An Experimental Study on the Explosion Disposal of the Boulder in the Metro Engineering Stratum

An Experimental Study on the Explosion Disposal of the Boulder in the Metro Engineering Stratum Shi Youzhi1,2 1. Associate Professor, School of Civil Engineering and Architecture, Xiamen University of Technology, Xiamen 361021，China 2. Post-doctoral, School of Naval Architecture, Ocean & Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 3 Hua Jianbing 3. Department of architectural engineering, Hefei University; Hefei, 230013 Corresponding author e-mail: [email protected] Lin Shuzhi4 4. Chief engineer , Xiamen Construction Bureau, Xiamen 361003, China Ge Xiurun2 2. Professor , School of Naval Architecture, Ocean & Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT When applying the shield method in the metro construction, such as the spherical weathering granite (also known as the boulder) in the stratum, the construction will face difficulties and risks. Ground explosion is usually applied for the pre-treatment of the boulder in the construction. Based on the Xiamen metro construction and field experiment, we studied the influence of the size of the boulder, burial depth, degree of the weathering, drilling distance of the powder, the amount of the powder and other parameters on the effect of the explosion to the boulder and thereby we concluded a series of the boulder explosion disposal industrial process. The study shows that: (1) the explosion effect of the larger size boulder is better than the smaller size of the boulder. More drilling holes are supposed to set for the small size boulders. In some local part of the boulders, the number of the drilling holes can be adjusted to 30cm; (2) the burial depth of the boulder has a significant influence on the explosion effect. The shallow boulder has a better explosion effect; (3) when using deep explosion method for the disposal of the boulder, the weathering degree of the boulder has a great effect on the explosion effect, the greater the weathering is, the worse effect of the explosion is; (4) the amount of the powder in one single hole has a great effect on the explosion effect. To better the explosion effect, we suggest that the amount of the powder in one single hole should be more than 4.0kg/m and adjust the amount of the powder properly according to the burial depth of the boulder; (5) the pipelines near the explosion field should be considered in the explosion construction and control the damage degree to the surroundings. The research results can provide reference for the metro construction in the similar place. KEYWORDS: Subway engineering; Boulder; Blasting control; Field investigation - 6707 - Vol. 21 [2016], Bund. 22 6708 INTRODUCTION Granite is widely distributed in the earth’s crust in southern China (Wang, et al., 2011). The protruding edges of massive rocks are susceptible to weathering, which leads to the tapering edges and tends to be spherical and finally becomes spherical-like weathering body, more commonly known as boulders (Zhang, et al., 2014; Zhang, et al., 2014), are formed underground. Boulders are normally 0.4m~4m in diameter, and 70~130MPa in terms of uniaxial compressive strength, with an average of 90MPa (Wu, et al., 2015). The presence of boulders can result in great risks and difficulties for subway construction projects where tunnel boring machines (TBMs) are used, for example, jamming the cutter head, causing wear and tear on the cutting tools, deviating the TBM from the tunneling axis, to name a few. In more severe cases, the ground surface would sink and collapse (Shirlaw, et al., 2000; Babendererde, et al., 2004). Blasting, as an efficient, economical and convenient way of processing boulders, has found its wide application. So far, there have been few studies of boulder blasting, and even fewer in-situ experimental studies of blasting parameters. As different boulder has different shape, mechanic properties and surroundings, each parameter, charge amount and charge structure needs to be adjusted when using blasting methods for the boulder. There would be severe consequences without proper design or construction. Therefore, the situ-experimental study of the boulder and the summary of the boulder blasting processing has significant meanings to solve the boulder problems and guarantee the construction safety. RESEARCH STATUS Domestic and foreign literature has various English names for spherical weathering body or boulder, such as “spheroidal weathering body”, “weathered granite residual”, “spherical weathered granite”, “globular weathering body”, “globular weathered body”, “corestones”, “bolas”, “boulder”, “solitary stone”, etc. This also shows that domestic and foreign study on the boulder is limited. In this paper we use the name of the boulder. As for the cause of the boulder, Røyne A, et al. (2008) believes boulder is formed by volume expansion and continuously layered peeling, a coupling result of physical and chemical weathering; Lan H X, et al.(2003) believes boulder’s physical and mechanical properties and weathering degree is connected to its location. Medium and weak weathering rocks have big difference to construction. As such rocks often have high strength and undeveloped cracks, they make it difficult for construction and the identify of bedrock surface. To reduce the adverse effect of the boulder on shield construction, we usually detect the position of the boulder by investigation in the first place. Then big boulder is decomposed into small pieces of boulder by pretreatment which cannot harm the shield machine. Before the shield construction, it is a common method in engineering to blast the boulder from ground boreholes. Zhu W B, et al.(2011) processed boulders by means of controlled drilling and blasting (drilling holes on the ground, filling with explosives and detonating explosives); You Y F, et al.(2012) systematically summarized the influencing factors of blasting and established a comprehensive assessment system for the risks associated with blasting of boulders and bedrock; on top of that, he defined the weight and membership function of each factor with reference to the analytic hierarchy process (AHP) and the fuzzy set method, and finally evaluated their respective risk Vol. 21 [2016], Bund. 22 6709 acceptance level. Wang G Z, et al. (2014) introduced a feasible scheme to blast the boulders at the intake tunnel of Taishan Nuclear Power Plant in Guangdong Province. He Q, et al.(2014) presented a set of safe blasting practices. Zheng L J (2014) and Lu Y B, et al. (2014) suggested a variety of boulder treatment methods during the construction of shield driven tunnels. Wu S F, et al.(2015) developed a model to test the density, moisture content and shear strength of soil before and after blasting, in order to reveal the range of blasting disturbance. The most frequently applied boulder blasting technique is called “shallow-hole blasting”, and the effect of blasting only requires being cracked or shattered rather than being smashed into pieces. Compared with large-scale rock blasting, boulder blasting needs lower doses of explosive. Since boulders are usually hard and brittle, if not enough explosive is used, the expected effect can hardly be achieved; by contrast, if the amount of explosive is excessive, flying stones are very likely to be produced, and due to their irregular shapes, it is difficult to determine the line of least resistance. Besides, because boulders vary from one another as regards form, mechanical property and ambient environment, different boulders should be processed differentially by adjusting various parameters, explosive load and explosive charge structure. Any defect or impropriety in design or construction can cause serious damages. Given the limited research thus far on the parameters of boulder blasting, this paper, revolving around the Xiamen Metro Project, studied the influence of an array of parameters (the size, burial depth and weathering degree of boulders, the space between boreholes, and the amount of explosive) on the effect of blasting through field tests, and summarized a full set of boulder blasting techniques, hoping to provide a reference and guidance for shield tunneling projects carried out in similar strata. The rest of the paper’s layout is as follow. Third chapter introduces the project overview and determines the boulder’s size, burial depth, shape and other parameters; fourth chapter introduces the situ-experiment of blasting, including experiment process, parameters design, experiment details, etc. Plus, it also evaluates the blasting effects; the last chapter summarizes the whole paper and gives relevant conclusion. PROJECT OVERVIEW Project Background The section between Software Park and Jimei Avenue stations is a part of the Xiamen Metro Line 1. The mileage of the start station is DK27+953.022 and that of the end station is DK28+897.069. According to the plan of this section, the route departs from Software Park, runs northeastward with a radius of 350m, and then enters Jimei Avenue with a radius of 800m, as shown in Figure 1. Vol. 21 [2016], Bund. 22 6710 Figure 1: Schematic maps of the shield method in Software park-Jimei Avenue zone According to the vertical profile of this section, the route takes the form of a V slope. The burial depth of the tunnel roof is about 4.4～12m. The maximum gradient is 26‰ and the minimum gradient is 2‰. Shield tunneling is employed in the construction of the left tunnel ZK27+998.023～ZK28+897.069 as well as the right tunnel YK27+989.323～YK28+897.069. The strata in this section

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