A support method for tunnels in karst caves

Abstract

The present invention relates to a method for supporting a tunnel in a karst cave, comprising: (1) backfilling the cave with slag from the bottom plate; (2) backfilling foam concrete to the design elevation of the tunnel top; (3) building reinforced concrete retaining walls at both ends above the foam concrete layer; (4) laying a layer of waste tires on the top of the foam concrete layer; (5) performing initial support and secondary lining, and reserving holes for anchor rods, anchor cables and grouting at the same time; (6) performing prestressed anchor rods to form a composite beam; (7) performing prestressed anchor cables to connect the composite beam with the deep stable rock mass of the karst cave; (8) injecting rock foam into the cavity of the karst cave; (9) performing expansion anchor rods and closing the holes for grouting to form a supporting overall structure. The present invention utilizes a prestressed anchoring structure and combines composite lining with multiple materials for filling to support the tunnel, which can reduce the volume of concrete in the tunnel support construction, making the construction process more green and environmentally friendly, and has a high degree of mechanization, convenient operation and wide applicability.

Description

Technical Field

[0001] The present invention belongs to the field of tunnel construction engineering, and particularly relates to a support method for a tunnel in a karst cave. Background Art

[0002] In tunnel construction in karst areas, the cavities behind the lining will not only affect the stress of the lining, causing local concentrated forces, but also directly expose the lining. The falling of deteriorated surrounding rock may directly impact the lining, thereby affecting the service life of the entire tunnel operation period. Therefore, in addition to using the lining as the main structure for tunnel support, corresponding repairs and rectifications should also be carried out for the connection between the double linings and the cavities behind the lining.

[0003] Composite lining refers to the tunnel lining constructed in two layers, the inner and outer layers, successively. After the excavation of the tunnel, the outer flexible support (generally shotcrete support) that is closely attached to the surrounding rock is constructed in a timely manner first, also known as the primary support, allowing the surrounding rock to undergo a certain amount of deformation without causing excessive deformation of the loosening pressure. After the deformation of the surrounding rock is basically stable, the inner lining (generally cast-in-place) is constructed, also known as the secondary support. Between the two linings, a waterproof layer is set as needed, or a waterproof concrete inner lining can be poured without setting a waterproof layer. The composite lining has the advantages of timely closure, reasonable structure, being beneficial to the stress of the lining, and being applicable to most surrounding rock conditions, and is a support technology widely used in mountain tunnels at present.

[0004] However, when a single composite lining passes through a karst cave, it does not form a combined support structure that is closely connected to the surrounding rock, and there is no buffer when encountering falling rocks, so the lining is easily damaged and the self-bearing capacity of the surrounding rock cannot be fully exerted. The cavity of the karst cave is too large, and it is difficult to directly construct a prestressed anchoring structure at positions such as the roof. If the prestressed anchoring structure is directly used to support the roof of the karst cave, the rock mass in the karst cave is relatively broken and difficult to form a whole, and it is difficult to obtain a satisfactory reinforcement effect. Summary of the Invention

[0005] The purpose of the present invention is to provide a support method for a tunnel in a karst cave, which uses a variable stiffness combined structure to support the tunnel in a super-large karst cave, further optimizes the support technology for the tunnel in a super-large karst cave, organically combines the traditional composite lining with the prestressed anchoring structure and the Rocchio foam filling technology, and uses a new variable stiffness combined structure to combine the three reinforcement means to ensure the safety of the tunnel, so as to solve the problems existing in the prior art.

[0006] A support method for a tunnel in a karst cave includes the following steps:

[0007] (1) The cavity of the karst cave is first backfilled with tunnel slag starting from the bottom plate until it reaches the bottom of the tunnel;

[0008] (2) Backfill foamed concrete to the designed elevation of the tunnel roof;

[0009] (3) Construct reinforced concrete retaining walls at both axial ends of the tunnel in the foamed concrete layer to prevent leakage of the later-filled Roxul foam and to prevent the broken rock mass from directly threatening the safety of the open cut section during later construction.

[0010] (4) Lay a layer of waste tires on the top of the backfill layer (i.e., the foamed concrete layer);

[0011] (5) Excavate and construct the primary support and secondary lining that are closely attached to the surrounding rock, and at the same time reserve the hole positions for prestressed anchor bolts, prestressed anchor cables and grouting;

[0012] (6) Install prestressed anchor bolts to tightly connect the primary support and the secondary lining to form a composite beam;

[0013] (7) Install prestressed anchor cables to connect the composite beam to the stable rock mass deep in the karst cave, and at the same time connect and reinforce the individual unstable rocks to the stable rock mass;

[0014] (8) Inject Roxul foam into the cavity of the karst cave through the reserved grouting hole positions, and wait for the foam to expand and harden to form a Roxul foam layer;

[0015] (9) Install expansion anchor bolts at the grouting hole positions and seal the grouting hole positions. While connecting the composite beam and the Roxul foam, it produces a squeezing effect on the Roxul foam to form an integral support structure.

[0016] In the present invention, the tunnel lining is constructed after the completion of the backfill of the tunnel slag. The tunnel lining adopts a composite lining. The primary support adopts shotcrete and steel arch frames, and the secondary lining adopts shotcrete. After the lining is closed, FC0.5 foamed concrete is filled into the cavity. The prestressed anchor bolts apply a pre-tightening force to the two layers of lining, so that they are tightly connected, and the multiple layers of lining are tightly connected to form a "composite beam"; it is beneficial for the lining structure to jointly bear the load and automatically distribute the load ratio. The prestressed anchor cables are used to connect the whole lining to the stable rock mass deep in the roof to control the settlement of the tunnel. The cavity is filled with Roxul foam mixed with waste tires, which not only reinforces the cavity, but also can absorb the energy of the falling rocks and reduce the impact of the falling rocks on the tunnel structure. The expansion anchor bolts are installed to reinforce the lining again, and at the same time produce a squeezing effect on the Roxul foam to increase its strength. Compared with the traditional composite lining, the present invention adds a prestressed anchoring structure and a polymer foaming resin.

[0017] The prestressed anchor bolts and prestressed anchor cables of the present invention constitute an active surrounding rock support scheme, which has advantages such as stronger ability to restrain the deformation of the rock mass compared with ordinary support structures, having an initial anchoring force to provide resistance in time, and being able to connect the support structure to the deep stable surrounding rock, etc.

[0018] Furthermore, in step (4), the waste tires are arranged in a staggered pattern, with the upper and lower layers, and they must be intact tires. Using waste tires for the support of tunnels in karst caves is also more environmentally friendly and cost-saving.

[0019] Furthermore, the Rocxiu foam layer is located above the waste tire layer, and the thickness ratio of the Rocxiu foam layer to the waste tire layer is 1.5 - 5:1. Waste tires have a greater self-weight compared to Rocxiu foam. If the waste tire layer is placed above the Rocxiu foam layer, it may compress the thickness of the Rocxiu foam layer.

[0020] Furthermore, the injection method of Rocxiu foam in step (8) is as follows: insert an injection iron pipe through the grouting hole position, the injection iron pipe passes through the waste tire layer, and then mix resin and catalyst in a ratio of 4:1 and inject them into the karst cave cavity to form Rocxiu foam; Rocxiu foam has high expansibility, and its volume after expansion is 25 - 30 times the original volume. The foam reacts rapidly, and the reaction can be completed within 20 - 30 seconds at room temperature. When the temperature is 10 - 15 degrees, the expansion ends within 3 - 5 minutes and hardens within 20 minutes. It does not require a leak-proof support, has good compressive strength, can withstand the movement of the rock formation, and does not spread flames.

[0021] Furthermore, the initial support uses C20 shotcrete, and the secondary lining uses C30 cast-in-place concrete. In the present invention, C20 shotcrete and C30 cast-in-place concrete can provide relatively high compressive strength and withstand the impact of large falling rocks without damage through their own rigidity. C20 shotcrete and C30 cast-in-place concrete are the results of joint selection based on strength and cost through multiple experiments in the present invention, and also to ensure the increase in stiffness of the entire support structure from the outside to the inside, mainly from the perspective of ensuring the safety of the lining.

[0022] Furthermore, the foamed concrete in step (2) is FC0.5 foamed concrete. FC0.5 foamed concrete has good fluidity, does not require large-scale equipment for backfilling, the number of construction workers under the dome is small, and the risk is easy to control. Moreover, FC0.5 foamed concrete has a low cost.

[0023] After the expansion anchor is installed, there will be a relatively large radial finite deformation as a whole, and it exerts an extrusion effect on the surrounding stable body to obtain the anchoring force.

[0024] Furthermore, when backfilling the tunnel muck in step (1), the accumulations at the bottom of the tunnel are not processed. Use the backfilled tunnel muck for surcharge preloading while monitoring the settlement; level the top of the backfilled tunnel muck.

[0025] Furthermore, the structures of the prestressed anchor rods, prestressed anchor cables and the length of the expansion anchor rods can be adjusted according to the actual situation, and the number and positions of the reserved hole positions can also be adjusted according to actual needs.

[0026] Furthermore, the prestressed anchor cables should penetrate through the composite lining, Roxtec foam, and waste tire layer and other composite support layers to reach the original top position of the karst cave. The number of holes should be arranged at 1 - 3 locations per cross-section according to the actual situation, and arranged in a plum blossom pattern. This can play a role in stabilizing the overall structure. The length of the expansion bolts should penetrate through the foamed concrete layer to reach the Roxtec foam layer. The number of holes should be arranged at 5 - 7 locations per cross-section according to the actual situation, and arranged in a plum blossom pattern.

[0027] The support structure formed by the support method for the tunnel in the karst cave described above.

[0028] Compared with the prior art, the present invention uses a prestressed anchoring structure and combines a composite lining with a variety of materials for filling to support the tunnel, which can effectively reduce the volume of concrete in tunnel support construction, make the construction process more environmentally friendly, and has a high degree of construction mechanization, which can effectively shorten the construction period, overcoming the defect that it is difficult to ensure the long-term operation safety of traditional tunnel linings when passing through super-large karst caves. Moreover, the construction technology of the present invention is easy to operate, easy to manufacture, and has a wide range of applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is the construction flow chart of the present invention;

[0030] Figure 2 is the schematic diagram of the overall tunnel lining of the present invention;

[0031] Figure 3 is the schematic diagram of the expansion bolt of the present invention;

[0032] Figure 4 is the schematic diagram of the anchoring force of the expansion bolt of the present invention;

[0033] Figure 5 is the numerical simulation cloud diagram of the displacement generated by the support structure of Example 1 of the present invention and a single tunnel lining under the impact of falling rocks;

[0034] Figure 6 is the numerical simulation cloud diagram of the displacement generated by Example 1 of the present invention and Comparative Example 1 under the impact of falling rocks;

[0035] Figure 7 is the numerical simulation cloud diagram of the displacement generated by Example 1 of the present invention and Comparative Example 2 under the impact of falling rocks;

[0036] Figure 8 is the numerical simulation cloud diagram of the displacement generated by Example 1 of the present invention and Comparative Example 3 under the impact of falling rocks;

[0037] Figure 9 is the numerical simulation cloud diagram of the displacement generated by Example 1 of the present invention and Comparative Example 4 under the impact of falling rocks;

[0038] Figure 10Numerical simulation nephogram of the displacement generated by the impact of falling rocks in Embodiment 1 of the present invention and Comparative Example 5;

[0039] Wherein: 1 is a prestressed anchor cable, 2 is Roxul foam, 3 is a layer of waste tires, 4 is an expanding bolt, 5 is C20 concrete, 6 is C30 concrete, 7 is FC0.5 foam concrete, 8 is a prestressed bolt, 9 is backfill with tunnel slag, 10 is the plug of the expanding bolt, 11 is the blind plug of the expanding bolt, 12 is the sealing layer of the expanding bolt, 13 is the protective layer of the expanding bolt, 14 is the anchoring layer of the expanding bolt, 15 is the sealing of the expanding bolt, 16 is the anchor head of the expanding bolt, 17 is the tray of the expanding bolt, and 18 is the nut of the expanding bolt. Detailed implementation manners

[0040] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings.

[0041] Embodiment 1

[0042] A support method for a tunnel in a karst cave, as Figure 1 shown, mainly includes the following steps:

[0043] (1) Starting from the bottom plate of the karst cave cavity, first carry out backfill with tunnel slag until the bottom of the tunnel; do not treat the accumulations at the bottom of the cave, use the backfilled tunnel slag for surcharge preloading, and at the same time monitor the settlement; level the top of the tunnel slag backfill.

[0044] (2) Backfill FC0.5 foam concrete to the designed elevation of the tunnel top.

[0045] (3) Build reinforced concrete retaining walls at both axial ends of the tunnel in the foam concrete layer to prevent leakage of the later filled Roxul foam and to prevent the broken rock mass during later construction from directly threatening the safety of the open cut tunnel section.

[0046] (4) Lay a layer of waste tires on the top of the backfill layer, with a thickness of 3 m, arranged in a plum blossom shape on the tire plane, and the tires are tightly connected horizontally and vertically with steel wires of Φ14. Tires of the same model are used in the same layer, and the upper and lower layers are arranged staggeredly.

[0047] (5) Excavate and construct the initial support (with a thickness of 28 cm) and the secondary lining (with a thickness of 80 cm) that are closely attached to the surrounding rock, and at the same time reserve the hole positions for bolts, anchor cables, and grouting.

[0048] (6) Install prestressed bolts to tightly connect the linings to form a "composite beam".

[0049] (7) Install prestressed anchor cables to connect the composite beam to the stable rock mass deep in the karst cave, and at the same time connect and reinforce individual dangerous rocks to the stable rock mass.

[0050] (8) Through the grouting holes reserved on the "composite beam", when injecting, use a special pneumatic pump to send resin and catalyst into the injection gun at a ratio of 4:1 to form Rockstop foam with a thickness of 6 m, and wait for the foam to expand and harden completely.

[0051] (9) Finally, install expansion bolts and seal the holes. While connecting the composite beam and the Rockstop foam, it produces a squeezing effect on the Rockstop foam to form an integral support structure, as Figure 2 shown.

[0052] After the integral support structure is constructed, there is still a tunnel cavity above, and the distance from the support structure is h (generally speaking, the smaller h is, the better. When h = 0, the passive support can be changed to active support, which is safer. However, the actual reserved space h still needs to be determined according to various factors such as the specific size of the karst cave, the evaluation result of the stability of the karst cave roof, the highway grade, the elevation of the road surface, and the cost budget). The mass of the falling rock at the top is m, the contact area with the support structure is a, the gravitational acceleration is g, and the displacement of the falling rock after hitting the energy-absorbing layer is z max ; The elastic moduli of Rockstop foam, tires, and foamed concrete are E1, E2, and E3 respectively, the thicknesses above the lining are s1, s2, and s3 respectively, and the friction factors with the falling rock are μ1, μ2, and μ3. Then the total impact energy released by the falling rock is E i = m·g·(h + z max ), and the maximum energy that the energy-absorbing layer can absorb is E a = a·s1·E1 + a·s2·E2 + a·s3·E3 + μ1·m·g·s1 + μ2·m·g·s2 + μ2·m·g·s2. When E i < E a , the falling rock will not contact the inner lining, and the impact is borne by the energy-absorbing layer; when E i > E a , the falling rock will penetrate the energy absorption, and the remaining impact energy is borne by the rigid lining. The ABAQUS software is used to perform numerical simulation on the integral support structure described in the present invention. The mass of the falling rock is 1 t and the height is 30 m. The displacements generated by the integral support structure of this embodiment and the single composite lining are as Figure 5 shown. After being impacted, the maximum displacement of the support structure of this embodiment is 196.2 mm, which is located at the impact point of the falling rock at the top of the support structure, and the displacement at the inner lining of the tunnel is 59.88 mm; while the traditional composite lining, after being impacted, generates the maximum displacement in the middle of the tunnel, reaching 445.9 mm, and actually has cracked.

[0053] Embodiment 2

[0054] In this embodiment, a series of exploratory experiments were conducted on the selection of shotcrete for primary support and cast-in-place concrete for secondary lining, as shown in Table 1. It can be seen from Table 1 that considering factors such as comprehensive strength and cost, the best choice is to use C20 shotcrete for primary support and C30 cast-in-place concrete for secondary lining.

[0055] Table 1 Comparison of Concrete Strength and Cost

[0056]

[0057] Comparative Example 1

[0058] Inject a layer of Roxtec foam on the top of the backfill layer, and then lay a layer of waste tires, that is, the Roxtec foam layer is located below the waste tire layer. Others are similar to Example 1.

[0059] Comparative Example 2

[0060] Do not include the waste tire layer, and others are similar to Example 1.

[0061] Comparative Example 3

[0062] Do not include the Roxtec foam layer, and others are similar to Example 1.

[0063] Comparative Example 4

[0064] Do not include the expansion anchor bolts, and others are similar to Example 1.

[0065] Comparative Example 5

[0066] Do not include the prestressed anchor bolts and prestressed anchor cables, and others are similar to Example 1.

[0067] For the five comparative examples set according to the present invention, numerical simulations were respectively carried out. Figure 6 For the comparison of the displacement results of the numerical simulation between Example 1 and Comparative Example 1, the maximum displacement of the overall structure in Example 1 is 196.2 mm, and the displacement at the lining is 59.88 mm, both of which are smaller than 213.6 mm of the overall structure and 63.23 mm at the lining in Comparative Example 1.

[0068] Figure 7 For the comparison of the displacement results of the numerical simulation between Example 1 and Comparative Example 2, the maximum displacement of the overall structure in Example 1 is 196.2 mm, and the displacement at the lining is 59.88 mm, while the overall structure in Comparative Example 2 is 184.6 mm, and the displacement at the lining is 70.06 mm. Example 1 has a smaller displacement at the lining, and the tunnel operation is safer.

[0069] Figure 8For the comparison of the displacement results of the numerical simulation between Example 1 and Comparative Example 3, the maximum displacement of the overall structure in Example 1 is 196.2 mm, and at the lining it is 59.88 mm, while for the overall structure in Comparative Example 3 it is 117.3 mm, and at the lining it is 72.17 mm. Example 1 has a smaller displacement at the lining, and the tunnel operation is safer.

[0070] Figure 9 For the comparison of the displacement results of the numerical simulation between Example 1 and Comparative Example 4, the maximum displacement of the overall structure in Example 1 is 196.2 mm, and at the lining it is 59.88 mm, both of which are less than 251.2 mm of the overall structure and 70.55 mm at the lining in Comparative Example 4.

[0071] Figure 10 For the comparison of the displacement results of the numerical simulation between Example 1 and Comparative Example 5, the maximum displacement of the overall structure in Example 1 is 196.2 mm, and at the lining it is 59.88 mm, both of which are less than 201.8 mm of the overall structure and 65.98 mm at the lining in Comparative Example 5.

[0072] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application. The present invention is equally applicable to the support of mine roadways, adits and other underground projects. When encountering extremely broken and soft surrounding rocks or karst caves, prestressed bolts and cables can be replaced with grouting bolts and cables according to the on-site situation to obtain better support effects.

[0073] For the convenience of description, if the words "upper", "lower", "left", and "right" appear in the present invention, they only indicate the same directions as the upper, lower, left, and right of the drawing itself, and do not limit the structure. They are only for facilitating the description of the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus cannot be construed as a limitation of the present invention.

Claims

1. A support method for a tunnel in a karst cave, characterized in that, It includes the following steps: (1) Backfill the cavity of the karst cave with muck starting from the bottom plate until it reaches the bottom of the tunnel; (2) Backfill foamed concrete to the designed elevation of the tunnel roof; (3) Construct reinforced concrete retaining walls at both axial ends of the tunnel in the foamed concrete layer; (4) Lay a layer of waste tires on the top of the foamed concrete layer; (5) Excavate and construct the primary support and secondary lining in close contact with the surrounding rock, and at the same time reserve the hole positions for prestressed anchor bolts, prestressed anchor cables and grouting; (6) Install prestressed anchor bolts to tightly connect the primary support and the secondary lining to form a composite beam; (7) Install prestressed anchor cables to connect the composite beam to the stable rock mass deep in the karst cave, and at the same time connect and reinforce individual unstable rocks to the stable rock mass; (8) Inject Rochew foam into the cavity of the karst cave through the reserved grouting hole positions, and wait for the foam to expand and harden to form a Rochew foam layer; (9) Install expansion anchor bolts at the grouting hole positions and seal the grouting hole positions. While connecting the composite beam and the Rochew foam, an extrusion effect is generated on the Rochew foam to form an integral support structure; In step (4), the waste tires are arranged in a staggered manner in the upper and lower layers; in step (8), the Rochew foam layer is located above the waste tire layer, and the thickness ratio of the Rochew foam layer to the waste tire layer is 1.5 - 5:1; The primary support uses C20 shotcrete, and the secondary lining uses C30 cast-in-place concrete.

2. The support method for the tunnel in the karst cave according to claim 1, wherein, The injection method of Rochew foam in step (8) is: put an injection iron pipe through the grouting hole position, the injection iron pipe penetrates through the waste tire layer, and then mix resin and catalyst in a ratio of 4:1 and inject them into the cavity of the karst cave to form Rochew foam; the reaction is completed in 20 - 30 seconds at normal temperature.

3. The support method for the tunnel in the karst cave according to claim 2, wherein, The foamed concrete in step (2) is FC0.5 foamed concrete.

4. The support method for the tunnel in the karst cave according to claim 1, characterized in that, When backfilling the muck in step (1), the accumulations at the bottom of the cave are not treated, and the backfilled muck is used for surcharge preloading while monitoring the settlement; the top of the muck backfill is leveled.

5. The support method for the tunnel in the karst cave according to claim 1, characterized in that, The structures of the prestressed anchor bolts, prestressed anchor cables and the length of the expansion anchor bolts are adjusted according to the actual situation, and the quantity and positions of the reserved hole positions are adjusted according to actual needs.

6. The support method for the tunnel in the karst cave according to claim 1, characterized in that, The prestressed anchor cables penetrate through the primary support, secondary lining, Rochew foam layer and waste tire layer to reach the original top position of the karst cave, and 1 - 3 hole positions are arranged in each section; the expansion anchor bolts penetrate through the foamed concrete layer to reach the Rochew foam layer, and 5 - 7 hole positions are arranged in each section.

7. The support structure formed by the support method for the tunnel in the karst cave according to any one of claims 1 - 6.

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