Hard basalt challenges foundation drilling on Mexico rail project

Deep foundation construction is progressing for Mexico's AIFA-Pachuca railway project, where 827 bored piles are being installed to support the new rail connection between Felipe Angeles International Airport and Pachuca de Soto.

The new railway will extend for approximately 65 km and forms part of Mexico's wider passenger rail expansion programme. For the foundation works, piles are being installed to depths of up to 33 m, with diameters ranging from 1.2 m to 1.5 m.

The most demanding part of the work is below ground. Between approximately 8 m and 20 m depth, drilling encounters an extremely hard basalt layer that places significant demands on both the drilling equipment and tooling.

The piles are being constructed using the Kelly drilling method, a common technique for large diameter bored piles where the bore is excavated progressively using telescopic Kelly bars and suitable drilling tools.

The geological conditions make the operation particularly challenging. Basalt can have very high strength and abrasivity, which can significantly reduce drilling rates, increase tool wear and place high torque demands on the rotary drive.

Three drilling rigs are being used for the works, including an LB 25 nicknamed "Mjolnir". The machine provides a maximum torque of 252 kNm and is being used to penetrate the hard basalt while maintaining the pile geometry required for the railway foundations.

For bored piles of this diameter and depth, maintaining verticality and bore stability is particularly important. Encountering a competent rock layer can also create abrupt changes in drilling resistance compared with the overlying ground, requiring careful control of penetration rates and tooling.

The pile diameter range of 1.2 m to 1.5 m indicates substantial foundation elements capable of transferring significant structural loads into deeper competent strata. However, the source information does not provide the final pile load capacities, reinforcement arrangement or detailed stratigraphy beyond the identified basalt horizon.

Deep foundation construction for the railway began in October 2025 and is scheduled to be completed in December 2026.

The project provides a useful example of how railway infrastructure can require substantial foundation engineering even before tracks, stations and superstructures become visible. Ground conditions ultimately determine drilling productivity, equipment selection, pile construction methodology and construction programme.

In this case, the key challenge is not simply achieving a 33 m pile depth. It is advancing large diameter bored piles through a thick, very hard basalt horizon while maintaining the required geometry and construction quality.