1. 1 project overview

Huangbizhuang Reservoir is located on the main stream of Hutuo River, 30 kilometers northwest of Shijiazhuang, the capital of Hebei Province, with a total storage capacity of1210 million m3, a design water level of 127.6m, a normal water level of 120.0m, and a main dam project of190 m. 1968, and the elevation of dam crest is expanded from 125m to128.7m.. The main dam is located at the foot of Ma 'anshan Mountain, starting from the left pier of the normal spillway in the south, crossing the Hutuo River bed in the north and connecting with the right pier of emergency spillway. The total length of the main dam1843m, and the dam height is 30.7m.

1.2 general situation of engineering geology

The pile number of the main dam project ranges from 0+156.038 ~1+999.076, and it also spans four geomorphic units, namely Ma 'anshan residual hill, first terrace, river bed and second terrace.

The 0+ 156 ~ 0+300 on the right bank of the river bed is the first terrace, with a red soil layer of 3.0m thick. The bedrock is marble phyllite and its interlayer, and marble phyllite is seriously dissolved.

The pile number 0+300 ~ 1+000 is the riverbed, and the riverbed elevation is1000 mm. The bedrock is the interbedding of Archean Pre-Sinian silicified limestone with phyllite, phyllite and marble. Above the bedrock are sandy pebbles, gravels and sand layers, and the covering layer is 7 ~ 16m thick.

The range of the left bank of the river bed 1+000 ~ 1+999 is a secondary terrace with an elevation of 1 17 ~ 125m. The bedrock is phyllite and marble, covered with laterite pebbles with a thickness of about 5m, and the surface is clayey silt and loam.

1.3 seepage control measures for main dam

The riverbed upstream of the main dam is covered with clay, with a length of 180m and a thickness of 1 ~ 3m, which is connected with the dam toe. There are two drainage ditches at the foot of the downstream slope of the dam, one for draining rainwater from the dam surface and the other for draining seepage from the dam foundation, both of which flow into the downstream Hutuo River. The downstream toe of 0+450 ~ 0+989 section of riverbed is pad drainage, and its foundation is connected with the coarse sand layer of natural foundation. There is a local horizontal drainage sand cushion downstream of the right bank dam axis and a drainage sand belt downstream of the left bank dam axis.

1.4 Arrangement of dam foundation seepage observation facilities

The main dam * * has seven dam foundation seepage observation sections, namely 0+258, 0+450, 0+705, 0+850, 1+050, 1+200 and 1+400, with a total of 26 * * meters.

2. Dynamic analysis of main dam groundwater

From the isobar of the main dam groundwater (reservoir water level 1 19.0m), it can be seen that the groundwater dynamics of the main dam are closely related to the hydrogeological conditions of each section of the foundation. Generally speaking, the groundwater level in the upstream is high, the groundwater level in the downstream is low, the groundwater level at both ends is high, and the groundwater level in the middle is low, but the two ends are asymmetrical, and the groundwater level in each section also has different changes. There is little difference between the upstream and downstream groundwater levels in the riverbed, which is generally 1 ~ 3m. The groundwater level on the right side is high and the groundwater level on the left side is low, but the water level difference is generally less than 2m. From the hydrograph of piezometric pipe water level and reservoir water level over the years, it can be seen that the change of pipe water level is closely related to reservoir water level, and it changes obviously with the rise and fall of reservoir water level. The lag time is only about two days or less than one day, and the fluctuation of pipeline water level is not large. Generally, the fluctuation value of the upstream piezometer water level is less than 2m, and that of the downstream piezometer water level is less than1m.. The piezometer near the drainage ditch is in the mattress drainage belt, so the water level changes little.

The groundwater level of the second terrace north of the left bank 1+000 is quite different from that of the river bed. With 1+200 as the center, the groundwater level drops slowly to both sides and sharply near the river bed. For example, when the section of No.24 pipe is 1+200, the upstream of dam axis is 12.25m, and the water level of reservoir is 1 19.2m, the water level of the pipe reaches 1 18.56m, and the riverbed is on the same axis. Generally speaking, the secondary terrace groundwater responds slowly to the change of reservoir water level, which is suitable for the geological conditions with weak rock permeability.

South of pile number 0+300 on the right bank, the groundwater level of dam foundation is also higher than that of riverbed section, but lower than that on the left bank. If the reservoir water level is above119.0m, the water level of 1 # pipe downstream of the dam axis in section 0+258 is about 6m higher than that of 12 # pipe at the same position in the riverbed section.

In the 0+450 section of the river bed, the water level of upstream piezometer No.5 is 12.25m away from the dam axis. It is found that every time the reservoir water level rises to about 1 18.0, the water level in the pipe suddenly changes, which is about 4m higher. The abrupt change of reservoir water level is not completely consistent, and the correlation and regularity between abrupt change and reservoir water level are poor. When the reservoir water level drops rapidly, the pipeline water level drops to the original correlation curve; When the reservoir water level drops slowly, the pipeline water level slowly returns to the position of the original correlation curve. The sudden change of pipeline water level may be caused by cracks in the dam above117.0m. When the reservoir water level is high, the reservoir water seeps into the pipeline along the pipeline wall, which makes the pipeline water level rise. The reason is that the 5 # piezometer is on the upstream slope protection of the dam, with the nozzle elevation of 120.84m and the elevation of the pipe body entering the dam soil surface of118.5m. When the water level of the reservoir is 1 18.0m, the dam soil is already very thin. When the pipe body is combined with the soil, In addition, when the reservoir water level drops, the pipeline water level drops with lag time, and the reflected correlation decreases linearly. However, when the reservoir water level drops slowly, the pipeline water level no longer drops slowly with the reservoir water level, but returns to the original correlation curve. The piezometers with the same section as the 5 # pipe and the same position of adjacent sections on both sides have no abnormal phenomena, so the abnormal response of the water level of the 5 # pipe is isolated and needs further analysis and research.

3. Correlation between piezometric tube water level in dam foundation and reservoir water level.

In order to find out the relationship between the reservoir water level and the piezometer water level of each section of the dam foundation, and then infer the seepage situation of the dam foundation at high water level in the future from the measured data. According to the existing measured observation data, several representative pipeline water levels corresponding to relatively stable reservoir water levels are selected, and the regression linear calculation is carried out on the above-mentioned measured data by mathematical statistics, so as to obtain the expression between the reservoir water level and the pipeline water level of each observation well, and the linear expressions are used to predict 124.0m, 126. 1m and 65438+.

3. Water level selection of different stable reservoirs1year

In the calendar year, several representative reservoir water levels are selected, and it is required to be stable at the reservoir water level for more than 5 days, that is, to be stable at the water level.

3.2 Selection of piezometric tube water level under different reservoir water levels over the years

According to different stable reservoir water levels, the pipeline water level is selected, and the measured pipeline water level value of the same day is selected on the same section, regardless of the lag time caused by the influence of wheelbase. For the pipeline water level with the lowest reservoir water level over the years, the corresponding lowest pipeline water level value is taken, and for other pipeline water level values, the rising reservoir water level is taken, so that the lag time is relatively consistent, and the process of reservoir water level rising is adopted during operation.

3.3 When predicting the water level of high reservoir, predict the piezometer water level of each section.

The predicted reservoir water level value is directly substituted into the regression equation h=a+b*H, and the corresponding predicted pipeline water level value can be obtained. Through regression analysis and calculation, the correlation coefficient is significantly correlated at α=0.0 1 level, mostly around 0.80. Predict the water level value of piezometer; Constant term; B regression coefficient; H reservoir water level),

4. Analysis of seepage stability calculation results of main dam foundation

For an earth dam built on a strongly permeable foundation, because the permeability coefficient of the fill in the dam body is quite different from that of the strongly permeable layer, the water in the dam foundation is the main factor in seepage analysis, and the dam body can be regarded as a relatively impermeable part, while the artificial cushion is still regarded as a permeable part because of its small thickness and long length. In order to determine the distribution of seepage pressure in each section of dam foundation and its influence on dam foundation and dam body, seepage analysis and calculation are carried out by using measured data. The main contents are as follows: When the reservoir water level is 1 18.0m and higher than 1 18.0m, the predicted high reservoir water levels are 124.0m, 126. 1m, respectively.

4. 1 Horizontal permeability gradient and escape gradient of dam foundation

The main dam bed section is characterized by soil, sand, gravel and sandy pebble layers from top to bottom, and the allowable permeability ratio of the dam foundation is reduced to 0. 1. When the water level of the reservoir is118.0m and exceeds118.0m, the horizontal permeability gradient of the riverbed section is calculated by using the measured data. It is predicted that the horizontal slope of dam foundation with reservoir water level 127.6m is only 0.037, which meets the requirements of safe operation of the project. In addition, all the sections except 0+850 escape, and the escape ratio drops to 0. 1765, which is less than the critical escape ratio. Therefore, under the action of high water level, the possibility of contact erosion between bedrock layers in the riverbed section of the main dam is very small, and no escape deformation will occur.

At the left and right ends of the main dam, the permeability and seepage conditions of the dam foundation are poor, and the downstream drainage conditions are also poor. In the calculation of measured data, the horizontal permeability of dam foundation is reduced to 0.0507, which is 0.02 ~ 0.03 higher than the riverbed section on average. Even if the predicted reservoir water level is 127.6m and the horizontal permeability gradient drops to 0.077, it still meets the design requirements.

4.2 Upstream bedding permeability gradient

When calculating the infiltration slope of upstream blanket, only natural soil layer and artificial blanket are considered for soil layer thickness, and natural siltation is not considered. The infiltration thickness is calculated according to its permeability coefficient. When the reservoir water level is 1 18.0 and higher than 1 18.0m, the infiltration ratio of the blanket in front of the dam is less than 3.0, and when the predicted water level is 127.6m, the infiltration ratio of the blanket drops to 3.925. In view of the fact that the riverbed dam base of the main dam is a good natural filter layer with good seepage control conditions. However, when the reservoir water level is high, monitoring should be strengthened.

4.3 Dam foundation seepage

The main permeable layers of the main dam are sandy pebbles and gravels in the riverbed, so this time only the seepage flow of the riverbed dam foundation is estimated. According to the measured data, when the reservoir water level is 1 18.0m or above18.0m, the estimated leakage of dam foundation is less than 0. 19m3/s, and when the design water level is 127.6m, the leakage of dam foundation is very small.

According to the seepage analysis results calculated from the measured observation data, the seepage stability calculation results of the main dam foundation meet the design requirements, and even when the reservoir water level is 127.6m, the seepage calculation results meet or approach the design requirements.

5. Concluding remarks

Through the analysis of the observation data of piezometric tube at the dam foundation of the main dam, it is considered that the seepage control performance of the dam foundation is effective after years of operation. The seepage of dam foundation is stable and tends to be safe. Even below the design water level, the bedrock of dam foundation will not be destroyed by erosion. However, considering the high groundwater level at the left end of 1+200, it is necessary to reinforce the upstream horizontal blanket and other anti-seepage measures.