Vertical drops in manholes

[looksatstars]

We are going to be field fitting alot of sewer pipes in manholes coming up in a recent job.
The preference is to come in at the bottom of the manhole with a pre-benched base, but we may have a scenerio where we will have to drill into the side of a manhole and come in above the bottom. Is a vertical drop a problem? if so, is there a certain depth i should be concerned about?

 

[dicksewerrat]

Who did the design on this project? You will have to talk to the owner. Drops cause outgassing and odors.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[bimr]

It is customary to have a drop manhole if the drop is greater than 2 feet entering the manhole.

You need to contact the regulatory authority having jurisdiction over the manholes in question. The regulatory authority will be able to tell you what the requirements are.

 

[brucedaengineer]

A "drop" in a manhole is constructed as a piped drop (say greater than 2 foot as bimr says) - your local authority will have standard drawings of how to construct the drop pipe - it will have a "gun-bend" at the bend down - allows access for sewer cleaning. The drop pipe is usually a size smaller than the sewer you are constructing. The details are important if it is to be done properly.

Drops in large sewers are a big deal and gas check manholes and other details are required and if extreme case (deep vertical drops) you may need hydraulic drop structures - but I assume you are talking about smaller sewers - less than 18 inch say

 

[rconner]

If you have not already done so you might want to obtain and read a copy of ASCE MOP No. 60 (WPCF MOP No. FD-5), "Gravity Sanitary Sewer Design and Construction". While the coverage is not in excruciating detail, several different configurations of drops are discussed and/or depicted on pages 137-138 of this MOP. Drops employed by different folks can incidentally be actually inside or outside of the manhole, and in lieu of a downbend at the top of the drop, tees (or even crosses with risers extending to ground level, supposedly to facilitate cleaning) are alternatively used by some folks at the top of the drop.
Concluding points are made indicating that in whatever configuration drops can be some problematic and should be used sparingly e.g. (I guess when it is not economically feasible to use vertical curves or steepen incoming sewers etc.)