Our 25 year old driveway unexpectedly and quickly started sinking about 3 months ago. About 3 weeks ago it dropped about 4" where the driveway meets the garage. The home is fine because there's a wall there, not a footing.

The contractor shows up this morning and starts breaking up the driveway. Rings my doorbell. Your concrete is about 2' thick! he says. No way says I!

Sure enough, the pour of the original driveway ranges anywhere from 4" to 2'. There are footings in the ground under parts of the driveway next to the garage. That is where the driveway sank the most. There's random bits of rebar, and it's thick, like 1" rebar hastely strewn about.

Now he's going to have to bring in a load of dirt and potentially rock to bring up the grade so he's only pouring a 4" thick pad.

Not to mention the extra trucks and labor to haul away the old.

How the hell do you pour a 2' thick driveway and it still settles?! Don't answer that, I already know.

quote:

Originally posted by Shaql:
Our 25 year old driveway unexpectedly and quickly started sinking about 3 months ago. About 3 weeks ago it dropped about 4" where the driveway meets the garage. The home is fine because there's a wall there, not a footing.

The contractor shows up this morning and starts breaking up the driveway. Rings my doorbell. Your concrete is about 2' thick! he says. No way says I!

Sure enough, the pour of the original driveway ranges anywhere from 4" to 2'. There are footings in the ground under parts of the driveway next to the garage. That is where the driveway sank the most. There's random bits of rebar, and it's thick, like 1" rebar hastely strewn about.

Now he's going to have to bring in a load of dirt and potentially rock to bring up the grade so he's only pouring a 4" thick pad.

Not to mention the extra trucks and labor to haul away the old.

How the hell do you pour a 2' thick driveway and it still settles?! Don't answer that, I already know.

Settles, has nothing to do with slab thickness.
Its all about site prep and drainage. A 2" slab on a properly preped surface will not sink or heave. However when large weights drive across it the concrete could crack. A thick slab reenforced with rebar can standup to the extra weight.

quote:

Originally posted by Shaql:
the pour of the original driveway ranges anywhere from 4" to 2'.

From 4 inches to 2 feet? Wouldn't two feet thick be like the thickness of a jet runway?

Betting the 2’ are the footing areas.

quote:

A 2" slab on a properly preped surface will not sink or heave.

Two FEET, not two inches. Clearly there were questions about the soil and their solution was to just plop in 2 footings and over pour it.

My driveway isn't anything out of the ordinary. The driveway climbs a slight rise where it meets a 3 car side entry garage. Maybe 50' to where the pad starts.

The 2' is indeed where the footings are but almost the entire pad is at least 18" thick. The edges are where it goes down to 4"- 8".

It's a residential home so there's no need for a 2' pour for heavy equipment.

when we had sections of the pool deck drop we had a company that does concrete lifting come in and pump a foam based lifting product, it fills all the voids and lifts the area.

We also had them do a sound test and when it rang back hollow they filled that area.

Years ago they did it with a concrete slurry mix, now days its done with that or synthetics.

Called a pumping company. They wouldn't touch my driveway. Said it was too far gone.

2 things come to mind for me:

The forms broke when they were pouring the footing.
The had extra concrete and dumped it there.

Make sure your new driveway is 6 inches thick over stone not dirt. When I was working with a mason it was always 6 inches thick which included the apron and the sidewalk that led to the driveway.

quote:

Originally posted by sigarmsp226:
Betting the 2’ are the footing areas.

My thought as well.

Personally I wouldn't pour anything less than 5" thick.

4" is fine for a residential driveway if the rest is right. What about drainage?

If the soil underneath isn't stable, it won't matter how thick it is. it will still sink.

If there isn't too much needed, crushed gravel and a very heavy roller are what you want under the concrete. Dirt also needs to be properly compacted. Moisture below all that may need to be addressed too.
Any trees nearby? Be sure the roots won't be an issue in the future.

About that time ( my house is from 97 ) Georgia still allowed a lot of construction waste to be buried on site. So you’ll find where a big pile of limbs, etc that got buried has taken around 25 years to rot enough to start caving in. That’s my guess.

Happened between my neighbor and I, but luckily no house problem. Still, it was around 5’ deep sinkhole. Couple of yards of fill and it’s been fine for the last 3-4 years.

We actually talked to a sinkhole specialist. He had pictures of his current jobs, didn’t wanna be those folks. Digging out stories below houses level of things. He was the one that suggested the fill first to see, because it was going to be a pile of money for him to start digging.

quote:

Originally posted by 220-9er:
4" is fine for a residential driveway if the rest is right.....

In my experience the thickness is nominal, so with 4" it may vary anywhere from 3 1/4" to 4 3/4", averaging 4". IMO the minimum thickness should be 4" which is why I generally form with 2x6 and pour 5 1/2" of 4,000 psi. The additional concrete doesn't cost that much more and labor is the same.

Mine is 6" but not due to my own doing. Previous homeowner had it poured because I believe he had a boat.

(Tom, the professional civil engineer has entered the room.)

220-9er has hit the nail on the head in that the subgrade (soil below the driveway) needs to be moisture conditioned and properly compacted within the soil's optimum moisture content range. when the soil is compacted as so it stands a better chance than most of differential settlement or worse failure.

Most home contractors don't know anything about compacting soil or even performing appropriate soil moisture testing prior to conducting any earthwork. Most just "eyeball" the compactive effort and pour concrete right on top of the soil.

The proper way to better assure a long lived, even concrete surface is to place non-woven geotextile fabric on top of the compacted soil, backfill and compact at least 4 inches of graded aggregate / crushed aggregate base course, and then install weld wire fabric or better yet rebar prior to pouring the slab. This may seem like overkill but it is money well spent. The geotextile helps to spread the load of aggregate and concrete evenly over areas that could potentially assymetrically settle and cause slab cracking and failure.

Given today's fly-by-night home contractors, it is better to buy once - cry once. If you dont have that kind of money even a thin layer of crushed aggregate on top of a good non-woven geotextile will provide great insurance.

Anyone can feel free to reach out if you have any questions.