What to do with a shallow buried concrete pipe? 5

[engr888]

What would be the best way to handle this? We are realigning an existing 36" RCP sewer line that currently has about 2' of cover. The new sewer will need to be installed beneath an access road that frequently has loading trucks.

Would it be better to just use a reinforced concrete jacket around the pipe or use a higher class of concrete pipe? How much more strength does a jacket provide?

 

[civilperson]

Back fill with lean concrete, (including one foot above and below), makes a thick concrete pipe capable of enormous loads.

 

[gbam]

I would calculate the 'D' load on the rcp to determine what class if any can be used before designing a concrete cap.

 

[GoldDredger]

Couple of questions come to mind. First, the 2' of cover, is that top of pipe to finished concrete, or TP to begining of subgrade? Also, are you planning on re-using the 36" concrete pipe or replacing it? Also what kind of surface above the pipe, is is HMAC or reinforced concrete?

I would follow gbam's suggestion of determining loads, maybe an H-20 loading situation or more. Second, the pipe manufacturers tend to be very helpful with this. They will have real world tests of pipe load capacity. You can use a higher class RCP (make it poly lined too, if it is wastewater application, hydrogen sulfides in ww are known to erode concrete)

My guess is you can find a new pipe that can handle the loads without going to special jacket. Probably be less expensive to install too, rather than re-use of old pipe (which may be old and deteriorated) with some form of jacket around it.

 

[Maury]

The American Concrete Pipe Association may be able to help you with the calculations or providing publications to guide you through the design. Horizonatal Elliptical or arch pipe can provide additional cover if they will work hydraulically. I would consider a reinforced concrete slab to bridge the pipe as an alternate to a cradle.

 

[engr888]

Thanks for all of the replies. To answer some of your questions, a new 36" pipe will be installed with PVC liner to resist corrosion. The 2' cover is measured from the top of pavement to the top of pipe so that includes the pavement thickness, subgrade, and backfill (thicknesses yet to be determined).

My initial discussions with the manufacturers is that we can get by with 1' of minimum cover over standard class 3 RCP. I'm hesitant mainly because the acceptable standard is to provide at least 4' of cover. So if we install it at a shallower depth and run into any problems, we're at risk for deviating from the standard.

Thanks again for the options... definitely food for thought.

 

[bimr]

What do you mean by access road? Are you referring to a dedicated road or driveway?

You also have not said anything about the underground soil conditions.

Another alternative that has not been proposed so far is that you can probably use two smaller pipes instead of the single larger one. For example, 2- 24" pipes instead of the 36". That may be your best option.

 

[bpattengale]

The class of pipe you need is going to be determined by the loading on the pipe (live and dead), the type of installation, and existing soil conditions. The American Concrete Pipe Association puts out a free design manual that includes load calculation and pipe strength requirements. It also includes design examples. This is the link to the design manual

http://www.concrete-pipe.org/designmanual.htm.

I'm going to assume you have access to information on the existing soil types at the site. Now the pipe is crossed by access road so you need to determine the loading. HS-20 is a standard load and should be fine unless you have evidence of larger trucks using the road. Next select a type of bedding based on the soil information and determine the D-load on the pipe. Then select a class of pipe that meets that load.

There are some excelent examples in the design manual. As gbam said it will likely be cheaper and easier to install a higher class of pipe rather than constuct a concrete cap. I've seen loaded twin axle dump trucks drive over class 5 rcp with 6-inches of cover with no ill effects on the pipe and Class 3 RCP will likely be sufficient but as stated before just run through the design calcs to make sure.

One thing to remember is if you specifiy a higher class of bedding you can use a lower strength pipe, but you'll have to have tighter controls on installation to make sure it is constructed correctly. Its usually better to assume a lower bedding class for design purposes as it will result in a more conservative design and account for any installation errors.

 

[bpattengale]

I just looked up the D-load for a class 3 RCP. Its 1350 pounds per linear foot per foot of diameter to produce a 0.01" crack and 2000 for the ultimate loading. So for a 36-inch class 3 RCP pipe the D-load for a 0.01" crack would be 4050 lbs per foot.

 

[marblaze]

Provide minimum 6 in granular back fill on top of the pipe then cast a reinforced slab on top of the fill with min 3' extensions on both sides, i.e. total length is 3' + pipe diameter + 3'. Design slab as one-way with appropriate traffic loading.

http://malilipotbuilders.blogspot.com/search/label/Good Engineering