Although limestone is not porous, vertical and horizontal cracks commonly occur through limestone layers. As groundwater flows through these cracks, carbonic acid slowly dissolves the limestone and enlarges the cracks. Eventually, a cavern may form.

Limestone caves, which are formed primarily by rainwater and snowmelt, are by far the most numerous of all cave types. Rainwater and snowmelt seeping into the ground absorb large amounts of carbon dioxide, which is a product of decaying organic matter. The water and carbon dioxide together form carbonic acid.

Cave Formation Working slowly over many years, groundwater travels along small cracks. The water dissolves and carries away the solid rock, gradually enlarging the cracks. Eventually, a cave may form (Figure below).

Carbonate Dissolution Carbonate rocks such as limestone, composed mostly of the mineral calcite (CaCO3) are very susceptible to dissolution by groundwater during the process of chemical weathering. Such dissolution can result in systems of caves and sinkholes.

What are the warning signs?

• Fresh cracks in the foundations of houses and buildings.
• Cracks in interior walls.
• Cracks in the ground outside.
• Depressions in the ground.
• Trees or fence posts that tilt or fall.
• Doors or windows become difficult to open or close.
• Rapid appearance of a hole in the ground.

Limestone is a rock that dissolves fairly easily, at least compared to other rocks. All it takes is very slightly acidic ground water moving through fractures in limestone and the rock begins to dissolve.

Sinkholes can vary from a few feet to hundreds of acres and from less than 1 to more than 100 feet deep. Some are shaped like shallow bowls or saucers whereas others have vertical walls; some hold water and form natural ponds.

Sinkholes of China

• Dragon Hole – the deepest underwater sinkhole (blue hole), located in the Drummond Island reef of the Paracel Islands in the South China Sea.
• Xiaozhai Tiankeng – the deepest sinkhole in the world (over 2,100 feet), located in Fenjie Count of Chongqing Municipality.

Xiaozhai tiankeng The largest known sinkhole in the world, up to 662 m deep and 626 m wide pit with vertical walls.

The largest known sinkholes, formed in sandstone, are in Venezuela. They also occur in areas of China and Mexico, specifically the Yucatan Peninsula and Tamaulipas where you can find the deepest water-filled sinkhole, Zacaton, which is 1,112 feet deep.

“The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.”

Sinkholes may occur along outside walls or in the lawn or garden. They can have various shapes and sizes and can grow or deepen either slowly or rapidly. In many cases, sinkholes can be repaired by the homeowner. Before any remediation work is done, the extent and cause of the sinkhole should be determined.

A circular hole typically forms and grows over a period of minutes to hours. Slumping of the sediments along the sides of the sinkhole may take approximately a day’s time to stop. Erosion of the edge of the sinkhole may continue for several days, and heavy rainfall can prolong the stabilization.

Sinkholes are particularly dangerous when they form instantaneously by collapse, and they often occur in significant numbers within a short time-span.

Effects. Smothering grass with a thick soil covering can kill the grass completely, but it is a slow process, especially if the grass is perennial and grows by aggressive stolon stems or fleshy roots.

A small sinkhole with minimal damage to the structure may cost anywhere from $10,000 to $15,000. However, sinkholes that cause extensive damage and need a significant amount of work to repair or revive the structure, may be much pricier, costing anywhere from $20,000 to $100,000, or more.

Start by combining soil, sand, and compost. This mixture should be poured on the low areas and spread out to evenly cover the ground in a layer about half an inch deep. Keep checking the area to see if it becomes more level and supports new grass growth.

Leveling Deeply Sunken Lawn Areas The trick to filling deep holes is to compact the soil gently and give it some water to settle it as you fill up the hole. If you simply pile on some loose dirt and plant your seed, the soil will settle, and you’ll be left with sunken area covered with nice, new grass.

To fill in lawn ruts and holes, blend planting soil with sand and/or compost. Usually blending equal parts of each material forms a mix that allows grass to root effectively through the mix into existing soil.

Things You’ll Need

1. Tape measure.
2. Fluorescent orange spray paint.
3. Hammer or sledgehammer.
4. 4 stakes.
5. String.
6. Sod cutter and shovel.
7. Soil.
8. Bow rake.