SINKHOLES FORMED BY CAPTURED SURFACE DRAINAGE<
BY: GREGG A. BOGIA, P.E.
A sinkhole is a depression in a land surface that is caused by water erosion of the bedrock below the ground surface. This process starts so gradually that little change is noticeable above ground. Sinkholes are formed by groundwater seeping through the land surface and dissolving the bedrock below. Sinkholes are most common when the bedrock is composed of limestone, sandstone, or salt. Over time the groundwater causes natural pores and cracks to enlarge. When the surface above can no longer be supported the ground eventually collapses.
In areas of Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania limestone is usually the first layer of bedrock underlying the soil. These states are more prone to sinkhole problems due to the subterranean soil layer. When an area is characterized by sinkholes, caves, or underground drainage systems it is referred to as Karst topography. These landscapes can be found all over the world. An example of Karst topography is Doberdo Lake in Italy. It is fed by an underground water source into a depression with no surface inlet or outlet. The largest and most impressive sinkhole in the United States is Kingsley Lake in North Central Florida. The lake is 2,000 acres in area, 90 feet deep and almost perfectly round.
In the United States caves that are large enough for human exploration were formed through limestone bedrock. The most dramatic cave system is the Mammoth Cave in Kentucky. The cave is over 350 miles long and contains remarkable structures created by the deposition of slowly dripping calcium carbonate solutions.
Man-made sinkholes can form because of water main breaks, sewer collapses, or even abandoned mines. Sinkholes can be induced when construction changes the weight of the land surface, loosens soil or leaves uncompacted soil behind. This creates depressions in the ground where water is being concentrated to one point. The movement of water is essential to sinkhole development. The construction of parking lots, buildings, and houses all disrupt natural water absorption. Prior to construction, drainage systems are designed to route water into a designated location. Man-made sinkholes can develop suddenly and expand rapidly while natural sinkholes tend to form more slowly over time. Other known causes that can trigger an underground collapses are improper installation of underground utilities and government required infiltration beds. Information is now being released that links a rise in air pollution to a higher acidity in rainfall which increases the rate of sinkhole creation. Sinkholes have recently become more common with the growing population and construction sites with unsuitable ground present.
Recognizing indicators of potential sinkholes can reduce the risk of a sudden collapse. It can be a warning sign if there is fresh cracks in a foundation or if a door frame or window does not shut properly. It can be an indicator if the landscape contains a hole or depression that appears after a heavy rain and continues to reappear even after being filled in. Watch for broken ground surfaces with void spaces in the soil that can be seen extending underground. There may be surface water or stormwater runoff disappearing into the hole. If you determine that a sinkhole is present, immediately rope off the area and mark it as unsafe.
The method chosen for sinkhole repair is best determined by a professional engineer based on a risk assessment of the area. An evaluation reveals the conditions and stability of the hole. During the evaluation a review of the topographical, geological, soil and groundwater maps, aerial photographs, soil information from existing subgrade investigations and any other data pertinent to the site is analyzed. The data collected is verified at the site during a test pit study and site reconnaissance. A plan is then designed to mitigate sinkhole activity based on the site conditions. Other considerations that assist in determining the best solution are the location and proximity to features such as underground utilities, existing foundations, and/or detention basins.
In the repair process it is typical to excavate the soft, loose or muddy soil and rock to expose the sinkhole. Next, the hole must be plugged with a boulder and covered with filter fabric to prevent soils from re-entering the area. After the hole is plugged a firm stone and dirt base must be constructed. When this is established it must then be backfilled with a low permeability soil mixed with bentonite. The backfill mixture must be placed at six inch lifts and compacted to 98% of maximum dry density. The purpose of a bentonite mixture is to fill any cracks that may develop as the hole settles. Bentonite expands to as much as four to five times its original size when wet to fill voids caused by eroding groundwater. If the sinkhole is occurring under or near a foundation it may be better to use compaction grouting to seal the limestone’s surface. This fills voids and pushes into the loose, weak soil to compact and strengthen it. A professional engineer should always be consulted before attempting to repair any sinkholes.
It is important to remember that during construction activities of any nature, a stone and filter membrane must be used to reduce the amount of sediment running across the topsoil. As a result the deterioration process of the underlying bedrock is slowed down, therefore minimizing the amount of sinkhole occurrences. It is essential for our industry to act responsibly and in compliance with the law in order to preserve our surroundings and protect our future.
EXPLOSIONS & PRE-EXISTING CONDITIONS
BY: STEPHEN TERNULLO, P.E. & FRANK STREHL, P.E.
The recent house explosion in Royal Oak is raising insurance claims from the owners of the structures surrounding the explosion. Many of these neighboring structures have experienced distress as a result of this event. Documenting the distress to these structures is a critical task which insurance companies must perform in a timely fashion. An important facet of this documentation process is knowledge of the condition of the structure prior to the explosion. Not all distress to a structure may be the result of this event and may have been in existence for some time prior to the event.
Many times an insured will examine their property following a catastrophic event, and observe distresses that they had not observed previously but were in existence. Because they have not observed these distresses before, they will usually attribute them to the event.
Types of distress most likely to be reported by insured property owners include:
Stephen Ternullo & Associates has the experience to determine if distresses to a structure are the result of a sudden event, such as this explosion, or if they have been in existence prior to the event. Our Licensed Professional Engineers will look for evidence of pre-existing conditions that may indicate the timing of the distresses in relation to the event, and for the cause of distress to determine if the distress is a result of a recent event or of some other cause.
Some of the considerations and methods utilized to age distress include:
If you have a claim resulting from this recent explosion in Royal Oak, or if you have a claim concerning foundation or structural issues, Stephen Ternullo & Associates has the expertise to determine the existence, extent of, and the cause and origin of the structural distress. We also have the expertise to prepare plans for repair of this distress in a manner that employs sound engineering principles and economic considerations.
Stephen Ternullo & Associates, Inc.