Section I – Trenching, Backfilling and Compacting For Utility Systems
PART 1 – GENERAL
1.01 WORK INCLUDED
A. The work included under this Section consists of clearing, excavating, grading and backfilling as required for the construction of the utility systems consisting of piping and appurtenances as shown on the Drawings and specified herein.
PART 2 – PRODUCTS
2.01 BEDDING MATERIAL
A. Bedding material for use below the water table or in wet trenches shall be pea rock, drain field lime rock or washed sand as approved by the Engineer. Pipe bedding material for use in dry trenches for water lines and force mains shall be lime rock screenings, sand or other fine inorganic material as approved by the Engineer. Bedding material for gravity sewer shall be angular rock or washed sand.
2.02 ADDITIONAL BACKFILL MATERIAL
A. Additional backfill material shall be classified as A-1, A-3 or A-2-4 in accordance with AASHTO Designation M 145 and shall be free from vegetation and organic material. No stones or rocks shall be larger than 6-inches in diameter, and when placed within 1-foot of piping and appurtenances stones or rocks shall be no larger than 2-inches in diameter (1-inch for PVC).
PART 3 – EXECUTION
3.01 CLEARING
A. The Contractor shall perform all clearing necessary for the proper installation of all piping and appurtenances in the locations shown on the Drawings.
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Plantings, shrubbery, trees, utility poles or structures subject to damage resulting from the excavation shall be transplanted, relocated, braced, shored, or otherwise protected and preserved and prior to project completion restored to their original condition unless otherwise directed by the Engineer.
3.02 EXCAVATION
A. The Contractor shall perform all excavation of every description and of whatever substances encountered, to the di- mensions and depth shown on the Drawings, or as directed. All existing utilities such as pipes, poles and structures shall be carefully supported and protected from injury, and in case of damage, they shall be restored at no cost to the Owner.
B. Trench walls shall be kept vertical, and, if required to protect the safety of workmen, the general public, this or other work or structures, or to maintain trench widths within the limits hereinafter specified, shall be properly sheeted and braced. Where wood sheeting or certain designs of steel sheeting are used, the sheeting shall be cut off at a level 2-feet above the top of the installed pipe and that portion below that level shall be left in place. If inter-locking steel sheeting is used, it may be removed providing removal can be accomplished without disturbing the bedding, pipe or alignment of the pipe. Any damage to the pipe bedding, pipe or alignment of the constructed utility caused by removal of sheeting shall be cause for rejection of the affected portion of the work. Not more than 100-feet of trench shall be opened ahead of pipe laying operations at one time unless a greater length of open trench is approved by the Engineer.
C. In areas where trench widths are not limited by right-of-way, and/or easement widths, property line restrictions, existing adjacent improvements, including pavements, structures and other utilities, and maintenance of traffic, the trench sides may be sloped to a stable angle of repose of the excavated material. A substantially and safely constructed movable shield, “box” or “mole” may be used in place of sheeting when the trench is opened immediately ahead of the shield and closed immediately behind the shield as pipe laying proceeds inside the shield.
D. Ladders or steps shall be provided for and used by workmen to enter and leave trenches.
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E. Pipe trenches for utility lines shall be excavated to a width within the limits of the top of the pipe and the trench bottom so as to provide a clearance on each side of the pipe barrel, measured to the face of the excavation or sheeting, if used, of 8-inches to 12-inches. Where the pipe size exceeds 12-inches, the clearance shall be from 12-inches to 18-inches.
F. Excavation for appurtenances shall be sufficient to provide a clearance between their outer surfaces and the face of the excavation or sheeting, if used, of not less than 12-inches. Manhole excavations shall be carried to sufficient depth to permit their construction on the undisturbed bottom of the excavation.
G. Materials removed from the trenches shall be stored and disposed of in such a manner that they will not interfere unduly with traffic on public streets and sidewalks and they shall not be placed on private property. In congested areas, such materials as cannot be stored adjacent to the trench or used immediately as backfill shall be removed to convenient places of storage.
H. All materials suitable for use as backfill shall be hauled to and used in areas where not enough suitable material is available from the excavation.
I. Excess suitable material and all unsuitable material shall remain the property of the Owner and shall be disposed of within the limits of the project as directed by the Engineer. The disposal area shall be finish graded upon completion of the work.
3.03 REMOVAL OF WATER
A. It is a basic requirement of these specifications that excavations shall be free from water before pipe or structures are installed.
B. Contractors will not be permitted to allow trench water to flow into the work. In order to accomplish this, the Con- tractor may install temporary dams or plugs within the pipe provided they are suitably removed and do not damage the lining of the pipe. Pipe joints must not be connected in water filled trenches.
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C. The Contractor shall provide all necessary pumps, underdrains, well-point systems, and other means for removing water from trenches and other parts of the work. The Contractor shall continue dewatering operations until the backfill has progressed to a sufficient depth over the pipe to prevent flotation or movement of the pipe in the trench and so that it is above the natural water table.
D. Water from the trenches and excavation shall be disposed of in such a manner as will not cause injury to public health, to public or private property, to the work completed or in progress, to the surface of the streets, or cause any interference with the use of the same by the public. The Contractor shall submit his proposed methods of handling trench water and locations at which the water will be disposed of to the Engineer for approval and shall receive approval before starting the excavation.
3.04 PIPE BEDDING
A. As described above, all pipe trenches shall be excavated to a level 6-inches below the outside bottom of the proposed pipe barrel. The resulting excavation shall be backfilled with approved pipe bedding material, up to the level of the lower one-third of the proposed pipe barrel. This backfill shall be tamped and compacted to provide a proper bedding for the pipe and shall then be shaped to receive the pipe. Bedding shall be provided under the branch of all fittings to furnish adequate support and bearing under the fitting. Bedding material for water lines and force mains may be composed of fine material from the excavation as approved by the Engineer. Bedding material for gravity sewer shall be pea rock, drainfield limerock, or similar materials as approved by the Engineer. Limerock screenings, or sand may also be used. (See detail on Page I – 9 hereof).
B. Any excavation below the levels required for installation of the pipe bedding, except for “Additional Excavation”, as hereinafter specified, shall be backfilled with approved bedding material, tamped, compacted and shaped to provide proper support for the proposed pipe, at no additional cost to the Owner.
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3.05 BACKFILL UNDER MANHOLES AND METER VAULTS
A. Any excavation below the levels required for the proper construction of manholes or meter vaults shall be filled with Class C concrete. (See Section III). The use of earth, rock, sand or other materials for this purpose will not be permitted. A 12″ thick angular rock foundation is required under all manholes and meter vaults.
3.06 TRENCH STABILIZATION
A. No claim for extras, or additional payment will be con- sidered for cost incurred in the stabilization of trench bottoms which are rendered soft or unstable as a result of construction methods, such as improper or inadequate sheeting, dewatering or other causes. In no event shall pipe be installed when such conditions exist and the Contractor shall correct such conditions so as to provide proper bedding or foundations for the proposed installation at no additional cost to the Owner.
3.07 BACKFILL
A. Backfilling of utility trenches will not be allowed until the work has been approved by the Engineer and the Engineer indicates that backfilling may proceed. Any work which is covered or concealed without the knowledge and consent of the Engineer shall be uncovered or exposed for inspection at no cost to the Owner. Partial backfill may be made to restrain the pipe during pressure testing.
B. Backfill material shall be noncohesive, nonplastic material free of all debris, organic material, clods and broken paving. Backfill material placed within 1-foot of piping and appurtenances shall not contain any stones or rocks larger than 2-inches in diameter (1-inch for PVC) and no stones or rocks larger than 6-inches in diameter will be permitted in any backfill. (See Section 3.04A)
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C. If a sufficient quantity of suitable backfill material is not available from the trench or other excavations within the site of the work, the Engineer will order the Contractor to provide additional material suitable for this purpose. The additional material shall be installed as specified herein. When the Engineer has determined that additional material is required, and has ordered the Contractor to furnish same, it shall be paid for at the unit price stated in the Proposal for “Additional Backfill Material”. However, no payment will be made for additional backfill material used to replace nonplastic material (rock) over 6-inches in diameter.
D. Selected backfill material containing no stone or rocks larger than 2-inches (1″ for PVC) shall be placed in 6-inch layers and thoroughly tamped to a depth of 12-inches over the top of the pipe. Particular attention and care shall be exercised in obtaining thorough support for the branch of all service connection fittings. Care shall be taken to preserve the alignment and gradient of the installed pipe.
E. After the backfill has been placed to a level 12-inches over the waterline or force main pipe, the remainder of the back-fill shall be placed in layers, not to exceed 9-inches, and compacted with mechanical vibrators or other suitable equipment to obtain a density of the backfilled material of not less than 95 percent of its maximum density as hereinafter defined. (See Sections IV, V, VI for locator tape specification).
F. After selected backfill has been placed to a depth of 12-inches over the sewer pipe, backfilling shall proceed to a depth of 30-inches over the pipe by placing the backfill material in 6-inch layers and thoroughly compacting it with mechanical vibrators. Backfill in this portion of the work shall be compacted to 90 percent of maximum density of the material as hereinafter defined.
G. After the backfill has been placed to a level 30-inches over the sewer pipe, the remainder of the backfill shall be placed in layers, not to exceed 9-inches, and compacted with mechanical vibrators or other suitable equipment to obtain a density of the backfilled material of not less than 95 percent of its maximum density as hereinafter defined.
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H. Within paved areas of trench excavation, the base and surfacing shall be reconstructed.
I. No more than 800-feet of trench with pipe in place shall be partially backfilled at any time.
3.08 COMPACTION AND DENSITIES
A. Methods of control and testing of backfill construction to be employed in this work are:
1. Maximum density of the material in trenches shall be determined by AASHTO Designation T 180-74.
2. Field density of the backfill material in place shall be determined by AASHTO Designation T 238-79.
B. Laboratory and field density tests which, in the opinion of the Engineer, are necessary to establish compliance with the compaction requirements of these specifications, will be conducted at Contractor’s expense. Tests will be made at depths and locations selected by the Engineer.
C. Trench backfill which does not comply with the specified densities, as indicated by such tests, shall be reworked and recompacted until the required compaction is secured, at no additional cost to the Owner. The costs for retesting such work shall be paid for by the Contractor.
3.09 ADDITIONAL EXCAVATION AND BACKFILL
A. Where organic material, such as roots, muck, or other vegetation matter, or other material which, in the opinion of the Engineer, will result in unsatisfactory foundation conditions, is encountered below the level of the proposed pipe bedding material, it shall be wholly or partially removed as directed by the Engineer and wasted. Sheeting shall be installed if necessary to maintain pipe trenches within the specified limits. The resulting excavation shall be backfilled with suitable backfill material, placed in 6-inch layers, tamped and compacted up to the level of the bottom of the proposed pipe bedding material. Sufficient compaction of this material shall then proceed in accordance with the provisions of Article 3.04 Pipe Bedding.
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B. Additional excavation shall be performed only when ordered by the Engineer. Where organic or other unsuitable material is encountered in the excavation, the Contractor shall bring the condition to the attention of the Engineer and obtain his determination as to whether or not the material will require removal, prior to preparing the pipe bedding.
C. Additional backfill material, if required, shall be furnished in accordance with the provisions therefore in Article 3.07 Backfill.
3.10 RESTORATION OF EXISTING SURFACES
A. Paved and grassed areas disturbed by the operations required under this section shall be restored as indicated on the Drawings and/or specified herein.
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