[garrett1812]

After digging next to my concrete patio, to see the extent of a rodent den, I discovered (besides that the rodent den was not too large) that there is about a 1" +/- gap between the bottom of the concrete and the top of the soil.

From the hole I dug I could insert a tape measure about 6ft deep under the patio. I do not know if it is localized here, or if this condition exists all around the patio.

Any idea what caused this? Erosion? Poor compact of soil before pouring?

And more importantly, what needs to be done to fix it?

Normally the patio is just used for a dining table and chairs, but I have been eyeing a stone grill kit from Menards, which is approx 4000lbs in stone. I do not want to get this and make it crack. The patio is approx 1.5-2 years old.

Thanks for the help!

 

[jlhaslip]

How old is the house? Was this slab poured when the house was built? What distance from the house is the slab? Usually the foundation excavation is about four feet outside the house edge and the excavation may have settled. This is normal.

I have used a grout pump to fill voids like this.
Start by drilling a couple of holes in the concrete to accept the nozzle of the grout pump. Mix the non-shrink grout to a watery consistency and pump it into the holes.
This method, if done properly with threaded anchors, would even allow for you to jack the slab if it is off level.
In order to do that, you need to drill and insert some threaded anchors which wedge into the slab and accept a threaded connection to the grout pump. It usually takes more holes to jack the slab than to simply fill it.

An alternative is to find an outfit locally which does this professionally, but they might cost a few bucks.

 

[danpik]

Soil settles over time under concrete. Even properly compacted it will do that. I wish we had digital photography years ago when we cut coolant trenches into the floors out in the plant. Building was built in 1949 and 45-50 years later, there were voids under the floors in some spots you could crawl into. Realizing we had heavy equipment sitting on these floors, we decided to have a company come in that did a sonar check of the floor and then pumped grout into all the voids. The interesting side effect from all of this was the 5% drop in ambient noise in the plant.

 

[garrett1812]

The house is about 1.5 years old. I believe the patio was pored some months after the foundation, not at the same time. The patio sits directly up against the foundation.

Settling would make sense as the cause. I am going to have the builder look at it, hopefully they cover it! Sounds like an expensive fix otherwise.