Hard rock stratum pipe jacking construction method

Abstract

The application discloses a hard rock stratum pipe jacking construction method, which comprises the following steps: determining the information of the hard rock height and trend of a receiving well, the hard rock jacking hole position and the hard rock jacking slope; setting multiple groups of cutters on a cutter head of a first pipe jacking machine according to the information; after the first pipe jacking machine jacks to a designed position, the pipe is retreated and slurry is filled; when the first pipe jacking machine retreats to a through-wall pipe position, the first pipe jacking machine is stopped to block the hole, the mixed slurry is solidified, and the first pipe jacking machine is withdrawn; the starting well restores the pipe jacking of a second pipe jacking machine, and the second pipe jacking machine jacks to the receiving well for receiving. The hard rock is broken by the first pipe jacking machine in the receiving well in a reverse direction, the first pipe jacking machine is then pulled out, and the slurry is injected while the first pipe jacking machine is pulled out, so that the hard rock is broken, the second pipe jacking machine with a large section can be normally jacked, and the project can be smoothly carried out.

Description

Technical Field

[0001] This invention relates to the field of trenchless underground engineering pipe jacking construction technology, and in particular to a pipe jacking construction method in hard rock formations. Background Technology

[0002] During the construction of large-section rectangular pipe jacking machines, changes in geological conditions are quite common, especially when encountering hard rock formations. This can easily lead to cutterhead jamming and work stoppages. Due to the design of large-section rectangular pipe jacking machines, it is impossible to effectively utilize the rock-breaking capabilities of the cutterhead for forced rock breaking. This can cause the cutterhead to loosen, or even detach, ultimately preventing normal cutterhead movement or even jamming, severely impacting construction progress and causing significant losses to the project. Furthermore, the existence of cutting blind zones during rectangular pipe jacking drilling can lead to drilling failures. Summary of the Invention

[0003] The purpose of this invention is to at least solve one of the technical problems existing in the prior art. To this end, this invention proposes a method for pipe jacking construction in hard rock formations, which can ensure the normal jacking of large-section pipes and guarantee the smooth progress of the project.

[0004] A method for tunneling in hard rock formations according to a first aspect of the present invention includes:

[0005] Determine the information on the hard rock height and orientation of the receiving well, the location of the hard rock jacking entrance, and the hard rock jacking slope;

[0006] Based on the above information, multiple sets of cutting tools are set on the cutterhead of the first pipe jacking machine;

[0007] After the first pipe jacking machine reaches the designed position, the pipe is withdrawn and grout is filled.

[0008] When the first pipe jacking machine retracts to the position of the through-wall pipe, stop retracting and block the hole with the first pipe jacking machine. After the mixed grout solidifies, withdraw the first pipe jacking machine.

[0009] The second pipe jacking machine resumed jacking at the starting well and jacked to the receiving well for receiving.

[0010] The hard rock tunneling method according to the first aspect of the present invention has at least the following beneficial effects: by using a first tunneling machine to break the hard rock in the receiving well in reverse, and then pulling out the first tunneling machine while grouting, the hard rock is broken, ensuring that the large-section second tunneling machine can advance normally and ensuring the smooth progress of the project.

[0011] According to the first aspect of the present invention, the method for tunneling hard rock formations, the method for determining the height and orientation of the hard rock in the receiving well includes drilling cores using a down-the-hole drill based on geological survey data, determining the rock height and orientation, analyzing and determining the rock sample and hardness, obtaining the rock location, orientation and height within the tunnel area based on the core sampling results, and drawing cross-sectional and longitudinal sections of the rock.

[0012] According to the first aspect of the present invention, the method for determining the location of the tunnel entrance in hard rock formation includes determining it based on the rock surface height and the tunnel entrance width, and planning the rock-breaking channel.

[0013] According to the first aspect of the present invention, the method for determining the jacking slope in hard rock formations includes determining the jacking slope of the first jacking machine based on the height of the second jacking machine and the height of the first jacking machine, combined with the rock strata and the rock breaking line conditions.

[0014] According to the first aspect of the present invention, the pipe jacking construction method for hard rock formations includes a main spoke plate and a secondary spoke plate disposed on the main spoke plate. The secondary spoke plate is provided with a roller cutter, and at least four cutters are disposed on the outer side of the roller cutter according to the hard rock information, wherein the cutters are alloy grain conical cutters.

[0015] According to the first aspect of the present invention, the method for tunneling in hard rock formations includes a method for the first tunneling machine to withdraw the pipe by welding a corbel to the outside of the steel pipe as a pullback point, and using steel section support on the inside. After each withdrawal stroke, a new corbel is welded.

[0016] According to the first aspect of the present invention, the pipe jacking construction method in hard rock formation uses a mixture of bentonite, cement, and calcium chloride as the grout for pipe jacking retreat. The first section of pipe retreat is 0-2m, and the grout ratio is bentonite:cement:calcium chloride = 3:1:0.2; the second section of pipe retreat is 2-9m, and the grout ratio is bentonite:cement:calcium chloride = 1:1:0.2; the third section of pipe retreat is 9m to the tunnel entrance, and the grout ratio is bentonite:cement:calcium chloride = 1:3:0.4.

[0017] According to the first aspect of the present invention, in the hard rock stratum pipe jacking construction method, the first pipe jacking machine has a slurry chamber pressure of 0.03-0.05 MPa under static conditions, and the grouting pressure is maintained at 0.06 MPa or above during retreat grouting.

[0018] According to the first aspect of the present invention, in the hard rock stratum tunneling method, the calcium chloride mixed slurry reaches the initial setting state after standing for 1.5 hours.

[0019] According to the first aspect of the present invention, in the method for tunneling in hard rock formations, after the first tunnel jacking machine blocks the tunnel entrance, the mixed grout solidifies for more than 12 hours before the first tunnel jacking machine is withdrawn, and after the first tunnel jacking machine is completely withdrawn, the tunnel entrance is sealed with quick-setting cement.

[0020] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments;

[0022] Figure 1 This is a schematic cross-sectional view of a hard rock borehole core sample taken from a receiving well according to an embodiment of the present invention;

[0023] Figure 2 This is a schematic cross-sectional view of the hard rock surface of the receiving well in an embodiment of the present invention;

[0024] Figure 3 This is a schematic diagram of a longitudinal section of the hard rock surface of the receiving well according to an embodiment of the present invention;

[0025] Figure 4 This is a schematic diagram of the cross-sectional layout of the first pipe jacking machine in an embodiment of the present invention;

[0026] Figure 5 This is a schematic diagram of the jacking line of the first pipe jacking machine and the position of the second pipe jacking machine in an embodiment of the present invention;

[0027] Figure 6 This is a schematic diagram of the improved cutterhead of the first pipe jacking machine according to an embodiment of the present invention;

[0028] Figure 7 This is a schematic diagram of the steel pipe removal and reinforcement method in the first embodiment of the present invention.

[0029] Figure label:

[0030] 1. Hole position; 2. Steel ring; 3. Rock stratum; 4. Second pipe jacking machine; 5. Height of the inner corner; 6. Pipe jacking tunnel; 7. Jacking direction of the first pipe jacking machine; 8. Receiving shaft; 9. Main spoke plate; 10. Secondary spoke plate; 11. Cutter; 12. Jack; 13. Steel support; 14. Bracket; 15. Steel pipe; 16. Hole #1; 17. Hole #2; 18. Hole #3; 19. Hole #4; 20. Hole #5. Detailed Implementation

[0031] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0032] In the description of this application, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0033] In the description of this application, "several" means one or more, "more than" means at least two, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is for the purpose of distinguishing technical features only and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0034] In the description of this application, unless otherwise expressly defined, terms such as "setup," "installation," and "connection" should be interpreted broadly. Those skilled in the art can reasonably determine the specific meaning of the above terms in this application after considering the specific content of the technical solution.

[0035] Reference Figures 1 to 7 The hard rock jacking tunneling method of the first aspect of this application is applied to the construction of large-section rectangular jacking pipes. The hard rock jacking tunneling method includes the following steps:

[0036] Determine the information regarding the hard rock height and orientation of receiving well 8, the location of the hard rock jacking entrance, and the hard rock jacking slope;

[0037] Based on the above information, multiple sets of cutting tools are set on the cutterhead of the first pipe jacking machine;

[0038] After the first pipe jacking machine reaches the designed position, the pipe is withdrawn and grout is filled.

[0039] When the first pipe jacking machine retracts to the position of the through-wall pipe, stop retracting and block the hole with the first pipe jacking machine. After the mixed grout solidifies, withdraw the first pipe jacking machine.

[0040] The second pipe jacking machine 4 resumed jacking at the starting well and jacked to the receiving well 8 for receiving.

[0041] By using the first pipe jacking machine to break up the hard rock in reverse at receiving well 8, and then pulling out the first pipe jacking machine while grouting, the hard rock is broken up, ensuring that the large-section second pipe jacking machine 4 can advance normally and ensuring the smooth progress of the project.

[0042] In some embodiments of this application, the method for determining the height and orientation of the hard rock in receiving well 8 includes drilling cores using down-the-hole drilling based on geological survey data to determine the rock height and orientation, analyzing and determining the rock sample and hardness, and obtaining the rock location, orientation, and height within the pipe jacking tunnel 6 based on the core sampling results, and drawing cross-sectional and longitudinal profiles of the rock. Further, the method for determining the location of the hard rock jacking entrance includes determining it based on the rock surface height and entrance width, and planning the rock-breaking channel, while ensuring that there are no dead angles for breaking through at the angle of each channel. Further, the method for determining the hard rock jacking slope includes planning the rock-breaking jacking slope of the first pipe jacking machine based on the height of the second pipe jacking machine 4 and the height of the first pipe jacking machine, combined with the rock strata 3 and the rock-breaking line conditions. The first pipe jacking machine is a circular hard rock pipe jacking machine, and the second pipe jacking machine 4 is a rectangular pipe jacking machine.

[0043] In some embodiments of this application, the cutterhead of the first pipe jacking machine includes a main spoke plate 9 and a secondary spoke plate 10 disposed on the main spoke plate 9. The secondary spoke plate 10 is equipped with a roller cutter, and at least four cutting blades 11 are disposed outside the roller cutter according to the hard rock information. The cutting blades 11 are alloy particle conical cutters. Adding four alloy particle conical cutters to the outside of the machine head, based on the original first pipe jacking machine, further ensures the breaking of hard rock.

[0044] In some embodiments of this application, the method for retracting the pipe using the first pipe jacking machine includes welding a bracket 14 to the outside of the steel pipe 15 as a pullback point, and using a steel section support 13 on the inside. After each retraction stroke, a new bracket 14 is welded. Using appropriate retraction techniques after the first pipe jacking machine reaches the designed position ensures construction quality.

[0045] In some embodiments of this application, the grout used for pipe jacking retreat is a mixture of bentonite, cement, and calcium chloride. The first retreat section is 0-2m, with a grout ratio of bentonite:cement:calcium chloride = 3:1:0.2; the second retreat section is 2-9m, with a grout ratio of bentonite:cement:calcium chloride = 1:1:0.2; and the third retreat section is 9m to the tunnel entrance, using a bentonite:cement:calcium chloride ratio of 1:3:0.4. Furthermore, under static conditions, the pressure in the slurry chamber of the first pipe jacking machine is 0.03-0.05 MPa, and during retreat grouting, the retreat grouting pressure is maintained at 0.06 MPa or higher. Furthermore, the mixed grout containing calcium chloride reaches its initial setting state after standing for 1.5 hours. Understandably, the retreat grouting process must ensure the stability of the soil layer above the hard rock pipe jacking machine, while also guaranteeing the smooth advancement of the second pipe jacking machine (4). The grout for the retreat jacking was determined through testing, and grout mix ratio tests were conducted in advance. Grout filling is crucial during the pipe retreat process. During retreat, the grouting pressure must be ensured to be greater than the static water and soil pressure to guarantee complete filling.

[0046] In some embodiments of this application, after the first pipe jacking machine blocks the tunnel entrance, the mixed grout is allowed to solidify for more than 12 hours before the pipe jacking machine is withdrawn. After the first pipe jacking machine is completely withdrawn, the tunnel entrance is then sealed with rapid-hardening cement. It is understood that the mixed grout used for the tunnel entrance sealing section has a mix ratio determined through testing. When the first pipe jacking machine retracts to the position of the through-wall pipe, it stops retracting, allowing the first pipe jacking machine to block the tunnel entrance. After the mixed grout solidifies, the first pipe jacking machine is withdrawn. After the first pipe jacking machine is completely withdrawn, the tunnel entrance is then sealed with rapid-hardening cement.

[0047] Furthermore, a specific implementation example is for a municipal rail transit subway line project. (Refer to...) Figures 1 to 7 A method for tunneling through pipe jacking in hard rock formations includes the following steps:

[0048] (1) Determine the height and orientation of the hard rock in receiving well 8. For example... Figure 1 , Figure 2 and Figure 3 As shown, where Figure 3 The height of the internal corner is shown as 5. After technical research and discussion, the project team decided to adopt a down-the-hole drilling and core sampling scheme, drilling and core sampling at intervals of 40cm. Figure 1 Hole 1, where core samples were taken, was shown. The height and orientation of the rock were determined, and the rock sample and hardness were analyzed. Based on the core sampling results, the actual length of the slightly weathered conglomerate within the rectangular pipe jacking tunnel 6 is approximately 12m. Rock layer 3 is distributed along a slope, with the left side higher than the right side from the receiving shaft 8 towards the starting shaft. The highest point of rock layer 3 within the pipe jacking area is 90cm, and the lowest point is 50cm. The approximate location, orientation, and height of the rock within the rectangular pipe jacking tunnel 6 were obtained, and cross-sectional and longitudinal sections of the rock were drawn.

[0049] (2) Determine the location of the entrance to the tunnel through the hard rock. For example... Figure 4 As shown, based on the rock surface height and the width of the rectangular opening, the rock surface height is mostly around 650mm. A first-generation pipe jacking machine with an outer diameter of φ1660mm is proposed to be selected, aiming to jack into 5 rock-breaking channels. Simultaneously, it is ensured that there are no dead angles in the angled sections of each channel. The following points are considered:

[0050] 1) Break the left steel ring 2 to ensure that the lower part of the first pipe jacking machine can cover the rock surface of the tunnel entrance;

[0051] 2) Increase the overlapping surface of each ring channel, and at the same time break the bottom side wall of steel ring 2, reducing dead corners.

[0052] 3) Remove the concrete at the bottom of the tunnel opening to increase the contact area between the first pipe jacking machine and the rock surface.

[0053] 4) Place the water-stop ring directly onto the bottom plate of the receiving well 8, adjust the angle of the water-stop ring to the required slope, and lay the guide rail with square steel and channel steel close to the bottom plate surface.

[0054] 5) The pipe jacking section uses steel pipe with an outer diameter of φ1620. The jacking sequence is skip-hole construction: hole 3 (18) → hole 5 (20) → hole 1 (16) → hole 4 (18) → hole 2 (17). The construction procedure for pipe jacking in hard rock is as follows: pipe jacking → jacking into place → pipe jacking retraction and grouting → sealing of the pipe jacking opening.

[0055] (3) Determine the slope for jacking hard rock. For example... Figure 5 As shown, in combination with the jacking direction 7 of the first pipe jacking machine, based on the height of the second pipe jacking machine 4 and the height of the circular hard rock jacking surface of the first pipe jacking machine, and to ensure that there is no dead angle after the rock breaking channel meets the second pipe jacking machine 4, it is necessary to ensure that the bottom of the first pipe jacking machine is 300mm below the bottom of the second pipe jacking machine 4, and the proposed rock breaking jacking slope is 5%.

[0056] (4) Modification of the cutterhead of the first pipe jacking machine. For example... Figure 6 As shown, the original first pipe jacking machine consisted of 2 main spokes (9) + 4 auxiliary spokes (10), with the cutter 11 arranged using the Archimedes single spiral method. Based on the roller cutter arrangement, the cutter 11 was added; the height difference between the roller cutter and the cutter 11 improved rock-breaking ability. The cutting path of the cutter almost covered the entire excavation face, ensuring complete cutting of the entire face. The cutter had an inner deflection angle, further improving its rock-breaking efficiency. Considering the specific characteristics of this project, certain adaptive modifications and improvements were made to the original hard first pipe jacking machine. Four alloy-granule conical cutters were added to the outside of the machine head; these cutters were 1.5 cm larger in diameter than the machine body, and the old cutters were replaced.

[0057] (5) Determine the hard rock pipe jacking retreat technology. For example... Figure 7 As shown, to ensure that the steel pipe 15 has sufficient length after the first pipe jacking machine reaches the designed position, allowing for one pipe retraction stroke, a bracket 14 is welded directly to the outside of the steel pipe 15 using structural steel as the retraction point. A structural steel support 13 is used on the inside. A new bracket 14 is welded after each stroke. When retraction reaches the machine head section or when the reserved length of the steel pipe 15 is insufficient, making it inconvenient to weld the bracket 14, an H-beam spreader is welded to the machine head or inside the steel pipe 15. The pipe retraction is achieved using a combination of hydraulic jacks 12 and H-beam spreaders.

[0058] (6) Grouting mix ratio and filling. The grouting process for the retreat must ensure the stability of the soil layer above the hard rock jacking pipe and also guarantee the smooth advancement of the second jacking machine 4. The grout used for the hard rock jacking retreat is a mixture of bentonite, cement, and calcium chloride, and a grout mix ratio test was conducted in advance. The grout mix ratio test results showed that the grout with calcium chloride placed reached its initial setting state after standing for approximately 1.5 hours; the grout without calcium chloride placed reached its initial setting state after standing for approximately 3 hours. The entire hard rock jacking retreat grouting process is divided into three stages. The grout mix ratio for the first stage (0-2m) is bentonite:cement:calcium chloride = 3:1:0.2; the grout mix ratio for the second stage (2-9m) is bentonite:cement:calcium chloride = 1:1:0.2; and for the third stage (9m to the tunnel entrance), to quickly seal the tunnel entrance and ensure strength, a bentonite:cement:calcium chloride = 1:3:0.4 ratio is proposed. During the pipe jacking process, grout filling is crucial. Under static conditions, the pressure in the slurry chamber of the first pipe jacking machine is 0.03-0.05 MPa. During pipe jacking, it is essential to ensure that the pressure during grouting is greater than the static water and soil pressure, maintaining a grouting pressure of 0.06 MPa or higher to ensure complete filling. In the latter half of the pipe jacking process, due to reduced frictional resistance, the first pipe jacking machine can also be retracted using grouting pressure.

[0059] (8) Tunnel Portal Sealing. The tunnel portal sealing section is from the 9th meter to the tunnel entrance, with a sealing length of approximately 3 meters. To quickly seal the tunnel portal and ensure strength, the following measures are taken: a mixture of bentonite, cement, and calcium chloride in a ratio of 1:3:0.4 is used. After the mixture with calcium chloride has been left to stand for about 1.5 hours, it reaches the initial setting state. When the first pipe jacking machine retreats to the position of the through-wall pipe, it stops retreating, allowing the first pipe jacking machine to block the tunnel portal. After the mixture has solidified for more than 12 hours, the first pipe jacking machine is withdrawn. After the first pipe jacking machine has completely withdrawn, the tunnel portal is sealed with quick-setting cement.

[0060] (9) The second pipe jacking machine 4 resumes jacking. After the first pipe jacking and retraction grouting of section 5 are completed, the starting well resumes the jacking of the second pipe jacking machine 4. The cutterhead of the second pipe jacking machine 4 starts and jacks smoothly, and jacks to the receiving well 8 for receiving.

[0061] When encountering hard rock that prevents jacking during the construction of large-section rectangular pipe jacking tunnels in urban areas, a smaller-sized hard rock pipe jacking machine is used for reverse rock breaking and jacking. Combined with pipe jacking retreat grouting and portal sealing techniques, the hard rock is broken, allowing the large-section rectangular pipe to be successfully jacked and reach receiving shaft 8. This method offers high construction safety, accelerates construction speed, and improves construction efficiency.

[0062] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0063] The embodiments of this application have been described in detail above with reference to the accompanying drawings. However, this application is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of this application.

Claims

1. A method for tunneling by pipe jacking in hard rock formations, characterized in that, Includes the following steps: Determine the information on the hard rock height and orientation of the receiving well, the location of the hard rock jacking entrance, and the hard rock jacking slope; Based on the above information, multiple sets of cutting tools are set on the cutterhead of the first pipe jacking machine; After the first pipe jacking machine reaches the designed position, pipe retraction and grouting are carried out. The grout used for pipe retraction is a mixture of bentonite, cement, and calcium chloride. The first retraction section is 0-2m, with a grout ratio of bentonite:cement:calcium chloride = 3:1:0.2; the second retraction section is 2-9m, with a grout ratio of bentonite:cement:calcium chloride = 1:1:0.2; and the third retraction section is 9m to the tunnel entrance, using a grout ratio of bentonite:cement:calcium chloride = 1:3:0.

4. When the first pipe jacking machine retracts to the position of the through-wall pipe, stop retracting and block the hole with the first pipe jacking machine. After the mixed grout solidifies, withdraw the first pipe jacking machine. The second pipe jacking machine resumed jacking at the starting well and jacked to the receiving well for receiving.

2. The method for tunneling in hard rock formations according to claim 1, characterized in that: The method for determining the height and orientation of the hard rock in the receiving well includes drilling cores using down-the-hole drills based on geological survey data, determining the rock height and orientation, analyzing and determining the rock sample and hardness, obtaining the location, orientation, and height of the rock within the pipe jacking tunnel based on the core sampling results, and drawing cross-sectional and longitudinal profiles of the rock.

3. The method for tunneling in hard rock formations according to claim 1, characterized in that: Methods for determining the location of the entrance to the tunnel in hard rock include determining it based on the height of the rock surface and the width of the entrance, and planning the rock-breaking passage.

4. The method for tunneling in hard rock formations according to claim 1, characterized in that: The method for determining the jacking slope in hard rock includes determining the jacking slope of the first pipe jacking machine based on the height of the second pipe jacking machine and the height of the first pipe jacking machine, combined with the rock strata and rock breaking route.

5. The method for pipe jacking construction in hard rock formations according to claim 1, characterized in that: The cutterhead of the first pipe jacking machine includes a main spoke plate and a secondary spoke plate disposed on the main spoke plate. The secondary spoke plate is provided with a roller cutter. At least four cutting blades are disposed on the outside of the roller cutter according to the hard rock information. The cutting blades are alloy grain conical cutters.

6. The method for tunneling in hard rock formations according to claim 1, characterized in that: The first pipe jacking machine's pipe retraction method includes welding a bracket to the outside of the steel pipe as a pullback point, using steel profiles for support on the inside, and welding a new bracket after each pipe retraction stroke.

7. The method for tunneling in hard rock formations according to claim 1, characterized in that: Under static conditions, the pressure in the slurry chamber of the first pipe jacking machine is 0.03-0.05 MPa. During backward grouting, the backward grouting pressure is maintained at 0.06 MPa or above.

8. The method for pipe jacking construction in hard rock formations according to claim 1, characterized in that: The mixed slurry containing calcium chloride reached the initial setting state after standing for 1.5 hours.

9. The method for tunneling in hard rock formations according to claim 1, characterized in that: After the first pipe jacking machine blocks the tunnel entrance, the mixed grout solidifies for more than 12 hours before the first pipe jacking machine is withdrawn. After the first pipe jacking machine is completely withdrawn, the tunnel entrance is sealed with quick-setting cement.

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