Understanding sinkholes and their solutions: Most can be mitigated and stabilized

In March 2013 a catastrophic sinkhole collapse swallowed a central Florida bedroom killing the occupant without notice. The home was a total loss and the event made national news making many property managers keenly aware of the costs and dangers of sinkholes. Understanding possible causes and remedies should help with decision making at all levels of property management and ownership. Two different conditions exist which both can cause damages to property and structures, subsidence and sinkholes. Subsidence generally occurs over a wide area with a gradual depressing, one example being the Leaning Tower of Piza. Sinkholes occur from unconsolidated overburden collapsing into a subsurface void. Subsidence is generally slow and occurs over a long time span while sinkholes appear more rapidly, even suddenly at times. Sinkholes are considered more dangerous and damaging which is why this article will focus on them. What are sinkholes? The dissolution of rock in water is the most common cause of sinkhole formation. Two basic classes of rocks are soluble in water, evaporite and carbonate. While dissolution is the most common, mining, fluid withdrawal, flooding, expansive clays and buried organics can also cause sinkholes. Sinkholes can be placed into three main categories; solution sinkholes, cover subsidence sinkholes and cover collapse sinkholes. Solution sinkholes occur where very little overburden exists above the rock and the rock surface slowly erodes forming a depression which gathers water and accelerates the dissolution. Cover subsidence sinkholes are formed where the cover material is permeable and probably sandy. Similar to solution sinkholes, these sinkholes occur gradually over long periods of time and appear as a gradual depression of the ground surface. Cover collapse sinkholes occur where the overburden provides some limited stability, usually in clay soils, where limited surface drainage exists. Over time, the rock dissolves in ground water leaving a void beneath the overburden which eventually collapses to partially fill the void. Cover collapse sinkholes are the most dangerous situation because they offer limited warning but all types impose hardships on property owners. A sinkhole exists, what now? With localized events, the damage from most sinkholes can be mitigated and structures stabilized. The sinkhole most likely happened because a void in the subsurface rock formation occurred. It is important to identify what caused the void first. This is where the GPR and SPT tools are indispensable. Once the limits of the active soils, voids, damage and potential damage are determined, remediation techniques can be analyzed for effectiveness. The most two common fixes for sinkholes are compaction grouting and underpinning. Engineers generally don't like compaction grouting because there is no method of analysis available. The contractor's experience is key in this type of remediation, where a concrete grout mix is pumped into the void to seal it from further raveling or dissolution and provide support for fill to be placed on top. In some shallow cases, compaction grouting can lift slabs (slab jacking) back into, or near to, their original positions. A newer type of grouting referred to as chemical grouting is becoming more common. Chemical grouting is commonly used in loose or sandy soils to densify the soils by injecting polyurethane foam into the affected area. Some contractors are combining compaction and chemical grout with apparent success. Underpinning lends itself to empirical design and contractors generally provide warranties because of this. Many types of underpinning can be designed. Pre-construction, if the site condition defects are already identified, standard driven or drilled steel, wood, or concrete piles can be designed and installed. Helical piles are turned in with a drill head mounted on a skid steer loader, wagon drill or excavator. They can be installed vertically or at an angle from outside or inside the structure. Micro piles are another option that can be fixed to the structure mechanically. Some warning signs to look for sinkhole potential: Areas of depression in landscaping, ponding which did not previously exist, displaced curbing, paving or walkways, new cracking in foundations and surface springs. Some investigation is possible by the layman. Search the web for sinkhole reports in your area and check with the local building officials regarding other problems they are aware of. If you still think something is wrong, it is probably time to engage an engineer to perform a professional study and make remedial recommendations. Sinkhole potential can and should be analyzed and remediated before catastrophic collapse occurs. The potential liability for damage to structures and life requires serious consideration. If you have any of the above mentioned indications of potential problems, Falcon Engineering is available for consultation. John McGowan is a project manager at Falcon Engineering, Architecture and Energy Consultants, Bridgewater, N.J.