Home driveway question 2

[knowlittle]

I need an expert's advice on my home driveway situation. My sewer drain pipe runs under the concrete driveway. After failing to remove the blockage in the sewer pipe, he is going to cut a hole in the driveway and do the necessary repair. He recommends a new driveway afterward. This is why. My concrete driveway is cracked into several large pieces (25 year old house). One piece is noticeably sunk. According to my plumber, the piece sank under vehicle load, moving the soil underneath, which in turn damaged the sewer pipe.

Here is my question. Did the sinking concrete move the soil underneath? Or did uneven settling of soil cause concrete pad cracking and sinking? If the former, I will need to get a new driveway. If the latter, I may not need a new one.

I live in Dallas. We have an expanding clay soil. As it goes through dry/wet periods, it can cause structural damage to homes, they say. Thank you.

 

[IRstuff]

My two cents: if it rains at all in your location, you should redo your driveway. Rainwater flow will undermine the soil under the driveway.

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[Ron]

This is a bit of a chicken or egg syndrome....

Most likely shrinkage cracks occurred in your slabs long ago. As time progressed the cracks widened. As they did so, they lost a lot if not all of their load transfer capability as wheels roll across the crack. This causes the slab to deflect differentially under the load. It also allows more water to get into the underlying soil. Both of these can cause soil displacement and depending on the depth of the pipe, can damage the pipe.

I would recommend replacing the driveway, making sure that properly spaced control joints are cut the same day as concrete placement. The contractor will balk at this but make him do it.

The slab should be consistent in thickness with a minimum thickness of 6 inches. The tolerance should be -1/4 inch, +3/8 inch. Wet cure the concrete for 7 or more days before putting load on it. Use 4000 psi concrete. You need the durability of the stronger concrete. Lower strength will do for the loading but will be less durable.

 

[PEinc]

Or, the sewer pipe failed after 25 years and the soil above and around the pipe is washing away into the pipe causing the driveway to settle. Fix the pipe and the section of driveway above the pipe.

http://www.PeirceEngineering.com

 

[oldestguy]

Living in an area with expansible clays must be difficult. Thus, the recommendations of Ron might be considered over doing it, yet that high expand-shrinkage potential soil must be a constant headache. Hopefully the new sewer pipe is tough stuff and also can be affected by that soil situation.

 

[r13]

I am here now. Made a mistake on posting

I think it is time to let go the old drive way, and provide a new. The linked paper discuss the design and construction of pavement over expansive clay. It's worth reading and take notes. Link

 

[caenright]

I agree with Ron and retired13. Multiple cracks and failures like that indicate a subbase and bearing soil problem, and replacing the driveway wholesale will probably be the best bet in the long run.

You'll want good new embankment or subbase material (at least 6" if you're doing a 6" slab) to help mitigate some of the expansive clays issue. 4000 PSI concrete is likely good; we use a 4500 PSI standard for concrete pavement on our highways in CO but I don't think you need the same strength. Could consider tooled joints versus sawn joints (tooled grooves 1/4 slab depth deep x 1/8" wide) without any expansion joints if the driveway is shorter than a couple hundred feet.

 

[BridgeSmith]

I noticed no one has mentioned reinforcement. Was it just assumed that the standard welded mesh would be in the new slab, or is there some reason why it shouldn't be? I'd consider it a small expense for a big gain in ability of the slab to handle movement of the subgrade due to the expansive clay.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dauwerda]

Posting some pictures of your existing driveway may help with the advice you are given. "Cracked into several large pieces" doesn't sound like a replacement is warranted to me, but what I am envisioning could be quite different than what is actually there.

The performance of the driveway (or any slab on grade) is very dependent on the subgrade preparation. In residential construction it is very common for driveways to move and crack due to poor subgrade preparation, expanding soils just compound this problem. Residential driveways and slabs on grade are usually unreinforced or minimally reinforced, so they will not bridge and distribute loads over any soft spots in the subgrade. All of this will lead to cracking and settlement you are seeing.

How deep below your driveway is the sewer pipe? Is it right below the concrete so it is depending on the driveway to essentially "bridge" over it? Has soil started eroding under the concrete due to water infiltration (from rain or a broken sewer pipe)?

If the cracks don't bother you and water infiltration has not been an issue, I see no reason to replace the entire driveway. I would still recommend sealing all of the cracks to help prevent moisture intrusion under the slab.

making sure that properly spaced control joints are cut the same day as concrete placement. The contractor will balk at this but make him do it

Ron, do you see contractors having issues with this and not doing it in your area? I would be quickly finding a new contractor if this were an issue for them in my area.

 

[oldestguy]

This old time engineer that has dealt with both rigid and flexible pavements disagree somewhat the statement "The performance of the driveway (or any slab on grade) is very dependent on the subgrade preparation". Fortunately rigid pavements can span a lot of poor subgrade. Thickness depends more on loads and frequency. Add another inch and amazing different life. As to reinforcing using the packaged wire placed in the ready mix load is an easy way and it can produce wonders.

 

[dauwerda]

You are correct oldestguy. In my statement I was specifically thinking about nominal 4" SOG that are typically found in residential construction for driveways, garage floors, basement floors, etc.

 

[r13]

I think good subgrade preparation and quality pavement with adequate thickness are hand in hand twins headed for success. Somehow I've never specified any concrete slab less than 6" thick w/wo reinforcement. I guess that's why my late boss had quit to ask me handle SOG design jobs

 

[BridgeSmith]

I had read somewhere when investigating paving my driveway with concrete (don't remember where at the moment) that a 4" reinforced slab (typical 6x6 mesh or fiber reinforced concrete) would be as good or better than 6" of plain concrete. I don't know how accurate that is, but that would seem to indicate reinforced would be a significant improvement, especially where the subgrade is questionable. At less than a quarter per square foot (for the mesh - around here it's similar for adding poly fibers), I'd not hesitate to throw it in, if opting to replace the driveway.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[dauwerda]

BridgeSmith, I agree, just make sure you are there when they are placing the concrete to make sure the wire mesh ends up located in the middle of the slab and not on the ground as is often the case.

 

[r13]

I don't against reinforcing. But I won't use 4" even with wire mesh, for inadequate cover in my opinion. I usually use #4 bars for light duty SOG, so you can see why I wanted 6" thick slab.

 

[oldestguy]

Fibermesh is a lower cost, labor-saving option for concrete reinforcement. Unlike rebar which must be set precisely before concrete can be poured, Fibermesh pours with the concrete saving time, as well. Overall, according to Propex Concrete Solutions (which manufactures Fibermesh), Fibermesh can be used in residential, commercial or industrial applications as the primary form of reinforcement without a rebar skeleton.

 

[BridgeSmith]

On a typical driveway, I agree with the 6" minimum thickness being prudent. With expansive clay (or any clay) in the subgrade, I'd say 6" thickness is a necessity.

In my case, I have a well-compacted in-situ sand-gravel base that I planned on keeping undisturbed, in a location inaccessible to anything larger than a full-sized pickup. I considered a 4" slab with carefully placed reinforcing mesh, and probably poly fiber, as well for cheap insurance (local ready-mix suppliers charge $10/CY to add fibers).

Rod Smith, P.E., The artist formerly known as HotRod10

 

[r13]

The reason I don't like mesh too much is construction concern. The rebars usually set tightly in place throughout the concrete pour, while the mesh, more often than not, will sink to the bottom due to weight of the wet concrete if attention is lacking. I often ask my foreman, where is my mesh? "Yes sir, here it is", a big smile and two muddy hands

 

[Ron]

Fibermesh is a lower cost, labor-saving option for concrete reinforcement.

OG...this might be a first....I disagree with you. I don't consider fiber in concrete, whether steel, glass or polypropylene, to be "reinforcement". Fiber is a mix enhancement that may increase compressive and tensile strength properties of the concrete, but doesn't truly reinforce it in the same manner as rebar or welded wire fabric.

 

[oldestguy]

Ron; I partly disagree.
Any attempt at "reinforcement" is better than none. For a case where normally none is needed, BUT the site is crummy, it is an easy way to improve things. I can only Point to my limited observations of a few such uses where I'd expect cracks and faulted sections to be plenty, and there are NONE.