Investigating the Hydrological Relationship between the North Taihang Tunnel and Tianshengqiao Nine Falls

Abstract

The impact of a tunnel construction on the groundwater system depends on various parameters and cannot be easily predicted. Along these lines, a deep understanding of the hydrological relationship between tunnels and surface water is considered of vital importance for ensuring safety during railway construction. Upon completion, the North Taihang Tunnel will be one of four extra-long railway tunnels running through the natural ecotope and level-3 protection areas of the Tianshengqiao National Geological Park in Fuping County, Hebei Province. It will be 1 km away from the Tianshengqiao Nine Falls, which is known as a breathtaking landscape feature in Northern China. Local government, societies, and railway design units have raised concerns about whether the construction and operation of the North Taihang Tunnel will affect the Tianshengqiao Nine Falls. To effectively address this issue, in this work, hydrogeological mapping and hydraulic potential-energy calculations were performed in conjunction with hydrochemical and geological structure analyses. The groundwater system units in the study area were divided and the water source of the nine-level waterfall was determined retrospectively. In addition, the recharge of groundwater to the nine-level waterfall was calculated, the hydrogeological properties of the linear structure were analyzed, and the dominant channels of underwater discharge in surface water were compared and studied. The extracted results indicated that: (1) The Tianshengqiao Nine Falls represent a seasonal fall landscape, which is mainly supplied by surface water formed by precipitation and a low proportion of groundwater supply. (2) The water bodies of the North Taihang Tunnel project and Tianshengqiao Nine Falls belong to two independent groundwater systems. (3) No linear structure that connects these two groundwater systems has yet been discovered. It is widely accepted that a minor possibility of hydraulic connection might be present between the North Taihang Tunnel and Tianshengqiao Nine Falls. This work analyzed the water quantity of Nine Falls, determined the hydraulic relationship between the tunnel project and the waterhead of the Nine Falls, and addressed all stakeholder concerns. The conclusions could provide technological support for the scheduled construction projects.

1. Introduction

1.1. The Origin of the Research Question

The planned railway is a crucial part of the Beijing–Kunming Corridor, a major high-speed railway of “eight horizontal and eight longitudinal” networks in the mid-term and long-term railway plan. It could significantly improve the high-speed railway network. The North Taihang Tunnel is one of four extra-long tunnels [1]. It is 12.5 km long and runs through the denudation Zhongshan Region of Taihang Mountains, which has complicated terrain and steep mountains, as well as ravines and gullies to cross. Both the inlet and outlet have clinoform valley slopes. As shown in Figure 1, the North Taihang Tunnel runs 5.2 km through the Tianshengqiao National Geological Park, where the upslope is 29‰ [1]. Moreover, the section next to the head groundwater units of Tianshengqiao Nine Falls is 1.6 km long. The shortest distance of the tunnel to Tianshengqiao and Tianshengqiao Nine Falls is 977 m and the buried depth in this section is higher than 200 m [2].

The Tianshengqiao Nine Falls in Fuping County is a geological landscape of a type that was discovered for the first time in China [3]. To date, it is also the largest known landscape feature created by an inherent stone bridge of gneiss. Water continuously falls beneath the Tianshengqiao, forming the Yaotai Fall with a fall head of 112.5 m. Due to the valley terrain control, another eight falls (Yinhe Fall, Qinglv Fall, Mawei Fall, Sandie Fall, Wangkui Fall, Qiongjiang Fall, Tianmen Fall, and Shuanglong Fall) are formed straight upward within 800 m [4]. With the Yaotai Fall, these form the Tianshengqiao Nine Falls. The planned railway tunnel is less than 1 km away from the Yaotai Fall [1]. Whether the tunnel construction and operation will affect the falls landscape is a key concern of local government, people from all walks of life, and railway design units. Currently, the above-mentioned issues have not been addressed. Therefore, the goal of this work was to provide a systematic analysis of the underlying origins of this effect.

1.2. Selection of Research Methods

A tunnel can be described as a new groundwater catchment area or drainage channel. A thorough understanding of the hydrological relationship between the tunnel and surface water is considered a hydrogeological necessity in railway construction. In addition to endangering the safety of workers, an inflow of water in the tunnel during the construction could cause exhaustion of shallow groundwater and surface water. This effect could lead to the acceleration of the water circulation and hydrogeochemical processes, and destroy the water-system balance [5,6]. Certain established research methods such as sodium fluorescein, have been extensively used to examine the source of inflow water in tunnels [7,8,9], enabling an understanding of the groundwater system distribution and determining the groundwater flow paths. Additionally, the variation characteristics of river flow have been measured and the infiltration groundwater supply has been analyzed by performing multi-phase flow measurements [10]. The controllable-source audio magnetotelluric sounding method has also been used to investigate the hydrological characteristics of tunnel-excavation regions and determine the possible water inflow areas [11]. According to the literature, the water bursting points in tunnels can be accurately spotted and the inflow water volume can be predicted. Although the application of the above methods provides data based on a single professional perspective, they are not compared or validated taking into account other technological methods. On top of that, the majority of the reported works are based on comprehensive methods. For example, Yan Changhe et al. [12] disclosed the hydrological relationship between the South Taihang Tunnel and the groups of springs that developed in the region. More specifically, the authors performed a comprehensive analysis of the geomorphological features, tectonic connection, lithological characteristics, and spring levels and supply, as well as runoff and drainage conditions. It was concluded that tunnel construction cut off the groundwater seepage channel, resulting in a significant inflow of water in certain sections. Luo Mingming et al. [13] analyzed the existence of various water inflow sources and processed them by combining hydrogeologic survey, hydrology, and hydrogeochemical approaches. The authors found that the concentrated inflow of water in tunnels mainly resulted from karstic water. Fracture water of clastic rocks was the major water head during the stable drainage phase, which increased the accuracy of predicting the inflow water head. In another interesting work, Zhou Xuejun [14] determined the karst zoning in the Dafangshan Karst Tunnel by combining engineering geology, hydrogeological mapping, physical prospecting, and drilling. The water inflow based on season change and horizontal circulation zones was also calculated. Based on the groundwater flow system theory, Zeng Bin et al. [15] explored hydraulic connection karst groundwater systems between the Shuanglong Cave and the tunnel site and estimated the possibility of water inflow by combining hydrogeochemical test results. The authors found that the tunnel construction only affected the groundwater in the finite range of the outlet section. Nonetheless, no threats to karst water in the Shuanglong Cave were demonstrated. Wei Huapeng et al. [16] built a three-dimensional unsteady-flow numerical model of groundwater by using exploration drilling data. In particular, the variation amplitude of the diversion fissure zone was predicted and the degree to which groundwater affected the water inflow under different working conditions was analyzed. Based on the exploration of drilling data, Liu Mou et al. [17] developed a relatively vivid three-dimensional unsteady flow numerical model by adding geophysical prospecting, pumping tests, and long-term observation of groundwater. This model was also used to predict mine inflow and groundwater drawdown under coal-seam mining conditions, while calculating groundwater evaporation, overflow, and aquifer storage under the attack of new water inflow. From the above-mentioned analysis, it can be inferred that a combination of hydrodynamic and hydrochemical fields has been employed to study water inflow properties. Based on the overall characteristics of the frequent transformation between shallow surface water and groundwater in the study area and the limited connection between deep and shallow water bodies, the water flow of Tianshengqiao Nine Falls was determined in this work. Moreover, the characteristics of the groundwater system were revealed and the hydrological relationship between the North Taihang Tunnel and Tianshengqiao Nine Falls was investigated. The concerns of all stakeholders are also addressed and a guarantee of the proper construction of vital projects is provided. The latter was achieved by combining groundwater system division, head recognition of the Tianshengqiao Nine Falls, and calculation of the surface water–groundwater transformation, as well as by performing cross-system water diversion channel and hydrodynamic analyses.

2. Geological Background of the Study Area

2.1. Natural Geography

The study area has been in a state of continuous stable rising since the Mesozoic era, accompanied by intermittent pauses, and local uneven settlement. Thus, the dominant role of high, middle, and low mountains, as well as complicated and changing landforms was taken into consideration. The terrain is generally high in the south and low in the north, high in the west and low in the east. Elevations in most regions are higher than 1200 m. The highest elevation is at the Baicaotuo (2144.5 m), which is the main peak of Tianshengqiao Ditch and the lowest is at the mouth (800 m) of Tianshengqiao Ditch, showing a relative difference of 1344.5 m. In the study area, the landform consists of peak valleys alternating with cliffs, walls, and platforms. The major landform combination consists of overlapping cliff platforms, deep-cutting gorges, U-shaped valleys, and enclosed valleys. Major landforms in the study area include erosional, glaciated, tectonic erosion, and gravity landforms [18].

The study area belongs to the semi-arid region of the central Hebei Province and has a distinctive continental climate. It has a cold, dry, and long winter with little rain and snow. Furthermore, it has a hot and wet summer, which is the main precipitation season and the main period to appreciate the Tianshengqiao Nine Falls. The annual average precipitation is 594.0 mm and the rainy season takes mainly place from June to September. The annual evaporation capacity is 2024.6 mm, reaching a maximum in June and a minimum in January [18].

2.2. Geological Conditions

The study area is located at the foreland on the east of the central orogenic belt in North China [19] and is mainly composed of sediments at passive continental margins, accompanied by tectonic activation or reconstruction of a basement complex [20]. It is less than 2 km from the Longquanguan shear zone, which is the largest shear zone in North China. The study area has undergone several tectonic changes. Structures such as folds and fractures formed by the Yanshan Movement form the major tectonic skeleton of the study area [20]. The Precambrian folds and fractures in the study area mainly originated from the Fuping tectonic movement [21]. A series of folds and fault structures centered at ductile shear zones have been also formed. The overall tectonic line in the study is related to the NWW–NW trend [22].

2.3. Hydrogeological Conditions

Groundwater type is mainly weathering fracture water of metamorphic rocks. Considering the local hydrogeological survey results and the water abundance of the aquifer (Figure 2), the study area could be divided into a medium-water regions and a poor-water regions [23]. The medium-water regions are mainly controlled by the degree of weathered crust growth and the thickness of the quaternary alluvial diluvium. Some are also controlled by faults and their distribution is scattered. The groundwater is generally at a depth of 1.46–5 m and the water inflow per well is about 100 m³/d. The water quality is good and HCO₃·SO₄-Ca is the dominant water type. The total dissolved solids is 260 mg/L. The poor-water region is mainly controlled by landforms, and there are bare bed rocks and steep terrain. The water inflow per well is lower than 10 m³/d.

Groundwater basic units contain water volume, quality and energy input, and migration and output, as well as a combination of these. These are independent units of common groundwater circulation in temporal and spatial distributions [24]. According to the Guideline for Groundwater System Division and Geology · Minerals · Environment of Hebei Province, the study area is located in the level-2 groundwater system of Hai River, level-3 groundwater system of Daqing River, level-4 groundwater system of the Dasha River–Ci River fracture karst, and level-5 fracture groundwater system of Beiliu River.

Tianshengqiao Nine Falls belongs to the Tianshengqiao fracture groundwater system, which is the subzone of level-5 fracture groundwater system of the Beiliu River. In contrast, the North Taihang Tunnel is located in the subzone of the Sandaoling fracture groundwater system. A distinctive border exists between these two groundwater systems (Figure 3). The boundary separating the Camel Yingke Peak and the Yangshuta Ridge forms the edge of the surface watershed. The formation lithology of the study area is dominated by Fuping gneiss and groundwater mainly consists of shallow weathering crust fracture water. Therefore, the groundwater division is consistent with the surface water division.

The Tianshengqiao and Sandaoling fracture groundwater system subzones both belong to the low–middle mountains, which are sliced shallowly by the etching structure, as well as by their spreading ditch and valley zones. They have the same groundwater system characteristics. Precipitation and ice-snow melted water form the groundwater supply sources. The groundwater occurrence and migration depth are controlled by weathering of gneiss in the crust. It presents layered aquifer characteristics and has tectonic control locally, showing veined enrichment or extreme belt shortage. According to the existing statistical results of hydrogeological survey data in similar regions of Fuping, the development depth of the weathering crust of metamorphic rocks is closely related to the landform. The weathering degree is highest in the valleys. Large weathering depths are 9.00–25.80 m and moderate weathering depths are 15.00–48.00 m [14]. Hence, the maximum depth of weathering crust fracture water is less than 50 m. The groundwater flow direction in the system is controlled by the terrain; it sinks in valleys along mountain slopes, and drains as discharge flows or springs. According to the survey data in similar regions of Fuping, the common spring flow is 0.02–0.12 L/s [4].

There are also some differences between the Tianshengqiao and Sandaoling fracture groundwater system subzones. The former has narrow and long valleys, which are mainly developed at steep cliffs, forming the group of falls. The latter has narrow and low valleys. There is a thick stacking of alluvial sediments, as well as frequent transformation between the groundwater and surface water.

Based on the above-mentioned analysis, Tianshengqiao Nine Falls and the North Taihang Tunnel can be divided into two independent groundwater systems. Moreover, the groundwater systems mainly consist of weathering crust fracture water with a buried depth of less than 50 m. A weak hydraulic connection also exists between the two groundwater systems.

3. Data Acquisition and Results

3.1. Waterheads and Volume Information of Tianshengqiao Nine Falls

No records or descriptions of waterheads or volumes of Tianshengqiao Nine Falls have yet been found in any of local hydrogeological data; hydrometeorological data; special data like the Special Research Report for Tianshengqiao National Geological Park in Fuping, Hebei; the Overall Tour Plan in Fuping Tianshengqiao Nine Falls; or the Fuping County Annals from the last 30 years. Therefore, the total flow rate of the nine falls and the flow rates of each fall were measured in May 2022 by using the section survey method with a Flowatch portable current meter. The total flow (1105 m³/d) of the nine falls during ice melting and snow during the middle ten days of May was obtained for the first time. Based on a survey of paths and source tracking, a comprehensive analysis of landforms, high-definition images of vegetation coverage, formation lithology, weathering crust thickness, surface watershed, temperature, and elevation differences, it can be concluded that the groundwater overflow belt has been formed below the alpine meadow cliff along Baicaotuo, Liaodaobei, and Yangshuta and is the groundwater source of Tianshengqiao Nine Falls.

3.2. Hydrochemical Information of Tianshengqiao Nine Falls

To analyze the headwater of Tianshengqiao Nine Falls, as well as its composition and water circulation depth, snow water, spring water, and water samples from the nine falls were collected from upstream to downstream of fracture groundwater in Tianshengqiao in May 2022. In addition, a water composition test and deuterium–oxygen isotope test were carried out. The water samples were tested at the Water Environment Laboratory, Ministry of Natural Resources. The properties of cations and anions were examined by an ionization emission spectrometer ICP-OES (ICP-OES: AVIO 500, Perkinelmer Co., Ltd., Waltham, MA, USA) and ion chromatograph (ICS-2100) (model: ICS-2100, Metrohm 883, Metrohm, Herisau, Inc., Switzerland), with a test accuracy of 0.001 mg/L. The analytical errors of cations and anions were both lower than 5%. The contents and features of major hydrochemical components from precipitation at the major peak to the first fall along the Tianshengqiao Ditch are presented in Figure 3 and

Table 1. Main hydrochemical indexes of precipitation, surface water, and groundwater in the Tianshengqiao Gully.

Parameters	Snow Water SY03	Surface Water					Groundwater SY05
		SY06	SY07	SY08	SY09	Average
pH	6.46	7.51	7.57	7.49	7.63	7.55	7.22
TR		88.05	97.28	98.36	100.12	95.95	71.34
TDS (mg/L)	12.3	118.7	127.7	130.3	137.3	128.50	100.70
HCO3 (mg/L)	6.70	59.68	73.08	76.12	71.86	70.19	49.02
CI (mg/L)	0.95	2.49	2.64	2.67	2.92	2.68	2.39
SO42 (mg/L)	0.50	26.58	29.05	29.30	31.26	29.05	23.59
F (mg/L)	0.01	0.12	0.13	0.13	0.14	0.13	0.32
NO3 (mg/L)	0.23	10.68	8.90	9.82	14.45	10.96	8.84
NO2 (mg/L)		-	0.106	0.079	0.026	0.211	0.005
Turbidity (mg/L)	3.65	0.15	0.25	0.49	0.21	0.28	0.29
Metasilicic acid (mg/L)	1.63	11.01	10.21	8.62	10.77	10.15	10.25
K+ (mg/L)		1.46	1.69	1.70	2.21	1.77	1.06
Na+ (mg/L)	0.70	2.60	2.64	2.49	2.74	2.62	2.76
Ca2+ (mg/L)		27.66	30.95	31.13	31.25	30.25	21.43
Mg2+ (mg/L)		4.53	4.78	4.93	5.28	4.88	4.27
18O (‰)	−5.71	−10.21	−9.87	−9.82	−9.1	−9.75	−10.33
D (‰)	−25.84	−68.48	−66.80	−66.41	−61.85	−65.88	−69.74

. The deuterium−oxygen isotopes were tested with a liquid-water stable-isotope analyzer (Picarro L2130-i, Picarro, Inc. Santa Clara, CA, USA.), and expressed as δD and δ¹⁸O based on VSMOW standards. The typical accuracies of the test were 0.3‰ and 0.08‰, respectively.

3.3. Fracture Measurement

By combining the statistical window method, network yield survey, network area survey, etc., we recorded, in detail, the serial number of each fracture, coordinates of the endpoints, relationships between the fracture and the survey network (inclusion, cutting, intersecting), yield, trace length, undulation, roughness, openness, filling, groundwater information, and fracture characteristics.

Using the equirectangular projection method, the measured data of the fractures were organized to generate a polar-density map, and then the grouping of the fractures was completed according to the different densities of the production projections.

4. Discussion

4.1. Proportion Analysis of the Surface-Water Source and Groundwater Source of Tianshengqiao Nine Falls

The water sources of Tianshengqiao Nine Falls differ in rainy and dry seasons. During the rainy season, surface water collected from rainwater is the major source, supplemented by groundwater, which is supplied continuously along the path above Da’ao of the major vein. During the dry season, the nine falls experience ice falls and they begin to melt after March. The flowing stream is mainly supplied by ice and snow water.

Precipitation is the major water source for Tianshengqiao Nine Falls, which is attributed to the high mountains, abundant water, rich precipitation, and limited thickness of the weathered crust of the hard metamorphic rocks. It lacks deep space and cannot accept precipitation or surface water infiltration for a prolonged time. As a result, most precipitation is transformed into surface water and sinks into the falls.

Considering the test data of the collected snow water, spring water, and Tianshengqiao Nine Falls water samples, the mixing ratio of different water bodies was calculated according to the binary mixing theory and differences in hydrochemical components of supply and runoff, and different drainage positions. The specific formula was

δ_m = λ × δ_A + (1 − λ) × δ_B

(1)

where δ_m is the mixed sample, δ_A and δ_B refer to two end elements, and λ stands for the mixing ratio.

Specifically, the snow water sample and spring water sample of Liaodaobei were chosen as the two mixing end elements to represent the isotope composition of precipitation and groundwater in the study area, respectively. The surface water sample was used as the mixing sample. Based on the above formula, the supply proportion of groundwater to water of the nine falls in the study area was calculated. The calculated results are presented in

Table 2. Calculation results of the mixing ratio of groundwater and Tianshengqiao Nine Falls.

Location	Proportion of Groundwater Mixed with the Nine Falls (%)
Above the 9th fall	2.67
6th fall to 7th fall	10.04
3rd fall to 4th fall	11.14
Below the 1st fall	26.57
Stream outlet	30.05

.

Based on the above-mentioned analysis, surface water was found to be the major water source of the nine falls and the tunnel construction had a small possibility of affecting the falls formed by the catchment of surface water.

4.2. Water Source Analysis of Tianshengqiao Nine Falls Based on Water Volume and Hydrochemical Information

By performing hydrogeologic surveys and analyzing the remote-sensing high-definition images, it can be concluded that the groundwater overflow belt formed below the alpine meadow cliff along Baicaotuo, Liaodaobei, and Yangshuta is the groundwater source of Tianshengqiao Nine Falls. This is supported by a comprehensive analysis of landform, vegetation coverage, formation lithology, weathering crust thickness, surface watershed, temperature, and elevation differences.

Hundreds of thousands of mu of original ecological park are present in the alpine meadow. As a result, a water conservation area with 6000 mu of artificial larch forest and 70,000 mu of white birch forest was formed, creating favorable conditions for a continuous supply of groundwater. The groundwater accumulates continuously along gullies. This result indicated that the falls are supplied by ice and snow-melted water moving downward along gullies. The stream flow was tested on 13 May 2022 after the melting of snow. The results showed that the water flow from the head to the mouth of the gully first increased and then decreased (Figure 4). The stream flow continuously increased from Da’ao to the nine falls and the flow peak was formed at the 8th fall, while the stream flow slightly decreased from the 7th fall to Jiangjun Stone. The water flow at the exit of the landscape decreased to 1/3 of that at the 7th fall due to the impact of the impounding reservoir below Jiangjun Stone. The water flow between the 6th and the 7th falls reached 16.1 L/s. The water flow between the 3rd and the 4th falls was 13.5 L/s and the water flow at Jiangjun Stone, below the 1st fall, was 12.792 L/s. Nevertheless, the 8th fall reservoir and impounding reservoir had a significant impact on the stream flows.

High elevation is present in the catchment area of Tianshengqiao Nine Falls. There was ice and snow coverage in shady areas with an elevation of 1600 m during the first ten days of June. Ice and snow melted water is the primary water source of the nine falls during the dry seasons. According to the test results of stream flows at Jiangjun Stone and stream flows between the 3rd and the 4th falls, the deuterium–oxygen isotope test data of the water samples in the two phases differed significantly. Deuterium and oxygen both showed enrichment characteristics in May compared with March. The data in March reflected that the ¹⁸O and D values of stream water at Jiangjun Stone were −10.19‰ and −69.92‰, respectively, which were lower than those between the 3rd and the 4th falls (−10.12‰ and −68.59‰). However, the numerical values were almost identical. This outcome showed that the increase in water supply from the 4th fall to the 1st fall was mainly attributable to ice and snow melting. According to data in May, the ¹⁸O and D values of stream water at Jiangjun Stone were −9.10‰ and −61.85‰, respectively, which was higher than between the 3rd and the 4th falls (−9.82‰ and −66.41‰). This proved that with the thawing of the active layer, the groundwater had participated in transformation after the melting of ice and snow in May. A comparison of the deuterium–oxygen isotopic characteristics of snow water, groundwater, and stream water is displayed in Figure 5. The deuterium and oxygen of top-down stream water from Da’ao to the landscape exit were continuously enriched and were above the global precipitation line. In addition, they showed some differences in the local average precipitation. However, the fitting trend line intersected with the local average precipitation. This effect demonstrates that stream water and spring water are both supplied by ice and snow-melted water. They also exhibited obvious elevation-based zoning characteristics. In addition, the downstream stream water is supplied not only by snow in May but also by snow in winter. The supply period is longer and the supply scope is wider while having continuous accumulation.

Furthermore, the collected hydrochemical data reflected small differences in the hydrochemical composition between the Tianshengqiao spring and stream water from the head. However, the hydrochemical components of stream water were also continuously enriched from upstream to downstream (Figure 6). γCl/γHCO₃ and γMg/γCa are the most commonly used methods to identify the source of water. According to the hydrochemical data of stream water at Jiangjun Stone, TDS was extremely close between the 3rd and 4th falls in May. The values of γCl/γHCO₃ were both 0.05, whereas the values of γMg/γCa were 5.55 and 5.62, respectively, which was also extremely close. This outcome also verified that the quality and source of these two water samples was the same. During this period, the stream water mainly originated from ice and snow-melted water, indicating that the shallow groundwater in this interval was extremely deficient or was still in the freezing period. A common characteristic of the waterhead for all falls during dry seasons could be found—according to the hydrochemical data of stream water in May, the values of γCl/γHCO₃ also remained at 0.04, while γMg/γCa decreased quickly to less than 0.2. Based on this analysis, it can be inferred that the water sources of each waterfall are homologous. Nevertheless, due to the thawing of ice water in the gravel–cobble layer and the weathering crust in the gully, the stream water is continuously transformed with groundwater along the way and the water−rock interaction is continuously enhanced. The content of calcium ions significantly increased compared with that of magnesium ions as the former could be dissolved faster. This phenomenon rapidly decreased the γMg/γCa values.

Additionally, nitrite was detected at multiple water-sampling sites. This effect further proved that that surface water or shallow groundwater has a closer relationship with human activities as the major water source. Based on the above-mentioned analysis, it can be argued that the water of Tianshengqiao Nine Falls mainly originates from surface water formed by precipitation and the tunnel’s construction will hardly affect the nine falls formed by the catchment of surface water.

4.3. Connectivity Analysis between the Tunnel and Waterhead of Tianshengqiao Nine Falls Based on the Linear Structure

Strong tectonic water control is present in the bedrock mountain areas, and linear structures often become the hydraulic channels that connect the different groundwater systems. To guarantee hydraulic relationships between the tunnel and the nine falls, which are located in different systems, there are two prerequisites: (1) there must be a water-head difference and (2) continuous hydraulic connection channels must be present.

Even though the water head difference is present, the energy consumption for downward drainage is significantly less than that for the infiltration supply channel. The shortest distance from the tunnel to the 1st fall is 977 m. The elevation of the tunnel is 1005 m and the elevation of the 1st fall is 1351 m. After the tunnel’s construction, a water head difference of 346 m could be formed between the 1st fall and the tunnel. However, there is a 112.5 m-long cliff downward along the 1st fall and the hydraulic slope is nearly 90°. The groundwater flow coincides with the theory of minimum energy consumption rate. Therefore, the discharge force of surface water from the 1st fall along the cliff is far stronger than that in the 977 m-long rock block region with a groundwater gradient of 346 m. The shortest distance from the upstream of the 1st fall (e.g., the 7th fall) to the tunnel is 1185 m. The elevation of the tunnel is 1014 m and the elevation of the 7th fall is 1543 m. After the tunnel’s construction, a water head difference of 531 m could be formed between the 1st and 7th fall. The tunnel follows the hypothesis that there are hydraulic connection channels between them. In addition, a surface slope of 304.5 m is present at the bottom of the 7th and the 1st falls. According to the minimum energy consumption of fluid (Figure 7), the hydraulic slope from the 7th fall to the 1st fall is 77°. Without considering the infiltration resistance, the hydraulic slope from the 7th fall to the nearest tunnel point is 25° and the gradient is 0.98, which is much higher than 0.45. Hence, the water may first choose the surface channel. Therefore, the water-head difference still exists. However, flowing water consumes more energy than the downward discharge along the slope. Hence, the water-head difference is not anticipated to affect the water flow of Tianshengqiao Nine Falls.

The rocks in the region of the North Taihang Tunnel mainly consist of weakly weathered biotite monzonite gneiss, biotite plagioclase gneiss, leptite, granulite, and plagioclase amphibolite. The statistical-window method was used to measure 188 fractures in 170 m², and the fracture development characteristics in the study area were obtained. According to the fracture measurement results, the dominant fracture group is 200~230°∠75~90° (Figure 8). Among these, 69.7% of the total fractures are closed and 54% of the fractures are shorter than 1 m.

Considering the local tectonic background analysis, the dominant fracture group is associated with a secondary, short, and small tectonic group of the regional tectonics, which is mainly developed in shallow weathering crust. It has no cross-system connection ability for groundwater flows.

Taking into account the local geological background and landform analysis, no connection is present between the surface ravines and valleys of the nine falls and the tunnel since they have different surface watersheds with no hydraulic connection. According to lithological analysis of the formation, the weathering degree of metamorphic crust in ravines and valleys is the highest with a weathering depth of 48.00 m. The buried depths at the points that have the shortest distance from the North Taihang Tunnel to Tianshengqiao and Tianshengqiao Nine Falls are all larger than 200 m. None of these points have hydraulic connections with groundwater in the Sandaoling weathering crust and the water of Tianshengqiao Nine Falls is in a different watershed. According to the analysis of geological structures, no linear structure that can connect the nine falls and the tunnel was found in the 1:50,000 geologic map or interpretation of the current remote sensing imagery. Based on the above analysis, the water of the nine falls would have first used surface channels to form the falls. Moreover, the lack of hydraulic connection between the Tianshengqiao Nine Falls and the tunnel means that there will not be the conditions to generate hydraulic connections between them.

5. Conclusions

In this work, the water head of Tianshengqiao Nine Falls was investigated by performing hydrogeologic surveys coupled with hydrogeochemical and tectonic analyses. The hydraulic relationship between the Tianshengqiao Nine Falls and the North Taihang Tunnel was thoroughly discussed. Some major conclusions are as follows:

(1)

The main water of Tianshengqiao Nine Falls is surface water. Shallow groundwater is the supplementary water source at downward discharge along ravines in rainy seasons. For the first time, the total flow of Tianshengqiao Nine Falls was measured and, during the middle 10 days of May 2022, it was 1400 m³/d.

(2)

Based on the division of the surface watershed and groundwater system, weathering crust thickness, and the buried depth of the tunnel, and from connectivity analysis of linear structures, it could be concluded that the planned North Taihang Tunnel and Tianshengqiao Nine Falls could be divided into two independent groundwater systems. A small possibility for the generation of hydraulic connections remains.

(3)

By analyzing the groundwater system division and taking into account the hydrochemical and isotopic approaches, it can be argued that the tectonic-stress field and potential-energy field are effective ways to recognize hydraulic relationships between different water sources. These should also be included in future associated studies.