Warning Signs That a Sinkhole May Be Forming on Your Property

Sinkholes can appear seemingly without warning. Suddenly, they’re swallowing up roads and homes in an instant and causing a major safety hazard. While the news may report a sinkhole suddenly appearing, the truth is they’ve likely been growing slowly underground for some time. By the time a sinkhole forms and the earth falls into it, the ground in and around the area is both unstable and dangerous.

Ground subsidence and instability are more common than you realise. Melbourne’s varied soil conditions mean that sinkhole risk is real, and is a concern for property owners all over this great state of Victoria. The consequences of undetected ground movement can be severe, from voids that form beneath roads and the shifting soils beneath a house causing potential collapse.

A professional geotechnical investigation by SITE Geotechnical is the most reliable way to truly know the risk you have of a sinkhole on your land. We can help you understand what’s happening beneath the surface, and help identify signs that happen above the surface as well.

What Is a Sinkhole?

A sinkhole is a depression or a hole in the ground caused by the collapse of the surface underneath. This collapse is usually caused by water dissolving soluble rock, such as limestone, salt or gypsum. Or it is caused by the erosion of subsurface soils.

While the process of forming a sinkhole can form over a short period in cases where soil can be easily transported via some form of conduit (such as a stormwater pipe) or take a long time, the sinkhole itself will often open up usually as a small hole to start with and eventually open up abruptly and poses a significant danger to person and property.

What Types of Sinkhole Are There?

There are three main kinds of sinkholes, categorised by the way they form. They differ in how quickly they form and if they involve bedrock dissolution or soil collapse.

• Solution Sinkholes. These form when acidic water dissolves soluble bedrock, such as limestone, at the surface. They create shallow depressions which get deeper over time. They often occur in areas where there is only a thin layer of topsoil covering the bedrock.
  
• Cover-Subsidence Sinkhole. They develop slowly over time when sandy, permeable soil washes into underlying fissures in the bedrock or underlying broken pipes or trench backfill.
  
• Cover-Collapse Sinkholes. This type of sinkhole is what you would see on the news. They form when a subterranean cavity forms and grows too large to support the overlying soil. The ‘bridge’ of soil collapses into the breach in a sudden drop out.

How Do Sinkholes Form?

The common causes of sinkholes in and around Melbourne include:

• Internal erosion of soils, such as tunnel erosion.
• Leaking stormwater or sewer pipes eroding the soil and causing a cavity.
• Poorly compacted or uncontrolled fill (sometimes trench backfill around pipes or underground infrastructure, easily eroding away.)
• Groundwater seepage softening soil beneath foundations.
• Excessive surface water infiltration.

Common Key Mechanisms of Sinkhole Formation

• Dissolution. As rain falls it absorbs contaminants from the air and soil, making it slightly acidic. This slowly dissolves soluble rock underground as the water seeps into the earth. Over time this creates voids below the surface.
• Suffosion. Water flowing through underground cracks, or burst pipes below the ground, wash away soil and sediment. This makes the above ground unstable and prone to sinkholes.
• Groundwater Changes. If the level of groundwater changes, due to drought or other sort of issue.
• Human Activity. Broken water mains, leaking sewerage pipes, overpumping of groundwater and even heavy construction can alter the structure of the earth and speed up the process of sinkhole formation.

What Warning Signs Are There to Indicate Sinkholes?

There are signs you can pick up on, above ground, that would indicate the formation of a sinkhole below ground. Leaning trees or fenceposts, new and long-lasting puddles, or ponds, that form after rainfall. Knowing the signs is helpful in preventing property loss or injury.

• Structural Damage. New cracks in your home’s foundations or walls, or cracks in your driveway or the concrete path can indicate significant movement below ground.
• Structural Movement. Doors and windows that become stuck indicate there is movement amongst your foundations which could indicate the forming of a sinkhole.
• Vegetation Changes. You may get circular areas of your lawn, or small circular patches of shrubbery dying off, as water is drained away from their roots.
• Craters in the Ground. Circular depressions in the ground beneath your lawns, or circular cracks in the soil or concrete. Alternatively, sometimes the ground may bulge upwards when there is a sinkhole forming.
• Landscape Leaning. You may notice fence posts or light poles leaning or falling over as the ground beneath them is giving way.
• New Pools of Water. After a heavy rain you notice new pools of water on the ground where there weren’t pools before.

How Does SITE Geo Investigate for Sinkholes?

SITE Geo has a 3-step process when it comes to investigating your site for sinkholes.

1. We conduct a comprehensive site inspection followed by a desktop study to discover any reason for depressions in the earth, cracking or erosion of the ground. One of our Experts will have a conversation with you so you understand the context of the study and of the ground beneath your project.
   
2. A subsurface investigation is then done using test pits, boreholes and/or Dynamic Cone Penetrometer testing.
   
3. The third step is a forensic engineering assessment. This is to determine the cause of any potential sinkhole and provide options to fix the problem. This can include more inspections and testing while repairs are carried out to ensure everything remains stable. More discoveries often occur while undertaking these repairs, so it is good practice to stay in touch with us at all times.

What Should You Do If You Have Noticed These Warning Signs?

If you have noticed any of the signs mentioned in this article- cracks in your foundation, doors and windows getting stuck, trees and fence posts all leaning in on each other, you must not ignore them. While it might seem like a minor inconvenience right now it could quite easily be a sign of something more serious happening below the ground.

Act quickly. Keep away from the affected area as more activity will accelerate the process. Rope it off if you can, and call SITE Geotechnical so we can investigate what is going on below the grass, and determine what needs to be done to fix it.

SITE Geotechnical has been helping property owners and building developers all over Melbourne, and greater Victoria, understand what lies beneath their feet for over 20 years. Our three-step investigation process gives you a clear picture of the ground conditions on your property and what risk you have for deterioration and sinkholes.

Sinkholes are not something you should leave to chance. They can bring large scale destruction to life and property if left unchecked or undiscovered. You simply must have SITE Geotechnical investigate your property before you build, to ensure your safety and the success of your build. Call 1300 557 260 or send us an email to discuss your needs and to get an obligation-free quote from us.