(a) General.
(1) Provide erosion control measures as required. Erosion control measures including seeding and mulching shall be designed, installed and maintained in accordance with the N.C. Department of Environment and Natural Resources, Land Quality Section's "Erosion and Sediment Control Planning and Design Manual". The Developer/Engineer is responsible for securing all required permits.
(2) Pipe installation shall meet the following general guidelines:
a. Handle pipe and accessories in accordance with manufacturer's recommendations.
b. Carefully inspect pipe immediately prior to laying. Do not use defective pipe. Replace pipe damaged during construction.
c. Lay pipe to design grade and alignment.
d. Provide proper equipment for lowering pipe into trench.
e. Provide tight closure pipe ends when work is not in progress.
f. Keep pipe interior free of foreign materials.
g. Do not lay pipe in water or when the trench or weather conditions are unsuitable for the work.
h. Clean bell and spigots before joining. Make joints in accordance with pipe manufacturer recommendation.
(b) Trenching for underground pipe installation.
(1) Definitions.
a. Backfill. A specified material used in filling the excavated trench and placed at a specified degree of compaction.
1. Materials. Materials listed herein include processed materials plus the soil classifications listed under the Unified Soil Classification System, (USCS) (Method D2487 and Practice D2488). The soil materials are grouped into five broad categories according to their suitability for this application.
i. Class I. Angular, 6 to 40-mm (1/4 to 1-1/2-in.), graded stone, including a number of fill materials that have regional significance such as coral, slag, cinders, crushed stone, and crushed shell.
ii. Class II. Coarse sands and gravels with maximum particle size of 40 mm (1-1/2 in.), including various graded sands and gravels containing small percentages of fines, generally granular and noncohesive, either wet or dry. Soil Types GW, GP, SW, and SP are included in this class.
iii. Class III. Fine sand and clayey gravels, including fine sands, sand-clay mixtures, and gravel-clay mixtures. Soil Types GM, GC, SM, and SC are included in this class.
iv. Class IV. Silt, silty clays, and clays, including inorganic clays and silts of medium to high plasticity and liquid limits. Soil Types MH, ML, CH and CL are included in this class. These materials shall not be used for bedding, haunching, or initial backfill.
v. Class V. This class includes the organic soils OL, OH, and PT as well as soils containing frozen earth, debris, rock larger than 40 mm (1 1/2 in.) in diameter, and other foreign materials. These materials shall not be used for bedding, haunching, or initial backfill.
2. Backfill zones. Each backfill zone shall extend the full width of the trench bottom.
i. Foundation. Extending down from the bottom of bedding zone as defined below.
ii. Pipe embedment.
A. Bedding. Extending from 4-inches below the pipe bottom to the pipe bottom for 30-inch diameter and smaller and 6-inches below the pipe bottom for pipes larger than 30-inches in diameter.
B. Haunching. Extending from the bedding (bottom of the pipe) to the pipe spring line.
C. Initial backfill. Extending from the haunching (pipe spring line) to 1 foot above the top of the pipe.
iii. Final backfill. Extending from the initial backfill to the finish ground elevation.
b. Laying conditions.
1. Type 1. Flat bottom trench with loose backfill.
2. Type 2. Flat bottom trench with backfill lightly consolidated to centerline of pipe.
3. Type 3. Pipe bedded in 4-inches minimum of loose soil and backfill lightly consolidated to top of pipe.
4. Type 4. Pipe bedded on Class I material to 1/8 pipe diameter (4-inch minimum) Backfill compacted to top of pipe a minimum of 80 percent of standard proctor.
5. Type 5. Pipe bedded in compacted Class I material to pipe centerline with 4-inch minimum under pipe. Backfill to top of pipe with Class I, II, or III and compact to 90 percent of standard proctor.
c. Compaction. Process of mechanically stabilizing a material by increasing its density at a controlled moisture condition. "Degree of compaction" shall be expressed as a percentage of the maximum dry density obtained by the test procedure presented in ASTM D698 (Standard Proctor).
d. Excavation. The removal of soil or rock to obtain a specified depth or elevation.
e. Hard material. Solid, homogeneous material which is not included in the definition of "rock" but which may require the use of heavy excavation equipment with ripper teeth. Amount must exceed 1 cubic yard in volume. Material having a standard penetration resistance as determined by ASTM D1586 between 60 and 150 blows per foot is defined as “hard material.”
f. Lift. Layer of soil placed on top of a previously prepared or placed soil.
g. Rock. Solid, homogeneous material which cannot be removed without the systematic drilling and blasting exceeding 1 cubic yard in volume. Material having a standard penetration resistance as determined by ASTM D1586 greater than 150 blows per foot is defined as “rock.” Removal of “hard material” will not be considered rock excavation because of intermittent drilling and blasting that is performed merely to increase production.
h. Pipe springline. A line running horizontally through the center of the pipe.
i. Topsoil. Natural, friable soil, representative of productive soils in the vicinity of the site. Topsoil shall be free from roots, stones larger than 1- inch, objectionable weed seeds, toxic substances, and materials that hinder grading, planting, and maintenance operations.
(2) Products.
a. Stone. Class I material shall be #67 or #78M stone in accordance with NCDOT specifications Section 1005, General Requirements for Aggregate.
b. Warning and identification tape.
1. Tape shall be a minimum 3-inch wide polyethylene plastic tape manufactured specifically for identification of buried utilities with means of enabling detection by a metal detector to a minimum depth of 3 feet. Tape shall be color coded and continuously imprinted with warning and identification markings in bold black letters to read "CAUTION - BURIED (utility) LINE BELOW." Color and printing shall be permanent, unaffected by moisture or soil and shall be as follows:
UTILITY | COLOR | MARKING |
UTILITY | COLOR | MARKING |
Water | Blue | Caution-Buried Water Line Below |
Storm/Gravity Sewer | Green | Caution-Buried Sewer Line Below |
Force Main | Green | Caution-Buried Force Main Below |
Electric | Red | Caution-Buried Electric Line Below |
Gas | Yellow | Caution-Buried Gas Line Below |
Telephone | Orange | Caution-Buried Telephone Line Below |
SCADA | Orange | Caution - Buried SCADA Line Below |
2. Tape shall be by Blackburn Manufacturing, Joseph G. Pollard Co., or Reef Industries Inc.
c. Tracer wire.
1. Tracer wire shall be #12 solid copper wire. All connections shall be by wire nuts and taped.
2. Splices in tracer wire are to be kept to a minimum and joined with copper split nuts of appropriate size.
(3) Project safety.
a. Contractor is responsible for project safety.
b. Perform work in conformance with applicable State and Federal safety regulations including, but not limited, to the following:
1. North Carolina Safety and Health Standards for the Construction Industry (29CFR 1926 Subpart P).
2. NC OSHA Industry Guide No. 14, Excavations.
3. NC OSHA Industry Guide No. 20, Crane Safety.
c. Provide barriers, warning lights, and other protective devices at excavations as necessary for safety of workers and the public.
d. Provide sloping of bank, shoring, sheeting, or other means of maintaining the stability of the trench in accordance with the requirements of the Associated Contractor's Manual of Accident Prevention OSHA, Part 1926.P.
(4) Protection of underground facilities.
a. Investigate underground facility location prior to start of construction.
b. Installer is required to contact North Carolina 811 prior to start of construction.
c. Repair damage to any existing facilities.
(5) Water control.
a. Prevent surface water from entering the trench.
b. When trench bottom is below the existing ground water table, install a dewatering system to maintain water table 1 foot below trench bottom. Provide a man experienced in dewatering work at the job site.
c. Maintain dewatering until backfilling has proceeded above the existing ground water level.
d. Dispose of water from dewatering operations in accordance with the North Carolina Sedimentation Pollution Control Act.
(6) Use of explosives.
a. Explosives may not be used on any excavation unless specifically approved by the city.
(7) Excavating.
a. Excavation shall be by open cut method. Short sections of trench may be tunneled or direct bored with the approval of the city.
b. Stockpile excavated material in such a manner that it will not obstruct the flow of runoff, streams, endanger work, impair the use or appearance of existing facilities, or be detrimental to the completed work.
c. Contractor shall segregate excavated material so as to maintain material suitable for backfill separate from material that is unsuitable.
d. Trench dimensions at the pipe embedment and foundation zone unless noted otherwise shall be as follows:
1. Minimum width. Pipe outside diameter plus 18-inches.
2. Maximum width. Pipe outside diameter plus 24-inches.
3. Sides shall be vertical to a minimum of one foot above the top of pipe.
e. Shape trench bedding to provide uniform bearing for the full pipe length. Bottom shall be free of protrusions that could cause point loading on pipe. Provide bell holes as required for properly making pipe joint.
f. Do not over excavate. Excavation below grade shall be backfilled with Class I material.
g. Undercut soils that become unsatisfactory by construction activity or by being left exposed to the weather and backfill with Class I material.
h. Remove shoring, bracing, and sheeting, unless otherwise noted, as the trench is backfilled.
i. Excavation of trench shall not advance more than 200 feet ahead of the installation. In no case should the excavation extend beyond that which can be backfilled by the end of the workday.
j. Correct unstable soil conditions encountered at trench foundation by one of the following methods: Excavate below grade as approved by Engineer and backfill with Class I material or approved substitute material.
k. Rock and hard material. Excavate rock and hard material to a minimum depth of 4 inches below the pipe for pipes smaller than 30 inches and 6 inches for pipes 30 inches and larger.
l. Gravity lines. Excavate trench to the design alignment and grade.
m. Utility structures. Provide a minimum of 12 inches below subgrade and backfill with Class I material compacted to 95% maximum density. If the soil conditions are found to be unsuitable for structural stability of the manhole, city may require additional depth of Class I material.
(8) Backfilling.
a. General.
1. Temperature must be above freezing and rising.
2. In windy, hot, or arid conditions with a high rate of evaporation add moisture to the material to maintain the optimum moisture content.
3. Do not proceed in rain or on saturated subgrade.
4. Do not place material on surfaces that are muddy, frozen, or contain frost.
5. Maintain backfill operation within 200 feet from pipe laying operation.
6. Backfill trench to existing ground surface with select excavated material at the specified compaction.
7. If excavated material is unsuitable to obtain specified compaction, provide suitable off-site borrow material for backfill.
8. Re-excavate trenches improperly compacted. Backfill and compact as specified.
9. Provide appropriate tamping equipment, and water to obtain proper moisture content, to achieve specified compaction of backfill.
10. Conduct operation of heavy equipment above pipe installation as to prevent damage to pipe.
11. Install warning/identification tape over utilities. Bury tape one foot below finished grade above the utility.
b. Backfill in pipe embedment zone (bedding, haunching, and initial backfill).
1. General.
i. Backfill with material as specified below. Material shall be free from objects larger than 2 inches.
ii. Where rock and hard material has been excavated below pipe bottom, backfill and compact bedding with Class I material. Class II or III material may be used for bedding with Engineer's approval.
iii. Place backfill material to assure placement of material under pipe haunches.
iv. Take care during placement and compacting of material to avoid movement of pipe.
2. Place backfill in bedding and haunching zones in 6-inch maximum lifts and compact to 90% density. Provide backfill material in pipe embedment zone as specified below.
i. Storm sewer lines, rigid pipe.
A. Excavation in Class I, Class II, Class III, and stable Class IV soils suitable for bedding, the bedding surface shall provide a firm foundation of uniform density. Backfill with select excavated material.
B. Excavation in Class V, unstable Class IV soils, running water, and other unstable soil conditions, excavate a minimum of 4 inches below pipe bottom and provide Class I material for bedding and haunch zone. Backfill with Class I, II, or III material in initial backfill.
ii. Storm sewer lines, flexible (HDPE).
A. Depth 0 to 14 ft: Provide Class I material for bedding and haunching. Backfill with Class I, II, or III material in initial backfill.
B. Depth over 14 ft: Provide Class I material for bedding, haunching, and initial backfill.
c. Final backfill.
1. Backfill with materials free of stones and free of debris larger than 6 inches in dimension. Place backfill in lifts not exceeding the thickness and compacted to the minimum density specified below.
2. Trench backfilled with non-cohesive materials may be compacted with water flooding; except under roadways, shoulders of roadways, and other areas subject to vehicular movement, provided the method of compaction is approved by the city and provides the degree of compaction required.
3. Lifts and density.
i. Undeveloped areas (i.e., forests, fields, and, croplands). Trench may be filled with bulldozer blade provided material fall will not damage pipe. Mound soil over the trench area sufficiently to settle level over time. Degree of compaction shall be 85%.
ii. Lawns. Backfill in 12-inch lifts and compact to 90%. Top 12-inches shall be free of material with a dimension over 2 inches.
iii. Roads (including rights-of-way), drives, parking areas (including areas within 20 feet), and adjacent to existing utilities: Backfill in 6-inch lifts compact to 95%.
iv. Within 20 feet of foundations: Backfill in 6-inch lifts compacted to 95%.
d. Utility structures. Bring backfill to grade in even lifts on all sides. Lift depths and compaction densities shall be as specified according to area of installation for pipe above. Backfill against cast-in-place concrete structure only after concrete has attained the specified 28-day compressive strength.
(Ord. of 11-7-16, No. 10-16)