# Application of Artificial Freezing Method in Deformation Control of Subway Tunnel

Authors: Jie Li; Jinke Li; Yuancheng Cai; Dingyi Wu; Chunxia Guo; Wencai Zhao; Kunjie Tang; Yuhang Liu
Journal: Advances in Materials Science and Engineering
 (2022)

---

## Abstract

The metro tunnel construction mainly passes through the downtown area, including ground buildings, infrastructure, underground pipelines, existing subway lines, and station floors. However, it is very difficult to control the ground deformation, especially in the water-rich soft ground or in the tunnel engineering construction with strict requirements for surface settlement control. The traditional stratum reinforcement method may not meet the engineering needs. In this case, the artificial freezing method (AFM) provides a solution for such engineering problems. This paper summarizes the basic principle and characteristics and application scope of AFM, and compared with the other three stratum reinforcement methods, AFM has better effect in controlling surface displacement and vault settlement, while grouting method, pipe shed method, and pipe curtain method have absolute advantages in controlling surface uplift. Then, through engineering examples, the reinforcement effect of AFM in the four aspects of subway connecting passage, shield inlet and outlet, interval tunnel, and damage repair is introduced. Through analysis, it is found that the reinforcement effect of AFM in water-rich weak stratum is very good. Finally, the future development of AFM is prospected in order to provide some experience and guidance for the follow-up research.


---

## Body

## 1. Introduction

The ever-increasing population will cause urban congestion, which in turn will affect the daily lives of human beings. In this background, in order to alleviate the problem of urban traffic congestion, the development of underground space is currently a practical and feasible method. The development and utilization of underground space continues to affect and change our daily lives, such as metros, tunnels, and other projects. However, because metro tunnels are generally long and the strata that they pass through are complex and diverse, the excavation, support, and waterproofing are greatly affected by engineering geology, hydrological conditions, and the surrounding environment [1]. Due to the complex topography and geomorphology of our country, tunnel engineering is greatly affected by engineering geological conditions and hydrogeological conditions. The complex topography is prone to produce undesirable geological phenomena such as karst, faults, and water gushing. In addition to the geological problems, the construction of metro tunnels in cities also has to face another bigger challenge. Most of them are how to control the stability of the ground and control the ground settlement caused by the tunnel construction within the allowable ground settlement limit [2]. Therefore, it is necessary to find a new safe and economical method for stratum reinforcement. The freezing method is currently one of the best methods for reinforcement. The AFM is not restricted by environmental conditions and can be applied under various complicated conditions. At the same time, the AFM has good water barrier effect, and it can effectively prevent the inrush of groundwater. So, the artificial freezing has good safety performance and has the advantages of less impact on the environment. In 1862, the British engineer South Wales used artificial refrigeration technology for the first time to strengthen the soil during the construction of the building foundation. In 1883, the artificial soil freezing method was first applied in the shaft construction of the Alrid coal mine in Germany and was successful [3]. Since then, this special ground reinforcement technology has been widely used in underground engineering construction. Particularly in metro tunnel engineering, both the cyclic freezing method and the direct freezing method are widely used.The freezing method has been widely used in various working conditions, such as the construction of contact channel [4, 5], end wells [6, 7], underpassing tunnels [8–10], and tunnel repair and reconstruction [11]. However, while playing its positive role, it will also have a certain negative effect on the surrounding environment. For stratums with complex geological conditions and poorly developed soils, the freezing method should be strictly controlled for frost heave and thawing settlement, and preventive work should be done to reduce its adverse effects [12]. For example, pressure relief holes can be set in the freezing curtain to reduce the impact of frost heave on the surrounding environment; secondly, prestressed supports are installed in the tunnels on both sides of the freezing and excavation to prevent the tunnel from being damaged due to deformation; besides, grouting method can also be used to reduce the influence of thaw settlement [13]. It can be seen from this that when we apply the freezing method in construction [14], we must not only consider the convenience it brings to us, but also prevent its drawbacks [15]. While summing up the experience of others, we must also strengthen our own research to make the AFM to be more mature and can be widely used in various countries [16]. In general, the advantages of freezing method in the construction of tunnels in water-rich and soft ground are difficult to be replaced by other construction methods. Compared with other special underground construction methods, it has technical advantages such as strong applicability, good barrier-free performance, no pollution, and convenient construction [17–19]. Metro tunnels face great risks when passing through complex and weak ground. In this case, artificial freezing is one of the best ways to solve the risks. At present, there is a lack of systematic research on the application of this method in controlling the deformation of tunnels, so this article focused on the following research: (1) The principle and application scope of AFM are briefly introduced; (2) the reinforcement effect of AFM in different construction parts of subway tunnel through practical engineering cases is expounded; (3) the reinforcement characteristics of AFM compared with grouting reinforcement, pipe shed method, and pipe curtain method are discussed (Figure1).Figure 1
Construction site of AFM [20].


## 2. Principle of AFM

### 2.1. Basic Principles

Artificially ground freezing techniques is a special ground reinforcement technology. Its principle is to use artificial refrigeration technology to circulate low-temperature refrigerant in the aquifer around the underground project to transfer cold energy to the ground and make the soil in the temperature field below the freezing point. The water in the soil is continuously frozen into ice so that the soil particles are cemented with ice to form an impermeable overall structure, which greatly enhances the strength, stability, and impermeability of the soil. At the same time, a closed and continuous freezing wall is formed to surround the underground engineering. It can isolate the groundwater and resist the geotechnical pressure. And then excavation and support are carried out under the protection of the freezing wall [21, 22].In an ideal situation, the freezing process includes four different stages, as shown in Figure2: (1) Precooling and supercooling stages: in this stage, the liquid water is cooled from the initial temperature to the freezing temperature and then continues to cool to let the temperature of liquid water be lower than the equilibrium freezing temperature until the crystal nucleates. (2) The reheating stage: during this period, the too low temperature in the soil causes the rapid formation of ice crystals, and the excess temperature is quickly consumed. When the excess temperature is completely consumed and the soil temperature reaches the equilibrium freezing temperature, this stage ends. (3) The freezing stage: the growth of ice crystals is controlled by the heat transfer rate when the liquid water is completely frozen. (4) The cooling or tempering stage: the temperature of the solid ice drops to close to the brine temperature [23–26] (Figure3).Figure 2
Formation diagram of frozen wall [21].
(a)(b)(c)Figure 3
Cooling-freezing curve of supercooled water [23].At present, scholars have conducted in-depth and complete research on the theory of freezing method. In the study of the freezing temperature field, based on the actual engineering parameters, the phase change of the water in the soil layer and the freezing temperature change with the freezing time are considered, and the time of freezing hole intersecting was obtained when the freezing pipe is deflected. The thickness of the frozen wall, the average temperature of the frozen wall, the calculation formula of the average temperature of the frozen wall under the deflection condition, and other main technical parameters were proposed [27]. For the one-dimensional freezing process of the soil, a semianalytical method for the calculation of the temperature field was established. And compared with the numerical solutions in relevant literature, the correctness of the method was verified [28]. Through the analysis of the traditional theoretical calculation method of frozen wall, the analysis and application of reliability analysis method in frozen wall was introduced, the maximum displacement equation of radial deformation was established, and the functional function and limit state equation of frozen wall reliability analysis were established [29]. In the study of frost heave characteristics, the changes of temperature field and groundwater flow field in the freezing process were analyzed, and the quantitative relationships between the limit value of freezing hole spacing and groundwater velocity, original temperature of saturated sand, outer surface temperature of frozen pipe, pipe diameter of frozen pipe, and thermal conductivity of unfrozen saturated sand were obtained [30]; through simulation tests based on similarity theory, the vertical frost heave of the soil under load and the horizontal frost heave under the constraints of the simulated tunnel lining were obtained [31]. Through experiments, it is found that the frost heave rate of the soil decreased linearly with the decrease of cold end temperature, increased linearly with the increase of soil moisture content, increased with the increase of dry density, and decreased exponentially with the increase of upper load [32]. In the process of studying the characteristics of frozen and thawed soil, the experimental results showed that after the soil frozen and thawed, the density, dry density, and plasticity index decreased slightly, and the void ratio and liquidity index increased slightly, while other physical indexes were basically the same. After freezing and thawing of the clay, the permeability was greatly increased, which was 3–10 times that of the undisturbed soil, while the sandy soil only slightly increased [33]. Through research, the soil quality, moisture content, and dry density had significant effects on the thermal conductivity. Within the range of moisture content and dry density, the thermal conductivity increased approximately linearly with the increase of moisture content and dry density [34]. The coupling principle of temperature field, stress field, and moisture field in the process of soil freezing was analyzed. According to the formation and development process of frozen soil, the axisymmetric coupling model of temperature, stress, and moisture field of single cold source freezing was established. By using the superposition principle of potential field, the three-field coupling problem in the case of single cold source was extended to the three-field coupling in the case of multiple cold sources, and the analytical solution of the plane axisymmetric problem of a single cold source was obtained [35].At present, the AFM used in metro projects mainly included three kinds of cyclic freezing method, direct freezing method, and mixed freezing method. The cyclic freezing method was the main type of AFM in China, while the direct freezing method was more widely used in other countries. The characteristics and advantages and disadvantages of the three freezing methods were shown in Table1 [15, 36–38].Table 1
The characteristics of the three artificial freezing methods.
Types of artificial freezingType of cold catalystFreezing temperatureAdvantageDisadvantageCyclic freezing methodAmmonia, brine−20∼−40°CMature technology, uniform and stable formation of frozen soilThe freezing period is long, the freezing speed is slow, and it may affect the frozen soil in the original stratumDirect freezing methodLiquid nitrogen, dry ice−70∼−90°CConvenient operation, fast freezing speed, short construction periodThe freezing of the soil is uneven, and the construction cost is highMixed freezing methodAmmonia, brine + liquid nitrogen, dry ice—The freezing rate is relatively fast, the freezing efficiency is high, and the formation of frozen soil is relatively uniform and stableIncreased construction procedures, numerous construction equipment, and immature construction technology


### 2.2. Scope of Application

As a special construction method, artificial freezing had been used in shaft engineering, tunnel engineering, shield pipe jacking in and out, shield docking, connection channel construction, foundation pit engineering, pipeline engineering, waste burying, barriers, and other fields. It had a wide range of applications. And it had the advantages of wide application range, high reinforcement strength, good sealing performance, good controllability, high safety, and small environmental impact [39–47]. But the reinforcement effect of artificial freezing was more affected by the surrounding environment. Therefore, only when the use of other construction methods was limited by environment and conditions or difficult to achieve results, the AFM could be considered. The applicable scope and characteristics of the freezing method were shown in Figure 4 below [48–50].Figure 4
The scope and characteristics of the freezing method [51–53].


## 2.1. Basic Principles

Artificially ground freezing techniques is a special ground reinforcement technology. Its principle is to use artificial refrigeration technology to circulate low-temperature refrigerant in the aquifer around the underground project to transfer cold energy to the ground and make the soil in the temperature field below the freezing point. The water in the soil is continuously frozen into ice so that the soil particles are cemented with ice to form an impermeable overall structure, which greatly enhances the strength, stability, and impermeability of the soil. At the same time, a closed and continuous freezing wall is formed to surround the underground engineering. It can isolate the groundwater and resist the geotechnical pressure. And then excavation and support are carried out under the protection of the freezing wall [21, 22].In an ideal situation, the freezing process includes four different stages, as shown in Figure2: (1) Precooling and supercooling stages: in this stage, the liquid water is cooled from the initial temperature to the freezing temperature and then continues to cool to let the temperature of liquid water be lower than the equilibrium freezing temperature until the crystal nucleates. (2) The reheating stage: during this period, the too low temperature in the soil causes the rapid formation of ice crystals, and the excess temperature is quickly consumed. When the excess temperature is completely consumed and the soil temperature reaches the equilibrium freezing temperature, this stage ends. (3) The freezing stage: the growth of ice crystals is controlled by the heat transfer rate when the liquid water is completely frozen. (4) The cooling or tempering stage: the temperature of the solid ice drops to close to the brine temperature [23–26] (Figure3).Figure 2
Formation diagram of frozen wall [21].
(a)(b)(c)Figure 3
Cooling-freezing curve of supercooled water [23].At present, scholars have conducted in-depth and complete research on the theory of freezing method. In the study of the freezing temperature field, based on the actual engineering parameters, the phase change of the water in the soil layer and the freezing temperature change with the freezing time are considered, and the time of freezing hole intersecting was obtained when the freezing pipe is deflected. The thickness of the frozen wall, the average temperature of the frozen wall, the calculation formula of the average temperature of the frozen wall under the deflection condition, and other main technical parameters were proposed [27]. For the one-dimensional freezing process of the soil, a semianalytical method for the calculation of the temperature field was established. And compared with the numerical solutions in relevant literature, the correctness of the method was verified [28]. Through the analysis of the traditional theoretical calculation method of frozen wall, the analysis and application of reliability analysis method in frozen wall was introduced, the maximum displacement equation of radial deformation was established, and the functional function and limit state equation of frozen wall reliability analysis were established [29]. In the study of frost heave characteristics, the changes of temperature field and groundwater flow field in the freezing process were analyzed, and the quantitative relationships between the limit value of freezing hole spacing and groundwater velocity, original temperature of saturated sand, outer surface temperature of frozen pipe, pipe diameter of frozen pipe, and thermal conductivity of unfrozen saturated sand were obtained [30]; through simulation tests based on similarity theory, the vertical frost heave of the soil under load and the horizontal frost heave under the constraints of the simulated tunnel lining were obtained [31]. Through experiments, it is found that the frost heave rate of the soil decreased linearly with the decrease of cold end temperature, increased linearly with the increase of soil moisture content, increased with the increase of dry density, and decreased exponentially with the increase of upper load [32]. In the process of studying the characteristics of frozen and thawed soil, the experimental results showed that after the soil frozen and thawed, the density, dry density, and plasticity index decreased slightly, and the void ratio and liquidity index increased slightly, while other physical indexes were basically the same. After freezing and thawing of the clay, the permeability was greatly increased, which was 3–10 times that of the undisturbed soil, while the sandy soil only slightly increased [33]. Through research, the soil quality, moisture content, and dry density had significant effects on the thermal conductivity. Within the range of moisture content and dry density, the thermal conductivity increased approximately linearly with the increase of moisture content and dry density [34]. The coupling principle of temperature field, stress field, and moisture field in the process of soil freezing was analyzed. According to the formation and development process of frozen soil, the axisymmetric coupling model of temperature, stress, and moisture field of single cold source freezing was established. By using the superposition principle of potential field, the three-field coupling problem in the case of single cold source was extended to the three-field coupling in the case of multiple cold sources, and the analytical solution of the plane axisymmetric problem of a single cold source was obtained [35].At present, the AFM used in metro projects mainly included three kinds of cyclic freezing method, direct freezing method, and mixed freezing method. The cyclic freezing method was the main type of AFM in China, while the direct freezing method was more widely used in other countries. The characteristics and advantages and disadvantages of the three freezing methods were shown in Table1 [15, 36–38].Table 1
The characteristics of the three artificial freezing methods.
Types of artificial freezingType of cold catalystFreezing temperatureAdvantageDisadvantageCyclic freezing methodAmmonia, brine−20∼−40°CMature technology, uniform and stable formation of frozen soilThe freezing period is long, the freezing speed is slow, and it may affect the frozen soil in the original stratumDirect freezing methodLiquid nitrogen, dry ice−70∼−90°CConvenient operation, fast freezing speed, short construction periodThe freezing of the soil is uneven, and the construction cost is highMixed freezing methodAmmonia, brine + liquid nitrogen, dry ice—The freezing rate is relatively fast, the freezing efficiency is high, and the formation of frozen soil is relatively uniform and stableIncreased construction procedures, numerous construction equipment, and immature construction technology


## 2.2. Scope of Application

As a special construction method, artificial freezing had been used in shaft engineering, tunnel engineering, shield pipe jacking in and out, shield docking, connection channel construction, foundation pit engineering, pipeline engineering, waste burying, barriers, and other fields. It had a wide range of applications. And it had the advantages of wide application range, high reinforcement strength, good sealing performance, good controllability, high safety, and small environmental impact [39–47]. But the reinforcement effect of artificial freezing was more affected by the surrounding environment. Therefore, only when the use of other construction methods was limited by environment and conditions or difficult to achieve results, the AFM could be considered. The applicable scope and characteristics of the freezing method were shown in Figure 4 below [48–50].Figure 4
The scope and characteristics of the freezing method [51–53].


## 3. Advantage of AFM over Other Methods

The complexity of geological environment determines that metro tunnel construction usually crosses large faults, fully differentiated, shallow buried sections, soft, water-rich, broken, rheological, and other complex soft surrounding rock geological sections. In the process of metro tunnel construction, the strength of geological self-stability is the factor inducing dangerous accidents for construction workers [12]. For unfavorable geological conditions, the commonly used advance support mainly includes advance grouting, pipe shed, pipe curtain, and AFM. Due to different stratum properties, advanced support technology with different support principle and construction technology has been formed in construction practice [54].


### 3.1. Grouting Reinforcement

Grouting reinforcement method is one of the main reinforcement methods in stratum reinforcement engineering at present. It has the characteristics of simple technology, low production cost, wide applicability, and uniform blank structure. However, it has some disadvantages, such as low production efficiency, certain pollution to the surrounding environment, and great influence by the surrounding environment [55–57]. The freezing method is smoke-free, vibration-free, noise-free, green, and environment-friendly, with good stratum reinforcement effect and strong applicability [16]. However, freezing will cause frost heave and stratum subsidence when melting, which will have a certain impact on the ground and buildings. At the same time, it will be difficult to freeze when the saltness of groundwater is high and the speed of groundwater flow is fast. In addition, the freezing method has a complex process, high requirements for freezing equipment, personnel quality and management level, long construction period, and high cost [56]. In contrast, the grouting method has lower cost and good reinforcement effect, but it is difficult to achieve the expected grouting effect in some special strata. Although the freezing method is expensive, it is suitable for any loose rock and soil layer with a certain water content, in soft soil, unstable aquifer layer, quicksand layer, gravel area, high water pressure, and high pressure formation conditions [58].During the construction of the tunnel at the rear connection section of Donggang Station, when the foundation pit was excavated to 9 m underground, water and mud poured out from the sidewall of the foundation pit in the direction of the rear connection on the east side of Donggang Station. Later, during the excavation of the tunnel in the rear wiring section of Donggang station in July and August 2015, there were two large water seepage phenomena in the tunnel. In order to ensure the stability of surrounding rock during tunnel excavation in the rear connecting section, through comprehensive comparison and selection, it is decided to use cement water glass double grout for surface grouting reinforcement and take auxiliary measures such as increasing traffic diversion, increasing drainage wells and water supply pipe hoops for comprehensive treatment. The grouting reinforcement scheme of the project was as shown in Table2.Table 2
Grouting reinforcement scheme of the project.
Type of projectParameterType of slurryCement water glass double liquid slurryGrouting range and spacingThe soil mass at the upper left of the interval from the north of the tunnel centerline to the 2 m outer contour of the tunnel, and the grouting depth is 0.5 m below the pipe bottom of the water pipeline to 1.5 m within the tunnel excavation outlineGrouting pressureThe grouting pressure shall be controlled at 1.5∼1.8 MPa, and the grouting amount per unit time shall be controlled at 15∼30 L/minSlurry materialThe water cement ratio (by mass) of cement slurry is 1 : 1, the concentration of water glass is 15∼20°Bé, and the comprehensive ratio (by volume) of cement slurry and water glass is 1 : 0.5The design grouting time for the tunnel repair project in the rear wiring section of Donggang Station was 15 days. To further test the effect of grouting reinforcement, settlement monitoring points were arranged on the top surface of the tunnel to monitor the displacement changes of each measurement point in real time during the excavation. The arch crown settlement monitoring test section was 20 m long, and a settlement marker was arranged on the arch crown every 4 m, with a total of 5 monitoring sections. The layout of surface and pipeline settlement measuring points was shown in Figure5 [59].Figure 5
Variation curve of tunnel crown settlement, surface and pipeline cumulative settlement with time (a). Surface displacement measuring point. (b) Variation curve of vault settlement. (c) Variation curve of cumulative settlement of pipeline measuring point (GS0X-1). (d) Variation curve of cumulative settlement of surface measuring point (DS0X-1).From Figure5, it can be seen that at the initial stage of tunnel excavation, the displacement and subsidence rate of arch crown was fast, up to 2 mm/d. As time goes on, the cumulative settlement of each measuring point quickly reached the maximum, and the total settlement of each measuring point of arch crown was between 8 and 11 mm. From Figure 5, it could be seen that there was a slight uplift on the surface, and the maximum uplift above the tunnel center line was about 3 mm; the maximum uplift above the pipeline was about 2.5 mm, and the surface uplift was very small. In the tunnel excavation stage, the surface uplift disappeared, and the cumulative surface settlement increases rapidly in the early stage of excavation. After the 30th day, the total settlement began to stabilize, and the maximum value did not exceed 6 mm.During the construction of Lanzhou Metro Line 1, there was a dangerous situation of mud and water inrush at a connecting channel of Donggang West Road and the tunnel in the wiring section behind Donggang Station. For the two dangerous situations, the construction team used freezing method and grouting for reinforcement, respectively. The longitudinal section of a connecting channel project of Donggang West Road was shown in Figure6. After the temporary steel frame support of the 6th ring section of the connecting channel was completed, water and sand gushing occurred during the coring of the horizontal exploration hole of the tunnel, and the amount of water gushing and sand gushing has been difficult to control. After comprehensive comparison and selection, it was decided to adopt the freezing method for stratum reinforcement. The thickness of the freezing wall of the connecting channel and pump house was designed to be 1.7 m, and the design average temperature of the freezing wall was not greater than −10°C. The elevation of the freezing wall was shown in Figure 6.Figure 6
Tunnel crown settlement and surface and pipeline settlement.The design active freezing time of the connecting channel project was 40∼45 d. During the construction of connecting channel and pump house, the tunnel deformation, ground and surrounding pipelines, building settlement, and so on were monitored. The monitoring range was 20 rings of tunnel segments on both sides of the connecting channel and within 21 m outside the ground projection directly above the connecting channel. The monitoring results were shown in Figure6 [55].The following conclusions can be drawn through the comparison of vault settlement and surface and pipeline settlement:(1) Grouting reinforcement method has obvious advantages in construction period, which can complete the reinforcement of the stratum in a short time. (2) The maximum vertical settlement caused by tunnel excavation using AFM is 2.47 mm, while the maximum arch crown settlement caused by tunnel excavation after grouting reinforcement method is 11 mm. In contrast, the reinforcement effect of freezing method is more obvious in controlling arch crown settlement. (3) The soil heave produced by grouting reinforcement method in the construction process is larger than that of AFM. The two auxiliary construction methods of stratum reinforcement have the same control effect on surface settlement in the process of tunnel construction.Compared with grouting reinforcement, AFM has some advantages in controlling vault settlement and surface uplift, but the construction period of AFM is longer and the cost is higher.


### 3.2. Pipe Shed Method

The pipe shed method can significantly improve the internal environment of surrounding rock, especially suitable for soft and broken surrounding rock tunnel engineering with high surrounding rock pressure and high requirements for surrounding rock deformation and surface deformation [60]. If supplemented with in-pipe grouting, it can also deal with extremely broken rock mass, collapse mass, and rock pile areas. However, the construction process of pipe shed method is complex and of high cost and high requirements for personnel operation [14, 61, 62]. Compared with the freezing method, the pipe shed method belongs to permanent and rigid support, which has good reinforcement effect on the stratum, but its water stop capacity is poor and has high requirements for personnel and on-site management. During the construction of the connect tunnel project between Shuangliu Airport Terminal 2 Station and Shuangliu west station of Chengdu rail transit line 10 and the tunnel project between Saiyuntai Station and West 2nd Road Station of North Station of Chengdu Metro Line 5, the ground settlement was strictly controlled. In the face of similar stratum conditions, freezing method and pipe shed method were used for reinforcement, respectively. The following will compare the reinforcement effects of the two reinforcement methods.The section from Saiyuntai station to West 2nd Road Station of North Station of Chengdu Metro Line 5 was constructed by shield method, which mainly crosses medium-dense pebble and dense pebble stratum. The buried depth of tunnel vault was about 14.14∼15.5 m. The section tunnel passed through the railway turnout area. Due to the complex structure of turnout area and low driving safety, it was a weak link of track. Once large settlement occurred, it was very easy to derail the train, and the consequences are unimaginable. After comparison, it was decided to use pipe shed method to strengthen the stratum.The main design parameters of pipe shed wereφ159 × 10 mm steel pipes, which were arranged in double rows of quincunx shape, with a single length of 56 m, and which were arranged within 180° along the upper half section of the tunnel. Two rows were used: φ 159 advance support of pipe shed and quincunx layout. At the same time, cement slurry was injected into the pipe shed (plum blossom hole layout 0.3 m × 0.3 m, hole diameter 8 mm, timely fill the gap between pipe shed and stratum).During the construction of the tunnel project between Saiyuntai Station and West 2nd Road Station of North Station of line 5, a horizontal monitoring point was arranged for the surface heave and subsidence monitoring point, and 9 measuring points were arranged in the section. See Figure7 for the specific layout location. A transverse monitoring section was arranged every 10 m in the railway area, and a total of 5 transverse measurement sections were arranged in the railway area. The specific location is shown in Figure 7. A total of 45 surface settlement observation points were arranged in the railway area.Figure 7
Layout of tunnel measuring points and cumulative settlement of each measuring point.After monitoring, the final surface settlement results of the five survey sections above the railway area after the shield tunnel passes were shown in Figure7. It can be seen that the cumulative settlement of each measuring point on the surface was between 3.2 and 5.8 mm.On the left side of the center of the double track tunnel, the difference between the cumulative settlement values of each section and measuring point was small, and the settlement curves crossed each other. On the right side of the center of the double track tunnel, sections2, 4, and 5 corresponded to points 7 and 8, which were about 1 mm different from sections 1 and 3. The difference between other points was small, and the cumulative settlement of each measuring point showed a certain regularity. The surface settlement of the double track shield tunnel was a typical “V” settlement curve. The maximum settlement value appeared near the center of the two tunnels, which was 5.8 mm, meeting the requirements of settlement deformation control [63].The tunnel crossing stratum between Shuangliu Airport Terminal 2 station and Shuangliu west station of line 10 was mainly pebble soil and moderately weathered mudstone. The plan of the section under the apron was shown in Figure8. Due to the lack of precipitation implementation conditions in the apron and the poor water plugging effect of grouting in the water rich sandy pebble stratum, it was difficult to achieve the complete water plugging effect, and the construction risk was high. After many arguments, it was decided to use AFM for stratum reinforcement. The design thickness of freezing curtain was 2 m, and the average temperature of freezing wall was −10°C. The strength index of frozen soil (−10°C) was as follows: uniaxial compressive strength 3.5 MPa, flexural strength 1.8 Mpa, and shear strength 1.5 MPa.Figure 8
Comparison between plane diagram of section tunnel and settlement trough curve.During the tunnel crossing stratum between Shuangliu Airport Terminal 2 station and Shuangliu west station of line 10 construction, the apron and taxiway were closely monitored. The measured monitoring results showed that the maximum settlement of the apron was 6.97 mm and the maximum settlement of the taxiway was only 1.98 mm. Two typical sections of apron at mileage YDK11 + 450 and taxiway at mileage YDK12 + 860 were selected for analysis.It can be seen from Figure8 that the affected width of the sedimentation tank was about 30 m on the left and right of the apron and about 40 m on the left and right of the taxiway. The numerical simulation curve presented a symmetrical distribution of left and right lines. The surface settlement above the right line was slightly greater than that above the left line, but the overall cumulative settlement met the control requirements [64, 65].By comparing the reinforcement effects of two stratum reinforcement methods in water-rich sandy pebble stratum, the following can be concluded:(1) The stratum settlement strengthened by AFM is 1.98 mm∼6.97 mm, and the stratum settlement strengthened by pipe shed method is 3.2∼5.8 mm. It can be seen that both AFM and pipe shed method have achieved good reinforcement effect in water-rich sandy pebble stratum. (2) Compared with the AFM, the pipe shed method has complex construction steps and great construction difficulty, resulting in long construction period and high cost. In contrast, the construction period of AFM is shorter. Generally speaking, the stratum reinforcement effect of AFM is similar to that of pipe shed method, but the construction difficulty of AFM is slightly less than that of pipe shed method.


### 3.3. Pipe Curtain Method

The pipe curtain method does not need to draw groundwater during construction, and the surface displacement is small. After construction, the steel pipe is connected into a rigid whole, which can effectively strengthen the soil layer. It can be used for tunnels of any section type, with good water stop effect and small construction disturbance. It is generally used to deal with special areas with high requirements for formation deformation [66, 67]. However, the pipe curtain method has many construction steps, complex construction, high requirements for construction quality, and high cost. Compared with the freezing method, the pipe curtain method has a relatively general reinforcement effect on the stratum, but the pipe curtain method has little impact on the surrounding stratum.The underground excavation part of Tianhe Park turn-back line of Guangzhou metro line 21 is located below Huangpu Avenue and crosses Huangpu Avenue, the main road in Guangzhou city. The road has heavy traffic and huge traffic flow and is equipped with Yuancun overpass viaduct and many municipal pipelines. Therefore, it has high requirements for tunnel construction and strict settlement control. The geological conditions and surrounding control conditions of the concealed tunnel on the turn-back line were shown in Figure9. Moreover, the concealed tunnel on the turn back line was a single tunnel and double-track tunnel with large section and less than twice the tunnel span, which belonged to super shallow buried large section tunnel. The geological rock surface in the area where the tunnel passed through fluctuates greatly, cracks were developed, and the stratum was weak. In addition, there was thick mucky soil layer on the vault of concealed tunnel. Once the vault support was unstable, it was easy to cause pipeline leakage and ground collapse, resulting in serious engineering accidents. In order to ensure the traffic safety of the above pipeline and Huangpu Avenue, it was decided to adopt the pipe curtain method for stratum reinforcement.Figure 9
Detailed drawing of tube curtain connection and settlement history of monitoring points.The selection of pipe curtain materials of the project was shown in Table3.Table 3
Selection of pipe curtain materials.
Project typeDesign parametersSpecification of steel pipeQ235B seamless steel pipe is adopted, with pipe diameter of 299 mm, wall thickness of 10 mm, and steel pipe spacing of 0.36 MSpecification of latchMale and female lock catch shall be welded between steel pipes for pipe lap. Unequal angle steel of Q235B steel with specification of 80 mm × 50 mm × 10 mmThe tunnel construction of the project would take 5 months. According to the monitoring data, the maximum settlement of W4 monitoring point above the concealed excavation vault was 17.45 mm, and its settlement trend was shown in Figure9.It can be seen from Figure9 that the settlement in the actual construction process was divided into four stages: (1) the surface settlement in the pipe curtain construction stage was about 4.06 mm, and the pipe curtain was in a relatively stable stage in the excavation stage; (2) when the tunnel was excavated to the area directly below the monitoring point, the surface settlement rate was fast, but eventually tended to be stable; (3) after the initial support of the tunnel was connected, the ground settlement was stable within 17.45 mm; (4) there was basically no change in ground settlement during the initial support removal and secondary lining construction stages. It could be seen that the pipe curtain advance support construction would cause a certain degree of ground settlement, but after the pipe curtain was closed into a ring, it could effectively control the maximum ground settlement and ensure the safety of the surrounding environment during the short-term shutdown of the project (e.g., the Spring Festival holiday) [68].The geology of the tunnel in the construction section of the turn back line of Tianhe passenger station of Guangzhou Metro Line 3 was mainly strongly weathered granite eluvium. The net section of the designed double-track tunnel was horseshoe shaped, with a net height of 9.246 m, a net width of 11.5 m, and a length of 147.8 m. The stratum conditions were shown in Figure10. The area had heavy surface traffic, dense buildings, crisscross formation pipelines, and poor geological conditions. After comparison, it was decided to use the freezing method to strengthen the stratum. Freezing drilling adopted pipe following drilling combined with advanced drilling deviation correction technology. The brine temperature in the design active freezing period was −25∼−-30°C, the brine temperature in the maintenance freezing period was −20∼−25°C, the active freezing period was 150 days, the design maintenance freezing period was 120 days, the design average temperature of the freezing wall was −8°C, and the one-time horizontal freezing length was 138.8 m.Figure 10
Surface deformation monitoring points and surface displacement curve, vault settlement, and horizontal convergence curve.A total of 28 surface deformation measuring points were arranged in the turn-back line project area of Tianhe passenger station of line 3, and the location of the measuring points was shown in Figure10. It showed the surface deformation of the three measuring points 10-1#, E#, and F# arranged along the centerline of the tunnel. In the construction stage of freezing hole, the maximum cumulative value of ground settlement at the south end was −7 mm, with an average of −3.6 mm. The average cumulative settlement at the north end was ±1 mm, and the surface deformation at this stage was mainly caused by borehole excavation. In the tunnel excavation stage, the surface deformation is between 60 mm and 110 mm. After the construction is completed, the cumulative surface settlement was between 70 mm and 370 mm. Near the end of excavation, the surface frost heave was obvious, and the frost heave was relatively large in the middle of the turn-back line and along the tunnel centerline.Taking the arch crown settlement monitoring of zone II at 14 m from the south part as an example, the analysis of Figure10 showed that the arch crown settlement was closely related to the construction progress and generally increases first and then decreased. After the tunnel was excavated to the monitoring section, the settlement increases sharply after the tunnel was excavated to the monitoring section, with a maximum of about 6.5 mm. Then, with the continuation of excavation, the vault settlement of this section decreased gradually. Later, due to the removal of temporary support, the settlement increased again, but the increased value was smaller than that during excavation. After the tunnel section was just excavated, the walls on both sides converged inward rapidly. Within 35 m from the south end, the average horizontal convergence of the tunnel was about 8 mm and the maximum was 14 m. Then, due to the vertical stress field and horizontal temporary rigid support, the horizontal convergence decreased. After the temporary support was removed, the convergence value increased again, and the average final convergence value was about 3 mm [17].After comparing the construction period and surface settlement, the following conclusions can be drawn:(1) The construction period of AFM and pipe curtain method is relatively long, about 150 d.(2) In the strongly weathered and water rich weak stratum, the control effect of AFM on surface displacement is much better than that of pipe curtain method.(3) AFM will cause formation uplift during active freezing and maintenance, while pipe curtain method will not have a great impact on the formation in the reinforcement stage.


## 3.1. Grouting Reinforcement

Grouting reinforcement method is one of the main reinforcement methods in stratum reinforcement engineering at present. It has the characteristics of simple technology, low production cost, wide applicability, and uniform blank structure. However, it has some disadvantages, such as low production efficiency, certain pollution to the surrounding environment, and great influence by the surrounding environment [55–57]. The freezing method is smoke-free, vibration-free, noise-free, green, and environment-friendly, with good stratum reinforcement effect and strong applicability [16]. However, freezing will cause frost heave and stratum subsidence when melting, which will have a certain impact on the ground and buildings. At the same time, it will be difficult to freeze when the saltness of groundwater is high and the speed of groundwater flow is fast. In addition, the freezing method has a complex process, high requirements for freezing equipment, personnel quality and management level, long construction period, and high cost [56]. In contrast, the grouting method has lower cost and good reinforcement effect, but it is difficult to achieve the expected grouting effect in some special strata. Although the freezing method is expensive, it is suitable for any loose rock and soil layer with a certain water content, in soft soil, unstable aquifer layer, quicksand layer, gravel area, high water pressure, and high pressure formation conditions [58].During the construction of the tunnel at the rear connection section of Donggang Station, when the foundation pit was excavated to 9 m underground, water and mud poured out from the sidewall of the foundation pit in the direction of the rear connection on the east side of Donggang Station. Later, during the excavation of the tunnel in the rear wiring section of Donggang station in July and August 2015, there were two large water seepage phenomena in the tunnel. In order to ensure the stability of surrounding rock during tunnel excavation in the rear connecting section, through comprehensive comparison and selection, it is decided to use cement water glass double grout for surface grouting reinforcement and take auxiliary measures such as increasing traffic diversion, increasing drainage wells and water supply pipe hoops for comprehensive treatment. The grouting reinforcement scheme of the project was as shown in Table2.Table 2
Grouting reinforcement scheme of the project.
Type of projectParameterType of slurryCement water glass double liquid slurryGrouting range and spacingThe soil mass at the upper left of the interval from the north of the tunnel centerline to the 2 m outer contour of the tunnel, and the grouting depth is 0.5 m below the pipe bottom of the water pipeline to 1.5 m within the tunnel excavation outlineGrouting pressureThe grouting pressure shall be controlled at 1.5∼1.8 MPa, and the grouting amount per unit time shall be controlled at 15∼30 L/minSlurry materialThe water cement ratio (by mass) of cement slurry is 1 : 1, the concentration of water glass is 15∼20°Bé, and the comprehensive ratio (by volume) of cement slurry and water glass is 1 : 0.5The design grouting time for the tunnel repair project in the rear wiring section of Donggang Station was 15 days. To further test the effect of grouting reinforcement, settlement monitoring points were arranged on the top surface of the tunnel to monitor the displacement changes of each measurement point in real time during the excavation. The arch crown settlement monitoring test section was 20 m long, and a settlement marker was arranged on the arch crown every 4 m, with a total of 5 monitoring sections. The layout of surface and pipeline settlement measuring points was shown in Figure5 [59].Figure 5
Variation curve of tunnel crown settlement, surface and pipeline cumulative settlement with time (a). Surface displacement measuring point. (b) Variation curve of vault settlement. (c) Variation curve of cumulative settlement of pipeline measuring point (GS0X-1). (d) Variation curve of cumulative settlement of surface measuring point (DS0X-1).From Figure5, it can be seen that at the initial stage of tunnel excavation, the displacement and subsidence rate of arch crown was fast, up to 2 mm/d. As time goes on, the cumulative settlement of each measuring point quickly reached the maximum, and the total settlement of each measuring point of arch crown was between 8 and 11 mm. From Figure 5, it could be seen that there was a slight uplift on the surface, and the maximum uplift above the tunnel center line was about 3 mm; the maximum uplift above the pipeline was about 2.5 mm, and the surface uplift was very small. In the tunnel excavation stage, the surface uplift disappeared, and the cumulative surface settlement increases rapidly in the early stage of excavation. After the 30th day, the total settlement began to stabilize, and the maximum value did not exceed 6 mm.During the construction of Lanzhou Metro Line 1, there was a dangerous situation of mud and water inrush at a connecting channel of Donggang West Road and the tunnel in the wiring section behind Donggang Station. For the two dangerous situations, the construction team used freezing method and grouting for reinforcement, respectively. The longitudinal section of a connecting channel project of Donggang West Road was shown in Figure6. After the temporary steel frame support of the 6th ring section of the connecting channel was completed, water and sand gushing occurred during the coring of the horizontal exploration hole of the tunnel, and the amount of water gushing and sand gushing has been difficult to control. After comprehensive comparison and selection, it was decided to adopt the freezing method for stratum reinforcement. The thickness of the freezing wall of the connecting channel and pump house was designed to be 1.7 m, and the design average temperature of the freezing wall was not greater than −10°C. The elevation of the freezing wall was shown in Figure 6.Figure 6
Tunnel crown settlement and surface and pipeline settlement.The design active freezing time of the connecting channel project was 40∼45 d. During the construction of connecting channel and pump house, the tunnel deformation, ground and surrounding pipelines, building settlement, and so on were monitored. The monitoring range was 20 rings of tunnel segments on both sides of the connecting channel and within 21 m outside the ground projection directly above the connecting channel. The monitoring results were shown in Figure6 [55].The following conclusions can be drawn through the comparison of vault settlement and surface and pipeline settlement:(1) Grouting reinforcement method has obvious advantages in construction period, which can complete the reinforcement of the stratum in a short time. (2) The maximum vertical settlement caused by tunnel excavation using AFM is 2.47 mm, while the maximum arch crown settlement caused by tunnel excavation after grouting reinforcement method is 11 mm. In contrast, the reinforcement effect of freezing method is more obvious in controlling arch crown settlement. (3) The soil heave produced by grouting reinforcement method in the construction process is larger than that of AFM. The two auxiliary construction methods of stratum reinforcement have the same control effect on surface settlement in the process of tunnel construction.Compared with grouting reinforcement, AFM has some advantages in controlling vault settlement and surface uplift, but the construction period of AFM is longer and the cost is higher.


## 3.2. Pipe Shed Method

The pipe shed method can significantly improve the internal environment of surrounding rock, especially suitable for soft and broken surrounding rock tunnel engineering with high surrounding rock pressure and high requirements for surrounding rock deformation and surface deformation [60]. If supplemented with in-pipe grouting, it can also deal with extremely broken rock mass, collapse mass, and rock pile areas. However, the construction process of pipe shed method is complex and of high cost and high requirements for personnel operation [14, 61, 62]. Compared with the freezing method, the pipe shed method belongs to permanent and rigid support, which has good reinforcement effect on the stratum, but its water stop capacity is poor and has high requirements for personnel and on-site management. During the construction of the connect tunnel project between Shuangliu Airport Terminal 2 Station and Shuangliu west station of Chengdu rail transit line 10 and the tunnel project between Saiyuntai Station and West 2nd Road Station of North Station of Chengdu Metro Line 5, the ground settlement was strictly controlled. In the face of similar stratum conditions, freezing method and pipe shed method were used for reinforcement, respectively. The following will compare the reinforcement effects of the two reinforcement methods.The section from Saiyuntai station to West 2nd Road Station of North Station of Chengdu Metro Line 5 was constructed by shield method, which mainly crosses medium-dense pebble and dense pebble stratum. The buried depth of tunnel vault was about 14.14∼15.5 m. The section tunnel passed through the railway turnout area. Due to the complex structure of turnout area and low driving safety, it was a weak link of track. Once large settlement occurred, it was very easy to derail the train, and the consequences are unimaginable. After comparison, it was decided to use pipe shed method to strengthen the stratum.The main design parameters of pipe shed wereφ159 × 10 mm steel pipes, which were arranged in double rows of quincunx shape, with a single length of 56 m, and which were arranged within 180° along the upper half section of the tunnel. Two rows were used: φ 159 advance support of pipe shed and quincunx layout. At the same time, cement slurry was injected into the pipe shed (plum blossom hole layout 0.3 m × 0.3 m, hole diameter 8 mm, timely fill the gap between pipe shed and stratum).During the construction of the tunnel project between Saiyuntai Station and West 2nd Road Station of North Station of line 5, a horizontal monitoring point was arranged for the surface heave and subsidence monitoring point, and 9 measuring points were arranged in the section. See Figure7 for the specific layout location. A transverse monitoring section was arranged every 10 m in the railway area, and a total of 5 transverse measurement sections were arranged in the railway area. The specific location is shown in Figure 7. A total of 45 surface settlement observation points were arranged in the railway area.Figure 7
Layout of tunnel measuring points and cumulative settlement of each measuring point.After monitoring, the final surface settlement results of the five survey sections above the railway area after the shield tunnel passes were shown in Figure7. It can be seen that the cumulative settlement of each measuring point on the surface was between 3.2 and 5.8 mm.On the left side of the center of the double track tunnel, the difference between the cumulative settlement values of each section and measuring point was small, and the settlement curves crossed each other. On the right side of the center of the double track tunnel, sections2, 4, and 5 corresponded to points 7 and 8, which were about 1 mm different from sections 1 and 3. The difference between other points was small, and the cumulative settlement of each measuring point showed a certain regularity. The surface settlement of the double track shield tunnel was a typical “V” settlement curve. The maximum settlement value appeared near the center of the two tunnels, which was 5.8 mm, meeting the requirements of settlement deformation control [63].The tunnel crossing stratum between Shuangliu Airport Terminal 2 station and Shuangliu west station of line 10 was mainly pebble soil and moderately weathered mudstone. The plan of the section under the apron was shown in Figure8. Due to the lack of precipitation implementation conditions in the apron and the poor water plugging effect of grouting in the water rich sandy pebble stratum, it was difficult to achieve the complete water plugging effect, and the construction risk was high. After many arguments, it was decided to use AFM for stratum reinforcement. The design thickness of freezing curtain was 2 m, and the average temperature of freezing wall was −10°C. The strength index of frozen soil (−10°C) was as follows: uniaxial compressive strength 3.5 MPa, flexural strength 1.8 Mpa, and shear strength 1.5 MPa.Figure 8
Comparison between plane diagram of section tunnel and settlement trough curve.During the tunnel crossing stratum between Shuangliu Airport Terminal 2 station and Shuangliu west station of line 10 construction, the apron and taxiway were closely monitored. The measured monitoring results showed that the maximum settlement of the apron was 6.97 mm and the maximum settlement of the taxiway was only 1.98 mm. Two typical sections of apron at mileage YDK11 + 450 and taxiway at mileage YDK12 + 860 were selected for analysis.It can be seen from Figure8 that the affected width of the sedimentation tank was about 30 m on the left and right of the apron and about 40 m on the left and right of the taxiway. The numerical simulation curve presented a symmetrical distribution of left and right lines. The surface settlement above the right line was slightly greater than that above the left line, but the overall cumulative settlement met the control requirements [64, 65].By comparing the reinforcement effects of two stratum reinforcement methods in water-rich sandy pebble stratum, the following can be concluded:(1) The stratum settlement strengthened by AFM is 1.98 mm∼6.97 mm, and the stratum settlement strengthened by pipe shed method is 3.2∼5.8 mm. It can be seen that both AFM and pipe shed method have achieved good reinforcement effect in water-rich sandy pebble stratum. (2) Compared with the AFM, the pipe shed method has complex construction steps and great construction difficulty, resulting in long construction period and high cost. In contrast, the construction period of AFM is shorter. Generally speaking, the stratum reinforcement effect of AFM is similar to that of pipe shed method, but the construction difficulty of AFM is slightly less than that of pipe shed method.


## 3.3. Pipe Curtain Method

The pipe curtain method does not need to draw groundwater during construction, and the surface displacement is small. After construction, the steel pipe is connected into a rigid whole, which can effectively strengthen the soil layer. It can be used for tunnels of any section type, with good water stop effect and small construction disturbance. It is generally used to deal with special areas with high requirements for formation deformation [66, 67]. However, the pipe curtain method has many construction steps, complex construction, high requirements for construction quality, and high cost. Compared with the freezing method, the pipe curtain method has a relatively general reinforcement effect on the stratum, but the pipe curtain method has little impact on the surrounding stratum.The underground excavation part of Tianhe Park turn-back line of Guangzhou metro line 21 is located below Huangpu Avenue and crosses Huangpu Avenue, the main road in Guangzhou city. The road has heavy traffic and huge traffic flow and is equipped with Yuancun overpass viaduct and many municipal pipelines. Therefore, it has high requirements for tunnel construction and strict settlement control. The geological conditions and surrounding control conditions of the concealed tunnel on the turn-back line were shown in Figure9. Moreover, the concealed tunnel on the turn back line was a single tunnel and double-track tunnel with large section and less than twice the tunnel span, which belonged to super shallow buried large section tunnel. The geological rock surface in the area where the tunnel passed through fluctuates greatly, cracks were developed, and the stratum was weak. In addition, there was thick mucky soil layer on the vault of concealed tunnel. Once the vault support was unstable, it was easy to cause pipeline leakage and ground collapse, resulting in serious engineering accidents. In order to ensure the traffic safety of the above pipeline and Huangpu Avenue, it was decided to adopt the pipe curtain method for stratum reinforcement.Figure 9
Detailed drawing of tube curtain connection and settlement history of monitoring points.The selection of pipe curtain materials of the project was shown in Table3.Table 3
Selection of pipe curtain materials.
Project typeDesign parametersSpecification of steel pipeQ235B seamless steel pipe is adopted, with pipe diameter of 299 mm, wall thickness of 10 mm, and steel pipe spacing of 0.36 MSpecification of latchMale and female lock catch shall be welded between steel pipes for pipe lap. Unequal angle steel of Q235B steel with specification of 80 mm × 50 mm × 10 mmThe tunnel construction of the project would take 5 months. According to the monitoring data, the maximum settlement of W4 monitoring point above the concealed excavation vault was 17.45 mm, and its settlement trend was shown in Figure9.It can be seen from Figure9 that the settlement in the actual construction process was divided into four stages: (1) the surface settlement in the pipe curtain construction stage was about 4.06 mm, and the pipe curtain was in a relatively stable stage in the excavation stage; (2) when the tunnel was excavated to the area directly below the monitoring point, the surface settlement rate was fast, but eventually tended to be stable; (3) after the initial support of the tunnel was connected, the ground settlement was stable within 17.45 mm; (4) there was basically no change in ground settlement during the initial support removal and secondary lining construction stages. It could be seen that the pipe curtain advance support construction would cause a certain degree of ground settlement, but after the pipe curtain was closed into a ring, it could effectively control the maximum ground settlement and ensure the safety of the surrounding environment during the short-term shutdown of the project (e.g., the Spring Festival holiday) [68].The geology of the tunnel in the construction section of the turn back line of Tianhe passenger station of Guangzhou Metro Line 3 was mainly strongly weathered granite eluvium. The net section of the designed double-track tunnel was horseshoe shaped, with a net height of 9.246 m, a net width of 11.5 m, and a length of 147.8 m. The stratum conditions were shown in Figure10. The area had heavy surface traffic, dense buildings, crisscross formation pipelines, and poor geological conditions. After comparison, it was decided to use the freezing method to strengthen the stratum. Freezing drilling adopted pipe following drilling combined with advanced drilling deviation correction technology. The brine temperature in the design active freezing period was −25∼−-30°C, the brine temperature in the maintenance freezing period was −20∼−25°C, the active freezing period was 150 days, the design maintenance freezing period was 120 days, the design average temperature of the freezing wall was −8°C, and the one-time horizontal freezing length was 138.8 m.Figure 10
Surface deformation monitoring points and surface displacement curve, vault settlement, and horizontal convergence curve.A total of 28 surface deformation measuring points were arranged in the turn-back line project area of Tianhe passenger station of line 3, and the location of the measuring points was shown in Figure10. It showed the surface deformation of the three measuring points 10-1#, E#, and F# arranged along the centerline of the tunnel. In the construction stage of freezing hole, the maximum cumulative value of ground settlement at the south end was −7 mm, with an average of −3.6 mm. The average cumulative settlement at the north end was ±1 mm, and the surface deformation at this stage was mainly caused by borehole excavation. In the tunnel excavation stage, the surface deformation is between 60 mm and 110 mm. After the construction is completed, the cumulative surface settlement was between 70 mm and 370 mm. Near the end of excavation, the surface frost heave was obvious, and the frost heave was relatively large in the middle of the turn-back line and along the tunnel centerline.Taking the arch crown settlement monitoring of zone II at 14 m from the south part as an example, the analysis of Figure10 showed that the arch crown settlement was closely related to the construction progress and generally increases first and then decreased. After the tunnel was excavated to the monitoring section, the settlement increases sharply after the tunnel was excavated to the monitoring section, with a maximum of about 6.5 mm. Then, with the continuation of excavation, the vault settlement of this section decreased gradually. Later, due to the removal of temporary support, the settlement increased again, but the increased value was smaller than that during excavation. After the tunnel section was just excavated, the walls on both sides converged inward rapidly. Within 35 m from the south end, the average horizontal convergence of the tunnel was about 8 mm and the maximum was 14 m. Then, due to the vertical stress field and horizontal temporary rigid support, the horizontal convergence decreased. After the temporary support was removed, the convergence value increased again, and the average final convergence value was about 3 mm [17].After comparing the construction period and surface settlement, the following conclusions can be drawn:(1) The construction period of AFM and pipe curtain method is relatively long, about 150 d.(2) In the strongly weathered and water rich weak stratum, the control effect of AFM on surface displacement is much better than that of pipe curtain method.(3) AFM will cause formation uplift during active freezing and maintenance, while pipe curtain method will not have a great impact on the formation in the reinforcement stage.


## 4. Reinforcement Effect of AFM

Since the advent of artificial freezing, with the efforts of many researchers and decades of development, its theoretical system and construction technology had become more and more mature. With the characteristics of environmental protection, low construction noise, no occupation of aboveground space, no impact on ground traffic and landscape, and little damage to the stratum, its application in subway tunnel engineering was gradually popularized. At present, the AFM was widely used in metro connection channel projects, shield import and export projects, and metro tunnel damage repair projects. Although there are some examples of application in subway tunnel section, it is less applied in section tunnel due to the limitation of freezing method technology.


### 4.1. Metro Contact Channel

The metro contact channel is an underground passage connecting the two-way adjacent sections of the metro. The two-line metro contact channel is generally located in the middle of each section tunnel. It is often built together with gathering and drainage pump stations. It is very important for disaster prevention and reduction and rescue evacuation of long-distance tunnel, and it is the key project of subway tunnel construction [69, 70]. When the connecting channel is under special working conditions and complex geological conditions, due to its low elevation, the unique cross-sectional shape of the connection is not conducive to the stress of the tunnel, and engineering accidents are likely to occur, resulting in serious consequences [71].There are 3 contact channels in the metro section tunnel between Nationalities University Station and Qingchuan Station of Nanning Metro Line 1, China. This paper took the No. 2 contact channel as an example to explore the effect of freezing through the water-rich pebble stratum. The cross-sectional view and the freezing hole layout and freezing wall design parameters of the 2-line contact channel between Nationalities University Station and Qingchuan Station were shown in Figure11 and Table 4.Figure 11
The cross-sectional view and the freezing hole layout and freezing wall design parameters of the 2-line contact channel between Nationalities University Station and Qingchuan Station.Table 4
Design parameters of the frozen wall of the connecting channel.
ParameterUnitNumerical valueSoil freezing temperature (tf)°C−1.64Initial soil temperaturetg°C15Brine temperaturets°C−28∼−30Thermal conductivity of frozen soilW/(m2·°C)1.965Specific heat of frozen soilkJ/(kg·°C)1.22Freezing coefficienta2—5This project evaluated the reinforcement effect of the freezing method by monitoring the accumulated settlement and surface displacement.


#### 4.1.1. Cumulative Settlement Monitoring

Figure11 is the time history curve of the deformation law in the tunnel of No. 2 contact channel in the Nationalities University and Qingchuan section.It could be seen from the temporal curve of arch crown settlement cumulative settlement of connecting channel and the temporal curve of tunnel clearance convergence cumulative settlement that the value of each measuring point had been fluctuating near zero, but the fluctuation range was not large, and the cumulative value did not exceed 5 mm.


#### 4.1.2. Surface Displacement Monitoring

The layout of field displacement measurement was shown in Figure11, and the surface displacement comparison curve was shown in it above.The center of the connecting channel was 23 m away from the ground surface. According to the layout of field surface displacement measurement in Figure11, the land settlement measurement point was located at the intersection of the centerline of the connecting channel and the surface, extending several meters outward. It can be seen from Figure 11 that the field measurement showed a quasinormal distribution of “large in the middle and small on both sides.” Field measurements show that when the active freezing time is about 43.22 days, the freezing wall reaches the expected design thickness, and the maximum cumulative settlement displacement actually monitored on the surface is +7.3 mm. The corresponding numerical simulation value was slightly larger than the actual monitoring value, but the curve change trend was basically consistent, which verifies the effectiveness of freezing reinforcement [72].Due to the particularity of the location of the connect channel, it was easy to become the collection place of groundwater, and it was also greatly affected by the excavation vibration of tunnel construction. Therefore, it was very easy to encounter adverse geological phenomena such as broken surrounding rock and strong water yield during the construction of the connecting channel, which met the construction conditions of freezing method. Relying on the connecting channel project of Nanning Metro Line 1, it was found that even in the water-rich sandy pebble stratum, the reinforcement effect of freezing method had achieved the expected effect, especially in the control effect of tunnel clearance convergence, and the control effect of freezing method had also achieved the expected effect in the surface settlement, indicating that the application of freezing method in the connecting channel of metro tunnel was feasible.


### 4.2. Entrance and Exit of Shield

With the improvement of shield technology, shield method is widely used in urban metro tunnel construction. However, in some weak and high water content special strata, the depth of shield starting and reaching the end well is generally large, which is prone to instability of tunnel ring face after tunnel portal breaking, excessive settlement of end stratum, difficult control of starting attitude, and initial kowtow. If the setting of drainage measures is unreasonable, it will also lead to problems, such as water inrush and sand gushing in the portal circle of shield tunnel [73]. The occurrence of the above disaster problems will not only affect the construction period, but also lead to waste of resources and even cause serious accidents [74].TA03 bid section of Nanjing Metro Line 10 (west extension line) started from the middle air shaft on the North Bank of the Yangtze River and ended at Jiangxinzhou station. There were many buildings along the project and the project environment is complex.The stratum at the shield launching shaft at the east end had the characteristics of large stratum permeability coefficient, high groundwater pressure, and poor stratum bearing capacity, which was easy to lead to excessive settlement of surrounding buildings and adverse geology in the shield launching shaft. Nanjing Yangtze River Tunnel and Pukou water plant near the starting well also have high requirements for settlement control. After various argumentation and analysis, it was decided to adopt triaxial mixing pile + vertical drilling freezing method for reinforcement. The following mainly studies the vertical drilling freezing method. The average temperature of the freezing curtain was designed to be −10°C, the effective thickness was 2.0 m, the width of the freezing wall was 21.2 m, and the depth was 31.152 m. The layout of vertical and horizontal freezing holes is shown in Figures12–14 [75].Figure 12
Plan of D10-TA03 bid section.Figure 13
Site layout of vertical freezing pipe.Figure 14
Cumulative settlement and surface displacement curve of No. 2 connecting channel in Minqing section.In order to evaluate the stability of the originating well, two surface soil displacement monitoring sections DB1 and DB2 are arranged along the tunneling direction in the frozen area. DB1 section was located between row A freezing holes and the enclosure structure, and DB2 section was located between row A and row B freezing holes. Each section is arranged with 13 monitoring points, symmetrically arranged on the axis of the tunnel center with a spacing of 1.9 m. The change of surface displacement in the vertical freezing area from the 6th day to the 56th day was shown in Figure14.Figure14 also showed that frost heaving occurred in both DB1 and DB2 sections. The freezing expansion amplitude of DB1 and DB2 reached the maximum value after 16 days of freezing, which was 4 mm and 7 mm, respectively. When freezing for 56 days, the frost heave of DB1 and DB2 sections decreased, which were 0∼2.3 mm and −0.4∼3.3 mm, respectively, meeting the expected requirements [76].The construction of shield departure and arrival end shaft was an important part of metro tunnel engineering. If the reinforcement measures of shield end shaft were not selected properly, it could delay the construction period or lead to end shaft collapse, water inrush, and even shield machine damage, resulting in serious consequences. In this project, the vertical freezing method was used to strengthen the stratum. After actual measurement, the maximum impact of the freezing method on the surface was only about 3 mm, and there were no accidents, such as water leakage and collapse of the end well in the subsequent construction. It could be seen that the application of freezing method in water rich muddy soil not only strengthened the end well, but also controlled the surface displacement within the allowable range and achieved the expected effect of freezing method.


### 4.3. Metro Interval Tunnel

Metro tunnels are generally long and the strata they pass through are diverse and complex. If the blind construction of stratum reinforcement is not carried out, or the selection of stratum reinforcement measures is unreasonable, it will not only lead to tunnel collapse, water inrush, and other disasters, but also a small construction disturbance will have a great impact when the metro shield construction passes through sensitive and high-precision areas [77]. Therefore, how to predict the settlement deformation trend in advance and take targeted protective measures in time has become a key technical problem to be solved in metro shield construction [78].Guangzhou Metro Line 2 crosses the Qingquan Street fault zone. The geological structure and stratigraphic lithology of this section were complex. There are fault fracture zones on the north and south sides and interlayer compression zones in the middle. The metro tunnel intersects obliquely with the Qingquan Street fault zone. Therefore, according to the geological conditions, it was decided to adopt the horizontal freezing method to strengthen the stratum, the shallow buried and concealed excavation method to excavate and lining, and the full section curtain freezing. The stratum was reinforced by freezing method to form a freezing circle, and the mining method was used for excavation and construction. The total length of freezing construction was 170.85 m, of which the freezing length of the right line tunnel in the north of the fan room was 63.45 m and the drilling length was 63.45 m. The freezing length of the left line tunnel was 53.10 mm and the drilling length was 53.60 m. Figure15 shows the schematic diagram of layout section of tunnel test elements and the layout of freezing holes in section tunnel.Figure 15
Tunnel ground displacement, convergence deformation, and surface heave and subsidence curve.Since the secondary lining was constructed after the frozen curtain is completely thawed, the monitoring focused on the initial support structure. Figure15 showed that the cumulative convergence value of the horizontal convergence deformation of the right line tunnel before the construction of secondary lining was 13.19 mm, while the maximum measured values of the horizontal convergence deformation of the three sections of ZDK14 + 748, ZDK14 + 760, and ZDK14 + 770 of the left line tunnel before the destruction of the measuring points were 31.03, 36.32, and 37.67 mm, respectively. After thawing, the surrounding rock deformation at the side walls on both sides of the tunnel would gradually spread to at least 3m depth. At this time, the displacement meter in 3 m stratum fails, and the horizontal convergence deformation and ground displacement of the tunnel on the left line were significantly greater than those on the right line. The analysis showed that the inverted arch had been built during the monitoring of the right line tunnel, and the initial support structure was in a closed-loop state. During the monitoring of the left line tunnel, the inverted arch had not been excavated, and the initial support structure was in an open-loop state. It was precisely because of the excavation of the inverted arch that the deformation of the left line was significantly greater than that of the right line.Figure15 monitoring results showed that there was no obvious uplift on the surface when the stratum was frozen, and the change during the excavation during the freezing period was also very small (the subsidence was about 5 mm). However, during thawing, the surface had a very significant thawing settlement phenomenon. After thawing for about one week, the surface was still in the trend of rapid subsidence. At this time, the subsidence rate was 1.4 mm/d and the cumulative subsidence was 13.1 mm [72].Due to the complexity and diversity of strata that metro tunnels passed through, stratum reinforcement measures should be tailored to the case. The project passes through the water rich fracture zone. Through the analysis of the field measured data, the surface displacement of the project during freezing was mainly composed of the deformation during thawing and the deformation after thawing. The deformation during freezing and maintenance included the impact of tunnel excavation on surface displacement, which had little impact on the surface, but there was significant thawing settlement in the thawing process, which was the most unfavorable link. Therefore, in the construction process of freezing method, we should not only pay attention to the influence of tunnel excavation on surface displacement, but also pay attention to the influence of thawing process on surface displacement.


### 4.4. Repair of Metro Tunnel Damage

With the increase of the number of metro tunnels, the construction of above ground engineering structures will inevitably have a certain impact on or even damage to the tunnel structure, especially in the case of complex and unstable geological conditions, such as water gushing, quicksand, and silt, which brings great difficulty to the repair project. Therefore, the design and construction technology of metro repair engineering has become the key in geotechnical engineering. With the wide application of artificial freezing technology in mine and underground engineering in China, the artificial freezing construction technology has reached a high level. For the tunnel repair project with poor geological conditions, the AFM is used to freeze the surrounding rock of the tunnel so as to form a frozen soil wall or frozen soil circle with high strength and good sealing around the damaged tunnel, which greatly improves the safety and efficiency of the repair project [16, 73, 79–82].During the pile foundation construction of an arch bridge project in a bid section of Beijing Shanghai high-speed railway, the Renmin Road metro tunnel under construction was partially damaged. The ground construction was shown in Figure16. The shield tunnel segment between ring 505 and ring 35 of the uplink was broken down by five PHC (prestressed high-strength concrete) pile foundations, causing soil to enter the tunnel. The damaged part of the uplink was completely blocked, with a length of about 42 m. The soil layer in the construction area of the interval tunnel repair section was gray silty clay.Figure 16
Vertical and horizontal displacement curves of tunnel segments.After many arguments of experts, due to the weak overburden of the metro and the use of other reinforcement methods, the surface displacement was not strictly controlled. Finally, it was decided to adopt the combination of horizontal freezing and vertical freezing to locally freeze the upper part of the damaged tunnel and within 3 m around. The horizontal freezing holes and vertical freezing holes were shown in Figure16.During the freezing process, the frost heave force of the soil will create compressive stress on the entire tunnel segment, it will lead to the deformation or even damage of the undamaged tunnel segment. The deformation of the tunnel in the project was reflected by the vertical and horizontal displacement of segments in the tunnel. The two measuring points A and B were arranged at the position where the horizontal and vertical directions pass through the center of the tunnel. This analysis intercepted the deformation monitoring of tunnel segments from the beginning of freezing to the 17th day of freezing, as shown in Figure16.As can be seen from Figure16, since the lower freezing wall was relatively weak, in order to strengthen the freezing, the horizontal freezer froze for 5 d before the ground vertical freezer. Therefore, due to the frozen soil force of frozen soil expansion, the tunnel segments began to move outward and floated upward after 4.5 d of freezing, but the maximum outward movement of the tunnel did not exceed 4 mm, and the maximum upward floating degree was about 6 mm. As the soil on the upper and both sides of the tunnel began to freeze, the deformation of the tunnel began to stabilize, and there was no obvious deformation [83].As a special auxiliary method of stratum reinforcement, AFM uses physical methods to reduce the stratum temperature and freeze the water in the stratum into ice so as to improve the strength of soil. From several aspects of the application of freezing method in subway engineering, freezing method is mostly used for subway contact passages and shield entrances and exits, but it is less used in the repair of interval tunnels and tunnel damage. This is due to the limitation of the reinforcement mechanism of the freezing method, which limits the scope of application of the freezing method, and because the cost of the freezing method and the long-distance freezing technology are not mature, the freezing method is currently applied to small- and medium-sized projects. From the above aspects, the AFM has good reinforcement effect in soft geology, such as water-rich sandy pebble stratum, water-rich fracture zone, water-rich sandy soil layer, and silty clay layer, the surface settlement control reaches the predetermined standard and has little impact on the surrounding strata.


## 4.1. Metro Contact Channel

The metro contact channel is an underground passage connecting the two-way adjacent sections of the metro. The two-line metro contact channel is generally located in the middle of each section tunnel. It is often built together with gathering and drainage pump stations. It is very important for disaster prevention and reduction and rescue evacuation of long-distance tunnel, and it is the key project of subway tunnel construction [69, 70]. When the connecting channel is under special working conditions and complex geological conditions, due to its low elevation, the unique cross-sectional shape of the connection is not conducive to the stress of the tunnel, and engineering accidents are likely to occur, resulting in serious consequences [71].There are 3 contact channels in the metro section tunnel between Nationalities University Station and Qingchuan Station of Nanning Metro Line 1, China. This paper took the No. 2 contact channel as an example to explore the effect of freezing through the water-rich pebble stratum. The cross-sectional view and the freezing hole layout and freezing wall design parameters of the 2-line contact channel between Nationalities University Station and Qingchuan Station were shown in Figure11 and Table 4.Figure 11
The cross-sectional view and the freezing hole layout and freezing wall design parameters of the 2-line contact channel between Nationalities University Station and Qingchuan Station.Table 4
Design parameters of the frozen wall of the connecting channel.
ParameterUnitNumerical valueSoil freezing temperature (tf)°C−1.64Initial soil temperaturetg°C15Brine temperaturets°C−28∼−30Thermal conductivity of frozen soilW/(m2·°C)1.965Specific heat of frozen soilkJ/(kg·°C)1.22Freezing coefficienta2—5This project evaluated the reinforcement effect of the freezing method by monitoring the accumulated settlement and surface displacement.


### 4.1.1. Cumulative Settlement Monitoring

Figure11 is the time history curve of the deformation law in the tunnel of No. 2 contact channel in the Nationalities University and Qingchuan section.It could be seen from the temporal curve of arch crown settlement cumulative settlement of connecting channel and the temporal curve of tunnel clearance convergence cumulative settlement that the value of each measuring point had been fluctuating near zero, but the fluctuation range was not large, and the cumulative value did not exceed 5 mm.


### 4.1.2. Surface Displacement Monitoring

The layout of field displacement measurement was shown in Figure11, and the surface displacement comparison curve was shown in it above.The center of the connecting channel was 23 m away from the ground surface. According to the layout of field surface displacement measurement in Figure11, the land settlement measurement point was located at the intersection of the centerline of the connecting channel and the surface, extending several meters outward. It can be seen from Figure 11 that the field measurement showed a quasinormal distribution of “large in the middle and small on both sides.” Field measurements show that when the active freezing time is about 43.22 days, the freezing wall reaches the expected design thickness, and the maximum cumulative settlement displacement actually monitored on the surface is +7.3 mm. The corresponding numerical simulation value was slightly larger than the actual monitoring value, but the curve change trend was basically consistent, which verifies the effectiveness of freezing reinforcement [72].Due to the particularity of the location of the connect channel, it was easy to become the collection place of groundwater, and it was also greatly affected by the excavation vibration of tunnel construction. Therefore, it was very easy to encounter adverse geological phenomena such as broken surrounding rock and strong water yield during the construction of the connecting channel, which met the construction conditions of freezing method. Relying on the connecting channel project of Nanning Metro Line 1, it was found that even in the water-rich sandy pebble stratum, the reinforcement effect of freezing method had achieved the expected effect, especially in the control effect of tunnel clearance convergence, and the control effect of freezing method had also achieved the expected effect in the surface settlement, indicating that the application of freezing method in the connecting channel of metro tunnel was feasible.


## 4.1.1. Cumulative Settlement Monitoring

Figure11 is the time history curve of the deformation law in the tunnel of No. 2 contact channel in the Nationalities University and Qingchuan section.It could be seen from the temporal curve of arch crown settlement cumulative settlement of connecting channel and the temporal curve of tunnel clearance convergence cumulative settlement that the value of each measuring point had been fluctuating near zero, but the fluctuation range was not large, and the cumulative value did not exceed 5 mm.


## 4.1.2. Surface Displacement Monitoring

The layout of field displacement measurement was shown in Figure11, and the surface displacement comparison curve was shown in it above.The center of the connecting channel was 23 m away from the ground surface. According to the layout of field surface displacement measurement in Figure11, the land settlement measurement point was located at the intersection of the centerline of the connecting channel and the surface, extending several meters outward. It can be seen from Figure 11 that the field measurement showed a quasinormal distribution of “large in the middle and small on both sides.” Field measurements show that when the active freezing time is about 43.22 days, the freezing wall reaches the expected design thickness, and the maximum cumulative settlement displacement actually monitored on the surface is +7.3 mm. The corresponding numerical simulation value was slightly larger than the actual monitoring value, but the curve change trend was basically consistent, which verifies the effectiveness of freezing reinforcement [72].Due to the particularity of the location of the connect channel, it was easy to become the collection place of groundwater, and it was also greatly affected by the excavation vibration of tunnel construction. Therefore, it was very easy to encounter adverse geological phenomena such as broken surrounding rock and strong water yield during the construction of the connecting channel, which met the construction conditions of freezing method. Relying on the connecting channel project of Nanning Metro Line 1, it was found that even in the water-rich sandy pebble stratum, the reinforcement effect of freezing method had achieved the expected effect, especially in the control effect of tunnel clearance convergence, and the control effect of freezing method had also achieved the expected effect in the surface settlement, indicating that the application of freezing method in the connecting channel of metro tunnel was feasible.


## 4.2. Entrance and Exit of Shield

With the improvement of shield technology, shield method is widely used in urban metro tunnel construction. However, in some weak and high water content special strata, the depth of shield starting and reaching the end well is generally large, which is prone to instability of tunnel ring face after tunnel portal breaking, excessive settlement of end stratum, difficult control of starting attitude, and initial kowtow. If the setting of drainage measures is unreasonable, it will also lead to problems, such as water inrush and sand gushing in the portal circle of shield tunnel [73]. The occurrence of the above disaster problems will not only affect the construction period, but also lead to waste of resources and even cause serious accidents [74].TA03 bid section of Nanjing Metro Line 10 (west extension line) started from the middle air shaft on the North Bank of the Yangtze River and ended at Jiangxinzhou station. There were many buildings along the project and the project environment is complex.The stratum at the shield launching shaft at the east end had the characteristics of large stratum permeability coefficient, high groundwater pressure, and poor stratum bearing capacity, which was easy to lead to excessive settlement of surrounding buildings and adverse geology in the shield launching shaft. Nanjing Yangtze River Tunnel and Pukou water plant near the starting well also have high requirements for settlement control. After various argumentation and analysis, it was decided to adopt triaxial mixing pile + vertical drilling freezing method for reinforcement. The following mainly studies the vertical drilling freezing method. The average temperature of the freezing curtain was designed to be −10°C, the effective thickness was 2.0 m, the width of the freezing wall was 21.2 m, and the depth was 31.152 m. The layout of vertical and horizontal freezing holes is shown in Figures12–14 [75].Figure 12
Plan of D10-TA03 bid section.Figure 13
Site layout of vertical freezing pipe.Figure 14
Cumulative settlement and surface displacement curve of No. 2 connecting channel in Minqing section.In order to evaluate the stability of the originating well, two surface soil displacement monitoring sections DB1 and DB2 are arranged along the tunneling direction in the frozen area. DB1 section was located between row A freezing holes and the enclosure structure, and DB2 section was located between row A and row B freezing holes. Each section is arranged with 13 monitoring points, symmetrically arranged on the axis of the tunnel center with a spacing of 1.9 m. The change of surface displacement in the vertical freezing area from the 6th day to the 56th day was shown in Figure14.Figure14 also showed that frost heaving occurred in both DB1 and DB2 sections. The freezing expansion amplitude of DB1 and DB2 reached the maximum value after 16 days of freezing, which was 4 mm and 7 mm, respectively. When freezing for 56 days, the frost heave of DB1 and DB2 sections decreased, which were 0∼2.3 mm and −0.4∼3.3 mm, respectively, meeting the expected requirements [76].The construction of shield departure and arrival end shaft was an important part of metro tunnel engineering. If the reinforcement measures of shield end shaft were not selected properly, it could delay the construction period or lead to end shaft collapse, water inrush, and even shield machine damage, resulting in serious consequences. In this project, the vertical freezing method was used to strengthen the stratum. After actual measurement, the maximum impact of the freezing method on the surface was only about 3 mm, and there were no accidents, such as water leakage and collapse of the end well in the subsequent construction. It could be seen that the application of freezing method in water rich muddy soil not only strengthened the end well, but also controlled the surface displacement within the allowable range and achieved the expected effect of freezing method.


## 4.3. Metro Interval Tunnel

Metro tunnels are generally long and the strata they pass through are diverse and complex. If the blind construction of stratum reinforcement is not carried out, or the selection of stratum reinforcement measures is unreasonable, it will not only lead to tunnel collapse, water inrush, and other disasters, but also a small construction disturbance will have a great impact when the metro shield construction passes through sensitive and high-precision areas [77]. Therefore, how to predict the settlement deformation trend in advance and take targeted protective measures in time has become a key technical problem to be solved in metro shield construction [78].Guangzhou Metro Line 2 crosses the Qingquan Street fault zone. The geological structure and stratigraphic lithology of this section were complex. There are fault fracture zones on the north and south sides and interlayer compression zones in the middle. The metro tunnel intersects obliquely with the Qingquan Street fault zone. Therefore, according to the geological conditions, it was decided to adopt the horizontal freezing method to strengthen the stratum, the shallow buried and concealed excavation method to excavate and lining, and the full section curtain freezing. The stratum was reinforced by freezing method to form a freezing circle, and the mining method was used for excavation and construction. The total length of freezing construction was 170.85 m, of which the freezing length of the right line tunnel in the north of the fan room was 63.45 m and the drilling length was 63.45 m. The freezing length of the left line tunnel was 53.10 mm and the drilling length was 53.60 m. Figure15 shows the schematic diagram of layout section of tunnel test elements and the layout of freezing holes in section tunnel.Figure 15
Tunnel ground displacement, convergence deformation, and surface heave and subsidence curve.Since the secondary lining was constructed after the frozen curtain is completely thawed, the monitoring focused on the initial support structure. Figure15 showed that the cumulative convergence value of the horizontal convergence deformation of the right line tunnel before the construction of secondary lining was 13.19 mm, while the maximum measured values of the horizontal convergence deformation of the three sections of ZDK14 + 748, ZDK14 + 760, and ZDK14 + 770 of the left line tunnel before the destruction of the measuring points were 31.03, 36.32, and 37.67 mm, respectively. After thawing, the surrounding rock deformation at the side walls on both sides of the tunnel would gradually spread to at least 3m depth. At this time, the displacement meter in 3 m stratum fails, and the horizontal convergence deformation and ground displacement of the tunnel on the left line were significantly greater than those on the right line. The analysis showed that the inverted arch had been built during the monitoring of the right line tunnel, and the initial support structure was in a closed-loop state. During the monitoring of the left line tunnel, the inverted arch had not been excavated, and the initial support structure was in an open-loop state. It was precisely because of the excavation of the inverted arch that the deformation of the left line was significantly greater than that of the right line.Figure15 monitoring results showed that there was no obvious uplift on the surface when the stratum was frozen, and the change during the excavation during the freezing period was also very small (the subsidence was about 5 mm). However, during thawing, the surface had a very significant thawing settlement phenomenon. After thawing for about one week, the surface was still in the trend of rapid subsidence. At this time, the subsidence rate was 1.4 mm/d and the cumulative subsidence was 13.1 mm [72].Due to the complexity and diversity of strata that metro tunnels passed through, stratum reinforcement measures should be tailored to the case. The project passes through the water rich fracture zone. Through the analysis of the field measured data, the surface displacement of the project during freezing was mainly composed of the deformation during thawing and the deformation after thawing. The deformation during freezing and maintenance included the impact of tunnel excavation on surface displacement, which had little impact on the surface, but there was significant thawing settlement in the thawing process, which was the most unfavorable link. Therefore, in the construction process of freezing method, we should not only pay attention to the influence of tunnel excavation on surface displacement, but also pay attention to the influence of thawing process on surface displacement.


## 4.4. Repair of Metro Tunnel Damage

With the increase of the number of metro tunnels, the construction of above ground engineering structures will inevitably have a certain impact on or even damage to the tunnel structure, especially in the case of complex and unstable geological conditions, such as water gushing, quicksand, and silt, which brings great difficulty to the repair project. Therefore, the design and construction technology of metro repair engineering has become the key in geotechnical engineering. With the wide application of artificial freezing technology in mine and underground engineering in China, the artificial freezing construction technology has reached a high level. For the tunnel repair project with poor geological conditions, the AFM is used to freeze the surrounding rock of the tunnel so as to form a frozen soil wall or frozen soil circle with high strength and good sealing around the damaged tunnel, which greatly improves the safety and efficiency of the repair project [16, 73, 79–82].During the pile foundation construction of an arch bridge project in a bid section of Beijing Shanghai high-speed railway, the Renmin Road metro tunnel under construction was partially damaged. The ground construction was shown in Figure16. The shield tunnel segment between ring 505 and ring 35 of the uplink was broken down by five PHC (prestressed high-strength concrete) pile foundations, causing soil to enter the tunnel. The damaged part of the uplink was completely blocked, with a length of about 42 m. The soil layer in the construction area of the interval tunnel repair section was gray silty clay.Figure 16
Vertical and horizontal displacement curves of tunnel segments.After many arguments of experts, due to the weak overburden of the metro and the use of other reinforcement methods, the surface displacement was not strictly controlled. Finally, it was decided to adopt the combination of horizontal freezing and vertical freezing to locally freeze the upper part of the damaged tunnel and within 3 m around. The horizontal freezing holes and vertical freezing holes were shown in Figure16.During the freezing process, the frost heave force of the soil will create compressive stress on the entire tunnel segment, it will lead to the deformation or even damage of the undamaged tunnel segment. The deformation of the tunnel in the project was reflected by the vertical and horizontal displacement of segments in the tunnel. The two measuring points A and B were arranged at the position where the horizontal and vertical directions pass through the center of the tunnel. This analysis intercepted the deformation monitoring of tunnel segments from the beginning of freezing to the 17th day of freezing, as shown in Figure16.As can be seen from Figure16, since the lower freezing wall was relatively weak, in order to strengthen the freezing, the horizontal freezer froze for 5 d before the ground vertical freezer. Therefore, due to the frozen soil force of frozen soil expansion, the tunnel segments began to move outward and floated upward after 4.5 d of freezing, but the maximum outward movement of the tunnel did not exceed 4 mm, and the maximum upward floating degree was about 6 mm. As the soil on the upper and both sides of the tunnel began to freeze, the deformation of the tunnel began to stabilize, and there was no obvious deformation [83].As a special auxiliary method of stratum reinforcement, AFM uses physical methods to reduce the stratum temperature and freeze the water in the stratum into ice so as to improve the strength of soil. From several aspects of the application of freezing method in subway engineering, freezing method is mostly used for subway contact passages and shield entrances and exits, but it is less used in the repair of interval tunnels and tunnel damage. This is due to the limitation of the reinforcement mechanism of the freezing method, which limits the scope of application of the freezing method, and because the cost of the freezing method and the long-distance freezing technology are not mature, the freezing method is currently applied to small- and medium-sized projects. From the above aspects, the AFM has good reinforcement effect in soft geology, such as water-rich sandy pebble stratum, water-rich fracture zone, water-rich sandy soil layer, and silty clay layer, the surface settlement control reaches the predetermined standard and has little impact on the surrounding strata.


## 5. Conclusion and Prospect

Based on the analysis of the reinforcement effect of AFM, the reinforcement effect of freezing method in metro engineering is ideal. By comparing the reinforcement effect of AFM with grouting reinforcement, pipe shed, pipe curtain, and other practical projects, it is found that the freezing method has obvious reinforcement advantages in water-rich soft strata. The following conclusions are drawn:(1)
AFM is a construction method to improve the strength of soil by cooling the soil and freezing the water in the soil into ice. At present, the research on the freezing mechanism of AFM has been relatively perfect, and three freezing construction methods have been derived: direct freezing method, cyclic freezing method, and mixed freezing method. Among them, cyclic freezing method is the most widely used.(2)
Due to its good reinforcement effect and little impact on the surrounding strata, the freezing method is widely used in metro engineering, including metro connecting channel engineering, shield entrance and exit, metro tunnel damage repair engineering, and AFM in metro section tunnel, and the reinforcement effect is good.(3)
In terms of reinforcement effect, the effectiveness of AFM and that of the other three ground strengthening methods in controlling surface displacement and tunnel envelope deformation were compared. The results show that AFM is more effective in controlling surface displacement and vault settlement, while grouting, pipe shed, and pipe curtain methods have absolute advantages in controlling surface uplift; in terms of construction difficulty, the freezing method is moderately difficult to construct and has the least impact on the surrounding stratigraphic environment.(4)
At present, the application of AFM in metro section tunnel has great limitations. At present, it is mostly used in small- and medium-sized projects in metro tunnel and has high requirements for water content in the stratum. At present, the construction efficiency has not reached the ideal goal. Therefore, AFM is a very potential stratum reinforcement method, which still needs further research.In recent years, in Shanghai, Guangzhou, Suzhou, and other water-rich soft soil areas with dense surface buildings, the existence of groundwater has put forward new requirements and challenges for the design and construction of underpass engineering [84–87]. With the development and improvement of communication technology, big data, and artificial intelligence technology, it is the future development trend to realize the informationization, automation, and intelligent construction and management of the whole life cycle of underpass tunnels, and it will develop toward high reliability, adaptation to complex environment, and multifunctionality [22, 39, 48, 71, 88–93] (Figure 17).Figure 17
Prospect of AFM.



---

*Source: 3251318-2022-08-04.xml*