In the construction of rock burst area of Erlangshan highway tunnel, effective rock burst prevention measures must be taken to ensure the safety of construction personnel and equipment, and ensure the smooth construction and timely completion of the tunnel.

7.3. 1 Comprehensive prevention and control measures of rockburst proposed by the design unit and their opinions.

Up to now, the shotcrete support design of most tunnel projects still relies only on the analogy method of similar projects. Firstly, the supporting parameters are selected by consulting relevant specifications and parameter tables, and then optimized and adjusted by monitoring and measuring. Generally speaking, it is still in the stage of "empirical or semi-empirical design". Similarly, the design unit of Erlangshan Highway Tunnel, the First Highway Survey and Design Institute of the Ministry of Communications, put forward the following three comprehensive prevention and control measures for rock burst in the design report.

7.3. 1. 1 Strengthen monitoring and forecasting, and strictly implement construction measures.

(1) Strengthen the prediction and analysis of initial stress and rockburst of rock mass;

(2) Smooth presplitting (weak) blasting;

(3) excavation step by step, short footage, multiple cycles and timely support;

(4) Strengthen construction organization and management, and improve safety awareness; Remove dangerous stones in time and strengthen the protection measures for personnel and equipment.

7.3. 1.2 initial support (surrounding rock reinforcement) measures

(1) Tunnel face is reinforced with φ40mm slotted anchor rod in advance, and the surrounding area is reinforced with φ22mm mortar anchor rod; In the system, a steel pad is added to the end of the anchor rod, and the subsequent part is prestressed;

(2) spraying C20 concrete and laying φ 600 ~ φ 8 mm steel mesh;

(3) The surrounding rock is reinforced by the combined support of shotcrete, anchor, net and grid steel frame, and controlled by monitoring and measuring the surrounding rock.

7.3. 1.3 Improve the stress condition of surrounding rock.

(1) Sprinkle water on the excavation face and tunnel face in time after blasting;

(2) Pre-injecting high-pressure water into the advanced slotted pipe anchor rod;

(3) The upper half of tunnel face is closed by shotcrete.

See Table 7-4 for the support and lining type parameters of rock burst areas at all levels proposed by the design unit.

To sum up, in the comprehensive prevention and control measures of rockburst proposed by the design unit, it is generally desirable to strengthen monitoring and forecasting, strictly construct and improve the stress conditions of surrounding rock, but the initial support and surrounding rock reinforcement measures obviously have the following shortcomings:

(1) As can be seen from Table 7-4, there is little difference in lining type parameters of rock burst support at all levels. Obviously, the prevention and control measures of rockburst at all levels lack pertinence and rationality;

(2) The span of moderate rockburst (Grade II) is too large, and the correct scale and operability are poor in the actual construction process, which is obviously not conducive to the reasonable optimization of engineering design and measures;

(3) Engineering measures for rock burst prevention at all levels are too conservative. For example, from weak rockburst (Grade I) to strong rockburst (Grade III), φ40mm advanced slotted pipe anchor is used to reinforce the tunnel face, and C20 concrete with a thickness of 4cm is sprayed to seal the tunnel face during the excavation period. It is considered that inverted arch can be set up when necessary to improve the stress condition of lining structure and prevent the tunnel floor from bursting and collapsing. Therefore, if these rock burst prevention measures are taken indiscriminately in rock burst areas at all levels, not only the financial and material conditions are not allowed, but also the construction progress and construction period will be seriously affected.

Table 7-4 Parameter Table of Lining Types of Rock Burst Support at All Levels in Erlangshan Highway Tunnel.

(According to the First Highway Design Institute of the Ministry of Communications)

7.3.2 rockburst prevention measures at all levels in RMS scheme and their trial results

In order to overcome the defects of rock burst prevention and control measures proposed by the original design unit, on the basis of fully soliciting the opinions of the tunnel construction unit, supervision unit and senior engineer Han Changling, and according to the key-solving practice of rock burst area construction at all levels of Erlangshan highway tunnel, Based on the experience of foreign countries in preventing rockburst, such as Heigula Highway Tunnel (Norway), Hogg-Lanfjord Tunnel (Norway), Gransaso Highway Tunnel (Italy), Guan Yue Highway Tunnel (Japan), Taipingyi and tianshengqiao hydropower station Diversion Tunnel, the prevention measures of rockburst at all levels corresponding to RMS rockburst intensity grading scheme are put forward.

7.3.2. 1 Improve the material properties and stress conditions of surrounding rock.

According to the experience of rock burst prevention and control at home and abroad, short footage should be adopted when drilling and blasting method is used in rock burst area; Reduce the charge and blasting frequency, control the smooth blasting effect, and reduce the stress concentration on the surface of surrounding rock. Mild rockburst (Grade I) and moderate rockburst (Grade II) areas: generally, the footage is controlled at 2 ~ 2.5m, and the whole section shall be excavated as far as possible to reduce the damage to the stress balance state of surrounding rock; Water is often sprayed on the tunnel face and tunnel wall, and the stress release method of drilling ahead is adopted when necessary to form a local stress release zone, thus reducing (weak) rockburst. Severe rockburst and severe rockburst (Grade IV) area: generally, the footage is controlled within 2m, and 1/3 can be reserved for excavation in two parts if necessary to reduce the damage degree of rockburst; Pre-drilling pressure relief, loose blasting or vibration blasting can be used to reduce the stress of rock mass and release energy in advance before excavation. When necessary, high-pressure water can be injected into the rock mass of the tunnel face evenly and repeatedly to reduce the strength of the rock mass.

Initial Support in 7.3.2.2 (Strengthening Surrounding Rock)

After the excavation in the rockburst area, steel wire mesh shotcrete and bolt support should be carried out in time to achieve the goal of "combining rigidity with flexibility"; From another point of view, even if rockburst happens again after the completion of wire mesh shotcrete and anchor support, they constitute the "first line of defense" and will not endanger the integrity of construction personnel and equipment. Initial support measures are as follows:

7.3.2.2.1Generally speaking, C20 concrete should be sprayed in three cycles.

(1) After each drilling and blasting, find the roof in time and remove dangerous stones, and then spray concrete on the vault for the first time (initial spray thickness is 5 cm); Slag operation is carried out after initial spraying to complete a cycle;

(2) During the second excavation, after the bench is in place, the system mortar anchor rod is laid in the first shotcrete area, and then the steel mesh is hung for the second shotcrete (thickness of 5cm), and then drilling and blasting → smoke exhaust → slag discharge are carried out to reinforce the surrounding rocks on both sides of the wall;

(3) During excavation, concrete should be sprayed in time according to the occurrence, development and evolution of rockburst, and the thickness of sprayed concrete layer is generally 2 ~ 5cm；; In areas with serious rockburst, steel mesh should be repaired in time to improve construction safety.

System anchor

According to the experience at home and abroad, the anchor rod of the system should not be too long, generally controlled at 2 ~ 3.5m, arranged in plum blossom shape, with higher density than ordinary anchor rod (i.e. dense anchor rod), and the length and spacing of anchor rod depend on the strength of rockburst. The purpose of using dense anchor is:

(1) easy to hang the net;

(2) Prevent the occurrence of large rocks from bursting, loosening, falling and ejection;

(3) Combined with spray net to form a system, surrounding rock can be fully reinforced.

Usually, after each workbench is in place, the system anchor is applied first.

steel mesh

The method of hanging "integral net" is adopted, instead of simply using the sheet hanging net proposed by the original design unit, that is, several long steel bars and anchor rods are welded vertically and horizontally to form a basic skeleton in the same period, and the prepared sheet hanging net is welded at the same time to form an integral structure, which is arranged close to the surrounding rock. The purpose of adopting "whole network" is:

(1) In this circulating operation, the steel mesh is fused with each other, and the rockburst surface will not slide down again due to local surrounding rock;

(2) connected with the anchor rod;

(3) Spray, anchor and cymbal are integrated.

After the anchoring of the system is completed, the steel mesh shall be placed immediately.

7.3.2.3 Secondary Lining (Permanent Support)

According to the Code for Design of Highway Tunnels (JTJ 026-90) and the model test data of surrounding rock and supporting structure of Erlangshan Tunnel of Southwest Jiaotong University (1998), after the initial support of surrounding rock in rockburst area, the tunnel can be permanently supported by plain concrete lining with a thickness of 35cm originally planned by the design unit.

See Table 7-5 for the specific prevention and control measures of rock burst at all levels corresponding to the root mean square value of Erlangshan Highway Tunnel proposed by our research group. These preventive measures were gradually tested, summarized, supplemented and improved with the construction unit in the tunnel construction practice, and achieved good trial results in the tunnel engineering practice (Table 7-6), which can be used as a reference for other similar underground engineering construction practices.

Table 7-5 Table 7-5 Rock Burst Prevention Measures at All Levels of RMS Scheme for Erlangshan Highway Tunnel

Table 7-6 Experimental Effect of Rock Burst Prevention Measures of RMS Scheme at Various Levels in Several Rock Burst Areas of Erlangshan Highway Tunnel Table 7-6 Rock Burst Prevention Effect of Erlangshan Highway Tunnel

sequential