Constructing Manhole Over Ex. Water Main

[chocobo1198]

Hi all,

I'm a waterworks guy with limited field experience.

I have a small project that involves the installation of a flow meter on an existing 36" (900mm) steel water main. To reduce costs, I am looking at using a 3050mm diameter reinforced concrete manhole (we typically go with chambers but they cost a lot more). I want the manhole to be sealed and as leak-tight as possible. The manhole also has to be able to withstand H20 loading. Products in my area are able to meet these requirements and thus are not design issues.

I would like advice on the method of construction. Would the contractor be able to achieve proper compaction below the existing main? Specifically we typically design for 95% modified proctor density. I would like to reduce stress resulting from settlement on the existing pipe as much as possible. Also, how would the manhole be constructed around the existing main if I would like it to be leak tight? I heard of doghouse manholes around sewers, but I don't know the details. I.e. Are they leak tight? Are they typical pre-cast pieces with holes knocked out of them (top and bottom over existing pipe)? How would that affect rebars and reinforcing? Or are they custom made? Mannfacturers around here don't seem to have typical detailed drawings of doghouse manholes.

My duties include preparing engineered plans and thus I need to provide details and specifications. Not sure if this is helpful but the concrete manhole I'm looking at is manufactured in accordance with ASTM C478 and CSA A257.3.

Thanks for any feedback.

 

[JedClampett]

When you say the manhole needs to be leaktight, do you mean from infiltration? Stormwater? Or is there water coming from another source?
As far as your other questions:
Instead of specifying compaction, I would require that they slurry or place Controlled Low Strength Material (CLSM) under the pipe.
If the pipe is already there, it's been subjected to settlement stresses before and should be able to take them again, if the backfill is properly placed. Almost every city has guidelines for this.
The manhole would have slots in it and would be lowered over the pipe. If you mortared the gaps, it would be relatively sealed, but it's unlikely to be bottle tight no matter what you did. I would cast a reinforced concrete slab on the bottom with a groove in it matching the manhole shape. Use Ram-Nek or similar to seal the groove. Or mortar it.
Even though the precast manufacturer's have catalogues, almost everything is custom made. Draw them a picture and they will figure it out.
As far as H20, not a problem. Make sure the ring is adjustable for repaving changes.
In your situation, you probably want a large diameter manhole (10 ft.) reducing down to an access shaft of a much smaller diameter, like 4'-0". If you try to have a 10 ft. access opening, you're going to have a lot of trouble covering it with something traffic rated. Most manhole lids top out at 48 inches. If the meter needs to come out, there's going to be some digging involved.

 

[chocobo1198]

Thanks for the info JedClampett.

I meant leaktight as in from infiltration from stormwater.

The comment about using a slotted manhole section and mortaring the gaps for seal is an approach that I am considering except I was thinking of pouring cast-in-place concrete in prepared formwork (which is a big headache and I need to go to a structural engineer).

 

[civilman72]

I was going to recommend what Jed said about a slurry or flow-fill material in place of compaction of soil beneath the pipe.

One concern I have is with a ten-foot manhole - this large diameter (could) leave a fairly large unsupported section of pipe. It would seem prudent to build the manhole so there are no water pipe joints in the manhole, or plan on somehow supporting the pipe in the manhole - which is fairly common in PRV vaults, etc. I'm not a struct. eng. so maybe Jed (or others) can clarify or shoot down this concern.

Another comment - considering the pipe may move, wouldn't you want a more flexible material (than mortar) to fill the gap between the pipe and manhole? Something like a rubber boot, which is typical for a sewer line.

 

[dicksewerrat]

How deep is your water main to invert? What type of soil? You can go to the precast manufacturer, show them what you want and they will make it. The precast is taken out of the form. Then high pressure water cuts a groove through the semi-cured concrete. The reinforing wires will hold the rest of the concrete in place until you want to remove it. Makes it easier to transport.
Vacuum excavate a small hole to locate Centerline of water main. Remove the pavement from the area for the manhole. Place the section with the cut outs where you want it, lining up the cutouts with the mainline. Bring in a vac truck and a jet machine. Water the soil and start removing it from the center of the manhole section out to the edges. You will wash and suck the soil out keeping pressure on the manhole sections to keep it plumb. As the soil is sucked out the manhole will settle. Keep putting more manhole sections on as you need them. Once you get close to the water main you will want to remove just enough soil to below the water main in the 4 corners of the coutouts. Place concrete footings 2-3 inches below the bottom of the water main. Remove the coutouts by cutting the reinforcing wire. Start removeing more soil and bring the manhole section down to the footings. Clean out under the main 12-15 inches including the areas bewteen the footing and past the outside edge of the manhole. Put some rebar, probably # 5's 1 foot each way 1 foot on center. Place good concrete to within 3 inches of the bottom of the main. Put a reducer cone on and topslab that has a hole large enought o remove your meter. Place ring and cover. You can seal around the main with mortar and some type of water stop gasket. I would make the seal only 2-3 inches thick at the otur edge of the manhole section for about a year just to make sure you have enough room for settlement of the manhole. Wash some soildown around the outside of the manhole to fill any voids caused by construction. Replace your pavement

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[JedClampett]

To answer civilman, 36 inch pipe can span a long way, well over 20 feet, even when it's filled with water. But I'd still add a support in the manhole. I'd have a little saddle fabricated out of pipe and steel plate and have the pipe supported off the floor slab.
The reason is that you don't have good support conditions defined beyond the manhole. As the pipe transitions from manhole, to precast concrete to soil, I'm not really sure where the support is. And if the soil under the pipe (hopefully CLSM or slurry) doesn't do a good job of supporting it, you might get some soil sloughing and progressive loss of support.

 

[chocobo1198]

Thanks guys.

JedClampett is right that a 36 inch pipe could span typically 20 ft, 40 ft, or 60ft, at least in my area anyway. I just found out that we are using an FPI series 394 McCrometer that is inserted vertically through the top of pipe. This means I can backfill with pipe bedding to springline inside the manhole. Actually I would be comfortable with the design even if there was no support and no bedding. I have decided to locate it away from traffic.

Dicksewerat - your method sounds feasible, but I'll have to find out how much space we need in order to carry out a 2" wet tap at the top of main. The space inside the manhole might not be good enough.

 

[dicksewerrat]

I'd use the local Water Dept. to do the tap.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com