sanitary sewer services-
sanitary sewer services-
(OP)
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?
What does a trench dam do, and is it used when the connection is to a manhole?





RE: sanitary sewer services-
RE: sanitary sewer services-
RE: sanitary sewer services-
I will like you await no doubt more knowledgeable responses.
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RE: sanitary sewer services-
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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..
RE: sanitary sewer services-
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.
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Stalagmites.
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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.
RE: sanitary sewer services-