Active Bearing Technology of Foot Steel Pipe Applied in Controlling the Large Deformation of Tunnels: A Case Study

Abstract

Foot steel pipe was the main arch foot supporting structure to control large deformation of loess tunnels, but the supporting effect was not ideal. Taking Yulinzi Tunnel in Qingyang, Gansu Province, as the engineering background, the design concept and implementation scheme of a foot steel pipe active bearing was put forward. The purpose was to solve the problem that it was difficult to control the surrounding rock settlement with the foot steel pipe. Numerical simulation and field experiments were used to verify the effect of the active bearing technology of the foot steel pipe. The main conclusions were as follows: (1) The effect of increasing the diameter of the foot steel pipe is better than that of increasing the number of foot steel pipes. (2) The active bearing mode of exerting its bearing capacity in advance by prepressing the foot steel pipe can effectively reduce the settlement of the vault. The settlement rate of the vault can be reduced by about 70% in 1–2 days and more than 50% in 1–3 days. (3) At the initial stage of surrounding rock deformation, this technology can provide a large bearing capacity, thereby reducing the overall deformation of the surrounding rock, slowing down the release of the surrounding rock pressure, and playing a positive role in the settlement control of the vault.

1. Introduction

As a particular soil, loess has unique engineering geological characteristics [1,2]. During the construction of loess tunnels, problems such as surface cracking, primary lining misalignment, lining deformation, and even landslides are common and need to be urgently addressed. In recent years, with the construction of railway line projects such as Zhengxi Passenger College in China, experts have conducted a series of technical research and achieved certain results. Zhao et al. systematically summarized the technical characteristics and main problems of the large-section loess tunnels on China’s high-speed railway [3]. Chen et al. studied the supporting theory, design method, and construction technology of highway loess tunnels [4]. They proposed the theory that the systematic bolt cannot restrain the deformation of the loess tunnels and established the method of combining steel frame and foot steel pipe (FSP) in the loess tunnels to control deformation. However, the supporting means are still not perfect, and the significant deformation problem of loess tunnels still occurs from time to time. The FSP is essential to the soft foundation tunnel support. It has the characteristics of convenient and fast construction, high rigidity, and remarkable effect and is often used for vault settlement control [5].

Many studies have been carried out on FSP at present. Regarding the stress and deformation characteristics of the FSP, Wu et al. believed that when the FSP is subjected to external loads, the deformation characteristics are similar to those of the bearing piles subjected to horizontal loads [5]. The supporting force mainly comes from three aspects: the normal pressure generated by the relative extrusion with the side wall of the FSP and the surrounding rock, the frictional resistance caused by the relative sliding of the surrounding rock and the side wall of the steel conduit, and the supporting force generated by the surrounding rock at the bottom of the FSP. Luo et al. used the elastic foundation beam to establish the mechanical calculation model of the FSP [6]. They used the series solution method to solve the beam’s differential equation to calculate the FSP’s stress state. Based on the ultimate bond strength between the surrounding rock and the cement mortar, it is judged whether there is penetration failure between the FSP and the surrounding rock when the anchor bolt is subjected to the axial force. Chen used the variable foundation coefficient method to derive the deformation characteristics of the FSP [7]. The research of Tzivakos et al., Chan et al., and Georgiadis et al. on reinforced concrete single piles in soft clay strata also have reference value for the study of FSP [8,9,10]. Mastering the force and deformation characteristics of the FSP helps evaluate and analyze its application effect in practical engineering.

Regarding the field application of FSP in loess tunnels, Chen et al. believed that FSP was not loaded independently but was welded together with the steel arch to carry the load as a whole [11]. Cao et al. also used structural mechanics and elastic foundation beam theory to establish a theoretical model of the overall bearing capacity of the tunnel support system under the shallow and weak surrounding rock [12]. Liang et al. used numerical analysis software to establish a three-dimensional calculation model of a loess tunnel, selected beam elements to simulate the FSP, and analyzed the influence of the length and insertion angle of the FSP on the deformation of the surrounding rock [13]. Cheng found through simulation that increasing the downward slope angle of the FSP has a certain effect on controlling the vertical displacement of the surrounding rock [14]. However, a smaller downward slope angle of the FSP is beneficial for reducing the horizontal convergence of the tunnel. The above analysis shows that different design parameters of the FSP lead to different supporting effects of surrounding rock.

Although much research on FSP has been carried out, the actual effect is still unsatisfactory in many cases. The reason is that only when the surrounding rock settlement is large can the FSP provide great bearing capacity. To play the role of rigid support in advance, WU et al. proposed the concept of applying pre-deformation to the FSP to achieve active load bearing [15]. The numerical simulation calculation shows that the vault settlement can be significantly reduced after preloading. With the increased stiffness of the FSP, the effect of controlling the settlement is more significant. However, the analysis of engineering practice and application effect of active load-bearing of FSP is almost blank in the world. Therefore, this paper puts forward the design concept and practical scheme of active load bearing. Numerical simulation and field measurement methods were adopted to verify the active bearing effect of FSP, which is of great significance for solving the problem of controlling the large deformation of loess tunnels.

2. Description of the Project

Yulinzi Tunnel is located in Qingyang, Gansu Province (Figure 1). It is a two-way, four-lane split tunnel with a length of 1900 m and a maximum buried depth of 112.35 m. the tunnel is located in the middle Pleistocene (Q²_eol) Lishi loess stratum. The longitudinal geological section is shown in Figure 2. A high-density electrical method and advanced geological prediction were used to conduct hydrogeological measurements. It was found that the groundwater was loose rock pore water, and the tunnel was buried at a depth of 100 m without passing through the underground water-rich area. During the tunnel construction, it was found that it was in a dry state during excavation. However, after the excavation, the tunnel gradually became wet, and the phenomenon of water and mud gushing continued. Finally, the tunnel face developed seepage, and the loess softened significantly. The primary lining was seriously deformed, and longitudinal cracks appeared in the inverted arch. Therefore, a supplementary investigation was carried out on the shallow water content of 100 m underground in the ZK280 + 200~ZK280 + 600 area. The survey found that with the disturbance of the surrounding rock mass caused by the tunnel excavation, groundwater began to infiltrate the vicinity of the tunnel excavation surface continuously, and many water-rich areas appeared in front of the tunnel, as shown in Figure 3.

The tunnel is constructed by a three-benches and seven-steps excavation method, and the support is a composite lining structure. The primary lining adopts a steel arch frame, FSP, and shotcrete, and no system bolt is set. Loess tunnels generally have the problem that the vaults sink significantly. Under the influence of water inrush on the tunnel face, there is also apparent water seepage on the primary lining surface of the Yulinzi Tunnel. The support structure appears to have severe deformation under the action of surrounding rock pressure. The average settlement of the primary lining was as high as 60–100 cm, and a landslide that penetrated through the ground (with a buried depth of nearly 100 m) occurred once at YK280 + 596.8. A secondary collapse occurred during the landslide treatment (Figure 4). When the primary lining deformation of the Yulinzi Tunnel was challenging to control, a large pipe shed was used for advanced support, which caused a severe uneven surface of the primary lining structure, as shown in Figure 5. Due to the continued softening of the surrounding rock under the action of groundwater, the deformation rate of the closed-loop support structure can still occur at 3–5 mm/d. If the above-mentioned significant deformation problem of the supporting structure is not solved, it is hard to ensure the safety of the tunnel excavation process.

3. Active Bearing Concept of the FSP

3.1. FSP and Its Action Mechanism

Due to the low bearing capacity and strong deformation capacity of surrounding rock, tunnels with weak surrounding rock such as water-rich loess are prone to large subsidence and even collapse [16]. FSP is a common method to control large settlement of tunnel. Its implementation scheme is as follows: during or after the primary lining construction of each step of the tunnel, steel pipes with larger diameter are driven horizontally or diagonally downward from the arch foot of the tunnel to the outside, and the steel pipe tail and the steel arch frame of the primary lining are firmly welded to achieve common deformation and bearing (Figure 6). Commonly used steel pipes have diameters of 42 mm, 50 mm, 89 mm, and 108 mm.

According to the related research [5], the supporting force of the FSP on the primary lining structure comes from three aspects: the normal pressure caused by relative extrusion between the surrounding rock and the steel pipe side wall, the friction resistance caused by relative slip between the surrounding rock and the steel pipe side wall, and the supporting force of the surrounding rock at the bottom of the steel pipe. The normal pressure caused by the relative extrusion between the surrounding rock and the steel pipe side wall is the main part. The FSP is driven in the surrounding rock on both sides of the tunnel, and its end bears the vertical load and bending moment transmitted by the arch frame. When the steel pipe has bending deformation, the surrounding soil will restrict the deformation of the steel pipe. Therefore, the steel pipe body will be supported by the uneven soil, and the supporting force is proportional to the deformation of the steel pipe. Therefore, when analyzing the internal force of the steel pipe bearing transverse load in the soil, the soil can be simplified into a spring only under compression. Based on the Winkle hypothesis, a semi-infinite length elastic foundation beam model can be established (Figure 7).

Numerical methods can also be used for simulation analysis. Considering the size characteristics and large transverse deformation of the FSP, finite element simulation using ABAQUS software can better solve the problems of geometric nonlinearity and material nonlinearity and improve the accuracy of numerical simulation results. Taking a steel pipe with a diameter of 50 mm as an example, according to the Saint Venant principle, in order to prevent boundary effects from affecting the analysis accuracy, the surrounding rock that exceeds the range of five times the diameter of the steel pipe is taken as the research object. Since the steel pipe is a slender column and the surrounding rock area is a cylinder, better grid quality can be obtained. The length of the rock and soil mass is 3000 mm, and the bottom radius is 500 mm. The area occupied by the steel pipe is excavated in advance to form a hollow structure. The outer diameter of the FSP is 50 mm, the wall thickness is 5 mm, and the length is 2500 mm. According to the symmetry of the model structure and stress deformation, a calculation model is established by taking half of the surrounding rock mass and steel pipe, as shown in Figure 8.

Under the influence of construction excavation, the stress state of the surrounding rock at the side of the tunnel is complex, but it can be determined that the density of surrounding rock along the longitudinal driving direction of the steel pipe is increasing, and the surface of surrounding rock exposed to the outside is unrestricted. This numerical simulation is simplified according to the stress state of the surrounding rock mass and steel pipe in the actual project. In the initial state, through the balanced geo-stress analysis step, the stress of the soil mass along the longitudinal driving direction of the steel pipe at the locking feet increases continuously, which is used to reflect the loose degree and stress release of the surrounding rock mass. Before the steel pipe is loaded laterally, there is a longitudinal dead weight stress, which is used to reflect the initial stress caused by the friction between the steel pipe and the soil during the driving process. Through the above analysis, it can be determined that the constraints of each component in the numerical simulation are as follows: the symmetry plane of the entire assembly only restricts the displacement in the x direction; the side of the cylindrical soil mass can produce longitudinal horizontal displacement but cannot produce transverse deformation; the soil bottom directly restricts the displacement in x, y, z directions; there is no displacement constraint on the top surface of soil mass; and the end of the steel pipe bears the vertical load, and there is friction contact with the side and bottom of the hollow soil component. The tangential friction coefficient is 0.39, and the normal direction adopts “hard” contact, and it is allowed to separate after contact. Both loess and steel pipe are simulated by 3D solid element C3D8. The loess parameters are determined by combining geotechnical experiments with on-site steel pipe lateral bearing tests. The joint development of loess is not considered, and the loess soil material is assumed to be isotropic without tectonic stress.

The formation of the supporting force depends on the bending deformation of the steel pipe end (Figure 9): In the bearing limit range, the greater the bending deformation, the greater the supporting force, and vice versa. When the deformation exceeds the plastic limit, the formed supporting force provides slow growth or no more growth.

3.2. The Problem of Passive Bearing of the FSP

It can be seen from the bearing mechanism that the vertical supporting force of the steel pipe depends on the downward bending deformation of its end. However, in the traditional construction technology of FSP, the FSP is firmly welded to the primary lining after installation, and the FSP has not been deformed at this time, so it cannot provide support force. Only when the primary lining continues to sink and the end of the FSP is deformed vertically, which leads to the bending of the steel pipe, can the supporting force be provided, and the FSP is in a passive deformation and bearing state.

Due to the soft surrounding rock tunnel loose range, loose surrounding rock pressure is often a nonlinear positive correlation with the surrounding rock and primary lining deformation. After tunnel excavation and primary lining, with the gradual deformation of the primary lining, the surrounding rock near the excavation profile first deforms and loosens. The larger the primary lining deformation, the more the surrounding rock loosens, and the increasing speed of the surrounding rock loosening is much faster than the increasing speed of the primary lining deformation. Therefore, the passive bearing of the FSP will greatly weaken its effect of controlling the settlement deformation of the primary lining.

3.3. The Concept and Method of the Active Bearing of Pre-Deformed FSP

In view of the problem of the passive bearing of the FSP under the traditional technology, it is extremely necessary to adopt specific technical measures to make the FSP take the initiative to bend downward and form an upward support force before further settlement of the primary lining and actively exert its bearing capacity to resist settlement so that the primary lining structure enters a stable state. The further settlement will not cause a wider range of deformation and loosening and greater surrounding rock pressure.

The specific process of the active load-bearing of the FSP is shown in Figure 10. The vertical support force provided by the active bearing is up to f₀ + f₂ − f₁ more than that provided by the passive bearing. The methods to realize pre-deformation and active bearing of FSP are as follows [17]: After the FSP is driven, the vertical load is applied to the end of the FSP through a special loading device to make it downward displacement and bending. Then, the FSP and the primary lining structure are connected to form a whole, and the preloaded load is released. The supporting force caused by the bending deformation of the FSP is directly transferred to the primary lining structure, so that the primary lining, which will continue to have settlement deformation, enters the equilibrium state, and the deformation is controlled.

4. Numerical Analysis of Active Bearing of the FSP

4.1. Numerical Model Building and Material Parameters

The plane numerical calculation model is established by ABAQUS software, and the model size is 80 m × 45 m. Since the influence of the base reaction force is generally ignored in the weak surrounding rock, the upper bench arch is suspended in the calculation part. The x-direction constraints are applied to the model on both sides. The x and y direction constraints are applied at the bottom. A vertical load is applied to the upper part to simulate the actual burial depth, and the numerical calculation model is shown in Figure 11.

The FSP is simplified in the calculation process, and the FSP considers only the vertical support force of the arch frame. Instead of establishing the beam element model separately, the FSP’s bearing capacity is extracted separately and directly applied to the arch foot. Considering the nonlinear characteristics of the load-bearing deformation curve of the FSP, the built-in nonlinear spring element in ABAQUS is adopted to simulate the vertical bearing capacity of the FSP, as shown in Figure 12. When a nonlinear spring is used to replace the FSP, only the restraint force in the direction is required. By moving the displacement of the fixed end of the spring, the active bearing effect of the steel pipe can be quickly simulated. The active bearing effect of the FSP was analyzed by controlling the preload of the FSP.

According to the basic requirements of the geotechnical test, physical and mechanical tests were carried out on the Q2 loess around Yulinzi Tunnel, including measuring the soil moisture content and natural density and measuring its cohesion and internal friction angle through the direct shear test. Partial material parameters can be determined as shown in

Table 1. Material parameter table.

Material	Density (kN/m3)	Elastic Modulus (MPa)	Poisson’s Ratio	Cohesive Forces (kPa)	The Angle of Internal Friction (°)	Constitutive Model
15% water content of loess	16	10	0.3	40	22	M-C
20% water content of loess	16	8	0.3	16	22	M-C
Foot steel pipe (HPB235)	78	206,000	0.3	yield stress: 235 MPa		elasticoplasticity

. The factors considered in the calculation of the working conditions mainly include two aspects: On the one hand, the change of the parameters of the FSP, considering that the diameter of the FSP is 42 mm, 50 mm, 89 mm, and the number of pipes is 2 and 4. On the other hand, different load release coefficients are selected.

4.2. Analysis of Active Bearing Effect of the FSP

(1) The influence of steel pipe length

To study the influence of the length to FSP’s bearing capacity, we explored the efficient use of the steel pipe length in practical engineering applications on the premise of not changing the parameters of rock and steel pipe materials, only gradually shortening the modeling of the steel pipe length, exerting the same displacement at the end of the steel pipe, through the bearing and deformation behavior of the steel pipe and contact stress analysis of the influence degree of the length of steel pipe bearing.

Figure 13 shows the bearing deformation curves of the 42 × 4 mm steel pipe with different pipe lengths in loess stratum with 15% water content. It can be seen from the comparison that when the length of the steel pipe is 1.0 m or above, the bearing curves basically coincide and have no influence on the bearing deformation characteristics. When the length of the steel pipe is in the range of 0.75 m~1.0 m, the pipe length only affects the shape and slope of the bearing curve of the steel pipe but does not affect the ultimate bearing capacity of the steel pipe. When the pipe length is less than 0.75 m, the pipe length has a great influence on the bearing curve and ultimate bearing capacity of the steel pipe.

Figure 14 compares the bending deformation patterns of steel tubes with different lengths under a transverse load. According to the bending form of the steel pipe, when the length of the steel pipe is less than 0.75 m, the steel pipe has no obvious bending phenomenon, and the soil cannot restrain the penetrating failure of the bottom of the steel pipe, and the bottom deformation can reach 0.02 m. When the length of the steel pipe is 0.75 m~1.0 m, with the increasing contact area between the soil and the upper part of the steel pipe, the steel pipe will gradually bend at a certain point, and the ultimate bearing capacity of the steel pipe is gradually independent of the length. When the length of steel pipe is greater than 1.0 m, the bearing characteristic curve of steel pipe is directly unrelated to the length of steel pipe, so the length of steel pipe commonly used in the tunnel has almost no influence on the bearing capacity.

(2) Active bearing effect of FSP under different parameters

Under the traditional bearing mode, the variation law of vault settlement with the load release process is obtained through long-term on-site monitoring and numerical calculation. Based on the load release law, the reduction range of the vault settlement under the preloading of 5 cm and 10 cm at the ends of various FSP was investigated. The change of the active bearing effect of the FSP during the gradual release of the surrounding rock load was compared and analyzed. The installation is as shown in the Figure 15.

Table 2. The influence of four active bearing FSPs of 42 × 4 mm on settlement of the arch vault.

Load Release Rate	The Normal State	Preload a Displacement of 5 cm at the End of the Steel Pipe		Preload a Displacement of 10 cm at the End of the Steel Pipe
	Settlement of the Vault/mm	Settlement of the Vault/mm	Damping/%	Settlement of the Vault/mm	Damping/%
5%	20.7257	18.3299	11.56	17.7929	14.15
10%	46.9458	45.3057	3.49	44.9062	4.34
15%	79.1534	78.4163	0.93	78.1401	1.28
20%	115.958	115.535	0.36	115.493	0.40

,

Table 3. The influence of four active bearing FSPs of 50 × 5 mm on settlement of the arch vault.

Load Release Rate	The Normal State	Preload a Displacement of 5 cm at the End of the Steel Pipe		Preload a Displacement of 10 cm at the End of the Steel Pipe
	Settlement of the Vault/mm	Settlement of the Vault/mm	Damping/%	Settlement of the Vault/mm	Damping/%
5%	20.0421	16.4076	18.13	15.1168	24.57
10%	45.5087	42.4936	6.63	41.6787	8.42
15%	76.6237	74.751	2.44	74.2158	3.14
20%	112.01	111.23	0.70	111.084	0.83

,

Table 4. The influence of four active bearing FSPs of 89 × 8 mm on settlement of the arch vault.

Load Release Rate	The Normal State	Preload a Displacement of 5 cm at the End of the Steel Pipe		Preload a Displacement of 10 cm at the End of the Steel Pipe
	Settlement of the Vault/mm	Settlement of the Vault/mm	Damping/%	Settlement of the Vault/mm	Damping/%
5%	16.1067	5.3998	66.47	0.0311	99.81
10%	37.1501	27.3783	26.30	23.4207	36.96
15%	63.0909	55.4235	12.15	52.9629	16.05
20%	92.7426	88.2744	4.82	86.5692	6.66

and

Table 5. The influence of two active bearing FSPs of 89 × 8 mm on settlement of the arch vault.

Load Release Rate	The Normal State	Preload a Displacement of 5 cm at the End of the Steel Pipe		Preload a Displacement of 10 cm at the End of the Steel Pipe
	Settlement of the Vault/mm	Settlement of the Vault/mm	Damping/%	Settlement of the Vault/mm	Damping/%
5%	18.6706	12.9227	30.79	10.8579	41.84
10%	42.695	37.9235	11.18	36.5386	14.42
15%	72.1146	68.9974	4.32	68.0879	5.58
20%	105.973	104.522	1.37	104.124	1.74

correspond to the changes in settlement of the vault under different parameters of the FSP.

By comparing the vault settlement with and without active bearing under various supporting conditions, it can be found that the active bearing of the steel pipe has a certain control effect on the vault deformation. By comparing the active bearing effect of the FSP under different load release rates, it is found that the lower the load release coefficient of the surrounding rock, the smaller the vault settlement after preloading the FSP and the greater the reduction of the vault settlement. After excavation, the FSP was installed as soon as possible, and the active bearing scheme was adopted, which significantly reduced the deformation of the support in the early stage. Taking the 50 mm diameter FSP as an example, when the end of the steel pipe is preloaded by 10 cm, and the load release rate of the surrounding rock is 5%, the settlement of the vault is reduced by 24.57%. When the load release rate is 20%, the settlement reduction of the vault is 0.83%, and the FSP’s active bearing effect is not apparent. The main reason is that when the deformation pressure of the surrounding rock is slight, the FSP can provide sufficient bearing capacity in preloading. Sufficient bearing capacity can effectively reduce the release of surrounding rock pressure, thereby controlling the settlement of the vault.

By comparing the control effects of different FSPs on the vault settlement after preloading, it can be seen that increasing the number of FSPs and the amount of preloading at the ends can reduce the vault settlement. Taking the 89 mm diameter of FSP as an example, when the load release rate is 5%, the preloading of the FSP increases from 5 cm to 10 cm, and the reduction of the vault settlement increases from 30.79% to 41.84%. After the FSP number is increased from 2 to 4, the vault settlement is reduced from 12.92 mm to 5.40 mm, and the reduction rate is doubled. Therefore, when the preload is constant, if the overall bearing stiffness of the FSP is more considerable, the active bearing effect will be more obvious.

(3) Variation of active bearing effect with the excavation time

After the tunnel is excavated, the surrounding rock stress is gradually released with the development of time. According to the on-site monitoring and calculation results, the change of the surrounding rock pressure with time is calculated. The surrounding rock pressure at the corresponding time is brought into the simulation calculation, and the variation law of the vault settlement with time under the active action of the FSP with or without the foot lock is obtained.

This method can more realistically simulate the effect of the active bearing of the FSP on the settlement of the vault after the tunnel is excavated on the upper benches. Figure 10 is a detailed comparison of the tunnel support effect of 42 × 4 mm FSP with or without active bearing based on the known controlling effect of 50 × 5 mm FSP.

As can be seen from Figure 16, with the steel pipe with small stiffness, that is, under the steel pipe diameter of 42 mm and the steel pipe diameter of 50 mm without active bearing, it can be seen that the settlement curve of the steel pipe with the diameter of 50 mm is located above the diameter of 42 mm, and when the steel pipe with the diameter of 42 mm is actively bearing, obviously the curve of the steel pipe with the diameter of 42 mm under the active bearing is located above the steel pipe with the diameter of 50 mm without active bearing. This shows that even the steel pipe with small stiffness can play a better supporting effect by preloading, which is feasible.

Figure 17 and Figure 18 compare the support effect of 89 mm steel pipe preloading displacement of 50 mm. At the end of the third day, the steel pipe was prepressed, and the active load was applied. After one day of application, it was found that the settlement of the vault was reduced by 17.3 mm and the cumulative difference of vault settlement within 11 days was 15.4 mm. The final vertical bearing capacity of the steel pipe increased from 137.1 kN to 158.6 kN, and the safety factor decreased from 1.236 to 1.068. It can be seen from this that the active bearing of the FSP can effectively control the settlement of the vault.

Therefore, the active bearing of the FSP is not to permanently control the deformation of the vault as the main structure but to enable the steel pipe to provide a strong bearing capacity as soon as possible. The active bearing can slow down the deformation rate of the tunnel in a short time, reduce the disturbance degree of the surrounding rock mass caused by the tunnel construction, and reduce the release coefficient of the surrounding rock pressure to ensure the construction safety of the tunnel during the excavation process. When the primary lining is closed, the bearing effect of the steel pipe can be ignored.

5. Field Application of Active Bearing of the FSP

5.1. Implementation Process of Active Bearing

After the FSP is installed in the tunnel, a simple loading device needs to be applied to preload the end of the steel pipe so as to achieve the active bearing effect of the FSP. At the same time, before the steel pipe preloading, the appropriate preloading amount shall be calculated according to the surrounding rock conditions, and the steel pipe preloading shall be carried out quickly on the premise of basically not interfering with the construction so as to better conduct the tunnel site test and application.

The tunnel construction environment is complex, and the setting position of the FSP and the included angle with the arch are different. What is worse, there is no stable loading platform in the tunnel. Therefore, it is the key factor to realize the active load-bearing of the FSP to preload the end of the FSP more conveniently in the tunnel.

According to the structural form of the arch frame and the loading test of the FSP, the loading support suitable for the steel arch frame is designed. The loading support is easy to install and can adapt to the vertical loading of the FSP at different positions of the arch frame. Details of the loading device can be seen in Figure 19.

To ensure that the active bearing design scheme and loading device of FSP can be used stably in the tunnel, the specific construction steps are as follows:

(1) After the excavation of the upper bench of the tunnel is completed, the steel arch shall be erected, and the FSP shall be driven. The end of the steel pipe shall be sealed before driving to prevent the soil from entering;

(2) A self-designed loading support is installed at a position 30 cm upward from the end of the steel pipe. One hand holds the handle of the loading base, hooks one end of the “7”-shaped screw to the arch or grating steel frame, and the other hand rotates the nut to temporarily fix it and then uses a wrench to tighten it [17]. The device can bear several tons of weight;

(3) A loading jack device and a connecting member between the steel pipe and the loading device are installed under the loading support;

(4) To ensure the safety of the loading process, adjust the angle adjusting screw at the bottom of the loading support to keep the loading device vertical or perpendicular to the steel pipe. The installation diagram can be seen in Figure 20;

(5) Start loading, and stop loading when the preload amount reaches the design requirements;

(6) If active loading of steel pipes is to be carried out in the middle of tunnel deformation, foam filling shall be carried out within 30 cm of the connection between steel pipes and steel arches to reserve the loading space. After the concrete spraying is completed and reaches a certain strength, the steel pipe can be loaded with the help of concrete as the loading surface;

(7) To achieve the active bearing effect, after loading, the steel arch is overlapped or welded with the steel pipe, and then the loading device is removed.

5.2. Field Application Process of the FSP

In the section from zk280 + 575 to zk280 + 590 on the left line of the Yulinzi Tunnel, two FSPs on both sides of the upper bench arch were continuously preloaded. According to the advanced geological forecast, the surrounding rock geological conditions in the test section were basically unchanged, and there was no fault fracture zone. During the continuous excavation of the upper bench of the tunnel the site photos were taken, which can be seen in Figure 21, and we started the test preparation. After the construction of the arch frame and the FSP was completed manually, we installed the loading support at the appropriate position of the arch frame (Figure 22). Then we adjusted the bearing panel angle of the support and then fixing the hand-held loading device and started to preload the ends of the FSP on both sides of the arch in sequence (Figure 23). The spacing between arches was 60 cm, and 16 steel pipes with a diameter of 50 mm were preloaded in total. The preloading amount was controlled at 10 cm. The length of the cavity formed by the steel pipe on the surface of the surrounding rock was about 3 cm, which meets the preloading amount limited in this paper. After the steel pipe preloading was completed, the steel pipe and the arch frame were welded together in the form of overlapping. After all the FSP of the two adjacent arches were preloaded, shotcrete was started, and the monitoring points for vault settlement was arranged to start deformation monitoring.

5.3. Application Effect Analysis of the FSP

To verify the application effect of the active bearing of FSP in tunnel deformation control, deformation monitoring was carried out with the help of a total station, and the variation law of vault settlement with time under the support of FSP was analyzed, as well as the condition without FSP.

After continuously monitoring the deformation of the arch frame with active bearing and the adjacent arch vault area without active bearing for two weeks, it was found that the amount and rate of the settlement of the arch vault were significantly reduced. The comparison of specific monitoring data can be seen in Figure 24.

The excavation direction is from ZK280 + 575 to ZK280 + 590, and three sections were selected, among which ZK280 + 575 is the first section, which uses passive bearing and has the largest settlement value; ZK280 + 585 is the second section, which uses active bearing and has the smallest settlement value; ZK280 + 595 is the third section. The passive load was used, but it was affected by the active load effect of the previous section ZK280 + 585 and was close to the second section. The deformation value was less than 575, which is an intermediate value.

Through the comparison of the settlement data of the arch vault, it can be found that after the active load is applied, the settlement and settlement value of the arch vault are significantly reduced within 1 to 3 days, and the settlement value is reduced by more than 50%. The main reason is that when the initial deformation of the surrounding rock just occurs, the FSP can provide more than 60% of the ultimate bearing capacity, and the deformation rate of the surrounding rock is limited obviously. With the continuous sinking of the arch vault, the lateral bearing stiffness of the steel pipe gradually decreases, and the vertical supporting capacity provided by the steel pipe gradually reaches the ultimate bearing capacity. Without other auxiliary measures, the deformation of the surrounding rock will not be effectively restrained until the surrounding rock pressure is slowly released through deformation and reduced to balance with the supporting force. When the middle bench is gradually excavated, the effect of the active bearing of the FSP on the upper bench is no longer obvious, but the overall settlement of the tunnel vault is smaller than that of the section without active bearing, with a reduction of about 2~5 cm, accounting for more than 20% of the total deformation during the excavation of the upper bench.

Therefore, it can be seen that the active bearing of the FSP can reduce the deformation of the surrounding rock. However, due to the limited bearing capacity of the steel pipe itself, the optimal action time in deformation control is relatively short. If the closed loop of initial support can be completed as soon as possible, the effect of the active bearing of steel pipe to control surrounding rock deformation will be more obvious.

6. Conclusions

In this paper, based on the theory of active bearing of FSP and relying on the Yulinzi Tunnel in Qingyang, Gansu Province, the research was carried out. The numerical simulation method was used to analyze the active bearing effect under different parameter conditions, and the specific implementation scheme of active bearing technology in the field was proposed, and the deformation control effect was verified. The main conclusions are as follows:

(1)

The bearing capacity process of the traditional FSP is slow, and the active bearing can provide the bearing capacity of the FSP as early as possible. Only when the surrounding rock settlement is large can the traditional FSP provide a large bearing capacity, while the active bearing can provide the bearing capacity in advance by preloading.

(2)

The nonlinear spring can be used to simulate the bearing capacity of the steel pipe when simulating the supporting effect of the FSP on the tunnel. The control effect of increasing the diameter of FSP on tunnel deformation is better than that of increasing the number of steel pipes. However, due to the limited bearing capacity of the steel pipe itself, the deformation of the surrounding rock cannot be largely controlled simply by increasing the number or diameter of the steel pipe, and other auxiliary measures need to be taken.

(3)

The self-designed loading support realizes the active bearing of FSP can completely fit the size of any arch and grid steel frame. After the construction of the arch frame and FSP is completed manually, the preloading process at the end of FSP can be quickly carried out during tunnel construction by selecting a suitable type of loading device.

(4)

By adopting the active bearing method of FSP, the settlement rate of the arch vault can be significantly reduced within 1~2 days, which can be reduced by about 70%. The settlement of the arch vault can be reduced by more than 50% within 1~3 days. Therefore, the active bearing technology of FSP can provide a large bearing capacity at the initial stage of the surrounding rock deformation. It has a positive effect on reducing the overall deformation of the surrounding rock, slowing down the release of the surrounding rock pressure, and controlling the settlement of the arch vault.