5. 1 Control the settlement and convergence of the arch foot

The difference between loess tunnel and stone tunnel is that the arch settlement and arch foot convergence are great. In view of this situation, we have taken the following measures.

5. 1. 1 monitoring measurement

In the process of tunnel construction, starting from the first group of lining, three observation points are buried in the vault, and two arches are buried in a section. The horizontal convergence value and vault subsidence value of the arches are observed by convergence meter and level respectively. With the progress of tunnel construction, an observation section is made every 10m, and the front section is closely followed by the tunnel face. Observe 1 time every day for the first 7 days, then 1 time every month, and then 1 time every month.

Through observation, it is found that from the completion of the first lining construction of the vault to the excavation of the side wall, the vault sinks 6-8 cm and the arch foot converges 1-2 cm. After the excavation of the side wall, the arch part sinks for the second time, the sinking amount is 6-8 cm, and the convergence amount is basically unchanged.

According to the observation results, timely feedback to the construction, change the original reserved settlement from 15cm to 17cm. And the thickness of the secondary lining is effectively ensured.

5. 1.2 Close the inverted arch in time.

According to the measurement, after the initial concrete support is completed, the tunnel vault sinks about 12- 16 cm. In order to avoid further deformation, inverted arch concrete should be closed in time. It is advisable to keep the distance between the inverted arch and the horse's mouth 20cm, not too big. Inverted arch is constructed by half-width method to ensure the operation of upper and lower pilot tunnels. When half of the construction is about 30cm, change to the other half.

5. 1.3 Close the permanent support of secondary lining in time.

When lining is capped once, the capped concrete may not be dense due to the limitation of working face. However, the pressure of inward extrusion at the arch waist on both sides is great. If the permanent support of secondary lining is not applied in time, it is easy to cause vault cracking. In view of the above situation, we equipped each hole with 1 side arch concrete lining steel formwork trolley during construction, and the secondary lining construction was carried out simultaneously with the upper and lower holes.

5. 1.4 Add locking foot anchor.

Add two foot lock anchors on each side of the arch foot. The anchor rod is φ40 steel pipe, 2m long, and is driven into the soil 1.5m perpendicular to the side of the arch foot. The remaining 0.5m shall be poured together with the initial lining concrete.

5.2 Prevention of Landslide

During excavation, the principles of short excavation and strong support should be followed, and the soil quality should be observed at any time. When the soil is loose and joints are developed, the circulating footage should be increased and adjusted, the number of steel pipes in the pipe shed should be increased, and the lining should be followed up in time to shorten the time interval between excavation and lining. In case of landslide, it must be tamped with concrete of the same strength grade.

6. Conclusion

The excavation method of loess tunnel with positive steps arch first and then wall, circular arch and core retaining soil has the following advantages:

(1) Compared with the anchor-shotcrete method, it has the advantages of fast construction speed, safety and low investment. The average monthly footage of a single port is 90m, and the monthly footage of a single port is 120m.

(2) It can effectively control the collapse, arch ring settlement and convergence, make the dimensions of each part of the tunnel meet the design requirements, and ensure the thickness of permanent supporting concrete.

Matters needing attention in loess tunnel construction

A, the construction preparation stage matters needing attention

1. Before tunnel construction, the technical personnel of the construction unit and supervision unit shall be organized to study the technical specifications for highway tunnel construction seriously and be familiar with the design documents. By inviting experts to give special lectures on loess tunnel construction technology, the technicians of construction units and supervision units can master the construction characteristics of loess tunnels and ensure the professional quality of employees to meet the requirements of loess highway tunnel construction.

2. Organize design technical disclosure and blue prints.the design, urge the construction unit to fully understand the spirit of the design documents, check the problems existing in the design drawings with the design unit, and organize the design and construction unit to check whether the topographic and geological conditions are consistent with the site conditions.

3. Check whether the construction control survey data of the construction unit is approved by the supervision unit and meets the specification requirements, and check whether the implementation construction organization design is scientific, reasonable and feasible.

4. Check whether the construction site layout of the construction unit is reasonable and safe, especially the setting of facilities such as dangerous goods storage warehouses and the handling of dangerous goods procurement procedures.

5. Check and accept the ancillary works, temporary facilities, equipment configuration and laboratory construction of the construction unit.

6, review the construction, supervision, test unit personnel, equipment in place, the establishment and improvement of system measures.

7, review the construction, supervision and other units of employees pre-job safety education and training and related certificates.

Second, the construction stage matters needing attention

1, check whether the construction method of the construction unit is consistent with the construction organization design.

2. Check the completeness of the shallow buried section of the hole, slope protection and surface drainage system. Check and accept the backfill treatment of loess cave at the top of the cave.

3. Check whether the open-cut tunnel construction is timely and standardized, and whether the supporting measures for the safety construction of the tunnel entrance are reasonable and effective.

4. Further verify and check the excavation construction methods, supporting methods, geological conditions, surface subsidence and other changes in the shallow buried section and reinforcement section of the portal, and verify whether the construction safety guarantee measures are effective and reliable.

5, check whether the building waterproof and drainage system is smooth, comfortable and perfect.

6. Check whether the circulating footage of tunnel excavation, the size of core soil, the length of side wall pilot tunnel and the length of makou excavation are reasonable and conform to the requirements of specifications.

7. Check whether the thickness of arch frame, advance support and lining concrete meets the design and specification requirements.

8. Check whether the surface subsidence and cracks at the top of the cave and the vault subsidence and convergence in the cave are monitored according to regulations, and learn more about the monitoring and measurement.

9. Check whether the anchor length, angle, spacing and steel arch spacing meet the design requirements.

10, check whether the closure of inverted arch and the distance between secondary lining and working face meet the design and stability requirements, and adjust them in time according to surrounding rock conditions and measurement data.

1 1, check whether the waterproof and drainage system of the structure meets the design requirements;

12, check the tunnel construction site layout, electricity, ventilation, water supply, lighting, drainage and other facilities are safe, reliable and standardized, and the brightness and ventilation effect in the tunnel meet the specification requirements.

13, check whether the drainage and supporting facilities of the auxiliary tunnel are standardized, reasonable, safe and reliable.

14, check whether the site information provided by the construction unit is timely and accurate, including whether the construction log, inspection application form, test data and geological records are complete. To be filled in and signed by the supervisor.

15, check whether the sampling data and supervision log of the supervision unit are complete.

16, check the implementation of supervision work instructions by the construction unit, whether the problem has been solved and whether the instructions have been closed.

17, check whether the construction unit has implemented the advanced geological forecast required by the design documents and construction specifications, and whether there are corresponding countermeasures and preparations for adverse geological conditions.

18, spot check the safety facilities and personnel in place of supervision, design and construction units, whether the personnel qualification meets the requirements, and whether the safety organization and system are established and improved.

19, check the contract implementation of supervision, design, construction and test units.

20. Timely investigate and coordinate the handling of geological diseases and engineering quality accidents in construction, and organize relevant departments to put forward prevention and treatment measures for corresponding problems existing in other tunnels.

Three. Matters needing attention in emergency handling of events

1, check whether the safety emergency rescue plan of the construction unit is effective, and urge it to carry out emergency rescue drills on schedule.

2. Supervise and organize the safety education and safety technical training of the project contractor's personnel.

3. For special events in construction, such as mud outburst, landslide, serious ground subsidence, cracking and collapse, etc., the construction unit should be urged to start the emergency plan, take corresponding construction measures in time to ensure that the accident will not continue to develop and deteriorate, and report the situation to the superior in time, so as to organize corresponding experts to deal with it in time.

4. According to the relevant regulations of the state, the safety accidents in the construction shall be reported step by step within the prescribed time limit, and shall not be concealed, and the construction unit shall be urged to protect the site and conduct search and rescue work for the trapped personnel in time.

5. In the process of handling safety accidents, we should actively assist the superior transportation departments and local safety production management departments, provide relevant design, construction and management information in time, and help analyze the cause of the accident.