2.1 Prototype description
YHS is located in Akesu Wushi County, Xinjiang Uygur Autonomous Region. AHS is located in the Tashigan River, the left bank of the Gobi Desert. The length of YHS Water Diversion Project reaches 55 km. Power station is superimposed with hydropower station tail water power. The diversion line is found along the left bank of the Gobi Desert from west to east along the contour layout1. The channel gap of power generation contains tail water. Yamansu Hydropower Plant is composed of pressure front pool, pressure steel pipe, main workshop, auxiliary workshop, drain trough, and tailrace canal, which is located in the lower part of the front of the mountain. The ground elevation is 1550–1540 m while the base elevation is 1479.67 m. The maximum depth of the excavation reaches 70 m. For a large project, lowering the groundwater level is crucial for excavation.
The outcrop strata of the plant are mainly sand gravel layer that is based on the Pleistocene alluvium in the Quaternary System (Q3al+pl). The sand gravel that is widely distributed in the engineering area has the following properties: gray to green gray; egg; 60% to 70% gravel content; round and second round; better local sorting; dense; the limestone composition; sandstone-dominated; has a general particle size of 2–15 cm; and mainly fine-grained fillings, thereby meeting the requirements of the foundation bearing capacity of the workshop. The local sand–gravel distribution of Quaternary Holocene alluvium (Q4al+pl) on the local gully site development has the following properties: has an average thickness of 0.5 m; gray to green gray; egg; 60% to 70% gravel content; round and second round; better local sorting; the limestone composition; sandstone-dominated; has a general particle size of 2–15 cm; and fine-grained fillings are mainly composed of silt and loose structure and need to be removed. In addition, according to the 1971–2010 data statistics of Wushi County Meteorological station: the years of average annual precipitation is 112 mm. Precipitation is mainly concentrated in May to September, accounting for approximately 73% of the annual precipitation.
According to the borehole data, the slope is composed of the alluvium (Q3al+pl) sand gravel in the Quaternary series, which is green gray, has a dense structure, poor sorting, and the secondary angular and round. Gravel with a general particle size of 6–15 cm and 3–5 cm has approximately 5%–10% and 65%–70% of limestone composition, mainly sandstone, respectively. The remaining composition correspond s to fine silt. No sand or sand lens body are observed in the 80-m depth of drilling, and it has the forming condition of the slope of the workshop.
From top to bottom, the powerhouse strata of YHS are the alluvium (Q3al+pl) sand gravel layer and the Quaternary lower-Middle Pleistocene (Q1-2gl) semi-cementation conglomerate in the Quaternary series. Meanwhile, the bearing layer is the fourth Upper Pleistocene (Q1-2gl) half cementation conglomerate. The hydraulic conductivity of sand gravel is 2×10−2 cm/s–9x10−3cm/s, thereby belonging to medium permeability, and the hydraulic conductivity of half cementation conglomerate is 1x10−2–8x10−3 cm/s. The basic conditions of the foundation pit of Yamansu hydropower plant are shown in Table 1.
YHS (Fig. 1) is located in Akesu, Xinjiang and belongs to the Ⅲ hydrogeological area. Its groundwater is phreatic water. The excavation of the foundation pit is approximately 70 m. Based on the hydraulic parameters calibrated in situ pumping tests, the use of finite difference method was suggested to validate the proposed dewatering scheme and to optimize the layout of the tube well. Moreover, the influence of the depth of the cutoff wall, the permeability coefficient, and the thickness on the drainage must be discussed urgently. The stability of the slope and the failure sensitivity of the tube are analyzed. Finally, the optimization suggestion of the dewatering scheme is given.
2.2 Concept model
The YHS foundation pit dewatering was divided into initial drainage and later dewatering stages. In the initial drainage stage, the tail canal that extends downstream was used as the drainage channel. Groundwater was drained to the canal and finally flown into the Tashigan river. In this stage, the groundwater level was lowered to the 1503.10 m workshop platform. The groundwater level was reduced from 1512.91 m to 1501.00 m. The drawdown was approximately 12.00 m. In the dewatering stage, the cutoff wall and pumping wells were installed on the 1502.80 m platform. The groundwater level was decreased to 1478.67 m (1.0 m below the building base surface). The excavation under the 1502.80 m platform was performed with the gradual descent of the foundation pit water level. The pumping and drainage due was about 12 months.
2.3 Mathematical model
Based on the concept model, the mathematical model was presented as follows:
where k is the hydraulic conductivity, (m/d); w is the amount of water flowing in or out of the aquifer from the vertical direction under unit time (inflow is positive, outflow is negative), (m3/d); Ss is the water storage rate; ω is the seepage region; h is water level, (m); t is time (d); and h0 is the initial time water level (m).
Several iterative solutions were used in the numerical simulation. To enable the program structure to satisfy any iterative solution format, the difference equation form of the unknown and known head are merged, the model that contained the calculated units written the difference equation, and then the linear equation group was obtained:
where {A} was the water head coefficient matrix, {h} is the desired head matrix, and {Q} is all the constant and known items contained in each equation. Based on the two matrices, the iteration method is used to solve {h}.
2.4 Numerical model
According to the ground topographic map of YHS workshop and excavation plan, the influence radius of dewatering was 1000 m, and the model size was 2300 m ×2300 m.
Non-uniform rectangular mesh was used in horizontal and vertical directions by using hexahedral mesh subdivision. First, the 23 m × 23 m grid was used to divide the three-dimensional model. To improve the calculation accuracy, the mesh in the vicinity of the foundation pit was encrypted as 1 m × 1 m. The planar meshes were divided into 526 rows and 526 columns. A total of 315,844 grids were generated (Fig. 2).
According to the investigation report, the initial groundwater level of the foundation pit is 1512.86 m, and the ground water table is not fluctuating. The initial head of the first stage drainage was set as 1512.86 m. According to the original drainage design scheme, the first stage of slope drainage groundwater level was reduced to 1503.00 m, and the initial head of the second stage was 1503.00 m.
The area of the excavation of the workshop was set as the constant head boundary. The head value was 1512.86 m. The bottom was the zero-flux boundary, and the top of the phreatic aquifer was the free surface boundary (0 flow boundary). Drain PLATFORM was set in 1503.10 m, and the drain water level was 1502.00 m. The hydraulic conductivity of the drain ditch was 0.02 cm/s. The hydraulic conductivity of the gravel layer was 2×10−2 cm/s.
2.5 Working condition
According to the dewatering and drainage schemes, the numerical simulations were carried out in three kinds of working conditions:
Working condition 1: Due to the initial use of slope drainage in the foundation pit of the workshop, the tailrace canal, which has been extended downstream, was used as the channel of groundwater drainage in the initial stage of foundation pit. By drainage method, the ground water level of the foundation pit was reduced from 1512.86 m to 1501.00 m, that is, to the plant platform (1502.80 m), to meet the requirements of drawdown, hydraulic gradient, and slope stability.
Working Condition 2: In the late stage, pumping wells were used to drain groundwater without cut-off wall to check whether the 40 pumping wells on 1502.86 m platform reduce water level to 1478.67m (1.0 m below the factory building).
Working Condition 3: If working condition 2 cannot meet the drawdown requirements, then simulation was performed to check whether the 40 pumping wells on the 1502.80 m platform with cut off wall can decrease groundwater level to 1478.67 m.
2.6 Parameter reversion and model calibration using field pumping test
2.6.1 Pumping and observation well
The arrangement of pumping and observation wells is shown in Fig. 3. Pumping wells S1 to S6 were used as pumping/observation wells. Observation well G1 was installed in the center of the rectangle layout. Pumping/observation well G2 was installed in the S1\S2\S4 extension line layout. Pumping/observation well G3 was installed in the direction of the extension line of S1\S3\S5. The ground settlement observation points were arranged on the opening line of excavation slope within the range of the pumping test.
According to the design requirements, the main tests in the first stage were performed on the north side. The depth of wells S1, S4, S5, G2, G3, and S6 was 56 m; the depth of wells S2, S3 was 52 m; and the depth of observation well G1 was 40 m. The main technical parameters of each well are shown in Table 2.
The experimental observation was equipped with a XRB30 series multi-channel data recorder, which automatically collected and stored the detection data transmitted by the water level meter and the flowmeter according to the set time and frequency. After the experiment was completed, screening and analysis was conducted. The whole experiment was set up at intervals of 1 minute, collecting and storing the water level gauge readings and pumping well flow rates of each well. After completing a set of tests, the data were exported and analyzed in a timely manner.
According to the design requirements, observation points were installed in the EL.1522 road and the opening line plant upstream to monitor pumping test and ground settlement. The settlement observation plan in the foundation pit right layout fixed the control points, and the total station set up regular settlement observation of settlement observation point.
2.6.2 Pumping test process
The water pumping test scheme and process are shown in Table 3.
First, single well pumping test with constant pumping rate was carried out. The water level was fully recovered between two pumping tests. In the course of the test, the water level and the observation well were synchronized with the water level observation, and the ground subsidence and synchronous monitoring were carried out.
When the pumping wells were open, the water level was observed at the specified time interval, including 1 ', 2', 3 ', 4', 6 ', 8', 10 ', 15', 20 ', 25', 30 ', 40', 50 ', 60', 90 ', and 120 ‘. The observation interval after 120 ‘ was 30 min. The observation from 480' to 1200 ' interval was 60 min. The observation interval until the pump was stopped was 2 h. The water level was recovered after the pump was stopped, and the time interval was recorded with the pumping test.
During the pumping test, the pumping rate was also being observed. The time interval was 30 min. The readings of the flow meter were used, and the accuracy was less than 0.1m3. If the pumping rate was too small and the water level decreasing rate was slow, then a larger flow pump was used. The observation times of the pumping rate were synchronized with the observation of the groundwater level. In the process of the whole pumping test, the pumping rate in the pumping well was kept constant.
On November 12, 2016, test pumping and observation wells were installed. On November 15, pumping equipment and instrument installation were completed. On November 18, the pumping test began. On December 2, the experiments were finished.
2.6.3 Parameter reversion
The information of single well pumping test (S5) is listed in Fig. 4. The hydrogeological parameters were calculated on the basis of steady and unsteady flow test data. The hydraulic conductivity of the aquifer was determined according to the specific hydrogeological conditions of the test field, and the corresponding methods were selected for calculation and comparison (Fig. 5).
Theis unsteady flow fitting calculation method was adopted using the preliminary data. According to the fitting curve, the relevant hydrogeological parameters were calculated as shown in Table 4.