Two motorway crossings in Wales: safely installing water mains below the M4
In October 2025, Brownline UK surveyed two parallel horizontal directional drills in South Wales. Accurate positional measurements and reliable pressure control were key requirements during the pilot phase. Brownline’s steering service ensured a successful and efficient pilot completion.
Project background
The bores were part of a water infrastructure renewal project. Horizontal directional drilling was used to install new water mains under motorway M4 to minimize disturbance on the surface. Two parallel crossings were designed with a length of 405 m / 1329 ft. These would pass at a depth of 15 m / 50 ft below the road.
Also notable in this project was the considerable elevation difference: the entry and exit points were on opposite hills overlooking the M4. The entry site was 24 m / 79 ft higher than the road, and the exit site was 10 m / 33 ft higher than the road. The elevation differences and the crucial crossing of the motorway required measures to control the downhole pressures during drilling, in addition to accurate steering.
Managing pilot bore challenges with Drillguide GST
Our client Gmac Utilities Ltd was able to rely on the Drillguide Gyro Steering Tool as a solution for both of these challenges. A mud motor was used to efficiently drill through the hard mudstone, while Brownline provided real-time positional measurements and steering advice. This ensured that the curves were drilled accurately and that the drill passed safely below the motorway at the appropriate depth.
Of the two parallel crossings, one was initially used as a mud return line. This meant that the drilling fluid and cuttings could be returned to the entry site for recycling without having to transport these by road.
Controlling fluid pressure using real-time measurements
During the pilot phase of the project, Brownline’s Pressure-While-Drilling service was used to minimize the risks of inadvertent fluid returns. Using highly accurate pressure sensors built into the Drillguide GST, both surveyor and driller are equipped with real-time measurements of the downhole fluid pressure. Careful observation of these measurements helped to control the fluid pressure, resulting in a safe and efficient completion of the pilot bores.
The success of this project underscores how key challenges during the pilot phase of HDD projects can be reliably addressed by Brownline’s steering service.
