During the 2000s, Spain undertook a substantial expansion of its AVE high speed rail network as part of a plan to bring all major cities within four hours travel time from Madrid.
This was in part due to the local geological conditions present, which is composed of various materials, including clay, anhydrite, karst, which exhibited unfavourable heterogeneous mechanical behavior for tunnelling.
[2][3] During 2001, the state-owned railway infrastructure organisation Gestor de Infraestructuras Ferroviarias (GiF) awarded separate construction contracts for each of the Abdalajís Tunnel's twin bores.
Both consortiums shared the main portal site and employed nearly identical double shield tunnel boring machines (TBMs) to perform the majority of the excavation work.
The use of the customised double-shield TBM solution was reportedly due to the expectation of delivering higher production rates and superior steerability, particularly when traversing weak ground formations.
[4] However, in the middle point of the excavation phase encountered numerous difficulties, including dangerous intrusions of gases such as methane, which forced a complete stoppage while the bore was ventilated to dissipate the threat of explosion.
[7] According to the journal Environmental Geology, the drilling activity had come into contact with an underground aquifer, after which remedial measures were enacted prior to the tunnel's completion; the natural equilibrium was forecast to recover as it is replenished by rainfall over time.