Liujiagou Tunnel is located about 300 meters southeast of Liujiagou Village, Liupanshan Town, Jingyuan County, Guyuan City, Ningxia. The tunnel is designed as a double-hole tunnel with separated upstream and downstream lines. The mileage pile numbers of the entrance and exit of the upstream line are SK 189+820 and SK 190+505, both of which are located on the curve with large radius, with a length of 685 meters, one-way slope and vertical direction. The mileage pile numbers of the downlink entrance and exit are XK 190+470, XK 189+780, with a length of 690 meters and a longitudinal slope of 1.7000%. The distance between the two shafts is about 50 meters, which belongs to the shallow-buried earth-rock mixed tunnel.
The tunnel runs through the mountains from east to west, with gentle terrain on the north slope and steep terrain on the south slope. Generally speaking, it belongs to the loess hilly region extending in the eastern part of Liupanshan Mountain, with developed valley terraces and deep cutting. The micro-geomorphic units in the middle of the tunnel belong to the loess plateau, hills, middle and low bedrock mountains and so on.
There is no large-scale adverse geology in the tunnel passing area, the slope at Guyuan end is gentle, the topographic contour lines intersect with the tunnel axis at a large angle, and the hole is cut with 25m and 20 m open-cut bamboo. The slope along the end of Chuanzi is steep, and the end-wall portal is adopted. There is a certain deviation at the exit of the downline, so the arch back is cultivated with M7.5 mortar flaky and 50cm clay. The eccentric pressure at the entrance of the upstream line is serious, and the C 15 flaky concrete retaining wall is set on the right side to form an eccentric pressure opening. The excavation design of the tunnel body is divided into Class I, II and III surrounding rocks.
The tunnel body is designed according to the new Austrian tunneling method and adopts composite lining. The primary support is mainly sprayed C20 concrete, anchor rod and steel mesh, supplemented by strengthening measures such as steel arch or steel grating, and the secondary lining adopts C25 reinforced concrete structure. The hole section is provided with a reinforced section lining, which is a composite lining. The initial support is reinforced by steel arch frame, supplemented by advanced large pipe shed advanced support. Class Ⅰ and Ⅱ surrounding rock sections are pre-supported by advanced small pipes, and reinforced concrete inverted arches are set in the reinforcement section and Class Ⅰ, Ⅱ and Ⅲ surrounding rock sections in the tunnel. In the process of construction, the following links were strictly controlled technically.
1. Mining
Excavation is the dominant factor in tunnel construction, and its quality, progress, safety and benefit are closely related to it. Only when the excavation meets the requirements can the support have a smooth guarantee foundation.
1. 1 The biggest difficulty in excavation is that the surrounding rock, geological structure and water seepage of the tunnel are complex and changeable, so it is difficult to fully understand it.
1. 1. 1 With the increase of tunnel excavation, a steel wire in a single hole is like a rope thrown out, and the longer it is, the easier it is to deviate from the control center axis. You can also substitute the measured wire value through the connecting hole to verify each other. When the length of tunnel excavation exceeds 500 meters, if the two ends can't rectify each other, two independent wires must be built in a hole to rectify the deviation.
1. 1.2 geological advance prediction in the process of tunnel excavation, every step forward has an unexpected situation, so advance prediction is particularly important. At present, the geological advance prediction methods are: drilling ahead of time, seismic wave method and ultrasonic detection method.
1. 1.2. 1 The advance drilling method should arrange at least three boreholes in the tunnel face: record the drilling speed of each borehole, the position of each different dust, the lithology of drilling powder and the water inflow of the borehole in detail. According to the drilling speed, the rock strength and the development of cracks and joints in different parts can be judged.
1. 1.2.2 Seismic wave and ultrasonic detection method judge the surrounding rock in front according to the propagation speed difference of wave velocity in rock mass, air, water and other media with different strengths. The conclusion drawn by this method is accurate within 30 meters from the transmitted wave.
1.2 excavation, usually by blasting, but special areas must be pried open with pneumatic pick or steel drill.
All surrounding rocks above1.2.1Ⅲ are excavated by full-face blasting. In order to ensure no under-excavation and less over-excavation, smooth blasting must be used to control the smooth surface. The hole spacing of the peripheral smooth blasting holes is about 50cm, and the decoupling charge is adopted, and the decoupling coefficient is 1.25 ~ 2.5 (calculated by ammonium nitrate explosive). The auxiliary holes are arranged between the smooth blasting hole and the cut hole, with the distance of 75 ~ 10~30cm centered on the cut hole. When the hole depth is less than 2m, the axial depths of smooth blasting hole, auxiliary hole and cut hole are the same. When the depth exceeds 2m, the depth of the cut hole will be appropriately deepened in the axial direction 10 ~ 30cm.
1.2.2 Class II weak or Class I surrounding rock can be pried. This kind of surrounding rock is extruded fault gouge or fault filling in large fault zone, and the shaft rock with large folds has no self-stabilizing ability after losing support; This kind of surrounding rock excavation method includes: retaining core soil method, small pilot hole method (also called eye method) and upper step method.
(1) The core part of tunnel face is excavated by the method of retaining core soil without moving. Dig a ring with a width of about 1 m in the circumferential direction with a pick or shovel, support the steel frame and spray steel fiber concrete for initial support, then excavate the position of the next grid steel frame, enlarge the circumferential opening while excavating, excavate a part of the core soil with mechanical cooperation after several cycles, and then proceed to the next cycle.
(2) Small pilot tunnel excavation (eye method) Excavate a pilot tunnel with a width of about 2 meters and a height of 2 meters on both sides. After excavation, support both sides first, then expand the excavation from both sides to the vault, and support while digging. After digging to a certain depth, it is generally 4 ~ 5 meters. When excavating, block the wall and advance accordingly.
(3) The upper abutment excavation method is to construct a retaining wall at the lower part, and manually excavate a pilot hole with a height of 1.5 ~ 2m at the arch part, first supporting the vault, then digging to a certain depth at the top, and then excavating and supporting to both sides in turn. This excavation method is not easy to make the pilot hole too deep.
2. Support includes pre-support, primary support and secondary support.
2. 1 Advance support Advance support must be used in two situations, one is that the tunnel needs advance support when it starts to form a hole, and the other is that it encounters wide weak zones, faults or folds, because in these two situations, not only can short footage be used to avoid landslides, but also support measures must be taken in advance, and advance small pipes or large pipe sheds must be drilled.
2. 1. 1 Small catheters are generally different in length, such as 3m, 4.5m and 6m. The specific length depends on the depth of the sliding surface and the broken surrounding rock. The diameter of conduit hole is 42mm, plum blossom hole is drilled in the hole wall, and grouting is carried out after entering the surrounding rock.
2. 1.2 The general length of large pipe shed is 20m, and the pipe diameters are φ 105, φ 135 and φ 150. The selection of pipe holes can be determined according to the existing construction conditions, which can generally meet the requirements. Large pipe shed is generally used in the construction of large fault zone, and the vault is driven along the axis of the hole with a spacing of 50 ~ 75 cm, and then grouting is carried out. Form a consolidation circle with a thickness of about 3m within 20m of the surrounding rock to be excavated, and then proceed to the next excavation.
2.2 Initial support Initial support includes steel fiber concrete, I-shaped steel frame and bolt support. Suitable for different kinds of surrounding rocks.
2.2.1Ⅲ surrounding rock support forms can be directly sprayed with concrete after excavation, and the spraying thickness is 10 cm ~ 14 cm. In order to ensure that the sprayed concrete reaches the standard and the rebound rate is within the control range, wet spraying machine spraying must be adopted, and the steel fiber is of Jiamike RC6535BN type, and the dosage is 30kg/. 3. The average thickness of the jet is greater than the design value, the minimum thickness is greater than 6cm, and the radial direction is locked with WTD25 anchor rod, and the length is 3 ~ 3.5 m. ..
2.2.2 Grade Ⅱ ~ Ⅲ surrounding rock support, after steel fiber concrete is sprayed, grid steel frame is erected, and the spacing is about 100cm.
The reserved deformation of surrounding rock is 0 ~ 8 cm when these two support forms are excavated, depending on the deformation of surrounding rock, so as to avoid vault shrinkage after excavation and intrusion into the clearance after secondary stable deformation.
2.2.3 The thickness of shotcrete is increased to 18 ~ 22 cm, and the steel fiber content can be increased to 35Kg/m according to the situation. 3. The grille becomes an I-beam, which can be 16.
~ ~22cm steel frame, the steel frame spacing is 50 ~ 80 cm, the reserved deformation is 8 ~ 65438+50~80cm, and the anchor length can be 3.5 ~ 6 m. If necessary, the anchor can be tensioned to reduce the stable deformation. During the whole initial supporting process, if there is a water-rich section, water must be diverted by a drainage pipe, and then concrete is sprayed.
2.3 secondary support secondary support is an important guarantee to strengthen primary support and completely prevent water leakage.
2.3. 1 Conditions of secondary support After the primary support is completed, secondary support, lining and inverted arch pouring are carried out, and the time of support and control is directly related to the safety of lining structure. Premature construction will make the secondary support and initial lining bear greater surrounding rock pressure; Too late support is not conducive to the stability of initial support, and the following points should be met during construction:
(1) The displacement rate is obviously slowed down and the surrounding rock is basically stable.
(2) The displacement has reached 80% ~ 90% of the total displacement.
(3) The side wall displacement rate is less than 0. 1 ~ 0.2 mm/d, and the vault subsidence rate is less than 0.07 ~ 0.15 mm/d. 。
2.3.2 pouring inverted arch The center of inverted arch is on the middle vertical line of tunnel highway, and the radius is 2 ~ 3 times of the radius of tunnel vault. The main consideration of increasing the radius is to reduce the amount of excavation and backfilling. The thickness of inverted arch is generally 40cm, with double reinforcement and upper and lower protective layers of 5 ~ 6 cm respectively. Fill with subgrade concrete. In this way, it can form a ring with the second lining concrete. As we know, the ring is the most ideal shape when it is subjected to peripheral pressure, and inverted arch construction is mainly based on this consideration.
3. Second lining concrete
The secondary lining concrete can reinforce the primary lining and completely prevent water seepage. If the tunnel excavated and initially lined is only a semi-finished product, then the secondary lining is basically a finished product. The secondary lining includes geotextile, waterproof board and secondary lining concrete.
3. 1 350g/m geotextile? 2, laid on the initial lining concrete, mainly for the protection of waterproof board.
3.2 Waterproof board, thickness 1mm, mainly plays a role in preventing leakage of secondary lining, and is an important part of secondary lining. The bonding along the extension direction of the tunnel should not be less than 50cm, and the circumferential lap should not be less than 10cm. Bonding must be dense, and there can be no holes and spots.
3.3 The secondary lining concrete is reinforced concrete, and its thickness varies according to the type of surrounding rock, ranging from 35 to 50cm. It integrates the short side wall, inverted arch and subgrade concrete to form a complete ring arch.
Between the primary lining and the secondary lining, it is necessary to lead the drainage pipe from behind the geotextile to the drainage ditch. Set a complete drainage pipe with the same pile number at the circumferential position of the secondary lining. Under normal circumstances, at least one drainage pipe should be set every 15m, and it can be added in special water-rich areas according to the actual situation to facilitate smooth drainage. Every two sections of secondary lining concrete joints must be provided with water stops to prevent water leakage at the joints.
In the whole tunnel construction process, excavation is the main control norm, and the main purpose of primary support and secondary support is to change passive support into active support. In order to achieve the purpose of water plugging, water diversion, drainage and leakage prevention.
4. Concluding remarks
Through the strict technical control of the above-mentioned key procedures of tunnel construction, the qualified rate of Liujiagou tunnel construction quality reaches 100%, which fully meets the design requirements.