## 1 Introduction  
The Urdinbide road tunnel is part of a new road communication route between Amorebieta and Muxika (Figure 1). It is in the northern part of the Iberian Peninsula, specifically in the Basque Country, running through the municipalities of Amorebieta-Etxano and Muxika in the province of Bizkaia (Spain).  
Figure 1: Location of the study area. Relative situation with respect to the metropolitan area of Bilbao, capital of Biscay, in the north of Spain.  
The environmental approval of the project was conditioned on the execution of the tunnel not significantly affecting the Autzagane aquifer, which it was going to pass through, as well as maintaining the existing water catchment and usage systems and the ecological flow of the Ategorri stream, which comes from the springs that supply this aquifer.  
During the design phase, the hydrogeological functioning of the aquifer was studied in depth. In addition, during its construction, a thorough hydrogeological monitoring was carried out, including the monitoring of piezometric levels and flow rates. The data obtained were used to calibrate the initial hydrogeological simulation of the aquifer. This hydrogeological monitoring made it possible to quantify the effectiveness of the proposed waterproofing solution both during the construction phase and in the final phase, once the tunnel was completed and waterproofed [1].  
There are many references in the scientific literature that analyze the internal dynamics of complex aquifers [2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14] and the influence of the construction of underground infrastructures in these highly permeable environments [15; 16; 17; 18] and that study possible strategies to mitigate this phenomenon [19; 20; 21].  
The Urdinbide tunnel project is characterized by its proximity to the Urdaibai area, of high ecological value, which was declared a biosphere reserve by UNESCO in 1984 [22]. In addition, it has the particularity that the Autzagane aquifer is used to supplement the water supply of an important population center in the municipality of Amorebieta-Etxano. To prevent water from entering the tunnel and draining the aquifer, a strategy of rock mass injection was employed during the construction of the tunnel, which in this particular case consisted of a combination of cement and microcement in stages. In addition, a completely impermeable lining was installed to ensure that the aquifer was not affected in service conditions.  
In the particular case of the Urdinbide tunnel, systematic, repetitive, and selective injections (IRS type) were used, with a high-pressure approach [23]. These injections were carried out using a redrilling system in the same boreholes, with incremental lengths in each drilling phase. These were truncated cone-shaped drilling and grouting crowns executed in a minimum set of four stages with cement grouting in the first stages and microcement grouting in the last stages. This set of injections also served to improve the geotechnical quality of the rock mass around the tunnel.  
The tunnel waterproofing system should ensure the recovery of the piezometric levels and the functioning of the streams in a normal rainfall regime, as well as maintaining the original compartmentalization of the aquifer and its natural hydrogeological functioning, as imposed by the environmental authorization of the project. The main purpose of this contribution is to describe the influence of the Urdinbide tunnel waterproofing solution on the hydrogeological functioning of the Autzagane aquifer.