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Proof without words: One house is temporarily deleted. The lines connecting the remaining houses with the utilities divide the plane into three regions. Whichever region the deleted house is placed into, the similarly shaded utility is outside the region. By the Jordan curve theorem, a line connecting them must intersect one of the existing lines.
Vertical pipes, usually made of precast concrete, called manholes, connect the mains to the surface. Depending upon site application and use, these vertical pipes can be cylindrical, eccentric, or concentric. The manholes are used for access to the sewer pipes for inspection and maintenance, and as a means to vent sewer gases.
A sewer pipe is normally at neutral air pressure compared to the surrounding atmosphere.When a column of waste water flows through a pipe, it compresses air ahead of it in the system, creating a positive pressure that must be released so it does not push back on the waste stream and downstream traps, slow drainage, and induce potential clogs.
To install this, the doghouse manhole is lowered to have the two openings to slide over the existing pipe. The concrete is poured in to form the base with slopes from the precast concrete wall down to the half point of the existing pipe. This forms a half bench for the manhole base. The top half of that existing pipe is cut open to form a channel.
Their planning and design requires the expertise of city planners and civil engineers, who must consider many factors, such as location, current demand, future growth, leakage, pressure, pipe size, pressure loss, fire fighting flows, etc.—using pipe network analysis and other tools.
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