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Section 02202 - Trenching and Backfill
PART 1 - GENERAL
1.01 WORK INCLUDED
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This section covers trenching and backfill
requirements for buried piping systems specified in Water
System - Section 02660; Storm Drainage - Section 02720;
Sanitary Sewers - Section 02730; Irrigation Systems -
Section 02910, Street and Safety Lighting - Section 16500.
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This Section also covers requirements for
excavation and for compaction of succeeding layers after
backfill has been placed around pipe.
1.02 APPLICABLE PUBLICATIONS
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The publications listed below form a part
of this specification to the extent referenced. The publications
are referred to in the text by the general designation
only.
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American Society for Testing and Materials
(ASTM) Publications:
C - 33 Specification for Concrete Aggregates
C - 136 Sieve Analysis of Fine and Coarse
Aggregates
C - 150 Portland Cement
C - 260 Air Entraining Admixtures
D - 424 Plastic Limit and Plasticity Index
of Soils
C - 618 Fly Ash and Raw or Calcined Natural
Pozzolan for use as a Mineral Admixture in Portland Cement
D -1557 Moisture-Density Relations of Soils
and Soil Aggregate Mixtures Using10-lb. (4.54 KG) Rammer
and 18-in. (457 mm) Drop
D - 2419 Sand equivalent Value of Soils and
Fine Aggregate
D - 2487 Classification of soils for Engineering
purposes
D - 3017 Density of Soil and Soil Aggregate
in Place by Nuclear Methods (Shallow Depth)
1.03 SUBMITTALS
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Certified test reports for the permeable
material backfill tested in accordance with ASTM C 136.
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Samples: Submit 1 gallon size sample of
permeable material for approval.
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Shoring and Sheeting Plan: Before starting
work submit a CAL-OSHA permit for the shoring and sheeting
plan when trench excavation is five feet deep or more.
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Dewatering Plan: If required in the Special
Conditions, before starting work submit a dewatering plan
describing the basic components of the dewatering including
silt control, etc.
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Traffic Plan: Where lane closures are anticipated,
submit a Traffic Control Plan 72 hours in advance for
approval prior to starting work.
1.04 QUALITY ASSURANCE
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Percentage of compaction specified shall
be the minimum acceptable. The percentage represents the
ratio of the dry density of the compacted backfill material
to the maximum dry density of the material as determined
by the procedure set forth in ASTM Designation D1557.
For field density tests, ASTM D-3017 may be used.
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D-load or class of pipe requirements shown
or called for on the plans shall be the minimum acceptable.
1.05 JOB CONDITIONS, PROTECTION, AND SHORING
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Existing Utilities:
1. Unless shown to be removed, protect active
utility lines shown on the Plans or otherwise made known
to the Contractor prior to excavating. If damaged, repair
or replace at the Contractor's expense. Pothole as required
to verify utility location. Contractor shall be responsible
for contacting all utility companies and coordinating any
work which requires relocation or abandonment of existing
utilities.
2. If active utility lines are encountered
and are not shown on the Plans or otherwise made known to
the Contractor, promptly take necessary steps to assure
that service is not interrupted.
3. If a known service is interrupted as a
result of work under this section, immediately restore service
by repairing the damaged utility at Contractor's expense.
4. If foreseen or unforeseen existing utilities
are newly found to interfere with the permanent facilities
being constructed under this Contract, immediately notify
the Engineer for directions.
5. Do not proceed with permanent repair or
relocation of utilities until written instructions are received
from the Engineer.
6. No construction water shall be disposed
of in the City's storm drain system.
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PROTECTION OF PERSONS & PROPERTY:
1. Install all necessary underpinning, shoring,
lagging, cribbing, and bracing of ample strength to support
adjoining soils, paving and structures. All such items shall
be so constructed that they will not interfere with the
building of any structural elements, and shall be removed
upon completion of the shoring operation.
2. Barricade open depressions and holes occurring
as part of this work, and post warning lights on property
adjacent to or with public access.
3. Operate warning lights during hours from
dusk to dawn each day and as otherwise required.
4. Protect structures, utilities, sidewalks,
pavements, and other facilities from damage caused by settlement,
lateral movement, undermining, washout, and other hazards
created by operations of Contractor.
5. No trenches shall be left open during non-working
hours.
6. Install fences and barricades to secure
the area from the public.
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SHORING
1. The Contractor is solely responsible for
all bracing and shoring. The Contractor shall forward their
application for shoring to the California Division of Industrial
Safety for their review. Contractor's application shall
include the basic design, assumed soils conditions and estimation
of forces to be resisted, together with plans and specifications
of the materials and methods to be used, and shall be prepared
by a Civil Engineer registered in California.
2. If an application for a shoring permit
is required, no excavation in trench section or around structures
shall proceed until the approved shoring plan has been received
by the Engineer.
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DEWATERING
1. Remove all water, including rain water,
encountered during trench and sub-structure work to an approved
location by pumps, drains, and other approved methods.
2. Keep excavations and site construction
area free from water.
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DUST CONTROL
1. Use means necessary to control dust on
and near the work, and on and near off-site areas, if such
dust is caused by the Contractor's operations during performance
of the Work, or if resulting from the condition in which
the Contractor leaves the site.
2. Thoroughly moisten surfaces as required
to prevent dust being a nuisance to the public, neighbors,
and personnel performing other work on the site.
3. Use dust palliatives or reclaimed water
(not potable water).
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Maintain access to adjacent areas at all
times.
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Maintain and/or replace all bench marks,
monuments, construction stakes and other reference points.
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Repair or restore damage to any portion
of the work resulting from movement of the sides or bottom
of trenches or other excavation which is attributable
to the Contractor's acts or omissions, whether sides are
braced or not.
PART 2 - PRODUCTS
2.01 GENERAL SOIL MATERIALS
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In general, soils used for backfill shall
be select material free of debris, roots, wood, scrap
material, vegetation, refuse, soft unsound particles,
frozen, deleterious, or objectionable materials, satisfactory
to the Engineer, free of stones or lumps exceeding 3 inches
in greatest dimension.
2.02 PIPE BEDDING AND INITIAL BACKFILL MATERIAL
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Pipe bedding and initial backfill up to
six inches above the top of the pipe shall be Class I,
Type A permeable material, conforming to Section 68-1.025
of the Standard Specifications, except that the gradation
for the #4 sieve shall be 30-55% passing, and gradation
for the #8 sieve shall be 5 - 10% passing
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Material shall contain at least 75% of the
particles having one or more fractured faces.
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Bedding and backfill material shall be subject
to the approval of the Engineer.
2.03 SELECT BACKFILL ABOVE INITIAL BACKFILL
OR BEDDING
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In non-paved areas unless otherwise shown
on plans, select backfill shall conform to the requirements
for soil materials above, and shall be classified as (GW),
(GP), (GM), (SW), (SP) or (SM) by ASTM D 2487 and meet
the following:
1. Sand equivalent shall not be less than
25 when tested in accordance with ASTM D 2419, plasticity
index shall not exceed 15 when tested in accordance with
ASTM D 424, and not more than 25% by weight shall be finer
than the No. 200 sieve.
2. On-site native material may be used as
backfill if it conforms to 2.03A.1. above.
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In paved areas, except Bay Mud, select backfill
shall be Class 2 aggregate base, 3/4" maximum size
gradation conforming to Section 25 of the Standard Specifications.
2.04 SPECIAL REQUIREMENTS FOR SELECT BACKFILL
IN BAY MUD CONDITIONS
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In areas where Bay Mud is prevalent, there
are two distinct types of soil material present, Bay Mud
and Imported Fill Material. Bay Mud consists of very soft,
cohesive, clayey material, and it may be encountered from
1 to 4 feet below grade, depending on location. (At Redwood
Shores for example, Bay Mud generally occurs at elevation
100). Excavated Bay Mud may be either segregated and hauled
off the site, or dried and conditioned for placement back
into the trench per Paragraph 3.07, at the Contractor's
option.
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The existing imported fill may be salvaged
and used for backfill if it meets with the requirements
of select backfill, per Paragraph 2.03.
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Suitable excavated Bay Mud from trenches
may be conditioned by aeration and used as backfill. To
accelerate drying to optimum moisture content for compaction
as backfill, spreading, disking or harrowing may be required.
2.05 SUBDRAIN MATERIAL
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Where required for trench drainage and for
subsurface drains, bedding shall conform to the requirements
of Class 1, Type A Permeable material per Section 68 of
State Standard Specifications.
2.06 CONTROLLED DENSITY FILL (CDF) (in trenches)
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Controlled density fill will be accepted
in lieu of the standard backfill specifications. It shall
be mandatory in trenches eight (8) inches wide or less
where the prevention of subsequent settlement after placement
of backfill is required. CDF shall conform to the following
requirements:
1. Strength Requirements
a. Non-structural CDF that can be excavated
by hand shall produce unconfined compressive 28 day strengths
from 50 psi to a maximum of 150 psi. CDF that is to be
excavated by hand shall contain aggregate no larger than
3/8" top size nor shall the 3/8" aggregate comprise
more than 30% of the total aggregate content.
2. Materials
a. Cement shall meet the standards as set
forth in ASTM C-150, Type II Cement.
b. Fly ash shall meet the standards as set
forth in ASTM C-618, for Class F pozzolans. The fly ash
shall not inhibit the entrainment of air.
c. Air entraining agent shall meet the standards
as set forth in ASTM C-260.
d. Aggregates need not meet the standards
as set forth in ASTM C-33. Any aggregate, producing performances
characteristics of the CDF, for any project will be accepted
for consideration as follows. The amount of material passing
a #200 sieve shall not exceed 12% and no plastic fines
shall be present.
3. Mix Proportions
a. CDF shall be a mixture of cement, Class
F pozzolan, aggregate, air entraining agent and water.
CDF shall be batched by a ready mixed concrete plant and
delivered to the job site by means of transit mixing trucks
except for
b. The actual mix proportions shall be determined
by the producer of the controlled density fill to meet
job site conditions, minimum and maximum strengths, and
unit weight. Entrained air content shall be a minimum
of 4.0%. The actual entrained air content shall be established
for each job with the materials and aggregates to be used
to meet the placing and unit weight requirements. Entrained
air content may be as high as 20% for fluidity requirements.
4. Mix Design
a. Mix design shall meet the Engineer's
approval.
PART 3 - EXECUTION
3.01 GENERAL TRENCHING AND EXCAVATING
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Trenches may be excavated either by hand,
or by machine. Trenches shall be cut with vertical sides,
and shall be of sufficient width to provide adequate space
for working therein; such space shall be a minimum clear
distance of six (6) inches of shoring and a maximum of
nine (9) inches clear of shoring on each side of the pipe
barrel when the pipe is properly placed and aligned in
conformity with the plans. Glory hole excavation or vee
trenches will not be allowed. Trench sides shall be parallel
to and at equal distance from the center-line of the pipe,
when aligned in conformity with the plans.
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Excavated material shall be loaded into
trucks immediately upon removal from the trench to prevent
stockpiling on roadways or walkways.
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Where the excavated trench exceeds the widths
specified above, furnish higher strength pipe, or other
methods of construction as approved by the Engineer, to
adequately provide for the increased loading, which the
trench widening will cause. Stepped trenches shall meet
the approval of the Engineer.
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Pipe trenches shall be excavated to a depth
below the bottom of the pipe sufficient to provide for
pipe bedding materials as required by Section 3.02.
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Where a trench has been excavated below
the designed grade, the bottom of the trench shall be
refilled to proper subgrade with approved material well
compacted in place, in an approved manner.
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The Engineer shall have the right to limit
the amount of trench which is opened or partially opened
at any one time; and also to limit the amount of trench
left without backfill, at any one time.
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No trench or holes shall be left open overnight.
Use steel pating to protect open trenches overnight.
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Excavation for thrust blocks shall be neat
to the line and dimensions shown or called for on the
plans.
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Provide for dewatering trenches and excavations
and subsequent control of ground water, utilizing such
pumps or other equipment as may be necessary to control
ground water and seepage until backfilling is completed.
3.02 GENERAL BEDDING
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Utilities shall be laid on a firm layer
of firm bedding material not less than four (4) inches
in depth as shown or as noted on the plans and detail
drawings, except that bedding shall not be required for
utilities two (2) inches or less in nominal diameter.
Compact as specified herein.
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Upon completion of bedding operations and,
prior to the installation of pipe or appurtenances, notify
the Engineer who will then inspect the bedding layer.
Pipe laying shall not commence until the bedding has been
approved.
3.03 GENERAL BACKFILLING
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Backfill shall be as shown on the plans.
Place in 6-inch maximum loose lifts to one foot above
pipe unless otherwise specified. Bring up evenly on each
side, and for the full length of the structure. Ensure
that no damage is done to structures or protective coatings
thereon. Place the remainder of the backfill in 8-inch
maximum loose lifts unless otherwise specified. Compact
each loose lift as specified in Paragraph "General
Compaction" before placing the next lift. Where unacceptable
settlements occur in trenches and pits due to improper
compaction, excavate to the depth necessary to rectify
the problem, then backfill and compact the excavation
as specified herein and restore the surface to the required
elevation.
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No backfill shall be placed until the line
has been inspected and approved for backfilling.
3.04 GENERAL COMPACTION
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Use hand-operated plate type vibratory or
other suitable hand tampers in areas not accessible to
larger rollers or compactors. Be careful to avoid damaging
pipes and protective pipe coatings. Compaction shall be
in accordance with the following unless otherwise specified.
If necessary, the Contractor's selected equipment and
construction procedure shall be altered, changed or modified
in order to meet the specified compaction requirements.
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Initial backfill and bedding shall be carefully
packed under the haunches of the pipe and brought up simultaneously
on both sides so as to obviate any displacement of the
pipe from its true alignment. Bedding shall be compacted
in layers not more than eight (8) inches in thickness
in a manner that will preclude moving the pipe, to not
less than 95% of maximum dry density as determined by
the procedure set forth in ASTM Designation D1557. Jetting
of backfill material will not be permitted.
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Select backfill above the initial backfill
shall be placed in loose lifts not exceeding eight (8)
inches in thickness before compaction, and compacted by
the use of pneumatic tampers or other mechanical means
approved. Water or dry, as required, to bring the soils
as close as practicable to the optimum moisture content
for proper compaction. Compaction equipment or methods
that produce horizontal or vertical earth pressures which
may cause excessive displacement or may damage the pipeline
will not be permitted. Lifts of backfill shall be compacted
to not less than 95% of maximum dry density as determined
by the procedure set forth in ASTM Designation D1557.
Jetting of backfill material will not be permitted.
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For flowable CDF, compaction is not necessary
for placement. Trench sections may be filled in one lift
above the initial backfill material.
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Backfill will be inspected and tested by
the Engineer during placement. Contractor shall cooperate
with the Engineer and shall provide working space for
such tests in his operations. Backfill not compacted in
accordance with these specifications shall be recompacted,
or removed as necessary and replaced to meet specified
requirements prior to proceeding with the work.
3.05 GENERAL BRACING AND SHORING
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The Contractor shall furnish, place and
maintain such bracing and shoring as may be required to
support the sides of the excavations for the proper protection
of workmen; to facilitate the work; and to prevent damage
to adjacent structures or facilities.
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Upon completion of the work, all bracing
and shoring shall be removed, unless otherwise directed
by the Engineer. Current requirements are for a maximum
depth of 5 feet without CAL-OSHA approved shoring.
3.06 SPECIAL REQUIREMENTS FOR SHORING AND EXCAVATING
IN BAY MUD
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Trenches or other excavations, where the
distance from the existing grade to the bottom of the
excavation exceeds eight (8) feet, shall be interlocking,
continuous solid sheathing. Said sheathing shall be designed
to resist an equivalent fluid pressure having a density
of at least 100-lbs.per cubic foot for the full height
of the trench and shall be installed for the full depth
of trench and to a sufficient depth below the bottom of
the excavation to prevent heaving of the bottom, and shall
be adequately supported in place with walers and braces.
Fill voids between sheathing and trench wall with pea
gravel.
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Mandatory use of solid sheathing at designated
locations shall not relieve the Contractor of the responsibility
to install sheathing at other locations required or where
necessary for safety of workmen or the general public.
Excavations below Bay Mud shall be accomplished between
previously placed sheathing and bracing in those areas
where solid sheathing is required. The required bracing
and sheathing will be installed prior to excavating below
braced elevation.
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Material which is salvaged from the trench
and stockpiled for later use as backfill, shall be stored
a minimum distance of 10 feet, but no less than 3 times
the trench depth from the edge of the excavation.
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Sheathing shall be withdrawn only after
backfill above the pipe has proceeded to a height equal
to or greater than 3/4 of the excavation depth. Bottom
cross bracing and walers may be left in place upon removal
of the sheathing. Backfill shall be brought to the level
of the cross-braces before these are removed.
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Install and extract the sheathing in a manner
which will not disturb the line, grade, backfill compaction
or operation of the utility being installed or adjacent
utilities and improvements.
3.07 SPECIAL REQUIREMENT FOR BACKFILL IN BAY
MUD CONDITIONS
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Bay Mud, if used as backfill at the Contractor's
option, shall be spread and dried to optimum moisture
content, then replaced in the trench to the same elevation
below grade as the existing Bay Mud, as directed.
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Imported fill material, which has been salvaged
from trench excavation for use as select backfill, shall
be stockpiled alongside the excavation unless otherwise
directed. Material shall not be stockpiled higher than
four (4) feet and shall be kept back from the trench a
horizontal distance at least equal to the depth of excavation.
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All material excavated within the limits
of work, not required as backfill, shall then be transported
and dumped or conditioned to optimum moisture content
and compacted as embankment at the site noted on the plans
or as directed.
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On-site native Bay Mud for use as backfill
shall be conditioned by means of watering or aeration
as may be required to bring the material to the range
of moisture content satisfactory for compaction to project
specification.
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Prior to placing conditioned material as
backfill, the material shall be thoroughly and uniformly
mixed to the satisfaction of the Engineer, and the moisture
content shall be within the allowable range for compaction
to project specification. Material which has dried to
less than allowable moisture content shall be reconditioned
to an acceptable moisture content and re-mixed to the
satisfaction of the Engineer.
3.08 SPECIAL REQUIREMENT FOR COMPACTION IN
BAY MUD
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Compaction procedures are as described in
paragraph "General Compaction" except that the
density of compaction shall be 90% as determined by ASTM
Designation D1557, except for the top 6" in paved
areas which shall be compacted to 95%.
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Remove shoring or sheathing as described
above, making sure that the pipe is not displaced.
3.09 SPECIAL EARTHWORK REQUIREMENTS FOR SUBSURFACE
DRAINS
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Excavate to the dimensions indicated.
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Provide a bedding surface of uniform density
consisting of permeable material as indicated.
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Backfill around and over the pipes after
pipe installation has been approved with permeable material
to the depth indicated. Place in maximum loose lifts of
8 inches.
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Compact each lift with mechanical tampers
or rammers. Compact bedding and backfill materials to
90% of ASTM D1557, Method D, maximum density. Place the
remainder of the trench backfill as specified.
3.10 SPECIAL REQUIREMENTS FOR CONTROLLED DENSITY
FILL (CDF)
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Applications of CDF include, but are not
limited to: backfills, structural fills, insulating fills,
road base, slab base, trench bedding, void and abandoned
tank fills caisson and pile fills, abandoned pipes and
culverts.
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CDF shall be discharged from the mixer by
any reasonable means into the area to be filled. CDF shall
be brought uniformly to the elevation as shown on the
plans. Trench sections to be filled with CDF shall be
contained at either end by bulkheads of earth fill.
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Permanent pavement may be placed directly
upon the CDF as soon as it has sufficiently self-consolidated
so that the surface will withstand the process of paving
without displacement or disruption. If the placement of
the CDF is not completed early enough to allow for permanent
paving to be completed the same day, the contractor shall
provide steel plates to span the trench and prevent traffic
contact with the CDF overnight or until permanent paving
can be placed.
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Compaction is not necessary when placing
CDF.
3.11 FIELD QUALITY CONTROL
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The Engineer will inspect, test and approve
trench backfill layers before further construction is
permitted thereon. Number of tests required will be determined
by the Engineer.
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If backfill has been placed, that is below
the specified density, provide additional compaction with
subsequent retesting until successful compaction is achieved.
3.12 DUST ALLEVIATION AND CONTROL
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Contractor shall be responsible for and
shall provide pollution and dust abatement and control
measures satisfactorily during the course of the work.
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The Contractor shall utilize reclaimed water,
or dust palliatives, in compliance with the City's Water
Conservation Ordinance.
3.13 FINISH OPERATIONS
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Pipes shall be laid to finished grades indicated
on the plans.
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Dispose of all surplus material or material
unsuitable for filling or grading off the site in a legal
manner.
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Satisfactorily restore any existing improvements,
paving, landscaping, and other utilities disturbed during
the course of constructing the improvements.
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Existing traffic markings and control devices
damaged or disturbed during construction shall be replaced
or repaired to the satisfaction of the Engineer.
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