For most Illinois driveways the workhorse is CA-6, a crushed rock with the fine dust left in so it packs into a hard, tight layer. Cleaner rock such as CA-7 earns its place where water has to pass through, and larger rock goes in first where the ground underneath is soft.
What CA-6 and CA-7 mean
In Illinois, quarries and contractors name rock by the Illinois Department of Transportation gradation codes. The CA codes are coarse aggregate gradations from IDOT's Standard Specifications for Road and Bridge Construction. Each code sets how much of the rock must pass through each size of screen.
That is useful for a homeowner. Ask a quarry for CA-6 in Montgomery County or Madison County and you get a known product, not a guess at what someone calls driveway gravel. The rock itself varies by source, and much of what is sold in this part of the state is crushed limestone. Ask the supplier for the spec sheet if you want the exact gradation for that pit.
CA-6 gravel: why it is the usual driveway rock
CA-6 is a dense mix. It has larger crushed pieces with the smaller chips and dust left in. When it is spread, wetted and packed, the fines fill the gaps between the stones and the whole layer locks together.
That makes it good at carrying weight and at staying put under tires. It is the standard base for drives, parking pads and shed floors around here, and many drives use it right to the top. Its weakness is dust in a dry summer and some mud if water is allowed to sit on it. Both come down to shape more than rock. A drive with a good crown sheds water off the CA-6 before it can soften.
Clean rock: where CA-7 earns its place
CA-7 is a cleaner stone with most of the fines screened out. Water runs straight through it. That is what you want around a drain pipe, in a French drain, or as a layer that lets water escape from under a slab.
On a drive surface it behaves differently. Clean stone does not lock together the way CA-6 does, so it rolls under tires and ruts on turns. Some people like a thin clean layer on top for less dust and a cleaner look. That only works over a firm, packed base. Clean rock poured straight onto soft ground just sinks. One IDOT district's own provisions list CA-7 as a crushed stone fill, which says a lot about its intended use.
Big rock for soft ground
Some drives cross ground too soft for a normal base. Low spots, old field ground, places near a creek or a pond, or a drive that has already pumped mud up through its rock all fall in this group.
There the fix is often a layer of larger rock first. The big stones bridge the soft ground and spread the load. CA-6 goes over it to fill the gaps and make a surface that packs. Getting the soft material out first still matters, because no rock floats on mud forever. If you have a spot that swallows every load you put on it, that spot is telling you what it needs. We look at those first when we quote.
Choosing the best gravel for a driveway on your ground
The right rock depends on three things. The soil under the drive. What drives on it. Where the water goes. A short town drive on firm ground may need nothing more than a packed CA-6 base. A long rural lane across wet farm ground may need big rock, CA-6 and some ditch work.
Avoid rounded rock, such as pea gravel or river gravel, as the main layer. Round stones roll against each other and never lock, so the drive feels loose and ruts easily. Crushed rock has sharp faces that grip. Tell us what the drive is for and what the ground is like, and we will tell you which rock suits. Our gravel driveway service covers the whole build from the base up.
Ordering the right amount
Rock is sold by the ton, and most people underestimate how much a drive takes. Measure the length and the width, and decide how deep the new rock should be. Give us those numbers and what the drive carries, and we work out the tonnage with you before the truck is loaded.
Ordering short means a second delivery. Ordering far too much leaves a pile in the yard. If you only need rock dropped and plan to spread it yourself, see rock and gravel delivery in Hillsboro. If the drive needs shaping, a base or a rebuild, our guide on how to build a gravel driveway walks through the steps.
The Short Version
- CA-6 is the standard Illinois driveway rock because its fines let it pack hard.
- CA-7 is clean stone that drains well but rolls and ruts as a top surface.
- Soft ground often needs larger rock under the CA-6.
- Crushed rock locks together; rounded pea gravel does not.
- Measure length, width and depth before ordering so the tonnage is right.
Quick Answers
What is CA-6 gravel?
CA-6 is an IDOT gradation for crushed rock with the fines left in. It packs into a hard layer and is the usual base for Illinois driveways.
Is CA-6 or CA-7 better for a driveway?
CA-6 for the base and usually the surface, because it locks together. CA-7 drains well but rolls under tires, so it suits drainage layers more than driving surfaces.
Can I use pea gravel on a driveway?
It is a poor choice for the main layer. Round stones never lock, so the drive stays loose and ruts. Crushed rock holds far better.
What is the lowest-cost way to make a gravel driveway last?
Get the soft ground out, build a packed base and give the drive a crown so water runs off. That does more than any choice of surface rock.
See our driveways & gravel service or call (217) 710-0606.