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

Hua Jianbing3 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

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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 are composed of, from top to bottom, artificial fill, alluvial silty clay, marine sludge, sand, eluvia sandy clay, and weathered granite. The weakly and spheroidal weathered granitic rocks developed in soil layers would entail a high risk for the construction of shield-driven tunnels. The unearthed boulders in similar strata are showcased in Figure 2.

Figure 2: the boulder condition of the similar revealed stratum

For better treatment of boulders by means of blasting and a comprehensive summary of blasting parameters through experimentation, the research was conducted in three stages: boulder detection, infill drilling (to determine the size and shape of boulders), and boulder blasting and post processing, as indicated in Figure 3. Vol. 21 [2016], Bund. 22 6711

Figure 3: schematic of the procedure of the ground deep explosion experiment

Boulder Detection Based on the geological condition unveiled at the detailed investigation stage, additional exploration drilling was performed. Out of the 31 exploration drill holes, 4 holes indicated the presence of boulders, which were concentrated in the area within a distance of 142 meters from the Software Park station in the section between Software Park and Jimei Avenue stations. The distribution of boulders revealed by additional exploration drilling is shown in Table 1. Within the site, the bedrock roof, whose burial depth is over 28.3m, is located beneath the tunnel trunk, exerting little influence on the shield tunnel construction.

Table 1: the revealed distribution of the boulder by complimentary detection Hole No. Depth (m) Thickness(m) Lithology Stratum 7.4-8.3 0.9 RJQ-BK-01 9.0-9.6 0.6 Light and middle RJQ-BK-02 6.8-8.6 1.8 Residual sticky weathering granite 6.0-8.3 2.3 sandy soil RJQ-BK-03 boulder 9.5-10.8 1.3 RJQ-BK-05 13.8-14.6 0.8

Determination of Boulder Boundary by Infill Drilling If boulders are found by exploration drilling to be present in the tunnel trunk, it is necessary to identify their shape and location. Since boulders are generally round-shaped and slightly different from each other in width and thickness, and given that drill holes can be reused during boulder treatment in order to cut costs, the drill holes shall be laid in accordance with the following procedures to explore the shape and location of the boulders:

① Drilling circles are arranged by centering on the point location of the explored boulder and 4 drill holes are equidistantly positioned in each circle; The first circle is arranged outwardly based on the central point, with a radius of 1/2 of the thickness of the explored boulder; ② If the first drilling circle cannot find the boulder boundary, then the second drilling circle is outwardly arranged 0.4m away from the periphery of the first circle; ③ If the second circle still cannot find the boulder boundary, then the third circle is outwardly arranged 0.4m away from the periphery of the second circle; ④ The exploration is conducted in a similar fashion until the boulder boundary is detected, as shown in Figure 4. ⑤ Vol. 21 [2016], Bund. 22 6712

Figure 4: Schematic of the determination on the border of the boulder

At the construction site, the boundary of the aforementioned 4 boulders was determined by positioning 54 infill drill holes. Table 2 shows the specifics of the boulders.

Table 2: relevant parameters of the boulder Boulder No. 1# 2# 3# 4# Plane 100×90 240×150 160×140 160×140 size(L×W)/mm Depth/m 11.6 4.4 6.4 5.05 Thickness/m 1.6 2.8 0.9 0.55

FIELD TEST OF BOULDER BLASTING

Boulder Blasting Process The boulder blasting process is indicated in Figure 5. Vol. 21 [2016], Bund. 22 6713

Figure 5: The process map of the explosion construction

Design of Blasthole Drilling Parameters In light of the size, shape and burial depth of the explored boulders at the infill drilling stage, the 4 boulders were numbered and at the same time blasthole drilling was schemed. For the convenience of construction and an accurate control of the drilling direction, the technique of vertical drilling was employed. The drilling machines were geological drilling rigs. The blastholes with a diameter of 100mm were arrayed in a quincunx, and the spacing between holes ranged from 0.5m to 0.8m. The plane layout is shown in Figure 6.

Figure 6: plain graph of the layout of the explosion hole

The blastholes were protected with slurry during drilling, and steel sleeves were inserted if necessary. When the holes were formed, PVC sleeves with a diameter of 80~90mm and with an end cap attached to the bottom were put into the holes for protection. Inside the holes were Vol. 21 [2016], Bund. 22 6714 charged PVC tubes with a diameter of 65~70mm. Blasting was performed by region and by row. After the first row was blasted, with the help of the free surface produced by the extrusion of soil, the second and subsequent rows were blasted orderly. The blastholes in the front row should be 0.2m deeper than those in the back row.

Design of Blasting Parameters

Calculation of Explosive Dosage The factors affecting the unit explosive consumption during underwater blasting include physical and mechanical properties of rocks, free surface conditions, water depth and overlying strata’s thickness, explosive performance, etc (Zhao, 2008). After a comparative analysis of all those factors, a modified Swedish empirical formula was used to calculate the unit explosive consumption (Lu, et al. 2012), as shown by the following equation (1):

q=q1+q2+q3+q4 (1) Wherein: q is the unit explosive consumption (kg/m3);

q1 is the base charge amount, usually 2~3 times of that used in bench blasting. For underwater vertical drilling, there should be an increase of 10% in amount. As far as this research is concerned, the target to be blasted is located at a depth of 6~10m underground, compounded with groundwater. So basically, it is underwater blasting. If the average unit consumption (q1) for the blasting of ordinary deep holes of hard rocks is 0.5kg/m3, then that for underwater drill holes is 1.0kg/m3, and for underwater vertical holes is 1.1kg/m3;

q2 is the increment of water pressure above the blasting area. q2=0.01h2, where h2 stands for water depth (m);

q3 is the increment of cover layer. q3=0.02h3, where h3 stands for the thickness (m) of the cover layer (silt, soil, sand, etc.);

q4 is the increment of rock expansion. q4=0.02 h4, where h4 is the bench height (m). The charge amount per blasthole is calculated as shown in equation (2) (Industrial Standards of the People's Republic of China , 2008):

Q＝qabH0 (2) Wherein: Q is the charge amount per blasthole (kg); a is the hole spacing (m); b is the row spacing (m). The charge amount per blasthole is decided by taking into account the crushing degree of rocks after blasting so as to ensure the smooth work of the shield boring machine. Meanwhile, the vibration of blasting should be monitored to avoid damages on the surrounding facilities. The amount of explosive should be adjusted according to the requirement specified in section of “3.3.4” of this paper. Vol. 21 [2016], Bund. 22 6715

To test the influence of different parameters such as blasthole spacing and charge amount on the effect of blasting, different spacings were assigned to the holes on the 4 boulders (1#~4#). The blasting scheme is presented in Table 3.

Table 3: the design table of the parameters for the explosion of the boulder Boulder No. 1# 2# 3# 4# Plane size 1.0m×0.9m 2.4m×1.5m 1.6m×1.4m 1.6m×1.4m Thickness 1.6m 2.8m 0.9m 0.55m Average burial depth 11.6m 4.4m 6.4m 5.05m Blasthole spacing 60cm×50cm 60cm×50 cm 80cm×70cm 80cm×70cm Explosive payload 2.0kg/m 5.0kg/m 3.0kg/m 4.0kg/m Amount of blastholes 3 11 3 3

Selection of Demolition Equipment and Materials No. 2 rock emulsion explosive of good water resistance, stability and explosion performance was chosen. Millisecond nonel detonators with a normal diameter of Φ60mm were used for hole- to-hole detonation. Adjustments were made to meet the requirements of the field work.

Charge Structure When a boulder’s thickness is smaller than the row spacing (0.6m), single-stage charge is advised, as indicated in Figure 7. If the proportions of explosive and slime water in the blasthole are very close, the charge would fail to sink or is unstable even after sinking. Thus the charge’s weight needs to be balanced for the purpose of anti-floating. Here, gravel with a diameter of 0.5cm and a density of 1.50g/cm3 was used for counterweight. As long as the total weight of explosive and gravel exceeds the buoyance of slime water, the charge would sink successfully. The counterweight can be adjusted in light of the blasting effect during construction. When a boulder’s thickness is larger than the row spacing, deck charge is advised, as indicated in Figure 8. The explosive charge was processed as shown in Figure 9.

Figure 7: Schematic of the single powder bag processing Vol. 21 [2016], Bund. 22 6716

Figure 8: Schematic of the powder filling structure of the explosion

Figure 9: Schematic of the sectionalized powder bags processing

Due to the uneven thickness of boulders, as well as the errors (less than 10cm accumulatively) arising from the process of measurement and charge hoisting, the length of charge should be the same with the boulder’s thickness during single-hole monomer blasting. However, during multiple-hole monomer blasting, any one of two adjacent balstholes should be drilled to the bottom surface of the boulder (in other words, piercing). And the charge is infilled to the bottom of the blasthole till the level that is 10cm under the top surface of the boulder (this space is free of explosive). For the adjacent hole, the bottom of the blasthole is 10cm away from the bottom surface of the boulder, and the charge is infilled to the bottom of the blasthole till the level that is 10cm under the top surface of the boulder (this space is free of explosive).

Verification of Blasting Safety To ensure the blasting safety and avoid damages on the surrounding buildings (especially those close to the blasting site), the safe distance for blasting must be worked out before construction. The charge should be strictly in accordance with the calculated amount of explosive that is deemed to be safe, and the cordon zone should be formed according to the calculated safe distance. The safe distance for blasting is calculated by the following equation (3):

R = 1 Q 1 (3) α K 3 wherein, �V� ∙ V is the safe seismic velocity (cm/s); Q is the maximum one-stage charge demanded by simultaneous blasting (kg); K is a coefficient related to geological conditions; Vol. 21 [2016], Bund. 22 6717

αis the blasting attenuation coefficient.

K and α are empirically valued, and should be further determined by monitoring the vibrations induced by blasting with using regression methods. Based on the above data and equation, the millisecond blasting was calculated with different distances from the blasting center to various buildings. The maximum one-stage charge (Qmax) was timely adjusted.

Implementation of Blasting

Charge Location and Protection After the charge was processed and prepared, two holes were punched in the sleeve wall and the charge was bound by iron wires. A string was attached to lay down the charge. As soon as the charge was positioned, the string was fastened to the sleeve wall with iron wires to stabilize the charge. Afterwards, blockage was conducted. Although no flying stones would be generated during underground blasting, the high pressure gas produced by blasting would squeeze out the mud inside the blashole. Therefore, to prevent the gushing mud from splashing, a united defense system was established, as shown in Figure 10. If any blasting operation has been previously conducted in the current blasting site, the residual blastholes need to be covered with sand bags to stop mud from ejecting.

Figure 10: schematic of the protection of the explosion

Detonation Network Given the large depth of the blasthole and the great burial depth of the boulder, the igniting primer was hung by a mild steel wire at the shot point on one end, and fastened to the opening of the blasthole on the other. Direct priming was used for detonation, and non-electrical denotation network was adopted. Detonator excitation pins were also used for detonation. Two detonators belonging to two detonation networks were installed in each blasthole. And explosion was triggered after the two detonation networks were combined. Figure 11 shows the detonation networks. Vol. 21 [2016], Bund. 22 6718

Figure 11: Schematic of the explosion net

Grouting Reinforcement of Blasted Body After boulder blasting, the blasted body should be grouted for reinforcement, in order to stabilize the excavation face, eliminate the negative influence brought about by blasting, and ensure the smooth operation of the shield boring machine.

Blasting Effect Analysis The 4 explored boulders were blasted separately in accordance with their corresponding schemes. And the blasting effect of each boulder is shown in Figure 12～15. The following table is a comparative analysis.

Vol. 21 [2016], Bund. 22 6719

(a) 1#Sketch of the boulder explosion

(b) 1#The minimum length of the boulder is 54 cm after the explosion Figure 12: 1#sketch of the explosion of the boulder (minimum length 98cm with the worst explosion effect) Vol. 21 [2016], Bund. 22 6720

(a) 2# Sketch of the boulder explosion

(b) 2# The maximum length of the boulder is 50 cm after the explosion Figure 13: 2#sketch of the explosion of the boulder (maximum length 50cm) Vol. 21 [2016], Bund. 22 6721

(a) 3# Sketch of the boulder explosion

(b) 3# The maximum length of the boulder is 98 cm after the explosion Figure 14: 3#sketch of the explosion of the boulder (maximum length 98cm) Vol. 21 [2016], Bund. 22 6722

(a) 4# Sketch of the boulder explosion

(b) 4# The maximum length of the boulder is 50cm after the explosion Figure 15: 4#sketch of the explosion of the boulder (maximum length 50cm with a good explosion effect)

As can be seen from Figure 12～15, the blasting effects of the 4 boulders can be ranked, from good to bad, as follows: 4#＞3#＞2#＞1#. (1) Boulder4# achieved the best blasting effect. Blasted bodies with a diameter of less than 30cm accounted for 80%, with a maximum length of 50cm. The charge amount was 4.0kg/m. The blasted boulder sample was analyzed to be slightly weathered spheroidal granite. There was a small loss in energy during blasting. (2) The charge amounted for Boulder2# is up to 5.0kg/m. The surface area of the boulder was relatively large, with a greater number of blastholes drilled. There were dense balstholes lying peripherally. The blasted bodies were measure maximally 50cm in length. The blasted boulder sample was analyzed to be slightly weathered spheroidal granite. There was a small loss in energy during blasting. Vol. 21 [2016], Bund. 22 6723

(3) Boulder1# and boulder3# had some characteristics in common: the charge amount was relatively small (2.0kg/m and 3.0kg/m respectively); the blasted boulder sample were analyzed to be heavily weathered granite, and there was a large loss of energy① during blasting; the size of the boulders were relatively small, with a smaller② number of blasholes drilled and distributed peripherally. Small spacing between holes resulted in small spacing between free③ surfaces. As a result, the blasting energy split the boulder into halves rather than into pieces.

CONCLUSIONS AND SUGGESTIONS To improve the boulder disposal effect and reduce the adverse effect to the shield construction, this paper studied the size, burial depth, weathering degree, borehole’s spacing, charge amount and other parameters’ influence on the blasting effect and reaches the conclusion and suggestions as follow: (1) We concluded the influence factor of the blasting effect. The blasting effect of a boulder is correlated with the size and burial depth of the boulder as well as the spacing between blastholes. A boulder with a large diameter can achieve a better blasting effect than a boulder with a small diameter. In case of a small diameter, infill drilling should be adopted, and the diameter can be adjusted in part to be 30cm. A boulder with a large burial depth can achieve a better blasting effect than a boulder with a shallow burial depth. A boulder’s degree of weathering can also affect its blasting effect. Generally, the higher degree of weathering, the poorer effect of blasting. (2) We introduced the suggestive value of the single charge amount. The charge amount per hole exerts a great influence on the effect of blasting. It is suggested that the charge amount per hole be more than 4.0kg/m and be adjusted according to the burial depth of a boulder. (3) The method of ground deep-hole blasting is not recommended for use in areas where pipelines are densely distributed and many hidden risks exist, because it can cause damages on sensitive structures. If this method is adopted, it is strongly advised that the charge amount be reduced. However, the blasting effect cannot be guaranteed. In areas free of pipelines and risk sources, when conducting deep-hole blasting of boulders, the charge amount should be properly adjusted if the intensity of drilling is not ensured to guarantee that a desirable blasting effect is achieved. By situ-experiment, the study concludes the process of the boulder blasting disposal and influence factors of the disposal effect. Relevant suggestions have significant value for the disposal of the boulder in similar zones. Due to the lack of the boulders, we failed to proceed more parameter design and draw more conclusions for the relationship between internal properties (such as strength, size, etc.) and blasting effects. In the future study, we will further enrich the experiment and have more accurate conclusions for the factors and regulations of the influence of boulder blasting effects.

ACKNOWLEDGEMENTS This study was supported by Natural Science Foundation of Fujian province (grant number 2016J01271), Science and Technology project of Housing and Urban-Rural Development Department of Fujian province (grant number 2015-K-38), Science and Technology Project of Housing and Urban-Rural Development Department of Fujian province (grant number 2016-K- 26) Vol. 21 [2016], Bund. 22 6724

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© 2016 ejge

Editor’s note. This paper may be referred to, in other articles, as: Shi Youzhi, Hua Jianbing, Lin Shuzhi, and Ge Xiurun: “An Experimental Study on the Explosion Disposal of the Boulder in the Metro Engineering Stratum” Electronic Journal of Geotechnical Engineering, 2016 (21.21), pp 6607-6725. Available at ejge.com.