Pre-Reinforcement Mechanism and Effect Analysis of Surface Infiltration Grouting in Shallow Buried Section of Long-Span Tunnel

Abstract

In order to solve the problem that the hole-forming rate of boreholes is low and it is difficult to reach the designed length when supporting a long pipe shed in loose stratum in a shallow buried section of a long-span tunnel, it is necessary to pre-reinforce the loose stratum in order to improve the strength and integrity of the surrounding rock. Relying on the grouting project of the shallow buried section at the exit of Botanggou tunnel, it is assumed that the grouting material is Newtonian fluid and the steel floral tube shows cylindrical infiltration and diffusion. Through the analysis of the structural characteristics of the injected stratum, the conceptual model of infiltration grouting is established. Twelve groups of test slurry were prepared with ordinary Portland cement and ultra-fine cement, and through the analysis of the slurry parameters of each group, ordinary Portland cement slurry was selected with a water–cement ratio of 1:1 plus 3% water glass to strengthen the gravel layer, and ultra-fine cement slurry with a water–cement ratio of 1:1 plus 3% water glass and 0.3% polycarboxylate superplasticizer to strengthen the fully and strongly weathered porphyritic granite layer. Through the on-site single-hole grouting test and combining with the empirical formula, the maximum diffusion radius of single-hole infiltration grouting is calculated, and the sliding width of the sidewall is deduced using Terzaghi theory. To ensure the grouting effect, the 5 m expansion of the excavation profile is taken as the grouting range. Grouting construction adopts the overall order of periphery and then interior, and three-sequence opening and grouting are adopted in the same row of grouting holes, which can effectively prevent grouting running and grouting. For the strata treated by surface grouting, the construction of the long pipe shed is smooth and reaches the designed length, and there is no large deformation of the surrounding rock when excavated using the CD method. The treatment effect is analyzed by the P-Q-t control method, excavation observation method, and deformation monitoring method. The results show that the injected stratum is fully infiltrated and gelled, forms an obvious grouting stone body, the integrity and strength of surrounding rock are obviously improved, and the convergence values of the tunnel surface, vault subsidence, and clearance do not exceed the alarm value of 60 mm. The research results provide some awareness and understanding of the grouting pre-reinforcement of loose stratum in a shallow buried section of a long-span tunnel in the future.

1. Introduction

The rock–soil mass in the shallow buried section of a long-span mountain tunnel is mostly completely and strongly weathered rock mass or residual and slope deposits, which are loose in structure and poor in integrity. Tunnel excavation without support often leads to geological disasters such as deformation of the working face, falling of vault, collapsing of side walls, ground subsidence, and even roof fall, bringing great inconvenience to construction and seriously threatening the safety of construction personnel [1,2,3,4,5,6]. How to pass the shallow buried section of a long-span tunnel safely, economically, and quickly has become a major subject for domestic and foreign engineers and technicians [5,6,7,8,9,10,11]. At present, the most common measure is to use the pipe shed pre-support to control the excavation deformation of the shallow buried section of the tunnel, so as to ensure safe entering and smooth tunneling. However, the hole-forming rate of boreholes is low and it is difficult to reach the designed length. Therefore, tunnel builders have been working hard to find another stratum pre-reinforcement measure to address the issue. In some projects, the surface grouting method has been tried to pre-reinforce the loose stratum in the shallow buried section to cooperate with the pipe shed forming.

The surface grouting pre-reinforcement method is used to inject cementitious materials into the cracks or holes of rock–soil mass under a certain grouting pressure. Then, a series of measures are taken to fill, infiltrate, compact, and split the injected rock–soil mass so that the particles or blocks are connected with each other. It is used to achieve the purpose of anti-infiltration and water blocking and increase the tensile and compressive strength of the rock–soil mass. This measure has the advantages of low cost, quick effect, and simple operation. It is increasingly applied to the stratum reinforcement and infiltration prevention of hydraulic engineering [12,13], tunnels [14,15,16,17], foundations [18,19], slopes [20], etc. In tunnel construction, surface grouting pre-reinforcement is mainly used for surrounding rock reinforcement in fault fracture zones, shallow buried sections of portals, and soft rock sections. Due to the concealment of grouting engineering and the complexity of the injected stratum structure, there are still many problems in surface grouting technology that need to be solved urgently.

In recent years, many scientific and technological workers have carried out a lot of research and practice on grouting projects in the shallow buried section of the tunnel, and made many valuable achievements [7,8,9,14,15,16,17]. Deng Kuang-hui et al. [14] obtained the foundation parameters of the rock–soil mass of the injected stratum by drilling test holes on site. The surface deep hole grouting was used to pre-reinforce the fault fracture zone of the first Yanlieshan tunnel with cement water glass double liquid material to cooperate with the pre-support of small conduits, so that the tunnel excavation could be carried out smoothly. Lai Hong-peng et al. [15] used cement single slurry and cement sodium silicate double slurry as grouting materials. The shallow broken surrounding rock of Guangfushan tunnel was treated by surface high-pressure pre-grouting. Through numerical analysis and on-site monitoring, it was found that the integrity and strength of the surrounding rock after reinforcement were significantly improved. The convergence values of clearance in tunnel surface and vault subsidence were small. Li Rong et al. [16] proposed the feasibility of grouting reinforcement in the fully strongly weathered granite stratum through the surface grouting test study of the Xiang’an undersea tunnel. Li Hua et al. [17] studied the surface grouting test in the shallow buried section of Tongluoshan tunnel. He found that low-pressure grouting can prevent the grout from spreading too far along long cracks or groundwater flow in the crack development section. However, from the analysis of the results from previous work, it can be seen that the pre-reinforcement parameters of surface grouting are taken according to the field test results or construction experience, and the basic characteristics of the injected medium are rarely considered, resulting in a poor grouting effect or waste of materials.

In order to obtain a good grouting effect, based on the cylinder diffusion theory, this paper analyzes the stratum structure characteristics of the shallow buried section of the Botanggou tunnel in Qinhuangdao to study its injectability. Twelve groups of test slurry were prepared with ordinary Portland cement and ultra-fine cement. Through the analysis of the slurry parameters of each group, two groups of infiltration grouting materials suitable for the shallow buried section of Botanggou tunnel are selected. During grouting construction and before and after tunnel excavation, three inspection methods, including the P-Q-t control method, deformation monitoring method, and excavation observation method, are used to evaluate the grouting effect. The research results are expected to provide a reference for the surface grouting pre-reinforcement of the shallow buried section of the same type of tunnel.

2. Overview of Project

The Botanggou tunnel under construction is an important control project of the Zunhua–Qinhuangdao expressway. It is located in Liujiang National Geological Park, Funing District, Qinhuangdao City. It is a separated two-hole six-lane long-span tunnel, of which the right line tunnel is 5386 m long and the left line tunnel is 5342 m long. The tunnel passes through Yanshanian porphyry granite and Archean homogeneous migmatite. The rock mass near the surface is seriously weathered, with a loose structure and poor integrity and continuity. The exit end of the tunnel in the direction of Qinhuangdao is a shallow buried section, with a length of about 50 m, a maximum buried depth of 21.6 m, and a minimum buried depth of 3.4 m. The stratum where the tunnel crosses in this section consists of a crushed stone layer (from the collapse of the perilous rock mass on the upper slope) and a completely strongly weathered porphyritic granite layer from top to bottom. The self-supporting capacity of the surrounding rock during the excavation is poor, which is very likely to cause large-scale collapse and roof fall accidents. When simply usinga long pipe shed to pre-support the tunnel entry in such stratum, the hole-forming rate of the boreholes is low and it is difficult to reach the designed length. The method of “grouting before pipe” is adopted for entering the tunnel smoothly and excavating safely. That is to say, the stratum is pre-reinforced by grouting to improve the integrity, compactness, and tensile and compressive strength of the surrounding rock, and then the tunnel is pre-supported by a long pipe shed without grouting.

3. Cylinder Diffusion Theory of Infiltration Grouting

According to its diffusion mode, grouting can generally be divided into infiltration, fill, compaction, and split, etc. However, the slurry diffusion in the stratum is a complex process. There is no obvious boundary between various diffusion modes, which often coexist in many ways. People always name it after some main diffusion mode.

The injected medium of this grouting project is gravel and completely strongly weathered porphyritic granite with a loose structure and high crack or hole rate. Therefore, the slurry diffusion mode can be classified as infiltration-dominated. Based on the cylinder diffusion theory, it is considered that the slurry is diffused as a cylinder in the stratum during steel floral tube grouting.

3.1. Basic Assumptions

(1)

The Grouting Material is Newtonian Fluid

It is assumed that the grouting material is Newtonian fluid. The rheological curve is a straight line through the origin, and its equation is expressed as follows [21]:

$$\tau =\mu \cdot (d{u}_x/dy)$$

(1)

where τ is the shear stress (it is the internal friction force per unit area, Pa), du_x/dy is the shear rate (s⁻¹), and μ is the viscosity coefficient (Pa·s. The common unit in engineering is “s”).

(2)

The Slurry Diffuses on a Cylindrical Surface

Under the action of a certain grouting pressure, the grout infiltrates and diffuses from the grouting hole along the crack or hole gap to the surrounding stratum. The movement rate of the grout is inversely proportional to the distance of the grouting hole. Finally, it stops advancing at a certain position away from the grouting hole and reaches the farthest diffusion distance. The grouting holes are arranged in a quincunx shape from top to bottom along the grouting tubes. The grout injected into the stratum is superposed and gelled with the rock–soil mass to form a stone body similar to a column (Figure 1).

3.2. Analysis of Injectability

When the grouting material is Newtonian fluid, the key to successful infiltration grouting is that the diameter of the infiltration channel of the rock–soil mass should be larger than the particle size of the grouting material. However, there is a certain relationship between the diameter of the infiltration channel and the particle size of the components of the rock–soil mass of the stratum. Therefore, the injectability of the stratum can be defined by the particle size of the rock–soil mass itself, which usually requires the following formula [22]:

$$D_{10}/G_{95}\ge 8$$

(2)

where D₁₀ is the diameter corresponding to the cumulative screen residue of the rock–soil mass accounting for 10%, and G₉₅ is the diameter corresponding to the cumulative screen residue of the grouting material accounting for 95%.

According to the basic parameters of the rock–soil mass of the injected stratum (

Table 1. Basic parameters of grouting stratum rock and soil mass.

Number	Lithology	Density ρ/(g.cm3)	Moisture Content w/%	Porosity n	Infiltration Coefficient K/(cm.s−1)	Accumulative Screen Residue/(%)
						10	50	90	95
①	Crushed stone	1.82	3.24	0.466	0.962	<9.5 mm	<37.5 mm	<75 mm	<90 mm
②	Completely weathered porphyritic granite	1.68	8.11	0.370	9.3 × 10−4	<0.05 mm	<0.75 mm	<2.0 mm	<2.8 mm
③	Strongly weathered porphyritic granite	1.76	5.26	0.439	5.8 × 10−2	<0.6 mm	<2.5 mm	<4.3 mm	<5.2 mm

), the maximum diameter of each layer that can achieve infiltration grouting is obtained through the back calculation of Formula (2). The first layer is 1.188×10³ μm. The second layer is 6.25 μm. The third layer is 75 μm. At present, the particle size of ordinary Portland cement produced by most cement manufacturers is 44μm~100μm, and the average particle size of ultra-fine cement is generally 5 μm below. Comprehensive analysis shows that ordinary Portland cement can be used as the main grouting material for the first layer, and ultra-fine cement can be used as the main grouting material for the second and third layers.

3.3. Calculation of Diffusion Radius

Infiltration grouting is a complex mechanical behavior. The size of the diffusion radius is affected by many factors. The decisive factors are as follows. The first is the internal structure characteristics of the injected medium, including mutual cementation between the rock–soil particles or blocks, crack or hole porosity, and permeability coefficient. The second is the properties of the grouting materials, including viscosity and gel time. The third is the constructing conditions, including grouting pressure and grouting rate. In addition, it is also affected by ground stress conditions and groundwater occurrence. In engineering practice, the diffusion radius of infiltration grouting is generally estimated by the following empirical formulae [23]:

$$r_1=\sqrt{\frac{2K{h}_{1}t}{n\beta \ln (\raisebox{1ex}{$r_1$}\!\left/ \!\raisebox{-1ex}{$r_0$}\right.)}}$$

(3)

$$t=\frac{n\beta r_1^2\ln (\raisebox{1ex}{$r_1$}\!\left/ \!\raisebox{-1ex}{$r_0$}\right.)}{2K{h}_1}$$

(4)

where K is the infiltration coefficient of the injected formation (cm/s); β is the viscosity ratio of slurry to water, β = μ_g/μ_w; r₀ is the radius of the grouting pipe (cm); r₁ is the diffusion radius of the slurry (cm); h₁ is the grouting pressure head (cm); t is the grouting time (s); n is the crack or hole clearance rate of the injected medium; μ_w is the viscosity coefficient of water (s); and μ_g is the viscosity coefficient of the slurry (s).

The principle of infiltration grouting is not to damage the stratum structure, and its grouting pressure is difficult to control. In order to obtain the maximum allowable grouting pressure of infiltration grouting, the relationship between the grouting pressure and grouting volume is analyzed through construction test holes before formal grouting. When the grouting pressure rises to a certain value p_f, the amount of grout absorption increases suddenly. It can be considered that the stratum structure has been damaged and the grout diffusion mode has changed from infiltration to splitting. Therefore, this inflection point value is defined as the extreme value of infiltration grouting (Figure 2).

4. Optimization of Grouting Materials

4.1. Selection of Grouting Materials

Based on the analysis of the structural characteristics of the injected stratum, it pays attention to energy conservation and environmental protection. The main materials for surface grouting at the shallow buried section at the exit end of Botanggou tunnel are ordinary Portland cement (P·O 42.5) produced by Qinhuangdao Lantu and ultra-fine cement (the fineness is 1250 mesh) produced by Shandong Wenqu new material. In order to shorten the initial setting time of the grouting material, a small amount of sodium silicate (45°Bé) produced by Shengpeng in Zaozhuang is added to the main material as the accelerator. The polycarboxylate superplasticizer produced by Shenyang Xingzhenghe is added into the ultra-fine cement slurry to increase the fluidity of the cement slurry.

4.2. Laboratory Test of Grouting Materials

The rock–soil mass structure of the injected formation is loose, for which the crack or hole rate is high. The infiltration grouting material should have good fluidity and stability, reasonable cementing time, and large solidification strength. Therefore, the laboratory test needs to test the viscosity of the grouting materials (the model 1006 mud viscosity is often used for measurement and the unit is “s”), water separation rate, gel time, stone rate, compressive strength, and other parameters to provide a basis for the rational selection of grouting materials.

A large number of experimental studies [19,20,21] show that the critical value of the water–cement ratio for cement slurry to convert Bingham fluid to Newtonian fluid is 1:1. Therefore, the water–cement ratio of the slurry in this test is close to or equal to the critical value. The water cement ratio is 0.8:1, 1:1, and 1.2:1. Performance analysis was conducted on twelve groups of proportioning materials under four conditions, namely, ordinary Portland cement slurry, ordinary Portland cement external accelerator slurry, ultra-fine cement slurry, ultra-fine cement external accelerator, and water reducer slurry (

Table 2. Grouting material laboratory test results.

Number	Name of Main Agent	Admixture/(%)		Water Cement Ratio	Viscosity /(s)	Syneresis Rate/(%)	Gel Time/min		Stone Rate/(%)	Compressive Strength/MPa
		Accelerator	Water Reducer				Initial Set	Final Set		3d	7d	28d
1	Portland cement	0	0	0.8:1	21.2	25	675	1278	92	4.15	7.09	14.66
2	Portland cement	0	0	1.0:1	12.1	34	867	1473	84	2.26	5.80	9.31
3	Portland cement	0	0	1.2:1	11.7	45	986	2109	77	2.05	2.23	3.45
4	Portland cement	3	0	0.8:1	24.3	17	111	302	91	7.43	11.24	14.11
5	Portland cement	3	0	1.0:1	13.9	26	156	323	86	4.37	7.19	9.06
6	Portland cement	3	0	1.2:1	13.4	38	193	407	78	2.42	2.88	3.52
7	Ultra-fine cement	0	0	0.8:1	187.2	3.2	298	524	97	14.76	15.72	28.5
8	Ultra-fine cement	0	0	1.0:1	19.1	5.3	502	711	94	7.15	14.87	25.54
9	Ultra-fine cement	0	0	1.2:1	15.6	12.9	582	967	90	5.26	7.32	9.86
10	Ultra-fine cement	3	0.3	0.8:1	142.3	2.3	72	227	96	19.18	22.43	27.89
11	Ultra-fine cement	3	0.3	1.0:1	15.4	4.2	106	296	93	9.33	17.58	25.10
12	Ultra-fine cement	3	0.3	1.2:1	11.9	9.1	135	361	88	6.82	8.51	9.55

).

4.3. Determination of Optimal Slurry Ratio

(1)

Selection of Optimum Proportion of Ordinary Portland Cement Slurry

The analysis of the laboratory test results shows that when the ordinary Portland cement slurry is not added with additives, the water separation rate is high, while the initial setting and final setting time are relatively long, which increase with the increase in the water–cement ratio. Such grouting materials cannot make the injected stratum form a high early strength. It may also cause grout flowing and block other grouting holes, directly affecting the grouting effect. When 3% sodium silicate is added to the cement slurry as the accelerator, the gel time of the slurry is obviously shortened. The water separation rate is reduced and the stability is increased. The stone body has a higher early strength. By comparing the properties of six groups of ordinary Portland cement slurry, it is considered that the fifth group can be used as the grouting material for the gravel layer.

(2)

Selection of Optimal Proportion of Ultra-Fine Cement Slurry

When the ultra-fine cement slurry is not added with additives, it has high viscosity, poor fluidity, and a long gel time. Then, the early strength formation is slow so it is difficult to form an obvious advantage in penetration grouting. When 3% sodium silicate is added to the ultra-fine cement slurry as the accelerator and 0.3% polycarboxylate superplasticizer is added, the performance of the ultra-fine cement slurry is greatly changed. The initial setting time is about 1~2 h, and the early strength is formed quickly. Under the effect of polycarboxylate superplasticizer, the viscosity of the slurry decreases significantly, which is conducive to the uniform penetration of the slurry in the injected formation. The comprehensive analysis shows that the 11th group of candidate materials is suitable for grouting reinforcement of fully strongly weathered porphyritic granite.

5. Design of Infiltration Grouting

5.1. Grouting Scope

The infiltration grouting is only carried out for specific areas outside the tunnel excavation contour line, while the specific scope can be determined by calculating the loose range of the tunnel excavation. Because the surrounding rock is composed of discontinuous rock–soil mass, Terzaghi theory is a good method to deduce the loose range of the surrounding rock. When the surrounding rock is subjected to the self-weight stress of the overlying rock–soil mass, it will deflect and deform, and then cause the block to move. When the internal friction angle of the surrounding rock is φ, the slip surface inclines from the tunnel bottom at 45° + φ/2 angle. After reaching the tunnel vault, it continues to the surface with appropriate curves CE and DF. For the convenience of calculation, it is approximately assumed that CA and DB are the two vertical lines for simplification (Figure 3). At this time, the sliding width outside the arch toe excavation line is set as b, and its value is as follows:

$$b=h\tan (45°-\phi /2)$$

(5)

where h is the height from the tunnel arch foot to the vault.

During calculation, the average value of the internal friction angle of completely and strongly weathered porphyritic granite is taken as φ = 44°. The height from the tunnel arch foot to the vault is taken as h = 9 m, and the sliding width b = 3.51 m can be calculated using Formula (5). In the grouting construction, the value of the grouting range on both sides of the tunnel excavation boundary should generally be greater than the sliding width to ensure the grouting effect. This grouting project takes the excavation boundary of the tunnel side wall expanded by 5 m as the grouting range. After the grouting is completed, the tunnel vault will still be pre-supported by a long pipe shed into the tunnel. Through the engineering analogy method [15,16], the vault grouting scope is considered to reserve 0.5 m construction space for long pipe shed drilling and then expand by 5 m.

5.2. Layout of Grouting Holes

The infiltration radius of single-hole grouting should be determined before the overall layout of grouting holes to determine the reasonable hole distance. Therefore, the single-hole grouting test is carried out in the fully strongly weathered porphyritic granite stratum with a small permeability coefficient. The test method is described in Section 3.3. Using the 11th group of grouting materials, the infiltration diffusion radius of single-hole grouting can be calculated using Formula (3).

When the grouting pressure is close to 0.38 Mpa, the amount of grout absorption is significantly reduced during the single-hole grouting test—less than 2 L/min. After continuous grouting for 5 min, the amount of grout absorption increases significantly, which can be considered that the infiltration grouting has changed to split grouting. This grouting pressure is regarded as the limit value p_f of the infiltration grouting pressure. This single-hole grouting test takes 35.5 min, and r₁ = 104.5 cm can be obtained through calculation.

When determining the spacing, we should not only maximize the role of each grouting hole, but also meet the mutual overlap between holes. The infiltration diffusion of some grouting holes is difficult to reach the maximum calculated radius during grouting. In order to fully overlap, the spacing of the grouting holes in the same row is d = 1.5 r₁, arranged in quincunx shape, that is 150 cm (transverse) × 200 mm (longitudinal). The surrounding holes also serve as the grout stop wall of the whole grouting area, which should be properly densified, and the hole spacing is 100 cm (as shown in Figure 4 and Figure 5).

5.3. Estimation of Grouting Amount

The amount of grout required for infiltration grouting is related to the structural characteristics of the injected stratum, grout performance, and construction technology. It is usually estimated using the following formula [24]:

$$Q=\pi r_1^2{h}_{2}n\alpha (1+\beta )/m$$

(6)

where r₁ is the diffusion radius of seepage grouting; h₂ is the length of the grouting section; n is the crack or hole rate of the rock and soil mass; α is the effective grouting coefficient, which as 0.8 according to the single-hole grouting test results; β is the loss rate—the larger peripheral hole loss rate is 0.2 and the smaller intermediate loss rate is 0.15; and m is the stone rate.

6. Construction Technology of Grouting

6.1. Setting the Grout Stop Slurry Pad

In order to prevent the surface from bleeding and rising during grouting, a grout stop slurry pad is set in the grouting area. The surface topsoil is cleaned first, and then 20 cm thick C20 concrete is poured in as the grout stop slurry pad. After grouting, the cultivated soil is backfilled in time and grass is planted for ecological restoration.

6.2. Drilling Hole

A geological drilling rig is used to drill the hole eccentrically, with the hole diameter of 91 mm, from end to end. The grouting hole at the vault is drilled to 0.5 m above the excavation contour line, and the grouting hole at the side wall is drilled to 2 m below the inverted arch excavation line. The drilling sequence adopts the three-sequence hole opening method, that is, in the same row of grouting holes, holes 1, 4, and 7 are opened first, and then holes 2, 5, and 8, and finally holes 3, 6, and 9.

6.3. Fabrication and Installation of Steel Floral Tube

The pre-reinforcement grouting pipe of the stratum adopts a Φ 50 × 5 mm steel floral tube. The flowered holes are drilled at the grouting section of the steel floral tube. The diameter of the flowered holes is 8 mm, and its spacing is 15 cm. The flowered holes are arranged in a quincunx shape. The steel floral tube is fed according to the depth of a single hole, directly inserted into the drilled sleeve. The wall protection sleeve is pulled out after installation for repeated use.

6.4. Construction and Its Completion Standards of Grouting

Before grouting construction, the gap of the orifice is blocked with quick-setting mortar to prevent grouting. In the grouting area, the overall sequence of grouting from the periphery to the interior can prevent the grout from permeating outside the grouting area. The grouting sequence of the same row of grouting holes is consistent with the drilling sequence, which can prevent grout from flowing. An automatic flow recorder is connected to the grouting pipe to record the changes of flow and pressure during grouting. The grouting rate should be controlled at about 10 L/min. The grouting pressure should be low first and then high, and the final pressure should not exceed the limit value of 0.38 MPa. When the grout suction reaches the designed grout demand, or under the limit pressure, the grout suction is less than 2 L/min for five consecutive minutes. It can be considered that the penetration grouting of this hole has been completed.

7. Analysis of Grouting Effect

7.1. Analysis of P-Q-t Curve

The P-Q-t curve analysis method is used to check the grouting effect by recording the grouting pressure P and grouting volume Q values with the grouting automatic recorder, drawing and analyzing the P-t and Q-t curves, so as to judge the grouting effect [25]. During the grouting construction, the starting grouting pressure is 0.1 MPa. When the grouting pressure reaches 0.38 MPa, the pressure rise will be stopped to keep the grouting machine under constant pressure. It can be seen from Figure 6 that the P-t curve shows a slow upward trend during grouting. The grout is evenly infiltrated into the injected medium. With the increase in the grouting time, the grouting pressure needs to increase gradually. When the grouting time reaches about 40 min, the grouting pressure reaches the designed final pressure. Figure 7 shows that the Q-t curve rises sharply at first and then drops slowly. This is because the permeability coefficient of the gravel layer is large, and the grout is easy to diffuse at the initial stage of grouting. As the pores of the injected medium are filled, the amount of grout absorption gradually decreases. The maximum amount of grout absorption occurs within 10 to 25 min after grouting, up to 80 L/min, and then starts to decline. When the grouting pressure reaches the design final pressure, the amount of grout absorption is about 2.5 L/min, and after 5 min, it can be considered that grouting has been completed. When the pipe is blocked, the P-t curve will rise sharply and the Q-t curve will remain unchanged. When the diffusion mode is changed to splitting or bleeding, the P-t curve will remain unchanged and the Q-t curve will jump up. It can be considered that the single-hole grouting has not achieved the expected effect.

7.2. Deformation Monitoring of Excavation

After the surface infiltration grouting pre-reinforcement is completed, the long pipe shed is used to pre-support the tunnel entry, and the CD method is used for excavation. In order to monitor the deformation during excavation, the K129 + 732 shallow buried grouting section is taken as the monitoring section to monitor the ground subsidence, vault subsidence, and horizontal convergence (Figure 8). The measurement frequency of the three monitoring options is 1 time/d. The deformation alarm value is set as 60 mm.

(1)

Monitoring of Surface Subsidence

Seven subsidence monitoring points were set on the surface, named YDB-01–YDB-07, respectively. In order to reflect the deformation characteristics of the surrounding rock under excavation conditions as much as possible, monitoring was carried out in time after the excavation. The first measurement was taken after the excavation of Part A-1 on 6 August 2020, with a duration of 37d. During the period from 6 August to 20 August, the cumulative subsidence value of each monitoring point on the surface showed an increasing trend, of which the maximum value was YDB-3, as 3.56 mm. During the period from 21 August to 5 September, the cumulative subsidence value of each monitoring point fluctuated repeatedly. After 5 September, the surface subsidence became stable, while the cumulative subsidence value did not exceed the alarm value by more than 60 mm (Figure 9).

(2)

Monitoring of Vault

Botanggou tunnel is a long-span tunnel. Three monitoring points for vault subsidence are set at the vault, named YGD-01–YGD-03, respectively. The measurement of YGD-01 began on 6 August 2020. The measurement of monitoring points YGD-02 and YGD-03 began immediately after the excavation of part B-1 had been completed on 17 August. According to the analysis of the measurement data, the cumulative subsidence value of YGD-01–YGD-03 increased gradually, and remained basically stable until 10 September. YGD-01 had the largest cumulative subsidence value of 10.25 mm. From continuous monitoring, it can be seen that the cumulative subsidence value of the three monitoring points for vault subsidence does not exceed the alarm value of 60 mm (Figure 10).

(3)

Monitoring of Horizontal Convergence

A total of four horizontal convergence survey lines were set in this section, which are named YSL1-1, YSL1-2, YSL2-1, and YSL2-2, respectively. The YSL1-1 and YSL1-2 measurements started on 6 August 2020. The convergence value increased linearly within 3 days after excavation, and then slowly increased. It was basically stable until 6 September. The cumulative convergence value of YSL1-1 was the largest for 10.78 mm. The measurement of YSL2-1 and YSL2-2 started on 17 August. The convergence value increased linearly within 4 days of excavation and then slowly increased, while it remained basically stable until 10 September. The accumulated value of the four horizontal convergence measuring lines does not exceed the alarm value of 60 mm (Figure 11).

7.3. Observation Method of Excavation

The observation and analysis of the tunnel excavation process is the most intuitive means to check the grouting effect. Figure 12 reflects the surrounding rock condition of the surface infiltration grouting vault. Although the bottom of the steel floral tube is 50 cm away from the excavation contour line, the slurry has still infiltrated to the vault, forming a stone body tightly cemented with the rock–soil mass, or infiltrating along the dominant fracture surface to form a filling vein. Figure 13 reflects the condition of the side wall after excavation. The side wall is close to the steel floral tube. The grout has sufficient penetration and cementation with the rock–soil mass, forming a large stone body.

8. Conclusions

(1) The stratum structure of the shallow buried section at the entrance of the Botanggou tunnel is loose. In order to improve the strength and integrity of the stratum and improve the hole-forming conditions of the long pipe shed pre-support, it is proposed to pre-reinforce the crushed stone layer with ordinary Portland cement single liquid grouting, and the fully strongly weathered porphyritic granite layer with ultra-fine cement single-liquid grouting. The results of the laboratory tests show that adding 3% sodium silicate into the single slurry of ordinary Portland cement with a water–cement ratio of 1:1 can obviously shorten the initial setting time of the slurry. When 3% sodium silicate and 0.3% polycarboxylate superplasticizer are added to the ultra-fine cement single slurry with a water–cement ratio of 1:1, the slurry has good fluidity and stability.

(2) The diffusion radius of infiltration grouting is mainly controlled by the structural characteristics of the injected medium and the performance of the grouting materials. When the empirical formula is used to calculate the infiltration diffusion radius of the grout, the extreme value p_f of the grouting pressure is obtained from the field test. It is reasonable to determine the grouting range according to the loose range of the surrounding rock derived from Terzaghi theory. Through the engineering analogy method and ensuring the grouting effect, the 5 m expansion of the excavation profile is taken as the grouting range.

(3) Grouting construction adopts the overall order of periphery and then interior. Three-sequence opening and grouting are adopted in the same row of grouting holes, which can effectively prevent grouting running and grouting. The grouting process adopts the “constant pressure limit” method. The control of grouting pressure and grouting volume is taken as the end standard of single-hole grouting.

(4) During grouting construction, the P-t curve shows a slow upward trend, and the Q-t curve first rises sharply and then drops slowly, which can be considered as a normal penetration grouting process. The monitoring and measurement of tunnel surface subsidence, vault subsidence, and horizontal convergence shows that the deformation value increases within 15 days after the completion of the excavation, and then fluctuates repeatedly. After 30 days, the deformation value tends to be stable, but ultimately does not exceed the alarm value of 60 mm. After the excavation is completed, it can be observed that the slurry has infiltrated into the vault, cemented closely with the rock–soil mass to form a stone body, or infiltrated along the dominant fracture surface to form a filling vein. It can be seen from the above three inspection methods that the effect of this infiltration grouting is significant.