1 Engineering significance

Han River to Wei River Water Diversion Project in Shaanxi Province is an inter-basin water diversion project approved by the State Council in 2005, which is a key hydraulic project in the 12th Five-Year Plan of China. It is expected to solve water resources shortage in the Guanzhong area of Shaanxi Province, effectively curb the deterioration of ecological environment in Wei River and reduce environmental geological disasters in the Guanzhong area. It is a strategic project for optimal allocation of water resources by adjusting the distribution of water resources in Shaanxi Province and promoting the economic development of the Guanzhong-Tianshui Economic Zone. Implementation of the project is of great importance to the sustainable economic and social development of the Guanzhong area. The project crosses the Yangtze River and Yellow River basins and passes through the Qinling Mountain. The huge-scale project has a profound historic impact on the economic development in the region.

2 Project overview

2.1 General layout

The general layout of Han River to Wei River Water Diversion Project is shown in Fig. 1. The project consists of the Golden Gorge hydro-junction, the super-long Qinling water diversion tunnel and the Sanhekou hydro-junction. The super-long Qinling water diversion tunnel (hereinafter referred to as the Qinling tunnel) is 81.779 km in length, with the maximum overburden of 2 012 m. Its average longitudinal slope is 1/2 500, the designed flow rate is 70 m3/s, and the average annual water transfer volume is 1.5 billion m3. The building grade of the tunnel is level 1.

Fig. 1 General layout of Han River to Wei River Water Diversion Project

The Qinling tunnel passes through the bottom of one of the ten super mountains in the world, Qinling Mountain, for the first time. It is the longest and deepest tunnel in the world. It is rarely reported that TBM advanced continuously by more than 20 km as achieved in the Qinling tunnel.

2.2 Engineering geology and hydrogeology

The geological profile of the Qinling tunnel is shown in Fig. 2. The Qinling tunnel mainly passes through metamorphic and magmatic rock strata dominated by quartzite, marble, gneiss and granite. The tunnel successively passes through 3 regional faults, 4 Class Ⅰ faults and 33 ordinary faults. Gullies are developed near the surface. The groundwater is dominantly phreatic water and supplemented by precipitation and surface water. The groundwater is not corrosive.The normal and maximum groundwater inflow rates are predicted as 87 340 m3/d and 196 160 m3/d, respectively. The entire tunnel is located in hard bedrock. 60 515 m long tunnel section or about 74% of the tunnel is constructed in Grade Ⅲ rock masses; 21 264 m long tunnel section or about 26% of the tunnel is excavated in Grade Ⅳ-Ⅴ rock masses. The surrounding rock masses mainly belong to Grade Ⅲ and are generally stable. The main problems relating to hydrogeology and engineering geology are the differences in geological conditions at great depth; potential groundwater burst and abnormality in the tectonic fracture zones; rockburst, soft rock deformation, rock mass instability, high rock temperature and high water head at deep tunnel sections.

Fig. 2 The geological profile of Qinling tunnel

2.3 Project layout

Proper project layout and construction method are essential prerequisites for the smooth implementation of the project. According to the geological condition, an cost-effective and feasible construction scheme with "2 TBMs + drilling and blasting" was adopted after comparing various combinations of different forms and number of branch tunnels and multiple construction methods including the drilling and blasting method and TBM tunneling. The ridge section of Qinling tunnel was excavated by TBM and the rest by the drilling and blasting method. The tunnel layout is shown in Fig. 3. 10 construction aidts were designed. The main tunnel exits, the branch tunnels and the main tunnel sections connecting to the branch tunnels were excavated by the drilling and blasting method.

3 Engineering problems and countermeasures

3.1 Problems

The super-long Qinling tunnel is 81.779 km long. It is the controlling and key project in the Han River to Wei River Water Diversion Project, characterized by difficulties in tunnel alignment, complex geology, great overburden and long-distance continuous tunneling by single TBM.

The main problems encountered during tunnel construction are: 1) long-distance construction ventilation, belt conveyor transportation and reverse slope drainage; 2) soft rock deformation under high in-situ stresses, rockburst and high rock temperature.

3.2 Countermeasures

3.2.1 Selection of tunnel alignment in complex mountain area

The alignment of the Qinling tunnel in complex mountain area was determined based on a comprehensive exploration approach involving invisible measurement, geological mapping, borehole drilling, geophysical prospecting, remote sensing and sample test. Comprehensive geological investigation was conducted for an area of 650 km2along the tunnel line. The geological conditions at each mountain pass were revealed. Various tunnel options were evaluated in 5 candidate passes. The total length of alternative tunnels was approximately 800 km. Four types of tunnel schemes across the Qinling Mountain were proposed. Finally, the tunnel alignment from the Pu Rvier pass at the north of the Qinling Mountain to the Hei River pass at the south of the Qinling Mountain was selected, due to relatively short tunnel length, favorable geological conditions along the tunnel and minimum environmental impact.

Fig. 3 Layout of the Qinling tunnel

3.2.2 Large excavation length by single TBM

The continuous tunneling distance by single TBM is an important index for selection of construction scheme. The maximum tunneling distance depends on the service life of the main bearing of TBM. The main bearing is the key part of a TBM due to the long time required for manufacturing (about half a year) and replacement (over 6 weeks). At present, the service life of the main bearing is generally 15 000-20 000 h (10 000-12 000 h on the conservative side). Therefore, a TBM could generally advance by 20-25 km without replacing the main bearing. Construction of 20.22 km and 18.87 km long tunnel sections by two TBMs at the ridge is technically feasible. However, as the tunneling length by the two TBMs in this project is among the front rank in the world and intact and hard rock strata are involved, certain construction risks exist. Therefore, it is necessary to closely monitor the construction conditions, conduct fault diagnosis of TBM and make proper daily maintenance plans, maintain and repair TBM before construction of next phase and ensure supply of TBM accessories in order to implement the project smoothly.

3.2.3 Construction ventilation

Ventilation is required for underground oxygen supply and dilution of harmful gases. To a certain extent, ventilation is a key factor influencing the layout of auxiliary tunnels and the project scale. The single-head ventilation distance of the two TBMs for the ridge section reached 16.2 km. In order to improve the ventilation effect， soft air ducts were adopted instead of false ceiling, after comparative analysis of environmental standards in the tunnel, 13 ventilation schemes and various materials. According to different air supply lengths, gallery ventilation combined with forced single-head ventilation and forced longitudinal relay ventilation was adopted. Applicable conditions for different air ducts and parameters for ventilation machines were then proposed based on comprehensive analyses of performance parameters, cost and ventilation distance. Site management of construction ventilation was strengthened. At present, the single-head ventilation distance is 6 493 m for the tunnel section excavated by the drilling and blasting method and 12.35 km for the tunnel section constructed by TBM. With the advanced ventilation equipment, the single-head ventilation distance will reach 16.2 km for tunnel sections excavated by TBM.

3.2.4 High in-situ stress and rockburst

Based on in-situ stress measurements, the maximum horizontal principal stress is 29.85 MPa, which could induce rockburst during tunnel construction. To prevent unnecessary loss caused by rockburst, the following measures were taken: 1) enhance equipment protection and provide safety education to workers; 2) apply micro-seismic monitoring for prediction of rockburst intensity, location and frequency in front of the tunnel face; 3) apply water spraying on rock surface, deep-hole water injection, stress release holes, shotcrete mesh and steel arches to reduce the exposure time of rock surface and to eliminate potential safety hazards.

3.2.5 Deformation in fault fractured zones and soft rock strata

1) In addition to the conventional advanced geological prediction methods, including TSP, HSP, geological radar and infrared detection, induced polarization, transient electromagnetics and 3D seismicity were adopted for sections with moderate to rich groundwater. The geological conditions and groundwater were further identified and treatment measures were determined accordingly.

2) Pre-grouting was carried out to reinforce the strata and prevent water inrush. Cement and sodium silicate mixed grout or relay pumping were undertaken for sealing or drainage, respectively, when needed.

3) Deformation monitoring was enhanced. Pre-grouting and primary support were conducted when the deformation or deformation rate was larger than the pre-defined threshold.

4 Technological innovations

4.1 Tunneling through the Qinling Mountain

The Qinling tunnel is a deep-buried super-long tunnel crossing the bottom of the Qinling Mountain. The tunnel passes through a number of geological units and tectonic zones with multiple large-scale faults and complex lithology. Many adverse geological conditions, such as water inrush, rockburst, large deformation of soft rock, high external water pressure and high rock temperature, were prominent. A set of comprehensive construction technology for deep tunnel was established to deal with the problems involving high in-situ stresses water inrush and high rock temperature, including prediction and treatment of water inrush at great depth, prediction and prevention of rockburst, prevention of large deformation of soft rock under high in-situ stress, long-distance ventilation, correlation between long-distance TBM excavation in hard rock and the rock properties, determination of external water pressure on tunnel lining and its countermeasures, design, analysis, evaluation accuracy control of measurements.

4.2 Design, analysis, evaluation and accuracy control of measurements

The exits of the main tunnel and branch tunnels are located in a steep and complex terrain. Therefore, it is difficult to select and set the measurement control points outside the tunnel. The maximum tunnel length is 27.259 km. A scientific and reasonable measurement control method was established after analyzing various measuring methods.

4.3 Construction ventilation

Based on the test results for various ventilation parameters in laboratory and the measurement results in the tunnel, the applicable conditions for different formulas of air leakage rate were suggested. Finally, the detailed environmental parameters in the tunnel were obtained during construction of long inclined shafts. The values of key design parameters for various air ducts were recommended.

4.4 Prediction of the external water pressure

The maximum overburden of the Qinling tunnel is 2 012 m and the water head is approximately 1 460 m. Proper evaluation of the external water pressure directly affects the structural form and the capital investment of the tunnel. Various treatment measures were suggested for different external water pressures so as to ensure the structural safety and cost effectiveness.

4.5 TBM-related technology

A set of construction technologies for long-distance TBM excavation at depth was established based on prediction of the TBM advance speed, analyses of construction parameters, prediction and back analysis of disc cutter wear, studies of TBM tunneling parameters and adaptability for various rock strata, analysis of TBM advance rate, cutter wear, utilization rate and lining strength, etc.

5 Project duration and progress

5.1 Project duration

The project was started in December 2011 and will be completed in the end of 2019.

5.2 Project progress

1) As of the end of 2017, the tunnel sections constrleted by the drilling and blasting method and 10 branch tunnels were completed. The main tunnel was excavated by 67.57 km, accounting for 86.88% of its total length and the branch tunnels were excavated by 26.551 km.

2) The maximum excavation speed of the drilling and blasting method was 230 m/month in the branch tunnel and 240 m/month in the main tunnel, respectively. The maximum excavation speed of adit was 380 m/month.

3) The maximum monthly and daily advance rate by TBM was 868 m/month and 50.5 m/d, respectively, as of the end of 2017.

4) A record of 5 820 m for branch tunnel was mude.

5) A record of 6 385 m for single-head ventilation distance was made in the branch tunnel excavated by the drilling and blasting method with trackless transport. The maximum ventilation distance was 12.35 km for TBM tunneling.

6 Award

The project was awarded the second prize of the National Excellent Engineering Consulting Achievements in 2012.

7 Project developer, design company and contactors:

Developer: Han River to Wei River Water Diversion Project Construction Co., Ltd.

Design company: China Railway First Survey and Design Institute Group Co., Ltd.

Contractors: Consortium of China Railway Tunnel Group and Sinohydro Corporation Engineering Bureau 15 Co., Ltd., China Railway 18 Bureau Group Co., Ltd., China Railway 17 Bureau Group Co., Ltd. and China Railway No.5 Engineering Group Co., Ltd.