Sunken Driveway Slab @ Garage Door

Problem: Driveway slab sinks as much as 2 inches at garage door.
Past Remedy: Mudjacked 3 years ago. Sunk again.
Comments: There's obviously a cavity being created by water seeping from somewhere under the slab.
Question: Aside from using expensive commercial sonar equipment that can't be rented nor is made available for residential use... and aside from removing the slab... how would you find:
1) Size of the cavity 2) Location of the cavity 3) Source of the water flow
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jackhammer slap and find out? sewer line run thru that area?
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you could drill some holes thru the slab and go look with a borescope camera
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"jaygreg" wrote

Hire a soothsayer. Without the proper gear, they have as good of a guess as any other.
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On 4/3/2011 6:29 PM, jaygreg wrote:

Might not be a discernable cavity or heavy water flow- could easily just be inadequate substrate prep when they put it in. The gravel or sand or whatever could just be compacting. When you say slab at garage door, do you mean the apron, or do you have a full concrete driveway or parking pad? If it is just the 2-foot apron that is slumped (like mine is) poorly compacted backfill around garage foundation is the likely cause, aggravated by the crack between the slabs directing water down there.
No cheap permanent cure that I am aware of. If you are curious, take posthole diggers and dig an inspection hole on both sides of driveway at the slumped spot, to see what is down there. If you go a foot or two down, and the digging suddenly gets easy, that tells you something. You can dig another hole someplace where the drive hasn't sunk, for a comparision. You can tell my apron and asphalt drive were put in by amateurs, and no rollers or jumping-jack compactors were used on the substrate- the ditch for the gas line and what looks like a rut where a truck got stuck, are clearly telegraphed. 2 different paving companies told me to not waste my money on sealcoating, just save up for a rip and replace, with proper substrate prep.
Sometimes tear it out and do it right is the only permanent cure. Stuff like this is why my father always recommended to his clients that they wait till the following spring to install the driveway and front walk, and live with gravel and planks till then. Things usually have stabilized by then.
--
aem sends...

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I appreciate the comments but the probability of finding this source by drilling just two inspection holes - one on each side of the double-car driveway slab that abuts the garage floor (no apron) - is about as like as my striking oil in the process. The probability may be increased by using some sort of vibrator that sets on the concrete and transfers vibrations downward that are then monitored with a stethoscope held right next to the unit. The shift in tone may be audible. But then.... I don't know what tool could be used to generate sufficient vibration without being too noisy.
Was hoping someone had similar experience or knowledge of a tool that may have application here.
Thanks for your suggestions though.
On 4/3/2011 7:53 PM, aemeijers wrote:

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wrote:

Ground penetrating radar is probably what you want. Have no idea of the cost or whether it's worth doing.
--Vic
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On 4/3/2011 6:29 PM, jaygreg wrote:

Drill multiple holes for mudjacking. Then drill down 10' or whatever until you hit bedrock, fill with concrete and rebar or better small steel I beam and concrete to make piers, then mudjack. Or use concrete filled steel posts made for basements for piers.
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