Large-slope-ratio downhill water-rich extremely-soft shallow stratum tunnel construction method

Abstract

The invention discloses a large-slope-ratio downhill water-rich extremely-soft shallow stratum tunnel construction method. The method comprises the steps that surface water is treated; a tunnel advance support measure is adopted; taking a measure of grouting around the interior of the tunnel; tunnel horizontal drainage holes and in-tunnel drainage measures are adopted; monitoring measures of ground surface settlement and in-hole primary support deformation are taken; and a measure of a back waterstop ring of a primary support is adopted. By means of the method, the risk existing in excavation of the large-gradient downhill water-rich extremely-soft shallow stratum tunnel with the gradient larger than 10% can be prevented and solved. The method is reasonable in treatment measure and more optimized in construction process, solves the problems of large deformation, collapse, mud burst, water gushing, soil and sand flowing and the like of the primary support of the tunnel caused by excavation of the tunnel under the large-slope-ratio downhill water-rich extremely-soft fully-weathered granite shallow buried stratum, and is suitable for the stratums such as scattered granular fully-weathered granite and the like which disintegrate and soften when encountering water. And safety and progress guarantee is provided for excavation of tunnel engineering under similar working conditions and geology.

Description

Technical Field

[0001] This invention relates to the field of shallow buried tunnel construction technology for mountain tunnels crossing gullies, and in particular to a method for constructing tunnels in water-rich, extremely soft, shallow buried strata on steep slopes. Background Technology

[0002] Completely weathered granite strata, due to their widespread distribution in China and their characteristics of low strength, poor integrity, and post-excavation weathering, especially softening and disintegration under the influence of groundwater, have become a major source of hazards leading to large deformations in the initial support of tunnels, mudslides, water inrushes, and quicksand flows. In the Lingnan region of Guangdong, granite is widely distributed in residual hilly terrain with undulating topography, well-developed valleys, abundant rainfall, and simultaneous rain and heat. Groundwater is replenished by atmospheric precipitation and pore water infiltration, dynamically changing with the seasons, draining to the surface along low-lying areas and flowing into nearby river valleys and gullies. Under these topographical, geomorphological, climatic, and groundwater conditions, the granite rock mass undergoes repeated weathering, resulting in a high degree of rock erosion. Because completely weathered granite strata have the characteristics of disintegration upon contact with water and physical transformation after interaction, their self-stabilizing ability below the groundwater level is extremely poor. When encountering such strata during tunnel construction, problems such as severe surrounding rock deformation, collapse, mudslides, water inrushes, and quicksand flows will be encountered, posing serious safety and quality hazards to the construction. Summary of the Invention

[0003] The purpose of this invention is to provide a construction method for tunnels in shallow, water-rich, extremely soft, and shallowly buried strata on steep slopes. This method can prevent and resolve the risks associated with tunnel excavation in shallowly buried strata of extremely soft, completely weathered granite on steep slopes, and is applicable to strata such as granular, completely weathered granite that soften and disintegrate upon contact with water. By proposing reasonable treatment measures and optimizing the construction process, this invention solves the problems of large deformation of the initial tunnel support, collapse, mudslides, water inrush, and quicksand caused by tunnel excavation in shallowly buried strata of extremely soft, water-rich, and completely weathered granite on steep slopes, providing safety and progress guarantees for tunnel projects under similar working conditions and geological conditions.

[0004] This invention is achieved through the following technical solution:

[0005] A method for constructing tunnels in shallow, water-rich, extremely soft, downhill strata with a steep slope includes the following steps:

[0006] S1: Surface water treatment; drain the water accumulated on the surface above the tunnel axis, and construct a herringbone clay slope and drainage system on the top surface of each side of the tunnel axis, 9m on each side.

[0007] S2: Take advanced support measures for tunnels; including: C25 shotcrete + double-layer steel mesh + temporary anchor + steel arch frame + orifice pipe for the guide wall of the pipe roof inside the tunnel; advanced support for the shallow buried section of the tunnel crossing the gully, long pipe roof is used for the sidewalls within 1.0m above the top arch and the bottom slabs on both sides, and advanced small pipe is used within 120° of the top arch.

[0008] S3: Take measures to grout around the tunnel; including: the first section of the tunnel grout-stopping rock wall is reinforced with C25 shotcrete + double-layer steel mesh + φ25 short anchor rod grouting, and the subsequent sections are reinforced with reserved rock strata for grouting around the tunnel face; the grouting range around the tunnel face is from the tunnel excavation outline to 4.0m outside the excavation outline.

[0009] S4: Take measures for horizontal drainage holes and internal drainage in the tunnel; including: at least three drainage holes are arranged on the left, middle and right sides of the steps on the tunnel face, with built-in perforated pipes. The outlet of the perforated pipes is connected with a flexible hose to drain the seepage water to the sump. A movable water collection tank is set at the lowest point of the tunnel, and the pumping and drainage are automatically controlled by a float switch.

[0010] S5: Take measures to monitor surface settlement and initial support deformation in the tunnel; including: setting up arch settlement observation points and clearance convergence points on the surface of the initial support; setting up surface settlement observation points on the surface of the tunnel axis;

[0011] S6: Adopt water-stop ring measures behind the initial support; install water-stop rings in the tunnel section where the initial support has been completed, one water-stop ring every 100m, and construct two drainage holes on each side wall 3.0m in front of the water-stop ring in the direction of the tunnel opening, with built-in perforated pipes, and wrap the perforated pipes with drainage geotextile.

[0012] Further, as a construction method for tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S1 specifically includes:

[0013] S1.1: Drain surface water near the tunnel axis;

[0014] S1.2: A herringbone-shaped clay slope with a slope ratio of 3% is constructed on the top surface of the tunnel axis, 9m on each side.

[0015] S1.3: A seepage-proof membrane shall be covered on the surface of the herringbone clay slope on both sides of the tunnel axis for 9m. The seepage-proof membrane shall be a polyethylene geomembrane with a thickness of not less than 0.8mm.

[0016] S1.4: A drainage system is installed around the 9m herringbone clay slope on both sides of the tunnel axis to ensure that the waterproof membrane can be smoothly connected to the drainage ditch, completely cut off the surface water supply, and smoothly connect the drainage ditch water flow to the natural drainage.

[0017] Further, as a construction method for tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S2 specifically includes:

[0018] S2.1: The guide wall of the pipe roof inside the tunnel adopts C25 shotcrete + double-layer steel mesh + temporary anchor bolts + steel arch frame + orifice pipe. The thickness of C25 shotcrete is ≥30cm; the double-layer steel mesh has a steel diameter of φ8 round steel, a mesh size of 150mm×150mm, and an overlap length of 20cm; the temporary anchor bolts have a diameter of φ25 threaded steel, a row spacing of 1.0m×1.0m, a single length of 1.5m, and the construction angle is perpendicular to the tunnel face; the steel arch frame uses 25a I-beams with 2 rings, a circumferential spacing of 50cm, installed in sections, and connected to the initial support steel arch frame with φ32 steel to form a whole; the orifice pipe has an outer diameter of 127mm, a wall thickness of 5mm, a single length of 0.7m, a circumferential spacing of 30cm, and is firmly welded to the 25a I-beams using "Ω" steel bars;

[0019] S2.2: Advance support for the shallow buried section of the tunnel crossing the gully. Φ108 long pipe roof is used for the sidewalls within 1.0m above the top arch and both sides of the bottom slab. φ42 advance small guide pipes are used within 120° of the top arch. The pipe roof has an outer diameter of 108mm, a wall thickness of 10mm, a circumferential spacing of 30cm, and a ring length of 15m / 26m / 30m, with an external insertion angle of 15° / 3° / 2°. The advance small guide pipe has an outer diameter of 42mm, a wall thickness of 4.0mm, a single length of 4.5m, a circumferential spacing of 30cm, and a longitudinal spacing of 2.0m.

[0020] S2.3: Adjust the φ42 advanced small pipe support measures according to the distance between the pipe roof location and the excavation outline. Do not install φ42 advanced small pipes 3m behind the guide wall and 3m in the gradual expansion section of the pipe roof working chamber.

[0021] Further, as a construction method for tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S3 specifically includes:

[0022] S3.1: The first section of the grout-stopping wall uses C25 shotcrete + double-layer steel mesh + φ25 short anchor rods. The thickness of the C25 shotcrete is 30-50cm. The double-layer steel mesh uses φ8 round steel bars with a mesh size of 150mm×150mm and an overlap length of 20cm. The temporary anchor rods are φ25 threaded steel bars with a spacing of 1.5m×1.5m and a single length of 1.5m. The construction angle is perpendicular to the working face. The subsequent sections of the grout-stopping wall use a 3-5m section of rock stratum reserved for grouting around the perimeter, and simultaneously use C25 shotcrete with a thickness of 30-50cm. It is equipped with a double-layer steel mesh with a diameter of φ8 round steel bars, a mesh size of 150mm×150mm, and an overlap length of 20cm.

[0023] S3.2: The grouting range around the tunnel face extends from the tunnel excavation outline to 4.0m outside the excavation outline. The circumferential spacing of the grouting holes is 1.00m, and the ring spacing is 1.00m. The grouting holes are arranged in 2.5 rings, with a half-ring at the tunnel invert. The bottom ring (f1-f8) has an outer deviation angle of 0° coinciding with the tunnel axis, the middle ring (e1-e25) has an outer deviation angle of 20°, and the inner ring (d1-d19) has an outer deviation angle of 15°. There are a total of 52 grouting holes, arranged in two sequences: Sequence I has 26 holes with a depth of 12m, and Sequence II has 26 holes with a depth of 15m. The grouting range extends from the tunnel excavation outline to 4.00m outside the excavation outline. The grout diffusion radius of each hole is ≥0.5m, and the grout from adjacent grouting holes is required to overlap.

[0024] S3.3: Two-component grout C-liquid is used for grouting, and the completion criteria for grouting are determined by the dual control indicators of grouting pressure and grout absorption rate; the mix ratio of the two-component grout C-liquid is as follows:

[0025] Liquid A (water glass slurry) = water glass : water = 1 : 1 (volume ratio);

[0026] B liquid (cement grout) = cement:water = 1:1 (mass ratio);

[0027] C liquid (cement-water glass mixture) = cement slurry : water glass slurry = 1:1 (volume ratio);

[0028] S3.4: The grouting sequence should be one hole at a time, with the grouting sequence from top to bottom and from outside to inside. The grouting holes should be filled in layers from the outside to the inside.

[0029] S3.5: Grouting shall be carried out using a retreating grouting method, and the grouting pressure shall be controlled within 0.8 to 1.5 MPa. The grouting pressure may be appropriately increased according to the gel time of the two-component grout, but the maximum grouting pressure shall ensure the safety of the tunnel and prevent excessive heave or cracking of the ground surface. During the grouting process, close monitoring and inspection of the ground surface and the tunnel interior are essential, especially the heave and grout leakage of the ground surface directly above the grouting section. At the same time, the tunnel face and initial support shall be monitored to monitor the uplift of the surrounding rock. If uplift is found, pressure shall be reduced in time and intermittent grouting shall be used for emergency treatment.

[0030] S3.6: If weak points or inadequate grouting are found in the grouting around the tunnel during the excavation process, local supplementary grouting can be carried out randomly to plug water and ensure the stability of the surrounding rock at the excavation face.

[0031] Further, as a construction method for tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S4 includes:

[0032] S4.1: Three drainage holes are arranged on the steps of the tunnel face. The drainage holes are 130mm in diameter, 15m in length, and have a horizontal upward angle of 3-5°. They are equipped with PVC perforated pipes with an outer diameter of 108mm. The outlet of the perforated pipes is connected with a flexible hose to drain the seepage water to the collection pit.

[0033] S4.2: Strengthen the drainage system at the working face to prevent water from soaking the initial support arch foot and softening and eroding the surrounding rock excavated from the sidewall.

[0034] S4.3: A movable water collection tank is installed at the lowest point of the tunnel, and a float switch is used to automatically control the pumping and drainage.

[0035] S4.4: Install a water flow meter at the tunnel entrance drainage pipe to count the tunnel seepage. The collected data is used to determine the relationship between the tunnel excavation length and the seepage, and to guide the design of the tunnel drainage system.

[0036] Further, as a method for constructing tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S5 includes:

[0037] S5.1: After the tunnel body is excavated and supported and before the secondary lining of the tunnel, the top arch settlement observation point and the clearance convergence point shall be arranged on the surface of the initial support to monitor the deformation of the initial support after excavation and the disturbance to the initial support during the construction of the middle and lower benches. The top arch settlement point shall be arranged with a cross section every 2.5m and the clearance convergence point shall be arranged with a cross section every 5.0m.

[0038] S5.2: Set up surface settlement observation points on the ground along the tunnel axis, with one cross-section every 5m along the tunnel axis;

[0039] S5.3: Compare and analyze the linkage between tunnel settlement and surface settlement, as well as the relationship between construction disturbance and settlement rate, based on the monitoring data, and guide tunnel excavation construction through the monitoring data.

[0040] Further, as a construction method for tunnels in water-rich, extremely soft, shallow buried strata on steep slopes, step S6 includes:

[0041] S6.1: The tunnel is constructed on a steep downhill slope with a gradient of 10.99%. Due to the excavation at the tunnel face, groundwater seepage will accumulate along the gaps behind the initial support towards the tunnel face, further eroding and softening the surrounding rock at the tunnel face. Therefore, it is necessary to install water-stopping measures in the completed initial support section. Water-stopping rings are installed every 100m, grouting holes are inserted 4m into the rock, the borehole diameter is not less than 48mm, the circumferential spacing is 3.0m, the grouting pressure is 0.5~1.0MPa, and the grout is a cement-water glass two-component grout with a grout volume ratio of 1:1.

[0042] S6.2: Construct two drainage holes on each side wall at a distance of 3.0m in front of the opening of the water-stop ring. The drainage holes are 75mm in diameter, 2m long, and 15° in elevation. They are filled with 50mm outer diameter PVC pipes and wrapped with a layer of drainage geotextile.

[0043] The fully weathered metamorphic quartz sandstone, a soft to relatively soft rock, exhibits well-developed joints and fissures, and a fractured to layered structure. It possesses excellent permeability, leading to significant water infiltration and a rise in the groundwater level during tunnel excavation. Under the erosive and weakening effects of water, the bearing capacity and strength of the fully weathered metamorphic quartz sandstone strata decrease substantially, resulting in large deformations, mudslides, water inrushes, collapses, and roof falls during tunnel excavation.

[0044] The construction method of this invention greatly reduces the damage of water to the fully weathered metamorphic quartz sandstone strata by setting up waterproof and drainage works inside and outside the tunnel, and reduces the loss of the strength of the strata rock mass.

[0045] The construction method of this invention further enhances the strength and stability of water-rich, fully weathered metamorphic quartz sandstone strata through advanced support and stratum reinforcement measures, ensuring the safe excavation of the tunnel.

[0046] The construction method of this invention monitors and obtains the intrinsic relationship between the deformation of the support structure and the construction disturbance during the tunnel excavation process in real time through surface settlement monitoring and initial support deformation monitoring measures. It can issue early warnings in a timely manner based on the deformation rate or cumulative deformation of the initial support structure to guide tunnel construction, thereby enabling early detection of risks, early handling of problems, reduction of losses, and ensuring the safety of tunnel excavation.

[0047] This invention provides a method to prevent and resolve the risks associated with tunnel excavation in shallow, water-rich, extremely soft, and shallowly buried strata with a slope greater than 10%. The method features reasonable treatment measures and an optimized construction process, solving problems such as large deformation of the initial tunnel support, collapse, mudslides, water inrush, and quicksand caused by tunnel excavation in shallowly buried, water-rich, extremely soft, and completely weathered granite strata on steep slopes. It is applicable to strata such as loosely granular, completely weathered granite that soften and disintegrate upon contact with water, providing safety and schedule assurance for tunnel projects under similar working conditions and geological conditions. Attached Figure Description

[0048] Figure 1 This is a cross-sectional view of the treatment of surface water bodies.

[0049] Figure 2 This is a longitudinal section view of the tunnel's pre-support structure.

[0050] Figure 3 This is a cross-sectional view of the tunnel's pre-support structure, i.e. Figure 2 Schematic diagram of section A1-A1.

[0051] Figure 4 This is a cross-sectional view of the tunnel's pre-support structure, i.e. Figure 2 Schematic diagram of section A2-A2.

[0052] Figure 5 This is a cross-sectional view of the tunnel's pre-support structure, i.e. Figure 2 Schematic diagram of section A3-A3.

[0053] Figure 6 This is a longitudinal section view of the grouting around the tunnel face.

[0054] Figure 7 This is a cross-sectional view of the grouting face inside the tunnel.

[0055] Figure 8 This is a cross-sectional view of the waterstop ring behind the initial support, i.e. Figure 2 Schematic diagram of section A4-A4.

[0056] In the diagram: 1-permeable membrane, 2-herringbone clay slope, 3-drainage system diversion channel, 4-pipe roof guide wall, 5-pipe roof working platform, 6-pipe roof orifice pipe, 7-φ42 advanced small guide pipe, 8-φ108 pipe roof, 9-φ25 temporary anchor, 10-guide wall H25 steel arch frame, 11-face drainage hole, 12-initial support steel arch frame, 13-φ32 threaded steel bar, 14-guide wall H25a I-beam, 15-[16a transverse connecting channel steel, 16-grout stop wall and guide wall, 17-advanced surrounding grouting hole, 18-water stop ring grouting hole. Detailed Implementation

[0057] The present invention will be further described below with reference to specific embodiments. These specific embodiments are further explanations of the principles of the present invention and are not intended to limit the present invention in any way. Any technology that is the same as or similar to the present invention does not exceed the scope of protection of the present invention.

[0058] In the description of this invention, it should be noted that the terms "axis," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0059] Example 1:

[0060] like Figures 1 to 8As shown, this embodiment is a construction method for tunnels in shallow buried strata with high slope and abundant water. It includes the following measures during tunnel excavation: taking reasonable measures such as surface water treatment, advanced support, surrounding grouting, horizontal drainage holes, monitoring of surface settlement and tunnel deformation, and water-stop rings to prevent the risks of large deformation, collapse, mudslides, water inrush and soil and sand flow in the initial support of the tunnel.

[0061] (1) Surface water treatment:

[0062] This embodiment addresses a 10.99% downhill slope with water-rich, extremely soft, completely weathered granite shallow-buried strata. This rock is extremely soft, exhibiting a granular structure, and readily disintegrates, softens, and deforms upon contact with water. The area experiences intense groundwater activity, resulting in significant surrounding rock stability issues. Therefore, surface water treatment is necessary to prevent it from seeping along fissures and eroding the surrounding rock, exacerbating waterlogging and softening, and causing instability of the tunnel arch and reduced bearing capacity. To ensure tunnel construction safety, specific measures are as follows:

[0063] Drain the water that has accumulated on the ground surface near the tunnel axis;

[0064] A herringbone-shaped clay slope with a slope ratio of 3% is constructed on the top surface of each side of the shallow buried section of the tunnel axis, 9m wide.

[0065] The surface of the 9m herringbone clay slope 2 on both sides of the shallow buried section of the tunnel is covered with a geomembrane 1. The geomembrane 1 is a polyethylene geomembrane with a thickness of not less than 0.8mm.

[0066] A drainage system is installed around the 9m herringbone clay slope 2 on both sides of the tunnel axis to ensure that the seepage prevention and drainage of the geomembrane 1 can be smoothly connected with the drainage ditch 3, completely cut off the surface water supply, and divert the water flow of the drainage ditch 3 to connect smoothly with the natural drainage.

[0067] (2) Advance support measures:

[0068] The large pipe roof pre-support structure is a rigid support structure composed of pipe roofs and grouting bodies. It shares the load of the upper soil, reduces stress release deformation of the surrounding rock after excavation, and forms an arched support shell to reinforce the unexcavated surrounding rock in advance, preventing tunnel face collapse and large deformation of the initial support. To ensure the stability of the tunnel face during excavation and prevent large deformation of the initial support, pre-support is required before excavation. Specific measures are as follows:

[0069] The guide wall 4 of the pipe roof inside the tunnel adopts C25 shotcrete + double-layer steel mesh + temporary anchor bolts + steel arch frame + pipe roof orifice pipe 6. The thickness of C25 shotcrete is ≥30cm; the double-layer steel mesh has a steel diameter of φ8 round steel, a mesh size of 150mm×150mm, and an overlap length of 20cm; the temporary anchor bolt 9 has a diameter of φ25 threaded steel, a row spacing of 1.0m×1.0m, a single length of 1.5m, and a construction angle perpendicular to the tunnel face; the steel arch frame consists of 2 rings of 25a I-beams with a circumferential spacing of 50cm, installed in sections, and connected to the initial support steel arch frame with φ32 steel to form a whole; the pipe roof orifice pipe 6 has an outer diameter of 127mm, a wall thickness of 5mm, a single length of 0.7m, a circumferential spacing of 30cm, and is firmly welded to the 25a I-beams using "Ω" steel bars.

[0070] The tunnel's shallow-buried section crossing a gully uses Φ108 long pipe roof 8 (within the range of the top arch and the side walls 1.0m above the bottom slab on both sides) + φ42 advance small pipe 7 (within the range of 120° of the top arch). The pipe roof 8 has an outer diameter of 108mm, a wall thickness of 10mm, a circumferential spacing of 30cm, and a ring length of 15m / 26m / 30m, with an external insertion angle of 15° / 3° / 2°. The advance small pipe 7 has an outer diameter of 42mm, a wall thickness of 4.0mm, a single length of 4.5m, a circumferential spacing of 30cm, and a longitudinal spacing of 2.0m.

[0071] Based on the distance between the pipe roof 8 and the excavation outline, the support measures for the φ42 advanced small guide pipe 7 were adjusted accordingly. The φ42 advanced small guide pipe 7 was not installed 3m behind the guide wall 4 and 3m in the gradual expansion section of the pipe roof 8 working room.

[0072] Based on the on-site excavation conditions, observe the rockfall situation at the tunnel arch and adjust the spacing of the φ42 advanced small guide pipe 7 supports in a timely manner to reduce the occurrence and severity of rockfall at the arch.

[0073] (3) Surrounding grouting measures:

[0074] Grouting around the tunnel face serves two main purposes: first, the grout seeps into cracks and pores in the soil, solidifies to form a unified structure, and strengthens the surrounding rock; second, in water-rich strata, grouting can seal groundwater channels, reducing the risk of mudslides and water inrushes. To ensure the stability of the tunnel face during excavation and prevent soil erosion, quicksand, mudslides, water inrushes, and large deformations of the initial support, grouting around the tunnel face is necessary before excavation. Specific measures are as follows:

[0075] The first section of the grout-stopping wall 16 uses C25 shotcrete + double-layer steel mesh + φ25 short anchor rods. The thickness of the C25 shotcrete is 30-50cm. The double-layer steel mesh uses φ8 round steel bars with a mesh size of 150mm×150mm and an overlap length of 20cm. The temporary anchor rods 9 use φ25 threaded steel bars with a spacing of 1.5m×1.5m and a single length of 1.5m. The construction angle is perpendicular to the working face. The subsequent section of the grout-stopping wall 16 uses a 3-5m section of rock stratum reserved for grouting around it, and at the same time, C25 shotcrete with a thickness of 30-50cm is arranged. It is equipped with a double-layer steel mesh with a diameter of φ8 round steel bars, a mesh size of 150mm×150mm, and an overlap length of 20cm.

[0076] The grouting range around the tunnel face extends from the tunnel excavation outline to 4.0m outside the excavation outline. The circumferential spacing of grouting holes 17 is 1.00m, and the ring spacing is 1.00m. Grouting holes 18 are arranged in 2.5 rings (half a ring is set at the tunnel invert arch). The outer deflection angle of the bottom ring f1 to f8 is 0° (coinciding with the tunnel axis), the outer deflection angle of the middle ring e1 to e25 is 20°, and the outer deflection angle of the inner ring d1 to d19 is 15°. There are a total of 52 grouting holes, arranged in two sequences: 26 holes in sequence I with a depth of 12m; and 26 holes in sequence II with a depth of 15m. The grouting range extends from the tunnel excavation outline to 4.00m outside the outer excavation outline. The grout diffusion radius of each hole is ≥0.5m. It is required that the grout from adjacent grouting holes 17 can overlap.

[0077] The grouting grout uses a two-component grout with the following mix ratio:

[0078] Liquid A (water glass slurry) = water glass : water = 1:1 (volume ratio);

[0079] B liquid (cement grout) = cement : water = 1 : 1 (mass ratio);

[0080] C liquid (cement-water glass mixture) = cement slurry : water glass slurry = 1:1 (volume ratio).

[0081] The grouting sequence should be one hole at a time, from top to bottom and from outside to inside. Grouting holes should be filled in layers from the outside to the inside. This ensures the quality of grouting and also allows for the inspection of the grouting effect.

[0082] Grouting should be performed using a retreating grouting method, with the grouting pressure controlled within 0.8–1.5 MPa. The grouting pressure can be appropriately increased based on the gelation time of the two-component grout, but the maximum grouting pressure should ensure tunnel safety and prevent excessive heave or cracking of the ground surface. During grouting, close monitoring and inspection of the ground surface and tunnel are essential, especially monitoring for heave and grout leakage directly above the grouting section. Simultaneously, monitoring should be conducted at the tunnel face and initial support to monitor the uplift of the surrounding rock. If uplift is detected, pressure should be reduced and intermittent grouting should be used as an emergency measure.

[0083] If weak points or inadequate grouting are found in the grouting around the tunnel during the excavation process, local supplementary grouting can be carried out randomly to ensure the stability of the surrounding rock at the excavation face and prevent mudslides and water inrushes.

[0084] (4) Horizontal drainage hole 11 at the working face and drainage measures:

[0085] The horizontal drainage hole 11 at the tunnel face can reduce groundwater pressure and the phreatic line at the tunnel face, prevent sudden water inrush, improve the stability of the surrounding rock, optimize the construction environment, and assist in geological exploration ahead of the tunnel face. Drainage within the tunnel primarily prevents the immediate disintegration and softening of the completely weathered granite upon contact with the freshly excavated rock surface, thus avoiding further softening and physical deformation of the rock mass around the tunnel excavation outline, and also improves the construction environment. The core of the horizontal drainage hole 11 and drainage treatment is dewatering and drainage diversion. Specific measures during construction are as follows:

[0086] Three drainage holes 11 are arranged on the steps of the tunnel face. The holes are 130mm in diameter, 15m in length, and 3-5° in horizontal elevation. They are equipped with 108mm outer diameter PVC pipes. The outlet of the pipes is connected to a flexible hose to drain the seepage water to the collection pit.

[0087] Strengthen drainage and water diversion at the working face to prevent water from soaking the initial support arch foot and softening and eroding the surrounding rock excavated from the sidewall.

[0088] A movable water tank is installed at the lowest point of the tunnel, and a float switch is used to automatically control the pumping and drainage.

[0089] A water flow meter is installed at the tunnel entrance drainage pipe to count the tunnel seepage. The collected data is used to determine the relationship between the tunnel excavation length and the seepage volume, and to guide the design of the tunnel drainage system.

[0090] (5) Monitoring measures for initial support deformation on the surface and inside the tunnel:

[0091] The core purpose of monitoring the initial support arch settlement within the tunnel is to analyze the response of the surrounding rock strata after excavation, assess the stability of the surrounding rock, and provide early warnings for construction safety; clearance convergence is used to determine the stress condition of the initial support structure and to control the shape of the excavation section; surface settlement monitoring mainly determines the stability of the overburden and the effectiveness of the initial support system. The ultimate goal of monitoring surface settlement, tunnel arch settlement, and initial support structure convergence is to guide safe tunnel excavation and ensure that the design section does not encroach on the design limits. Specific measures during construction are as follows:

[0092] After the tunnel body is excavated and supported and before the tunnel is lining for the second time, observation points for the top arch settlement and clearance convergence points are set up on the surface of the initial support to monitor the deformation of the initial support after excavation and the disturbance to the initial support during the construction of the middle and lower benches. A cross section is set up for the top arch settlement every 2.5m and for the clearance convergence every 5.0m.

[0093] Surface settlement observation points are set up on the ground surface along the tunnel axis, and a cross section is set up every 5m along the tunnel axis.

[0094] By comparing the monitoring data, we can analyze the linkage between tunnel settlement and surface settlement, as well as the relationship between construction disturbance and settlement rate. The monitoring data can be used to guide the safe construction of tunnel excavation and ensure that the tunnel design section does not encroach on the limit.

[0095] (6) Initial support backwater sealing ring measures:

[0096] The water-stop ring behind the initial support of the tunnel involves grouting at intervals behind the completed initial support section in the downslope direction. This divides the seepage channels behind the initial support into several segments, diverts ground water, reduces the amount and pressure of seepage at the construction face, and prevents erosion and hollowing of the rock strata behind the initial support. Ultimately, it protects the effectiveness of the initial support structure and avoids further settlement and deformation after the initial support is closed. Specific measures during construction are as follows:

[0097] The tunnel is constructed on a downhill slope with a gradient of 10.99%. As the tunnel face is excavated, groundwater seepage will accumulate along the gaps behind the initial support towards the tunnel face, further eroding and softening the surrounding rock. Therefore, it is necessary to install water-stopping measures behind the completed initial support structure. A water-stopping ring should be installed every 100m. The grouting holes should penetrate 4m into the rock, with a borehole diameter of not less than 48mm, a circumferential spacing of 3.0m, and a grouting pressure of 0.5~1.0MPa. The grout should be a cement-water glass two-component grout with a volume ratio of 1:1.

[0098] Two drainage holes are constructed on each side wall 3.0m in front of the water-stop ring (in the direction of the opening). The drainage holes are 75mm in diameter, 2m long, and 15° horizontally upward. They are filled with 50mm outer diameter PVC pipes and wrapped with a layer of drainage geotextile.

[0099] This invention method is applied to a tunnel in a certain field. The tunnel is a main water conveyance line with a designed flow rate of 70 m³ / s. The shallow-buried pre-support section is from GH44+087.00 to GH44+204.00, with a length of 117 m. The lithology of this section is metamorphic quartz sandstone of the Lower Devonian Lianhuashan Formation (D11). The tunnel roof is mainly strongly weathered, with local weak weathering. It is soft to relatively soft rock with well-developed joints and fissures, and a fractured to layered structure. The overall rock mass is relatively broken, and groundwater activity is mainly seepage to dripping. Based on the exploration results, combined with the actual exposed engineering geological conditions at the tunnel face and comprehensive analysis of advanced geological prediction, the surrounding rock has dense joints and low strength. The surrounding rock is classified as Class V (soft rock), and there is a possibility of large deformation of the initial support, collapse, roof fall, and mudslides. The burial depth is 7 m to 21 m.

[0100] During the excavation of a certain tunnel from GH44+222.00 to GH44+204.00, the initial support triggered a deformation warning. Multiple cracks appeared on the initial support surface inside the tunnel, and surface settlement occurred outside the tunnel. The initial support encroached on the limit, with the maximum encroachment exceeding 15cm. In addition, with the 25cm settlement reserved for excavation, the maximum settlement exceeded 40cm. This posed a major risk of tunnel collapse, roof fall, mudslide, and water inrush during construction, and settlement and deformation became the norm.

[0101] In response to the risks of large deformation, collapse, roof fall, and mudslide / water inrush during tunnel excavation in water-rich, shallowly buried metamorphic quartz sandstone strata, the construction team adopted various measures, including surface water treatment, advanced support, surrounding grouting, horizontal drainage holes and tunnel drainage, surface settlement monitoring, and initial support deformation monitoring, to prevent the risks of large deformation, collapse, roof fall, mudslide / water inrush, and ensure safe tunnel excavation.

[0102] This embodiment addresses the problems of large deformation, collapse, roof fall, and mudslide / water inrush in the initial support of the water-rich, soft, shallow-buried metamorphic quartz sandstone strata in the GH44+087.00~GH44+204.00 section of a certain tunnel. It implements the surface water treatment, advanced support, grouting around the tunnel, horizontal drainage holes and tunnel drainage, surface settlement monitoring, and initial support deformation monitoring methods of this invention. Tests have shown that surface water treatment significantly reduces the tunnel drainage flow, greatly reducing water erosion and softening of the metamorphic quartz sandstone; horizontal drainage holes at the tunnel face effectively reduce groundwater levels, increasing face stability and preventing mudslide / water inrush; advanced support effectively curbs overall rock settlement and roof collapse during construction; grouting around the tunnel face reinforces the integrity and strength of the surrounding rock along the excavation outline and has a significant water-blocking effect. The combined use of these measures ensures safe construction and stable initial support for this type of tunnel excavation.

Claims

1. A method for constructing tunnels in shallow, water-rich, extremely soft, downhill strata with a steep slope, characterized in that... Includes the following steps: S1: Surface water treatment; drain the water accumulated on the surface above the tunnel axis, and construct a herringbone clay slope and drainage system on the top surface of each side of the tunnel axis, 9m on each side. S2: Take advanced support measures for tunnels; including: C25 shotcrete + double-layer steel mesh + temporary anchor + steel arch frame + orifice pipe for the guide wall of the pipe roof inside the tunnel; advanced support for the shallow buried section of the tunnel crossing the gully, long pipe roof is used for the sidewalls within 1.0m above the top arch and the bottom slabs on both sides, and advanced small pipe is used within 120° of the top arch. S3: Take measures to grout around the tunnel; including: the first section of the tunnel grout-stopping rock wall is reinforced with C25 shotcrete + double-layer steel mesh + φ25 short anchor rod grouting, and the subsequent sections are reinforced with reserved rock strata for grouting around the tunnel face; the grouting range around the tunnel face is from the tunnel excavation outline to 4.0m outside the excavation outline. S4: Take measures for horizontal drainage holes and internal drainage in the tunnel; including: at least three drainage holes are arranged on the left, middle and right sides of the steps on the tunnel face, with built-in perforated pipes. The outlet of the perforated pipes is connected with a flexible hose to drain the seepage water to the sump. A movable water collection tank is set at the lowest point of the tunnel, and the pumping and drainage are automatically controlled by a float switch. S5: Take measures to monitor surface settlement and initial support deformation in the tunnel; including: setting up arch settlement observation points and clearance convergence points on the surface of the initial support; setting up surface settlement observation points on the surface of the tunnel axis; S6: Adopt water-stop ring measures behind the initial support; install water-stop rings in the tunnel section where the initial support has been completed, one water-stop ring every 100m, and construct two drainage holes on each side wall 3.0m in front of the water-stop ring in the direction of the tunnel opening, with built-in perforated pipes, and wrap the perforated pipes with drainage geotextile.

2. The method for constructing tunnels in shallow, water-rich, extremely soft strata on steep slopes according to claim 1, characterized in that, Step S1 specifically includes: S1.1: Drain surface water near the tunnel axis; S1.2: A herringbone-shaped clay slope with a slope ratio of 3% is constructed on the top surface of the tunnel axis, 9m on each side. S1.3: A seepage-proof membrane shall be covered on the surface of the herringbone clay slope on both sides of the tunnel axis for 9m. The seepage-proof membrane shall be a polyethylene geomembrane with a thickness of not less than 0.8mm. S1.4: A drainage system is installed around the 9m herringbone clay slope on both sides of the tunnel axis to ensure that the waterproof membrane can be smoothly connected to the drainage ditch, completely cut off the surface water supply, and smoothly connect the drainage ditch water flow to the natural drainage.

3. The method for constructing tunnels in shallow, water-rich, extremely soft strata on steep slopes according to claim 1, characterized in that... Step S2 specifically includes: S2.1: The guide wall of the pipe roof inside the tunnel adopts C25 shotcrete + double-layer steel mesh + temporary anchor bolts + steel arch frame + orifice pipe. The thickness of C25 shotcrete is ≥30cm; the double-layer steel mesh has a steel diameter of φ8 round steel, a mesh size of 150mm×150mm, and an overlap length of 20cm; the temporary anchor bolts have a diameter of φ25 threaded steel, a row spacing of 1.0m×1.0m, a single length of 1.5m, and a construction angle perpendicular to the tunnel face; the steel arch frame uses 25a I-beams with 2 rings, a circumferential spacing of 50cm, installed in sections, and connected to the initial support steel arch frame with φ32 steel to form a whole; the orifice pipe has an outer diameter of 127mm, a wall thickness of 5mm, a single length of 0.7m, a circumferential spacing of 30cm, and is firmly welded to the 25a I-beams using "Ω" steel bars; S2.2: Advance support for the shallow buried section of the tunnel crossing the gully. Φ108 long pipe roof is used for the sidewalls within 1.0m above the top arch and both sides of the bottom slab. φ42 advance small guide pipes are used within 120° of the top arch. The pipe roof has an outer diameter of 108mm, a wall thickness of 10mm, a circumferential spacing of 30cm, and a ring length of 15m / 26m / 30m, with an external insertion angle of 15° / 3° / 2°. The advance small guide pipe has an outer diameter of 42mm, a wall thickness of 4.0mm, a single length of 4.5m, a circumferential spacing of 30cm, and a longitudinal spacing of 2.0m. S2.3: Adjust the φ42 advanced small pipe support measures according to the distance between the pipe roof location and the excavation outline. Do not install φ42 advanced small pipes 3m behind the guide wall and 3m in the gradual expansion section of the pipe roof working chamber.

4. The method for constructing tunnels in shallow, water-rich, extremely soft strata on steep slopes according to claim 1, characterized in that... Step S3 specifically includes: S3.1: The first section of the grout-stopping wall uses C25 shotcrete + double-layer steel mesh + φ25 short anchor rods. The thickness of the C25 shotcrete is 30-50cm. The double-layer steel mesh uses φ8 round steel bars with a mesh size of 150mm×150mm and an overlap length of 20cm. The temporary anchor rods are φ25 threaded steel bars with a spacing of 1.5m×1.5m and a single length of 1.5m. The construction angle is perpendicular to the working face. The subsequent sections of the grout-stopping wall use a 3-5m section of rock stratum reserved for grouting around the perimeter, and simultaneously use C25 shotcrete with a thickness of 30-50cm. It is equipped with a double-layer steel mesh with a diameter of φ8 round steel bars, a mesh size of 150mm×150mm, and an overlap length of 20cm. S3.2: The grouting range around the tunnel face extends from the tunnel excavation outline to 4.0m outside the excavation outline. The circumferential spacing of the grouting holes is 1.00m, and the ring spacing is 1.00m. The grouting holes are arranged in 2.5 rings, with a half-ring at the tunnel invert. The bottom ring (f1-f8) has an outer deviation angle of 0° coinciding with the tunnel axis, the middle ring (e1-e25) has an outer deviation angle of 20°, and the inner ring (d1-d19) has an outer deviation angle of 15°. There are a total of 52 grouting holes, arranged in two sequences: Sequence I has 26 holes with a depth of 12m, and Sequence II has 26 holes with a depth of 15m. The grouting range extends from the tunnel excavation outline to 4.00m outside the excavation outline. The grout diffusion radius of each hole is ≥0.5m, and the grout from adjacent grouting holes is required to overlap. S3.3: Two-component grout C-liquid is used for grouting, and the completion criteria for grouting are determined by the dual control indicators of grouting pressure and grout absorption rate; the mix ratio of the two-component grout C-liquid is as follows: Liquid A (water glass slurry) = water glass : water = 1 : 1 (volume ratio); B liquid (cement grout) = cement:water = 1:1 (mass ratio); C liquid (cement-water glass mixture) = cement slurry : water glass slurry = 1:1 (volume ratio); S3.4: The grouting sequence should be one hole at a time, with the grouting sequence from top to bottom and from outside to inside. The grouting holes should be filled in layers from the outside to the inside. S3.5: Grouting shall be carried out using a retreating grouting method, and the grouting pressure shall be controlled within 0.8 to 1.5 MPa. The grouting pressure may be appropriately increased according to the gel time of the two-component grout, but the maximum grouting pressure shall ensure the safety of the tunnel and prevent excessive heave or cracking of the ground surface. During the grouting process, close monitoring and inspection of the ground surface and the tunnel interior are essential, especially the heave and grout leakage of the ground surface directly above the grouting section. At the same time, the tunnel face and initial support shall be monitored to monitor the uplift of the surrounding rock. If uplift is found, pressure shall be reduced in time and intermittent grouting shall be used for emergency treatment. S3.6: If weak points or inadequate grouting are found in the grouting around the tunnel during the excavation process, local supplementary grouting can be carried out randomly to plug water and ensure the stability of the surrounding rock at the excavation face.

5. The method for constructing a tunnel in a shallow, water-rich, extremely soft stratum on a steep downhill slope according to claim 1, characterized in that... Step S4 includes: S4.1: Three drainage holes are arranged on the steps of the tunnel face. The drainage holes are 130mm in diameter, 15m in length, and have a horizontal upward angle of 3-5°. They are equipped with PVC perforated pipes with an outer diameter of 108mm. The outlet of the perforated pipes is connected with a flexible hose to drain the seepage water to the collection pit. S4.2: Strengthen the drainage system at the working face to prevent water from soaking the initial support arch foot and softening and eroding the surrounding rock excavated from the sidewall. S4.3: A movable water collection tank is installed at the lowest point of the tunnel, and a float switch is used to automatically control the pumping and drainage. S4.4: Install a water flow meter at the tunnel entrance drainage pipe to count the tunnel seepage. The collected data is used to determine the relationship between the tunnel excavation length and the seepage, and to guide the design of the tunnel drainage system.

6. The method for constructing a tunnel in a shallow, water-rich, extremely soft stratum on a steep downhill slope according to claim 1, characterized in that, Step S5 includes: S5.1: After the tunnel body is excavated and supported and before the secondary lining of the tunnel, the top arch settlement observation point and the clearance convergence point shall be arranged on the surface of the initial support to monitor the deformation of the initial support after excavation and the disturbance to the initial support during the construction of the middle and lower benches. The top arch settlement point shall be arranged with a cross section every 2.5m and the clearance convergence point shall be arranged with a cross section every 5.0m. S5.2: Set up surface settlement observation points on the ground along the tunnel axis, with one cross-section every 5m along the tunnel axis; S5.3: Compare and analyze the linkage between tunnel settlement and surface settlement, as well as the relationship between construction disturbance and settlement rate, based on the monitoring data, and guide tunnel excavation construction through the monitoring data.

7. The method for constructing a tunnel in a shallow, water-rich, extremely soft stratum on a steep downhill slope according to claim 1, characterized in that... Step S6 includes: S6.1: Water-stopping rings shall be installed in the initial support section after completion. One water-stopping ring shall be installed every 100m. The grouting hole shall be inserted 4m into the rock with a diameter of not less than 48mm and a circumferential spacing of 3.0m. The grouting pressure shall be 0.5~1.0MPa. The grout shall be cement-water glass double-liquid grout with a volume ratio of 1:

1. S6.2: Construct two drainage holes on each side wall at a distance of 3.0m in front of the opening of the water-stop ring. The drainage holes are 75mm in diameter, 2m long, and 15° in elevation. They are filled with 50mm outer diameter PVC pipes and wrapped with a layer of drainage geotextile.

Images