The stability of high slopes has long been an important issue in civil engineering, and slope instability has brought inestimable losses to people's lives and property. Bolt support is a general term for support structures with bolts as the main body. It includes support forms such as rock bolts, shotcrete with rock bolts, and shotcrete with rock bolts and wire mesh. It involves drilling inclined holes in the soil layer, embedding bolts, and then grouting cement (or cement mortar). The friction between the anchor body and the soil, the bond between the tension rod and the anchor body, and the strength of the tension rod work together to bear the load acting on the support structure. In recent years, there have been increasingly more engineering examples using bolt technology in China, and its application scope is not limited to foundation pit support but is also often used for emergency reinforcement or landslide treatment when foundation pits become unstable. For example, the bolt (cable) support reinforcement of the Hero Slope and Beacon Tower high slope in Zheng Chenggong Park.

 

 

　　The beacon towers on both sides of a park's city wall are located on the south and north sides of Hero Slope. The south beacon tower is designed to be about 14 m long and 10.60 m wide, with a ground design leveling elevation of 28.13 m; the north beacon tower is designed to be about 10 m long and 9.60 m wide, with a ground design leveling elevation of 37.57 m. The designed height of the city wall is 18 m for both, and the maximum load is 2500 kPa. Both the beacon towers and Hero Slope are located on a hillside with relatively high terrain, and their construction will form slopes about 15-30 m high.

 

　　2 Key Technologies in High Slope Support Design

 

　　1) Emphasis on drainage engineering. Drainage holes are provided on the slope surface to discharge groundwater into drainage ditches. The drainage pipes use Φ100 PVC flexible permeable pipes 6 m long. A water-shedding slope and interception ditch are provided at the top of the slope to prevent surface water from flowing in and infiltrating. Drainage ditches are provided at the bottom of the slope. Reverse filter layers should be provided for some slope surface structures to prevent loss of fine-grained soil. Reliable anchoring measures should be provided between slope surface structures and the slope surface.

 

　　2) Material requirements. The bolt body material uses prestressed steel strands with a strength grade of ≧1860 MPa, and the anchors are OVM series anchors. The cement slurry for grouting uses grade 42.5 ordinary Portland cement, and the centering brackets are welded from HPB235 grade (Grade I steel Φ6.5).

 

　　3) Drilling requirements. The hole diameter for anchor cables is ≧130 mm. Before drilling, hole positions are determined and marked according to design requirements, and drilling must not disturb the surrounding strata. The error in horizontal and vertical hole spacing of bolts shall not exceed 100 mm, and the drill bit diameter shall not be less than the designed hole diameter by 3 mm.

 

　　4) Assembly and insertion. Materials shall be cut strictly according to design dimensions, and the allowable error in the length of each steel strand is 50 mm. Before assembly, oil stains and film rust on the surface of the bolt body shall be removed. The free section of the bolt shall be coated with two coats of anti-corrosion paint, wrapped with grease and a sleeve, and fixed with engineering adhesive tape. The fabricated length of the bolt shall be the design length plus the working length for jack tensioning and locking. The spacing of bolt positioning brackets is 1500 mm, and the grouting pipe and exhaust pipe shall be firmly tied to the rod body. When inserting the bolt body, twisting and bending of the bolt body shall be prevented, the insertion angle of the rod body shall be consistent with the drilling angle, and the protective layer thickness of the rod body shall not be less than 25 mm.

 

　　5) All anchor cables are prestressed anchor cables with an inclination angle of 15° and a hole diameter of 130 mm. Anchor cable construction adopts dry drilling technology and secondary high-pressure grouting. During bolt construction, boulders and slightly weathered bedrock may be encountered inside the slope. If the bolt enters boulders and slightly weathered bedrock to a depth of ≧6 m, drilling can be terminated; otherwise, construction shall continue according to the design length.

 

　　6) Ecological protection engineering design. In order to minimize the damage of engineering construction to the natural ecological environment, ecological protection engineering should be done as well as possible.

 

　　3.1 Good construction preparation

 

　　Prevent falling objects from injuring people during construction. At the same time, prepare construction materials and drilling and grouting machinery and equipment; lay ventilation and water supply pipelines. Slope bolt construction shall use access roads or erect scaffolding according to site conditions. The layered height of scaffolding generally shall not exceed 1.8 m, and horse walkway boards shall be laid; both ends of the horse walkway boards shall be firmly tied with galvanized iron wire to form a drilling and grouting construction platform.

 

　　3.2 Key technologies for support

 

　　1) Slope support shall be carried out layer by layer during layered excavation. The upper layer support shall ensure the safe and smooth progress of the lower layer excavation, and at the same time ensure that the height difference between the bolt system and shotcrete in permanent support and the excavation working face should not be greater than 5 m; the height difference between the prestressed anchor cables in permanent support and the excavation working face should not be greater than 10 m, and weak rock parts shall be supported immediately following the excavation working face.

 

　　2) The construction of prestressed anchor cables and bolts shall proceed according to the following procedures. Install the dowel bars for the scaffolding for support layer bolt construction, which shall be constructed simultaneously after the support layer excavation and muck removal and the lower layer rock removal. The erection of scaffolding and the drilling of support layer anchor cables shall be carried out after the lower layer excavation rock removal. The subsequent work of anchor cables, including hanging geogrid and three-dimensional geomat and grass planting, shall be carried out on the scaffolding.

 

　　3.3 Bolt construction process control

 

　　Slope bolt construction process: erect scaffolding → hang safety net → clean rock surface → arrange drilling hole positions → set up operation platform → prepare drilling tools, blower, and water → drill → install bolts → grout cement mortar → cure.

 

　　1) Drilling and hole cleaning. The opening of bolt holes shall be carried out according to the drilling positions arranged in the construction drawings, and the hole position deviation shall not be greater than 100 mm. During construction, surveyors shall set out and position points on the slope surface according to drawing requirements; for special parts, technical personnel shall determine them on site according to technical needs. The axial direction of bolt holes shall meet the requirements of the construction drawings. When not specified in the construction drawings, the axial direction of system bolts shall be perpendicular to the excavation surface; the axial direction of local reinforcement bolts shall be opposite to the dip direction of the possible sliding surface, and the angle between them and the sliding surface shall be greater than 45°.

 

　　2) Mortar preparation and grouting. ① The mix proportion of cement mortar for bolt grouting shall be selected through tests within the following specified ranges: cement:sand = 1:1 to 1:2 (by weight); cement:water = 1:0.35 to 1:0.4; ② The mortar shall be mixed on site by a 400 L forced mixer. The bonding mortar shall be mixed evenly, and its workability shall be adjusted. It shall be used as mixed, and the mortar mixed at one time shall be used up before initial setting; ③ During grouting, first insert the bolt and grouting pipe. The bolt shall be inserted to the bottom of the hole and centered, and the grouting pipe shall be inserted to 5-10 cm from the bottom of the hole. During grouting, after thick slurry emerges from the hole opening, slowly withdraw the grouting pipe.

 

　　3) Bolt installation. The bolt rod body shall be inserted centered, and after insertion, the rod body shall be fixed at the hole opening; after the bolt is installed, it shall not be knocked, collided with, or pulled at will.

 

　　3.4 Key technologies for prestressed anchor cable construction

 

　　1) Drilling hole positions. Drill according to the positions required by the design drawings, ensuring the opening deviation is not greater than 10 cm. The hole positions for anchor cable drilling shall be measured and positioned using a total station, and the drill inclination shall be controlled using a level.

 

　　2) Drilling method. During drilling, the rock stratum conditions in the hole shall be recorded in detail, with particular attention to describing and judging the rock stratum conditions of the inner anchorage section, ensuring that the inner anchorage section is located on hard and complete rock strata; otherwise, deepening treatment shall be carried out and reported to the command headquarters in a timely manner.

 

　　3) Treatment of special situations. During drilling, when fault fracture zones cause hole collapse, block falling, and large seepage, the following measures shall be taken: immediately stop drilling, and adopt consolidation grouting and other measures to improve the stability of the hole wall in the fracture zone and drilling efficiency. Adopt the method of drilling a pilot hole first, perform consolidation grouting on the pilot hole, and after setting, perform coaxial reaming drilling.

 

　　4) Anchor pier casting. First, install embedded parts such as anchor bearing plates, steel sleeves, and structural steel bars at the hole opening. When installing the anchor bearing plate, the plane of the bearing plate must be perpendicular to the hole axis, the sleeve must be centered with the hole axis, and the gap between the sleeve and the hole shall be sealed. To ensure that the early strength of the anchor pier concrete can meet the needs of anchor cable tensioning, an appropriate amount of early-strength water-reducing agent shall be added to the concrete mix.

 

　　5) Anchor cable grouting. To ensure that the strength of the grouting body in the anchorage section reaches the design requirements in time, an anchoring agent is planned to be added to the slurry.

 

　　6) Tensioning. Tensioning method for ordinary tensile anchor cables: single-strand pre-tensioning and overall tensioning. During tensioning, when the load is stable at each tension level, measure the elongation of the steel strand to verify the tensioning force.

 

　　7) Anchor head protection. 24 hours after the grouting backfill of each anchor cable hole is completed, use a hand-held cutting machine to cut off the excess steel strand outside the working anchor. The remaining length of the cut steel strand outside the working anchor shall be 15 cm, and then cast concrete for permanent protection of the outer anchor and steel strand. The thickness of the concrete protective layer shall not be less than 10 cm.

 

 

　　In summary, bolt support technology is a new type of high slope support technology that has emerged in recent years and is gradually maturing. Through the reinforcement treatment of this high slope, this paper proves that bolts (cables) can better control the deformation of strata and structures and can detect whether the bearing capacity of bolts meets design requirements, which is conducive to stratum stability. It also proposes design and construction quality control points that should be noted and corresponding reinforcement treatment countermeasures, which are of great significance to the national economy and people's livelihood.