| (ii) The "n" value must take into consideration the
slime, grit, and grease layers that will affect hydraulics or hinder
flow as a pipe ages.
(B) If a velocity greater than 10 feet per second will
occur when a pipe flows full, based on Manning's formula, shown in
Equation C.2. in Figure: 30 TAC §217.53(l)(2)(B), using the "n"
value for new pipe recommended by the manufacturer, the collection
system must be designed to protect against pipe and bedding displacement.
Attached Graphic
(m) Alignment.
(1) Alignment Requirements. A gravity collection system
must be laid with a uniform grade and straight alignment between manholes,
if possible. All deviations from uniform grade and straight alignment
must be justified in the engineering report.
(2) Prohibited Deviations.
(A) Deviation from uniform grade (e.g., grade breaks
or vertical curves) without manholes is prohibited if the open cut
construction method is used, except as provided in subparagraph (B)
of this paragraph.
(B) For segmented pipe, a horizontal curve must not
be accomplished by bending the pipe unless the pipe joints are welded
or heat-fused. Horizontal curves must be accomplished using additional
manholes or joint offset. Horizontal curves for non-segmented, welded,
or heat-fused pipes must follow the manufacturer's recommendations.
(C) A construction method that bends a pipe joint is
prohibited, unless the joints are offset less than the least of the
following:
(i) five degrees of deflection;
(ii) 80% of the manufacturer's recommended maximum
joint offset; or
(iii) 80% of the appropriate ASTM, AWWA, ANSI, or other
nationally established standard for joint offset.
(3) Calculations and Plan Drawings. The calculations
for horizontal curvature must be included in the engineering report.
Details of the proposed curvature must be displayed on the plan drawings.
(4) Manhole Spacing. The maximum allowable manhole
spacing for collection systems with horizontal curvature is 300 feet.
A manhole must be at the point of curvature and the point of termination
of a curve.
(n) Inverted Siphons and Sag Pipes.
(1) A sag pipe must include:
(A) two or more barrels;
(B) a minimum pipe diameter of 6.0 inches; and
(C) the necessary appurtenances for convenient and
routine flushing and maintenance.
(2) A manhole must include adequate clearance for
rodding and cleaning.
(3) Sag pipes must be sized and designed with sufficient
head to achieve a velocity of at least 3.0 feet per second at initial
and design flows.
(4) The arrangement of inlet and outlet details must
divert the normal flow to one barrel.
(5) A portion of the collection system with the inverted
siphon must allow any barrel to be temporarily taken out of service
for cleaning.
(6) Provisions must be made to allow cleaning across
each bend with equipment available to the entity operating the collection
system.
(7) A sag pipe must prevent nuisance odors.
(8) Inverted siphons and sag pipes must be pressure
tested according to the requirement of §217.57 of this title
(relating to Testing Requirements for Installation of Gravity Collection
System Pipes).
(o) Bridged Sections.
(1) Piping with restrained joints or monolithic pipe
across a bridged section requires a manhole on each end.
(2) A bridged section must withstand the hydraulic
forces, including buoyancy, applied by the occurrence of a 100-year
flood event.
(3) A bridged section must be capable of withstanding
impacts from debris.
(4) Bank sections must be stabilized to prevent erosion.
(5) Bridge supports must be designed to ensure that
a pipe has adequate grade, slope, and structural integrity.
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