A design and construction method for strengthening the tunnel stratum in the karst underground river area with sand layers that can be lost

Abstract

A design and construction method for reinforcing the stratum of a tunnel in a karst underground river area with a sand layer that can be lost, the steps are as follows: one or more rings of large-pipe shed grouting reinforcement are applied at the large and small mileage ends of the underground river section, and the application range is the tunnel arch and side walls. The tunnel base of the entire underground river section is reinforced with rotary grouting bite piles; multiple rings of curtain grouting are applied in the middle section of the underground river, and a grouting wall and a grouting rock plate are respectively arranged in front and behind the middle section, and multiple circles of grouting holes are arranged on the grouting wall, and the diffusion radius of each hole is about 1.5m; the present invention can avoid water gushing accidents in the tunnel section of the karst cave underground river area during the construction process, improve the water pressure resistance of the tunnel structure, improve the bearing capacity of the foundation, and ensure construction safety and safety during operation.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel engineering, and in particular to a design and construction method for reinforcing a tunnel stratum in a karst underground river area with a sand layer that can be lost. Background Art

[0002] The terrain conditions in southwest my country are difficult, and karst phenomena are widely distributed. The size and shape of karst in different regions are different. With the development of the western region, the problem of tunnels passing through karst strata is also urgently needed. Especially for tunnels that pass through underground water systems and underground rivers with well-developed karst caves, the karst problem during construction and operation is more prominent. When tunnels pass through karst caves filled with sand layers that can be lost, water and mud outbursts and landslides may occur. In addition, the karst fillings around the tunnel will be lost under the action of underground water, changing the stress field around the tunnel during the construction period, resulting in the possibility of collapse of the karst cavity, which has a great impact on the stress of the tunnel lining structure.

[0003] The existing construction technologies for karst water-rich tunnels include grouting reinforcement, changing the excavation method, etc.; base reinforcement includes concrete replacement, pile foundation support beams, jet grouting piles, micropiles, CFG piles, sleeve valve pipe foundation reinforcement, etc. However, due to the development of underground water systems and underground rivers in some karst tunnels, the karst cave fillings around the tunnels are easy to lose. During the flood season, there is still a risk of underground river water scouring the karst stratum sand filling and losing it, and the tunnel body is subjected to high water pressure. Summary of the invention

[0004] In view of the problems existing in the prior art, the purpose of the present invention is to propose a design and construction method for strengthening the stratum of tunnels in karst underground river areas with loseable sand layers, aiming to avoid water inrush accidents in tunnels in karst underground river areas during construction, improve the water pressure resistance of tunnel structures, improve the bearing capacity of foundations, and ensure construction safety and safety during operation.

[0005] The object of the present invention is achieved by: a design and construction method for reinforcing the stratum of a tunnel in a karst underground river area with a sand layer that can be lost, comprising the following steps:

[0006] ① From the starting mileage and the ending mileage of the dark river section, one or more rings of large pipe sheds are grouting reinforced to the large mileage end and the small mileage end. The specific scope of the large pipe shed is the arch and the side wall. A steel cage is added inside the pipe shed. A small guide tube with a large external angle is added between the large pipe sheds in the sand layer section for grouting reinforcement;

[0007] ② The tunnel base treatment from the starting mileage to the ending mileage of the dark river section is reinforced with jet grouting interlocking piles. The above-mentioned jet grouting interlocking piles are composed of several evenly distributed units. Each unit has 7 jet grouting piles, 6 of which are arranged at the 6 corner points of a regular hexagon, and one is arranged at the center of the regular hexagon. The interlocking surface of the jet grouting pile at the center is the same as that of the other 6 jet grouting piles, and the above-mentioned interlocking surface is also the same as the interlocking surface between the other 6 jet grouting piles. The bottom of the pile is embedded in the bedrock for not less than 0.5m. If the tunnel bottom filling is less than 1.5m, concrete replacement is adopted. The construction sequence of the jet grouting piles is: 1) site leveling; 2) water quality review; 3) process test piles; 4) clearing ground and air obstacles; 5) measurement and layout; 6) jet grouting 7) Quality inspection of pile construction; automatic slurry metering device should be configured in the construction of jet grouting piles; the holes of jet grouting piles are generally drilled by vibration drilling rigs. When encountering relatively hard strata, geological drilling rigs should be used for drilling; the distance between the jet hole and the high-pressure grouting pump should not be greater than 50m, and the deviation between the pile position and the designed position should not be greater than 50mm; the jet grouting in the hole should be lifted evenly from bottom to top, and the lap length of the jet pipe lifted in sections should not be less than 100mm. For parts that need to expand the reinforcement range or increase the strength locally, re-spraying measures should be adopted; the jet pipe should be pulled out quickly after the high-pressure jet grouting is completed, and if necessary, measures such as grouting backfilling or secondary grouting should be taken at the original hole position to prevent the slurry from solidifying and shrinking and affecting the pile top elevation;

[0008] ③ Multi-ring curtain grouting is carried out in the middle section of the tunnel excavation to the underground river. The length of each cycle of grouting is 15-30m. The grouting stop rock plate with a thickness of 4-5m is determined and retained according to the actual construction environment. A grouting stop wall should be set up for grouting. The thickness of the grouting stop wall should not be less than 2m. The orifice pipe of the grouting borehole is pre-buried in the grouting stop wall. The drilling and grouting construction can only be started after the concrete strength of the grouting stop wall reaches 75% of the design strength. The diffusion radius of a single hole is about 1.5m, and the spacing between the bottoms of each ring hole is not more than 2.5m, so that the single-hole grouting diffusion circles are effectively overlapped to form a curtain to effectively reinforce the rock mass, that is, the curtain grouting consolidation circle. The grouting reinforcement range is 4-6m outside the lining excavation contour line. After the grouting is completed, it should be inspected Check the grouting effect. After inspection, the tunnel can be further excavated only after the grouting effect meets the design requirements. To ensure the stability of the stop wall, a ring of grouting steel flower pipes is set in front of the stop wall for grouting reinforcement. The grouting drilling adopts a circular hole arrangement. The number of holes and the number of holes are determined according to the actual situation. The grouting sequence is first the outer circle and then the inner circle, first the near and then the far. The same circle of holes is constructed at intervals, and the upper and then the lower in the construction of the same circle of holes. The outer circle drilling adopts forward grouting, and the inner circle drilling adopts backward grouting. The grouting material is mainly cement slurry, and the water-cement ratio is 0.8:1~1:1. During construction, the grouting section, grouting method, grouting material, etc. should be appropriately adjusted according to the results of advanced prediction and forecasting.

[0009] ④ After the last round of curtain grouting is completed, a large pipe shed (10) is constructed for grouting reinforcement near the terminal section of the underground river area.

[0010] The outer contour of the grouting wall is embedded in the tunnel excavation contour line by 50 cm; the grouting holes are evenly distributed on the grouting wall according to 6 concentric circles, and the spacing between two adjacent concentric circles is equal, and the grouting holes on the same circumference are evenly distributed.

[0011] The quality inspection method of the jet grouting pile is as follows:

[0012] ① After the jet grouting pile is constructed, the grouting effect should be inspected. The number of quality inspection points should not be less than 2% of the number of construction holes. The content should include the integrity, uniformity, strength of the pile body, bearing capacity of the foundation, etc.;

[0013] ② Within 7 days after the pile is constructed, the uniformity of the pile body shall be checked by low strain, and the number of piles to be checked shall be 2% of the total number of piles, and no less than 3;

[0014] ③ Within 7 days after pile construction, shallow excavation of the pile head (depth exceeding 0.5m below the grouting surface) is used to visually inspect the uniformity of mixing and measure the diameter of the pile. The number of piles to be inspected is 2‰ of the total number of piles, and no less than 3 piles;

[0015] ④ 28 days after the pile is built, a double-tube single-action sampler should be used to vertically drill and core at 1 / 4 of the pile diameter and within the length of the pile to observe the integrity and uniformity of the pile body. Take no less than 3 specimens at different depths for unconfined compressive strength test. The number of specimens to be tested is 2‰ of the total number of piles, and no less than 3 piles;

[0016] ⑤ The bearing capacity test of the jet grouting piles should be carried out 28 days after the piles are completed. Single pile or composite foundation load test should be adopted. The number of piles to be tested should be 2‰ of the total number of piles and no less than 3.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] 1) The design and construction method for reinforcing the lost sand layer in a karst underground river area of ​​the present invention is to fully mix the slurry and the lost sand layer at the bottom of the tunnel through the rotary grouting bite piles, so that the foundation is reinforced and the bearing capacity is improved, and the loss of the sand layer is avoided;

[0019] 2) The design and construction method for reinforcing the tunnel stratum in the karst underground river area with sand layers that can be lost described in the present invention strengthens the rock strata around the tunnel through measures such as pipe-roof grouting, curtain grouting, and rotary grouting interlocking piles, blocks groundwater, and effectively improves the anti-seepage and water pressure resistance of the tunnel structure, and can avoid the occurrence of water gushing, water inrush, slurry overflow, and mud bubbling;

[0020] 3) The design and construction method for reinforcing the sand layer in a tunnel in a karst underground river area described in the present invention reinforces the sand layer that was originally lost through curtain grouting, improves cohesion, and enhances self-stabilizing ability, which can effectively stabilize the working surface and facilitate construction and excavation. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 To the grouting wall layout drawing. Figure 2 Design drawing of curtain grouting longitudinal section

[0022] Figure 3 Schematic diagram of grouting end hole

[0023] Figure 4 Plane diagram of rotary grouting interlocking pile

[0024] Figure 5 Schematic diagram of the cross section of the rotary grouting interlocking pile

[0025] Figure 6 Schematic diagram of the longitudinal section of the tunnel (including the underground river area)

[0026] Explanation of main reference numerals: 1- tunnel excavation outline, 2- stop wall, 3- grouting hole, 4- steel flower pipe, 5- stop rock plate, 6- curtain grouting consolidation ring, 7- rotary grouting bite pile, 8- karst underground river section, 9- sand layer, 10- large pipe shed DETAILED DESCRIPTION

[0027] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0028] See also Figure 1 , the arch and side walls inside the tunnel excavation outline have secondary lining (including initial support, waterproof board, reserved deformation, lining backfill of the same grade concrete, and secondary lining of 50cm, C35 reinforced concrete). The present invention provides a design and construction method for strengthening the stratum of a tunnel in a karst underground river area with sand layers that can be lost, including a large pipe shed 10 applied on both sides of the tunnel crossing the underground river range, wherein the large pipe shed 10 is set at the starting section of the underground river range 8 and at the end section near the underground river 8;

[0029] The present invention provides a design and construction method for reinforcing the stratum of a tunnel in a karst underground river area that can lose sand layers, including multi-ring curtain grouting applied to the middle section of an underground river 8, wherein the curtain grouting is provided with a grouting stop wall 2 with a thickness of not less than 2m and a grouting stop rock plate 5 with a thickness of 4 to 5m, and multi-ring grouting holes 3 are provided, the outer contour of the grouting stop wall 2 is embedded in the main tunnel excavation line 1 by not less than 0.5m, and a grouting steel flower pipe 4 is provided on the grouting stop wall 2;

[0030] The present invention provides a design and construction method for reinforcing the stratum of a tunnel in a karst underground river area that can lose sand layers, including base jet grouting bite pile reinforcement applied to the entire range of an underground river section 8. The jet grouting bite piles 7 are arranged in a plum blossom shape, with the pile bottom embedded in the bedrock by no less than 0.5m. The specific parameters of the jet grouting bite piles 7 are selected according to the site environment and construction technical requirements.

[0031] Furthermore, the construction sequence of jet grouting piles is: 1) site leveling; 2) water quality review; 3) process test piles; 4) clearing ground and air obstacles; 5) measurement and layout; 6) jet grouting pile construction 7) quality inspection.

[0032] Furthermore, an automatic slurry metering device should be installed during the construction of rotary jet piles.

[0033] Furthermore, jet grouting piles are usually drilled using a vibration drill. When encountering relatively hard strata, a geological drill should be used.

[0034] Furthermore, the distance between the injection hole and the high-pressure grouting pump should not be greater than 50m, and the deviation between the pile position and the designed position should not be greater than 50mm.

[0035] Furthermore, the jet grouting in the hole should be lifted evenly from bottom to top, and the overlap length of the segmented lifting of the jet pipe should not be less than 100mm. For areas that need to be partially expanded in reinforcement range or increased in strength, repeated spraying measures can be adopted.

[0036] Furthermore, the jetting pipe should be pulled out quickly after high-pressure jet grouting is completed. If necessary, measures such as grouting backfilling or secondary grouting should be taken at the original hole position to prevent the grout from solidifying and shrinking and affecting the pile top elevation.

[0037] Multi-ring curtain grouting is carried out in the middle section of the tunnel excavation to the underground river. The length of each grouting cycle is 15 to 30 meters, and the 4-5m thick grouting rock plate is determined according to the actual construction environment and retained. Grouting should be provided with a grouting wall, the thickness of which should not be less than 2 meters. The orifice pipe of the grouting drilling hole should be pre-buried in the grouting wall. The drilling and grouting construction can only be started after the concrete strength reaches 75% of the design strength. The single hole diffusion radius (referring to the radius of the slurry diffusion range) is about 1.5 meters, and the spacing between the bottoms of each ring hole is not more than 2.5 meters, so that the single hole grouting diffusion circle overlaps effectively to form a curtain to effectively reinforce the rock mass. The grouting reinforcement range is 4 to 6 meters outside the lining excavation contour line. The grouting effect should be checked after the grouting is completed. After inspection, the tunnel can be continued to be excavated only after the grouting effect meets the design requirements.

[0038] Furthermore, in order to ensure the stability of the slurry stop wall, a ring of grouting steel flower pipes is set in front of the slurry stop wall for grouting reinforcement.

[0039] Furthermore, the holes are arranged in a circular pattern, and the number of holes and the number of circles are determined according to the actual situation. The grouting sequence is outer circle first, inner circle second, near first, far later, the same circle of holes are constructed at intervals, and the upper part is constructed first, the lower part is constructed later. Forward grouting is used for the outer circle drilling, and backward grouting is used for the inner circle drilling.

[0040] Furthermore, the grouting material mainly uses cement slurry, and the water-cement ratio is 0.8:1 to 1:1. During construction, the grouting section, grouting method, grouting material, etc. should be appropriately adjusted according to the results of advanced prediction and forecasting.

[0041] The last round of curtain grouting has been completed, and a large pipe shed is used for grouting reinforcement near the terminal section of the underground river area. The construction scope covers the arch and side walls, and steel cages are added inside the pipe shed. Small guide tubes with large external angles are added between the large pipe sheds in the sand layer section for grouting reinforcement.

[0042] See also Figure 1 Figure 2 The inclination angles of all grouting holes on the same circle on the grouting wall (referring to the angle between the grouting hole and the axis centerline, the same below) are the same. The inclination angle of the grouting holes on the circle with a larger diameter is larger, and the inclination angle of the grouting holes on the circle with the largest diameter is less than 90°.

[0043] Example

[0044] The Lujiazhai Tunnel of Wengma Line passes through the sand section of the underground river (D1K52+910~D1K53+031). From DK52+910, a ring of Φ108 large pipe shed is constructed towards the large mileage end, with 25 pipes in each ring and a single pipe length of 20m. A steel cage is installed inside, and grouting is injected inside the pipe, but no grouting is injected outside the pipe. The mileage of the heading face at the exit of Lujiazhai Tunnel is D1K53+010. From the heading face to the small mileage, Φ89 advanced large pipe shed grouting reinforcement is applied. The construction scope is the arch and side walls. Steel cages are added inside the pipe shed with a circumferential spacing of 30cm and a single pipe length of 25m. Φ42 large external angle small guide tubes are added between the large pipe sheds in the sand layer section for grouting reinforcement. 4.5m; when the heading face of Lujiazhai Tunnel is constructed to D1K52+945, curtain grouting is carried out from the heading face of the cross tunnel end toward the direction of large mileage. A C25 concrete stop wall is set for curtain grouting with a thickness of not less than 2m. The outer contour of the stop wall is embedded in the main tunnel excavation line by 50cm. The main tunnel excavation length is 15m. The tunnel is constructed in both directions. When the exit work area is constructed to D1K52+990, the next cycle of curtain grouting is carried out toward the small mileage section. When excavating to DK52+975 (leaving a 5M stop rock plate), the next cycle of curtain grouting is carried out again toward the small mileage end. There are a total of 3 rings of curtain grouting in this section (D1K52+945~+990). The surrounding rock grade of D1K52+910~D1K53+010 section (100m in total) was adjusted from Grade III to Grade V, and the lining type was changed from the original Type IIIb composite lining to Type Vt composite lining. The D1K53+010~+016 section (6m) adopted Type Vc composite lining, and the D1K53+016~+031 section (15m) adopted Type IVc composite lining. The base of D1K52+917~D1K53+024.4 section was reinforced with Φ120cm rotary grouting bite piles, with a spacing of 1.0×1.0m (plum blossom arrangement), and the bottom of the piles embedded in the bedrock was not less than 0.5m. The bottom filling of the tunnel of D1K52+915~+916.4 and D1K53+925~+927 sections was less than 1.5m, and C25 concrete was used for replacement. During the curtain grouting, large pipe shed and the drilling of the center holes of each row of rotary jet piles, detailed drilling records must be made for each drilling. If any abnormality is found, it must be reported to the participating construction units in a timely manner for processing.

[0045] Φ120cm jet grouting interlocking piles, arranged in a plum blossom shape with a spacing of 1.0×1.0m, totaling 687 piles, with a single pile length of 4 to 18m, an average length of 14.50m, and a total of 9960.56m. The main grouting material is ordinary Portland cement, and the recommended water-cement ratio is 1 to 1.5. Technical requirements for the construction of jet grouting interlocking piles:

[0046] The construction sequence of jet grouting piles: site leveling → water quality review → process test piles → clearing ground and air obstacles → measurement and layout → jet grouting pile construction → quality inspection.

[0047] Construction precautions:

[0048] (1) An automatic slurry metering device should be installed during jet grouting pile construction.

[0049] (2) The holes for spray piles are usually drilled with a vibration drill. When encountering a relatively hard stratum, it is advisable to use a geological drill.

[0050] (3) The grouting pressure of spray piles should be greater than 20Mpa.

[0051] (4) The distance between the injection hole and the high-pressure grouting pump should not be greater than 50m, and the deviation between the pile position and the designed position should not be greater than 50mm.

[0052] (5) The jet grouting in the hole should be carried out evenly from bottom to top. The overlap length of the segmented jet pipe should not be less than 100 mm. If it is necessary to expand the reinforcement range or improve the strength locally, repeated jetting measures can be adopted.

[0053] (6) If there is a sudden drop, rise or abnormal grouting of pressure during high-pressure jet grouting, the cause should be identified and timely measures should be taken.

[0054] The outer contour of the stop wall is embedded 50cm from the excavation line of the main tunnel. The stop wall is made of C25 concrete with a thickness of 2m. The orifice pipe of the grouting drilling hole is pre-buried in the stop wall according to the "D1K52+665 Lujiazhai Tunnel Crossing the Dark River Section Advance Curtain Grouting Design Drawing". The drilling and grouting construction can only be started after the concrete strength reaches 75% of the design strength. In order to ensure the stability of the stop wall, a ring of radial 42 grouting steel flower pipes is set in front of the stop wall for grouting reinforcement, with a circumferential spacing of 1.0m, a single spacing of 1.0m, and a single length of 4.5m. The grouting order is first the outer circle and then the inner circle, first the near and then the far, and the same circle of holes is constructed at intervals, and in the construction of the same circle of holes, the top is first and then the bottom. The 1#~96# holes are grouted in the forward direction, and the 97#~145# holes are grouted in the backward direction. The diameter of the grouting hole shall be no less than 89mm, and the final hole diameter shall be no less than 75mm. The drilling holes shall be arranged in a circular pattern. During construction, Zhongke shall adjust the hole opening position according to its own mechanical configuration and construction convenience, but the final hole position of the drilling hole shall be guaranteed.

[0055] Grouting parameters:

[0056] (1) The diffusion radius of a single hole is 1.5 m, and the distance between the bottoms of each ring hole is no more than 2.5 m, so that the diffusion circles of the single hole grouting overlap effectively, forming a curtain to effectively reinforce the rock mass.

[0057] (2) The grouting reinforcement range is 5m outside the lining excavation contour line.

[0058] (3) Grouting pressure: Normal grouting pressure is hydrostatic pressure + 0.5 MPa. The final grouting pressure shall not be greater than 2.5 times of the hydrostatic pressure. It shall be adjusted during construction based on on-site grouting tests.

[0059] Grouting material: mainly cement slurry. Cement: 425 ordinary Portland cement, water-cement ratio 0.8:1~1:1. After the grouting is completed, the grouting effect should be checked. After the inspection, the tunnel can be continued only after the grouting effect meets the design requirements.

Claims

1. A design and construction method for strengthening the tunnel stratum in a karst underground river area with a sand layer that can be lost, characterized in that: The following steps are involved: ① From the starting mileage and the ending mileage of the dark river section (8), one or more rings of large pipe sheds (10) are grouting reinforced towards the large mileage end and the small mileage end. The specific scope of the large pipe shed (10) is the arch and the side wall. A steel cage is added inside the pipe shed. A small guide tube with a large external angle is added between the large pipe sheds in the sand layer section for grouting reinforcement; ② The tunnel base treatment from the starting mileage to the ending mileage of the dark river section (8) is reinforced by jet grouting interlocking piles (7). The jet grouting interlocking piles (7) are composed of a number of evenly distributed units. Each unit has 7 jet grouting piles, 6 of which are arranged at the 6 corner points of a regular hexagon and one is arranged at the center of the regular hexagon. The interlocking surface of the jet grouting pile at the center is the same as that of the other 6 jet grouting piles. The interlocking surface is also the same as that of the other 6 jet grouting piles. The bottom of the pile is embedded in the bedrock by no less than 0.5m. If the tunnel bottom filling is less than 1.5m, concrete is used for replacement. The construction sequence of the jet grouting piles is: 1) site leveling; 2) water quality review; 3) process pile test; 4) clearing ground and air obstacles; 5) measurement and layout ; 6) Jet-jet pile construction 7) Quality inspection; Automatic slurry metering device should be configured in the construction of jet-jet piles; Jet-jet pile holes are generally drilled by vibration drilling rigs. When encountering relatively hard strata, geological drilling rigs should be used for drilling; The distance between the jet hole and the high-pressure grouting pump should not be greater than 50m, and the deviation between the pile position and the designed position should not be greater than 50mm; The jet grouting in the hole should be lifted evenly from bottom to top, and the lap length of the jet pipe lifted in sections should not be less than 100mm. For parts that need to expand the reinforcement range or increase the strength locally, re-jetting measures should be adopted; After the high-pressure jet grouting is completed, the jet pipe should be pulled out quickly. If necessary, slurry backfilling or secondary grouting measures should be taken at the original hole position to prevent the slurry from solidifying and shrinking and affecting the pile top elevation; ③ Multiple rings of curtain grouting are applied from the tunnel excavation to the middle section of the underground river. The length of each grouting cycle is 15-30m. The grouting rock plate (5) with a thickness of 4-5m is reserved according to the actual construction environment. A grouting wall (2) should be set for grouting. The thickness of the grouting wall (2) should not be less than 2m. The orifice pipe of the grouting borehole (3) is pre-buried in the grouting wall. The drilling and grouting construction can only be started after the concrete strength of the grouting wall reaches 75% of the design strength. The diffusion radius of a single hole is about 1.5m. The spacing between the bottoms of each ring hole is not more than 2.5m, so that the single hole grouting diffusion circle effectively overlaps to form a curtain grouting consolidation circle (6). The grouting reinforcement range is 4-6m outside the lining excavation contour line. After the grouting is completed, the grouting effect should be checked. After the grouting effect meets the design requirements, the tunnel can be further excavated. In order to ensure the stability of the grouting wall (2), a ring of grouting steel flower pipe (4) is set in front of the grouting wall (2) for grouting reinforcement. The grouting holes (3) are arranged in a circular pattern. The number of holes and the number of circles are determined according to the actual situation. The grouting sequence is outer circle first, inner circle second, near circle first, far circle second. The holes in the same circle are constructed at intervals. In the same circle, the upper part is constructed first, the lower part is constructed later. The outer circle holes are constructed by forward grouting, while the inner circle holes are constructed by backward grouting. The grouting material is mainly cement slurry, with a water-cement ratio of 0.8:1 to 1:

1. During construction, the grouting sections, grouting methods, and grouting materials should be appropriately adjusted according to the results of advance prediction and forecasting. ④ After the last round of curtain grouting is completed, a large pipe shed (10) is constructed for grouting reinforcement near the terminal section of the underground river area.

2. According to claim 1, a method for designing and constructing tunnel strata reinforcement in a karst underground river area with sand layers that can be lost is characterized by: The outer contour of the grouting wall (2) is embedded in the tunnel excavation contour line (1) by 50 cm; the grouting holes (3) are evenly distributed on the grouting wall in 6 concentric circles, and the spacing between two adjacent concentric circles is equal, and the grouting holes on the same circumference are evenly spaced.

3. According to claim 1, a method for designing and constructing tunnel strata reinforcement in a karst underground river area with lost sand layers, characterized in that: The quality inspection method of the jet grouting pile is as follows: ① After the jet grouting pile is constructed, the grouting effect should be inspected. The number of quality inspection points should not be less than 2% of the number of construction holes. The content should include the integrity, uniformity, strength and bearing capacity of the pile body. ② Within 7 days after the pile is constructed, the uniformity of the pile body shall be checked by low strain, and the number of piles to be checked shall be 2‰ of the total number of piles, and shall not be less than 3; ③ Within 7 days after pile construction, shallow excavation of the pile head is carried out to a depth of more than 0.5m below the grouting surface. Visual inspection is conducted to check the uniformity of mixing and measure the diameter of the pile. The number of piles to be inspected is 2‰ of the total number of piles, and no less than 3 piles; ④ After 28 days of pile construction, a double-tube single-action sampler should be used to sample the pile in the direction of the pile diameter. Drill vertical holes within the length of the pile to observe the integrity and uniformity of the pile body, and take no less than 3 specimens at different depths for unconfined compressive strength test. The number of specimens to be tested is 2‰ of the total number of piles, and no less than 3 piles; ⑤ The bearing capacity test of jet grouting piles should be carried out 28 days after pile construction. Single pile or composite foundation load test should be adopted. The number of piles to be tested should be 2‰ of the total number of piles, and no less than 3.

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