capacity of 4" driveway slab

[mike deiters]

Hello everyone, new here and have a question for the SMEs on this board

first I am a PE in GA in Civil Engrg. Recently ,I inspected a homeowners driveway after a tree removal company brought a 68,000 lb truck crane in to remove a 44" white oak that fell on his house as well as a 50,000 lb grapple truck to haul the logs.

the work caused some settlement / lateral movement of the compacted fill subgrade as evidenced by displacement of granite cobble curbing on the driveway edges

granite curbing on each side of the driveway was displaced 1-2" from the driveway edge. as built's show a 1/2" construction joint between granite cobble and concrete and the new gap is obviously new and due to the excess loading

the load also caused some cracking in a few places but primary issue is the long crack / separation between the curbing and concrete drive which will lead to water infiltration, freeze thaw, undermining the aggregate base over time.


so after that background, I would like to know the approximate capacity of the 4" slab / 6" aggregate. Slab is reinforced 2' o.c. each way.
assume soil bearing capacity 2,000 psf.

I have read that these common driveway slabs are built for 8-10K lbs which is a passenger car, obviously not a large piece of const equipment.

Anyway I need a definitive reference to cite, I dont want to go thru a long structural calculation to prove the slab was not designed for these types of loads, I am looking for some references.


thanks guys

 

[jayrod12]

Regardless of what the numbers end up saying, in my experience in these types of disputes, if the owner doesn't have proof of condition prior to the purported vehicle damage then they'll have a tough time recouping repair costs.

That being said, so much of the load carrying capacity of a slab-on-grade is in the base prep that it's tough to talk about capacity without knowing how well compacted and uniform the gravel (and soil beneath that) is in-situ.

 

[JLNJ]

4" of pavement isn't good for much, so I doubt that one could show that a truck wheel load would be OK.

It's just a matter of figuring out whose responsibility it is.

 

[dik]

Was there any other means of removing the tree? What did they do with the lumber... that's a lot of what could be really fine wood.

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[hokie66]

I assume homeowners insurance will pay for repairs to the house. Repairs to the driveway should also be covered, along with the tree removal. Assessing a 4” slab for those loads is fruitless.

 

[TLHS]

There's no definitive resource to cite that will just give you a thickness vs load. Even for a simple slab on grade design, it's heavily subgrade and load shape dependent. The thinner the slab is, the more subgrade dependent it becomes.

Common practice might be 4, 6 or 8 inches for a certain use and it'll generally work, but the capacities may vary by a factor a 10 in different conditions. I generally wouldn't go below 5-6 inches for something seeing moderate trucks regularly, but part of that is durability rather than a structural failure.

You'd have to go through a calculation for your specific loading condition and site conditions, especially with the confounding issue of a crane truck. The point loads from that are a little trickier.

You can take a look at:

https://www.concrete.org/store/productdetail.aspx?ItemID=36010&Language=English&Units=US_AND_METRIC

https://wbdg.org/FFC/ARMYCOE/COETM/ARCHIVES/tm_5_809_12.pdf

From my past work, I know that a truck on the edge of a 4" slab with moderate soil likely won't calc out with a safety factor I'd want if at all, using the methods I'd pick. A crane operating off the side would be more of an issue.That being said, there's certainly a soil condition where it would as you move towards gravels and rock. There are also a wide variety of different calculation methods with varying levels of aggressiveness.

If I were writing an assessment, I would might not make a direct statement of capacity unless it really helped. I'd likely make a statement that:

-A 3.5" or 4" slab is the practical minimum thickness for concrete pavement
-it is often used for light vehicle traffic and pedestrian areas although it may support higher loads depending on soil conditions
-it is generally more vulnerable to damage from large equipment than thicker slabs

Then I would describe the failure that I saw and state whether it's reasonable that it could have been caused by overload.

If I were to present calculated values, I'd likely calculate the point load capacity as described by a certain method for a whole range of soil conditions, including a set of conditions that would make the load work, display that as a table or graph, and then describe the observed soil conditions. Any way you decide to do it, just make sure you clearly communicate the sungrade sensitivity and envelope a reasonable range in your statements.

I would also likely check what the local building regulations say for residential driveway thickness, commercial driveway thickness and sidewalks, as they provide good context to the reader. However, it could also be bad for the homeowner if the current requirements are higher than 4". These comparisons might be enough to do what you're trying to do without hard capacities.

Beyond that, though, this sounds like it's not really a vertical slab failure. It sounds like if it's an overload of the slab the failure is because of some sort of edge effects. The soil at the edge of the slab isn't confined vertically, so it has nothing stopping it from pushing up and sideways and displacing the curb. This isn't a traditional slab failure, it's closer to a bearing failure with a rotational slip. The literature isn't going to handle this well, but some methods have suggestions for edge effects. For this type of failure, you'd expect that the driveway would have to rotate. Maybe it rotated as a unit if it's not very wide, but is there a crack along the top of the slab somewhere running roughly parallel to where you see the distress?

Out of curiousity, what service are you providing? If it's just to document that something is damaged and that it could have been due to the actions of the contractor, I'd personally keep it simple and just document the condition, explain a potential failure mechanism and state whether it could reasonably have been caused by a large load being placed on the driveway.

 

[Tomfh]

We recently had similar situation. We had a large dead tree removed, with access via our 100mm driveway. There were 4 large logs (“sticks” as the logger called them), 3 tonnes each, which a large grapple truck came and collected in one hit. The driveway moved a bit here and there where the subgrade isn’t as good. One crack worsened slightly. I expected as much would happen.

Most basic guides will say ~100mm for light vehicles, ~150mm for heavier vehicles, but it’s very difficult to provide definitive axle limit, as it all depends on the subgrade and the frequency of loading.

 

[mike deiters]

Everyone,

thank you very much for the responses, I have learned a good deal from this discussion