Septic tanks, whether they are made of plastic or concrete, are designed to be durable and long-lasting. However, there are some situations that can cause them to crack, which will result in sewage leaking out into your yard. 

1. Shifting Soil

Ground movement is often the culprit when it comes to cracks in a septic tank. Although major movement, such as from an earthquake, may cause damage, even minor movement from shifting soil can cause problems. This includes major vibrations from nearby areas, such as if construction or heavy equipment is causing soil vibrations. Sometimes the rocks in the soil that are up against the tank cause cracks when pressure from small movements in the soil pushes them into the tank walls. 

2. Tree Roots

Roots from a tree can spread out quite widely, so the tree doesn't have to be growing directly over the tank to pose a problem. Roots can cause cracks simply by the pressure they exert as they push through the soil or against the tank walls. Roots will also sometimes work their way into small chips on the exterior of a concrete tank until they have forced their way through by forming a crack. 

3. Excess Weight

No buildings should be erected over the site where your septic tank is buried, nor should vehicles be driven over or parked in the area. Heavy weight can put downward pressure on the tank, which can cause it to crack. Both concrete and plastic tanks are at risk. It's generally safe to walk over a tank as well as to use lightweight equipment, like a lawnmower, but nothing heavy should ever be placed on the site.

4. Concrete Deterioration

One issue that only affects concrete tanks is the slow deterioration of the concrete itself. Concrete is very slightly porous, so older tanks may begin to crumble naturally due to years of moisture exposure from both the tank contents and the soil surrounding it. Sometimes newer tanks will also deteriorate quickly and develop cracks if the moisture-to-concrete ratio was off during the pouring and curing process.

5. Groundwater Pressure

In areas with a high water table, groundwater can be the cause of a cracked septic tank. When the groundwater levels are high, the saturated soil particles will swell up and exert pressure on the tank from all sides. Plastic tanks may develop splits along any seams. Concrete can be a bit more durable against groundwater pressure, but even this material can develop cracks if the pressure is severe enough or is a frequent problem.

Contact a septic service if you suspect a possible crack in the tank.