Settlement Characteristics of Urban Underground Comprehensive Pipe Gallery under Different Backfill Conditions

In order to solve the problem of diferential settlement of pavement after the backflling 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 backflling process was simulated by ABAQUS, and the pavement settlement and the change of the pipe gallery model under diferent construction conditions were simulated and verifed. Te results show that many external environmental factors have an impact on pavement settlement, and methods such as increasing the compaction degree of backfll soil and the slope of the foundation pit, reducing the width of the pit foor, and controlling the vehicle load after work can avoid the pavement damage caused by uneven settlement.


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.Te 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 backflled after the acceptance of the pipe gallery structure, the backfll material is generally selected as suitable excavated soil or qualifed external transportation materials, and a certain proportion of gray soil is added according to the site situation to improve the strength, but under the infuence of the upper load or backflling process, uneven settlement of the surface is prone to occur.If the uneven settlement is not efectively 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.Terefore, it is necessary to study the settlement characteristics and its infuencing factors of the backfll above the pipe gallery, and the problem of uneven settlement caused by it needs to be solved urgently.
Terefore, many scholars at home and abroad have done a lot of research based on engineering examples.Arsyad et al. [2] studied the infuence of geotextile in backfll soil on the stability of embankment by feld monitoring and numerical calculation.Zhong et al. [3] carried out the dynamic triaxial test of backflling 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 infuence of backfll 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 infuence of diferent backfll materials on pavement settlement through numerical simulation and optimized the construction method and backfll 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 fnal 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 afect 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 backfll soil at the back of the ditch.Huang et al. [9] based on the longterm monitoring of the pipe gallery and surrounding roads studied the settlement characteristics of the pipe gallery foundation pit in soft soil and discussed and verifed the rationality of the existing design scheme.Chen [10] determined the optimal combination of parameters for imperfect trench mounting methods based on modern fnite element analysis of deep buried concrete box culverts.Zhang [11] verifed the vertical earth pressure coefcient through numerical analysis results and experimental results under diferent 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 afected by the soil moisture content and the fault ofset 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 infuence 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 fnite element (FE) continuum model to derive hyperbolic and bilinear models of vertical upward forces and displacements.Kianian and Shiri [16] studied the infuence of lateral pipe-backfll-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 fnite element analysis of deep buried concrete box culverts.
To sum up, in view of the technical difculties in the settlement control of foundation pit backfll in utility tunnel, comprehensively applying the intelligent monitoring and numerical calculation analysis method of the Internet of Tings of onsite backfll soil layer settlement and deformation and carrying out the research on the diferential settlement and deformation law of soil layer in the layered backfll process can provide certain theoretical guidance for similar process pipe gallery backfll engineering, which has important engineering signifcance and reference.

Project Overview
Xiong'an New Area startup area is east to the middle of the ffth 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.Tis 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.Te design service life of the structure is 100 years.Te 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 backfll of pipe gallery foundation pit.Te thickness of each layer is not more than 250 mm by manual compaction and 300 mm by mechanical compaction.Te compaction coefcient should be ≥0.93, and the compaction coefcient should meet the requirements of road design.Te 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.Te special rock and soil distributed in the proposed site are mainly artifcial fll layers, mainly plain fll ①-② layers.Te general thickness of this large layer revealed by drilling is 0.30∼1.30m.Tis 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 efects on foundation uniformity, foundation pit stability of open-cut grooving construction, and design and construction of the supporting structure.Te schematic diagram of foundation pit backflling 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 backflling, the feld test and numerical calculation analysis method are comprehensively adopted to explore the deformation characteristics of the backfll 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 backfll settlement of the foundation pit in the utility tunnel, i.e., Xiong'an startup area.

Monitoring and Analysis of Settlement of Backfill Soil Layer of Deep Foundation Pit in Utility Tunnel
By embedding strain sensors in the feld, the settlement and deformation of the backfll soil are monitored in real time in the whole process, and the settlement characteristics of backfll 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 backflling stage.

Implementation Plan
3.1.1.Monitoring Purpose.On-the-spot backfll soil settlement monitoring can monitor the backfll soil subgrade settlement of municipal pipe gallery, reveal the deformation 2 Advances in Materials Science and Engineering law of backfll soil and the infuence efect of diferential settlement, and then predict whether the settlement is stable and whether the postconstruction settlement meets the specifcation 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.

Monitoring Plan.
Te monitoring of settlement and deformation of municipal pipe gallery backfll mainly includes diferential settlement between the top of pipe gallery and the backfll around pipe gallery and diferential settlement between backfll and the original subgrade on both sides.Te 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.
Te monitoring instrument adopts the XHX-21XX seriesconnected 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 defection observer, which measures the vertical deformation of each level meter relative to the reference point.It is widely used in bridge defection, pavement alignment, or dam alignment settlement measurement.Te specifc 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 foor.After each layer of soil is backflled, sensors are installed, all sensors are connected to the solar base station through data lines, and fnally the data are successfully output to realize real-time monitoring of soil settlement at diferent depths.Te frequency of monitoring is 1-2 times a day within three months after backflling construction.After backflling for three months, gradually reduce the frequency of monitoring.Based on similar projects, the monitoring period is estimated to be three years.

Initial Stage Analysis.
Figure 4 shows the settlement and deformation analysis diagram at the initial stage.Regarding the fuctuation 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 fgure that (1) the settlement characteristics of backfll 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 fll 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) Infuenced by the three construction backflling 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.Te steeper the broken line, the faster the settlement development.(3) Te backfll soil layer above the pipe gallery and the backfll soil layer on both sides have obvious diferential settlement, and the maximum settlement is about 25 mm.

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 fuctuation, 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 fuctuates from June 30 to July 25, 2022, but its fuctuation range gradually increases, and it fuctuates greatly on July 30 and then returns to the normal fuctuation range.
As can be seen from Figure 7, the fuctuation 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 fuctuation range gradually slows down after November 2, 2022.Advances in Materials Science and Engineering settlement diference developed rapidly within three days after the backflling 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 backfll 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 backfll work, and the development speed of settlement slows down after 26 days; and (3) the backfll transport path is close to the location of monitoring point 10, which produces a large settlement under the crushing of the flling vehicle, so that monitoring point 10 has increased signifcantly after 40 days.

Numerical Calculation Model and Analysis Method.
In order to better simulate the strain infuence of the pipe gallery structure with backfll, a representative paragraph of EA1 section in Xiong'an New Area startup area is taken, and a simplifed model is established by using ABAQUS fnite element analysis software.Te model consists of natural soil, utility tunnel, and backfll, of which the main calculation component is backfll, which is divided into three layers, namely, high, middle, and low according to the site construction process.Te model does not consider the infuence of groundwater, and the backfll uses the Mohr-Coulomb model.Te overall model is shown in Figure 9. Te 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 backfll part, in which the grid density of the backfll 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 backfll 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 backfll soil and its settlement changes during backfll construction, in order to improve the calculation efciency and make the result more stable, the pipe gallery structure is appropriately simplifed in the attribute setting and its attribute is set as the elastic structure.Te main parameters of the soil mass refer to the "Geological Survey Report of EA1 Section of the Start-up Area," and the specifc parameters are shown in Table 2.
Te numerical model of backfll soil is divided into upper, middle, and lower layers.In order to better simulate the construction state, the backfll soil model is vertically divided into fve zones, backflled in a ladder manner from front to back, a total of 7 construction sections as follows: the frst section backflls the bottom layer of zone 1, the second construction section will backfll the middle layer of zone 1 and the bottom layer of zone 2, the third section backflls 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 backfll efect of on-site   6 Advances in Materials Science and Engineering construction.In step 1, the backfll soil is set to "fail" by model change and then the backflled soil is set to "restart" in each step so as to simulate the soil backflling during construction, and in order to reduce the infuence 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 fgure, the settlement data of each monitoring point are brought into the fgure to obtain the point data in Figure 10.It can be seen that as of 90 days after construction, the diference between the measured settlement data and the numerical simulation results is less than 15%, and the numerical simulation results have a good ftting efect, which is feasible and can meet the research requirements of backflling engineering.Advances in Materials Science and Engineering   It can be seen from the fgure that, with the decrease of the covering soil thickness at the top of the corridor, the settlement of the fll 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 infuence 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 afects the diferential settlement at the side wall of the pipe gallery.Te greater the thickness of the covering soil at the top of the pipe gallery, the smaller the diferential settlement at this place and the smaller the infuence of the pipe gallery structure.

Infuence of Compaction Degree of Backfll Soil on
Settlement Characteristics of Backfll Soil.By setting different parameters for the backfll soil, the settlement of the fll soil under diferent compaction conditions can be simulated, and the data are from literature [6], and the soil parameters under diferent compaction degrees are shown in Table 3.When diferent compression modulus Es is set for the flled soil in area I, the displacement vector of the whole flled soil area, the vertical displacement nephogram of the model, and the settlement curves of the flled soil at diferent depths within the load range are shown in Figure 12.As can be seen from the fgure, the displacement mode of the fll near the foundation pit slope is because the rigidity of the rock foundation is much greater than that of the fll and the settlement deformation of the fll under the load is greater than that of the rock foundation.Te deformation of the fll 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 backfll surface becomes smaller and the constraint of the foundation pit slope on flling soil increases, which leads to the deformation of flling soil mainly in the vertical direction.Te settlement characteristics of each compaction degree are similar.With the increase of the compaction degree, the settlement of pavement decreases signifcantly.Proper compaction after the completion of backfll construction has a signifcant efect on reducing the uneven settlement of pavement in the later period.
Figure 13 shows the comparison of settlement curves of diferent compaction degrees at diferent depths.It can be seen from the fgure that when the compaction degree of fll 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 fll decreases from −45.37 mm to 25.42 mm within the load range.When the compaction degree of fll 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.Te maximum settlement of fll 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 fller can reduce the settlement of flled soil, it will reach the limit when it increases to a certain extent.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 diferent base widths at the depth of 4.5 m that the diference of settlement velocity of fll at the side wall of pipe gallery caused by pipe gallery structure becomes larger.Te width of the bottom of the foundation pit mainly afects the settlement of the fll 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 fll in the foundation pit decreases, resulting in the increase of the maximum settlement and the deviation of its position from the pipe gallery.

Te Infuence 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 diferent slopes of foundation pit, respectively.I is the ratio of width to height of the foundation pit slope.It can be seen from the fgure 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.Te smaller the slope is, the smaller the settlement diference between the starting point and the end point is.Te infuence of slope change of the foundation (2) Te main infuence 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 signifcant.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 infuence of vehicle load on settlement, the greater the infuence of pipe gallery structure on settlement.
In view of the factors afecting pavement settlement, scholars put forward diferent research directions; literature [6] discusses the infuence of diferent backfll materials, literature [8] studies the infuence of diferent backfll depths and trench bottom bedding, and this paper studies the infuence of backfll soil compaction, pit foor width, foundation pit slope, vehicle load, and other factors on pavement settlement through numerical simulation and fnally obtains the corresponding results.Literature [9] studies the infuence of diferent elastic modulus of fll and concludes that with the increase of the elastic modulus of fll, the reduction efciency of pavement settlement slows down, which is similar to this paper, but the concept of compaction introduced in this paper can more truly refect the flling 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.Advances in Materials Science and Engineering

Conclusion
Aiming at the technical problems of foundation pit backfll settlement control in the utility tunnel, the Internet of Tings intelligent monitoring and numerical calculation analysis method of onsite backfll soil settlement deformation is comprehensively used to study the diferential settlement deformation law of soil layer under layered backfll process.Te main conclusions are as follows: (1) Te intelligent monitoring method of Internet of Tings for the settlement and deformation of the backfll soil layer on site can efectively warn and analyze the realtime state and overall trend of the settlement change of the backfll soil layer and can provide efective deformation control basis for site construction.(2) Te limiting conditions of foundation pit flling include the compaction degree of backfll soil, the thickness of covered soil on the top of gallery, the width of basement, the slope of foundation pit, and the vehicle load.Te main reasons for the diferential settlement of foundation pit backfll ground are the diference of backfll compaction, slope gradient, and vehicle load.Teoretically, the pavement settlement caused by vehicle load in the later period has little efect on the pipe gallery structure.(3) Te maximum settlement of the flled road surface decreases with the increase of backfll 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.Terefore, 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 sufcient backflling 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 backfll and then settle.It can be seen that the infuence 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 fller in other literature, the 6% lime soil used in this project has a smaller settlement after backflling, and the numerical simulation shows that the compaction degree of the fller is greater than 90% through the rolling construction, which can efectively reduce the uneven settlement of the later pavement.

igure 1 :
Figure 8  shows the settlement characteristics within 90 days after foundation pit backflling.It can be seen from the fgure that (1) the Schematic diagram of backflling.

Figure 9 :
Figure 9: Assembly model of pipe gallery foundation pit backfll.

Figure 10 :Figure 11 :
Figure 10: Comparison chart of simulated data and monitoring data.(a) Top contrast.(b) Middle contrast.(c) Bottom contrast.

Table 3 :
Calculation parameters of soil fll with diferent compaction degrees.