Some Natural, Some Not so Natural
It was not an unusual spring morning in the South Cumbria region of England. Farmers in this area near the scenic Lake District tended their usual morning chores. Yet one farmer received a nasty surprise while checking his fields when the ground below his quad bike opened up and swallowed him. The weight of the farmer’s bike triggered a breakthrough into a sixty-foot-deep sinkhole, which suddenly appeared on his land. Fortunately, he survived, and a rescue crew retrieved him from the bottom of the hole.
About two months earlier, a 60-foot-deep and 160-foot-wide sinkhole suddenly opened beneath a hospital parking lot in Naples, Italy. Cars, buildings, and concrete plummeted to the bottom, and the collapse severed local electricity services. No one was injured due to blind luck. Sinkholes occur worldwide, and the frequent and steady reports of collapses in Florida might lead one to believe large portions of the State sit atop a giant sinkhole.
But the risk from sinkholes is not evenly spread, nor is the formation of these pits a random event. Despite the unwanted surprise people get when a new sinkhole seemingly appears at random, geological processes were at work long before the danger became apparent.
One of the first points for clarification is, not all sinkholes are caused by human activity, and many occur in natural settings away from human influences. Sinkholes rely on natural geological processes, and the formation of sinkholes involves two main components. The first is rock and soil upon which we build our homes, roads, parking lots, and more — terra firma. The second is water, but not just any water, flowing water.
In rural settings, sinkholes are often identified as depressions in the landscape with no outlet for flowing water, yet there is no pond at the bottom. The implication is, water flows to the depression’s bottom and then disappears through the rock and soil. This phenomenon is particularly prevalent in limestone terrains where caves easily form.
The physics is simple; flowing water erodes soil and dissolves rock. But this process happens below the ground, out of sight from prying human eyes. When enough soil and rock erode or dissolve, a cavity develops, and the structural integrity of the remaining near-surface cover weakens. Eventually, the soil over the cavity gives way, and “ka-boom,” a hole filled with rubble appears at the ground surface, taking with it homes, roads, parking lots, and even quad bikes. We only notice the sinkhole at the end of the process, so from our viewpoint, it suddenly appears.
Usually, there are warning signs before a collapse, but we tend not to notice them. New cracks in the ground or asphalt accompanied by a subtle change in grade along the ground surface are indicators of trouble, particularly if the overlying ground develops a shallow depression where rainwater temporarily puddles.
Because sinkholes result from the simple physics of earth and water interactions, they can form naturally or as a result of human activity as we change the physical conditions in the ground.
Creating a Sinkhole
Construction has a profound effect on both key factors controlling sinkholes. One of the most notable effects of urban construction is forced changes in water drainage patterns. Homes and pavement block rainwater from entering the soil, creating runoff. When this runoff eventually finds its way into the ground, the volume of water is greater than under natural conditions. Increased water volume translates into higher flow rates, and the faster water flows, the greater its ability to erode. As drainage patterns in the subsurface change, the flowing water may remove soil from beneath roads and other structures, creating cavities and hence sinkholes.
Soil management during construction is also an issue. The science of soil engineering explores many ways to create dams and construction fills that withstand the effects of increased groundwater flow. Unfortunately, these techniques are not always used on run-of-the-mill construction projects. The specifications are often generalized and fail to recognize when special or additional engineering is needed.
Changes in drainage patterns or poor construction techniques can result in the unwelcome surprise of finding your car or home at the bottom of a newly formed sinkhole. Nature deserves some respect and there are consequences from building over natural sinkholes or creating new ones through negligent construction practices.
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Farmer riding quad bike ‘swallowed up’ by 60ft sinkhole in Barrow-in-Furness field (by Manpreet Kaur Sachdeva, Sky news)