A gradient shotcrete structure for single-layer tunnel lining and its construction method

Abstract

This invention discloses a gradient shotcrete structure for single-layer tunnel lining and its construction method. The shotcrete structure of this invention uses the tunnel surrounding rock surface as a base. The concrete structure sequentially comprises a mixed steel fiber shotcrete layer, a water-retaining shotcrete mortar layer, and a low-shrinkage shotcrete layer. The mixed steel fiber shotcrete layer includes cementitious materials, mixed fibers, a liquid early-strength agent, a liquid alkali-free quick-setting agent, aggregates, and water. The water-retaining shotcrete layer includes ordinary Portland cement, silica fume, fine aggregates, heavy calcium carbonate powder, a liquid water-reducing agent, a water-retaining modifier, and a liquid alkali-free quick-setting agent. The low-shrinkage shotcrete layer includes cementitious materials, a shrinkage-reducing agent, a liquid water-reducing agent, a liquid alkali-free quick-setting agent, and aggregates. The single-layer shotcrete structure of this invention exhibits excellent mechanical bearing strength and toughness, low shrinkage cracking, and good impermeability, thereby improving the adaptability of single-layer tunnel lining structures under complex terrain conditions.

Description

Technical Field

[0001] This invention belongs to the field of tunnel lining concrete technology, specifically, it relates to a gradient shotcrete structure and construction method for single-layer tunnel lining. Background Technology

[0002] Tunnel concrete lining refers to the support structure constructed immediately after tunnel excavation. It provides temporary (initial support) or permanent support (secondary lining) to the tunnel surrounding rock surface, forming an integral load-bearing structure with the surrounding rock and controlling its relaxation and deformation. The lining structure is primarily a composite lining form under the New Austrian Tunneling Method (NATM). Unlike composite lining structures, single-layer linings fully consider the performance margins and support strength of shotcrete in their structural design, using it as a permanent support system and eliminating the need for isolation layers such as waterproofing membranes. The secondary lining structure also uses shotcrete or cast-in-place concrete layers (thicker than the NATM secondary lining concrete layer). Therefore, it has advantages such as integrated load-bearing capacity, sufficient interlayer stress transfer, simplified construction process, reduced lining thickness, and high cost-effectiveness.

[0003] my country has a vast territory and a complex and diverse geological environment, which makes the geological and hydrological conditions of the surrounding rock of tunnels more complex. The shotcrete material used in single-layer lining currently has problems such as low load-bearing strength (mostly C25 / C30 strength grade), slow early strength development, shrinkage cracking and leakage, and insufficient flexural toughness. In addition, there are weak stress zones at the interface between single-layer lining concrete layers, and leakage problems caused by the development of fissure water are common. The self-waterproofing and seepage resistance of lining concrete materials urgently need to be improved.

[0004] Chinese patent application CN109626914A discloses a material and preparation method for a waterproof single-layer lining of hard rock tunnels. It uses polyolefin fibers, silica fume, and ordinary silicate 52.5 cement to prepare a fiber-reinforced shotcrete material. However, the high grade and large quantity of cement (980-1020 parts) easily cause shrinkage cracking in the shotcrete material. Patent CN110359915A discloses a waterproof single-layer lining structure and its manufacturing method suitable for double-track tracks in Class IV surrounding rock. It uses coarse polyolefin fibers, fine polypropylene fibers, and Ω-shaped spring drainage half-pipes to prepare a double-layer fiber-reinforced shotcrete single-layer lining structure. However, the process also involves smooth blasting, drainage pipe laying, and grid arch frame construction, resulting in high engineering costs and complex processes and materials. Therefore, there is an urgent need to develop a single-layer lining concrete structure with excellent mechanical bearing strength and toughness, low shrinkage cracking, and good impermeability to improve the adaptability of single-layer lining structures for tunnels in complex terrain conditions. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a gradient shotcrete structure for single-layer tunnel lining and its construction method. The concrete gradient adopts a "mixed steel fiber shotcrete layer + water-retaining shotcrete mortar layer + low-shrinkage shotcrete layer", aiming to meet the mechanical bearing strength, interface impermeability and deformation resistance of single-layer lining under the complex terrain conditions of tunnels, while simplifying the construction process and lining thickness, saving project costs and ensuring the service life of the lining structure.

[0006] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0007] This invention provides a gradient shotcrete structure for single-layer tunnel lining, comprising three layers of shotcrete or mortar materials arranged in a gradient manner. The gradient is determined by the selection, order, and thickness of each layer. Using the tunnel surrounding rock as a base, the concrete structure sequentially includes a mixed steel fiber shotcrete layer, a water-retaining shotcrete mortar layer, and a low-shrinkage shotcrete layer. The concrete structure is prepared using a wet shotcrete process. Specifically, the mixed steel fiber shotcrete layer serves as the first layer, directly contacting the tunnel surrounding rock to provide early support and improve structural toughness. The concrete layer has a thickness ranging from 50 to 150 mm, depending on the surrounding rock grade. For example, 50 mm is recommended for Grade II surrounding rock, while 150 mm is recommended for Grade V and above. The water-retaining shotcrete layer serves as the intermediate layer, forming good adhesion with the first layer, improving defects in the shotcrete surface, and providing water retention and impermeability. The thickness of the mortar layer is 20-50 mm. The low-shrinkage shotcrete layer serves as the third layer and is also the surface layer of the single-layer lining structure. It features low shrinkage, deformation resistance, and high later-stage strength. The thickness of the concrete layer is 100-150 mm.

[0008] The mixed steel fiber shotcrete layer of the present invention comprises cementitious material, mixed fiber, liquid early-strength agent, liquid alkali-free quick-setting agent, aggregate, and water. The aggregate includes coarse aggregate and fine aggregate. The mass of each component per cubic meter of mixed steel fiber shotcrete layer material is as follows: cementitious material 400-450 kg, mixed fiber 20 kg-45 kg, liquid early-strength agent 8 kg-15 kg, liquid alkali-free quick-setting agent 25 kg-35 kg, and the cement-aggregate ratio is 1:3.5 to 1:5.0, the sand ratio is 55-65%, and the water-cement ratio is 0.38-0.45. The total aggregate = cementitious material / cement-aggregate ratio, the fine aggregate = total aggregate * sand ratio, and the coarse aggregate = total aggregate - fine aggregate.

[0009] The cementitious material comprises the following components by mass percentage: 70%-90% cement and 10-30% active early-strength powder;

[0010] The cement is selected from any one of ordinary Portland cement, slag Portland cement, and composite Portland cement, and the cement strength grade is 42.5 with a loss on ignition of less than 3%.

[0011] The active early strength powder is a mixture of silica fume, metakaolin and granulated blast furnace slag powder, wherein the mass ratio of silica fume: metakaolin: granulated blast furnace slag powder is (0.5-1):1:(0.5-1).

[0012] Both the coarse and fine aggregates are obtained by crushing, screening, and grinding the tunnel slag parent rock. The compressive strength of the tunnel slag parent rock used is greater than 80 MPa, the mica content is less than or equal to 0.2 wt%, and the alkali-silica reactivity is less than 0.1%.

[0013] Furthermore, the coarse aggregate has a maximum particle size of no more than 10 mm, a needle-like / flaky content of less than 3 wt%, and a crushing index of less than 10%; the fine aggregate has a stone powder content of less than 5 wt% and a loose bulk density of greater than 1600 kg / m³. 3 .

[0014] The mixed fiber is a mixture of steel fiber and non-steel fiber, wherein the volume ratio of steel fiber to non-steel fiber is 1:0.5-1; the steel fiber is a coated and modified profiled steel fiber, preferably one of hooked, bundled, or wavy shape, with a diameter of 0.4-0.6 mm, a length of 15-30 mm, and a tensile strength greater than 1200 MPa; the coating material used for the steel fiber is one of epoxy resin, tin plating, or copper plating; the non-steel fiber is selected from any one of imitation steel fiber, glass fiber, or polyoxymethylene monofilament fiber, with a diameter of 50-100 μm, a length of 8-12 mm, and a tensile strength greater than 800 MPa;

[0015] The modification method of the coating-modified shaped steel fiber is to physically coat the surface of the steel fiber with a coating material and then dry it. This is used to improve the service life and toughening properties of the fiber.

[0016] The liquid early-strength agent has a solid content of 30-50%, and its solute component is a composition of early-strength polycarboxylate superplasticizer, calcium lignosulfonate or calcium lignohumate, and inorganic calcium salt; wherein the mass ratio of early-strength polycarboxylate superplasticizer: calcium lignosulfonate or calcium lignohumate: inorganic calcium salt is 5:(1-2):(3-5); the inorganic calcium salt is one of calcium nitrate, calcium formate, and calcium nitrite; the 1-day compressive strength ratio of the early-strength polycarboxylate superplasticizer is greater than 180%, and the 28-day compressive strength ratio is greater than 150%.

[0017] The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and the equivalent content of fluorine are both less than 0.05 wt%.

[0018] The water-cement ratio in the water-retaining sprayed mortar layer of this invention is no greater than 0.4, and the raw materials, excluding water, are composed of the following parts by weight:

[0019]

[0020]

[0021] The material of the water-retaining sprayed mortar layer has a binder-to-mortar ratio of 1:1.5 to 1:2.5.

[0022] The ordinary Portland cement has a strength grade of 52.5 and a specific surface area of ​​350-400 m². 2 / kg, C3S content not less than 65wt%;

[0023] The silica content in the microsilica powder is greater than 95 wt%, and the maximum particle size of the microsilica powder is not greater than 200 nm.

[0024] The fine aggregate is selected from one or a mixture of two types of river sand and manufactured sand, with a maximum particle size of less than 1.6 mm;

[0025] The calcium carbonate powder contains more than 98 wt% calcium carbonate and the maximum particle size is not higher than 60 μm.

[0026] The liquid water-reducing agent is selected from any one of polycarboxylic acid, melamine, and naphthalene sulfonic acid water-reducing agents, and the water reduction rate is not less than 20%.

[0027] The water-retaining modifier is a mixture of dispersible latex powder, cellulose ether, and polyacrylamide, wherein the mass ratio of dispersible latex powder:cellulose ether:polyacrylamide is 1:0.1-0.5:0.02-0.1; the cellulose ether is selected from any one of hydroxypropyl methylcellulose ether, hydroxyethyl cellulose, and hydroxyethyl methylcellulose ether, with an average relative molecular weight of not less than 200,000; the dispersible latex powder is a mixture of acrylic acid copolymer and polyvinyl alcohol, with a maximum particle size of not more than 50 μm;

[0028] The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and the equivalent content of fluorine are both less than 0.05 wt%.

[0029] The low-shrinkage shotcrete layer of the present invention comprises cementitious materials, shrinkage-reducing agents, liquid water-reducing agents, liquid alkali-free quick-setting agents, and aggregates, wherein the aggregates include coarse aggregates and fine aggregates; the mass of each component per cubic meter of low-shrinkage shotcrete layer material is as follows: cementitious materials 450-550 kg, shrinkage-reducing agents 5-10 kg, liquid water-reducing agents 3-6 kg, liquid alkali-free quick-setting agents 30-50 kg, and the cement-aggregate ratio is 1:2.0 to 1:3.5, the sand ratio is 50-60%, and the water-cement ratio is not greater than 0.4.

[0030] The cementitious material comprises the following components by mass percentage: 85%-95% cement and 5%-15% expansion compensation material;

[0031] The cement is selected from any one of ordinary Portland cement, pozzolanic Portland cement, and fly ash Portland cement, with a strength grade of not less than 42.5 and a C3A mineral content of not less than 3.5 wt%, wherein pozzolanic Portland cement is preferred.

[0032] The expansion compensation material is selected from any one of calcium-magnesium composite, calcium sulfoaluminate, calcium oxide, and calcium sulfoaluminate-calcium oxide expansion agents, with a specific surface area of ​​300-400 m². 2 / kg, with a 7-day limiting swelling rate of 0.15-0.25% in water;

[0033] Furthermore, the expansion compensation material is more preferably calcium sulfoaluminate-calcium oxide, wherein the mass proportion of calcium sulfoaluminate is 15-25%, and the mass proportion of calcium oxide is not less than 40%.

[0034] Both the coarse and fine aggregates are obtained by crushing, screening, and grinding the tunnel slag parent rock. The compressive strength of the tunnel slag parent rock used is greater than 80 MPa, the mica content is less than or equal to 0.2 wt%, and the alkali-silica reactivity is less than 0.1%.

[0035] Furthermore, the coarse aggregate has a maximum particle size of no more than 10 mm, a needle-like / flaky content of less than 3 wt%, and a crushing index of less than 10%; the fine aggregate has a stone powder content of less than 5 wt% and a loose bulk density of greater than 1600 kg / m³. 3 .

[0036] The shrinkage reducing agent is a polyether-based shrinkage reducing agent with a relative molecular weight of no more than 500.

[0037] The liquid water-reducing agent is a polycarboxylate high-performance water-reducing agent with a water reduction rate of not less than 28%.

[0038] The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and the equivalent content of fluorine are both less than 0.05 wt%.

[0039] Another object of the present invention is to provide a construction method for the above-mentioned gradient shotcrete structure for single-layer tunnel lining, which includes the following steps:

[0040] S1. Preparation of mixed steel fiber shotcrete layer:

[0041] (1) In a twin-shaft forced mixer, coarse and fine aggregates and steel fibers are fully mixed and dispersed, and then active early strength powder, cement, water, non-steel fiber materials and liquid early strength agent are added in sequence. Mixing is continued to obtain mixed fiber concrete mixture.

[0042] (2) After the tunnel surrounding rock surface is alternately flushed with 0.4-0.8MPa air pressure and 0.2-0.4MPa high-pressure water, steel arch frame, steel mesh or anchor rod are erected, and liquid alkali-free quick-setting agent is added to the quick-setting agent storage tank or pipeline of the wet spraying equipment. Then, the above-mentioned mixed fiber concrete mixture is loaded into the hopper of the wet spraying equipment, mixed with liquid alkali-free quick-setting agent at the nozzle and sprayed onto the tunnel surrounding rock surface. After the spraying thickness is reached, the first layer of mixed steel fiber sprayed concrete of the concrete structure of the present invention is obtained. Water curing is carried out for 24 hours with a water temperature not lower than 10℃.

[0043] S2. Preparation of water-retaining sprayed mortar layer:

[0044] (1) Add the fine aggregate, cement, water, liquid water-reducing agent, silica fume, water-retaining modifier, and heavy calcium carbonate powder weighed according to the mass ratio to a twin-shaft forced mixer, and mix thoroughly to obtain water-retaining mortar.

[0045] (2) Clean the surface of the first layer of mixed steel fiber shotcrete layer with 0.4-0.8MPa air pressure, add liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, and then load the above water-retaining mortar material at the nozzle and mix it with the liquid alkali-free quick-setting agent before spraying it onto the surface of the first layer of mixed steel fiber shotcrete layer. After reaching the spraying thickness, the second layer of water-retaining shotcrete layer is obtained. Sprinkle water for 24 hours and the water temperature is not lower than 10℃.

[0046] S3. Preparation of low-shrinkage shotcrete layer:

[0047] (1) Add coarse and fine aggregates, cement, expansion compensation material, shrinkage reducer, liquid water reducer and water according to the mass ratio to a twin-shaft forced mixer, and mix thoroughly to obtain low-shrinkage shotcrete mixture.

[0048] (2) After the water-retaining sprayed mortar layer has been formed for 3 days, add the liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, load the above-mentioned low-shrinkage sprayed concrete mixture and mix it with the liquid quick-setting agent at the nozzle and spray it onto the second layer of water-retaining sprayed mortar layer. After reaching the spraying thickness, the gradient sprayed concrete structure for single-layer tunnel lining described in this invention is obtained.

[0049] The beneficial technical effects of this invention are as follows:

[0050] Through reasonable concrete layer design and material configuration, a highly adaptable gradient shotcrete structure for single-layer tunnel lining was obtained.

[0051] In terms of concrete layer design, mixed steel fiber shotcrete is used as the first layer to directly contact the surrounding rock of the tunnel. The active early strength powder and liquid early strength agent in the material composition, together with the fluorine-free and alkali-free quick-setting agent, can quickly achieve the hourly strength development of shotcrete. Under the effect of the uniform random distribution of mixed fibers, the toughness of concrete materials is improved and the growth of microcracks is controlled.

[0052] Water-retaining sprayed mortar, as an intermediate layer, utilizes water-retaining modifiers and microsilica to improve particle size distribution, resulting in good interfacial bonding strength and impermeability. It can effectively improve the porosity defects on the surface of the mixed fiber sprayed layer, play a good role in connecting the upper and lower layers, and improve the load transfer performance between single-layer lining gradient concrete layers.

[0053] Low-shrinkage shotcrete, as the third layer, utilizes high-performance expansion-compensating inorganic materials and organic shrinkage-reducing agent components to improve the tendency of shotcrete to dry shrinkage in the later stage, thereby enhancing the overall mechanical strength of the single-layer lining gradient concrete.

[0054] Ultimately, the single-layer lining gradient concrete achieved an 8-hour compressive strength greater than 15.0 MPa, a 28-day compressive strength greater than 45.0 MPa, a 28-day flexural strength greater than 5.5 MPa, a 28-day drying shrinkage rate less than 200 micro-strain, an interfacial bond strength greater than 1.5 MPa, and no seepage after 30 days under a constant seepage pressure of 1.2 MPa. This effectively meets the requirements of single-layer lining structures in environments with large deformation, high surrounding rock grade, and abundant water, in terms of support toughness, bearing strength, and service durability. In addition, the construction process is simplified (only spraying and mixing equipment are used, and no formwork is required) and the project cost is low. Attached Figure Description

[0055] Figure 1 This is a schematic diagram of the gradient concrete structure for single-layer tunnel lining described in this invention.

[0056] Figure 2 This is a construction flowchart of the single-layer gradient concrete lining structure for tunnels as described in this invention.

[0057] Figure 1 The layers are labeled as follows: 1. Tunnel surrounding rock surface, 2. Mixed steel fiber shotcrete layer, 3. Water-retaining shotcrete layer, 4. Low-shrinkage shotcrete layer. Detailed Implementation

[0058] The embodiments of the present invention will now be described in detail. However, the present invention can be implemented in many different forms, and should not be construed as being limited to the specific embodiments set forth herein. Rather, these embodiments are provided to explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the various embodiments of the invention and various modifications suitable for particular intended applications.

[0059] The following will describe the components of a gradient shotcrete structure for a single-layer tunnel lining provided by the present invention through specific embodiments. However, those skilled in the art will understand that the following embodiments are only specific examples of gradient concrete for single-layer lining of the present invention and are not intended to limit all of them.

[0060] Tables 1 to 3 below show the composition and mix proportions of each layer of the single-layer lining gradient concrete structure provided in Examples 1 to 3:

[0061] Table 1. Material composition and dosage of each layer in the single-layer lining gradient shotcrete structure of Example 1

[0062]

[0063] Liquid alkali-free quick-setting agent: Add at a dosage of 25 kg per cubic meter, pH=3.5, containing fluorine and alkali.

[0064]

[0065]

[0066] Table 2. Material composition and dosage of each layer of single-layer gradient concrete lining in Example 2

[0067]

[0068] Cellulose ether: Hydroxyethyl methyl cellulose ether, with an average relative molecular weight of 350,000;

[0069] Dispersible latex powder: maximum particle size 35μm

[0070]

[0071] Table 3. Material composition and dosage of each layer of single-layer gradient concrete lining in Example 3

[0072]

[0073] Fine aggregate made from mixed fibrous slag: stone powder content 3.0%, loose bulk density 1800 kg / m³ 3

[0074]

[0075]

[0076] As described in Tables 1-3 above, gradient shotcrete materials for single-layer tunnel linings in Examples 1-3 were used. To verify the performance of the single-layer lining gradient concrete provided in the above embodiments of the present invention, test blocks were prepared using the following construction method. The method includes the following steps:

[0077] S1. Preparation of mixed steel fiber shotcrete layer:

[0078] (1) In a twin-shaft forced mixer, coarse and fine aggregates and steel fibers are fully mixed and dispersed, and then active early strength powder, cement, water and non-steel fiber remaining materials are added in sequence and the mixture is stirred to obtain a mixed fiber concrete mixture.

[0079] (2) After the tunnel rock surface is alternately flushed with 0.4-0.8MPa air pressure and 0.2-0.4MPa high pressure water, steel arch frame, steel mesh or anchor rod are erected, liquid alkali-free quick-setting agent is added to the quick-setting agent storage tank or pipeline of the wet spraying equipment, mixed fiber mixture is loaded, and sprayed onto the rock surface after mixing with quick-setting agent at the nozzle. After the spraying thickness is reached, the first layer of mixed fiber sprayed concrete is obtained. Water curing is carried out for 24 hours with water temperature not lower than 10℃.

[0080] S2. Preparation of water-retaining sprayed mortar layer:

[0081] (1) Add the fine aggregate, cement, silica fume, water-retaining modifier and heavy calcium powder weighed according to the mass ratio to the twin-shaft forced mixer and mix thoroughly to obtain water-retaining mortar.

[0082] (2) Use 0.4-0.8MPa wind pressure to clean the surface of the first layer of mixed fiber shotcrete, add liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, load water-retaining mortar and mix it with the quick-setting agent at the nozzle and spray it onto the rock surface. After reaching the spraying thickness, the second layer of water-retaining shotcrete layer is obtained. Sprinkle water for curing for 24 hours, and the water temperature is not lower than 10℃.

[0083] S3. Preparation of low-shrinkage shotcrete:

[0084] (1) Add coarse and fine aggregates, cement, expansion compensation shrinkage material, shrinkage reducer and water reducer weighed according to the mass ratio to a twin-shaft forced mixer, and mix thoroughly to obtain low-shrinkage shotcrete mixture.

[0085] (2) After the water-retaining sprayed mortar layer has been formed for 3 days, add liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, load low-shrinkage sprayed concrete mixture and mix it with quick-setting agent at the nozzle and spray it onto the rock surface. After reaching the spraying thickness, a gradient sprayed concrete structure for single-layer tunnel lining is obtained.

[0086] In Examples 1-3, while carrying out single-layer lining shotcrete construction of the solid tunnel, concrete specimens were prepared according to Appendix L of GBT 50086-2015 "Technical Specification for Rock and Soil Anchors and Shotcrete Support Engineering" for performance testing.

[0087] Comparative Example 1:

[0088] Referring to CN109626914A "A material and preparation method for waterproof single-layer lining of hard rock tunnel", a polyolefin fiber silica fume reinforced shotcrete was prepared. The single-layer lining uses a single layer of shotcrete. Concrete specimens were made according to Appendix L of GBT50086-2015 "Technical Specification for Rock and Soil Anchor and Shotcrete Support Engineering" as comparative test blocks.

[0089] Comparative Example 2:

[0090] Referring to CN110359915A "A Waterproof Single-Layer Lining Structure Applicable to Double-Track Tracks in Class IV Surrounding Rock and Its Manufacturing Method", a polyolefin fiber (coarse fiber) shotcrete and a polypropylene fiber (fine fiber) shotcrete were prepared. The concrete structure of the single-layer lining was "coarse fiber shotcrete + shotcrete mortar layer + waterproof coating + fine fiber shotcrete". Specimens were made using a solid cutting method as comparative test blocks.

[0091] Performance testing

[0092] Based on the concrete specimens prepared in Examples 1-3 and Comparative Examples 1-2 above, the mechanical properties and toughness behavior of the shotcrete were evaluated according to the compressive and flexural tests in GB / T50081-2002 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete". The water permeability resistance of the shotcrete was evaluated according to the water permeability test in GBT50082-2009 "Test Methods for Long-Term Performance and Durability of Ordinary Concrete", where the water permeability height test was performed with a fixed water permeability pressure of 1.2 MPa. The drying shrinkage rate was evaluated according to the drying shrinkage test. The interlayer bond strength of the single-layer lining concrete was evaluated according to the core pull-out test in GB 50086-2015 "Technical Specification for Rock and Soil Anchors and Shotcrete Support Engineering".

[0093] The performance test results of the single-layer lining gradient concrete in Examples 1 to 3 and the comparative single-layer lining shotcrete in Comparative Examples 1 to 2 are shown in Table 4 below.

[0094] Table 4 Performance test results of single-layer lining gradient concrete structures in Examples 1-3 and comparative single-layer lining shotcrete in Comparative Examples 1-2

[0095]

[0096] As shown in Table 4, the performance test results indicate that, compared with Comparative Examples 1 and 2, the single-layer lining gradient concrete of this invention exhibits a compressive strength greater than 15.0 MPa at 8 hours, a compressive strength greater than 45.0 MPa at 28 days, a flexural strength greater than 5.5 MPa at 28 days, a drying shrinkage rate less than 200 microstrain at 28 days, an interfacial bond strength greater than 1.5 MPa, and no water seepage after 30 days under a constant seepage pressure of 1.2 MPa. This demonstrates that it effectively meets the requirements for support toughness, bearing strength, and service durability of single-layer lining structures in environments with large deformation, high surrounding rock grade, and abundant water. Furthermore, the construction process is simplified (using only spraying and mixing equipment, without the need for formwork), and the rebound rate of the sprayed concrete is low (<10%), effectively saving on project costs.

[0097] Compared with the use of single-layer polyolefin fiber silica fume reinforced shotcrete as a single-layer lining structure concrete in Comparative Example 1, the early strength of the shotcrete was only 5.7 MPa. This indicates that the active early strength powder and liquid early strength agent used in this invention, together with the fluorine-free and alkali-free quick-setting agent, can rapidly achieve the early strength development of the shotcrete. It has good early support performance under high surrounding rock grade or soft rock deformation, and establishes the first key protective layer for tunnel construction, ensuring the safety of personnel and equipment in subsequent construction operations.

[0098] Comparative Example 2 uses a single-layer lining concrete structure consisting of "coarse fiber shotcrete + shotcrete mortar layer + sprayed waterproofing + fine fiber shotcrete". The interfacial bond strength of the concrete layer is 0.89 MPa, while the embodiments of the present invention are all at 1.5 MPa. Based on the interfacial fracture mode, it can be seen that the water-retaining shotcrete used in the present invention utilizes the effect of water-retaining modifier and microsilica to improve the particle size distribution, which has good interfacial bond strength and impermeability. It can effectively improve the porosity defects on the surface of the mixed fiber shotcrete layer and improve the load transfer performance between the single-layer lining gradient concrete layers.

[0099] The results of the concrete drying shrinkage test show that the present invention uses low-shrinkage shotcrete as the third layer and utilizes high-performance expansion-compensating inorganic materials and organic shrinkage-reducing agent components to improve the tendency of shotcrete to dry shrinkage in the later stage. This results in a 28-day drying shrinkage rate of less than 200 microstrain for the single-layer lining gradient concrete material, which is a significant improvement over comparative examples 1 and 2, and further reduces the cracking risk of the single-layer lining structure during service.

[0100] Although the invention has been shown and described with reference to specific embodiments, those skilled in the art will understand that various changes in form and detail may be made herein without departing from the spirit and scope of the invention as defined by the claims and their equivalents.

Claims

1. A gradient shotcrete structure for single-layer tunnel lining, characterized in that: The shotcrete structure consists of three layers of shotcrete or mortar material in a gradient manner, with the tunnel surrounding rock surface as the base. The concrete structure includes, in sequence, a mixed steel fiber shotcrete layer, a water-retaining shotcrete mortar layer, and a low-shrinkage shotcrete layer. The concrete structure is prepared by a wet shotcrete process. The mixed steel fiber shotcrete layer serves as the first layer, directly contacting the surrounding rock of the tunnel. The thickness of the concrete layer ranges from 50 to 150 mm. The materials of the mixed steel fiber shotcrete layer include cementitious materials, mixed fibers, liquid early-strength agent, liquid alkali-free quick-setting agent, aggregates, and water. The aggregates include coarse aggregates and fine aggregates. The mass of each component per cubic meter of mixed steel fiber shotcrete layer material is as follows: cementitious materials 400-450 kg, mixed fibers 20 kg-45 kg, liquid early-strength agent 8 kg-15 kg, liquid alkali-free quick-setting agent 25 kg-35 kg, with a cement-aggregate ratio of 1:3.5 to 1:5.0, a sand ratio of 55-65%, and a water-cement ratio of 0.38-0.

45. The cementitious material used in the hybrid steel fiber shotcrete layer includes the following components by mass percentage: 70%-90% cement and 10-30% active early-strength powder; Both the coarse and fine aggregates are obtained by crushing, screening, and grinding the tunnel slag parent rock. The compressive strength of the tunnel slag parent rock used is greater than 80 MPa, the mica content is less than or equal to 0.2 wt%, and the alkali-silica reactivity is less than 0.1%. The mixed fiber is a mixture of steel fiber and non-steel fiber, wherein the volume ratio of steel fiber to non-steel fiber is 1:0.5~1; The liquid early strength agent has a solid content of 30-50%, and its solute components are a composition of early strength polycarboxylate superplasticizer, calcium lignosulfonate or calcium lignohumate, and inorganic calcium salt in a mass ratio of 5:(1-2):(3-5). The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and equivalent fluorine content are both less than 0.05 wt%. The water-retaining sprayed mortar layer serves as an intermediate layer, with a thickness of 20-50mm. The water-cement ratio in the water-retaining sprayed mortar layer is no greater than 0.4, and the raw materials, excluding water, are composed of the following parts by weight: 30-50 parts of ordinary Portland cement 10-20 parts of microsilica powder 60-80 parts of fine aggregate, 10-25 parts of heavy calcium carbonate powder Liquid water-reducing agent, 0.2-0.4 parts. 5-10 parts of water-retaining modifier 2-6 parts of liquid alkali-free quick-setting agent; The material of the water-retaining sprayed mortar layer has a binder-to-mortar ratio of 1:1.5 to 1:2.

5. The ordinary Portland cement used in the water-retaining sprayed mortar layer has a strength grade of 52.5 and a specific surface area of ​​350-400 m². 2 / kg, C3S content not less than 65wt%; The low-shrinkage shotcrete layer serves as the third layer and is also the surface layer of the single-layer lining structure, with a thickness of 100-150mm. The materials of the low-shrinkage shotcrete layer include cementitious materials, shrinkage-reducing agents, liquid water-reducing agents, liquid alkali-free quick-setting agents, and aggregates, including coarse and fine aggregates. The mass of each component per cubic meter of low-shrinkage shotcrete layer material is as follows: cementitious materials 450-550kg, shrinkage-reducing agents 5-10kg, liquid water-reducing agents 3-6kg, liquid alkali-free quick-setting agents 30-50kg, and the cement-aggregate ratio is 1:2.0~1:3.5, the sand ratio is 50-60%, and the water-cement ratio is not greater than 0.

4. The cementitious material used in the low-shrinkage shotcrete layer comprises the following components by mass percentage: 85%-95% cement and 5%-15% expansion compensation material.

2. The gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, In the mixed steel fiber shotcrete layer, the cement is selected from any one of ordinary Portland cement, slag Portland cement, and composite Portland cement, and the cement strength grade is 42.5 with a loss on ignition of less than 3%. The active early strength powder is a mixture of silica fume, metakaolin and granulated blast furnace slag powder, wherein the mass ratio of silica fume: metakaolin: granulated blast furnace slag powder is (0.5-1): 1: (0.5-1).

3. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, In the mixed steel fiber shotcrete layer, the maximum particle size of the coarse aggregate is no greater than 10 mm, the content of needle-like and flaky particles is less than 3 wt%, and the crushing index is less than 10%; the stone powder content of the fine aggregate is less than 5 wt%, and the loose bulk density is greater than 1600 kg / m³. 3 .

4. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, In the mixed steel fiber shotcrete layer, the steel fibers in the mixed fibers are coated and shaped steel fibers with a diameter of 0.4-0.6 mm, a length of 15-30 mm, and a tensile strength greater than 1200 MPa; the coating material used for the steel fibers is one of epoxy resin, tin plating, or copper plating. The non-steel fiber is selected from any one of imitation steel fiber, glass fiber, and polyoxymethylene monofilament fiber, with a diameter of 50-100μm, a length of 8-12mm, and a tensile strength greater than 800MPa.

5. A gradient shotcrete structure for single-layer tunnel lining according to claim 4, characterized in that, In the mixed steel fiber shotcrete layer, the steel fibers are either hook-shaped, bundled, or wavy.

6. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, In the mixed steel fiber shotcrete layer, the inorganic calcium salt in the liquid early strength agent is one of calcium nitrate, calcium formate, or calcium nitrite; The early-strength polycarboxylate superplasticizer has a 1-day compressive strength ratio greater than 180% and a 28-day compressive strength ratio greater than 150%.

7. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, The silica content in the microsilica powder is greater than 95 wt%, and the maximum particle size of the microsilica powder is not greater than 200 nm. The fine aggregate is selected from one or a mixture of two types of river sand and manufactured sand, with a maximum particle size of less than 1.6 mm; The calcium carbonate powder contains more than 98 wt% calcium carbonate and the maximum particle size is not higher than 60 μm. The liquid water-reducing agent is selected from any one of polycarboxylic acid, melamine, and naphthalene sulfonic acid water-reducing agents, and the water reduction rate is not less than 20%. The water-retaining modifier is a mixture of dispersible latex powder, cellulose ether, and polyacrylamide, wherein the mass ratio of dispersible latex powder: cellulose ether: polyacrylamide is 1:0.1~0.5:0.02~0.1; The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and the equivalent content of fluorine are both less than 0.05 wt%.

8. A gradient shotcrete structure for single-layer tunnel lining according to claim 7, characterized in that, In the water-retaining sprayed mortar layer, the cellulose ether in the water-retaining modifier is selected from any one of hydroxypropyl methylcellulose ether, hydroxyethyl cellulose, and hydroxyethyl methylcellulose ether, and the average relative molecular weight is not less than 200,000. The dispersible latex powder is a mixture of acrylic copolymer and polyvinyl alcohol, with a maximum particle size of no more than 50 μm.

9. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, Both the coarse and fine aggregates are obtained by crushing, screening, and grinding the tunnel slag parent rock. The compressive strength of the tunnel slag parent rock used is greater than 80 MPa, the mica content is less than or equal to 0.2 wt%, and the alkali-silica reactivity is less than 0.1%. The shrinkage reducing agent is a polyether-based shrinkage reducing agent with a relative molecular weight of no more than 500. The liquid water-reducing agent is a polycarboxylate high-performance water-reducing agent with a water reduction rate of not less than 28%. The liquid alkali-free quick-setting agent is fluorine-free, with a pH value greater than 3.0, and the alkali content and the equivalent content of fluorine are both less than 0.05 wt%.

10. A gradient shotcrete structure for single-layer tunnel lining according to claim 9, characterized in that, In the low-shrinkage shotcrete layer, the cement is selected from any one of ordinary Portland cement, pozzolanic Portland cement, and fly ash Portland cement, with a strength grade of not less than 42.5 and a C3A mineral content of not less than 3.5 wt%. The expansion compensation material is selected from any one of calcium-magnesium composite, calcium sulfoaluminate, calcium oxide, and calcium sulfoaluminate-calcium oxide expansion agents, with a specific surface area of ​​300-400 m². 2 / kg, with a 7-day limited swelling rate of 0.15-0.25% in water.

11. A gradient shotcrete structure for single-layer tunnel lining according to claim 10, characterized in that, In the low-shrinkage shotcrete layer, the cement is pozzolanic silicate cement; The expansion compensation material is calcium sulfoaluminate-calcium oxide, wherein the mass ratio of calcium sulfoaluminate is 15-25% and the mass ratio of calcium oxide is not less than 40%.

12. A gradient shotcrete structure for single-layer tunnel lining according to claim 9, characterized in that, In the low-shrinkage shotcrete layer, the maximum particle size of the coarse aggregate is no greater than 10 mm, the content of needle-like and flaky particles is less than 3 wt%, and the crushing index is less than 10%; the stone powder content of the fine aggregate is less than 5 wt%, and the loose bulk density is greater than 1600 kg / m³. 3 .

13. A gradient shotcrete structure for single-layer tunnel lining according to claim 1, characterized in that, The thickness of the mixed steel fiber shotcrete layer is selected as the first layer according to the surrounding rock grade. When the surrounding rock grade is II, the thickness is 50mm; when the surrounding rock grade is V or above, the thickness is 150mm.

14. A construction method for a gradient shotcrete structure for single-layer tunnel lining according to any one of claims 1 to 13, characterized in that, It includes the following steps: S1. Preparation of mixed steel fiber shotcrete layer: (1) In a twin-shaft forced mixer, coarse and fine aggregates and steel fibers are fully mixed and dispersed, and then active early strength powder, cement, water, non-steel fiber materials and liquid early strength agent are added in sequence, and the mixture is stirred to obtain a mixed fiber concrete mixture. (2) After the tunnel surrounding rock surface is alternately flushed with 0.4-0.8MPa air pressure and 0.2-0.4MPa high pressure water, steel arch frame, steel mesh or anchor rod are erected, and liquid alkali-free quick-setting agent is added to the quick-setting agent storage tank or pipeline of the wet spraying equipment. Then, the above-mentioned mixed fiber concrete mixture is loaded into the hopper of the wet spraying equipment, mixed with liquid alkali-free quick-setting agent at the nozzle and sprayed onto the tunnel surrounding rock surface. After the spraying thickness is reached, the first layer of mixed steel fiber sprayed concrete is obtained. Water curing is carried out for 24 hours with a water temperature not lower than 10℃. S2. Preparation of water-retaining sprayed mortar layer: (1) Add the fine aggregate, cement, water, liquid water-reducing agent, silica fume, water-retaining modifier and heavy calcium carbonate powder weighed according to the mass ratio to a twin-shaft forced mixer, and mix thoroughly to obtain water-retaining mortar; (2) Clean the surface of the first layer of mixed steel fiber shotcrete layer with 0.4-0.8MPa air pressure, add liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, and then load the above water-retaining mortar material and mix it with the liquid alkali-free quick-setting agent at the nozzle and spray it onto the surface of the first layer of mixed steel fiber shotcrete layer. After reaching the spraying thickness, the second layer of water-retaining shotcrete layer is obtained. Sprinkle water for 24 hours and the water temperature is not lower than 10℃. S3. Preparation of low-shrinkage shotcrete layer: (1) Add coarse and fine aggregates, cement, expansion compensation material, shrinkage reducer, liquid water reducer and water according to the mass ratio to a twin-shaft forced mixer, and mix thoroughly to obtain low-shrinkage shotcrete mixture; (2) After the water-retaining sprayed mortar layer is formed for 3 days, add the liquid alkali-free quick-setting agent to the quick-setting agent storage tank or pipeline of the wet spraying equipment, load the above-mentioned low-shrinkage sprayed concrete mixture and mix it with the liquid quick-setting agent at the nozzle and spray it onto the second layer of water-retaining sprayed mortar layer. After reaching the spraying thickness, the gradient sprayed concrete structure for the single-layer lining of the tunnel is obtained.

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