It is quite a shock when the ground suddenly disappears under your feet, your house or your field leaving a hole. This hole may be thousands of meters or more deep, eventually leading into a cavity that may extend hundreds of meters underground.
We call those sinks, and they are a global problem. Sometimes sinkholes are a purely natural phenomenon, but they may also be related to previous industrial activities, most commonly mining. So how do scientists like us detect sinkholes before they appear on the surface?
The geology of the rocks below is a clue to the possibility of sinkholes. Limestone is prone to dissolution by groundwater which, over time, can create vast networks of underground caverns known as karst. These can collapse due to gravity, leading to large surface depressions and subsidence damage and even total loss of homes.
Historical mine workings in coal, salt, potash, tin and copper often leave voids in the ground that have not been mapped and recorded correctly. These voids may surface over time as the roof gradually collapses. These falls can be gradual, or they can occur suddenly, with surface weaknesses appearing overnight without warning. Such rapid events are often associated with changes in groundwater or during excessive rainfall events.
In densely populated areas, such as the UK, a lack of available land leads to the use of brownfield sites, often without sufficient prior knowledge or land enquiry. Compensation is available for some types of falls, such as coal mining liabilities which are covered by The Coal Authoritybut insurers see many of the other causes as “Acts of God” and coverage is expensive or difficult to obtain.
We both worked on the sink problem everywhere from gold mines in australia to the Middle East, especially in Kuwait and the Dead Sea, and the Bahamas. We also traveled around the UK and Ireland, looking at mining pits in South Wales, Yorkshire and the Potteries, and we fell for robbing medieval chalk for mortar under a main road. near London.
Over the years, we have become experts in measuring Earth’s gravity with ultra-high accuracy to a few parts per billion. It is given microgravity. We can use this to detect a cavity, or even a partially-filled an area with less density than the surrounding rocks, long before any fall reaches the surface.
Often, we are brought in after the first drop has occurred to detect all the other potential sinkholes nearby. Developers need to start thinking about this kind of work before construction begins.
An additional innovation we have developed is to repeat time-varying 4-D microgravity surveys over months and years. This enables us to perceive changes in gravitywhich suggests that the cavities are propagating towards the surface and may become unstable.
We have looked a article problems of the Trent and Mersey Canal in Cheshire since 2002 and have carried out repeated readings on a more or less annual basis. Over this period, the microgravity results indicate that anomalies have increased and that the underlying salt mines that caused the canals to be built in the first place are becoming less stable. This could be due to leakage from the canals themselves or more frequent rains as the climate changes.
Recently fall part of the same canal nearby in Middlewich, which was very disruptive and almost fatal, our work is in sharp focus. The proposed route of Phase 2 of HS2 is proposed to cross a significant part of this Cheshire Salt Field where subsidence is very common and the engineering challenges for this high speed line are great. We have added new Channel 5 series on the sink holes which covered this issue and our work in detail. We hope to be able to provide guidance on how to set up the area and where this technique could be used to map other vulnerable areas along this waterway network. After all, something can be done about sink holes – if they can be detected in time.