That Sinking Feeling

Posted on February 21, 2017 by Telesis

That Sinking Feeling

At Telesis Collision Center in Palmdale, safety always comes first. After the heavy rains we’ve been experiencing, we’d like to shed a little light on something we don’t usually have to contend with, here in Southern California. Sinkholes! A common naturally occurring geologic phenomenon, sinkholes are classified as a geologic hazard that can cause extensive damage to structures and roads resulting in costly repairs. They even threaten water supplies by draining unfiltered water from streams, lakes and wetlands directly into the underground water supply!

Sinkholes can appear to have opened overnight, but the majority of them evolve over time, developing long before any surface evidence is detected. Areas where there’s a lot of underground rock and clay soil are ideal for sinkholes.  A common misunderstanding is that a sinkhole is the actual hole in the rock. But in reality, the sinkhole is what we see on the ground surface because of the hole in the rock below. The void in the rock takes hundreds or thousands of years to form.

Sinkholes can come in many different sizes and shapes.  They range from shallow depressions a few inches deep and several feet across to giants that can swallow multiple houses.  The sides of the sinkhole may be gently sloping or they may be vertical.  There are generally three types of sinkholes:

• Limestone Solution Sinkholes – When limestone is exposed at the surface or is covered by a thin layer of soil, the limestone subject to both physical and chemical processes that break down the rock. When this breakdown occurs, it usually forms a saucer or bowl-shaped depression. Due to the natural dissolving of limestone, these sinkholes develop continuously, but slowly.

• Cover-Subsidence Sinkholes – Where the sand layer may be as thick as 50 to 100 feet, with very little clay below it, the dissolving limestone is replaced by granules of sand that cascade down to fill the void. This type of sinkhole is referred to as a cover-subsidence sinkhole. These sinkholes are only a few feet in diameter and depth. Their small size is due to the fact that the cavities in the limestone cannot develop to appreciable size before they are filled with sand.

• Cover-Collapse Sinkholes – Generally, the deeper the soil, more clay is present. This clay provides some cohesiveness to the soil material above it, allowing it to bridge an existing cavity in the limestone. If this “bridge” collapses, it results in what is called a cover-collapse sinkhole. The size of the sinkhole depends upon the size of the cavity. Cover -collapse sinkholes form the same way as cover-subsidence sinkholes but differ mainly in the bearing strength of the soil above the cavity and whether the sinkhole subsides slowly or collapses abruptly.