Research on Drilling and Blasting Tests of Rock Drilling Rigs in the Taixing Mountain Tunnel of the Xikang High-Speed Railway

1. Current Application Status of Rock Drilling Rigs

1.1 Application History

In 1979, a batch of large construction machinery was imported during the construction of the double-track line from Hengyang to Guangzhou, including a four-arm full-hydraulic rock drilling rig used for direct hole slot drilling, achieving full-section one-time blasting. This technology was subsequently promoted in the Jundu Mountain Tunnel of the Datong-Qinhuangdao Railway. By the late 1990s, the imported equipment had gradually aged, and the maintenance costs became high. Due to the abundance of domestic labor, handheld drills were reintroduced, and simple racks were developed. Using handheld drills and simple racks for tunnel drilling and blasting significantly improved construction efficiency. In 2000, domestic rock drilling rigs were introduced, entering a rapid development phase after 2015. Tunnel rock drilling rigs are generally equipped with three drill arms, with some models featuring two or four arms; configurations with five arms are rare.

1.2 Existing Problems

1. Lack of Professional Drill Operators: Drill operators require specialized training and practical experience, but such training is currently scarce due to the limited large-scale application of rock drilling rigs.
2. High Over-Excavation Rates: Operators with average technical skills using rock drilling rigs for drilling and blasting typically result in a linear over-excavation of 35-45 cm. Even highly skilled operators achieve an average over-excavation of 13-18 cm, roughly twice that of handheld drills. The over-excavated sections need to be filled with shotcrete, increasing excavation costs.
3. Mismatch Between Explosive Cartridge Diameter and Drilling Diameter: The explosive consumption per cubic meter of rock is significantly higher for rock drilling rig operations compared to handheld drill operations. For similar surrounding rock conditions, the explosive consumption for rock drilling rig and handheld drill operations is 1.02 kg/m³ and 0.64 kg/m³, respectively.

To address these issues, a drilling and blasting test was conducted on the left tunnel entrance of the Taixing Mountain Tunnel of the Xikang High-Speed Railway (DK23+075 to DK23+155) to propose over-excavation control measures and determine the appropriate explosive cartridge diameter.

2. On-Site Test

2.1 Project Overview

The Taixing Mountain Tunnel is a single-line double-tube tunnel, with the left and right tubes being 16.906 km and 16.923 km long, respectively. The maximum distance between the left and right tubes is 71 meters, with a distance of 18 meters at the entrance section. The tunnel’s surrounding rock grades II, III, IV, and V extend 15.120 km, 16.430 km, 3.268 km, and 0.905 km, respectively, accounting for 42%, 46%, 9%, and 3% of the total length. The surrounding rock is mainly composed of mixed granite and mixed gneiss, with quartz, feldspar, amphibole, and mica as the primary mineral components.

2.2 Rock Drilling Rig Modification

The rock drilling rig used is the JTH3200 three-arm rig, with overall dimensions of 17.1 m (length) x 3.1 m (width) x 4.2 m (height), and a propulsion beam length of 7.1 m. Due to the single-line tunnel span of 10.8 m, the outer insertion angle during perimeter hole drilling is large, and the space is limited during slot hole drilling, with a maximum drilling depth of 3.5 m, which does not meet rapid construction requirements. Therefore, the mechanical arm length on both sides was shortened to 6.1 m. After modification, the outer insertion angle during perimeter hole drilling was reduced, and the drilling depth could reach 4.0 m.

2.3 Precision Drilling Measures

The blasting effect of the slot holes significantly impacts subsequent holes and the overall blasting advance. The amount of over-excavation and under-excavation mainly depends on the blasting effect of the perimeter holes. Therefore, the drilling precision of the slot holes and perimeter holes is crucial.

To ensure drilling precision, the following measures were adopted:

1. Before drilling, accurately mark the tunnel centerline, foot arch line, and perimeter contour line on the tunnel face with red oil paint, indicating the positions and spacing of slot holes, perimeter holes, and auxiliary holes.
2. Drill holes in the order of slot holes, perimeter holes, and auxiliary holes. To ensure the blasting effect of the slot holes and the over-excavation of the perimeter holes is within the design range, drilling should strictly follow the blasting design in terms of spacing, depth, and angle. The opening position error for slot holes and auxiliary holes should be within 5 cm and 10 cm, respectively. The opening position for perimeter holes should be 5 cm inward from the contour line, with the hole bottom not exceeding 15 cm from the excavation contour line.
3. Since the drilling rig control room is far from the tunnel face, a technician should be positioned at the face to guide the drilling of slot holes and perimeter holes to ensure precision.

2.4 Determining Blasting Parameters

Under the condition of ensuring drilling precision, the blasting effect of the rock drilling rig mainly depends on the initiation sequence, timing, and slot hole configuration.

When using a rock drilling rig for smooth blasting, the initiation sequence follows the order of slot holes, auxiliary holes, inner circle holes, perimeter holes, and bottom holes.

Electronic detonators do not have preset initiation times upon delivery and need to be set based on the on-site rock conditions. For convenience, the initiation times for electronic detonators were set to match those of odd-numbered stages of non-electric detonators (MS1, MS3, …, MS17).

Since direct hole slotting requires high drilling precision and is difficult to meet design requirements, inclined hole slotting is commonly used with rock drilling rigs for tunnel blasting. Therefore, inclined hole slotting was adopted in the test section. Based on the experience of handheld drill operations in similar rock grades, and after three cycles of drilling and blasting tests, a two-stage inclined hole slotting configuration was determined.

The hole layout and initiation sequence are shown in Figure 1. The blasting parameters for various holes in the tunnel are listed in Table 1. A total of 149 holes were drilled, with a total explosive charge of 257 kg.

2.5 Test Results

The overall integrity of the grade III surrounding rock was good. After three cycles of blasting tests, the perimeter hole blasting marks retention rate was over 90%. In one cycle, the linear over-excavation ranged from 3.3 to 46.2 cm, with an average of 19.9 cm.

Due to the general diameter of the on-site explosive cartridges being 32-35 mm, matching the handheld drill hole diameter of 40 mm but not the rock drilling rig hole diameter of 50 mm, the slot hole blasting effect was unsatisfactory. Based on the matching relationship between handheld drill hole diameter and cartridge diameter, the cartridge diameter for rock drilling rigs should be between 40-45 mm.

The rock drilling rig used has two working platforms, each accommodating up to two people for charging. The charging and blasting time was around 150 minutes. To shorten construction time, a simple rack was used in conjunction with the rock drilling rig for blasting operations, requiring eight people for charging and wiring. The remaining test sections were completed following the test scheme and parameters. The blasting advance for the test section was 3.4-3.6 m, with an average drilling time of 224 minutes and an average charging and blasting time of 90 minutes, totaling 314 minutes for drilling and blasting operations.

3. Conclusion

1. To address the large outer insertion angle of perimeter holes and the depth limitation of slot holes, the mechanical arm length of the rock drilling rig was reduced from 7.1 m to 6.1 m. After modification, the outer insertion angle during perimeter hole drilling was reduced, and the drilling depth could reach 4.0 m.
2. The drilling precision of slot holes and perimeter holes is crucial for the blasting construction quality of rock drilling rigs. Before drilling, mark the tunnel centerline, foot arch line, and perimeter contour line; drill in the order of slot holes, auxiliary holes, inner circle holes, perimeter holes, and bottom holes; position a technician at the face to guide the drilling of slot holes and perimeter holes. The average linear over-excavation after blasting was only 19.9 cm.
3. When using a rock drilling rig for single-line tunnel drilling and blasting, a two-stage inclined hole slotting configuration can be adopted, with a blasting advance of 3.4-3.6 m.
4. Using a simple rack in conjunction with a rock drilling rig for blasting operations can shorten the drilling and blasting construction time. The cartridge diameter for rock drilling rigs should be between 40-45 mm.

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