I. The Role of Rock Bolts in Tunnel Support

 

　　In the current tunnel support system, rock bolts are a very effective support method, widely used and available in many types. However, the exact role that rock bolts play in the tunnel support system is not yet fully clear. It is generally believed that rock bolts have four reinforcement effects: suspension effect, enhancement effect, arching effect, and internal pressure effect. In addition to these four effects, in weak surrounding rock sections with poor geological conditions, rock bolts also stabilize the initial support steel arches. The presence of rock bolts not only facilitates the on-site installation of steel arches but also, to a certain extent, stabilizes the relatively thin initial support under high pressure, preventing local or overall instability. At the same time, rock bolts can reduce the relative displacement between the initial support and the surrounding rock, preventing excessive overall settlement of the initial support.

 

　　In fact, the reinforcement effect of rock bolts is the result of multiple effects acting simultaneously. Under different bolt arrangement patterns and different geological conditions, one effect will play a dominant role while others are secondary. For local bolts in relatively intact Grade I and II surrounding rock sections, the main function is suspension, primarily reinforcing unstable blocks; for systematic bolts in relatively fractured Grade III and IV surrounding rock sections, the main function is to form a load-bearing arch with a certain capacity; and for very weak Grade V and VI surrounding rock, the main function of rock bolts (anchor pipes) is to stabilize the initial support steel arches. At the same time, the effect of rock bolts is related to their arrangement: local bolts mainly exert a suspension effect, while systematic bolts mainly exert an arching effect. The main research project of this paper will combine the design and construction practice of highway tunnels to deeply analyze the working principle and adaptability of systematic rock bolts.

 

　　II. Application of Rock Bolts in Tunnel Support

 

　　(I) Project Overview

 

　　A certain tunnel has a total length of 1385 meters; it is a municipal highway traffic tunnel connecting the new urban area and the old urban area of H City. The entrance section is located in a busy downtown area, and the tunnel passes through buildings such as the steps in front of the railway station, the square, the station, the waiting room, railway tracks, and an amusement park. The surrounding rock types are mainly loose artificial fill, gravel-cobble-mudstone layers, mudstone layers, and argillaceous siltstone. The 153 m entrance section, with a burial depth of 10–16 m, has surface buildings that remained intact and undamaged during construction, making construction extremely difficult and a controlling challenge for the entire tunnel. The tunnel is designed as a composite lining based on the New Austrian Tunneling Method.

 

　　In response to the geological characteristics of the entrance section, after careful analysis and comparison, we adopted the WTD25 radial rock bolt system on the basis of advanced support using long pipe roofs and short pipe roofs. This is because WTD rock bolts have the following characteristics: WTD rock bolts, together with specially made anchor heads, grout stop plugs, bearing plates, and nuts, are constructed using the anchor-first-then-grout method. Under the combined action of the anchor head and grout stop plug, cement mortar can fill the lock hole and, as the grouting pressure increases, a large amount of cement mortar can penetrate into the surrounding rock fissures, consolidating the formation and improving the overall stability of the surrounding rock. This integrates grouting and anchoring into one, thoroughly solving the fundamental problems of insufficient grout filling and no grouting pressure in ordinary mortar rock bolts and ZW cartridge rock bolts. The continuous threads on the surface allow the rock bolt to be lengthened via coupling sleeves, and it can be very easily made into a prestressed rock bolt with a certain tonnage of prestress applied.

 

　　(II) Construction Parameter Design

 

　　In the entrance section of Tunnel H, WTD rock bolts are mainly used for radial initial support. The method is to mark the bolt positions on the excavated contour surface according to the design, drill bolt holes, and then inject cement grout, thereby forming an integral support structure outside the excavation contour line of the tunnel, controlling the deformation of the surrounding rock, reducing the settlement of the surrounding rock, and protecting the safety of surface buildings.

 

　　1. Initial Support Parameters

 

　　Systematic rock bolts use 3 m long WTD25 hollow grouting rock bolts, with longitudinal and circumferential spacing of 100 cm, arranged in a quincunx pattern; steel grid arches are installed on the arch walls at 50 cm spacing, and at each side of the arch foot, 4 m long WTD25 hollow grouting locking rock bolts are installed, arranged in a quincunx pattern on both sides of the steel grid arches with a circumferential spacing of 50 cm; double-layer ∮6 steel wire mesh with a grid size of 15 cm × 15 cm is hung, and the shotcrete thickness is 25 cm.

 

　　2. Selection of Shotcrete Materials and Equipment

 

　　(1) Equipment

 

　　The Bz-5 type concrete spraying machine is used for shotcrete, with a pressure of 0.2–0.4 MPa.

 

　　(2) Cement and Fine Aggregate

 

　　Grade 425 ordinary Portland cement is used; the fine aggregate is Xinkou sand from Yuanping City, Shanxi Province, with a sand ratio controlled at 50% and a mud content ≤3%.

 

　　(3) Coarse Aggregate

 

　　Crushed stone with a size of 7–15 mm is used; through testing, crushed stone produced from limestone meeting all design requirements was selected.

 

　　(4) Thickener

 

　　STC type thickener is selected. Field tests show that the optimal dosage is 10% of the cement weight, with initial setting in 3 min and final setting in 6 min, and it can greatly reduce rebound.

 

　　(5) Water-Cement Ratio

 

　　Too high or too low a water-cement ratio will increase concrete rebound and waste a large amount of material; through multiple field tests, a water-cement ratio of 0.47 was determined to give the best shotcrete effect.

 

　　3. Shotcrete

 

　　After excavation, to shorten the exposure time of the surrounding rock and prevent further weathering, initial shotcrete of 3–5 cm thick must be applied first to seal the surrounding rock; after the steel grid arches and steel wire mesh are installed, another 10–12 cm of shotcrete is applied; finally, in the next cycle, shotcrete is applied in two layers to the design thickness. (1) A semi-wet shotcrete process with STC thickener is adopted to reduce dust pollution and rebound in the tunnel. (2) Before spraying, use high-pressure air to blow away dust and debris from the rock surface. Cement, coarse and fine aggregates are dry-mixed with a small amount of water in a mixer; the water amount is 20% of the total water required for the concrete at the water-cement ratio; after mixing, the dry material is transported to the spraying site for manual mixing, and the thickener is added at 10% of the cement weight. (3) Spraying is carried out in sections and panels, from bottom to top in successive layers, with each section 3 m long. To accelerate the increase of concrete strength and improve the spraying effect, multiple iodine-tungsten lamps are used to raise the working environment temperature. (4) The nozzle spraying direction should be at an angle of less than 10° to the rock surface, with an included angle of 45°; the distance from the nozzle to the sprayed surface is 0.6–1.0 m, and the nozzle moves slowly and evenly in a spiral pattern, generally with a circle diameter of about 0.4 m.

 

　　4. Drilling and Installation of Rock Bolts

 

　　The position error of bolt holes shall not exceed ±15 cm, arranged in a quincunx pattern. The allowable deviation of bolt hole position is ±150 mm. The allowable deviation of drilling depth is ±50 mm. Rock bolts shall be installed perpendicular to the excavated rock surface. The hole depth shall be 10 cm greater than the design length of the bolt, and the length of the bolt inserted into the hole shall not be less than 95% of the design length. After drilling, the rock powder in the hole must be washed clean with high-pressure water. When drilling to the design depth, if the rock powder flowing out is yellow or brown, or if the drill bit impact is abnormal, it indicates encountering softer rock or a fractured interlayer, and the bolt depth and position shall be changed. For bolts with bearing plates, the bearing plate shall be tightly against the surrounding rock; if the rock surface is uneven, it shall be leveled with M10 mortar. Grout with a 1:1 (weight ratio) cement slurry shall be injected into the bolt hole, and the grout shall be dense and full. The exposed length of the bolt body beyond the rock surface shall not exceed the thickness of the shotcrete layer.

 

　　5. Grouting

 

　　Before grouting, first check whether the exhaust pipe is unobstructed; if the exhaust pipe is blocked, clear it before starting grouting. If grouting is started or stopped for more than 30 minutes, the grouting tank and its pipelines shall be lubricated with water or thin cement slurry. During grouting, the grouting pipe shall be inserted to 50–100 mm from the bottom of the hole. For permanent support rock bolts grouted first, the bolt body shall be inserted immediately after the hole is filled with grout; for permanent support rock bolts grouted later, grouting shall be carried out immediately after the bolt is installed. If no mortar overflows from the hole opening, measures shall be taken promptly for supplementary grouting.

 

　　III. Construction Quality Correction and Preventive Measures

 

　　1. Install rock bolts according to design requirements, drill holes with a rock drill, and clean the holes after drilling. Insert the hollow grouting rock bolt with the anchor head installed to the bottom of the hole, install the grout stop plug, bearing plate, and nut, then connect the grouting pipe and inject grout into the hole through the tail using a grouting pump. The grout is cement mortar, and the grouting pressure is controlled at 0.3–1.0 MPa. Grouting is carried out from bottom to top, one by one. After grouting, insert the grout stop plug into the drilled hole and seal the hole with quick-setting cement.

 

　　2. To prevent grout leakage between holes, skip-hole construction can be adopted, i.e., first construct odd-numbered holes, then even-numbered holes.

 

　　3. Treatment measures after grout leakage occurs: grout the leaking hole and the grouting hole simultaneously, using a grout distributor to grout multiple rock bolts at the same time with one grouting pump.

 

　　4. When a large amount of grout leakage occurs, the following principles shall be adopted: use low pressure, thick grout, limited flow, limited quantity, and intermittent grouting for injection, or inject other filling materials to block the large channels first and then adopt the first method.

 

　　5. Water inflow treatment: in grouting hole sections with water inflow at the hole opening, measure and record the water inflow pressure and quantity before grouting, and then select the following comprehensive measures according to the water inflow conditions. First, use higher pressure and grout in sections from top to bottom. Second, use thick grout for screen grouting for 1–24 hours, then close the grout and wait for setting. Third, use pure pressure grouting. Fourth, use quick-setting grout.