Analysis of Seepage Field Characteristics of Water Diversion Power Generation System of Pumped Storage Power Station

Based on the diversion power generation system project of the Hami pumped storage Hydropower Station, the drainage substructure method is used to accurately simulate the drainage holes of the plant area, and the three-dimensional finite element model of the plant area including the main faults and reflecting the seepage characteristics of complex rock mass is established. Based on the inversion analysis of the natural seepage field, the distribution characteristics of the three-dimensional seepage field in the plant area were studied, and the rationality of the layout of the anti-seepage drainage system was evaluated. The results show that under the action of the anti-seepage drainage system, the groundwater level of the mountain is greatly reduced, and the descending funnel is formed in the three caverns, the seepage flow is controllable overall, and the permeability stability meets the engineering safety requirements.


Drainage Hole Simulation Method
There are many simulation methods for drainage holes, and there is a contradiction between simulation accuracy and calculation amount [1].On the one hand, in order to meet the needs of engineering practicability, the drainage hole should be simulated by equivalent method as far as possible to reduce the complexity of modeling and avoid the appearance of a large number of grids, such as rod element, pipe instead of hole and joint instead of well.Such methods are small in calculation, but the equivalent simulation reduces the calculation accuracy.On the other hand, if the simulation accuracy is considered, the physical size of the drainage hole must be accurately simulated, so it is inevitable that the grid near the drainage hole needs to be finely divided, which increases the modeling difficulty and calculation amount, but the calculation accuracy is high.Commonly used methods such as air unit method and drainage substructure method, and many scholars have proposed some other methods.Such as improved drainage substructure method, drainage hole quasi analysis method, manifold unit method, etc. [2][3].
This paper focuses on two drainage hole simulation methods, "drainage substructure method" and "fracture-substituting well method", and compares the two simulation methods through theoretical As a strong drainage medium distributed in rock mass, drainage holes essentially play a role in reducing pressure through the boundary of holes.In order to evaluate the effect of the drainage system, the boundary conditions of the drainage holes must be reflected correctly during the numerical analysis.When the drainage hole intersects with the free surface, the boundary conditions of the drainage hole are more complex, and there are three categories as follows [6].The first type of boundary condition is the escape type boundary, as shown in Figure 1 (a), the seepage flow discharged from the drainage hole can always be discharged through the drainage gallery at the lower end, and the boundary condition of the drainage hole AB section satisfies φ<Z,q_n=0, while the boundary condition of the drainage hole section satisfies Bccφ =Z,q_n≤0.For example, the vertical drainage hole between the dam body and the two drainage galleries satisfies such boundary conditions.The second category is the constant head boundary condition, as shown in Figure 1 (b), whose head value generally depends on the floor elevation of the drainage gallery connected to it.The third type of boundary is a mixed boundary composed of the first type and the second type of boundary conditions, and belongs to the boundary conditions for which the total flow rate Q is known.As shown in Figure 1 (c), the boundary of the AD section satisfies the escape boundary of the first type, while the rest of the DC satisfies the water head boundary conditions.In the third type of boundary, the water head value in the drainage hole is generally unknown, which is determined by the total displacement Q of the iterative algorithm.

The Comparison between the Method of "Replacing Well with
Fracture" and the Method of "Drainage Substructure"

Example Verification Analysis
The cubic rock mass is simulated.The length, width and height of the model are 60m, 30m and 20m respectively, as shown in Figure 2. The permeability coefficient of the homogeneous rock mass is 2×10 - 7 m/s.The drainage holes were simulated by "drainage substructure method" and "fracture-substituting well method" respectively.A total of 5797 nodes and 4800 elements are divided in the finite element model of "replacing well with fracture method".In order to make the change of the groundwater surface line more obvious, a fixed head of 18m was selected for the upstream surface and 15m for the downstream surface.The bottom of the comprehensive and equivalent narrow slit of the drainage hole was the seepage boundary, and the bottom and left and right boundaries of the model were treated as impervious boundaries.From the perspective of well array and permeability distribution of rock mass, the water conductivity of each drainage hole is shown as a pulse type mutation, and the isobar of the drainage hole profile is shown in Figure 3.This mutation results in the emergence of a "fundive-shaped" head depressurization zone, forming a "wavy" head line distributed along the direction of Wells in the seepage field.The "trough" is located at the drainage hole, and the "crest" is located in the middle of the two drainage holes.The head difference between "trough" and "crest" gradually decreases with the filling of the drainage holes.The water level of this wave will be close to a straight line, and the characteristics of the seepage field will be consistent with the seepage field under the action of a narrow slit.In the numerical simulation of "drainage substructure method", a "wavy" water head line distributed along the direction of Wells is formed in the seepage field.From the perspective of the permeability structure, the water conductivity shows a pulse-type mutation, which makes the permeability of rock mass extremely uneven along the direction of Wells.As can be seen from the figure 4 below, the infiltration line at the drainage hole drops more rapidly and the water level decreases greatly.The dip of the wetting line is slow and the drop of water level is low.The saturation line of the profile at the hole drops to the lowest point at the drainage hole.Due to the different water heads on the upstream and downstream surfaces, the contact position between the upstream and the drainage hole is higher, while the downstream contact position is lower.The interhole profile wetting line drops to the lowest at the lower part of the drainage hole, and the interhole profile wetting line is slightly higher than that at the hole, as shown in Figure 4.Although the scale of the drainage hole in the rock mass is much larger than that of the pore pipe in the porous medium, the structural characteristics are similar.From a more macroscopic perspective, ignoring the personality of the drainage hole and the rock mass between them, the corresponding permeability coefficient can also be assigned to a certain section of the drainage hole screen as a relative whole under the condition that the seepage flow rate and water head are equivalent.In this way, it is equivalent to the existence of a strong permeability guiding belt in the rock mass, and its function is exactly equivalent to that of a narrow water guiding slit.The equivalent joint profile isobaric line is shown in Figure 5. Through calculation, a stable seepage field is finally obtained, and the distribution of the head line of the section X=6m intercepted is shown as follows: X=6 of the "drainage substructure method" is the section of the drainage hole, where the head drops significantly under the strong water conduction effect of the drainage hole, and the seepage rate of the drainage hole is 11.41m 3 /d.The potential line distribution of the head of the two simulation methods is shown as Figure 6.Through manual trial and error, the permeability coefficient of the equivalent joint is 3.73×10 -5 m/s, the permeability flow rate is 11.70 m 3 /d, and the relative error is 2.54%.

Calculation Model
The fine simulation of the drainage holes of the three caverns, the lower and middle horizontal sections was carried out, and the distribution of the seepage field under various working conditions in the plant was deeply analyzed, which laid a foundation for further water exosmosis analysis and optimization of seepage control scheme in the water delivery tunnel.According to the general principles and calculation requirements of seepage analysis, combined with the actual situation of the project, a three-dimensional integral finite element model reflecting the topography, geological structure, plant layout and antiseepage drainage system is constructed.The method and scope of the finite element model for seepage analysis during the operation period are consistent with that of the natural period model, and hexahedral and tetrahedral elements are adopted in the whole model.A total of 3539304 units and 714238 nodes are divided, as shown in Figure 7 and 8.

Computational Boundary
The boundary types of the stable seepage calculation model mainly include known head boundary, seepage boundary and impervious boundary: (1) Known water head boundaries include upstream cut-off boundary, downstream cut-off boundary, drainage hole, and areas below the natural water table on both sides of the model.The upstream boundary takes the value of the water head at the watershed.The cut-off boundary of the downstream model takes the value of the reservoir water head under the running period.The cut-off boundary below the natural water table on the left and right sides of the model is the boundary of the given water head (take the water table interpolated by the inversion analysis).
(2) Impervious boundary includes the partial boundary of the interception boundary on the left and right sides of the model except for the given groundwater level and the bottom surface of the model.
(3) Except for the above boundaries, the drainage holes (middle flat section, lower flat section and workshop area) are all seepage boundaries; The surface of the hillside above the natural groundwater level is the boundary of seepage, and other (drainage galleries, workshops, diversion pipes) are also considered as the boundary of seepage.

Effect Analysis of Anti-seepage Drainage System
In order to accurately evaluate the rationality of the layout of the anti-seepage drainage system and the occurrence state of the groundwater in the mountain during the operation period of the plant, the seepage finite element calculation method was used to calculate and analyze the seepage of the power generation system based on the permeability coefficient and boundary of each zone obtained from the inversion of the natural groundwater level.According to the normal operation conditions, the seepage characteristics of the plant area were analyzed.In order to more intuitively display the distribution law of the seepage plants in the plant area, four typical profiles of the water diversion power generation system were selected to display the calculation results.The specific positions of each typical profile are shown in FIG. 9.The distribution characteristics of seepage field in the plant area under normal operation of drainage holes are analyzed.As can be seen from the distribution of total water potential lines in each section, the infiltrating surface will drop significantly after passing through the drainage hole, and the curtain of the drainage hole plays a good drainage effect to prevent groundwater leakage from the surrounding rock mass to the factory.The distribution of total water potential lines in each section of the anti-seepage drainage system is shown in FIG.10. (1) Seepage Field of Water Transmission Tunnel During Operation of the Plant After the excavation of the underground powerhouse, an "underground descent funnel" is formed.Under the action of the drainage system, the groundwater penetrates from the surrounding mountain to the powerhouse.Without considering the water seepage in the tunnel, the section H1 with large seepage potential is selected as the representative to analyze the seepage potential of the tunnel.The pressure water head at the end of the horizontal section of the middle horizontal section is 0, and the osmotic pressure at the beginning and end of the horizontal section of the tunnel is little different, and the external water pressure drops from 0.17MPa at the beginning of the middle horizontal section to 0PMa at the end, that is, under the action of the drainage hole, the infiltrating surface drops below the middle horizontal section of the tunnel.From the middle horizontal section to the lower horizontal section, the external water pressure of the tunnel increases rapidly, reaching a maximum value of 1.07MPa at the initial end of the lower horizontal section, and decreasing to 0.29MPa at the end of the lower horizontal section through the action of the drainage hole.The pressure reduction effect of the drainage hole is significant.
(2) Infiltration Gradient The permeability of each part of the plant is low, and the penetration slope span is small.The maximum penetration slope occurs in the lower horizontal section of the water transmission tunnel, and its maximum value is 1.28, which is smaller than the permeability slope of the rock mass, and is at a low level, meeting the requirements of permeability stability.When the plant is in the normal operation period, the total water head distribution can be obtained from the H1 section of the axis position of the water transmission tunnel.Since the upper flat section of the water transmission tunnel is located above the underground water surface and is not affected by the drainage hole, its penetration gradient is the smallest in each key part.Under the action of the drainage hole, the infiltration surface drops obviously, the total water head line is encrypted in the middle flat section, the permeability gradient increases, and the permeability slope increases.At the flat section under the pressure steel pipe, due to the strong drainage effect of the "human" shaped drainage hole, the infiltrating surface drops rapidly, and its infiltration slope increases again, reaching the maximum value of various key parts, and forming a drop funnel in the plant.When the infiltrating surface reaches the cavern of the main powerhouse, its water head is low after the action of the drainage holes in the plant area.Due to the overall quality of the rock mass, there is no obvious leakage point, and the infiltrating surface continues to fall to the elevation level of the cavern floor of the main powerhouse.
(3) Seepage Flow Regardless of the internal water exosmosis condition, it can be seen from Table 1 that the seepage flow of the middle horizontal section, the lower horizontal section, the middle layer of the workshop and the lower gallery are 32.47 m 3 /d, 203.62 m 3 /d, 619.36 m 3 /d and 874.23 m 3 /d, respectively, and the total seepage volume of the gallery is 1729.68 m 3 /d.The calculation results show that the drainage hole screen plays a good role in the plant.The lower the elevation of each drainage gallery, the greater the seepage flow.The seepage flow of the middle and lower drainage galleries in the plant area accounts for a relatively large proportion, the seepage flow of the middle corridor accounts for 35.8% of the total seepage flow of the corridor, and the seepage flow of the lower corridor in the plant area accounts for 50.54% of the total seepage volume of the corridor, and the leakage volume of other parts is small on the whole.The total leakage volume of the plant area has been effectively controlled, and the drainage arrangement is reasonable and effective.

Conclusion
Based on equivalent continuous model, the drainage substructure method is adopted.Based on the inversion analysis of the natural seepage field, the effect of the drainage curtain during the operation was analyzed, and the seepage control effect of the anti-seepage drainage system in the whole underground powerhouse area was evaluated.When the anti-seepage drainage system runs normally, the external water pressure of the water delivery tunnel is at a low level, and the depressurization effect of the drainage hole is remarkable.The seepage slope of each key part is smaller than the allowable seepage slope of rock mass, and the seepage water is discharged through each drainage hole through the drainage gallery.The results show that the drainage hole screen plays a good role in the plant area, the total leakage of the plant area is effectively controlled, and the drainage arrangement is reasonable and effective.It is reasonable and effective to adopt the anti-seepage drainage design principle of "plugging and drainage outside the plant, supplemented by drainage inside the plant" [7], which can provide a reference for similar projects.In the operation process of pumped storage hydropower station, the underground seepage field is often dynamic.The numerical simulation of the seepage field of the underground powerhouse based on the stable seepage field in this paper has certain limitations, and the model does not simulate the traffic tunnel, ventilation tunnel and self-contained drainage tunnel, nor does it study the drainage state of each cavity in operation, so it cannot truly and comprehensively simulate the actual situation.Further research should be carried out.
saturation permeability coefficient tensor; 3 i k -The value of the permeability coefficient in the saturation permeability coefficient tensor related only to the third axis; r k --Relative water permeability; β --saturation-unsaturated selection constant, equal to 0 in the unsaturated region and 1 in the saturated region; s S --Elastic water storage rate ; Q --Source sink item.

Figure 3 .
Figure 3. Fine simulation of Y=0 water head line in the drainage hole.

Figure 4 .
Figure 4. Comparison of infiltration lines between pores and pores in the substructure.

Figure 6 .
Figure 6.Distribution of equipotential lines at X=6 for different simulation methods.

Figure 8 .
Figure 8. Layout of drainage holes in the factory area.

Figure 9 .
Figure 9. Schematic diagram of typical section location.

Figure 10 .
Figure 10.Potential line distribution of total water head of each section in normal operation of impermeable drainage system.