A. General Requirements
An important and high priority goal of the City of Sierra Vista is to assure proper utility trench backfill operations within public streets and/or rights-of-way when trenching is allowed. MAG Specifications (Section 601) is the basic premise used by the City to control backfill operations, and for developing a utility trench backfill policy that will minimize trench settlement and street repair costs to the City.
Utility trenching in public rights-of-way can be categorized under three different situations:
1. Trenching outside of an existing or proposed street prism.
2. Trenching within an existing street prism.
3. Trenching within a proposed street prism to be constructed as part of a new development/subdivision.
A street or roadway prism is defined as any existing or proposed pavement, curb, gutter, sidewalk, or such construction specified in a construction contract, or when any part of the trench excavation is within two feet of the improvements previously mentioned.
Utility trench excavation, pipe bedding, and shading requirements for trenching activities conducted under any of the three categories previously mentioned shall comply with MAG Specifications outlined in Section 601. However, utility trench "backfill requirements," as outlined in MAG Specifications under Section 601, are hereby modified below for each of the three utility trench categories in order to achieve and maintain the City's goal of minimizing utility trench settlements and street repair costs caused by failed utility trench crossings.
B. Utility Trench Backfill Requirements Outside an Existing or Proposed Street Prism
Utility trench excavations and backfill requirements pertaining to mainline sewers, lateral sewer lines, water, power, gas, telephone, and cable underground installations that lie outside and/or parallel to an existing or proposed street prism, but are located within a public utility easement, public street right-of-way, or unpaved alley, shall conform with MAG Specifications, as modified by the following requirements:
1. Utility Trench Backfill Material.
a. Native material may be used as backfill material in utility trenches in unpaved alleys and in public utility easements provided the native material meets the following criteria:
(1) Material is free of broken concrete, broken pavement, rubbish, chunks of clay, wood, organic material, or other deleterious material.
(2) Material does not contain rock or stones larger than three inches.
(3) Offsite material will qualify as acceptable native material if it meets this criteria and is identified in the native fill report.
b. Aggregate base course (ABC) complying with MAG Specifications, Section 702, may be used as backfill material in trenches in unpaved public alleys or in public utility easements.
c. Non-shrink, cement slurry mix with a minimum 28-day compressive strength of 100 psi, but not more than 150 psi, may be used as a backfill material in trenches in unpaved public alleys or in public utility easements, provided a slurry mix design, as prepared and submitted by a civil engineering materials testing laboratory is approved by the City Engineer.
2. Placement and Compaction of Utility Trench Materials
a. A selected backfill material meeting the criteria listed under A., above, shall be processed, placed, and compacted with construction equipment furnished by the contractor. Compaction equipment will be determined adequate provided the material can be placed and compacted in horizontal lifts not exceeding two feet in depth for ABC backfill material, or one foot for native material, and the required moisture and density can be achieved throughout each lift. Non-shrink, cement slurry backfill material may be placed in the entire depth of the trench up to within 18" of finished grade. The last 18" may be compacted native material or ABC.
b. The minimum percent compaction for native material or ABC material placed outside the roadway prism shall be 95% relative to the maximum density determined in accordance with AASHTO T-99, up to finished grade.
c. No water consolidation, jetting or "settling" will be allowed as a means of trying to achieve compaction.
d. The method of compaction of backfill material shall be with mechanical work methods using equipment such as rollers, pneumatic tamps and hydro-hammers, pneumatic sheep's foot, or other approved devices which secure uniform and required density without injury to the pipe or related structures.
3. Minimum Quality Control Testing Requirements
a. A civil engineering materials testing firm/company licensed in the State of Arizona shall be retained by the contractor/developer to provide quality control testing during utility trench backfilling operations. Materials testing technicians shall be certified by the National Institute for certification in Engineering Technologies (NICET). Their certification shall be in the field of Construction Materials Testing and sub-field of Soils. The technician shall work under the direction of a professional engineer who is licensed in the State of Arizona and employed on a full-time basis by the same company.
b. As a minimum, the following number of tests are to be performed during full-time observation and testing of the backfill operation:
(1) One field density test per lift for every 400 feet (horizontally) of trench, but not less than one density test per lift for shorter trenches.
(2) The field density test shall be taken at every one-foot lift (vertically) of native backfill material, or one every two-foot lift (vertically) of ABC backfill material, starting 18 inches above the top of the pipe shading material. The density tests shall be staggered in order to distribute testing throughout the lifts of backfill material placed in the trench.
(3) One maximum density (Proctor) test for each soil type of native backfill material encountered. One maximum density (Proctor) test and one PI for each source of ABC backfill material used.
Field density testing shall be performed in accordance with either ASTM D1556 with AASHTO T-217 or ASTM D2922 with D3017 or AASHTO T-217.
c. Two field cylinders of non-shrink backfill slurry mix shall be collected and compression- tested for every 18 cubic yards placed.
d. Results of any failing quality control tests shall be reported immediately to the Public Works Construction Inspector so remedial action can be relayed to the contractor. Written results of all quality control test results shall be sent to the Public Works Construction Inspector within five working days from the date the tests were taken.
4. Workmanship Guarantee
The contractor shall guarantee the utility trench backfill work for a period of two years from the date the Right-of-Way Permit is signed by the Public Works Construction Inspector as being completed. The guarantee shall protect against any trench settlement and repair any settlement at no cost to the City.
C. Utility Trench Backfill Requirements Within an Existing Paved City Street Prism
Utility trench cuts within any existing paved City street or alley, if allowed by Right-of-Way Permit, shall be performed in accordance with MAG Standard Detail 200 ("T" Top Pavement Replacement Detail) and the following requirements:
1. Utility Trench Backfill and Pavement Materials
a. Backfill material shall be aggregate base course (ABC) complying with MAG Specification 702. A non-shrink, cement slurry mix may be considered as a suitable backfill material on local, low volume streets or alleys, if approved by the City Engineer. Native material will not be allowed as an acceptable backfill material under existing paved City streets or alleys.
b. Asphalt concrete pavement material shall be a hot dense-graded asphaltic concrete mix complying with MAG Specification 710 (D-1/2), or as may be approved by the City Engineer. A cold asphaltic concrete mix may be used as a temporary pavement patch if a hot asphaltic concrete mix is not readily available and hot mix is placed immediately upon availability.
2. Placement and Compaction of Utility Trench Materials
a. All existing asphalt shall be saw cut and removed.
b. ABC backfill material shall be uniform in gradation, have suitable moisture content to obtain minimum specified density, and may be placed in lifts not exceeding two feet in thickness. The backfill material should be placed and compacted with suitable compaction equipment that will achieve a minimum of 95% of the maximum density determined in accordance with AASHTO T-99 from 18 inches above the pipe/ conduit to a level of two feet below finished surface. ABC backfill material placed two feet below the finished base course shall be compacted to a minimum of 100% of the maximum density.
c. Hot mix asphaltic concrete surface course shall be placed and compacted with proper equipment in accordance with MAG Specification 321 and compacted to a minimum of 95% relative maximum density. The minimum pavement thickness shall be two inches or the thickness of the existing asphalt pavement, whichever is thickest.
d. No water consolidation, jetting or “settling” will be allowed as a means of trying to achieve compaction.
e. The method of compaction of backfill material shall be with mechanical work methods using equipment such as rollers, pneumatic tamps and hydro-hammers, pneumatic sheep's foot, or other approved devices which secure uniform and required density without injury to the pipe or related structures.
3. Minimum Quality Control Testing Requirements of Materials
a. Field density tests on ABC backfill material shall be taken, as a minimum, one for every lift of backfill material placed beginning 18 inches above the pipe/ conduit and every 50 feet or less (horizontally) of trench length. Tests shall be staggered at various lifts in order to distribute testing throughout the lifts of backfill material placed in the trench.
b. Nuclear field density tests on asphaltic concrete material placed as a final surface course shall be taken, as a minimum, one every seven square yards of pavement area replaced. Cores may be required if thickness of the asphalt pavement replaced is in question.
c. Results of any failing quality control tests shall be reported immediately to the Public Works Construction Inspector so remedial action can be relayed to the contractor. Written results of all quality control test results shall be sent to the Public Works Construction Inspector within five working days from the date the tests were taken.
4. Workmanship Guarantee
The contractor shall guarantee the utility trench street cut for a period of two years from the date the Right-of-Way Permit is signed by the Public Works Construction Inspector as being completed. The guarantee shall protect against trench settlement and pavement deterioration.
D. Utility Trench Backfill Requirements Under A Proposed City Street Prism Within New Developments
Utility trench excavations and backfill requirements pertaining to mainline sewers, lateral sewer lines, water, power, gas, telephone, and cable underground installations that cross or lie under a proposed paved street, as designated by subdivision or development improvement plans, shall conform with MAG Specifications as modified by the following requirements:
1. Utility Trench Backfill Material
Acceptable backfill material for utility trenches in a new subdivision or development may consist of native material, aggregate base coarse (ABC), and/or a non-shrink cement slurry mix, provided the following requirements are satisfied:
a. If native material is to be used as a backfill material, a developer shall retain a civil engineering materials testing laboratory/firm licensed in the State of Arizona, and capable of analyzing existing, on-site soils and/or any soil/material that may be imported and used as backfill material for utility trenches that lie under a proposed paved street or alley. The materials engineering firm shall determine if the native soils to be used as native backfill material meet the following criteria:
(1) Material is free of broken concrete, broken pavement, rubbish, chunks of clay, wood, organic material, or other deleterious material.
(2) Material does not contain rock or stones larger than three inches.
(3) Material does not have a PI greater than 15, and the total sum of the percent passing the No. 200 sieve and the Plasticity Index (PI) shall not exceed 45.
(4) Offsite material will qualify as acceptable native material if it meets this criteria and is identified in the native fill report.
b. If the materials engineering firm determines the native soils to be suitable as backfill material, a certified report shall be submitted for approval to the City Engineer showing evidence that the soil meets the criteria stated in 1., above, specifying the optimum moisture content, and a suitable moisture content range that will result in minimum specified compaction, and any special processing procedures that may be required to achieve the minimum specified compaction.
c. Aggregate base course (ABC) complying with MAG Specifications, Section 702, may be used as backfill material.
d. Non-shrink, cement slurry mix with a minimum 28-day compressive strength of 100 psi, but not more than 150 psi, may be used as a backfill material in trenches under proposed paved local streets or alleys provided a slurry mix design, as prepared and submitted by a civil engineering materials testing laboratory is approved by the City Engineer.
2. Placement and Compaction of Utility Trench Materials
a. Based on the certified engineer's report prepared under A.2., above, native backfill material shall be processed, placed, and compacted with construction equipment furnished by the contractor. Compaction equipment will be determined adequate provided the material can be placed and compacted in horizontal lifts not exceeding 12 inches in depth, and the specified moisture content and density is obtained throughout the lift.
b. ABC backfill material shall be processed, placed, and compacted with construction equipment furnished by the contractor. Compaction equipment will be determined adequate provided the material can be placed and compacted in horizontal lifts not exceeding two feet in depth, and the required moisture and density can be achieved throughout the lift.
c. Non-shrink cement slurry backfill material shall be mixed in accordance with the approved mix design and may be placed the entire depth of the trench up to finished sub-grade of the proposed roadway.
d. The minimum percent compaction for any backfill material shall be 95% relative to the maximum density determined in accordance with AASHTO T-99, up to finished sub-grade of the proposed roadway.
e. No water consolidation or "settling" will be allowed as a means of trying to achieve compaction.
f. The method of compaction of ABC or native backfill material shall be with mechanical work methods using equipment such as rollers, pneumatic tamps and hydro-hammers, pneumatic sheep's foot, or other approved devices which secure uniform and required density without injury to the pipe or related structures.
g. The Items 1 through 6, mentioned above, shall apply to all utility crossings/installations including sewer laterals off the main sewer line that will lie under the new roadway pavement.
3. Minimum Quality Control Testing Requirements
a. A civil engineering materials testing firm/company licensed in the State of Arizona shall be retained by the contractor/developer to provide full-time, quality control observation and testing during all back filling operations. Materials testing technicians shall be certified by the National Institute for certification in Engineering Technologies (NICET). Their certification shall be in the field of Construction Materials Testing and sub-field of Soils. The technician shall work under the direction of a professional engineer who is licensed in the State of Arizona and employed on a full-time basis by the same company.
b. As a minimum, the following number of tests are to be performed during full-time observation and testing of the backfill operation:
(1) Native Backfill Material
(a) One field density test per lift for every 300 feet (horizontally) of trench, but not less than one density test per lift for shorter trenches.
(b) The field density test shall be taken at every one foot lift (vertically) of backfill material placed starting 18 inches above the top of the pipe shading material. The density tests shall be staggered in order to distribute testing throughout the lifts of backfill material placed in the trench.
(c) If other soil types are encountered, other than the soil types approved in the initial geotechnical report, then one maximum density (Proctor) test, one PI and minus 200's test shall be performed for each soil type of native backfill material encountered to determine if the native material meets the "suitable criteria" outlined under subsection A.1.
(d) Field density testing shall be performed in accordance with either ASTM D1556 with AASHTO T-217 or ASTM D2922 with D3017 or AASHTO T-217.
(2) ABC Backfill Material
(a) One field density test per lift every 400 feet (horizontally) of trench, but not less than one per trench length for shorter trenches.
(b) The field density test shall be taken at every two-foot lift (vertically) of ABC material placed, starting one foot above the top of the pipe shading material. The density tests shall be staggered in order to distribute testing throughout the lifts of backfill material placed in the trench.
(c) Field density testing shall be performed in accordance with either ASTM D1556 with AASHTO T-217 or ASTM D2922 with D3017 or AASHTO T-217.
(3) Non-Shrink Cement Slurry Backfill Material
A set of four field cylinders of non-shrink backfill slurry mix material shall be collected and compression tested for every 18 cubic yards of material placed. Minimum 28-day strength shall be 100 psi, but not greater than 150 psi.
(4) Report Requirements
(a) Results of any failing quality control tests shall be reported immediately to the Public Works Construction Inspector so remedial action can be relayed to the contractor.
(b) At the end of the project, a certified report shall be prepared by the civil engineering materials testing company/firm and shall contain, as a minimum, each of the field density test results and a letter stating that utility trench back filling operations were observed and completed in a manner that met the project specifications and Engineer's recommended backfill operation requirements. The report shall be sealed and signed by the professional engineer, and three copies submitted to the City prior to acceptance of any public works improvements for maintenance.
(5) Workmanship Guarantee
The developer and contractor shall execute a warranty agreement that will guarantee repair of any utility trench settlement under a paved roadway constructed within the development for a period of two years. The guarantee shall include costs associated with saw cutting pavement, reworking and re-compacting backfill material, restoring the pavement surface, and any other incidentals associated with repairing utility trench settlements, at no cost to the City.
('76 Code, Art. 12-1) (Ord. 743, passed 4-10-86; Am. Ord. 934, passed 10-18-93; Am. Ord. 1043, passed 9-11-97; Am. Ord. 2012-004, passed 6-28-12)