After detecting the existence of karst caves, after discussion and analysis by the owner, designer, supervisor and construction party, it is considered that large-scale karst caves should be constructed in accordance with the scheme of "long pipe shed advanced support and small pipe grouting reinforcement" to achieve the purpose of "safety and mud outburst prevention".
Drilling and grouting construction of large pipe shed was carried out from February 4 to February 14, 2002. After the large pipe shed construction and grouting reinforcement, the exploration and excavation began on February 18, with a penetration of 0.5 m per cycle. When excavated to PDK 354+255.8, the lithology of the tunnel face is Wujiaping Formation shale, brown, calcareous cemented, massive structure, moderately to strongly weathered, and the weathered surface is rust-colored; Joints and cracks are developed, and some cracks are filled with soft plastic clay with a width of 5 ~ 20 cm, and the rock mass is broken. Soft plastic mud has been exposed in the lower right corner of tunnel face, with an area of about 0.5 m2, which is presumed to have entered the tunnel body. At about 1 1: 00, at the end of the caving face, it was found that the displacement of the two arches in the working face was about 1 m, and the right tunnel wall began to collapse when the inverted arch was welded, and then the arch began to deform and broke at the arch shoulder, and the collapsed body was about 5 m3. After the collapse, a radial crack can be observed in the depression of the right wall, with a width of 5 ~ 15 cm. No filler is found, and a small amount of water emerges. The surface of the crack is weathered in rust color, which is a structural crack. There are several cracks in the direction of tunnel face, the widest of which is about 20cm. After testing, the length is more than 5 m, and the crack surface is fresh. It is speculated that it is caused by plastic deformation and collapse of excavated rock mass. Such a long and wide crack also shows that the joints and cracks in the surrounding rock in front are extremely developed and the rock mass is loose.
According to the problems in the project, the tunnel face shall be closed immediately, and the excavation section behind 10m shall be supported and reinforced. Subsequently, at 0: 30 on February 23, when PDK 354+255.8 inverted arch leg joint was constructed, mud burst occurred in the lower right corner of horizontal heading face. Mud outburst is located at the junction of sandbags piled up on the tunnel face and the right side wall. After spraying for more than ten cubic meters, it stopped and then sprayed again. At the same time, insert a 5-meter-long bolt for spewing, and the whole process is about 10min. The spray is brownish yellow muddy clay, which is plastic and smelly. No gravel was found and no water gushing occurred. The total mud inflow is about 40m3, and the water inflow from the tunnel face is still 40 ~ 60m3/h ... Through analysis, it is considered that the change of water in the tunnel and the hydraulic action of the water outlet from the tunnel face cause the instability of the filling material flow in the tunnel. Mud break has a certain pressure, and this plastic filling exists in a certain range in the upper part of the horizontal conduit, and it is possible to continue to break at any time.
Due to the mud gushing out, it is impossible to ensure the safe excavation ahead. After the demonstration of the four-way scheme of participating in the construction, it is decided to adopt full-face advanced pre-grouting construction to expand the scope of grouting reinforcement and achieve the purpose of "consolidating mud and blocking water and safe construction".
(2) Rear reinforcement
During the excavation of PDK 354+240 ~+255 section, steel+shotcrete was used for initial support; The excavated PDK 354+245 ~+255 mudstone section is reinforced by radial grouting with small pipes.
1) The range of radial grouting reinforcement is 3 m outside the excavation contour line.
2) Radial grouting is arranged in Kong Meihua, with circumferential spacing of orifices 1 m and row spacing 1 m, and grouting holes are arranged perpendicular to the excavation contour line.
3) After the layout of grouting pipe is completed, spray concrete around grouting pipe to block it, so as to prevent slurry from escaping during grouting and ensure the grouting effect; In order to ensure the rigidity of grouting pipe, grouting pipe can be arranged near the arch frame. After the construction is completed, measures can be taken to weld with the arch frame.
4) What did grouting pipe adopt? Make 42mm welded steel pipe, the grouting pipe is 3 m long, of which the flower pipe is 2 m long, and the plum blossom layout is arranged every 20cm in the flower pipe part. 4mm~？ 6mm overflow hole, the front end of grouting pipe is tapered and sealed, and the tail of the pipe adopts double channels? 6mm round steel welding hoop, one of which is used to connect the grouting core pipe and the other is used to stop grouting after winding cotton yarn.
5) The grouting hole is drilled by pneumatic drill. What is the aperture? 45mm, run in after hole formation? 42mm grouting pipe, using the whole hole one-time grouting method for grouting.
6) The grouting materials are ordinary cement-sodium silicate double slurry and ordinary cement single slurry, with double slurry as the main grouting material. The mixture ratio of double liquid slurry is: water cement ratio of cement slurry is 0.8 ∶ 1 ~ 1∶ 1, volume ratio of cement slurry to water glass is 1∶ 1, and water glass concentration is 35Be'. The slurry ratio of ordinary cement single slurry is: water cement ratio 0.8∶ 1.
7) Grouting control is carried out by combining constant pressure with quantitative standard, with constant pressure grouting as the main method, and the final grouting pressure is 2 ~ 3 MPa. The principle of single-hole grouting quantity is no more than 5 m3.
8) The drilling and grouting sequence shall be carried out in the direction of PDK 354+245 →+255, and interval hole jumping and compaction grouting measures shall be adopted.
(3) Advanced large pipe shed
Grouting reinforcement can not only increase the supporting rigidity of the pipe shed, but also reinforce the muddy clay around the pipe shed by grouting, forming a continuous closed pipe shed trumpet-shaped supporting structure, so as to avoid or reduce the harm caused by mud and karst water pouring into the excavation space through the gaps between the pipe sheds during construction. Grouting construction for large pipe shed installation started on February 4th, 2002 and ended on February14th, lasting 1 1 day. The installation and grouting construction technology of large pipe shed is as follows.
1) C20 shotcrete is used to close the tunnel face with a thickness of 50cm.
2) Construction of circumferential dense pipe shed around the excavation section. The pipe shed is made of seamless steel pipe, with a diameter of φ 108mm, a wall thickness of 6mm, and a section length of 3 m, and an overflow hole of φ 5 mm ~ φ 10 mm is set on the outside. The pipe shed spacing is 20cm, and the extrapolation angle is 3. According to the drilling situation, the length of pipe shed is based on the principle of drilling 2 ~ 3 m into hard rock.
3) The grouting material is ordinary cement-water glass double slurry, and the ratio of the double slurry is: water cement ratio of cement slurry is 0.8 ∶1~1,volume ratio of cement slurry to water glass is 1∶ 1, water glass concentration is 35Be', and retarder content is/kloc-.
(4) Full-face pre-grouting in advance
In order to ensure the safety of construction in karst cave area, advance pre-grouting is carried out for the whole section, and the reinforcement range is 5 ~ 8 m outside the excavation face and excavation contour line. The advance pre-grouting design is shown in figure 10-44. The full-face advance pre-grouting construction started on February 28th, 2002 and ended on March 23rd, which lasted for 24 days. The construction technology of full-face advance pre-grouting is as follows.
Figure 10-44 Full-face Advance Pre-grouting Design Drawing
(Unit: cm)
1) C20 molded concrete with a thickness of 80cm is used for the grout stop wall. When constructing the grout stop wall, the gushing water will be discharged from the orifice pipe to ensure the construction quality of concrete and grouting.
2) The grouting hole is drilled by MKD-5S drilling rig, and installed after drilling. 108mm orifice tube, the length of orifice tube is 2m; When the orifice pipe can't be installed, the water bag type slurry stop plug is used for grouting construction.
3) The drilling depth should be 2 ~ 3m into the rock stratum.
4) According to the requirement of water plugging by grouting, the grouting material is ordinary cement-sodium silicate double slurry. The mixing ratio of cement-water glass double slurry is: water cement ratio of cement slurry is 0.8 ∶ 1 ~ 1∶ 1, the volume ratio of cement slurry to water glass is1,the concentration of water glass is 35Be', and the dosage of retarder is 0 ~ 2%. The gel time of cement slurry is controlled at 30 seconds and 3 minutes, which can be adjusted according to the site construction situation.
5) Grouting construction sequence is basically carried out according to the following two principles.
① Divergent constraint grouting. That is, grouting is carried out according to the principle of C ring →A ring →B ring ... First, divergent grouting is carried out for C ring and A ring, and then constrained grouting is carried out for B ring to achieve the purpose of diffusion-compaction.
(2) Fully consider the influencing factors of water source, and carry out grouting from bottom to top and from left to right.
6) The grouting process adopts gradual and segmented grouting, and the grouting interval is 5 m. ..
7) The grouting end standard is given priority to with constant pressure, and the final grouting pressure is water pressure +2 ~ 3 MPa. When the pressure does not rise for a long time during grouting, the gel time of the slurry should be shortened, intermittent grouting measures should be taken, and the grouting amount should be controlled at the same time.
8) Special case handling.
(1) The grouting quantity in the design is controlled at 24 m3/ section. From February 28th to March 4th, 2002, grouting construction was carried out with the designed grouting quantity. During the grouting construction on March 4th, a large amount of water and mud gushed out from the waist of the left arch and the right vault. Through analysis, it is considered that it may be caused by the serious blockage of the large water flow channel at the bottom during the floor grouting construction. This was verified in the subsequent A8 drilling and grouting construction. Therefore, in the later grouting construction, the grouting quantity control standard of 12 m3/ section is adopted at the bottom, and the grouting quantity control principle of 12 m3/ section on the left side and 24 m3/ section on the right side is adopted above the bottom.
(2) During drilling and grouting construction, water and mud gushed from holes A 1, B7 and A 12. The patent technology TSS one-way sleeve valve grouting pipe of scientific research institute is adopted for reverse segmental grouting construction, and good grouting effect is obtained.
(5) Pre-support of small catheter
After the advance pre-grouting is completed, before the excavation construction, small duct supplementary grouting should be carried out for the parts with insufficient grouting or grouting blind spots.
1) The grouting pipe is 6 m long and each section is 2 m long? Threaded connection of 42mm welded steel pipe. The front end of grouting pipe is tapered and sealed. The flower tube part is 3 m long and 30cm apart on the cross section of the flower tube, which is arranged in a plum blossom shape. 4mm~？ 6mm mud overflow hole. There are two ways to use the tail of the pipeline? 6mm round steel welding hoop, one of which is used to connect the grouting core pipe and the other is used to stop grouting after winding cotton yarn.
2) The grouting material is ordinary cement-water glass double slurry, the slurry ratio is: water cement ratio of cement slurry is 0.8 ∶1~1,the volume ratio of cement slurry to water glass is 1∶ 1, the concentration of water glass is 35Be', and the dosage of retarder is 0 ~ 2.
3) The grouting end standard adopts the principle of combining constant pressure with quantification, and the final grouting pressure is 2 ~ 3 MPa, and the single-hole grouting quantity is 0.2~0.3 m3.