| Faced rockfill dam has been widely used in China this year because of its characteristics of saving investment,good safety and strong adaptability.With the development of dam construction technology and the accumulation of dam construction experience,faced rockfill dam gradually develops from high dam to extra high dam,the amount of filled square is increasing,the construction period is getting longer and longer,and it needs to span multiple flood seasons.Moreover,faced rockfill dam is mostly concentrated in the western mountainous and canyon areas,and the flood and low water level and flow rate change greatly and quickly during flood season,which is easy to cause problems of permeability stability.Therefore,it is of great theoretical significance and application value to study the seepage stability of ultra-high faced rockfill dam during the construction period.The main research results of this paper are as follows:(1)The seepage stability of the cushion during the construction period of ultra-high faced rockfill dam are studied under different working conditions.The seepage stability is judged by whether the hydraulic slope exceeds the critical hydraulic slope and whether the anti-buoyancy of the panel exceeds the effective stress.Working conditions including direct water retention during the construction period,the combined water retention during the first phase of the construction period and the combined water retention during the second phase of the construction period.The results found that the maximum water head difference of the three upstream and downstream gradually increases to 75m,102.4m and 200m respectively during the construction period,but the water head and slope value increase and the isoline is more densely distributed near the waterproofing layer.However,the decrease of velocity or discharge indicates that the three retaining effects are enhanced successively,and the dam body is in a stable state of permeability.(2)The seepage stability of the cushion during the construction period of ultra-high faced rockfill dam are studied when the reservoir water level is sudden drawdown under different working conditions.The safety factor is used to judge the anti-sliding stability of the dam slope.The results found that the infiltration line of the cushion layer in direct water retention,the panel and cushion layer combined water retention in the first phase and the panel and cushion combined water retention in the second phase all decreased with the sudden drop of reservoir water level,but the time required to fall to the lowest water level was different.During the first phase of construction,the total seepage flow rate of the combined panel and cushion layer is 4.84×10-5 L/s,which is significantly lower than the total seepage flow rate of the direct cushion layer is 2.06×10-4 L/s,and the total seepage flow rate of the combined panel and cushion layer is 7.49×10-5 L/s between the two.The relation of hydraulic slope of the three is similar to that of seepage flow.With the sudden drop of reservoir water level,the safety factor of upstream dam slope decreases,while that of downstream dam slope increases,and both the upper and downstream dam slope safety factors meet the requirements of dam slope stability.(3)The paper further investigates the influence of the permeability coefficient of key materials and the fall speed of reservoir water level on the permeability of the dam body during the construction period of ultra-high face rockfill dam.The results show that the dam head difference,hydraulic gradient and flow velocity increase gradually with the increase of the permeability coefficient of cushion area,transition area and main rockfill.However,because of the different functions of the three dam materials,the seepage parameters of the dam body are slightly different,and the dam body is in a stable state of permeability.The faster the water level of the reservoir in front of the dam,the shorter the time required for the dam body to drop to the lowest water level and the greater the hydraulic slope drop,the more detrimental to the seepage stability of the upstream dam slope.Meanwhile,the shorter the time required for the upstream dam slope to reach the minimum safety factor,the safety factors of the upstream dam slope meet the requirements of dam slope stability. |
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