Abstract
In order to solve the problem of differential settlement of pavement after the backfilling of urban underground comprehensive pipe corridors based on a pipe gallery project in Xiong’an New Area, the mechanical calculation model of the pipe gallery foundation pit backfilling process was simulated by ABAQUS, and the pavement settlement and the change of the pipe gallery model under different construction conditions were simulated and verified. The results show that many external environmental factors have an impact on pavement settlement, and methods such as increasing the compaction degree of backfill soil and the slope of the foundation pit, reducing the width of the pit floor, and controlling the vehicle load after work can avoid the pavement damage caused by uneven settlement.
1. Introduction
An urban underground utility tunnel refers to a modern and intensive urban infrastructure formed by setting more than two kinds of urban pipelines in the same underground space. The underground utility tunnel of the city will organically and intensively lay all kinds of municipal pipelines such as water supply, drainage, telecommunications, electricity, gas, and heat in the same tunnel to realize centralized management and maintenance of municipal pipelines [1]. When the pipe trench is backfilled after the acceptance of the pipe gallery structure, the backfill material is generally selected as suitable excavated soil or qualified external transportation materials, and a certain proportion of gray soil is added according to the site situation to improve the strength, but under the influence of the upper load or backfilling process, uneven settlement of the surface is prone to occur. If the uneven settlement is not effectively controlled, the road surface will collapse at the later stage, resulting in ground cracks, which will lead to unpredictable economic losses and potential safety hazards. Therefore, it is necessary to study the settlement characteristics and its influencing factors of the backfill above the pipe gallery, and the problem of uneven settlement caused by it needs to be solved urgently.
Therefore, many scholars at home and abroad have done a lot of research based on engineering examples. Arsyad et al. [2] studied the influence of geotextile in backfill soil on the stability of embankment by field monitoring and numerical calculation. Zhong et al. [3] carried out the dynamic triaxial test of backfilling with earth-rock mixture under the cyclic load and studied the dynamic characteristics of earth-rock mixture under the cyclic load. Ardah et al. [4] evaluated the performance according to the strain, lateral deformation, uplift, and lateral pressure at the intersection of approach bridge slabs. Khosrojerdi et al. [5] studied the influence of backfill constitutive behavior on the vertical and horizontal deformation of geosynthetic reinforced soil (GRS) pier under static axial load by numerical calculation and analysis method. Zhao et al. [6], aiming at utility tunnel of Beijing Daxing New Airport, discussed the influence of different backfill materials on pavement settlement through numerical simulation and optimized the construction method and backfill scheme. Shen and Ruan [7] aimed at the municipal road construction in soft soil stratum, established a numerical model by ABAQUS to calculate, and summarized the final settlement of typical subgrade. Zhou et al. [8] took the utility tunnel project in a lake area as an example, carried out the simulation of the construction process of pipe gallery and site soil, and put forward three factors that affect the settlement of the base as follows: the buried depth of utility tunnel, the gravel cushion at the bottom of the ditch, and the compactness of the backfill soil at the back of the ditch. Huang et al. [9] based on the long-term monitoring of the pipe gallery and surrounding roads studied the settlement characteristics of the pipe gallery foundation pit in soft soil and discussed and verified the rationality of the existing design scheme. Chen [10] determined the optimal combination of parameters for imperfect trench mounting methods based on modern finite element analysis of deep buried concrete box culverts. Zhang [11] verified the vertical earth pressure coefficient through numerical analysis results and experimental results under different working conditions, and this equation is suitable for estimating the vertical earth pressure of existing or newly designed HFCCCTs. Guo et al. [12] studied the hazard of earthquake ground faults as continuous buried pipelines through numerical simulation, and the results showed that the pipeline behavior was nominally not affected by the soil moisture content and the fault offset rate. Kong et al. [13] derived a semianalytical solution for the spatial distribution of horizontal transverse ground deformation (HTGD) of buried pipelines to understand its influence on the behavior of buried pipelines. Huang et al. [14] studied the upper bound plastic analysis of partially embedded pipes on undrained soil, and the results showed that the resistance to invasiveness varied nonlinearly with embedded and vertical loading. Watson and Crick [15] evaluated the uplifted soil-pipe interactions in granular soils using a two-dimensional finite element (FE) continuum model to derive hyperbolic and bilinear models of vertical upward forces and displacements. Kianian and Shiri [16] studied the influence of lateral pipe-backfill-trench interaction on soil failure mechanism and lateral soil resistance under shallow and deep buried conditions, and the results can improve the prediction of the lateral response of buried pipelines in the near future. Kim and Yoo [17] determined the optimal combination of parameters for imperfect trench mounting methods based on modern finite element analysis of deep buried concrete box culverts.
To sum up, in view of the technical difficulties in the settlement control of foundation pit backfill in utility tunnel, comprehensively applying the intelligent monitoring and numerical calculation analysis method of the Internet of Things of onsite backfill soil layer settlement and deformation and carrying out the research on the differential settlement and deformation law of soil layer in the layered backfill process can provide certain theoretical guidance for similar process pipe gallery backfill engineering, which has important engineering significance and reference.
2. Project Overview
Xiong’an New Area startup area is east to the middle of the fifth group in the startup area, south to Baiyangdian, west to the third group in the startup area, and north to Rongwu Expressway, with a planned area of 38 square kilometers and a planned construction land of 26 square kilometers. This project is the utility tunnel project of Xiong’an New Area startup area EA1, EA2, NA10, EC2-1, and EC3-1. Among them, EA1 is about 1.77 km long and has the following three cabins: power cabin, comprehensive cabin, and gas cabin. The design service life of the structure is 100 years. The standard depth of frozen soil in this area is 0.6 m, which belongs to seasonal frozen soil. Good soil mixed with ash (6%) is used for backfill of pipe gallery foundation pit. The thickness of each layer is not more than 250 mm by manual compaction and 300 mm by mechanical compaction. The compaction coefficient should be ≥0.93, and the compaction coefficient should meet the requirements of road design. The groundwater aquifer of the proposed project site is caused by alluvial and diluvial deposits, which are silt and sand layers, and the groundwater mainly comes from atmospheric precipitation. The special rock and soil distributed in the proposed site are mainly artificial fill layers, mainly plain fill ①-② layers. The general thickness of this large layer revealed by drilling is 0.30∼1.30 m. This layer of soil has uneven spatial distribution, uneven soil quality and messy components, and poor engineering properties. It has the characteristics of large deformation, high compressibility, low strength, relatively loose structure, poor self-stability, and so on, which has adverse effects on foundation uniformity, foundation pit stability of open-cut grooving construction, and design and construction of the supporting structure. The schematic diagram of foundation pit backfilling is shown in Figure 1.
In order to ensure the safety and structural stability of the foundation pit in the utility tunnel and meet the requirements of pavement design after subgrade backfilling, the field test and numerical calculation analysis method are comprehensively adopted to explore the deformation characteristics of the backfill soil layer of the foundation pit in the utility tunnel, which provides an important theoretical basis for solving the technical problems such as the safety, structural stability, and backfill settlement of the foundation pit in the utility tunnel, i.e., Xiong’an startup area.
3. Monitoring and Analysis of Settlement of Backfill Soil Layer of Deep Foundation Pit in Utility Tunnel
By embedding strain sensors in the field, the settlement and deformation of the backfill soil are monitored in real time in the whole process, and the settlement characteristics of backfill soil in the foundation pit of the pipe gallery are analyzed. In order to verify the rationality of the numerical model, it is also compared with the numerical simulation results of the pipe gallery foundation pit backfilling stage.
3.1. Implementation Plan
3.1.1. Monitoring Purpose
On-the-spot backfill soil settlement monitoring can monitor the backfill soil subgrade settlement of municipal pipe gallery, reveal the deformation law of backfill soil and the influence effect of differential settlement, and then predict whether the settlement is stable and whether the postconstruction settlement meets the specification requirements so as to guide the schedule of the later permanent road construction. Also, they can provide timely feedback information for municipal road construction so as to achieve information construction. On the other hand, designers can modify and improve the original design scheme through the measured results so as to ensure the safety and smooth progress of road network construction.
3.1.2. Monitoring Plan
The monitoring of settlement and deformation of municipal pipe gallery backfill mainly includes differential settlement between the top of pipe gallery and the backfill around pipe gallery and differential settlement between backfill and the original subgrade on both sides. The monitoring arrangement is shown in Figure 2, and in order to better display the location of the monitoring point, the proportion of the foundation pit in the picture has been adjusted appropriately.
The monitoring instrument adopts the XHX-21XX series-connected tube vibrating wire static level, which is composed of a plurality of high-precision liquid level sensor positioning substrates and connected tubes. It is a surface-mounted settlement deflection observer, which measures the vertical deformation of each level meter relative to the reference point. It is widely used in bridge deflection, pavement alignment, or dam alignment settlement measurement. The specific parameters of the monitor are shown in Table 1.
Figure 3 shows a photograph taken during the installation of monitoring point no. 6 on the middle floor. After each layer of soil is backfilled, sensors are installed, all sensors are connected to the solar base station through data lines, and finally the data are successfully output to realize real-time monitoring of soil settlement at different depths. The frequency of monitoring is 1-2 times a day within three months after backfilling construction. After backfilling for three months, gradually reduce the frequency of monitoring. Based on similar projects, the monitoring period is estimated to be three years.
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3.2. Monitoring Data Analysis
3.2.1. Initial Stage Analysis
Figure 4 shows the settlement and deformation analysis diagram at the initial stage. Regarding the fluctuation of data on sensor no. 2, it is speculated that this phenomenon is related to rainfall after the installation of the instrument. It can be seen from the figure that (1) the settlement characteristics of backfill soil in the monitoring section are related to the depth of the soil layer, and the settlement of the soil layer increases with the increase of depth; with the increase of fill height, the settlement will also increase, that is, the settlement of surface soil is the largest and the settlement of bottom soil is the smallest. (2) Influenced by the three construction backfilling stages from October 28th to November 1st, November 7th to November 9th, and November 17th to November 18th, the settlement curves of the monitoring points all changed obviously. The steeper the broken line, the faster the settlement development. (3) The backfill soil layer above the pipe gallery and the backfill soil layer on both sides have obvious differential settlement, and the maximum settlement is about 25 mm.
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3.2.2. Early Warning Analysis
Figures 5–7, respectively, show the settlement curves of three early warning situations.
It can be seen from Figure 5 that (1) the settlement of No. 1 monitoring point on the side of the utility tunnel is unstable and the degree of settlement is large while that of No. 2 monitoring point on the top of the utility tunnel is relatively stable. After December 28, 2021, the deformation curve of No. 1 monitoring point changed obviously, showing a large amplitude of unstable fluctuation, and (2) from December 17 to January 8, 2021, the whole period of settlement is relatively stable and the amount of settlement is small. No. 10 monitoring station keeps a relatively fast settlement rate in the whole period, and the early settlement rates of No. 8 and No. 9 monitoring stations are slow.
As can be seen from Figure 6, the curve of No. 3 monitoring point fluctuates from June 30 to July 25, 2022, but its fluctuation range gradually increases, and it fluctuates greatly on July 30 and then returns to the normal fluctuation range.
As can be seen from Figure 7, the fluctuation range of the settlement of No. 7 and No. 10 monitoring points on the utility tunnel side is large, which is in an unstable state, and the fluctuation range gradually slows down after November 2, 2022.
3.2.3. Analysis of General Change Trend
Figure 8 shows the settlement characteristics within 90 days after foundation pit backfilling. It can be seen from the figure that (1) the settlement difference developed rapidly within three days after the backfilling of the bottom soil and then the settlement development showed a slow and stable trend, in which the No. 1 measuring point maintained a faster settlement rate than No. 2 measuring point; (2) the overall settlement of backfill soil can be divided into two stages as follows: rapid settlement period and stable period; the rapid settlement period is within 26 days after the completion of backfill work, and the development speed of settlement slows down after 26 days; and (3) the backfill transport path is close to the location of monitoring point 10, which produces a large settlement under the crushing of the filling vehicle, so that monitoring point 10 has increased significantly after 40 days.
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4. Numerical Calculation and Analysis of the Settlement of Backfill Soil Layer in Utility Tunnel Deep Foundation Pit
4.1. Numerical Calculation Model and Analysis Method
In order to better simulate the strain influence of the pipe gallery structure with backfill, a representative paragraph of EA1 section in Xiong’an New Area startup area is taken, and a simplified model is established by using ABAQUS finite element analysis software. The model consists of natural soil, utility tunnel, and backfill, of which the main calculation component is backfill, which is divided into three layers, namely, high, middle, and low according to the site construction process. The model does not consider the influence of groundwater, and the backfill uses the Mohr–Coulomb model. The overall model is shown in Figure 9. The model is 80 m long (Z axis), 180.5 m wide (X axis), and 28.5 m high (Y axis).
In order to better simulate the settlement of soil, the grid is encrypted by mainly analyzing the backfill part, in which the grid density of the backfill part is 0.6 m, the density of the natural soil part is increased to 3 m, the total number of grids is 132770, and the element type used by all the components is C3D8R. Regarding boundary conditions, constrain the Z displacement on the front and rear sides of the model, constrain the X displacement on the left and right sides of the model, and constrain the displacement in the X, Y, and Z directions at the bottom of the model.
In order to better simulate the settlement change of backfill soil, numerical simulation is carried out according to the Geological Survey Report of Section EA1 of Startup Area as the main parameter of soil. As the main research object is backfill soil and its settlement changes during backfill construction, in order to improve the calculation efficiency and make the result more stable, the pipe gallery structure is appropriately simplified in the attribute setting and its attribute is set as the elastic structure. The main parameters of the soil mass refer to the “Geological Survey Report of EA1 Section of the Start-up Area,” and the specific parameters are shown in Table 2.
The numerical model of backfill soil is divided into upper, middle, and lower layers. In order to better simulate the construction state, the backfill soil model is vertically divided into five zones, backfilled in a ladder manner from front to back, a total of 7 construction sections as follows: the first section backfills the bottom layer of zone 1, the second construction section will backfill the middle layer of zone 1 and the bottom layer of zone 2, the third section backfills the top layer of zone 1, the middle layer of zone 2, the bottom layer of zone 3, and so on, and 8 analysis steps are set in the numerical simulation to simulate the backfill effect of on-site construction. In step 1, the backfill soil is set to “fail” by model change and then the backfilled soil is set to “restart” in each step so as to simulate the soil backfilling during construction, and in order to reduce the influence of setting boundary conditions on the simulation results, the longitudinal section of the middle of the third section is taken as the result cloud.
Taking the settlement data of the bottom, middle, and top planes of the cloud as the figure, the settlement data of each monitoring point are brought into the figure to obtain the point data in Figure 10. It can be seen that as of 90 days after construction, the difference between the measured settlement data and the numerical simulation results is less than 15%, and the numerical simulation results have a good fitting effect, which is feasible and can meet the research requirements of backfilling engineering.
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4.2. Analysis of Numerical Calculation Results
4.2.1. The Influence of the Thickness of Covered Soil on the Settlement Characteristics of Filled Soil
According to the site backfilling situation, set different thicknesses of the covered soil on the gallery roof and get the vertical displacement nephogram of the filled soil model as shown in Figure 11. It can be seen from the figure that, with the decrease of the covering soil thickness at the top of the corridor, the settlement of the fill at the beginning of the path, that is, at the top of the pipe corridor, decreases. On the side of the pipe gallery, the greater the depth, the smaller the influence of the thickness of the covered soil on the settlement curve. From the analysis of Figure 11, it can be seen that the thickness of the covering soil at the top of the pipe gallery mainly affects the differential settlement at the side wall of the pipe gallery. The greater the thickness of the covering soil at the top of the pipe gallery, the smaller the differential settlement at this place and the smaller the influence of the pipe gallery structure.
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4.2.2. Influence of Compaction Degree of Backfill Soil on Settlement Characteristics of Backfill Soil
By setting different parameters for the backfill soil, the settlement of the fill soil under different compaction conditions can be simulated, and the data are from literature [6], and the soil parameters under different compaction degrees are shown in Table 3.
When different compression modulus Es is set for the filled soil in area I, the displacement vector of the whole filled soil area, the vertical displacement nephogram of the model, and the settlement curves of the filled soil at different depths within the load range are shown in Figure 12. As can be seen from the figure, the displacement mode of the fill near the foundation pit slope is because the rigidity of the rock foundation is much greater than that of the fill and the settlement deformation of the fill under the load is greater than that of the rock foundation. The deformation of the fill near the foundation pit slope near the ground is the downward sliding along the foundation pit slope. With the increase of depth, the width of backfill surface becomes smaller and the constraint of the foundation pit slope on filling soil increases, which leads to the deformation of filling soil mainly in the vertical direction. The settlement characteristics of each compaction degree are similar. With the increase of the compaction degree, the settlement of pavement decreases significantly. Proper compaction after the completion of backfill construction has a significant effect on reducing the uneven settlement of pavement in the later period.
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Figure 13 shows the comparison of settlement curves of different compaction degrees at different depths. It can be seen from the figure that when the compaction degree of fill increases from 87% to 90%, the overall form of settlement curve changes little, and the maximum settlement is located outside the side wall of pipe gallery, and the maximum settlement of fill decreases from −45.37 mm to 25.42 mm within the load range. When the compaction degree of fill increases from 90% to 93%, the overall form of the settlement curve changes little, and the maximum settlement is located outside the side wall of pipe gallery. The maximum settlement of fill within the load range decreases from 25.42 mm to 25.13 mm, but the settlement at the road boundary changes greatly and its settlement decreases from 12.44 mm to 8.98 mm. When the degree of compaction increases from 87% to 90%, the maximum settlement decreases the most. When the degree of compaction is greater than 90%, the maximum settlement decreases only slightly, which shows that although increasing the compression modulus of filler can reduce the settlement of filled soil, it will reach the limit when it increases to a certain extent.
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4.2.3. The Influence of the Bottom Width of Foundation Pit on the Settlement Characteristics of Filled Soil
When different widths B are set at the bottom of the foundation pit, the vertical displacement nephogram of the whole filled area model and the settlement curves at different depths of filled soil within the load range are shown in Figures 14 and 15. It can be seen from the figure that, with the increase of the width of the base, the settlement at the starting point of the path in each depth range is basically unchanged, while the settlement at the end point and the maximum value of the settlement increase with the increase of the depth, and the position of the maximum value of the settlement gradually moves to the left. The end point of each settlement curve is located on the slope of the foundation pit, so the path length increases with the width of the bottom of the foundation pit, the settlement values at the start and end points of the path are unchanged, and the settlement at the same position on the path increases with the width of the base. It can be seen from the settlement curves of different base widths at the depth of 4.5 m that the difference of settlement velocity of fill at the side wall of pipe gallery caused by pipe gallery structure becomes larger. The width of the bottom of the foundation pit mainly affects the settlement of the fill near the slope of the foundation pit. With the increase of the width of the foundation, the restriction of the foundation pit slope on the settlement of the fill in the foundation pit decreases, resulting in the increase of the maximum settlement and the deviation of its position from the pipe gallery.
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4.2.4. The Influence of Slope Gradient of Foundation Pit on Settlement Characteristics of Filled Soil
Figures 16 and 17 show the nephogram and curve of soil layer settlement with different slopes of foundation pit, respectively. I is the ratio of width to height of the foundation pit slope. It can be seen from the figure that as the slope of the foundation pit becomes slower, the maximum settlement of the pavement at each depth will also increase, while the maximum settlement at the end point will decrease. Within the depth range of h < 3.0 m, the settlement at the starting point of the path is basically unchanged, while the settlement at the end point and the maximum value of the settlement become larger, and the position of the maximum value of the settlement gradually moves to the left. At the beginning of the settlement curve in the deeper position (h ≥ 3.0 m), there is obvious settlement change, which mainly shows that the larger the slope is, the larger the settlement at the beginning is, and the length of the path increases with the width of the foundation pit bottom. The smaller the slope is, the smaller the settlement difference between the starting point and the end point is. The influence of slope change of the foundation pit on the settlement characteristics of filled soil is similar to that of the base width. When the slope becomes slower, the constraint of the foundation pit on filled soil becomes weaker, and the settlement of filled soil near the slope becomes larger, resulting in the maximum settlement of filled soil and its position deviating from the pipe gallery structure.
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4.2.5. The Influence of Vehicle Load on the Settlement Characteristics of Filled Soil
When the adjacent loads of different sizes are set, the vertical displacement nephogram and the settlement curve of each depth of the filled soil model are shown in Figures 18 and 19. It can be seen from the figure that the (1) under the condition of constant vehicle speed, with the increase of axle load, the subgrade settlement gradually increases, and the maximum settlement values corresponding to axle loads of 100, 125, 150, 185, and 200 kN are 33.3, 35.9, 38. 9, 40.7, and 43.1 mm, respectively, 0.5 mm outside the vehicle load. (2) The main influence of the vehicle load on road settlement lies in the middle of the road. With the increase of the horizontal distance from the load center, the settlement decreases but the decrease is not significant. When the driving speed is constant, the subgrade settlement curve under various axle loads along the vehicle driving area is consistent, and the settlement curve presents a V-shaped single-peak distribution. With the increase of depth, the smaller the influence of vehicle load on settlement, the greater the influence of pipe gallery structure on settlement.
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In view of the factors affecting pavement settlement, scholars put forward different research directions; literature [6] discusses the influence of different backfill materials, literature [8] studies the influence of different backfill depths and trench bottom bedding, and this paper studies the influence of backfill soil compaction, pit floor width, foundation pit slope, vehicle load, and other factors on pavement settlement through numerical simulation and finally obtains the corresponding results. Literature [9] studies the influence of different elastic modulus of fill and concludes that with the increase of the elastic modulus of fill, the reduction efficiency of pavement settlement slows down, which is similar to this paper, but the concept of compaction introduced in this paper can more truly reflect the filling situation in construction than the elastic modulus. In addition to the factors mentioned in this paper, the size of the pipe gallery and the structural deformation also have an impact on the settlement of the pavement, which needs to be further studied.
5. Conclusion
Aiming at the technical problems of foundation pit backfill settlement control in the utility tunnel, the Internet of Things intelligent monitoring and numerical calculation analysis method of onsite backfill soil settlement deformation is comprehensively used to study the differential settlement deformation law of soil layer under layered backfill process. The main conclusions are as follows:(1)The intelligent monitoring method of Internet of Things for the settlement and deformation of the backfill soil layer on site can effectively warn and analyze the real-time state and overall trend of the settlement change of the backfill soil layer and can provide effective deformation control basis for site construction.(2)The limiting conditions of foundation pit filling include the compaction degree of backfill soil, the thickness of covered soil on the top of gallery, the width of basement, the slope of foundation pit, and the vehicle load. The main reasons for the differential settlement of foundation pit backfill ground are the difference of backfill compaction, slope gradient, and vehicle load. Theoretically, the pavement settlement caused by vehicle load in the later period has little effect on the pipe gallery structure.(3)The maximum settlement of the filled road surface decreases with the increase of backfill compaction, increases with the increase of the base width and adjacent load, decreases with the increase of corridor top covering soil thickness and slope of foundation pit, and increases with the increase of vehicle load. Therefore, it is necessary to put forward the construction measures to reduce the uneven settlement of the pavement, including controlling the width of the bottom of the foundation pit, the slope of the foundation pit, and sufficient backfilling and rolling.4.For the late pavement load, under the action of vehicle load after construction, the settlement of subgrade will increase by 32.1%∼71.0%, and the vehicle load will further compact the backfill and then settle. It can be seen that the influence of the vehicle load on the total settlement of subgrade at various depths decreases with the increase of the compaction degree.5.Compared with the filler in other literature, the 6% lime soil used in this project has a smaller settlement after backfilling, and the numerical simulation shows that the compaction degree of the filler is greater than 90% through the rolling construction, which can effectively reduce the uneven settlement of the later pavement.
Data Availability
The monitoring instrument adopts Internet cloud technology, which can update the data in real time on the website, but the account password is required to log in to view the monitoring data of this study, and if the account password is needed, please ask the first author by email, and the data URL is https://project.zhiwucloud.com/signin?p=664b11d0-7572-4bdc-baad-125d87215fe3.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
Acknowledgments
This work was supported by the Jinan High-Value Patent Incubation Project (GJZ20201008-2).