SECTION 674 - PREFABRICATED CONCRETE MODULAR GRAVITY WALL
674.01 Description This work shall consist of designing, fabricating, furnishing, and
constructing a prefabricated modular reinforced concrete gravity wall in accordance with these specifications and in reasonably close conformance with the lines and grades shown on the Plans, or established by the Resident. Included in the scope of the Pre fabricated Concrete Modular Gravity Wall construction are: all grading necessary for wall construction, excavation, compaction of the wall foundation, backfill, construction of leveling pads, placement of geotextile, segmental unit erection, and all incidentals necessity to complete the work. 6-199 The Prefabricated Concrete Modular Gravity Wall design shall f ollow the general dimensions of the wall envelope shown in the contract plans. The top of the leveling pad shall be located at or below the theoretical leveling pad elevation. The minimum wall embedment shall be at or below the elevation shown on the plans. The top of the face panels shall be at or above the top of the panel elevation shown on the plans. The Contractor shall require the design-supplier to supply an on-site, qualified experienced technical representative to advise the Contractor concerning proper installation procedures. The technical representative shall be on-site during initial stages of installation and thereafter shall remain available for consultation as necessary for the Contractor or as required by the Resident. The work done by this representative is incidental.
674.02 Materials Materials shall meet the requirements of the following subsections of
Division 700 — Materials:
G r a v e l B o r r o w 703.20 Preformed Expansion Joint Material 705.01 Reinforcing Steel 709.01 Structural Pre-cast Concrete Units 712.061 Drainage Geotextile 722.02 The Contractor is cautioned that all of the materials listed are not required for every Prefabricated Concrete Modular Gravity Wall. The Contractor shall furnish the Resident a Certificate of Compliance certifying that the applicable materials comply with this section of the specifications. Modify requirements in 712.061 as follows: Concrete Units: Concrete shall be Class P. The concrete shall contain a minimum of 5.5 gallons per cubic yard of calcium nitrite solution. The minimum permeability of the concrete as indicated by Surface Resistivity shall be 17 KOhm-cm. Defects Defects which may cause rejection of precast units include, but are not limited to, the following: Any discontinuity (crack, rock pocket, etc.) of the concrete which could allow moisture to reach the reinforcing steel. Rock pockets or honeycomb over 6 square inches in area or over 1 inch deep. Edge or corner breakage exceeding 12 inches in length or 1 inch in depth. Any other defect that clearly and substantially impacts the quality, durability, or maintainability of the structure, as determined by the Fabrication Engineer. Repair honeycombing, ragged or irregular edges and other non-structural or cosmetic defects using a patching material from the Department Qualified Products List (QPL). The repair, including preparation of the repair area, mixing and application and curing of the patching material, shall be in accordance with the manufacturer's product data sheet. 6-200 Corners that are not exposed in the final product may be ground smooth with no further repair necessary if the depth of the defect does not exceed 1/2 inch. Remove form ties and other hardware to a depth of not less than 1 inch from the face of the concrete and patch the holes using a patching material from the Department QPL. Repair structural defects only with the approval of the Fabrication Engineer. Submit a nonconformance report (NCR) to the Fabrication Engineer with a proposed repair procedure. Do not perform structural repairs without an NCR that has been reviewed by the Fabrication Engineer. Structural defects include, but are not be limited to, exposed reinforcing steel or strand, cracks in bearing areas, through cracks and cracks 0.013 inch in width that extend more than 12 inches in length in any direction. Give the QAI adequate notice prior to beginning any structural repairs. Tolerances In addition to meeting the requirements of 712.061 , all prefabricated units shall be manufactured with the following tolerances. All units not meeting the listed tolerances will be rejected. 1.All dimensions shall be within (edge to edge of concrete) ±3/16 in. 2.Squareness The length differences between the two diagonals shall not exceed 5/16 in. 3.Surface Tolerances For steel formed surfaces, and other formed surface, any surface defects in excess of 0.08 in. in 4 ft will be rejected. For textured surfaces, any surface defects in excess of 5/16 in. in 5 ft shall be rejected. Joint Filler (where applicable) Joints shall be filled with material approved by the Resident and supplied by the approved Prefabricated Concrete Modular Gravity Wall supplier. 4 in. wide, by 0.5 in. preformed expansion joint filler shall be placed in all horizontal joints between facing units. In all vertical joints, a space of 0.25 in. shall be provided. All Preformed Expansion Joint Material shall meet the requirements of subsection 502.03. Woven Drainage Geotextile Woven drainage geotextile 12 in. wi de shall be bonded with an approved adhesive compound to the back face, covering all joints between units, including joints abutting concrete structures. Geotextile seam laps shall be 6 in., minimum. The fabric shall be secured to the concrete with an adhesive satisfactory to the Resident. Dimensions may be modified per the wall supplier’s recommendations, with written approval of the Resident. Concrete Shear Keys (where applicable) Shear keys shall have a thickness at least equal to the pre-cast concrete stem. Concrete Leveling Pad Cast-in-place concrete shall be Fill Co ncrete conforming to the requirements of Section 502 Structural Concrete. Leveling pad shall have a minimum thickness of 4 inches. The horizontal tolerance on the surface of the pad shall be 0.25 in. in 10 ft. Dimensions may be modified per the wall supplier’s recommendations, with written approval of the Resident. 6-201 Backfill and Bedding Material Bedding and backfill material p laced behind and within the reinforced concrete modules shall be gravel borrow conforming to the requirements of Subsection 703.20. The backfill materials shall conform to the following additional requirements: backfill and bedding material shall only contain particles that will pass the 3-inch square mesh sieve and the plasticity index (PI) as determined by AASHTO T90 shall not exceed 6. Compliance with the gradation and plasticity requirements shall be the responsibility of the Contractor, who shall furnish a copy of the backfill test results prior to construction. Materials Certificate Letter The Contractor, or the supplier as his agent, shall furnish the Resident a Materials Certificate Letter for the above materials, including the backfill material, in accordance with Section 700 of the Standard Specifications. A copy of all test results performed by the Contractor or his supplier necessary to assure contract compliance shall also be furnished to the Resident. Acceptance will be based upon the materials Certificate Letter, accompanying test reports, and visual inspection by the Resident.
674.03 Design Requirements The Prefabricated Concrete Modular Gravity Wall shall be
designed and sealed by a Professional Engineer licensed in the State of Maine. The design to be performed by the wall system supplier shall be in accordance with AASHTO LRFD Bridge Design Specifications, current edition, except as required herein. Design calculations that consist of computer generated output shall be supplemented with at least one hand calculation and graphic demonstrating the design methodology used. Design calculations shall provide thorough documentation of the sources of equations used and material properties. The design by the wall system supplier shall consider the stability of the wall as outlined below:
A.Stability Analyses Wall systems shall be investigated for be aring resistance failure, lateral sliding and overturning failure in accordance with LRFD Pullout resistance shall be investigated at every module level using nominal resistances and forces in accordance with Paragraph C., below. The ratio of the sum of the nominal resistances to the sum of the nominal forces shall be greater than, or equal to, 1.5. Traffic impact loads transmitted to the wall through guardrail posts shall be calculated and applied in compliance with LRFD with the exception that Article
11.10.10 2 is modified such that the upper 3.5 ft of concrete modular units shall be
designed for an additional horizontal load of γP H1, where γPH1=300 lbs per linear ft of wall.
B.Backfill and Wall Unit Soil Parameters For overturning and sliding stability calculations, earth pressure shall be assumed acting on a vertical plane rising from the back of the lowest wall stem. For overturning, the unit weight of the backfill within the wall units shall be limited to 96 pcf. For sliding analyses, the unit weight 6-202 of the backfill within the wall units can be assumed to be 120 pcf. Both analyses may assume a friction angle of 34 degrees for backfill within the wall units. These unit weights and friction angles are based on a wall unit backfill meeting the requirements for select backfill in this specification. Backfill behind the wall units shall be assumed to have a unit weight of 120 pcf and a friction angle of 30 degrees. The friction angle of the foundation soils shall be assumed to be 30 degrees unless otherwise noted on the plans. Internal Stability Internal stability of the wall shall be demonstrated using accepted methods, such as Elias’ Method, 1991. Shear keys shall not contribute to pullout resistance. Soil-to-soil frictional component along stem shall not contribute to pullout resistance. The failure plane used to determine pullout resistance shall be found by the Rankine theory only for vertical walls with level backfills. When walls are battered or with backslopes greater than 0 degrees are considered, the angle of the failure plane shall be per Jumikus Method. For computation of pullout force, the width of the backface of each unit shall be no greater than 4.5 ft. A unit weight of the soil inside the units shall be assumed no greater than 120 pcf when computing pullout. Coulomb theory may be used.
D.Safety against Structural Failure Prefabricated units shall be designed for all strength and reinforcement requirements in accordance with LRFD.
E.External loads which affect the internal stability such as t hose applied through piling, bridge footings, traffic, slope surcharge, hydrostatic and seismic loads shall be accounted for in the design.
F.Stability During Construction Stability during construction shall be considered during design, and shall meet the requirements of the AASHTO LRFD Bridge Design Specifications, Extreme Limit State.
G.Hydrostatic forces Unless specified otherwise, when a desig n high water surface is shown on the plans at the face of the wall, the design stresses calculated from that elevation to the bottom of wall must include a 3 ft minimum differential head of saturated backfill. In addition, the buoyant weight of saturated soil shall be used in the calculation of pullout resistance.
H.Not more than two vertically consecutive units shall have th e same stem length, or the same unit depth. Walls with units with extended height curbs shall be designed for the added earth pressure. A separate computation for pullout of each unit with extended height curbs, or extended height coping, shall be prepared and submitted in the design package described above.
674.04 Submittals. The Contractor shall supply wall design computations, wall details,
dimensions, quantities, and cross sections necessary to construct the wall. Thirty (30) days prior to beginning construction of the wall, the design computations and wall details shall be 6-203 submitted to the Resident for review by the Department. The fully detailed plans shall be prepared in conformance with Subsection 105.7 of the Standard Specifications and shall include, but not be limited to the following items:
A.A plan and elevation sheet or sheets for each wall, contain ing the following: elevations at the top of leveling pads, the distance along the face of the wall to all steps in the leveling pads, the designation as to the type of prefabricated module, the distance along the face of the wall to where changes in length of the units occur, the location of the original and final ground line.
B.All details, including reinforcing bar bending details, sha ll be provided. Bar bending details shall be in accordance with Department standards.
C.All details for foundations and leveling pads, including de tails for steps in the leveling pads shall be provided. The maximum calculated factored bearing pressure under the Prefabricated Concrete Modular Gravity wall shall be indicated on the wall plans.
D.All prefabricated modules shall be detailed. The details s hall show all dimensions necessary to construct the element, and all reinforcing steel in the element.
E.The wall plans shall be prepared and stamped by a Professio nal Engineer.
F.Four weeks prior to the beginning of construction, the cont ractor shall supply the Resident with two copies of the design-supplier’s Installation Manual. In addition, the Contractor shall have two copies of the Installation Manual on the project site.
674.05 Construction Requirements
Excavation The excavation and use as fill or disposal of all excavated material shall meet the requirements of Section 203 -- Excavation and Embankment, except as modified herein. Foundation The area upon which the modular gravity wall struc ture is to rest, and within the limits shown on the submitted plans, shall be graded for a width equal to, or exceeding, the length of the module. Prior to wall and leveling pad construction, this foundation material shall be compacted to at least 95 percent of maximum laboratory dry density, determined using AASHTO T180, Method C or D. Frozen soils and soils unsuitable or incapable of sustaining the required compaction, shall be removed and replaced. A concrete leveling pad shall Leveling pad shall have a minimu m thickness of 4 inches and shall be constructed as indicated on the plans. The leveling pad shall be cast to the design elevations as shown on the plans, or as required by the wall supplier upon written approval of the Resident. Allowable elevation tolerances are +0.01 ft and -0.02 ft from the design elevations. Leveling pads which do not meet this requirement shall be repaired or 6-204 replaced as directed by the Resident at no additional cost to the Department. Placement of wall units may begin after 24 hours curing time of the concrete leveling pad. Method and Equipment Prior to er ection of the Prefabricated Concrete Modular Gravity Wall, the Contractor shall furnish the Resident with detailed information concerning the proposed construction method and equipment to be used. The erection procedure shall be in accordance with the manufacturer’s instructions. Any pre-cast units that are damaged due to handling will be replaced at the Contractor’s expense. Installation of Wall Units A field representative from the wa ll system being used shall be available, as needed, during the erection of the wall. The services of the representative shall be at no additional cost to the Department. Vertical and horizontal joint fillers shall be installed as shown on the plans. The maximum offset in any unit joint shall be 3/4 in. The ove rall vertical tolerance of the wall, plumb from top to bottom, shall not exceed 1/2 in per 10 ft of wall height. The prefabricated wall units shall be installed to a tolerance of plus or minus ¾-in in 10 ft in vertical alignment and horizontal alignment. Select Backfill Placement Backfill placement in the interior of the wall units and behind the wall shall progress simultaneously following the erection of each row of prefabricated wall units. The maximum lift thickness shall be 8 in., loose measure, and be thoroughly compacted by mechanical or vibratory compactors. The Contractor shall decrease the lift thickness if necessary to obtain the specified density. Gravel borrow backfill shall be compacted in accordance with Subsection 203.12 except that the minimum required compaction shall be 92 percent of maximum density as determined by AASHTO T180 Method C or D. Backfill compaction shall be accomplished without disturbance or displacement of the wall units. Sheepsfoot rollers will not be allowed. Puddling for compaction will not be allowed. Whenever a compaction test fails, no additional backfill shall be placed over the area until the lift is recompacted and a passing test achieved. The moisture content of the backfill material prior to and dur ing compaction shall be uniform throughout each layer. Backfill material shall have a placement moisture content less than or equal to the optimum moisture content. Backfill material with a placement moisture content in excess of the optimum moisture content shall be removed and reworked until the moisture content is uniform and acceptable throughout the entire lift. The optimum moisture content shall be determined in accordance with AASHTO T180, Method C or D. At the end of the day’s operations, the Contractor shall shape the last level of backfill so as to direct runoff of rain water away from the wall face.
674.06 Method of Measurement Prefabricated Concrete Modular G ravity Wall will
be measured by the square foot of front surface not to exceed the dimensions shown on the contract plans or authorized by the Resident. Vertical and horizontal dimensions will be from the edges of the facing units. No field measurements for computations will be made unless the Resident specifies, in writing, a change in the limits indicated on the plans. 6-205
674.07 Basis of Payment The accepted quantity of Prefabricate d Concrete Modular
Gravity Retaining Wall will be paid for at the contract unit price per square foot complete in place. Payment shall be full compensation for furnishing all labor, equipment, materials, professional services, and incidentals necessary for designing, fabricating, furnishing and installing the prefabricated concrete modular gravity wall and accessories. Cast-in-place concrete leveling pad, pre-cast concrete units, backfill, bedding material, hardware, joint fillers, woven drainage geotextile, cast-in-place coping, traffic barrier, and technical field representative will not be measured and paid for separately, but will be incidental to the square foot pay item. There will be no allowance for excavating and backfilling for the Prefabricated Concrete Modular Gravity Wall beyond the limits shown on the approved submitted plans, except for excavation required to remove unsuitable subsoil in preparation for the foundation, Payment for excavating unsuitable material shall be full compensation for all costs of pumping, drainage, sheeting, bracing and incidentals for proper execution of the work, and will be paid as common excavation in accordance with Section 203. Payment will be made under: Pay Item P a y U n i t
674.10 Prefabricated Concrete Modular Gravity Wall Square Fo ot
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Source: Standard Specifications — Highways and Bridges, 2020 Edition. Pages 668–675 of 817. View official source ↗