A sink hole is the erosion of land. It forms when underground water dissolves minerals in the ground. This creates a buried hole.  Eventually, the hole becomes large enough that the land on top caves in.  Sinkholes are common in land containing lime stone,  because limestone is soft and has water holes like a sponge. It also dissolves in water.

One definition of a “sinkhole loss” is: “actual physical damage to the property covered arising out of or caused by sudden settlement or collapse of the earth supporting such property only when such settlement or collapse results from subterranean voids created by the action of water on a limestone or similar rock formation.”

Three General Classifications of Sinkholes:

1. Collapse Sinkhole:

Sinkholes develop quickly over a period of hours and can cause major damage. These are the types of sinkholes which make the news. These usually happen where the ground has a large amount of clay. Water flows underground and fills a cavity. As it continues the ground forms a structural arch. The cavity moves upward by progressive roof collapse. Then the cavity breaks through to the surface and creates sudden, dramatic sinkholes.  These types of sinkholes have been known to swallow homes, swimming pools, buildings, roadways, and bridges

FIGURE 1:  There is no evidence of land subsidence, but small to medium size cavities have already formed in the rock matrix.  Water from surface percolates through to rock, and the erosion process begins.

FIGURE 2: Cavities in the rock matrix continue to grow larger but remain filled with water.  This water pressure helps to support the thinner, weaker roof of the enlarged cavity.

FIGURE 3: As the water level drops during the dry season, or is lowered due to pumping it out, the weight of the overburden exceeds the strength of the cavern roof, and the overburden collapses into the cavern, forming a sinkhole.

2. Subsidence Sinkhole:

The progression of a subsidence sinkhole is illustrated below in figures 4-6. Rainwater percolates through overlying sediments (usually thin) and reaches the limestone, dissolving the rock and gradually weakening its structural integrity.  Gradually subsiding sinkholes commonly form where slow dissolution takes place, mostly along joints in the limestone. These sinkholes tend to form naturally and are not greatly affected by human activities.

FIGURE 1:  Initially the limestone contains fractures and small cavities that have formed by dissolution, but no subsidence has occurred.

 FIGURE 2: Small cavities and cracks grow larger as time progresses, and water moving through the rock erodes the rock matrix. Sediments from above are carried by groundwater to fill the voids forming in the rock.

 FIGURE 3:  Sediments from the upper layers continue to fill in the openings in the limestone, causing a depression at the land surface. If water collects in the depression, a new lake is formed.

2. Solution Sinkhole:

If the overburden is thin or absent, the surface of the limestone bedrock is broken down by erosion from wind and surface water. A bowl-shaped depression, or solution sinkhole, naturally forms slowly and continuously as chemical and physical processes erode the rock.

Cracked foundation

Building separating from the     foundation

Cracked walls

Cracked floors

Cracking around door frames

Cracking around window frames

Cracked pool deck

Cracked pavement

Slumping, sagging, or slanting     trees

Water pooling

Slumping, sagging, or slanting     fence posts

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