Overview+of+the+Causes+and+Remediation+of+Sinkholes

Overview of the Causes and Remediation of Sinkholes

//Sikandar S. Porter-Gill, M.A.E./B.A.E., Penn State 2013 //

Introduction
toc Sinkholes can form naturally or can be caused by human factors introduced into the terrain of a landscape. They can form on both land and sea and are generally circular voids in the ground, but can also take other shapes based on soil conditions and geological formations. They form due to the dissolution of strata below the soil layer of the Earth, or due to the weakening of the soil from man-made elements, such as mining, construction, and the manipulation of ground and surface water.



Sinkholes, naturally and artificially induced, can cause damage to buildings. Most often, a portion of the foundation system or wall façade sinks, slumps or sags causing damage. However, in some cases an entire section of a building can be encompassed by a sinkhole—as was the case with the Summer Bay Resort in Florida or the apartment building of Guangzhou, China. Presently, the town of Bayou Corne, Louisiana is being threatened by a sinkhole that is steadily growing in a neighboring swamp.

Sinkhole prone areas are studied extensively before a building is constructed; however, unfortunately they are also studied after the fact—after the sinkhole has been identified and is shown to be a possible danger to public safety. Geotechnical engineering and forensic research on sinkholes study how and why they form. There is no definitive way to go to a site and conclude a sinkhole will not form there in the future—as was the case with the Summer Bay Resort in Florida, whose geotechnical report stated stable ground existed below the building. Ground conditions can change due to a rise in the water table or as a result of one of many human factors induced on the ground—that are common in cities.

This article will focus on elements that lead to sinkhole formation, potential solutions to attempt to facilitate a site that has a sinkhole, and preventative measures that can be taken to avoid sinkholes forming under or near building structures. Sewer lines, electric and cable wiring, underground subways, traditional and deep foundation systems, water pumps, mining and clearing natural foliage, all may lead to a larger potential of sinkholes forming (Wines 2013; McGhee 2013).


 * Keywords:** sinkhole, karst, natural, artificial, cover-subsidance, cover-collapse, ground radar surveys, penetration test borings, grouting, stabilization, dynamic, throat, piping, dissolution

Sinkhole Formation
Sinkhole formation occurs over the course of many years, several decades to centuries, but the impact is the same—a large depression in the ground. Karst Regions, a landscape underlain with large amounts of soluble rock that is dissolved by rain and groundwater, are the most susceptible to sinkholes (Karst Is a Landscape). When water moves through this type of soil, the rock begins to dissolve, forming potentially large voids under the ground. These voids, or caverns, eventually become so large, that the soil above no longer is able to support itself—causing a large depression to form on the landscape (Waltham 2005 pp1).



This depression can occur quickly or can be relatively stable for several years. If the latter occurs, properties can be developed over the site adding more weight and stresses to the soil. When a sinkhole forms under a building, it could merely cause aesthetic concerns, or more severely, structural issues (Zisman 2009 pp224). In the worst case, an entire building could be engulfed by a sinkhole, potentially causing significant loss of life (USGS The Science of Sinkholes; USGS Sinkholes; The Dangers of Sinkholes – Insurance – United States). The size of a sinkhole can vary widely in both depth and width—varying from just a few feet to several hundred feet. A visually small sinkhole on the surface can potentially penetrate hundreds of feet into the ground. On the other hand, a sinkhole hundreds of feet across could only affect the first 10-30 feet of soil. Any ratio of depth to width is possible for a sinkhole.





Types of Sinkholes
When water moves through soil, the rock begins to dissolve, forming potentially large voids under the ground. These voids, or caverns, eventually become so large, that the soil above them is no longer is able to support itself—causing a large depression to form on the landscape. This is referred to as a cover-subsidence sinkhole or a cover-collapse sinkhole. Cover-collapse sinkholes may form in a matter of hours and cause a catastrophic breakdown of the ground. Another, more likely, way a sinkhole forms is through the loss of soil and or rock material. This loss of material slowly erodes the top surface layer, creating a void on the surface—a dissolution sinkhole. Depressions can occur quickly or can be relatively stable for several years.

[[image:failures/portergill_usgs_dissolution.gif align="right" caption="Figure 6: Dissolution sinkhole formation (U.S. Geological Survey)"]]
Dissolution sinkholes are just what there name describes —the dissolution of soils and rock. Soluble rock, carbonate, will slowly erode causing a cavity on the surface to form. This usually occurs because there is a thin layer of overburden, or topsoil, over the carbonate rock. Dissolution sinkholes can form more quickly if there is running water, or a place for water to easily collect, that will increase the process of disintegration of the rock. The figure to the right shows a dissolution sinkhole that occurs due to the introduction of water to a soluble rock (top) or the ponding affect of water (bottom) and the dissolution of the rock that will occur. This ponding, or even worse flowing water, may be caused by small crevices, trenches or openings in the landscape (Waltham 2005 pp9; Khomenko 2008 pp269; USGS Sinkholes).

Cover-subsidence Sinkholes
When a larger layer of material exist over soluble rock, the cover material can begin filling in the voids of the rock. This fill material is composed mostly of sandy-like soil. When the sediment slowly starts filling up the voids created in the carbonate rock layer, the sediment can slowly erode downward, forming small depressions. These depressions can vary in both depth and width. The cover-subsidence process is depicted in the figure below. The first stage involves overburden spilling into cavities of the carbonate layer. The second stage continues overburden spilling in the rock layer, and allows a column to form in the vacated space below. This is known as //piping//. The process persists into the third stage, where a noticeable depression is formed in the landscape surface. The dissolution and infilling process still continue at this stage. The final stage involves the formation of a surface depression and a slowing of the erosion. This leaves an unstable ground condition that is not safe (Waltham 2005 pp43; USGS Sinkholes).



Cover-collapse Sinkholes
When the layer of material over the carbonate rock is a clayey material, a cover-collapse sinkhole may form. This is a very dangerous type of sinkhole because of the limited visual indication that something is wrong with the landscape. Similar to cover-subsidence sinkholes, a cover-collapse sinkhole begins filling in voids of a carbonate rock layer. However, as the voids are filled in below, the overburden layer above shows no visual indication of a depression or caving in. This is caused by the cohesiveness of the clayey material that forms the overburden layer. The cavity formed in the overburden layer migrates upward toward the surface. Eventually, a limit is met where the soil layer is no longer able to span the cavity that has formed, collapsing. This collapse is usually very sudden and can cause a dramatic change in the landscape, destruction of land and property, and a loss of life. Whereas, a cover-subsidence sinkhole forms a gradual depression in the ground that can be visually seen (Waltham 2005 pp58). Cover-collapse sinkholes may form in a matter of hours and cause a catastrophic breakdown of the ground (USGS Sinkholes; Currens KGS).



Natural vs. Artificial Sinkholes
There are two main classifications of sinkholes, natural and artificial. Artificial sinkholes are becoming more common due to the complexity of ground modifications throughout the world’s cities.

Natural Sinkholes
Karst regions cover approximately 20% of the United States, mostly affecting the states of Florida, Texas, Alabama, Missouri, Kentucky, Tennessee and Pennsylvania. While karst regions are the most susceptible to sinkholes, sinkholes form in other locations too. In addition, areas were water dissolves rock is not the only location sinkholes form. Droughts can play a role in sinkhole formation—the water table drastically is reduced due to drought, and the ground gives way because of a void created under the ground by the absence of water. Two types of sinkholes exist, Cover-Collapse and Cover-Subsidence sinkholes (USGS The Science of Sinkholes).



The map above shows regions in the United States that are composed of rock vulnerable to breaking up when exposed to water. The figure defines three various regions that are prevalent in the United States.
 * 1) Evaporite rock of salt and gypsum
 * 2) Karst from evaporite rock
 * 3) Karst from carbonate rock

An evaporite rock is formed when an aqueous solution evaporates from a solution and leaves only the salt behind (Martinez 1998 pp38; USGS Sinkholes). Figure 9 depicts one major fact: a large area of the United States is susceptible to sinkhole formation.

**Artificial Sinkholes** Sewer lines, electric and cable wiring, underground subways, and traditional and deep foundation systems each modify the ground in some way. This could potentially disturb underlying soils, causing voids to form. In addition, alteration of ground water supplies, well water pumps, and the creation of water storage ponds and lakes can change the soil weight distribution over a region, creating an area that can not be supported by the soil beneath. Mining can leave large voids under the ground and clearing natural foliage allows soil to be swept away more easily, allowing a sinkhole to more easily form ( USGS The Science of Sinkholes; Kochanov 1999 pp21).

Facilitating Buildings with Sinkholes
Both permanent and temporary solutions for stabilizing buildings, after a sinkhole forms under or near the foundation, exist. This section will discuss treatments to the soil that can be performed to stabilize structures on or near a sinkhole.



Grouting the soil of a site is a lengthy process involving surveying the ground, pinpointing locations on the site to stabilize, boring holes, finding the proper mix of grout and actually grouting the site. The point of grouting is to stabilize the soil so it is less likely for a sinkhole to form in the future. The grout is a mix of cement, sand, gravel and pulverized fuel ash. This slurry mix is pumped into the ground by bored holes. These holes are usually placed in grid system, varying from 10 to 30 feet apart. The actual boring into the ground does not reinforce the soil. Rather, the void that the bored holes lead to, which will fill up with a slurry mix of grout, will reinforce the ground. This technique can be combined with a pressurization stage, pressurizing the surrounding soil and causing it to compact. This compaction further reinforces the soil (Basile 2008 pp568; Waltham 2005 pp228). This injection of a grout material into the ground helps with mitigating the sinkhole from expanding and also reinforces the sinkhole for future loads that may be placed on it (Miluski 2008 pp672).
 * Grouting Karst Terrains**

Surgical grouting can also be performed to a small section of the site; however, it is preferred to grout the entire site under consideration for safety concerns (Perlow 2010 pp2442).

Using dynamic motion and kinetic energy, compaction energy can be added to the soil under consideration to reinforce it from sinkhole formation. This is carried out, most of the time, by dropping a heavy weight on the ground from a crane (Waltham 2005 pp231). Dynamic stabilization is not the most common facilitating measure for buildings.
 * Dynamic Stabilization**

When a large sinkhole opens underneath a structure or road, a different treatment for the sinkhole can be implemented. If the sinkhole is wide and deep enough, it can be filled with large rocks to effectively block the throat of the sinkhole. This is topped with a reinforced concrete slab. An engineered profile is formed around the original sinkhole profile, and this is filled with a silty clay backfill. An engineered gravel layer is placed over the clay backfill and is topped with a low permeable fill to slow water penetration. A restored road or building foundation can now be placed on this site, where a large sinkhole previously existed (Waltham 2005 pp235).
 * Large Sinkhole** **Stabilization**



Damages Caused by Sinkholes
Sinkholes, naturally and artificially induced, can cause damage to buildings. Most often, a portion of the foundation system or wall façade sinks, slumps or sags causing damage (Zisman 2009 pp224). However, in some cases an entire section of a building can be encompassed by a sinkhole. These damages can be classified into the following categories:
 * 1) Cosmetic Appearance of the Building
 * 2) Functionality of the Building
 * 3) Structural Integrity of the Building

There are several precursors to a sinkhole interacting with a building that can be used to save lives. Many local governments, particularly in Florida, have recommendations on what to look for in a sinkhole formation:
 * Windows and doors that fail to close or change behavior (remain open or remain closed)
 * Cracks in walls, pavements and floors
 * Well water being pumped is cloudy
 * Increased or decreased moisture content, locally on a section of the site
 * Ponding of water where water did not use to collect in the past
 * Popped fence posts
 * Odd conditions around the foundation or a tree
 * Glass may also begin to crack

If these precursors are noted, they should be taken seriously, and a registered engineer or knowledgeable person should investigate them (Lake County Florida Public Works; McGhee 2013).

Preventative Measures for Buildings
The best thing to do to prevent a sinkhole from forming is understanding the site you are working on. “Starting with science,” is the recommendation in USGS’s article The Science of Sinkholes. Properly mapping a site and its surroundings will go a long way to understanding what types of soil and rock exist under an existing or future building. With the proper understanding, a good effort can be made to determine if it is likely a sinkhole will form on the site in the future. In addition, being aware of what is occurring under the site from human factors—sewer lines, additional underground services and modifications to the water table—will all go a long way to reducing the likelihood of a sinkhole forming on the site.

Several technologies exist that attempt to give engineers a better sense of what is underneath a site —what is it that will support a building. The site's history and geological surveys can be studied to understand how soil and rock conditions may be changing under the site. Several penetration surveys, or boring surveys, can be used to get virtual or physical evidence as to what comprises the soils below. These are known as Ground Penetration Radar Surveys and Standard Penetration Test Borings. These techniques can be used to evaluate how the building may be under stress in the future or may currently be under stress. With a better understanding of how the site is placing forces on the building, a better plan can be devised to try to alleviate these induced stresses (Zisman 2005 pp1; Zisman 2010 pp1202).