Repair measures and control of local collapse of slopes in power stations

　　1Overview

　　The power station is located near Chipei City in Kachin State in northern Myanmar, at the confluence of Chipei River, a primary tributary of Nmai Kha River, the upper main stream of the Irrawaddy River. It is the construction power source for the Myitsone and Myitkyina Hydropower Station, a recent development project in the Irrawaddy River basin above Myitkyina. The power station is a diversion hydropower station with an installed capacity of 99MW. The project consists of a dam, a power generation and water diversion system, and a power station plant. The dam is located on the Chipei River, about 12km away from Chipei Town; the power station plant is located on the left bank of the Nmai Kha River, about 9km away from Chipei Town upstream, about 62km away from the Myitsone Hydropower Station downstream, and about 20km away from the Chipei Hydropower Station upstream. The dam is a concrete gravity dam, mainly composed of the left bank non-overflow dam section, the overflow dam section, the flood discharge and sediment discharge hole dam section, and the right non-overflow dam section. The dam top elevation is 747.5m, the lowest foundation elevation of the riverbed is 700m, the maximum dam height is 47.5m, and the total length of the dam top is 220m. The power generation water diversion system consists of three parts: the open pipe section of the water inlet, the water diversion tunnel, and the pressure steel pipe. The water diversion tunnel is 11103.4m long. The power plant is a ground-type plant, and the buildings in the plant area include the main plant, installation site, auxiliary plant, tailwater channel and other buildings. According to the requirements of the power station design department, it is necessary to repair the local collapse of the slope below the right bank slope road during the reservoir storage period. The treatment range is the slope range between the right bank slope road and the riverbed elevation of 723m, and the slope range of -8.59--77.61m on the dam, with a treatment area of ​​about 3991m2. The treatment measures mainly include slope excavation, slope anchoring, drainage and concrete slope protection.

　　2 Main design contents of slope restoration

　　(1) Slope excavation: The slope ratio of the dam head connection section is controlled at 1:1.2, and the slope ratio of the collapsed part is changed from 1:1.5 to 1:1, with a slope change elevation of 731m; a tooth groove is set at the slope foot, with a groove bottom width and height of 2.5m and a groove bottom elevation of 721.5m; (2) Slope anchoring and drainage measures: Three types of Φ32mm anchor rods are arranged on the slope, with a lock anchor of 15m, a middle slope anchor of 12m, and a lower slope and slope foot anchor of 9m. The spacing between rows is 2m; the diameter of the drainage holes on the slope is not less than Φ56, the hole depth is 5m, the spacing between rows is 3m, the drainage pipe is a Φ50mm PVC pipe, and the industrial filter cloth is wrapped on the outside; (3) Concrete slope protection: tooth groove concrete 2.5m×2.5m, embedded 1.5m in the foundation surface, and cast integrally with the slope concrete; the slope ratio of the slope concrete is 1:1.436-1:1.6, the thickness is 0.5-1.0m, and the concrete grade is C20.

　　3 Construction methods[1-5]

　　3.1 Construction Procedure The basic construction procedure for collapsed slope treatment is: technical briefing → measurement and layout → slope excavation → tooth groove construction → slope concrete construction → anchor rod and drainage hole construction → completion acceptance.

　　3.2 Slope excavation Slope excavation adopts layered excavation, backhoe material transfer and loading, and dump trucks transport to the waste dump downstream of Labang Bridge. The height difference of slope excavation is controlled at 5m. Before excavation, measure the excavation boundary line, and chisel or cut the original cast slope protection concrete in the excavation range. Use the ramp on the right bank of the reservoir area to an elevation of 723m, and then build a zigzag road to the top of the collapsed part for excavation. It is planned to use one backhoe for slope repair and material transfer, and one backhoe with dump trucks for loading and transportation. During the excavation process, for the parts with isolated rocks, smooth blasting is used for blasting, and the control line charge is not more than 200g/m, the unit charge is not more than 0.45kg/m3, and the one-way charge is not more than 15kg.

　　3.3 Tooth groove excavation: Accurately measure the tooth groove position, excavate the groove according to the designed size by backhoe, and manually build the slope; in the area where there are isolated rocks, blast the isolated rocks that occupy the structure according to the designed structural size.

　　3.4 Concrete construction

　　3.4.1 Formwork installation and removal (1) The panel shall adopt 2.1m×3m flip formwork, and the transverse joint formwork shall adopt P3015, P2015, and P1015 small steel formwork. Wooden formwork shall be used to fill the joints in some parts, and asphalt fir board shall be used to separate the joints; (2) The formwork shall adopt pre-buried dowel bars with square timber or steel pipe support, and the panel shall be coated with mold release agent to avoid contamination of the casting bin surface; (3) The panel shall be lifted by a 25t crane, and shall be lifted or removed from the two working surfaces of the slope foot and the slope top. The transverse joint formwork shall be installed and disassembled manually.

　　3.4.2 Concrete mixing and transportation (Reprinted from: http://www.21ks.net1) Horizontal transportation. Aggregates are produced by the Peihekou aggregate processing system, mixed in the mixing system, and transported to the construction site by three 6m3 mixer trucks; (2) Vertical transportation. Chutes or chutes are mainly used for transportation. Rebars and steel pipes are installed on the slope for fixing. Partitions are installed in the chute to prevent aggregate separation. The chute outlet is no more than 1.5m away from the casting bin surface;

　　3.4.3 Concrete pouring (1) Concrete pouring follows the method of pouring the tooth groove first, pouring the slope later, and pouring in sections and compartments. The length of the section is generally controlled at 12m; (2) During construction, pour to the top of the slope as much as possible at one time. If there is a construction joint during the construction process, the joint surface is required to be perpendicular to the slope surface or the joint surface is pre-inserted with steel bars and roughened to ensure good bonding between the joint surfaces; (3) Concrete vibration is carried out using a 50mm hose vibrator. The exposed surface should be vibrated more to ensure the density of the concrete; (4) According to the weather conditions in the rainy season, the poured concrete should be watered and maintained in time, and the maintenance time should not be less than 7 days.

　　3.5 Anchor and drainage construction

　　3.5.1 Anchor construction (1) During the pouring of slope concrete, the anchor hole position is reserved according to the anchor layout diagram in the design drawing, and the control size is 20cm×20cm×15cm. The hole is accurately laid out during drilling; (2) Drilling and blowing holes: The anchor holes are drilled with 100B fast drill or TY-28 hand pneumatic drill. The drill bit diameter is Φ42-91mm, the hole position deviation is not more than 100mm, and the hole depth deviation is -50-200mm. In the location with isolated rocks or bedrock, the 100B fast drill is used to drill holes, and then the TY-28 hand pneumatic drill is used to send the rod body into the hole; before installing the anchor, use high-pressure air to punch and clean the hole to blow out the dust and sewage in the hole to ensure that the rod body and the inner hole of the drill bit are unobstructed; (3) Grouting: After the rod body reaches the designed hole depth, use the MZ-1 grouting machine to pour M30 mortar. The sand particles are required to be no larger than 1mm, and the water-cement ratio should be 0.4-0.5; (4) Anchor bolt fixing: After grouting is completed, the rod body is temporarily fixed, and the chiseled concrete surface is leveled with M30 mortar before installing the gasket and nut. The nut can be tightened only after the cement slurry strength reaches 5MPa; (5) Second phase concrete pouring: After the anchor bolt is fixed, the concrete surface of the reserved drilling position is roughened, the floating slurry and soil on the surface are removed, and concrete with the same strength as the slope concrete is poured.

　　3.5.2 Construction of drainage holes The drainage holes are drilled with a 100B fast drill, the hole diameter is Φ56mm, and the drilling inclination and hole position deviation must meet the requirements of the design drawings and the technical terms of the bidding contract. After the drilling is completed and the hole is cleaned, the hole device is installed manually, and then the gap between the drill hole and the hole device is sealed with mortar to ensure that the slope seepage seeps out along the hole. 4 Quality assurance measures (1) Establish a quality management system with the project manager as the core leadership, and strictly implement the "three inspections system"; strictly control the purchase of raw materials, carefully inspect the raw materials, and prevent unqualified materials from entering the construction site; (2) Strictly excavate the slope according to the designed slope ratio, take rain protection measures to prevent rainwater from eroding the slope; (3) The panels where the template contacts the concrete and the joints of each template are required to be flat and sealed. During the concrete pouring process, a special person is assigned to check and take corrective measures immediately if the formwork is found to be deformed; (4) The test personnel shall conduct random sampling at the mixer mouth and the concrete construction bin surface in strict accordance with the sampling frequency required by the specification; during concrete pouring, the vibrating of the formwork should be strengthened to reduce bubbles and improve the appearance quality of the concrete; (5) Rain protection measures shall be taken during the transportation and pouring of concrete, and rainwater shall be prohibited from invading the transported or poured concrete to prevent the strength of the concrete from decreasing; (6) The position, depth and directional deviation of the anchor rod and drainage hole shall be strictly controlled to ensure that they are within the control deviation range allowed by the specification; (7) Before grouting the anchor rod, check the smoothness of the anchor rod body and drill bit. If blockage is found, appropriate measures shall be taken to deal with it; after grouting, avoid collision before the slurry strength reaches the strength requirement.

　　5 Conclusion

　　In response to the problem of local collapse of the slope below the right bank inclined road during the water storage period of the power station, measures such as slope excavation, slope anchoring and drainage, and concrete slope protection were adopted to carry out engineering repair of the collapsed part, and good results were achieved. The experience has certain promotion and reference significance in the construction of similar projects.