There is nothing more frightening than the ground literally opening up and giving way under your feet or some form of transport like a car or bus. This time last year on May 11, 2015, residents of Granbury in Hood County learned how frightening a sinkhole can be when a huge one measuring approximately 40 by 40 feet wide made an appearance overnight in a regularly used grocery store parking lot after heavy rainfall pounded the area. Thankfully, no one was hurt by the incident and police were able to quickly secure the site to prevent injury and vehicle damage. Yet, this was not the only incident that month:
North Texas is well known to have plenty of voids underground that can lead to this type of incident. In fact, another large sinkhole approximately 25 by 25 feet wide opened up a mere 15 days later on May 26 at the Dallas-Forth Worth International Airport approximately 250 feet between a taxiway and the 18-L runway. It is not difficult to imagine the type of horror airplane passengers or runway crews might experience if a sinkhole opened up under them. This particular sinkhole forced airport officials to close off the 18-L runway, which is one of the airport’s major runways, even though the runway and sinkhole both appeared stable. Although the shutdown did not have an incredibly noticeable impact on the more than 170,000 people who use it daily, the sinkhole still caused many concerns, property damage and subsequent financial losses to the airlines and airport.
What Causes a Sinkhole?
A sinkhole is literally a hole that appears at the surface of the ground after the surface collapses or “sinks.” The surface of the ground we walk and travel on every day is not always as sturdy as it appears. Many voids exist underground such as natural caverns. Caverns form from water eroding rock and other natural processes. Limestone, which is found underground throughout North Texas, is one type of rock that water erodes easily. Over time, heavy impacts on the surface from people merely traveling from one place to another coupled with water erosion can cause a cavern ceiling to collapse.
Underground erosion can also happen when man-made structures flood – for example, leaking underground water and waste pipes can gradually or rapidly erode an area and cause a surface collapse. The sinkhole last year at the DFW Airport happened after damage to a storm drain caused underground flooding. Even as repair crews attempted to fix the drain, another storm that came through the area caused a complete collapse of the drain two days later forcing a second shutdown of the 18-L runway.
Other Types of Voids
Natural and pipe subsurface voids are not the only ones that people need to worry about causing a collapse. Tunnels made by pests, mines and any other hidden or forgotten underground structures like wells and eroding tanks can cause sinkholes.
These voids can also cause building foundation and wall instability that eventually results in one or more sides of a building sinking and structural tilt. A void that causes ground subsidence over time can even cause the collapse of an entire building. Additionally, new construction on certain types of ground can literally change how the ground moves and lead to above-ground voids underneath new home floors, driveways, pavements and roads.
Some types of soils shift more than others when people build upon them. Ground beneath any new structure is also impacted by the weight of that structure and the quality of the compaction processes used to make the soil foundation sturdy. Additionally, weight on the surface soil can push water out of subsurface soils causing unintentional subsidence. Lastly, the roots of trees and other plants can shift the ground beneath any structure. All of these natural process can result in an unplanned gap that makes the structure unstable.
Advantages of GPR
Ground penetrating radar is a tool that helps trained technicians find voids and other hidden structures. It is been in use since the 1970’s in different applications across industries. GPR equipment sends a pulse of electromagnetic radio waves into the ground. The signals bounce harmlessly off structures including voids and then echo back to the equipment. Although some subsurface materials absorb radio waves, modern GPR technology can detect most common voids through a variety of natural materials, such as soil, sand, silt, gravel, granite, limestone, shale, clay, wood and other plant matter, and man-made materials like asphalt and concrete. When GPR alone is not enough, professional inspectors use other tools with GPR to complete the picture.
For more information about detection of voids or to schedule an inspection, contact our experienced team at Wood Inspection Services Inc. today.