setting. From top to bottom, the main strata of the foundation pit
are fill (10 m), sandy soil (3 m), gravel (6 m), and fully weathered to
slightly weathered argillaceous (sandy) slate (41 m).
The horizontal direction of the foundation pit model extends by
approximately 3 times the excavation depth of the foundation pit around
the boundary, and the vertical direction extends downward along the
boundary by approximately 2 times the excavation depth of the foun-
dation pit. The width of the model is 100 m, and the height is 60 m. The
unit size within the depth range is 1 m wide and 1 m high. The size of the
unit outside the excavation range becomes 2 m wide and 2 m high.
There are 56,070 meshes. The X-direction and Y-direction of the
bottom Z ¼0 of the model are fixed constraints. The Y-direction of the
left and right sides of the model is constrained. Apply corresponding
horizontal constraints on the plane of symmetry according to each for-
mation parameter. The X-direction [16] of the model is constrained
before and after the model. The geometric model of the numerical
simulation of the foundation pit of the Juzizhou subway station is shown
in Fig. 4.
3.2. Simulation of foundation pit excavation
The excavation process is simplified, and the main construction pro-
cess is extracted to analyze its construction process. The simulation steps
of the excavation process are as follows:(see Table 1).
3.3. Calculation parameters
The engineering site of the Juzizhou station belongs to the Xiangjiang
terrace. According to geological surveys and borehole exposures, the
buried strata at this station are mainly fill, sandy soil, pebble soil, and
fully weathered to slightly weathered argillaceous (sandy) slate, and the
stress-strain relationship of the soil is approximately that of the Mohr-
Coulomb model. The field tests of the soil layers and the reinforcement
soil are carried out, and the calculation parameters of the main soil layers
and supporting structure are shown in Table 2 (see Table 3).
4. Impact of reinforcement measures outside the pit on the
stability of the deep foundation pit
Based on monitoring of the Juzizhou foundation pit, the water level in
the foundation pit is mainly 9 m–14 m below the ground level, and the
change in the groundwater level is large. The change in the water level
has a great influence on the safety of the foundation pit [17]. To ensure
the safety of the excavation under the high water level, it is necessary to
take reinforcement measures outside the pit to strengthen the foundation
pit support system.
Fig. 1. Juzizhou subway station project location.
W. Wang et al. Heliyon 5 (2019) e02836
2
Fig. 2. Foundation pit support structure of the Juzizhou subway station.
Fig. 3. Juzizhou foundation pit reinforcement plan.
Fig. 4. Geometric model of numerical simulation for Juzizhou station.
W. Wang et al. Heliyon 5 (2019) e02836
3
Based on the original reinforcement scheme, six kinds of calculation
conditions of the height of the water level (6 m, 8 m, 10 m, 12 m, 14 m
and 16 m) are set up to study the control effect of the reinforcement
measures outside the pit on the stability of the deep foundation pit. The
calculated results of the deformation the diaphragm wall and the anti-
overturning stability of the foundation pit [16] are shown in Fig. 5 and
Fig. 6.
The sixth working condition is selected to analyze the variation in the
bending moment of the wall before and after reinforcement, as shown in
Fig. 7.
As shown in Fig. 5,Fig. 6 and Fig. 7:
1) The change in the water level of the Juzizhou foundation pit has an
obvious effect on the deformation and stability of the foundation pit.
When the water level is high, the change in the water level has a great
influence on the deformation and stability of the foundation pit.
However, when the water level drops to a certain position, the water
level will continue to decrease, and the influence of a change in the
water level on the stability of the foundation pit is very small. When
the water level drops below 14 m, changes in the water level have
little effect on the stability of the foundation pit. When the water level
is raised from 16 m to the 8 m below the surface, with the rein-
forcement measures the foundation pit is within the allowable ranges
of deformation and stability [18,19]. In the absence of reinforcement
measures, when the water level ranges from 12 m to 10 m below the
surface, the influence of the groundwater level changes on the sta-
bility is the most obvious. When the water level rises to 10 m below
the surface, the foundation pit is already in a dangerous state, and
with the rise in the water level, the safety of the foundation pit is
continuously reduced. It can be seen that with the increase in the
groundwater level, without reinforcement measures the groundwater
warning level should be 10 m below the surface, and when the
groundwater level is higher than 10 m, the possibility of the insta-
bility of the foundation pit is greatly increased. Therefore, the
groundwater level that is higher than 10 m below the surface is
defined as a high water level. To ensure the stability of the deep
foundation pit construction under high water levels, it is necessary to
adopt strong measures outside the pit to reduce the influence of the
high water levels on the stability of the foundation pit.
2) According to the maximum horizontal displacement of wall defor-
mation, the maximum horizontal displacement of the grouting rein-
forcement is smaller than that of the nongrouting reinforcement.
After the foundation pit excavation, the maximum horizontal
displacement of the wall strengthened by grouting is 22.7 mm, the
maximum horizontal displacement of the wall not strengthened by
grouting is 37.2 mm, and the maximum horizontal displacement of
the wall strengthened by grouting decreases by nearly 48%. The
maximum bending moment of the wall under grouting reinforcement
is 1300 kN m after the foundation pit excavation is completed, and
the maximum bending moment of the wall without grouting rein-
forcement is 1950 kN m. The maximum bending moment of the wall
under grouting reinforcement is reduced by 33%. Therefore, the ef-
fect of grouting reinforcement on reducing wall deformation and
bending moments is very obvious.
3) The maximum displacement of the diaphragm wall under the original
reinforcement measures is 22.7 mm, which is much less than the early
warning value of the first-grade foundation pit diaphragm wall
deformation, indicating that the diaphragm wall deformation has a
larger surplus or a higher safety factor. It further shows that the
grouting parameters adopted in the original scheme have more room
for optimization. To design the reinforcement parameters of the
foundation pit more reasonably, it is necessary to add more grouting
to the original scheme. Fixed parameters are properly optimized to
ensure that the design is economical and safe.
5. Optimization of the reinforcement parameters outside the pit
The above analysis has shown that the soil reinforcement outside the
pit is of great significance to the stability of the foundation pit. The
feasibility of the design of the original grouting reinforcement scheme is
considered to further study the influence of the reinforcement on the
stability of the foundation pit to optimize the reinforcement parameters.
Based on the above simulation model and the original reinforcement
scheme, first, the influence of the reinforcement zone width (width of 2
m, 4 m, 6 m, 8 m and 10 m) on the stability of the foundation pit is
studied in the case of the original reinforcement depth. Based on the
economic law of diminishing marginal returns [20,21,22], combined
with safety, the grouting reinforcement width is optimized to obtain a
reasonable value. Then, the influence of the reinforcement depth (depth
Table 1
Simulation of foundation pit construction.
Simulation
steps
Simulation content
0 Setting the groundwater level, balancing the initial stress, and
constructing the site with walls
1 Excavating the second layer to -3 m, doing the first support, and
calculating the balance
2 Excavating the second layer to -8 m, doing the second support, and
calculating the balance
3 Excavating the third layer to -14 m, doing the third support, and
calculating the balance
4 Excavating the fourth layer to -19 m, doing the fourth support, and
calculating the balance
5 Excavating the fifth layer to -25 m, doing the fifth support, and
calculating the balance
6 Excavating the sixth layer to -31 m and calculating the balance
Table 2
Main physical and mechanical indexes of the soils.
Geotechnical names Thickness Р
(kg/m
3
)
Es
(MPa)
C
(kPa)
Φ
()
Void ratio K
0
Poisson's ratio
Filled earth 10 m 1940 7.50e6 15 20 0.829 0.45 0.30
Fine sand 3 m 2000 2.08e7 50 24 0.655 0.36 0.30
Gravel 6m 2030 2.27e7 0 35.0 0.877 0.33 0.28
Completely weathered slate 11 m 2700 7.58e7 800 43 0.12 0.30 0.28
Intermediately weathered slate 9 m 2720 1.79e8 1000 55 0.03 0.28 0.22
Slightly weathered slate 21 m 2760 4.76e8 2500 45/65 0.01 0.26 0.22
Table 3
Main physical parameters of the supporting structure.
Project name Р
(kg/
m
3)
Section
hw
(m)
Es
(Mpa)
Poisson's
ratio
Horizontal
spacing support
Diaphragm wall 2500 371 30e3 0.2
First concrete
support
2500 1.00.6 30e3 0.2 8 m
Second to fifth
concrete
supports
2500 1.20.7 30e3 0.2 4 m
W. Wang et al. Heliyon 5 (2019) e02836
4
Fig. 5. Influence of water level variation on the deformation and stability of the foundation pit with or without reinforcement.
Fig. 6. Influence of reinforcement measures on the horizontal displacement of the wall under high water. (a) With reinforcement, (b) Without reinforcement.
W. Wang et al. Heliyon 5 (2019) e02836
5
of 12.5 m, 15 m, 17.5 m, 20 m, 22.5 m and 25 m) on the deformation
stability of the foundation pit is discussed, and the depth of grouting
reinforcement is optimized from economic and safety considerations.
5.1. The impact of the width of the reinforcement outside the pit
Analysis of Fig. 8 shows the following:
(1) The maximum horizontal displacement of the diaphragm wall
decreases as the reinforcement width increases. After the foun-
dation pit excavation is completed, the maximum horizontal
displacement of the diaphragm wall is 35.3 mm, 34.48 mm, 29.76
mm, 27.7 mm and 27.0 mm at reinforcement widths of 2 m, 4 m, 6
m, 8 m, and 10 m, respectively. When the reinforcement width
ranges from 4 m to 6 m, the horizontal displacement of the wall
decreases obviously. When the reinforcement width is 4 m, the
displacement value of the wall is 34.48 mm, which is more than
30 mm [19]. As the reinforcement width continues to increase, the
maximum horizontal displacement of the wall gradually decreases
from 6 m to 8 m. Considering the effect of the reinforcement width
on the deformation of the ground wall, the reinforcement width of
6 m is the reinforcement "limit value".
(2) The anti-overturning stability coefficient of the foundation pit
increases with increasing reinforcement width. When the width
value is small, the reinforcement width has a significant effect on
the stability coefficient. When the width is more than 6 m, as the
reinforcement width continues to increase, the stability coefficient
gradually increases. When the width value is 4 m, the stability
factor is 0.76, which is less than the minimum pit stability
requirement at a safety factor of 1.25. As the reinforcement width
continues to increase from 4 m to 6 m, the increase in the stability
factor is the largest. When the width value is 6 m, the stability
factor is 2.4 meeting the stability requirements of a foundation pit
safety factor of 1.25 [18]. Considering the effect of the rein-
forcement width on the stability of the foundation pit, there is a
limit value of the reinforcement width, and the reinforcement
width of 6 m is the reinforcement "limit value".
In view of the controlling effect of the reinforcement width on the
deformation and stability of the foundation pit, there is a limit; when the
reinforcement width is more than the limit, continuing to increase the
reinforcement width plays a small role in the controlling effect, which
follows the economic law of diminishing marginal returns. Based on the
law of diminishing marginal returns and considering the safety re-
quirements of the deep foundation pit construction under high water
levels, the reasonable reinforcement width is 6 m. Increasing the scope of
the reinforcement not only has little impact on the wall deformation but
also increases the cost of the project.
5.2. The impact of the depth of the reinforcement outside the pit
Fig. 9 shows the following:
(1) The maximum horizontal displacement of the diaphragm wall
decreases with increasing depth of reinforcement; however, after
Fig. 7. Influence of reinforcement measures on the bending moment of the wall
under high water.
Fig. 8. Influence of the reinforcement width on the deformation and stability of the foundation pit.
W. Wang et al. Heliyon 5 (2019) e02836
6
the reinforcement increases to a certain depth, the trend of
decreasing displacement lessens. When the depth of reinforce-
ment increases from 12.5 m to 15 m, the maximum horizontal
displacement of the wall is reduced by 5%; when the depth of the
reinforcement is increased from 15 m to 20 m, the maximum
horizontal displacement of the diaphragm wall is reduced by 29%,
which is very significant. When the depth of reinforcement
increases from 20 m to 25 m, changes in the maximum horizontal
displacement of the wall are very small. The main reason is that
when the reinforcement depth is less than 20 m, the reinforcement
area is mainly in the soft soil layer. With the excavation of the
foundation pit, the internal and external soil and water pressure
increase, and the wall deformation increases, thus weakening the
reinforcement effect. When the reinforcement depth is increased
Fig. 9. Influence of reinforcement depth on the deformation and stability of the foundation pit.
Fig. 10. Typical reinforcement.
W. Wang et al. Heliyon 5 (2019) e02836
7
to 20 m, the reinforcement area enters the slate from the soft soil
layer, and the reinforcement area and the slab layer are connected
together, which greatly improves the deformation resistance of
the wall. When the reinforcement depth is between 10 m and 15
m, the maximum horizontal displacement of the wall is more than
the wall warning value of 30 mm. When the reinforcement depth
increases to 17.5 m, the maximum horizontal displacement of the
diaphragm wall is reduced to 28.2 mm, which satisfies the limits
of the horizontal displacement of the wall. Considering the effect
of the reinforcement depth on the deformation of the foundation
pit, the reinforcement depth has a limit value, and the reinforce-
ment depth of 20 m is the reinforcement "ultimate depth".
Considering the effect of the reinforcement depth on the defor-
mation of the diaphragm wall, the reasonable depth of rein-
forcement is 17.5 m–20 m.
(2) The anti-overturning stability coefficient of the foundation pit
increases with increasing reinforcement depth. When the depth is
less than 20 m, the effect of the reinforcement depth on the sta-
bility coefficient is significant; when the depth is more than 20 m,
with increasing reinforcement depth, the increases in the stability
coefficient gradually become smaller. With the strengthening
depth increasing from 15 m to 20 m, the stability factor increases
significantly. When the reinforcement depth is 15 m, the stability
factor is 0.61, which does not meet the foundation pit stability
safety factor of 1.25. When the reinforcement depth is 17.5 m, the
stability factor is 1.37, which meets the stability requirements of a
foundation pit safety factor of 1.25; when the reinforcement depth
is more than 20 m, increasing the depth of reinforcement to
control the stability of the pit is of little significance. Considering
the effect of the reinforcement depth on the stability of the
foundation pit, the reinforcement depth has a limit value, and the
reinforcement depth of 17.5 m–20 m is the reinforcement "ulti-
mate depth". Based on the above analysis, the effect of the rein-
forcement depth on the deformation and stability of the
foundation pit is in accordance with the influence of the rein-
forcement width. Based on the economic law of diminishing
marginal returns, considering the safety requirements of deep
foundation pit construction under high water levels, when the
grouting reinforcement does not enter the slate layer, the rein-
forcement depth is stronger than that of the ground wall and the
stability coefficient of the foundation pit. After the reinforcement
area enters the slate layer, the effect of the reinforcement depth on
the stability of the foundation pit is not obvious. The reasonable
reinforcement depth of the pit is in the range of 17.5 m–20 m.
(3) Comparing the influence of the reinforcement width on the
deformation of the connected diaphragm wall, when the width of
the reinforcing increases from 2 m to 6 m (limit value), the
maximum horizontal displacement of the diaphragm wall de-
creases from 35.3 mm to 29.76 mm, and the amplitude is reduced
by 15.1%. That is, for each additional 1 m reinforcement width,
the displacement is reduced by 3.77%. When the depth of rein-
forcement increases from 12.5 m to 20 m (ultimate depth), the
maximum horizontal displacement of the wall is reduced from
36.69 mm to 24.5 mm, and the amplitude is reduced by 31.81%.
That is, for each additional 1 m of reinforcement depth, the
displacement is reduced by 4.56%. Increasing the depth of rein-
forcement to control the pit deformation is much better than
increasing the width of reinforcement. Under the conditions of the
site regulations and the capacity of the mechanical equipment, the
actual engineering grouting design should control the grouting
reinforcement depth and control the reinforcement width as the
supplement.
6. Analysis of the safety control effect on site construction
To reduce the influence of the Xiangjiang River on foundation pit
excavation, the optimized reinforcement parameters are applied to the
foundation pit reinforcement of the Juzizhou station. The width and
depth of the diaphragm wall around the foundation pit are 6 m and 20 m,
respectively. The typical reinforcement section is shown in Fig. 10.(see
Fig. 11)
According to the actual situation of the foundation pit, there are 14
inclinometer tubes around the foundation pit, of which 8 monitoring
points can normally be tested. To avoid repetitive analysis, this paper
takes the standard section in the middle of the foundation pit as the
research object, while the damage at points C004 and C005 in the middle
Fig. 11. Diaphragm wall horizontal displacement monitoring point C003.
Fig. 12. Layout diagram of the horizontal displacement monitoring points of the diaphragm wall.
Table 4
Comparison of the effects of the optimized reinforcement scheme.
Reinforcement scheme Diaphragm wall displacement (mm) Grouting volume
(m
3
)
Original scheme 22.7 41730
Optimized scheme 24.85 23180
W. Wang et al. Heliyon 5 (2019) e02836
8
of the foundation pit cannot normally be tested during construction, so
the inclinometer C003 near the section is selected to proceed. Defor-
mation characteristics of the row diaphragm wall are analyzed. The
arrangement of the inclinometer tube is shown in Fig. 12.
According to the engineering design requirements, the slurry filling
rate should be more than 70%. Combined with the pit size and grouting
design geometric parameters, the grouting amount is calculated before
and after the optimization of the reinforcement, which is shown in
Table 4. The calculated values and measured values of the maximum
horizontal displacement of the diaphragm wall under the optimized
reinforcement measures are shown in Fig. 13.
Table 4 indicates that the grouting amount of the original reinforce-
ment scheme is 41730 m3, and the optimized grouting design needs
23180 m
3
. Compared with the original grouting reinforcement scheme,
the optimized grouting reinforcement scheme can reduce the grouting
amount by 45%. It can be seen that the optimized grouting reinforcement
design can save a significant amount of grouting material and grouting
costs.
Fig. 13 and Table 4 also indicate that after the excavation of the
foundation pit, the maximum horizontal displacement of measuring
point C003 on the diaphragm wall is 24.85 mm, which is less than the
warning value of 30 mm, and the diaphragm wall is in a safe and stable
state. The maximum horizontal displacement of the diaphragm wall
caused by the excavation under the original reinforcement scheme is 22.7
mm. The horizontal displacement of the diaphragm wall with the opti-
mized reinforcement scheme is 9.5% higher than that of the original
scheme, but the displacement control is within the range of the warning
value. The grouting reinforcement measures can meet the safety factor
requirement of the foundation pit construction of the Juzizhou subway
station under high water levels.
As shown in Fig. 13, considering the deformation mode of the dia-
phragm wall at different stages of excavation, the results of the numerical
simulation are basically consistent with the measured results. From the
numerical results, the measured maximum horizontal displacement is
24.85 mm, but the maximum horizontal displacement calculated is 29.5
mm. The horizontal displacement values in the simulation are slightly
larger than the measured values. The relative error between the
measured results and the calculated results is less than 20%. From the
engineering point of view, the relative error is also within the allowable
range, so that the simulation results and experimental results are
consistent, which illustrates the validity of the calculation model.
7. Conclusions
(1) Water level changes outside the Juzizhou subway station have a
significant impact on the stability of the foundation pit. The
warning water level of the deep foundation pit without rein-
forcement is 10 m below the ground surface. There is a great risk
in the foundation pit construction exceeding the warning water
level. It is necessary to adopt soil-reinforcement measures outside
the pit to strengthen the support system.
(2) There are reasonable limits for the width and depth of the soil
reinforcement. For the Juzizhou station pit, the reasonable value
of the width is 6 m, and the depth is 17.5 m–20 m. The change in
the reinforcement depth of the pit has a greater influence on the
stability of the foundation pit than the change in reinforcement
width. The engineering grouting design should first control the
Fig. 13. Comparison and analysis of the reinforcement simulation and site-measurement results. (a) Simulation results, (b) Site-measurement results.
W. Wang et al. Heliyon 5 (2019) e02836
9
grouting reinforcement depth and then control the reinforcement
width as the supplement.
(3) The optimized grouting parameters, which can meet the safety
factor requirement for the construction of the foundation pit
under high water levels, are applied in the reinforcement of the
Juzizhou subway station pit, and the grout injection volume can
be reduced by 45% compared with the original grouting design.
Declarations
Author contribution statement
Wei Wang: Conceived and designed the experiments; Analyzed and
interpreted the data; Wrote the paper.
Zhao Han: Analyzed and interpreted the data; Wrote the paper.
Jun Deng: Conceived and designed the experiments; Performed the
experiments.
Xinyuan Zhang &Yanfei Zhang:Performed the experiments;
Contributed reagents, materials, analysis tools or data.
Funding statement
This work was supported by the Hunan Natural Science Foundation,
China (2018JJ2519), the China Railway No.5 Engineering Group Co. Ltd
(2010JK3173), and the China Railway Tunnel Group Co., Ltd. Hangzhou
Zizhi Tunnel Engineering Civil Engineering VI Project Management
Department (20140018012).
Competing interest statement
The authors declare no conflict of interest.
Additional information
No additional information is available for this paper.
References
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