Gravity sensing for rail construction demonstration

Understanding the subsurface is critical to rail construction. A better understanding can reduce the risk of unknowns, saving time and money through the design, construction and lifetime of rail assets. In the UK rail industry this is clearly demonstrated by HS2\. In HS2 Phase One naturally occurring karsts in the Chilterns have led to subsidence and adverse publicity for HS2\. For HS2 Phase 2 specific challenges lie ahead in Cheshire were old salt mines present a significant hazard as do old coal mines in the Black Country.

Detecting underground voids poses a considerable challenge that requires specialised technologies. Various methods, such as ground penetrating radar, electromagnetics, and seismic surveys, are used but they have limitations and depend on specific environmental conditions. In this project, we proposed the utilisation of gravity surveying as a novel approach to identify subsurface hazards, offering several advantages over existing techniques such as immunity to environmental ground conditions.

Gravity surveying involves measuring minute variations in gravitational forces to image the subsurface. Traditionally, it has been widely used in large area surveys conducted from aerial platforms. However, its application to smaller area surveys has been limited due to the lack of a user-friendly and commercially viable sensor. To address this gap SMG, has developed a portable gravity sensor based on resonant MEMS (Micro-Electro-Mechanical Systems) technology.

In Phase One two types of subsurface hazard were identified in the Washings Area of the GCRE site. The first were shallow pipes which ran across the site but were not present on any maps. The second was old coal mine shafts at a depth of approximately 10m which are very difficult to detect and had only been found by drilling multiple boreholes. The feasibility study gravity forward modelled these hazards and showed that gravity sensing should be able to detect these hazards.

There are likely to be other old coal minings on the site that remain unidentified and present a hazard for construction. This project builds on the feasibility study by proposing the gravity survey the majority of the Washing Area to a) demonstrate that gravity sensing can detect the known features and then b) survey the remainder of the site to look for presently unknown hazards. This will reduce the risk of unknowns during future construction, reducing cost and increasing assurance.

The demonstration will have direct benefits for GCRE but also provide a case study for the global rail industry.