Han Zongze
Shanxi Road and Bridge Construction Group Co., Ltd. general contracting branch
Abstract: In order to solve the problem of rock burst that may occur in the construction of highway tunnel, based on the law and classification of rock burst occurrence, combined with the actual situation of a highway tunnel project, and on the basis of clarifying its rock burst situation and characteristics, relevant prevention and control measures are proposed, such as doing a good job in construction organization, promoting high ground stress release, avoidance measures, strictly controlling excavation inlet and blasting, and strengthening the primary support. Practice has shown that these rockburst prevention and control technologies are feasible and effective.
Keywords: highway tunnel; Tunnel rock burst; rock burst prevention;
About author:HAN Zongze (1986—), male, from Yangquan, Shanxi, engaged in highway engineering construction and management.
0 Introduction
Due to the large burial depth in the middle section of the highway tunnel, rock bursts are likely to occur when the surrounding rock integrity is good. Due to the sudden nature of rock bursts, it is difficult to predict the direction and distance of the thrown stones, which poses a safety threat to tunnel construction personnel and equipment, so it is necessary to explore effective prevention and control measures based on understanding the characteristics of rock bursts.
1 Rockburst law and classification
1.1 Rockburst law
Rock bursts mainly have the following rules:
(1) Rock bursts usually occur in tunnels with relatively large buried depths, because only the buried depth of the surrounding rock is large, it is possible to produce in-situ stress that causes rock bursts, and the probability of rock bursts increases due to the increase of the buried depth of the surrounding rock.
(2) Rock burst is related to the lithology of surrounding rock. Rock bursts usually occur in surrounding rocks with hard and brittle texture, good integrity and relatively high grade, such as quartzite, granite and limestone, which are all surrounding rock types with a high probability of rock eruption.
(3) Surrounding rocks in the joint development and fault fracture zone usually do not occur rock bursts, which shows that rock bursts are affected by the surrounding rock structure [1].
(4) Rock burst and surrounding rock water content also have a certain relationship. When the buried depth of the tunnel, the lithology and structure of the surrounding rock are identical, the drier surrounding rock is more prone to rock burst, and the surrounding rock of the section with groundwater is basically not rock burst.
(5) When the tunnel excavation work face is relatively rounded and the smooth surface blasting effect is relatively significant, once a rock burst occurs, the rock burst intensity will be much higher than the relatively large surrounding rock section under excavation.
(6) The surrounding rock section with a relatively high probability of rock eruption is usually not more than 300% of the excavation width at a distance from the face of the face, that is, the closer the distance between the face and the face of the face, the more prone to rock burst.
(7) The location prone to rock bursts on the excavation surface is mainly the arch and the upper part of the side wall, that is, the larger the distance from the ground, the more likely it is to occur rock bursts.
(8) In terms of time, rock burst mainly has the following rules: the first 3h of blasting and excavation construction on the palm face is the most prone to rock burst, and the rock burst intensity is large; Within 7 days of blasting and excavation, although rock bursts still occurred frequently, the intensity gradually decreased. Within 2 months of blasting and excavation construction, rock bursts may still occur; After 2 months of blasting and excavation, rock bursts are not prone to occur. In addition, with the passage of time, the location of rock eruption will continue to develop deeper, mainly manifested by internal fractures of the rock formation, and muffled noises can be heard inside the rock formation.
(9) Rock bursts are often accompanied by bursting sounds: if there is a tearing sound, it means that a rock burst caused by compression has occurred, because this rock burst can only occur on the surface of the surrounding rock, so the sound is crisp, the cracks generated by the rock burst are parallel to the rock wall, and after the rock burst is over, the rock block will appear flaky; If there is a dull sound, it indicates that a rock burst has been cracked by compression shear, and the rock block will appear lumpy after the end of the rock burst, and many arc-like cracks can be seen on the rock surface, and the cracks are accompanied by scratches. Typically, the former type of rockburst is not long-lasting and not very destructive, while the latter is relatively long-lasting and can cause more serious damage [2].
1.2 Classification of rock bursts
Depending on the intensity, rockbursts can be divided into the following 3 categories:
(1) Weak rock burst: the sound is small, the rock block generated after the rock burst is not large in size, the number is small, and the form of production is mainly dropping.
(2) Medium and strong rock burst: manifested as the sound similar to firecrackers emitted inside, the diameter of the rock pit after the rock burst can reach 1~2m, accompanied by a large number of rock blocks, and the form of ejection is the mainstay.
(3) Strong rock burst: manifested as a violent sound inside, the diameter of the rock pit generated after the rock burst exceeds 2m, accompanied by large rocks that pop out quickly [3].
2 Rock burst prevention
A tunnel is a split double-hole tunnel, with left and right line lengths of 7 097.897m and 7 120m respectively, and the buried depth is large, reaching 1 286m at the maximum point. The surrounding rock types in the middle section of the tunnel include limestone and limestone, and the production is thick bedded, large masonry form, good integrity, strong weathering resistance, and almost no groundwater. Based on the law and classification of rock bursts, effective rock burst prevention and control measures are discussed based on the actual situation of the expressway tunnel.
Among the rockbursts that occurred during the construction of the tunnel, the medium strong type had the largest range, followed by the weak rockburst, and the stronger rockburst had the smallest range. The rock burst intensity is mainly affected by the buried depth of the tunnel, and the burial depth of the weak rock explosion location is usually 800~950m. The burial depth of the eruption formation position of medium-strong rock is 950~1200m. The burial depth of the eruption sites of the stronger rocks generally exceeds 1 200 m.
According to the requirements of the specification, for tunnels where rock bursts may occur, the basic principle of "prevention first, prevention and control" should be strictly followed in construction. Combined with the rock eruption generation law of the tunnel, the following treatment measures were taken during the construction.
2.1 Do a good job in construction organization
During the construction of the section where a rock burst occurs, special personnel should be arranged to be on duty all day at the face of the face, and a special monitoring working group should be set up to carry out special safety education for all participants in the rock burst prevention and control work, and at the same time strictly implement all kinds of equipment and materials required for rock burst prevention and control.
2.2 Promote high stress release
(1) On the tunnel face with the help of a DTH drilling rig to continuously drill out advanced drilling, the hole depth is controlled by 10~30m, the hole diameter is controlled by 70~90mm, and the number of holes needs to reach 5~8 to promote the early release of high ground stress.
(2) For the surrounding rock section where a strong rock burst occurs, the excavation method needs to be changed, that is, the excavation of the full section is changed to the excavation of the lower guide pit in the lower part of the section, and its width and height are controlled by 5m and 4m respectively. After the excavation of the lower guide pit is completed, the excavation section can be expanded. In addition to promoting the release of high ground stress, the excavation of the guide pit can also weaken the destructive force of flying stones, thereby reducing the hazard of rock burst, and the treatment of dangerous stones is more convenient than the full-section method. After the above treatment, no strong rock burst occurred during the entire construction process. When excavating the lower guide pit, in order to prevent the rock burst from intensifying, first do a good job of roof protection, and then carry out ballast, the specific methods are: (1) blasting and excavation according to the design requirements; (2) When using thin steel plates for protection, spray water on the arches and side walls of the surrounding rock that are directly exposed in the distance; (3) Concrete spraying of wet arches and side walls, with a spray thickness of 70mm, using concrete with strength grade C25 and mixed with steel fiber; (4) After the completion of concrete spraying, set protective anchors on the arch and side walls, and hang steel mesh; (5) After the above roof protection construction is done, the ballast can be started; (6) Set protective anchors in the lower part of the side wall and hang steel mesh.
(3) For other types of rock bursts except for strong rock bursts, it is necessary to set drill holes with a depth of 10m and a hole diameter of not less than 70mm to the arch, side wall and excavation direction on the face of the face, and inject water into the borehole at high pressure to continuously spray cold water for the face and the arch and side wall of the surrounding rock, so as to soften the surrounding rock, promote stress release, and reduce the probability and intensity of rock eruption.
(4) When conditions permit, cracks can be created inside the surrounding rock by loose blasting to promote the release of high geostress.
2.3 Avoidance Measures
(1) Considering that the probability of rock bursting is the largest in the first 3h after the start of blasting, construction is suspended during this period to reduce or avoid the harm caused by rock bursts.
(2) When constructing the surrounding rock section where the rock burst occurs, special safety personnel should be arranged at the face of the palm to observe and record the actual situation of the rock burst, and be responsible for listening to the sound of the rock burst, grasping the situation of the surrounding rock arch and side wall, and once the danger is found, it should immediately warn and organize all construction personnel to evacuate in an orderly and rapid manner.
(3) When constructing the surrounding rock section where a weak or medium-strong rock burst occurs, the following protective measures shall be taken as appropriate: (1) Install steel plates specially designed to prevent flying stones from hitting machinery such as drilling trolleys and transport vehicles; (2) Set temporary anchors in the arches of the surrounding rock and the upper part of the side wall, and hang steel reinforcement nets at the same time; (3) Concrete spraying of surrounding rock arches prone to rock bursts with the help of jet machinery.
(4) When constructing the surrounding rock section where a strong rock burst occurs, in addition to excavating the lower guide pit, it is also necessary to set up wedge pipe anchors in the lower guide pit and hang steel reinforcement nets to avoid accidents caused by flying rocks. In addition, if necessary, advance support should be done to reduce the hazard of rock burst by setting up steel frame supports.
2.4 Strengthen the initial branch
For different rockburst types, the initial branch can be strengthened according to the parameter requirements shown in Table 1.
Table 1 Initial branch reinforcement parameters of each rockburst type Download the original figure
3 Conclusion
In the construction of the expressway tunnel, nearly 860m long surrounding rock section occurred rockburst, through the analysis and mastery of the rockburst law during the construction, learn from the relevant experience of other projects, put forward the above prevention and control measures, fundamentally reduce the rock burst hazard, ensure the construction safety and progress.
bibliography
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[2] WANG Dengmao, TENG Zhennan, TIAN Zhiyu, et al. Disease treatment and design reflection of unconventional rock burst section of Bamiao tunnel from Taoyuan to Bazhong Expressway[J].Modern Tunnel Technology,2019(4):62-68.)
[3] XING Jun, WANG Jianbin, JIANG Lei, et al. In-situ stress characteristics and rockburst prediction of Guigala Expressway tunnel[J].Hydrogeology & Engineering Geology,2019(2):170-178.)
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