TRENCH AND BACKFILL

PART 1 - GENERAL

PART 2 - PRODUCTS

PART 3 - EXECUTION

    3.01 PREPARATION

      A. When natural soils for Class II, III and IV backfill material is required as specified in Figure 3010.1 the Contractor will provide written certification from a testing laboratory that the material meets the class specified if so requested by the Jurisdictional Engineer.

      B. Locate, mark, and protect existing utilities and facilities in the work area.

      C. Provide access to utility service locations, such as valves, manholes and utility poles.

      D. Identify owners of utilities on or near the site, and notify them of operations to occur.

      E. Protect existing facilities and landscaping features, or replace as shown on the plans.

      F. Protect bench marks, control points and land survey monumentation, or replace at Contractor's expense.

    3.02 TRENCH EXCAVATION

      A. Notify Jurisdictional Engineer prior to start of excavation activities.

      B. Remove and stockpile top 6 inches of topsoil for subsequent reuse.

      C. Place excavated material away from trench. Grade spoil piles to drain.  Do not allow spoil piles to obstruct drainage.

      D. Remove rock, rubbish, boulders, debris, and other unsuitable materials at least 6 inches below, and on each side of the pipe. Restore grade using soil suitable for backfill.

      E. Correct unauthorized excavation at no cost to Jurisdiction, using bedding or stabilization materials.

      F. Provide protective fences and barricades around open excavations, appropriate to the surrounding area.

      G. Provide weight tickets for stabilization material to the Jurisdictional Engineer at the time of delivery.

      H. Provide safety fence around open excavations.

      I. Trench Excavation for Sanitary Sewers, Storm Sewers and Water Mains:

        1. Maximum pipe trench width: Limit width at top of pipe to diameter of the pipe, plus 24 inches, plus minimum clearance for shoring, sheeting, or trench box (if any).

        2. Minimum pipe trench width: Pipe nominal diameter plus 18 inches, but not less than 24 inches total.

        3. Flat trench bottom, conduit bearing directly on trench bottom (not applicable for rock excavation) for water main pipe only with bell hole shaping.

          a. Shape trench bottom to support pipe around 1/4 of perimeter for the full length of the pipe barrel.

          b. Provide bell holes.

        4. Trench bottom, conduit supported by bedding material.

          a. Excavate trench as shown in Detailed Drawings.

          b. Install bedding material to support the full length of the pipe barrel.

        5. Trench depth:

          a. See Figures 3010.1.

          b. For those material types not shown in Figure 3010.1 and maximum height of bury will be 20 feet without a designed trench with engineer's certification.

         6. All trench operations shall conform to current OSHA regulations.

      J. Trench Excavation for Pipe Culverts:

        1. Width of Trench:  The width of trench in which pipe is placed shall meet Section 3010, 1.08 B and shall be sufficient to permit thorough tamping of bedding material under and around the pipes.  At the Contractor’s option, the trench may be cut wide enough to accommodate a tamping type roller on each side of the pipe.

        2. Preparation of Base:  The surface upon which pipe sections are to rest shall be brought to a suitable elevation to fit the desired grade and camber, and the base shall be prepared as shown in the contract documents.

      K. Structure Excavation:

        1. For concrete structures and parts of structures without footings, 18 inches outside the horizontal projection of the structure.

        2. For concrete structures with footings, 18 inches outside the footings.

        3. For anchor rods, 12 inches on each side of the rod.

        4. For buried anchors, the face of the buried anchor on one side and 24 inches outside the buried anchor on the other face.

    3.03 ROCK OR UNSTABLE SOILS IN TRENCH BOTTOM

      A. Notify Jurisdictional Engineer prior to over-excavation.

      B. Jurisdictional Engineer shall determine the need for trench bottom stabilization prior to installation of pipes and structures.

      C. Remove rock or very hard clay or soft areas of the trench bottom to a depth of one foot below the base of the pipe or structure, and backfill with earth or stabilization materials as required.

      D. Provide weight tickets for the stabilization material to the Jurisdictional Engineer at the time of delivery.

    3.04 SHEETING, SHORING AND BRACING

      A. Provide and install sheeting, shoring and bracing, or trench boxes as necessary to facilitate construction.

      B. Leave in place all temporary sheeting below 2 feet over top of pipe unless sheeting removal plan is approved by Jurisdictional Engineer.

      C. Move trench boxes carefully to avoid excavated wall displacement or damage.

      D. All trench operations shall conform to current OSHA regulations.

    3.05 DEWATERING

      A. Do all work in dry conditions; do not install pipes on excessively wet soil.

      B. Submit dewatering methods to Jurisdictional Engineer for review.

      C. Adequate dewatering is the Contractor’s responsibility unless otherwise stated in the contract documents.

      D. Install dewatering system appropriate for the soil conditions.

      E. Maintain water levels sufficiently below the bottom of trench excavation to prevent upward seepage.

      F. Provide for handling water encountered during construction:

        1. Prevent surface water from flowing into excavation.  Remove water as it accumulates.

        2. Do not use sanitary sewers for disposal of trench water.  Discharging water into storm sewers requires Jurisdictional Engineer's approval.

        3. Do not discharge water onto adjacent property without property owner's approval.

      G. Backfill pipes prior to stopping dewatering operations.

    3.06 PIPE PLACEMENT FOR STORM SEWER, SANITARY SEWER AND WATER MAINS

    Refer to Figure 3010.1, as appropriate for the installation being made. Use only the types of materials shown for each position within the trench, for the given groundwater conditions, for the compaction to be provided and for the type of pipe being installed.

      A. Pipe Bedding:

        1. Shape pipe bed to evenly support pipe at the proper line and grade, with full contact under the bottom of the pipe.

        2. Install pipe and system components.

        3. Place bedding simultaneously on both sides of the pipe.  Correct any pipe displacements before proceeding.

        4. Place bedding in lifts not greater than 6 inches thickness and compact to 90% Standard Proctor Density or 45% Relative Density using vibratory compactor.

        5. Concrete encasement: Install where shown in the Plans.

        6. If required in the contract documents or if approved by the Jurisdictional Engineer, flowable mortar or controlled low strength material may be used in lieu of other bedding material types.

        7. Secure pipe against displacement or flotation prior to placing flowable mortar or concrete encasement.

      B. Haunch Support:

        1. Refer to Figures 3010.1 and 3010.2, as appropriate for the installation being made.

        2. Place haunch material in lifts not greater than 6" thickness and compact to minimum 90% Standard Proctor Density using vibratory compactor for granular soils.

        3. If required in the contract documents, or if approved by the Jurisdictional Engineer, flowable mortar or controlled low strength material may be used in lieu of other haunch material types.

        4. Secure pipe against displacement or flotation prior to placing flowable mortar, Controlled Low Strength Material, or concrete encasement.

      C. Primary and Secondary Backfill (Pipe Cover):

        1. Refer to Figures 3010.1, 3010.2, and 3010.3 as appropriate for the installation being made.

        2. Place pipe cover material in 6" lifts and compact to densities required according to class of material.  See Section 3010, 2.04, 2.05, and 2.06.

        3. If required in the contract documents, or if approved by the Jurisdictional Engineer, flowable mortar or controlled low strength material may be used in lieu of other cover material types.

        4. Secure pipe against displacement or flotation prior to placing flowable mortar or concrete encasement.

        5. Special Pipe Support: If required, provide special pipe support as shown on the plans (See figures 3010.2-7).

      D. Final Trench Backfill:

        1. Backfill trench immediately after recording locations of connections and appurtenances, or at Jurisdictional Engineer’s direction.

        2. Backfill structures immediately after concrete has reached design strength and connecting work has been completed.

        3. Allow no more than 100 feet of trench to be open overnight or when work is not in progress except as provided on the Plans.

        4. Backfill with suitable excavated earth materials:

          a. Carefully place backfill over top of pipe and around structures.

          b. Compact as required.

        5. Compaction:

          a. Place backfill more than 3 feet below pavement structure in uncompacted lifts not thicker than 12 inches. Compact each lift to at least 95 percent of maximum Standard Proctor Density.

          b. Place backfill in remainder of excavation in 6 inch lifts.  Compact each lift to at least 95 percent of maximum Standard Proctor Density. Terminate backfill at 6 inches below finish grade in areas to remain unpaved, and to subgrade elevation in areas to be paved.

          c. When crossing under levees, railroads and State or Federal highways, compaction requirements of these jurisdictions shall apply, if more stringent than these requirements.

          d. For VCP, all heavy compaction equipment shall be kept 5 vertical feet above the top of the pipe.  In the area less than 5 vertical feet, hand held compactors shall be used.  Care should be taken to not allow the compactor to come in contact with the pipe.

        6. Dispose of surplus and unsuitable materials.

        7. Hydraulic compaction is not allowed unless authorized by the Jurisdictional Engineer.

    3.07 CULVERT BEDDING AND BACKFILL 

    Following trench excavation and except as otherwise indicated in the contract documents or in special provisions, pipe culverts shall be installed in accordance with the following requirements.  Where a new fill is being constructed, roadway pipe may be placed in a trench only when the total fill over the pipe is five feet or less.

      A. Bedding:

        1. When specified, the base shall be Class B bedding. When not specified, the base shall be Class C bedding.

        2. Unless bedding is specifically designated in the contract documents, Class C bedding will not be required for entrance pipe 24 inches or less in diameter.  In lieu thereof, the pipe may be bedded carefully in suitable material and the backfill compacted with a mechanical tamper to mid-height elevation of the pipe.

      B. Placing Pipe Sections:  (See Section 4020, 3.03)

      C. Backfilling Pipe Culverts:

        1. Culverts shall be thoroughly tamped under and around the pipe and in layers not to exceed eight inches for the full length and width of the culvert.

        2. Earth shall be filled and thoroughly tamped around and over the culvert for its full length, so that adjacent to the pipe on each side of the culvert there shall be an embankment of thoroughly tamped or undisturbed earth.

        3. The completed embankment shall extend on both sides of the culvert from the original ground line to at least one foot above the top of the pipe with a slope as shown in the contract documents.  The width of this fill at its top shall be not less than the outside diameter of the culvert and shall extend one-half its width on each side of the culvert center line.  The height of fill shall be increased, if necessary to accommodate construction traffic to the nominal diameter of the pipe or 3 feet, whichever is greater.

        4. When pipe are laid wholly or partly in a trench, granular backfill material may be required for backfill.  The remainder of the fill, to at least one foot above the top of the pipe, shall be compacted earth with slopes as outlined above.

        5. If the trench has been cut wide enough to permit use of a roller, after the pipe is bedded, the backfill material under and alongside the pipe shall be thoroughly tamped with a mechanical tamper to the mid-height elevation of the pipe.

        6. The contract documents may require placement of culvert pipe with moisture control.  When not required, roadway pipe culverts placed after construction of an embankment shall be placed by methods which will produce results equivalent to those required for construction of the embankment, except that moisture determinations will be waived for backfilling completed within 48 hours after excavation.

        7. In addition to the normal backfilling requirements, the Contractor, when ordered by the Jurisdictional Engineer, shall build such approach fills as will provide a roadway 10 feet in width over the culvert with grades not steeper then 10 percent.

    3.08 STRUCTURE BEDDING

      A. Bedding For Structures Bearing On Undisturbed Soils:

        1. Shape the bottom to accurate grade and size.

        2. Remove loose material, large clods, stones and foreign materials.

        3. In unstable soils condition see Section 3010, 3.03 for stabilization requirements.

        4. If rock, clay or unstable soils are encountered the material below the structure shall be excavated to a depth at least one foot and backfilled with material specified in Section 3010, 2.02.  Bedding will be required as shown in Figure 3010.1.

      B. Bedding For Structures Bearing On Bedding Material:

        1. Over excavate to minimum of 8 inches or as specified in the contract documents.

        2. Place bedding material for structures in accordance with the contract documents and  with the material and control specified in Figure 3010.1.

    3.09 STRUCTURE BACKFILL

      A. Removal of Forms and Falsework: Unless otherwise indicated in the contract documents, forms and falsework shall be removed and may normally be removed in accordance with the following provisions.  In cool or unfavorable weather, the Jurisdictional Engineer may require forms to remain in place for longer periods.

        1. Forms which may be removed in less than 5 calendar days:

          a. Forms for ornamental work, railings, parapets, curbs, and any other vertical surfaces may be removed whenever the concrete will not be damaged by so doing, but not less than 12 hours after the concrete is placed.

          b. Forms for roofs of culverts may be removed when the concrete has attained an age of 3 calendar days and a flexural strength of 350 psi for spans of 4 feet or less, 400 psi for spans 4 to 6 feet, and 450 psi for spans exceeding 6 feet.

        2. Forms which must remain in place 5 calendar days or longer:

          a. Except when form removal is permitted in less than 5 calendar days, forms may be removed as soon after 5 calendar days as the concrete has attained the required strength.

          b. When strength is not determined, forms for box culverts 4 feet or less in width may be removed after the concrete has attained an age of 7 calendar days, and forms for other concrete may be removed after the concrete has attained an age of 14 calendar days.

        3. Use of Fly Ash: Class F fly ash is used and strength is not determined, the ages for form removal, described above, shall be increased to 8 calendar days and 15 calendar days, respectively.

      B. Backfill Placement:  Backfill immediately after structure concrete has reached design strength and connecting work has been completed.

      C. Backfill Against Two Walls:

        1. Where backfill is required on both sides of a concrete wall, or other monolithic structure, filling operations shall proceed simultaneously on both sides of the structure so that the two fills are kept at approximately the same elevation at all times.  For the purposes of this paragraph, concrete box, arch, and circular culverts shall be considered to be monolithic structures.

        2. Compaction of the 3 feet closest to these walls or wing faces shall be done by pneumatic or hand tampers only.

      D. Backfill Against one Wall:

        1. Where backfill is to be placed on one side only of retaining walls, or wingwalls of culverts 8 feet or more in height, rolling shall not be done within 3 feet of the wall face.

        2. Compaction of the 3 feet closest to these walls or wing faces shall be done by pneumatic or hand tampers only.

      E. Moisture Control and Density:

        1. When moisture control is required backfill material shall be placed in layers not more than 8 inches in loose thickness.

        2. Each layer shall be reasonably leveled and thoroughly compacted before the next layer is placed.

        3. A tamping type roller shall be used, and compaction shall be not less than one roller passage per inch of loose thickness. Excavation shall be to a width to accommodate the roller to be used and to provide a reasonably level area for the roller operation for the first and all subsequent layers.

        4. A mechanical tamper shall be used for each layer in areas inaccessible to the roller.

      F. Backfilling with Excavated Material:

        1. When flowable mortar, controlled low strength material, or granular backfill is not specified in the contract documents, excavated material shall be used.

        2. Backfilling shall continue to natural ground line unless there is inadequate quantities of suitable excavated material.

        3. If there is inadequate quantities of suitable excavated material, backfilling shall be continued until the suitable material is depleted.

        4. Material used for backfilling shall be free from organic material, boulders or broken concrete over 6 inches in the greatest dimension, or frozen material.

        5. At the time soil is placed for backfill, its moisture content shall be suitable for compaction.  When compaction with moisture control is required by the contract documents, the moisture content in the backfill material shall be increased or reduced, as necessary, to bring the moisture within the designated moisture limits prior to and during placement and compaction.

      G. Backfilling with Granular Backfill:

        1. When granular backfill material is specified, backfill material shall meet the following requirements.  When the volume of material excavated exceeds that of backfill required by the contract documents, the quantity of granular backfill material furnished shall be increased to provide backfill for the excess volume of excavation.

          Sieve

          Percent Passing

          3 inch

          100

          No. 8

          20 to 100

          No. 200

          0 to 10

          IDOT Gradation #32 (Iowa DOT 4109 and 4121)
          Note:  Crushed stone shall have 100% passing the 1" sieve.

        2. Granular backfill shall be constructed in layers of not more than 8 inches.  Each layer shall be thoroughly tamped or vibrated to insure compaction.  The Engineer may require granular material to be moistened with water during backfilling operations.

        3. Backfill in Water:

          a. Backfill material may be placed in water only when removal of water from the area to be backfilled is impractical, within the waterway or in other deep excavations where removal of cofferdams is required before backfilling and such removal prevents dewatering.

          b. Backfill material placed in water need not be consolidated.

          c. When specified, all backfill material placed under water, over which a subsequent embankment is to be placed, shall be granular backfill material.

        4. Backfill above Water:

          a. Backfill material placed above the water line shall be placed in layers not more than 8 inches in loose thickness.

          b. Each layer shall be thoroughly compacted before material for the next layer is placed.

          c. All compaction shall be accomplished by rolling with an approved roller or by tamping with a mechanical tamper.  Pneumatic tampers shall be operated according to the manufacturer’s recommendations.

      H. Compaction Outside Road Right-of-Way in Unpaved Areas:

        1. Place backfill in lifts not thicker than 8 inches, to 6 inches below finish grade.

        2. Obtain at least 90 percent of maximum Standard Proctor Density using compaction equipment.

      I. Compaction Within Road Right-of-Way, in Paved Areas, and in Areas to be Paved:

        1. Place backfill more than 3 feet below pavement structure in lifts not thicker than 8 inches. Compact each lift to at least 95% of maximum Standard Proctor Density.

        2. Place backfill in the remainder of the excavation in lifts not thicker than 6 inches.  Compact each lift to at least 95 percent of Standard Proctor Density.

    3.10 FIELD QUALITY CONTROL

      A. References:

        1. ASTM C136; Standard Method for Sieve Analysis of Fine and Coarse Aggregates.

        2. ASTM D698; Standard Test Methods for Moisture-Density Relations of Soils and Soil-Aggregate Moisture Using 5.5 lb. (4.54 kg) Rammer and 12 inch (305 mm) Drop.  (Standard Proctor Method)

        3. ASTM D2922 and D3017; Test Methods for Density of Soil and Soil-Aggregate in Place and Water Content of Soil and Rock by Nuclear Methods (Shallow Depth).

        4. ASTM D 4253 and D 4254, Test Methods for Maximum Index Density of Soils using a Vibratory Table and Minimum Index Density of Soils and Calculation of Relative Density.

      B. Compaction Testing:  Provide compaction testing of backfill, using the services of an independent testing laboratory approved by the Jurisdictional Engineer, unless testing is provided by Jurisdictional Engineer.

      C. Moisture Range:  Obtain required compaction within a soil moisture range of optimum moisture to 4 percentage points above optimum moisture content.

      D. Schedule Testing: Notify Jurisdictional Engineer when work is prepared for testing.

      E. Soil Testing:

        1. Cohesive soils: Determine moisture-density relationships by ASTM D698 (Standard Proctor).  Perform at least one test for each type of cohesive soil used.

        2. Cohesive soils: Determine in-place density and moisture content using ASTM D1556 (sand-cone method) and D2216 or ASTM D2922 and D3017 (nuclear).

        3. Non-cohesive soils: Determine maximum and minimum index density and calculate relative density using ASTM D4253 and D4254 (cohesionless soils).

        4. Gradation: Test in accordance with ASTM C136.

      F. Testing Frequency:  Unless otherwise provided by the jurisdiction the contractor will complete the following tests:

        1. Make one test per each 2 vertical feet of consolidated fill at each street crossing.

        2. Make one test per each 2 vertical feet of consolidated fill for each 200 linear feet of trench.

        3. Additional testing may be required by Jurisdictional Engineer if non-compliance or a change in conditions occur.

      G.  Test Failure:  Rework, recompact, and retest as necessary until specific compaction is achieved in all areas of the trench.

      H. Retesting:  In event of failed tests, Jurisdictional Engineer may retest as deemed necessary.  Costs of such retesting shall be paid by the Contractor.  Refer to Section 3010, 1.08 G.

      END OF SECTION