CHAPTER 1200. Final Backfill.
1201. Final Trench Backfill, Above Pipe Zone. Backfill above the pipe zone shall be of one more of the following classes as herein required or as shown on the Plans, or as specified in the Special Provisions.
1202. Class "A" Backfill (Mechanically Compacted). This class of backfill is intended for use on paved streets or other areas where immediate and thorough compaction and completion of the trench backfill is required. The entire depth of the trench above the pipe zone shall be backfilled and compacted in eight inch layers with each layer being approximately parallel to the pipe grade longitudinally and level transversely. The Backfill material for each layer shall be evenly wetted with enough water where necessary to assure it being within plus or minus 2% of its optimum moisture content and compacted by mechanically operated tampers or vibrators until it acquires a compacted density of at least 95% of its maximum density (as determined by AASHTO. Designation T‑180 Method "A" (Modified Proctor Density).
1203. Compaction of Backfill. Backfill shall be compacted by means of sheepsfoot rollers, pneumatic tire rollers, vibrating rollers or other mechanical tampers of size and type approved by the Engineer.
Where compaction methods are used, the material shall be placed at a moisture content such that after compaction the required relative densities will be produced; also the material shall be placed in lifts which, prior to the compaction, shall not exceed 8 inches.
Prior to compaction, each layer shall be evenly spread, moistened, and worked by disk harrowing or other means approved by the Engineer.
If the required relative density is not attained, test sections will be required to determine adjustments in compacting equipment, thickness of layers, moisture content and compactive effort necessary to attain the specified minimum relative density.
Approval of equipment, thickness of layers, moisture content and compactive effort shall not be deemed to relieve the contractor of the responsibility for attaining the specified minimum relative densities. The contractor, in planning his work, shall allow sufficient time to perform the work connected with test sections, and to permit the engineer to make tests for relative densities.
All relative density tests shall be made by the owner’s project engineer at no expense to the city. (Ord. 98-03)
1204. Imported Backfill Material. In the event the native excavated material is not satisfactory for backfilling, the contractor shall provide imported granular material. This granular material shall pass a three inch square sieve and shall not contain more than fifteen percent (15%) of material passing a 200 mesh sieve, and shall be free from sod, vegetation and other organic or deleterious materials.
1205. Disposal of Excess Materials. All excess materials shall be hauled away from the construction site and disposed by the contractor.
1206. Class B Backfill (Water Densified).After completion of the compaction within the pipe zone, the balance of the trench shall be loosely filled with the excavated material and thoroughly saturated with water either by flooding, jetting, or other such means necessary to assure the required backfill settlement. Water shall be applied in such quantity and at a rate sufficient to thoroughly and completely saturate the entire thickness of the lift being densified. Where the trench is on a steep slope and water saturation of the backfill material is difficult due to the steepness of the slope, jetting pipes or nozzles shall be inserted into the backfill or water shall be applied to the backfill at sufficiently close intervals along the trench to ensure that the backfill will be evenly and thoroughly saturated and uniformly consolidated or densified.
The depth of backfill shall not exceed that which can be readily densified by the flooding or jetting procedure, but in no case shall the undensified lift exceed 10 feet for flooding or 15 feet for jetting. If the trench depth exceeds these limits it shall be backfilled and densified in two or more lifts of approximately equal thickness.
In the event that application of water alone fails to produce the required relative compaction throughout the entire thickness of the lift, the flooding or jetting shall be supplemented by the use of vibratory compaction equipment. Vibratory compaction or densification may be applied by the use of probe‑type vibrators to the surface of the backfilled trench.
Class B" Backfill (Water Densified) may be used up to within one foot of the roadway subgrade. After the water densified backfill has cured or dried out enough to be stable, the top one foot of backfill immediately below the existing or proposed roadway subgrade shall be compacted to 95% of AASHTO T‑180 Method "A" (Modified Proctor Density).
Water settlement of trenches is not recommended in native clay, silt, or silty clay materials & will be strictly prohibited on the public way where traffic loads are fairly heavy, or anticipated in the near future, unless otherwise approved by the City Engineer.
1207. Class "C" Backfill. This class of backfill is intended for use through open or undeveloped areas where subsequent trench settlement would not be objectionable. It is not to be used within existing or proposed street or alley rights‑of‑way unless specifically authorized by the City Engineer.
After completion of the backfilling and compaction within the pipe zone, the excavated trench material may be pushed back into the trench by mechanical or other means to a depth that is 12 inches below the top of the trench. The trench backfill shall then be thoroughly saturated with water, either by jetting, puddling, or other such means necessary to assure the required backfill settlement.
After settlement has taken place, the remaining depth of trench shall be backfilled with the remaining excavated material, or imported material if necessary and pneumatically rolled by truck or roller to provide a relatively hard, compacted trench surface that is at, or slightly above, the natural ground elevation.
1208. Class "D" Backfill. This class of backfill is intended for use through dedicated easements where lawn sod, shrubs, topsoil, fences, and other items must be removed and replaced by the Contractor during the trench excavation and backfill operation.
In areas where Class "D" backfill is required, the Contractor shall first notify the affected property owner(s) of the impending construction work through the easement. The Contractor shall then carefully remove all shrubs, fences, and other above‑ground items. Carefully cut and remove all lawn sod and remove the topsoil for a depth of at least 12 inches (or the depth of the actual topsoil if less than 12 inches) with the width of the lawn sod and topsoil removal being at least two (2) feet wider (one foot per side) than the excavated trench width. The topsoil material shall be piled separately from and shall not be mixed with the remainder of the excavated material.
After completion of the backfilling and compaction of the pipe zone, the remainder of the trench shall be backfilled in eight inch lifts with the material in each lift being evenly wetted and compacted to at least 90% of its standard maximum density. The top twelve (12) inches of the trench (or actual topsoil depth referred to above) shall be backfilled in 6 inch lifts with the removed topsoil. The topsoil backfill shall be evenly wetted and compacted to 80‑85% of its standard maximum density. The Contractor shall then replace the lawn sod, shrubs, fences, or other items that may have been removed from within the easement area and shall clean up and remove any rocks, dirt, or other debris that remain from the construction work. Upon conclusion of the work within the easement, the Contractor shall secure a written release from all affected property owners stating their acceptance of the work. In lieu of removing and replacing the lawn sod, the Contractor may, with the agreement of the property owner, replant the lawn or pay the property owner to replant the lawn. Such agreement shall be documented in the above referenced final letter of acceptance from the property owner(s).
1209. Precautionary Measures. In all water densified final backfills, all precautions necessary shall be taken by the Contractor to prevent damage and movement (including floating) of the pipeline, structures and existing adjacent improvements and utilities. The allowance of the use of consolidation methods shall not be construed as guaranteeing or implying that the use of such methods will not result in damage to adjacent ground. The Contractor shall make his own determination in this regard, and shall assume all risks and liability for settlement or lateral movement of adjacent ground, or improvements, or utilities, either on the surface of the ground or underground.