312317_Trenching_Utilities.pdf

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WATER SYSTEM TRENCHING, BEDDING AND BACKFILLING

FOR UTILITIES

SECTION 312317

TRENCHING, BEDDING AND BACKFILL FOR UTILITIES

PART 1 - GENERAL

1.1 SUMMARY

A. The work in this Section includes excavation, dewatering, preparation of the trench bottom, supply and installation of bedding and backfill materials; and disposal of waste material for the installation of utilities and their related appurtenances.

1.2 QUALITY ASSURANCE

A. The Contractor shall conduct compaction tests as necessary to monitor the installation procedure and assure the quality of the work.

B. All trench excavation shall be performed in accordance with minimum requirements of OSHA Regulations. The Contractor shall do all shoring, bracing and tight sheeting required to prevent caving and to protect his workmen in accordance with OSHA requirements and to protect adjacent property and structures.

C. Discharge from dewatering operations shall be directed to approved natural drainages as appropriate. Discharge from dewatering operations shall be in accordance with rules and regulations established by the Colorado Water Quality Control Division.

1.3 SUBMITTALS

A. Contractor shall submit to the Contracting Officer for approval the source of pipe bedding material and backfill materials and certified sieve analysis.

1.4 PROJECT CONDITIONS

A. The type, size and location of all known underground utilities have been shown on the drawings; however, no guarantee is made as to the true location of such utilities.

B. It shall be the Contractor’s responsibility to verify the existence and location of all underground utilities at the project site.

C. Existing Utilities: Do not interrupt utilities serving facilities occupied by the

Government or others unless permitted in writing by the Contracting Office (CO) and then only after arranging to provide temporary utility services. Notify the CO not less than 72 hours in advance of proposed utility interruptions.

1.5 DEFINITIONS

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A. Unclassified excavation shall consist of the excavation and disposal of all material encountered in the work, including excavation obtained from borrow source.

B. Rock excavation shall consist of the removal and disposal of igneous, metamorphic, and sedimentary rock which cannot be excavated without blasting or using rippers, and all boulders or other detached stones each having a volume of one-half cubic yard or more.

C. Muck excavation shall consist of the removal and disposal of saturated organic mixtures of soils or organic matter which requires additional work or equipment which would not normally be required for unclassified excavation.

PART 2 - PRODUCTS

2.1 BACKFILL MATERIALS, GENERAL

A. Excavated material may be processed and used for backfill if the Contractor can show compliance with the material specified herein to the satisfaction of the Contracting

Officer. If excavated material is not sufficient to meet requirements, Contractor shall import needed material.

B. Satisfactory Soils: ASTM D 2487 soil classification groups GW, GP, GM, SW, SP, and SM, or a combination of these group symbols, free of rock or gravel larger than 3 inches in any dimension, debris, waste frozen materials, vegetation, and other deleterious matter.

C. Unsatisfactory Soils: ASTM D 2487 soil classification groups GC, SC, ML, MH, CL, CH, OL, OH, and PT, or a combination of these group symbols.

1. Unsatisfactory soils also include satisfactory soils not maintained within 2 percent of optimum moisture content at time of compaction.

2.2 BACKFILL AND FILL

A. Satisfactory soil materials.

2.3 BEDDING

A. Naturally or artificially graded mixture of natural or crushed gravel, crushed stone, and natural or crushed sand, ASTM D 2940, except with 100 percent passing a ½-inch sieve and not more than 8 percent passing a No. 200 sieve. On site materials may be used with approval of the Contracting Officer.

2.4 INITIAL BACKFILL

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A. Naturally or artificially graded mixture of natural or crushed gravel, crushed stone, and natural or crushed sand, ASTM D 2940, except with 100 percent passing a ½-inch sieve and not more than 8 percent passing a No. 200 sieve. On site materials may be used with approval of the Contracting Officer.

2.5 SUBSEQUENT BACKFILL

A. Subsequent backfill shall be excavated material or borrow material, free from rock larger than 6 inches in the greatest dimension, frozen material, clods, debris, or organic material. Borrow material used as subsequent backfill shall have a gradation and character suitable to achieve the compaction specified in this Section.

2.6 WARNING TAPE

A. Detectable Warning Tape: Acid- and alkali-resistant polyethylene film warning tape manufactured for marking and identifying underground utilities, minimum 3 inches wide and 4 mils (0.1 mm) thick, continuously inscribed with a description of utility, with metallic core encased in a protective jacket for corrosion protection, detectable by metal detector when tape is buried up to 30 inches deep, colored as follows:

1. Red: Electric.

2. Blue: Water systems.

3. Green: Sewer systems.

PART 3 - EXECUTION

3.1 ALIGNMENT AND GRADE

A. The location of all pipelines and utility structures shall be staked out and grades established by the Contractor. The Contracting Officer shall approve locations before excavation is started.

3.2 PREPARATION

A. Protect structures, utilities, sidewalks, pavements, and other facilities from damage caused by settlement, lateral movement, undermining, washout and other hazards created by earthwork operations.

B. Protect subgrades and foundation soils against freezing temperatures or frost. Provide protective insulating materials as necessary.

C. Provide erosion-control measures to prevent erosion or displacement of soils and discharge of soil-bearing water runoff or airborne dust to adjacent properties and walkways.

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3.3 SHEETING AND SHORING

A. Provide adequate shoring and bracing as required by OSHA and other local governing regulations.

B. Excavations and sides of trenches shall be supported with adequate materials as necessary for the safety of personnel and protection of the utility before backfilling.

C. The installation and removal of all sheeting, shoring, piling, and bracing shall be such as to maintain the required trench or excavated section, and to maintain the undisturbed state of the soils at and below the bottom of the excavation.

D. Sheet piling and timbers in trench excavations shall be withdrawn in such a manner as to prevent subsequent settlement of the pipe or additional backfill loadings which might overload the pipe.

E. All sheeting and shoring shall be removed when safe to do so.

3.4 DEWATERING

A. Prevent surface water and ground water from entering excavations, from ponding on prepared subgrades, and from flooding Project site and surrounding area.

B. Protect subgrades from softening, undermining, washout, and damage by rain or water accumulation.

1. Reroute surface water runoff away from excavated areas. Do not allow water to accumulate in excavations. Do not use excavated trenches as temporary drainage ditches.

2. Install a dewatering system to keep subgrades dry and convey ground water away from excavations. Maintain until dewatering is no longer required.

3.5 EXPLOSIVES

A. Do not use explosives.

3.6 EXCAVATION, GENERAL

A. If excavated materials intended for fill and backfill include unsatisfactory soil materials and rock, replace with satisfactory soil materials.

B. Topsoil shall be removed from the area to be excavated and from the area where excavated material will be piled prior to excavation. Topsoil shall be stored as specified below.

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C. Maintain the excavations to guard against and prevent injury to employees and the public. Provide adequate shoring and bracing as required by OSHA and other local governing regulations.

D. Excavations left open at the end of the working day shall be fenced to protect the public.

E. Notify the Contracting Officer when excavation is completed. Do not place pipe, bedding or backfill until the excavation is approved by the Contracting Officer. Do not cover pipe until placement has been approved by the CO.

3.7 EXCAVATION FOR UTILITY TRENCHES

A. Excavate trenches to indicated gradients, lines, depths, and elevations.

B. Excavate trenches to uniform widths to provide a working clearance on each side of pipe or conduit. Excavate trench walls vertically from trench bottom to 12 inches higher than top of pipe or conduit, unless otherwise indicated.

1. Clearance (General): 6 inches on each side of pipe or conduit.

2. Clearance from Electrical: 12 inches on each side of pipe or conduit

C. Trench Bottoms: Excavate and shape trench bottoms to provide uniform bearing and support of pipes and conduit. Shape subgrade to provide continuous support for bells, joints, and barrels of pipes and for joints, fittings, and bodies of conduits. Remove projecting stones and sharp objects along trench subgrade.

1. For pipes and conduit less than 6 inches in nominal diameter hand-excavate trench bottoms and support pipe and conduit on an undisturbed subgrade.

2. For pipes and conduit 6 inches or larger in nominal diameter, shape bottom of trench to support bottom 90 degrees of pipe circumference. Fill depressions with tamped initial bedding course.

3. Excavate trenches 6 inches deeper than elevation required in rock or other unyielding bearing material to allow for initial bedding course.

3.8 FIELD QUALITY CONTROL

A. Testing Agency: Contractor shall engage a qualified independent geotechnical engineering testing agency to perform field quality control testing.

B. Allow testing agency to inspect and test subgrades and each fill or backfill layer.

Proceed with subsequent earthwork only after test results for previously completed work complies with requirements.

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C. Testing agency will test compaction of soils in place according to ASTM D 2922.

Tests will be performed at the following locations and frequencies:

1. Trench Backfill: At each compacted layer (bedding and backfill), at least one test for every 75 feet or less of trench length, but in no case fewer than two tests.

D. When testing agency reports that fills or backfills have not achieved degree of compaction specified, scarify and moisten or aerate, or remove and replace soil to depth required; recompact and retest until specified compaction is obtained.

E. Excessive settlement or other evidence of improper backfill shall be corrected by reopening the trench or excavation to the depth required for proper compaction and then shall be refilled and satisfactorily compacted.

F. The correction and retesting of unacceptable work shall be paid by the Contractor at no expense to the Government.

3.9 STORAGE OF SOIL MATERIALS

A. Stockpile, borrow materials and satisfactory excavated soil materials. Stockpile soil materials without intermixing. Place, grade, and shape stockpiles to drain surface water. Cover to prevent windblown dust.

1. Stockpile soil materials away from edge of excavations. Do not store within drip line of remaining trees.

B. Topsoil shall be kept separate from trench-excavated material by stockpiling. Topsoil will be reused after backfilling on those areas from which it came.

3.10 UTILITY TRENCH BACKFILL

A. As soon as possible after individual pipe is jointed, sufficient backfill material shall be placed along each side of the pipe to offset conditions that might tend to move the pipe off line and grade.

B. Backfill shall not be completed until the pipeline installation is approved by the

Contracting Officer and pressure and/or leakage tests have been performed and approved by the Contracting Officer. Sufficient backfill shall be placed to prevent pipe movement under pressure tests. This backfill shall be tamped to the density specified in this section. All joints, valves, etc., shall remain uncovered for inspection purposes during testing.

C. Place and compact initial bedding course on trench bottoms and where indicated.

Initial bedding shall be installed to the full width of the trench bottom to a minimum depth of four inches. Shape initial bedding course to provide continuous support for bells, joints, and barrels of pipes and for joints, fittings, and bodies of conduits.

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D. The initial backfill and bedding material shall be placed by hand and tamped to the density specified in this Section. Carefully compact initial backfill and bedding under pipe haunches and compact evenly up on both sides and along the full length of utility piping or conduit to avoid damage or displacement of piping or conduit.

E. Place and compact initial backfill to a height of 12 inches over the utility pipe or conduit. Place initial backfill and bedding in layers not to exceed 4 inches in loose depth.

F. Place and compact subsequent backfill to final subgrade in layers not to exceed twelve inches in loose depth. Each layer shall be compacted as specified in this section. No large rock remaining in the backfill material and approved by the Contracting Officer for placement with the backfill shall be placed within three feet of the pipe.

G. Backfill trenches excavated under footings and within 18 inches of bottom of footings with initial backfill; fill with concrete to elevation of bottom of footings.

H. Fill voids with approved backfill materials while shoring and bracing, and as sheeting is removed.

I. Install warning tape directly above utilities, between 4 and 8 inches below finished grade.

J. Six inches of topsoil shall be deposited as final backfill in trench areas where this material had been previously stripped and stockpiled. This topsoil layer shall be compacted sufficiently to remain in place but remain loose enough to allow plant growth.

3.11 MOISTURE CONTROL

A. Uniformly moisten or aerate subgrade and each subsequent fill or backfill layer before compaction to within 2 percent of optimum moisture content.

1. Do not place backfill or fill material on surfaces that are muddy, frozen, or contain frost or ice.

2. Remove and replace, or scarify and air-dry, otherwise satisfactory soil material that exceeds optimum moisture content by 2 percent and is too wet to compact to specified dry unit weight.

3.12 COMPACTION OF BACKFILLS AND FILLS

A. The minimum degree of compaction required shall be a percent of the maximum laboratory density obtained by the standard proctor test AASHTO T-99 or ASTM

D698. The in-place field density shall be determined by AASHTTO T238 or ASTM

D2922.

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3.13 DISPOSAL OF SURPLUS AND WASTE MATERIALS

A. Disposal: Remove surplus satisfactory soil and waste material, including unsatisfactory soil, trash, and debris, and legally dispose of it off Government’s property.

END OF SECTION