“Sinkhole” is becoming a household word in many areas of the country.  From California to Florida, sinkholes are causing major property damage.  In urban areas, the sudden appearance of a sinkhole is a hazard that can disrupt utility services, hamper transportation, and cause severe damage to nearby homes, commercial and municipal structures.

Each year our nation’s City, State and Federal transportation departments, Water & Sewer departments and private industry spend hundreds of millions of dollars repairing damaged streets and pipelines caused by roadway collapses.  These problems are becoming more widespread and more expensive to repair. Why? The answer lies in understanding the problem and in knowing prevention and repair options.

So, what is a sinkhole?   Why are we seeing more of them now?

 

There are two basic ways that sinkholes occur:

1 – They can be formed through natural occurrences where subsurface rock is water-soluble.  (ie: limestone, carbonate rock, and salt beds) As the rock dissolves, usually over millions of years, spaces and caverns develop underground.  Once the void gets large enough, the materials above then collapse to expose the sinkhole.

2 – Research in the 1980’s by EnTech Engineering, Inc., Sverdrup & Parcel (Now Jacobs) Engineering and the Metropolitan Sewer District of St. Louis (MSD) showed that failures in roadways often begin with small corrosion holes in underground water and sewer pipelines.

Over 5 to 20 years, these leaks liquefy the surrounding soils and this combination of water and loose soil builds up pressure that has to go somewhere.  The typical escape route is a nearby sewer line that has cracked or failed joints where the soil and water enters and is then washed downstream without notice.  Given enough time, the void enlarges to the extent that the soil above it collapses to form an “overnight” sinkhole.

What can we do to stop erosion voids and sinkholes from forming?

 

 

Erosion voids and sinkholes are ALWAYS less expensive to prevent than to fix. The process to prevent these expensive major problems is simple:

 

1. Locate the leaks in the water and sewer pipelines, then;

 

2. Fix them

 

Several methods exist to locate pipeline leaks, but there is no magic technique that works in all situations.  EnTech Engineering, Inc., through its almost 30 years of research and experience, has developed a procedure that fuses data from the best of these techniques to produce results for the lowest overall cost.

1.     Use EnTech’s EnSITE Infrared emissions sensors mounted on a van or helicopter to locate leaks and erosion voids on both the water and sewer pipelines.  This technique is extremely accurate and the least expensive base upon a cost per foot of pipeline.  It also allows prioritizing the problem areas as to their relative size and whether voids have already begun to form in the flow path.

2.     Use sonic, ultrasonic and auto correlator instruments to confirm these leaks. These tools are widely used by water pipeline owners because they are good for locating small corrosion caused leaks, but are of limited use for larger crack caused leaks (the most serious) because they do not generate noise in the ultrasonic frequencies.  They are also very labor intensive and require multiple connections to the pipelines.  Most importantly, these traditional testing techniques can be used to confirm the EnSITE IR findings before expensive digging is performed.

Using these combined techniques, up to 10 miles of pipelines can be investigated per day by vans; or 100 miles per day by helicopter.

3.     One of the advantages of the EnSITE IR emissions leak & erosion void detection system is that all of the potential problem areas can be plotted on a map of the investigation area.  This mapping process can be extremely helpful in determining the best method to perform system rehabilitation.

Clustered leaks can be fixed by such techniques as:

Trenching & pipe replacement (most expensive);slip-lining; cured-in-place; or chemical grouting (least expensive and fewest hassles)

The sinkhole shown below, near Highway 40 & Mason roadways in St. Louis, MO was initially located during a drive-by investigation with EnTech’s EnSITE V, IR Pipeline Leak & Erosion Void Detection System.  During this drive-by no visual cues of the upcoming pavement collapse were visible to the eye.

The IR energy patterns clearly signaled that the underground sewer pipeline (shown in red in the upper right area of the data picture) had an anomalous area in the center section of the pipeline IR data image (as shown by the red and black areas in the data image).

A year had passed between the time the IR data image was collected and the visual picture of the collapse was taken.  Imagine what would happen if this deterioration continues beneath the adjoining major 10-lane Interstate Highway.