3.8.1 Placement of Backfill Material shall be performed in accordance with these Specifications and as identified in the Standard Plans. Trench backfill shall consist of two stages: (1) Bedding Material as described in Section 3.7, to a level 12 inches above the top of the pipe, and (2) Backfill Material as described herein, from the top of the Bedding Material to the subgrade for roadway or landscape restoration.
3.8.2 Backfill Material shall consist of granular soil and/or aggregate which is free of deleterious material and is non-plastic. Deleterious material includes wood, organic waste, coal, charcoal, or any other extraneous or objectionable material. The material shall be considered non-plastic if the percent by weight passing the U.S. No. 40 sieve cannot be rolled, at any moisture content, into a thread as prescribed in Section 4 of AASHTO T 90.
3.8.3 Unless otherwise shown on the Approved Drawings or Right-of-Way permit, Backfill Material shall meet the requirements of Aggregate for Gravel Base as specified in Section 9-03.10 of the WSDOT Standard Specifications, excepting however that 100 percent of the material shall pass a 4-inch square screen in conformance to the following gradation:
GRAIN SIZE
PERCENT PASSING BY WEIGHT
4” square 100
2” square 75 – 100
U.S. No. 4 22 – 100
U.S. No. 200 0 – 10
Dust Ratio: ⅔ max.
Sand Equivalent: 30 min.
3.8.4 No pipe shall be backfilled above the top of the pipe until the pipe joints, elevations, gradient, and alignment have been checked by the Contractor and the Work has been approved by the County. However, partial backfill to protect the pipe will be permitted
immediately after the pipe has been properly laid in accordance with the Approved Drawings and these Specifications.
3.8.5 Excavations for manholes and structures shall be backfilled as soon as the
structures have developed sufficient strength to resist backfilling loads and forces. Backfill Material shall not be placed on or against any Portland cement concrete until the strength of the concrete is sufficient to support the imposed loads.
3.8.6 Construction Requirements and Compaction Testing
3.8.6.1 Water settling and jetting of Backfill Material are not acceptable. Mechanical methods shall be utilized for placement and compaction of all Backfill Material unless otherwise approved in writing by the County or governing jurisdiction. The Contractor shall use compacting equipment approved by the County.
3.8.6.2 The Contractor shall retain the services of an independent, third-party soils testing laboratory to conduct compaction testing of the Backfill Material and subgrade to ensure compliance with these Specifications. The testing laboratory shall meet the
“Recommended Requirements for Independent Laboratory Qualifications” as published by the American Council of Independent Laboratories (ACIL) and be approved by the County.
3.8.6.3 In-place density shall be determined by nuclear methods in accordance with WSDOT FOP for AASHTO T310, unless otherwise approved by the County. The maximum dry density of the Backfill Material shall be determined in accordance with ASTM D1557, and corrected for oversize in accordance with ASTM D4718. The moisture content of the material shall not vary more than 3 percent above or below optimum as determined in accordance with ASTM D1557.
3.8.6.4 Shallow Trench Zone Restoration (trench depth less than 8 feet)
1. The backfill material shall be placed in maximum 1-foot lifts and compacted to at least 95% maximum dry density, except within easements where no
vehicular traffic is anticipated where the minimum compaction shall be 90%
maximum dry density.
2. The minimum allowable compaction testing frequency in the shallow trench zone shall be three tests per 100 lineal feet of trench. The three tests should be distributed uniformly over the full extent (depth) of the trench zone (e.g., 8 feet deep, 4 feet deep, and subgrade).
3.8.6.5 Medium Trench Zone Restoration (trench depth greater than or equal to 8 feet but less than 16 feet)
1. The backfill material shall meet the requirements of Section 3.8.3, with the exception that 100 percent of the material shall pass the 6-inch square sieve.
All other requirements in Section 3.8 shall remain in effect.
2. The backfill material shall be placed in maximum 2-foot lifts and compacted to at least 95% maximum dry density, except within easements where no
vehicular traffic is anticipated where the minimum compaction shall be 90%
maximum dry density.
3. The minimum allowable compaction testing frequency in the medium trench zone shall be one test per 100 lineal feet of trench.
3.8.6.6 Deep Trench Zone Restoration (trench depth greater than or equal to 16 feet) 1. The backfill material shall meet the requirements of Section 3.8.3, with the
exception that 100% of the material shall pass the 6-inch square sieve. All other requirements in Section 3.8 shall remain in effect.
2. The backfill material shall be placed in maximum 4-foot lifts and compacted to 95% maximum dry density, except within easements where no vehicular traffic is anticipated where the minimum compaction shall be 90% maximum dry density.
3. The minimum allowable compaction testing frequency in the deep trench zone shall be one test per 100 lineal feet of trench.
3.8.6.7 The Contractor shall provide the County with results of compaction testing within 5 business days of the tests being performed, and prior to placement of surfacing materials and hot mix asphalt, whichever occurs first.
3.8.7 Alternative Trench Backfill Material
3.8.7.1 Backfill Material that does not meet the requirements of Section 3.8.3 may be conditionally accepted as “Alternative Trench Backfill Material”, provided that the following conditions are met:
1. A soils analysis and report must be provided to the County prior to utilizing a material that does not meet the specifications stated above. At a minimum, the report shall provide the following information:
a. Grain size (sieve) analysis including dust ratio and sand equivalency demonstrating the material is non-plastic. The material shall be
considered non-plastic if the percent by weight passing the U.S. NO. 200 sieve does not exceed 15 percent, or if the soil fraction passing the U.S.
No. 40 sieve cannot be rolled, at any moisture content, into a thread as prescribed in Section 4 of AASHTO T 90.
b. Optimum moisture content and proctor (ASTM D1557).
c. Construction requirements that address means for placing and
compacting the material, maximum lift thickness, frequency of compaction testing, protecting material from wet weather conditions, etc. In no case shall the requirements stated in the soils report be less restrictive than the requirements stated herein.
2. The soils analysis and report shall be stamped by an engineer who has an expertise in soil mechanics and who is licensed in the State of Washington.
3. An engineer who has an expertise in soil mechanics, and who is licensed in the State of Washington, shall monitor the placement and compaction of the material. The engineer shall be on the site each day during backfilling operations to monitor moisture content and to observe field operations to ensure compliance with the soils report.
4. Random verification sample(s) of the trench backfill material shall be taken during construction at a frequency of 1 sample per 500 lineal feet of trench, and a minimum of 1 sample per Project. The sample shall be taken by the engineer who stamped the original soils report, or their duly appointed representative. The samples shall be analyzed by the engineer. As a minimum, the engineer shall test and report the results for comparison purposes with the requirements for gravel base per Sections 3.8.2 and 3.8.3.
If appropriate (more than 15% by weight passing the 200 sieve), the engineer shall report the results of AASHTO T 90.
5. In the event that test results or construction observations verify lack of compliance with the soils report or construction requirements, the material shall be removed, replaced and re-compacted to the satisfaction of the engineer.
6. At the completion of construction, a report that summarizes the observations associated with the trench backfill, results of the soils analysis, and
compaction results shall be submitted to the County. Included as an
appendage, include copies of the daily Project diaries, soil testing reporting forms, and compaction reports. The licensed engineer described above shall affix their seal, sign and date the end of Project report.
7. A 2-year warranty, from the date of Final Acceptance of the Project, shall be provided to guarantee satisfactory road restoration in the case of settlement or pavement distress. The warranty amount shall be 10% of the total
construction costs.
8. As appropriate, the governing jurisdiction or Property Owner shall indemnify and hold the Sewer Utility harmless from loss or damages arising out of the use of alternative trench backfill material. A notarized “Hold Harmless”
agreement shall be in force prior to commencing Work.