Q&A / 

Dumping Driveway Gravel on Grass

DEAR TIM: I had a contractor come out to increase the size of a parking area next to my driveway. He just dumped smaller gravel, the size of golf balls, onto the grass. This gravel did have some smaller crushed stone and sand mixed in. Then he rolled over it with his backhoe saying all would be fine. Is this the right way to add gravel to make a driveway extension? I ask because my existing driveway has larger stones the size of softballs as the base. John O., Southern Ontario, Canada

DEAR JOHN: The contractor you hired did the wrong thing. It sounds like a classic dump and run scam. He rushes out with a dump truck or two, unloads the gravel, spreads it out, drives over it and cashes your check. There's a remote chance his work may stand up to moderate truck traffic, but only in very dry conditions.

Road building, and driveways are just small roads, was perfected thousands of years ago. The Romans knew how to build roads that have stood the test of time, and many of their roads still exist today. To get lasting results, just mimic what they did.

This gravel base was dumped directly onto grass. That’s not acceptable. Photo Credit: John O.

This gravel base was dumped directly onto grass. That’s not acceptable. Photo Credit: John O.

Here's an analogy to help explain why your contractor's work is going to fail. If you try to drive a heavy truck onto sand, there's a very good chance the truck will get stuck. This is especially true if the sand is composed of tiny rounded pieces of rock. Some sand is made from angular pieces of rock or sea shells and you can drive on it with some success.

At the other extreme is driving on solid bedrock. You can drive an Abrams M1A1 battle tank that weighs over 62 tons and the rock will not sink. The Romans figured this out, but their chariots didn't weigh 62 tons!

As you dig into regular soil, you'll discover that it's usually pretty easy to cut into the top layer, often referred to as the A profile. This top soil is fluffy, it's got organic material in it and insects help aerate the soil. This is why rain water soaks into top soil readily. Topsoil is very weak. It can't support very much weight.

But as you go deeper into soil, it contains less air, and it becomes more compact. This is why you're able to build buildings on the subsoils that are deeper in the ground. Some subsoils can support tremendous amounts of weight depending on the makeup of the soil.

Knowing this, your contractor should have come out and excavated out all of the top soil where the new gravel was to be placed. He should have then used a compacting roller to compress and compact the disturbed subsoil.

I feel he should have then installed a layer of geotextile fabric. This is newer technology that helps keep mud from fouling the base layers of gravel roadbeds. If mud seeps up into a gravel roadbed, and it does when heavy vehicles pass over the road, the mud lubricates the interlocking pieces of gravel making the roadbed loose strength.

Once the geotextile fabric is spread out, you then place the softball-sized angular stones on top of it. I would put in at least a 6 to 8-inch layer of this stone. This stone is made from bedrock. It's important that these stones are very angular so they interlock with one another.

Never use rounded rocks of any size for the gravel roadbed. The rounded stones don't interlock, and they move around like ball bearings when driven upon.

After the 6-inch layer of stone is installed, use a roller compactor to get the stones to interlock as much as possible. This layer of stone is your simulated bedrock, and it's the foundation of your new driveway.

You then start to add layers of more crushed stone or rock, but the size of the stones gets progressively smaller as you get closer to the surface. Add about 4 inches per level and compact each layer. The smaller stones on top of the larger stones below choke in the larger stones making it nearly impossible for them to move when weight is applied from above.

This methodology produces an extremely strong roadbed or driveway as it creates simulated bedrock that's got a very smooth top surface. Keep in mind that the smaller the stones are at the surface, down to sand-sized particles, the less water it takes to wash them away. Be sure that the top surface of the driveway doesn't have too much angular sand in it, or a heavy rain will cause erosion.

The final reason why the contractor should have never poured the gravel onto the grass is that you never ever want organic material under a roadbed. This material will decompose and cause the roadbed to fail. The organic material will also become a lubricant and can cause the gravel to easily move and shift when weight is applied to it.

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