Research on Dynamic Response and Construction Safety Countermeasures of an Adjacent Existing Line Foundation under the Influence of a New Railway Line

Abstract

The excavation of a new high-speed railway causes the side slope adjacent to the existing line foundation to become airborne, and the excessive dynamic deformation or cumulative deformation caused by the dynamic load of trains will affect the normal service of the subgrade, even leading to its instability. To date, there are no relevant experimental data regarding this, and there is also a lack of corresponding specifications. The only available numerical simulation research results need to be verified in practice. Therefore, this study relies on the Shanghai–Nanjing intercity high-speed railway construction project adjacent to the existing Beijing–Shanghai line to carry out a subgrade dynamic response test to ensure the safe operation of the existing line. The test obtained the vibration displacement, frequency, acceleration, and other parameters of the existing subgrade construction in three stages: subgrade excavation, pile formation, and subgrade filling. From the test results: During the test period, the vertical surface vibration displacement and vibration acceleration have a certain attenuation along the depth direction. In the stage of subgrade excavation, the vibration displacement and vibration acceleration generated are the largest. The vertical vibration displacement amplitude reaches 1.9 mm, and the horizontal vibration displacement amplitude reaches 0.15 mm. The vibration frequency of the roadbed under the action of the train load is concentrated in the range of 0–50 Hz, and the vibration energy at the peak value is relatively large, which reflects the load action frequency of the train, and the peak value is mainly concentrated in the range of 20–40 Hz. These results show that the maximum vibration response peak appears in the subgrade excavation stage, that is, the most dangerous stage of the existing subgrade. The vibration acceleration and vibration displacement of each dynamic response parameter are important in that they reflect the dynamic performance of the subgrade and establish the index control standard, which can be used as a control index for roadbed dynamic stability monitoring. The dynamic test of the subgrade state provides data support for the reasonable opening of the construction skylight and the protection of the excavation slope. Taking into account the impact of piling vibration, technical measures such as static pressure, jumping construction, and setting up stress relief holes are adopted. The test results and engineering measures ensure the safe operation of the existing subgrade, and have important theoretical significance for guiding the construction of the new subgrade adjacent to the existing line.

1. Introduction

The service life of large civil structures such as railways usually lasts for decades, or even hundreds of years. During the service period, structures are inevitably affected by external environmental erosion, material aging, etc., so that the engineering body becomes damaged to varying degrees. With the accumulation of these damages, the bearing capacity of engineering structures is reduced, and their safety and normal function are affected. In recent years, China has built a large number of high-speed railways, and it is inevitable that the new lines will run parallel to the existing lines in normal operation. The new Shanghai–Nanjing Passenger-Dedicated Line was built for public transportation between Nanjing and Shanghai. The total length of the line is 300.209 km, and the construction technical standard includes: a railway passenger dedicated line, a speed target value of 300 km/h; and a ballastless track. Some sections of the newly built railway are parallel to the existing railways in Beijing and Shanghai. The parallel section of the new and old lines is approximately 170 km long, the closest distance is only 4–5 m, and a drainage ditch is shared. During the construction of the Shanghai–Nanjing Line, a foundation pit was excavated close to the existing line causing the existing line foundation to be up in the air on one side, which significantly changed the stress field and displacement field of the existing line foundation. Therefore, compared with ordinary railways, the Shanghai–Nanjing section of the Beijing–Shanghai railway has more obvious characteristics of external interference. The existing Beijing–Shanghai line, which was built in the 1960s, is one of the busiest railway lines in the country, and has been in a saturated state for a long time. With the long-term effects of the construction of the new line and the long-term dynamic load of the trains, the service status of the existing roadbed needs to be carefully evaluated.

Research on the deformation of adjacent structures caused by construction has attracted the attention of engineers and technicians; for example, research on the stability and safety of railways under tunnels, which are now common [1]. Through the real-time monitoring of parameters, such as settlement deformation and horizontal displacement of adjacent structures, timely engineering measures can be carried out according to the monitoring to ensure the stability of structures during construction [2,3,4]. System safety assessment can be performed through efficient probabilistic stability analysis of engineering slopes along new railway lines [5]. The author carried out deformation monitoring of the existing line foundation, mainly based on the horizontal displacement and lateral stress changes, on the Shanghai–Nanjing Intercity Railway Project [6]. The above studies are all analyzed from a static point of view; however, research on the stability of the roadbed on one side facing up under the action of the train load belongs to the category of dynamic analysis. The dynamic response of the subgrade under the action of a dynamic load has become the basis for evaluating the subgrade condition [7,8], especially under conditions where the excavation on the side of the existing line base causes the stress field of the existing line to change. The dynamic deformation is an important evaluation index reflecting the state of the subgrade [9,10]. Ye et al. [11] combined field measurements and finite element calculations of dynamic response to analyze the amplitude characteristics of dynamic stress, dynamic deformation and vibration acceleration of ballastless track road foundations, and revealed the distribution law of stress and strain in the foundation bed under train load. Han [12] studied the dynamic characteristics of the roadbed of the Beijing-Shanghai high-speed railway, and concluded that the acceleration, dynamic stress, and displacement decrease nonlinearly with the increase of depth, and the dynamic stress in the roadbed increases with the increase of train speed. Bi [13], through the combination of three-dimensional finite difference software and theory, carried out the numerical simulation of the heavy-haul railway roadbed, analyzed the dynamic response law of the multi-layer roadbed system, and obtained the lateral distribution law of the dynamic stress on the roadbed surface. In the field of subgrade dynamic test research, in the early stages of the research on the dynamic characteristics of track subgrades, the test method was mostly used, and the dynamic response law of the subgrade was obtained by statistical analysis of the test data. Okumura analyzed the field test data of eight ordinary lines in Japan, and believed that the distance from the track, the structure of the track, the type of train, the speed of the train, the length of the train, and the natural vibration characteristics of the ground are the six main factors that affect the vibration level of the roadbed. Madshus proposed a semi-empirical model of subgrade low-frequency vibration, which includes five independent influencing factors: train type, speed, distance, track state, and the influence of structures. Roadbed dynamic tests have been conducted in China many times. Yang et al. [14] obtained the following from the actual measurement of subgrade dynamic stress and vibration: the irregularity of the line has a great influence on the subgrade dynamic stress, an increase in the driving speed can lead to an increase in the subgrade dynamic stress, and the subgrade dynamic stress and vibration acceleration show an exponential decay law in the depth direction. Mao [15] surveyed the ground vibration attenuation curves of 109 trains, and obtained empirical formulas for ground vertical and horizontal vibration attenuation consistent with the principles of vibration propagation energy diffusion and energy attenuation through composite regression. In research on the dynamic stability of roadbeds, Nie et al. [16] used the critical dynamic stress method (dynamic strength) and the dynamic shear strain method (dynamic deformation) to preliminarily evaluate the long-term dynamic stability of the red clay roadbed of the Wuhan–Guangzhou high-speed railway ballastless track. Wang [17] established a three-dimensional finite element numerical model of a ballastless track-roadbed system to simulate the running process of eight marshalled bullet trains. Combined with the measured data, the effects of track irregularity, train speed, axle load and depth on vertical dynamic stress were analyzed. Nie [18] developed the track-roadbed dynamic test system and applied it to the long-term dynamic performance tests of the dynamic response of many heavy-haul railway and high-speed railway subgrade projects.

In general, there are few studies on the dynamic performance of roadbeds. Restricted by objective conditions, most of the vehicle speeds in China’s track subgrade dynamic response tests are not high, and cannot reflect well the subgrade dynamic characteristics of the trains at high speed. Due to the lack of high-speed test data, the dynamic response law of track subgrades cannot be summarized with high reliability. In addition, testing of the dynamic deformation of the subgrade requires the arrangement of measuring rods or embedded components in the subgrade drilling [19,20], which is costly and cumbersome, and is not suitable for promotion and application in the public works sector. Moreover, this type of subgrade dynamic response research is tested under the normal operating state of the subgrade body.

There are few relevant experimental studies on the dynamic testing of the existing line foundation under the influence of construction, and only numerical simulation methods can be used to analyze such problems. Bi et al. [21] used the finite element method to simulate a wheel-rail excitation load for the foundation pit construction of a nearby operating subway station in Shanghai, analyzed the dynamic response of the existing station structure during the excavation of the deep foundation pit, and used displacement and vibration acceleration to evaluate the subway’s degree of impact. In addition, there are also research results on the use of numerical models to calculate the impact of foundation pit excavation on existing lines [22]; however, no experimental data were used to prove it.

In order to ensure the construction quality and construction period of the new line, and to ensure the safe operation of the existing line on the basis of static tests, this study carried out dynamic testing of the existing line foundation operation state close to the construction site. The test design takes the existing line operating trains as the vibration source, and the existing line base dynamic response as the goal to test the existing line base dynamic response; obtains the vibration displacement, frequency, acceleration and other subgrade dynamic performance indicators; and based on the test data, optimizes the construction technology and proposes construction safety measures adjacent to the existing subgrade. The test results and engineering measures provide a scientific basis for the safe operation and smooth construction of the existing line, and have important theoretical significance for guiding the construction of new subgrades adjacent to the existing line.

2. Project Overview

The nearest road section between the old and new lines was selected as the test section on the site to obtain data on the largest impact of new railway construction on the service status of the existing lines. The test section is located at DK95 + 256 of the Shanghai–Nanjing Line, corresponding to K1274 + 128 of the existing Beijing–Shanghai Line, and is composed of silty clay with a thickness of 18–20 m underneath. The two lines are only 4–5 m away from each other and share a drainage ditch. The existing Beijing–Shanghai railway line was built in the 1970s using natural foundations. The post-construction settlement control of the newly built Shanghai–Nanjing Railway is strictly regulated, with the requirement being not greater than 15 mm. Therefore, the new pile-raft composite foundation adopted by the Ministry of Railways was introduced for soft soil foundation reinforcement. Taking into consideration that it was close to the existing line, the pile type adopted was that of static pressure construction, the pipe pile was 18 m long, the raft thickness was 0.5 m, and the gravel cushion was laid underneath. Temporary protection was provided before excavation and replacement, and a row of rail piles was installed outside the excavation range for protection. The distance between rail piles was 2.0 m, and the pile length was 5.0 m. The design section of the test section surface is shown in Figure 1.

Subgrade dynamic analysis research is an important technical guarantee to reduce the damage of railway subgrades. The dynamic response of the subgrade includes the deformation, dynamic stress, acceleration, etc., of the subgrade. The magnitude and process of the dynamic response of the subgrade are directly related to the strength and fatigue characteristics of the subgrade, the cumulative deformation, and its dynamic stability [23,24]. Therefore, how to test and evaluate the stable state of the roadbed under the action of train power was the main problem to be solved.

3. On-Site Power Test

3.1. Test Content

The dynamic response of the subgrade under the action of a dynamic load (that is, a change in parameters such as vibration acceleration, vibration velocity, vibration flat rate, and vibration displacement of the subgrade) includes change over time, attenuation, and transmission and diffusion along the horizontal and vertical directions of space. The dynamic characteristics of the subgrade are exhibited by the changes in these dynamic parameters and directly affect the operation of the road. Field testing was undertaken to test the dynamic changes in these parameters over time and space under load. The dynamic test acquisition system is shown in

Table 1. Dynamic testing of collecting systems.

Equipment Name	Quantity
Donghua DH-5935 dynamic signal test and analysis system	1
DH-5935 solid state acceleration shock pickup	4
imc16 channel dynamic signal tester	1
imc accelerometer	6
891-Ⅱ type vibration sensor (horizontal, vertical)	6
RDP displacement sensor	5
Laptop	1
Desktop PC	1

.

The on-site in situ dynamic test mainly tested the following: (1) the distribution and changing law of soil dynamic parameters in the subgrade under a specific train operating speed; and (2) the distribution and variation law of dynamic displacement and overall displacement under different train operating speeds.

There was a large number of construction machines and personnel on the project site, and the test environment was very complex. During the test process, attention had to be paid to the safety of personnel and equipment, and various interferences in the process of power data acquisition had to be eliminated as much as possible. Combined with the actual situation on site, the overall test idea adopted an “embedded vibration measuring pile + sensor”. Between the new line raft and the existing line base guardrail, three steel pipes with different depths were buried along the line, with depths of 1, 2, and 3 m, respectively, as permanent monitoring piles. In addition, at each construction stage, a section of temporary observation piles was buried.

The tested and operated trains were mainly the “Harmony” series of power-dispersed EMUs, with two 12-car trains as the standard. During the test, a particular person was responsible for measuring the speed of the vehicle and recording the vehicle information at the beginning/continuing/end of each construction stage. In addition, the test included parameter changes in the case of natural environment excitation, construction machinery vibration, etc.

3.2. Test Methods

When the existing train passed by, the test software was started to collect the vibration data. The channel adopted 1–4 piezoelectric sensor channels, which could obtain vibration acceleration, dynamic displacement, and vibration speed. The on-site power monitoring is shown in Figure 2.

The data collection was designed according to the plan, and the sensor was fixed at the measuring point. In the cross-section direction, measuring points were arranged at different distances from the existing line to monitor the distribution law of the dynamic characteristics of the embankment soil in the cross-section direction. Acceleration sensors were arranged on the top of steel pipes with different depths to monitor the distribution of the dynamic characteristics of the embankment at different depths. Displacement sensors were arranged at different heights of steel pipes and slopes to monitor the distribution law of dynamic displacement when the train passed by. The layout of the measuring points is shown in Figure 3.

4. Analysis of Test Results

According to the collation and analysis of the test results, the data were analyzed in terms of vibration frequency, acceleration, velocity, and displacement. In the current vibration research, the vertical vibration generally dominated. Therefore, the vertical vibration was used as the evaluation index for the analysis in this paper.

4.1. Vibration Frequency

4.1.1. Subgrade Natural Frequency

The natural frequency of the structure is the basis of dynamic calculations. Before the dynamic calculation, frequency extraction and analysis must be carried out to obtain the mode shape and natural frequency of the structure. The natural vibration frequency of the existing line base depends on the subgrade filler, the subgrade soil properties, and the surrounding soil conditions. The dynamic test studies of high-speed railway foundations, such as Wuhan–Guangzhou and Qinshen, show that the [17,18] high-speed railway subgrade is an engineering structure with a low natural vibration frequency. The subgrade natural vibration is mainly a low-frequency natural vibration, ranging from 0 to 40 Hz, and there are five large peaks in between, namely 11.8, 20.1, 22.6, 27.9, and 30.9 Hz, which all represent the subgrade natural frequency. In a field resonance test of an embankment conducted in Japan on the Tokai Shinkansen, the resonance frequency of the embankment was measured at 15–20 Hz.

The train wheel group forms a load series through the subgrade section, so that the subgrade is subjected to repeated loads to generate forced vibrations, and the excitation frequency changes with the train speed. When the excitation frequency is equal to or close to the natural vibration frequency of the roadbed, resonance will occur and the vibration of the roadbed will be aggravated. Therefore, during the test, attention was paid to the coincidence of the natural frequency of the roadbed and the external excitation frequency.

4.1.2. Train Excitation Frequency

The dynamic loads imposed by trains on the roadbed are complex, and mainly include: the vibration of the sprung and unsprung mass of the vehicles, the impact of the trains on the track caused by the uneven surface of the wheel and the rail, and the periodic load caused by the movement of the wheel axle load, etc. These excitations can be regarded as the superposition of harmonic components of different frequencies, in which the shaft load movement mainly produces a low frequency (0–40 Hz). The unevenness of the wheel and rail mainly produces the middle frequency band (40–100 Hz). According to the measured data of the dynamic stress spectrum of the subgrade, the fundamental frequency (that is, the passing frequency of the vehicle) has the greatest impact on the subgrade. The self-power spectrum is obtained by filtering and eliminating the trend. The typical test spectrum is shown in Figure 4.

As shown in Figure 4, the vibration frequency of the subgrade under the train load is concentrated in the 0–50 Hz range, the vibration energy at the peak value is relatively large, reflecting the load action frequency of the train, and the peak value is mainly concentrated in the 20–40 Hz range. At different speeds, the soil response frequency has two peaks, verifying the double-peak phenomenon of the dynamic response of the high-speed railway subgrade mentioned in the literature [5]. The excitation frequencies all move to the high frequency direction with the increase in speed, and the energy corresponding to the peak value increases. The in situ monitoring went through the process of pouring the bearing plate, embankment filling, and preloading in the test section. The vibration frequency of the foundation soil caused by the train load was kept at 20–40 Hz. Among these, the excitation frequency at the foundation excavation stage is relatively low, maintained at approximately 20 Hz, which is close to the natural frequency of the roadbed obtained by the Japanese test in the existing research.

The impact or vibration frequency of the axle of high-speed trains on the roadbed is determined by the speed of the vehicle and the structure of the roadbed. The test vibration frequency is close to the natural vibration frequency of the roadbed. When the natural vibration frequency of the roadbed is consistent with the action frequency of the train, resonance will occur, which will intensify the vibration of the train and the track, increase the sinking deformation of the track, and affect the stability of high-speed driving. From the perspective of vibration frequency control, the safety monitoring of the dynamic performance of the existing line base should be further strengthened, and necessary engineering safety protection measures should be taken.

4.2. Vibration Displacement

Vibration displacement is generated briefly when a train passes by. Generally speaking, train speed, axle load, line smoothness, and the properties of roadbed materials are the main factors affecting vibration displacement. However, in the case of excavation at the immediate position, the stress state of the subgrade changes greatly, and attention should also be paid to the change in vibration displacement of the subgrade. The line/vehicle vertical coupled vibration analysis model is used by Southwest Jiaotong University to calculate the subgrade dynamics. It is believed that, under the action of the train load, the elastic deformation of the subgrade is the result of the interaction of all the parts of the wheel/rail/track bed/subgrade system. It is proposed that the dynamic elastic deformation value corresponding to subgrades with differing stiffness should be controlled within 3.5 mm. According to the compaction standards and wheel load conditions proposed in the “Interim Regulations on the Design of Bridges, Tunnels and Stations on the Beijing-Shanghai High-speed Railway Line”, when the height of the embankment is 5–10 m, the elastic deformation of the subgrade surface is within the range of 1.32–2.25 mm.

As shown in Figure 5 and Figure 6, during the construction excavation stage, the test vibration displacement was basically in the elastic stage, the vertical vibration amplitude reached 1.9 mm, and the horizontal vibration amplitude reached 0.15 mm. The maximum vibration displacement occurred in the surface soil of the slope during the excavation stage. After the embankment was filled in the later stage of the test, the vibration displacement of the foundation soil was basically in the 10–4 m range. Richart compiled data on the allowable vertical amplitude of vibration for a specific frequency, and found that when the operating frequency is 40 Hz (2400 cycles/min), the displacement amplitude limit is approximately 0.018 inches (0.457 mm). Based on this preliminary judgment, the dynamic characteristics of the foundation soil between the two lines tended to be stable.

In addition, during the test period, the vertical vibration displacement of the ground had a certain attenuation along the depth direction. When the trains ran at low speed, the surface vibration mainly represented the local elastic deformation caused by the axle load, the surrounding surface fluctuation was very small, and the vibration was a quasi-static response. When the train speeds reached the foundation shear wave speed of 80 m/s, the vibration increased, the Mach effect was obvious, and the vibration influence range became larger.

The fast Fourier transform (FFT) was used to calculate the convolution integral, finding that when the excitation frequency is larger, the fluctuation of the surface displacement is larger. This can be explained as follows: after the vibration wave is transmitted to the underlying soil, it is reflected on the surface and interferes with the original wave. As the distance increases, the vertical amplitude will fluctuate to a certain extent, instead of being monotonically attenuated. This phenomenon increases with the excitation frequency, and the increase becomes more and more obvious.

As shown in Figure 7, taking the peak value of vibration displacement at the same measuring point in each construction stage, it is found that the vibration displacement generated is the largest in the stage of subgrade excavation, and the dynamic deformation gradually decreases with the completion of subgrade filling.

4.3. Vibration Acceleration

The magnitude of the vibration acceleration of the subgrade is the main parameter for judging the effect of vibration on track damage [16], and the amplitude of the vibration acceleration is closely related to the dynamic characteristics of the subgrade and the frequency of load action. In Japan, vibration tests have been conducted on the subgrade surface of Shinkansen high-speed trains, and the maximum acceleration is 5–20 m/s². From the acceleration time–history curve, the peak periodicity of the acceleration waveform under the action of the dynamic load of the running train can be clearly distinguished.

The data show that there is a certain correlation between the peak vibration acceleration, the speed of the train, and the depth of the soil layer, showing a law similar to that of dynamic stress. When the train speed was 30 km/h, the vibration acceleration peak depth range corresponding to buried depths of 2, 1, 0.5, and 0.3 m was attenuated by 67%. At a burial depth of 0.5 m, the train speed increased from 30 to 120 km/h, and the ground response vibration acceleration peak increased by 44%. The vibration acceleration distribution and vibration acceleration-train speed change curves are shown in

Table 2. Vibration acceleration distribution.

Train Speed km/h	Buried Deep (m)	Content Test	Numeric Value m/s2
30	2	Vibration acceleration	0.01029
	1		0.01736
	0.5		0.01874
	0.3		0.03202
120	0.5		0.03375

and Figure 8.

The foundation soil is non-uniform and non-isotropic, and is an inelastic medium. Vibration waves propagate in the soil layer, and different types of vibration waves will be reflected and refracted when they encounter the soil layer interface. In the process of wave propagation, even a very small plastic deformation will cause energy dissipation due to the internal friction of the soil. With an increase in the distance of the train excitation vibration source, the wave energy is dissipated, and the acceleration vibration amplitude is also obviously attenuated. The vibration acceleration-depth distribution is shown in Figure 9.

According to the regression analysis of the measured data, the vertical and horizontal vibration acceleration attenuation formulas of the foundation soil of the Shanghai–Nanjing and Beijing–Shanghai projects can be summarized as follows. The measured vibration acceleration curve is shown in Figure 10.

Horizontal vibration acceleration distance attenuation formula:

$$\mathrm{a}=0.145\times {\mathrm{e}}^{-0.20\mathrm{s}}$$

Vertical vibration acceleration attenuation formula:

$$\mathrm{a}=0.031\times {\mathrm{e}}^{-0.17\mathrm{s}}$$

The variation law of the vibration velocity distribution can be obtained by the convolution and integration of the vibration acceleration, as shown in Figure 11.

Many domestic and foreign scholars or codes have given structural vibration limits based on the peak ground vibration velocity (PPV). The blasting vibration effect is shown in

Table 3. Blasting vibration effect.

Particle Velocity/mm·s−1	Vibration Effect
<1	Hard to feel
1	Humans can feel weak vibrations
5	Makes people feel uncomfortable, vibrate
10	Disturbing, with obvious vibration
33	Gives a strong sense of vibration
50	Safe vibration limit of general civil residential buildings
100	Safe vibration limit of reinforced concrete structure and tunnel support structure
140	Cracks in rock medium and expansion of old cracks
190	General civil buildings are severely cracked and damaged
300	Vibration and shedding of rock in unsupported tunnel
600	New cracks in rock

.

German researchers obtained the relationship between vibration speed and the stability of railway subgrade at different depths using tests, as shown in Figure 12. The critical vibration speed of coarse-grained soil is approximately Vkrit = 30 Ip (mm/s), while for mixed-grained clay soil, it is Vkrit = 25 Ip^1.5 (mm/s).

As peak velocity is less affected by frequency relative to peak acceleration and peak displacement, the dynamic test found that the attenuation of the vibration velocity in the horizontal amplitude is much faster than that in the vertical amplitude. The data show that the attenuation of soil vibration velocity along the depth direction is similar to the vibration acceleration, that the maximum vibration velocity of the surface is 5.04 × 10⁻³ m/s, and that the influence of vehicle speed on the vibration velocity is not obvious. Referring to the vibration velocity limit of the blasting vibration effect, there is an obvious vibration sense at the test site, but it is within the structural engineering safety limits.

Extracting the amplitudes of the dynamic response parameters in the three construction stages of roadbed excavation, piling, and roadbed filling, it can be seen that the vibration acceleration and vibration velocity show similar laws. That is, from the beginning of the excavation construction measurements, the vibration response tends to decrease, and the vibration response is the largest in the subgrade excavation stage. In the pile formation stage, due to the disturbance to the foundation soil, there is a slight rebound, and until the subgrade is filled, the vibration parameter gradually stabilizes. The trend of peak vibration acceleration in different construction stages is shown in Figure 13.

To sum up, the construction in the immediate position has a significant impact on the dynamic performance of the existing line foundation. Referring to the relevant regulations, the vibration displacement and vibration acceleration of the roadbed do not exceed the standard, but they specifically involve the dynamic stability of the existing line foundation. As there is no norm or experience in this area for reference, from the perspective of construction safety and existing line operation safety, close attention should be paid and construction safety measures should be adopted.

5. Test-Based Construction Safety Measures

5.1. Subgrade Excavation Stage

The stability of the roadbed was monitored through real-time dynamic tests during construction, and the test data were analyzed and fed back to the project site in real time. On one hand, protection measures for the subgrade slope were proposed, while at the same time, the road sections with abnormal stability and dangerous driving safety were found in the test to provide a basis for a reasonable opening time for the emergency construction skylight.

5.1.1. Excavation Roadbed Slope Protection

The test shows that the stability of the existing line foundation deteriorates after excavation. In addition, the exposed soil of the silty clay slope becomes looser under the influence of rain erosion and soaking, so the slope should be protected accordingly. On the project site, measures such as shotcrete plastering and hanging nets are mainly used to reinforce the excavation slope. Shotcrete plastering can effectively prevent the infiltration of rainwater and surface water, produce an embedded effect, improve cohesion, and improve the properties of the soil. Hanging nets can help strengthen the overall strength of the slope.

5.1.2. Reasonable Opening of Skylights and Determination of Construction Blocking Time

The “gap” required for maintenance and construction is reserved on the train operation map, and this gap is called the “skylight”. The skylight is the comprehensive embodiment of the coordination between construction organization and transportation organization. In order to minimize the interference of the construction blockade on the existing line transportation, the arrangement of the blockade time is a core issue.

In the construction operation in the parallel section of the operation line, the normal operation of the existing line should be maintained as much as possible; however, from the perspective of safe driving, once the test finds that the stability of the subgrade is abnormal, emergency repairs should be carried out. The dynamic test provides a data basis for determining a reasonable opening time for the skylight, the timely forecasting of the road section with the subgrade stability exceeding the standard, and formulating the skylight opening time as soon as possible, so as to save valuable time for the transportation organization and the construction organization. The time actually occupied by the skylight consists of the following three parts: (1) the opening time of the fixed skylight; (2) the safety time reserved before and after the skylight; and (3) the additional time for the train to run after the skylight is opened.

5.2. Engineering Measures in the Piling Stage

In the Shanghai–Nanjing Intercity Railway DK92 + 700~DK93 + 515, DK95 + 404.3~DK95 + 619.54, and DK95 + 656.56~DK95 + 692, a total of 1065.62 m of subgrade foundation was reinforced with PC-A500 high-strength prestressed concrete pipe piles. The concrete strength grade was C60, the wall thickness of the pipe pile was 100 mm in a square layout, the pile spacing was 2.2~2.4 m, and the pile length was 15~37 m. The three sections of subgrade were all parallel to the existing Shanghai–Nanjing Railway, and the distance from the center of the outermost row of pipe piles to the center of the existing Shanghai–Nanjing Railway line was less than 20 m, with the nearest one being 6.8 m.

Engineering practice shows that prefabricated pipe piles are widely used because of their good bearing capacity, scale of production and construction, and easy detection of quality. The vibration, soil squeeze, and noise caused by the construction process have a negative impact on the surrounding environment; in particular, the stress waves generated by piling will cause a strong vibration of the surrounding soil, which will affect the normal use and safety of the surrounding buildings and underground facilities. A schematic diagram of the impact of piling is shown in Figure 14.

The vibration method of pile driving is similar to an earthquake or blasting. Energy is transmitted in the form of waves from the earthquake source through the rock and soil, thus causing the vibration of the foundation, existing bridges, and buildings. Although the duration of the single-click vibration is short, the amplitude and period of the impact energy load generated by the mechanical action are almost unchanged during the load action due to the small interval and the large number of strikes. According to the existing research results [25], the pile side soil can be divided into four areas according to the degree of disturbance from the pile driving process: Zone—the hard layer area along the edge of the pile body, which is formed under the action of the extrusion force along the pile, with most of the pore water pressure in the soil being squeezed out; Zone II—known as the remodeling of the compacted zone, where the soil mass is disturbed and reshaped under the action of the compressive force, and the compaction is improved, while the shallow soil above the ground uplifts; Zone III—the disturbance zone, where the thickness is approximately 3.5 times the diameter of the pile, the average density of the soil is reduced, the water content is large, and the pore water pressure is increased; and Zone IV—the less affected zone, where the additional stress and pore water pressure caused by the pile driving are relatively small, and the original structure of the soil remains basically unchanged.

In order to prevent the impact of piling construction on the vibration and extrusion of the existing line, it was proposed that the static pressure method be adopted to construct the prestressed pipe pile and that stress relief holes be set up on the side of the existing line with a spacing of 3 to 5 m and a diameter of 0.5 m, a depth not less than half the length of the pipe pile and not less than 10 m, and a general length of 10 to 19 m, which effectively minimizes the impact of the construction of adjacent prestressed pipe piles on the existing line. A schematic diagram of the optimization technology for pile construction is shown in Figure 15.

The piling construction safety technology reduces the impact on the existing lines from three aspects:

(1)

By setting up stress release holes on the side adjacent to the existing line, the soil-squeezing effect of the pipe pile construction can be effectively released, and the continuous transfer of soil to the existing line side can be blocked to ensure the safety of the existing line.

(2)

Using static pressure construction instead of dynamic piling.

(3)

The jumping sequence is adopted to form the pile, and the purdah is formed as soon as possible.

The specific construction technology is as follows. Before constructing the pipe piles, construct stress-relief holes first, using a gravel grouting pile machine to extract the soil, with a diameter of 50 cm, laying them at 1–2 m outside the pipe pile closest to the existing line, parallel to the existing line 3–5 m, with a hole depth of 50 cm. At half the length of the adjacent piles, and not less than 10 m, use a short-screw earth-boring rig to borrow soil to form a hole, filling it with gravel over time after the drilling is completed. After the construction of the stress relief holes is completed, the pipe pile is then constructed. Its setting enables a drainage channel to dissipate the excess pore water pressure generated by the construction of the pipe pile. When constructing the pipe piles, the same row of jumping is used; that is, the No. 1, 3, 5, 7, etc., piles are constructed first, and the No. 2, 4, 6, 8, etc., piles are constructed after 24 h of no change from roadbed observation and rail surface observation. The second row of piles and the third row of piles are constructed according to this method, so that the constructed pipe piles on the side of the existing line form a pile group enclosure, suppress the effect of soil squeeze on the side of the existing line, and provide sufficient dissipation time for the excess pore water pressure.

After the construction was completed, the displacement test showed that the construction of the adjacent pipe piles did not cause obvious extrusion deformation of the existing line. Due to the use of the static pressure construction method, there was no adverse vibration effect.

6. Conclusions

In this study, combined with a passenger-dedicated line construction project, experimental research on the dynamic stability of an existing line foundation under the influence of a new line construction was carried out, and the construction technology was optimized according to the experimental analysis. The following conclusions were obtained:

(1)

A set of dynamic performance testing methods for railway subgrade in operation are proposed to obtain the dynamic performance indicators of the subgrade, such as vibration displacement, vibration frequency, and vibration acceleration of the existing subgrade under the influence of construction.

(2)

During testing, the peak value of the vibration response parameter in the subgrade excavation stage was the largest, which was the most dangerous stage of the subgrade dynamic stability. Subgrade vibration acceleration as a common index of structural dynamics has similar standards to follow, which can directly reflect the dynamic performance of the test body, and vibration displacement can be used as a dynamic deformation condition to evaluate the dynamic stability of the subgrade. Therefore, it is suggested that these two parameters be used as the dynamic stability control index of the existing roadbed under the influence of excavation.

(3)

During testing, the excitation frequency of the train to the subgrade is close to the natural frequency of the railway subgrade, and the possible resonance will lead to the aggravation of the subgrade vibration. This situation mainly occurs in the foundation excavation stage, and it was found that it is affected by the changes in the foundation stress field and displacement field. Based on this, it is suggested that the engineering construction department take preventative measures, such as strengthening the slope and speeding up the construction.

(4)

Dynamic testing provides a forecast and an early warning for the stability of the subgrade slope, and supplies data support for the protection of the subgrade slope and the reasonable opening of skylights. The piling construction technology is optimized from the perspective of subgrade dynamic stability, and the adverse effects, such as the extrusion and vibration of the adjacent existing line foundation, are eliminated by setting up stress relief holes, static pressure pipe piles, and jumping construction.

The test results provide data support for the safety evaluation of the existing subgrade. The construction safety measures proposed based on the test results ensure the safe excavation of the adjacent existing subgrade, which has important theoretical significance for guiding the construction of new subgrades adjacent to the existing subgrades in the future.