Void under driveway

Voids are pretty common under concrete driveways.  Voids are pockets of air below concrete which cause it to not be supported by the underlying ground. 

There are two common causes of a void under driveway slabs:  erosion & settlement


Erosion can use a void under driveway slabs to form by washing away the soil under the concrete.  Erosion under concrete driveways is usually caused by downspouts discharging next to the edge of a driveway that’s not backfilled properly.  If a driveway is very steep, water can also cause erosion under the driveway by getting under the driveway by flowing through the expansion joints in the driveway.  

A void under driveway concrete slabs which is caused by erosion is most common in areas with non-cohesive soils (like sand), because the soil particles can easily wash away.  Because sand grains are tiny, they can easily be washed away by flowing water.

Most of the time, erosion washes the material under your driveway onto the surface of the ground somewhere adjacent to the driveway.  Sometimes, however, if there’s a broken pipe under or next to a driveway, erosion can wash the soil into the pipe.  Therefore, when determining the cause of a void under driveway slabs, it’s important to know what utilities are in the area, particularly sanitary and stormwater pipes.


Settlement of backfill material around utilities and homes is another common cause of a void under driveway slabs.  Settlement usually takes place in the first 10 years after the soil is installed.  Most of the settlement occurs fairly quickly, though, with less and less settlement happening the closer you get to the 10 year mark.  

Settlement if the backfill material under a driveway will cause a void to form under the driveway slabs.  Sometimes the settlement which causes the voids can be compounded by erosion.  This is because as the soil settles, it can redirect water flow into the settled area, where it can cause erosion.

How To Fix A Void Under Driveway Slabs

Fixing a void under a driveway slab is pretty straightforward.  There are two processes which are most appropriate:  foam jacking and mud jacking.  The names of both of these processes imply that they lift the slabs back up. However, for void filling applications, it’s possible to inject material fill the voids without causing the concrete to lift.  If the slabs have settled a bit because of the voids, either one can be used to correct the settlement while also filling the voids under driveway slabs.

Foam jacking is the preferable method for repairing a void under driveway slabs.  Foam jacking uses a high density polyurethane which won’t erode, and is very lightweight.  Because it won’t wash away, it’s great for addressing voids caused by erosion.  Because it’s very lightweight (but still plenty strong to hold up the concrete and whatever you drive over it), it is great for cases with backfill settlement, because it places less weight on the underlying soil.  Another great thing about using foam jacking for driveway repair is that the foam cures within a few seconds of being injected, so you can drive on your driveway immediately after the repair is done.

Mud jacking (also called slab jacking or concrete leveling), works similarly to foam jacking.  However, with mud jacking the material injected under the driveway to fill voids is either a sand or lime-based grout.  Cement has to be added to the mix to make sure that it won’t wash away in cases of erosion causing voids under driveways.  Mud jacking is also appropriate for filling voids that result from backfill settlement.  However, it does place additional weight on the underlying soil compared to foam jacking, and you usually have to wait at least 24 hours to drive over a slab that’s been repaired with mud jacking.

Cost To Fix A Void Under Driveway Slabs

The biggest driver of void filling costs is the size of the void.  The larger the void, the more material it will take to fill.  One thing is for certain; attempting to pack material under the edge of a driveway to fill a void under the driveway isn’t a long-term solution.  Because the bottom surface of the concrete isn’t smooth, it’s not possible to adequately pack material in and ensure that it won’t suffer settlement