sanitary sewer services- 1

[kedpe]

Why are manhole connections not allowed (discouraged) for sanitary sewer services? A standard drawing for a lateral connection shows a "trench dam" at 10' from the connection.
What does a trench dam do, and is it used when the connection is to a manhole?

 

[civilperson]

Who or what agency does not allow or discourages manhole connections for sanitary sewer?

 

[kedpe]

Just for the laterals, not mainlines.

 

[rconner]

I don't know the answer to this question I had not heard before, unless the authority(ies) involved may be worried about in addition to other problems from private services/construction they might have to be worried about increased connections/sources of I/I due to the extra service manholes and what is involved getting pipes in an out of same (over which they might have little control), or maybe the fact that some service manholes might even be on private property without easement for their ready access etc.? I did notice some time ago Ten State Standards apparently don't like "cleanouts" at the end of very long laterals or services, but I kind of thought this was possibly due to lack of effectiveness in cleaning obstructions in the very small pipes normally involved for the very long distances and/or perhaps etc. as well the aforementioned points etc.
I will like you await no doubt more knowledgeable responses.

 

[CarlB]

The public utility here also discourages service connects to manholes. I believe their primary concern has to do with the invert/flow channel of the main line not being interrupted, and if services enter MH at higher elevations they make maintenance operations more difficult.

 

[eddieng27]

CarlB hit on one problem and that is related to maintenance operations that will be conducted from the manhole. Another stems from the ownership of the service laterals and making service connections to "the main." If the building owner is responsible for the entire service from its "connection to the main" all the way to the building, the sewer utility will not want the building owner disturbing the manhole if/ when the service line and connections need repair or replacement; and neither will the building owner. Also the sewer utility would prefer to have a contractor make a saddle connection to main line piping rather than a connection into a manhole for a service. I know of sewer utilities, however, that require larger than minimum sized service connections (larger than 4-inches for an 8-inch main) to connect into manholes. This minimum service piping size is usually dependent upon the size of the main where the service would connect (generally not a good idea for a large service to connect to a relatively small main line). The specific requirements are to connect into the manhole above the "bench" with a formed "cuvette" in the bench curving into the pathway of the flow esentially the same way that a main line "cuvette" is formed from the incoming pipe to the outgoing pipe.

 

[Mack454]

Here in NJ all the sewerage authorities I have dealt with do not allow direct connections to manholes.

 

[LCruiser]

It gets messy...

 

[bimr]

There are lots of reasons:

1. The drop into the manhole will cause solids to drop out and odors.

2. Long distances between the house and manholes.

3. Manholes cost more than pipe and you would have to put more in as distances in the laterals increase.

4. More manholes make street repairs more costly and snow plowing problems as the plows hits the manholes..

 

[divad41]

Most, or all, public utilites I have worked with do not allow laterals into manholes... my understanding is it is at least in part a hydraulic issue, as dicussed elsewhere.

However, the Navy at Pearl Harbor prefers laterals into manholes. It was surprise when I learned this. No one really had a good explanantion, but one of our guesses is that their systems stay in place so long, and the have such maintenance problems (manpower etc), that maybe they want to be able to "see" all the laterals, in cases where they actually have to do something relating to them.

 

[LCruiser]

I think bimr's #1 is the real reason and can be summarized in one word:

Stalagmites.

 

[rconner]

I think I initially misunderstood the question. For some crazy reason I was envisioning the poster wanting to put a new manhole in the service line itself, when I now realize they were probably only talking in slight short-hand form about simply running the service line in a new penetration through the wall of an (existing?) mainline manhole, I guess likely some distance above the invert. There are good responses on the list to this issue that I understand would generally and understandably be discouraged.

 

[kedpe]

Thanks, guys-yes, the existing lateral penetrated an existing mainline manhole. A new penetration was proposed because it was originally installed incorrectly (trying to make "it" flow uphill). So, a new inside drop was proposed ~2' lower in the manhole. The standard drawing for a service lateral shows a "trench dam". I didn't know its purpose, so I didn't know if it would be required when a lateral penetrates a manhole. I didn't think it would apply.

 

[CarlB]

I would guess a "trench dam" is an impermeable barrier constructed in a trench to prevent water/groundwater migration along the trench, through more permeable backfill such as pipe bedding. I've seen this only for a utility passing through a wetlands area that authorities didn't want being drained by water flowing along the pipe. May be other purposes as well. Something like a sand/bentonite mix was used.

 

[bmike]

Trench dams are frequently specified here in Ottawa (Canada) for water and sewer laterals, and put near the property line.

I do not get involved too much in wetland preservation, but have seen cases where they have been used to mitigate drainage for major sewer trunks.

This city has several areas of young, sensitive, and compressible clay (high water content) up to 100m thick. Lighter structures (like houses) are often founded on spread footings in the sitff upper dessicated crust of this clay.

The idea of the dam is that this will prevent drainage of the clay into the deeper mains trenches, with the resulting induced consolidation of the clay and settlement of the house.

Studies have shown that they are generally ineffective, largely as a result of their absence or poor quality/workmanship (surely not self-policed residential developer QC!).

Trees have historically been considered the culprit, and suspect species are routinely cut down along city streets when they reach a particular size, and replaced with younger, less thirsty stock.

I think it would be safe to say that the jury is out on the cause and solution, but as the city grows into previously less desirable ground, there has been a rise in the incidence of differential settlement and cracked foundations. You can imagine that if the ground was draining toward the street, the settlement would progress from the street toward the house, which is consistent with reports.

My house is about ten years old. Three years ago there was a major increase in housing development, together with a bunch of regional large, depp utilities. Last year the city came and lowered the valve caps for everyones curbstops. Mine is now about 1/2" above the driveway surface again. Something is causing a reduction in the height between the ground surface and water lateral (2.5m deep).

Global warming has also been speculated as the cause, but you would have to ask Al Gore about that.

Personally, I think there a number of contributing factors: increased hard surfacing - reduced recharge, installation of deep (>25m) utility arteries through the clay, as well as the traditional tree transpiration, climate change, general drainage of the land through utility trenches, etc.

Without some kind of regional groundwater inventory and monitoring, I doubt this will ever be settled, except by the loudest voices and the theory of the month.

Anyway that's the idea behind trench dams here. I would not be surprised that they pay attention to this in Boston as well, where falling groundwater levels have had an impact on timber piled foundations in the downtown.

 

[Romeosperson]

While all of the above is true, I don't think anyone mentioned the other reason. My utility guys would kill me if we changed our regulations and permitted laterals into the manhole. Generally they come in above the hydraulic gradeline - read drop manhole. When one of the guys is in the manhole, clearing roots, obstructions, removing random shop towels or myriad of other entertaining objects that make it down there, or using our camera, they prefer not to get a shower from the lateral when someone flushes. Yes, we can dam the lateral, but that doesn't always work for longer term work.