subsurface drainage and slab movement

[EG]

Hi All,

We have been asked to review 'drainage' for a townhouse project in our ski resort community. Several residents have noticed that generally during the winter season, there is substantial (5-inches or so) movement between the edge of the heated (hydronic) concrete driveway and a valley pan that parallels the asphalt road. In other words, the driveway slab abuts the concrete valley pan which abuts the edge of the asphalt. The concrete valley pan is NOT heated.

There was never any sealant installed along the joints. Snow from the plowed street gets piled up on either side of the driveway apron. This snow melts and generally infiltrates into the ground. Runoff from melting snow/rain also sheets across the driveway and gets into the joint. Clearly, the valley pan has heaved (and continues to heave) letting water into the subgrade/base materials. For some reason though, the pan has NOT moved relative to the asphalt.

The residents are planning on replacing the valley pan this summer and appropriately sealing the new joints. We have been asked for any additional drainage recommendations. Surface drainage appears to work fine. In other words, there is positive drainage and no low points and the facilities appear to be able to accommodate rainfall events.

I was considering installing an underdrain below the new pan and into the areas below the piled snow and daylight the pipe responsibly but not sure about other improvement options. Cannot dowel into the slab due to the hydronic coils. Any thoughts are appreciated. Thanks!

EG

 

[civilman72]

This is a fairly common issue, and was discussed here:

Link

Your movement issue is caused by heating one surface and not the other. I'm not convinced an underdrain below the pan is the answer or will stop the movement, and you may quickly lose your new seal.

I've seen this happen before, but a 5" movement is significant and probably indicates a substantial amount of water infiltration. The first thing that I would recommend is that the snow adjacent to the pan and driveway entrance is relocated/removed, particularly on the uphill side.

The thread above discusses potential ways to address the issue, but it requires a significant amount of excavation, import, insulation, and if done correctly, will probably require that you cut into the asphalt road.

 

[EG]

Thanks Civilman72-

What is weird is that this occurs in 2 driveways out of about 20...all of the rest are similar in construction, but not exposure. I will inquire about the snow indicator and see if the heat runs too much (their utility bill should reflect this as well). Additional NFS material seems like a reasonable approach...especially since they will be removing the pan and can install it then...

 

[EG]

After giving this more thought, I just can't believe that the 'heat' from the slab is influencing the heave. Let me frame the question in a different way: What would you do to remedy the heaving problem of the ribbon/valley pan along the slab, knowing you are not able to dowel into the driveway slab (due to the hydronic tubing)?

EG

 

[civilman72]

The heat makes the basecourse and subgrade under the concrete react differently (i.e. frost-heave) than the frozen basecourse/subgrade adjacent to it. I do not believe you will solve this issue by only removing/replacing the concrete pan. You need to create a gradual transition in the basecourse/subgrade to solve the issue.

Look at the recommendations from "oldestguy" on the thread I linked above. Based on the extent of excavation recommended in his posts, it will more than likely require that you remove/replace a portion of the heated driveway and the asphalt road. If you can't cut into the asphalt road, you should consider removing more of the existing driveway. When you replace it, leave a portion of the concrete driveway unheated at the road entrance and install the insulation and tapered gravel basecourse (recommended by oldestguy) between the heated and unheated portion of the driveway.

 

[civilman72]

I should note, removing a portion of the existing heated concrete could cut off the flow through the hydronic tubes. It will be based on how these tubes were originally placed.