Sinkhole - Solving Drainage and Erosion Problems
What is a Sinkhole?
Depressions often develop in suburban yards. In Fairfax County, these depressions technically are not real sinkholes. True sinkholes are formed in areas with limestone rock and karst geology, which can be found elsewhere in Virginia, but not in our region. However, water can burrow through loose layers of soil or poorly compacted areas to create underground voids. These voids can become so large that the soil and grass “roof” sinks or collapses to form large holes. Since homeowners commonly call them sinkholes, we will also.
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.
First, use a probe to investigate the hole. How deep is it? How wide is it? Is there a solid floor?
Be very careful when examining or working in sinkholes. Collapse of the sinkhole walls can lead to injury or death. Never enter a sinkhole that is deeper than chest height. The deeper the hole and the steeper the walls are, the greater the danger of collapse. If you are at all nervous about repairing a sinkhole yourself, hire a professional to do the work for you. Landscaping companies should be able to do most jobs. If you think the sinkhole may be contaminated, as may be the case with old underground storage tanks, contact the Fairfax County Health Department (703-246-2411, TTY 711) and/or an environmental remediation firm.
Choose One: Where is the Sinkhole?
1. Sinkholes Near Outside Walls or Under Stairs
Sinkholes occurring near outside walls or under outdoor staircases usually develop because the soil along the structure was poorly compacted. Over several years, the soil settles and causes a depression or hole.
Tips for Filling in the Sinkhole
- Clean the area. Remove any trash, grass or other debris from the depression.
- Try to determine the extent of the hole by careful excavation and probing
- Incrementally fill the depression with fill soil that has a high amount of clay and low amount of sand. Do not use gravel or rock as a fill material. If you do, water will trickle through the gaps in the gravel and create an even deeper cavity underneath. Shovel a few inches of the fill soil into the bottom of the depression and compact it thoroughly with a soil tamper.
- Continue this process until the depression is flush with the ground. You may want to overfill the hole since the fill soil may compact and settle over time. Remember to make sure the land slopes away from the foundation so that water does not pool next to foundation walls. Ground should slope away from foundation walls, ideally with a grade of 5% over a length of 10 feet. Allow the area to sit for a month or two to ensure that a new sinkhole does not form.
- Before planting, you may want to lay down a few inches of topsoil or potting soil above the filled area. Topsoil can help you establish vegetation.
- Plant seeds or use your choice of landscaping.
2. Sinkholes Under Sidewalks
Sinkholes under sidewalks or walkways. Sometimes depressions or holes can form under sidewalks or other walkways, with uneven concrete slabs creating a trip hazard. There are two possible solutions.
- Concrete under pressure can be pumped under the sunken section of the sidewalk until it rises to the normal level. This process is called concrete slab jacking and a professional concrete company is required.
- Another option is to grind the upper level slab down so that it becomes flush with the sunken slab, eliminating the trip hazard. A landscaping company experienced in using a grinder tool should be able to perform this operation.
There are many reasons for sink holes occurring in the yard. It is important to determine the cause; that will determine the steps needed to fix the situation.
These three scenarios have different causes, but the solutions are the same.
Buried, rotting debris. Often a sinkhole develops in a large open area of a lawn or garden. A common cause of this is buried brush, stumps or debris from past construction or other activities. Over several years, the brush and debris settle or decompose, leaving an underground void. The ground surface settles into the void, causing a depression on the surface. Remnants of the debris will often be evident with a little digging.
Tree Stumps Not Removed. Tree stumps that are left in the ground after a tree is cut down can rot and cause a sinkhole to form. Portions of the rotting stump may be found in the hole, or rot patterns on the surface may indicate the presence of an old stump.
Large Rocks in the Yard Fill. During home construction, when the yard is graded and compacted to its final shape, large rocks may be buried in the soil. Rocks prevent even compaction of the yard. The soil next to the rocks will be shielded from the compacting force of heavy machinery and remain lighter and less dense than the surrounding soil. Over time, this poorly compacted soil can become a preferred conduit for water which can scour out cavities and lead to a depression.
Tips for Filling in the Sinkhole
- Clean the area. Remove all brush, grass and other debris from the depression.
- Try to determine the extent of the hole by careful excavation and probing.
- Incrementally fill the depression with fill soil that has a high amount of clay and low amount of sand. Do not use use gravel or rock as a fill material. If you do, water will trickle through the gaps in the gravel and create an even deeper cavity underneath. Shovel a few inches of the fill soil into the bottom of the depression and compact it thoroughly with a soil tamper.
- Continue this process until the depression is filled in. You may want to overfill the hole and create a small dome since it is likely the fill soil will compact and settle over time. Allow the area to sit for a month or two to ensure that a new sinkhole does not form.
- You may want to lay some topsoil or potting soil on top. Topsoil can help you establish vegetation.
- Cover the filled-in depression with your choice of landscaping. Remember to make sure the new grading does not direct water towards foundation walls.
A sinkhole may occur near a buried stormwater, drinking water, or wastewater line, either because of poorly compacted soil or because of a leaking underground pipe.
Poorly compacted soil around buried drainage pipes, water or sewer lines. Depressions sometimes develop in utility easements, such as water and sewer lines, or stormwater drainage pipes. This usually occurs when the soil fill over the buried pipes was not properly placed and compacted. The poorly compacted soil settles over several years, causing a hole or shallow, linear trench on the ground surface.
Use a plat to see if there are easements on your property. The plat of your lot will indicate the presence of any easements on your property. The pipes running through easements generally serve multiple residences. Copies of plats are often included in residential loan closing documents. If you do not have a copy, one can be obtained from the Zoning Permit Review Branch at 703-222-1082, TTY 711. Not all plats are available from the County. If the County does not have a copy of your plat, contact the lending institution from which the mortgage was obtained. The conservation district can also help you determine if you have an easement on your property.
If the sinkhole is in an easement, who do you call? Knowing the locations and types of pipes and easements will help you determine if the pipes are indeed responsible for the depression, and if they are, who is responsible for repairing the damage. If the pipe is in an easement, the County or a utility company may be responsible for fixing the damage, although you may still be charged for some repairs. Contact the Public Works – Maintenance and Stormwater Management Division at 703-877-2800, TTY 711 for reporting stormwater pipe problems and 703-323-1211, TTY 703-239-8498 for sewer line problems.
Is the sinkhole near a private line? Lines serving individual homes are generally not situated in an easement and will not appear on a plat. Private lines are smaller and shallower and have less of a chance of causing sinkholes, but if you think your private line is causing a depression, you can contact Miss Utility to map your water line or the County’s Public Works Department at 703-324-5015, TTY 711 to map your sewer line.
Leaking sewer or water pipes. On rare occasions, leaking sewer or water lines cause surface depressions. Normally, they can be identified by sewer odors and/or flowing water on the surface. If you think there may be a leak in the water line but there is no evidence of flowing water, check your recent water bills to see if there is an unusual increase. You can also use a plat to see if there are easements on your property. Please call the County Wastewater Collection Division (703-250-2003, TTY 703-239-8489) if you suspect a sewer leak or Fairfax Water (703-698-5800, TTY 711; or 703-698-5613, TTY 711 after hours) if you think you have a water leak. In either case, do not try to make repairs yourself.
Sometimes running water can be heard in a sink hole. If a sewer or water pipe leak is not suspected, the water is likely from an underground spring that is uphill from the depression. It has eroded away soil under the surface causing an underground cavity. The ground surface settles into the cavity causing the depression.
Controlling an Underground Spring. In these cases, the water flow cannot be stopped, but it can be controlled. Installing a small French drain or catch basin and pipe uphill of the erosion site to capture the flow and replacing the eroded area with fill soil can solve the problem. In some cases, the flow can be captured and used as a source of water for irrigation or other purposes. The flow should be routed carefully to a suitable area so as to not cause erosion and flooding elsewhere. If the affected area is large, you may want to hire a landscaping or drainage company to do the work.
This case is the least common and also the most dangerous. Homes were built in some areas before public utilities were provided, so the owners had wells, septic systems, cisterns and buried fuel oil tanks which were left unused when public utilities came to the area. Abandoned wells and cisterns were sometimes backfilled before they were abandoned. Without maintenance, these former utilities can deteriorate and collapse, causing a sinkhole. It usually is not possible to determine the exact cause without excavation, but pieces of the old utility are sometimes visible within the sink hole. A ring of bricks is often indicative of an old well; rusting metal can be a remnant of a cistern or fuel tank. The cavity caused by any of these can be hazardous, particularly for a child. It may be large, collapse easily, and it could contain rusted metal or contaminants such as fuel. If the collapse of one of these structures is suspected, you should get professional assistance.
If you believe there is an abandoned well or cistern on your property, call the County Health Department at 703-246-2435, TTY 711. For fuel tanks or other potentially contaminated sites, you may call the Health Department to see if any help is available, but you will likely need to contact an environmental remediation company.