Anti-seepage Grouting Of Strongly-unloaded Slope Rock At A Hydropower Station

Rock fissure as one kind of geological discontinuities is a main factor controlling the rock mass strength and flow characteristics. Cement grouting under pressure is used to fill the rock fissures, forming a certain thickness of continuous rock body to achieve the purpose of plugging seepage.Yangqu hydraulic power station on the Yellow River is located at the junction place of Xinghai and Guinan of Qinghai province. The downstream water runs into the with Longyangxia reservoir. The station is the last station planed between the Ciha and Banduo of Yangqu river, with a normal water level of 2715m, and a total reservoir storage capacity of 2.305 billion m3. The total generation capacity is 1200MW (300MW by 4 generating units) and the multi-year average power output reaches 4.91 billion kw·h. The power station is a first-class project, composed mainly by dam, flood discharge structures and electric generating structures.Rock slope at the dam site of Yangqu station is characterized by unloaded fractures with width of 10-30cm, and a maximum of 50cm. If using the conventional method of curtain grouting, cement slurry might spread farther along fractures, resulting in large quantity of cement leakage because of the difficulty for spread control. Therefore, grouting pressure can not meet the design requirements, and ground water seepage can not be cut off effectively.By studying the geological conditions of experimental area, this test try to investigate the grouting process through one-site inspection after grouting test, so to evaluate the curtain reliability, improve the grouting process and parameters. Grouting test is conducted from the top to the bottom within closed bore holes. The grouting holes are designed in four rows, of which the first row and the fourth row of holes on both sides of the test area are used to form a relatively closed area for controlling the spread of slurry during the next grouting. The second and third rows of the holes are used to create major curtain. The distance between two rows is 60cm, and the space between two hole within the same row is 1.0m. Grouting is done in three sequences wit a maximum pressure of 4.0MPa. In case that a larger amount of cemen injection occurred, grouting is improved by a series methods such a thick paste, addition of water glass, or by intermittent and comprehensive grouting.After grouting, detection is conducted by means of a five-point water pressure test, inspection of hole core, geophysical P-wave test, drill hole digital imaging technology. Test results showed that the lugeon value of the rock after grouting is significantly reduced. For the holes that was characterized previously before grouting by no return of drilling water or by zero water pressure during grouting, the water permeability tends to infinity, and the lugeon value become less than 1.0 after grouting. Geophysical exploration showed that the longitudinal wave speed increase to 3000-5000m/s after grouting, and the core rate recovery increase more than 95%. Drill hole digital imaging results showed that the cemented stone fills the fissures in a good condition. As final conclusion, grouting designed get a reliable achievement.