The purpose of this chapter is to outline the requirements for
proper storm sewer pipe sizing, construction and inspection.
Storm water facilities shall be designed in accordance with
the procedure of the Henderson Water Utility Technical Stormwater
Manual. A complete set of design calculations for the storm drainage
facilities shall be provided. As per the Stormwater Manual, depths
of flow and velocity for various design flows for the storm sewers,
culverts and constructed channels proposed shall be provided in tabular
form. Plans shall show the location of all streams, swales and springs
within the limits of or adjacent to the proposed work.
Pipe for storm sewers shall be circular reinforced concrete
pipe, ductile iron pipe, solid wall PVC pipe, or HDPE corrugated pipe
(smooth interior wall), as per the requirements of the applicable
Technical Stormwater Manual. Joints and pipe bedding shall conform
to manufacturer's recommendations. Where a storm sewer crosses or
lies within an arterial roadway, or where there is less than twelve
inches (12") cover over the pipe (as measured from the top of the
pipe to subgrade elevation of the pavement), reinforced concrete pipe
shall be required.
Unless specifically directed otherwise by the Engineer, not
more than 100 feet of trench shall be opened ahead of the pipe laying
work of any one crew, and not more than 100 feet of open ditch shall
be left behind the pipe laying work of any one crew. Watchmen or barricades,
lanterns and other such signs and signals as may be necessary to warn
the public of the dangers in connection with open trenches, excavation
and other obstructions, shall be provided by and at the expense of
the developer.
When so required or when directed by the Engineer, only one-half
of street and road crossings shall be excavated before placing temporary
bridges over the side excavated, for the convenience of the traveling
public. All backfilled ditches shall be maintained in such a manner
that they will offer no hazard to the passage of traffic. Excavated
materials shall be disposed of so as to cause the least disruption
to pedestrian and vehicular traffic, and in every case the disposition
of materials shall be satisfactory to the Engineer.
Trenches in which pipes are to be laid shall be excavated to
the depths shown on the approved plans or as specified by the Engineer.
The minimum allowable trench width shall not be less than the outside
diameter of the pipe plus eight (8) inches. Where rock is encountered
it shall be removed to a minimum depth of four (4) inches below the
pipe bells.
Unless specifically authorized by the Engineer, trenches shall
in no case be excavated or permitted to become wider than 2 feet 6
inches plus the nominal diameter of the pipe at the level of or below
the top of the pipe. If the trench becomes wider than 2 feet 6 inches
at the level of or below the top of the pipe, special precautions
may be necessary, such as providing compacted granular fill up to
the top of the pipe or providing pipe with additional crushing strength
as determined by the Engineer after taking into account the actual
trench loads that may result and the strength of the pipe being used.
The developer shall bear the cost of such special precautions as necessary.
Where conditions exist that may be conducive to slides or cave-ins,
proper and adequate sheeting, shoring and bracing shall be installed
to provide safe working conditions and to prevent damage to work.
Adequate and proper shoring of all excavations, and design and fabrication
of all sheeting and shoring systems shall be the entire responsibility
of the Developer.
Trenches shall be kept free of water during the laying of the
pipe and until the pipeline has been backfilled. Suitable dewatering
equipment shall be provided, and water generated by dewatering shall
be contained and treated as required.
All storm sewer pipes shall be laid using bedding of No. 9 crushed
stone placed to a minimum depth of four inches (4") below the bottom
of the pipe barrel. Unstable soil shall be stabilized by over-excavating
and placing a layer of No. 3 crushed stone below the 4 inches of No.
9 bedding.
The laying of sewer pipe in finished trenches shall be commenced
at the lowest point so that the spigot or tongue ends point in the
direction of flow. No pipe shall be laid resting on solid rock, blocking,
or other unyielding objects or materials.
Contractors may use a laser instrument to set the grades on
sewer lines in lieu of using a grade string and batter boards set
from grade stakes. In using such an instrument, the developer shall
be responsible for maintaining grades and elevations as called for
on the drawing profiles, and any variances found shall be corrected
by the developer at his expense.
All pipe lengths shall be properly joined and true to line and
grade. Supporting of pipe shall be as set out hereinbefore under "Pipe
Bedding" and in no case shall the supporting of pipe on blocks be
permitted.
Fittings for storm sewers shall be provided and laid as and
where directed by the Engineer or shown on the plans.
Before each joint of pipe is lowered into the trench, it shall
be thoroughly inspected to insure it is clean. Jointing before placement
in the trench and subsequent lowering of more than one section will
not be allowed. No piece of pipe or fitting which is known to be defective
shall be laid or placed in the lines. If any defective pipe or fitting
is discovered after the pipe is laid, it shall be removed and replaced
with a satisfactory pipe or fitting without additional charge. In
case a length of pipe is cut to fit in a line, it shall be so cut
as to leave a smooth end at right angles to the longitudinal axis
of the pipe and repair type coupling used as a splicing device.
When pipe installation is stopped for any reason, the exposed
end of such pipe shall be closed with a plug fitted into the pipe
bell, so as to exclude earth or other material, and precautions taken
to prevent flotation of pipe by runoff into trench.
When located near water lines, the horizontal separation between
water lines and storm sewers shall be at least 10 feet measured from
the outside edges of the two pipes. Should conditions prevent this
horizontal separation, deviation may be allowed at the discretion
of the Engineer, provided the storm sewer is laid in a separate trench,
or if in the same trench, with the water line located on a bench of
undisturbed earth. In any case, the elevation of the top crown of
the storm sewer shall be at least 18 inches below the bottom of the
water line.
When storm sewers cross water lines, maintain 18 inches of separation
between the pipes, measured from the edges of the two pipes. Storm
sewers crossing water lines shall be constructed of reinforced concrete
pipe or cement coated ductile iron pipe for a distance of 10 feet
either side of the water main. At all crossings, one full stick of
the water main pipe shall be located so that both joints are as far
from the storm sewer pipe as possible. Special structural support
for the water main and storm sewer may be required at the direction
of the Engineer.
All backfilling shall be accomplished in accordance with Section
6.7 for Backfilling in Open Terrain (Method A), Backfilling under
Sidewalks and Unpaved Driveways (Method B), or Backfilling under Streets
and Paved Driveways (Method C).
When directed by the Engineer, the developer shall add water
or dry out the material when needed to attain a condition near optimum
moisture content for a maximum density of the material when it is
compacted. The developer shall obtain a compaction of the backfill
of at least 95 percent of standard (ASTM D698) Proctor density where
mechanical compaction of backfill is required.
Before final acceptance, the developer will be required to level
off all trenches or to bring the trench up to the level of the surrounding
terrain. The developer shall also remove from roadways, rights-of-way,
and/or private property all excess earth or other materials resulting
from construction.
When pavement is not placed immediately following trench backfilling
in streets and highways, the developer shall be responsible for maintaining
the trench surface in a level condition at proper pavement grade at
all times.
In all cases walking or working on the completed pipelines except
as necessary in compacting and backfilling, will not be permitted
until the trench as been backfilled to a point one foot above the
top of the pipe. The filling of the trench and the compaction of backfill
shall be carried on simultaneously on both sides of the pipe in such
a manner that the completed pipeline will not be disturbed and injurious
side pressures do not occur.
Shall be accomplished in accordance with the approved plans
and specifications, and as directed by the Engineer.
Concrete cradle, anchors, or encasement for sewer lines shall
be placed where shown on the plans, required by the specifications,
or as directed by the Engineer. Concrete shall be Class "B" and shall
be mixed sufficiently wet to permit it to flow under the pipe to form
a continuous bed. In compacting concrete, care shall be taken not
to disturb the grade or line of the pipe or injure the joints.
Manholes of the form and dimensions shown on the plans shall
be built as directed. They shall be constructed on 3000 psi concrete
foundations with adequate structural and geometrical capacities, and
in conformance with ASTM C-478.
5.9.1.
Precast Concrete Rings. Precast concrete rings for manholes shall
conform to ASTM C-478.
5.9.2.
Precast Concrete Cones. Precast concrete cones shall be of the size
and shape shown on the plans and shall conform to ASTM C-478.
5.9.3.
Manhole Inverts. Manhole inverts shall be formed from 3,000 psi concrete
as shown on the plans. Inverts for a "straight-through" manhole may
be formed by laying the pipe straight through the manhole, pouring
the concrete invert, and then cutting out the top half of the pipe.
Curved invert shall be constructed of concrete, as shown, and shall
form a smooth, even half-pipe section. The inverts shall be constructed
when the manhole is being built using prefabricated forms. Precast
inverts will be allowed if approved by the Engineer.
The excavation shall be kept free of water while the manhole
is being constructed and the manhole shall not be backfilled until
inspected by the Engineer.
5.9.4.
Manhole Frames and Covers. Manhole castings shall consist of cast
iron frames and covers, weighing not less than 300 pounds per frame
and cover, dimensioned as shown in the standard drawings. Manhole
covers must sit neatly in the rings, with contact edges machined for
even bearing and tops flush with ring edge. They shall have sufficient
corrugations to prevent slipperiness. The lids shall have two pick
holes about 1-1/4 inches wide and 1/2 inch deep with 3/8 inch undercut
all around.
Shall be constructed to forms and dimensions shown on Standard
Drawings, or as shown on plans approved by the Engineer. Headwalls
may be required at the discretion of the Engineer on all storm drains
which terminate in an existing or proposed open ditch or waterway.
All concrete for inlets and headwalls shall have a minimum 28 day
compressive strength of 3500 psi. Reinforcing steel shall be ASTM
A-615, Grade 40 and the size and layout approved by the Engineer.
Upon completion of the installation of the storm sewers and
appurtenances, the developer shall remove all debris and surplus construction
materials resulting from the work. The developer shall grade the ground
along each side of the pipe trench in a uniform and neat manner.
