RETAINING WALL ITEM 423
CITY OF ABILENE
ITEM 423
RETAINING WALL
423.1 DESCRIPTION.
This Item shall govern for furnishing the materials and constructing retaining walls as shown on the Plans and required by this Item. Spread footing retaining walls shall consist of reinforced concrete footings and reinforced concrete stems as shown on the Plans. Mechanically Stabilized Earth (MSE) walls shall consist of a volume of select backfill with tensile reinforcing elements distributed throughout and a concrete facing. Other types of retaining walls shall be as shown on the Plans.
423.2 MATERIALS.
All materials shall conform to the pertinent requirements of the following Items:
Item 420, “Concrete Structures” Item 421, “Portland Cement Concrete” Item 440, “Reinforcing Steel” TxDOT Item 445, “Galvanizing” TxDOT Item 458, “Waterproofing for Structures” TxDOT Item 556, “Pipe Underdrains”
Unless otherwise shown on the Plans, concrete for retaining walls shall conform to the following:
Cast-in-Place, Reinforced Class “C” Cast-in-Place, Non-reinforced Class “A” Precast Class “H”, f'c=4000 psi
Filter fabric material shall conform to TxDOT Departmental Material Specification D-9-6200.
Materials for earth reinforcements shall be as shown on the Plans. All steel elements in contact with soil shall be galvanized or epoxy coated. Epoxy coating shall be in accordance with Item 440, “Reinforcing Steel”, except that the coating thickness shall be a minimum of 18 mils.
Joint fillers, pads, waterstops, and other incidental materials shall be as shown on the Plans, or approved by the Engineer.
BACKFILL MATERIAL.
(1) Backfill for spread footing retaining walls shall be in accordance with Item 132, “Embankment”, Type B, unless otherwise shown on the Plans.
(2) Backfill for MSE walls shall be free from organic or otherwise deleterious materials, and shall conform to the following gradation limits as determined by Test Method Tex-110-E:
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Sieve Size Percent Passing
Type A 3 inches 100 No. 40 0-60 No. 200 0-15
Type B 6 inches 100 3 inches 75-100 No. 200 0-15
Type B material not meeting the No. 200 sieve gradation limit stated above may be used if it conforms to the following requirements:
The percent passing the No. 200 sieve as determined by Test Method Tex-110-E shall not exceed 25 percent.
The Plasticity Index (P.I.) as determined by Test Method Tex-106-E shall not exceed six.
The material, when compacted to 95% of Da density as determined by Test Method Tex-114- E at optimum moisture content, shall exhibit an angle of internal friction of not less than 344 degrees as determined by Test Method Tex-117-E.
When the backfill gradation results in 15 percent or less material passing the No. 40 sieve, when tested in accordance with Test Method Tex-110-E, the backfill will be considered rock backfill.
Unless otherwise shown on the Plans, Type B backfill shall be used.
The pH range shall be from 5.5 to 10.0 as determined by Test Method Tex-128-E.
The resistivity shall not be less than 3000 ohms-cm as determined by Test Method Tex-129-E. Material with a resistivity between 1500 and 3000 ohms-cm may be used provided the chloride content does not exceed 100 ppm and the sulphate content does not exceed 200 ppm as determined by Test Method Tex-620-J.
MSE well systems using nonmetallic or epoxy coated metallic reinforcements may use backfill which does not comply with the pH and resistivity requirements. Epoxy coated metallic reinforcements may be used only when shown on the Plans or approved by the Engineer. All connection hardware used with nonmetallic or epoxy coated reinforcements shall likewise be nonmetallic or epoxy coated. When nonmetallic or epoxy coated reinforcements are used, the maximum allowable backfill particle size shall be three-quarter (3/4) inch.
(3) Unless otherwise shown on the Plans, the Contractor shall have the option of stabilizing backfill for MSE walls with five (5) percent Portland Cement by dry weight of the backfill material.
When Type A backfill is shown on the Plans, the Contractor may use Type B backfill stabilized with five (5) percent Portland Cement by dry weight of the backfill material.
When cement stabilized backfill is used, special drainage provisions shall be provided as shown on the Plans.
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When cement stabilized backfill is used, gradation, pH, and resistivity testing shall be run on the raw, unstabilized backfill material. Testing for angle of internal friction will not be required.
423.3 GENERAL.
(1) Options. When optional or alternate design details are shown on the Plans, the Contractor will have the option of constructing any of the types of retaining wall shown. The Contractor will be required to use the same facing design within an area of continuous retaining walls.
When the Contractor proposes two (2) or more systems, or when the Plans require a special surface finish, the Engineer may require drawings indicating the proposed design arrangement for his approval.
The Contractor's attention is directed to the fact that retaining wall options shown on the Plans may be proprietary. The Contractor shall provide for use of these systems in accordance with Part I, General Provisions – Division I, General Requirements & Covenants.
(2) Working Drawings. When proprietary wall systems are used, or when otherwise shown on the Plans, the Contractor shall prepare and submit working drawings and design calculations. Prior to fabrication, the Contractor shall submit to the Engineer seven (7) sets of casting drawings, seven (7) sets of construction drawings and two (2) sets of design calculations. Upon completion of construction, one (1) set of reproducible as-built drawings shall be submitted to the Engineer.
Casting drawings shall include all information necessary for precasting wall elements, including railing and coping when it is to be prefabricated. Casting drawings shall reflect shape and dimensions of panels; size, quantity and details of the reinforcing steel; the quantity, type, size, and details of connection and lifting hardware, and any additional details necessary.
Construction drawings shall include a numbered panel layout and shall reflect horizontal and vertical alignment of the walls as well as the existing and proposed ground lines, as shown on the Plans. The drawings shall also include all information needed to erect the walls including the proposed leveling pad elevations; the type and details of the soil reinforcing system (if applicable); the details and manufacturer of all pads, fillers, and filter fabric; the limits and dimensions of structural backfill; details necessary to incorporate coping, railing, drainage and electrical conduit as shown on the Plans; and any additional details necessary to complete the work.
Leveling pad elevations may vary from the elevations shown on the Plans. Unless otherwise noted on the Plans, one (1) foot minimum cover shall be provided from the top of the leveling pad to finish grade.
Design calculations shall include a summary of all design parameters used, including material types, strength values and assumed allowables; assumed loads and loading combinations; and factor of safety parameters.
Calculations shall be submitted covering the range of heights and loading conditions on the project. Both internal and external stability calculations will be required.
Construction drawings and design calculations shall bear the seal of a Registered Professional Engineer.
423.4 CONSTRUCTION METHODS.
(1) General. Construction of retaining walls shall conform to the design and details shown on the Plans and to the pertinent requirements of the following Items:
Item 110, “Excavation”
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Item 132, “Embankment” Item 400, “Excavation and Backfill for Structures” TxDOT Item 424 TxDOT Item 458 TxDOT Item 556
Any required piling or drilled shafts shall be in accordance with the pertinent specification.
(2) Mechanically Stabilized Earth (MSE) Walls.
The foundation for the structure shall be graded level for a width equal to or exceeding the approved length of the reinforcing system or as shown on the Plans. Prior to wall construction, the foundation shall be compacted with a smooth wheel vibratory roller or other roller approved by the Engineer. Any foundation soils found to be unsuitable shall be removed and replaced.
Drilled shafts and piling located within the MSE volume shall be placed prior to construction of the retaining wall.
At each foundation level, a concrete leveling pad shall be provided as shown on the Plans. The leveling pad shall be given a wood float finish and shall be in place a minimum of 24-hours before panel erection begins. No curing or strength testing of the leveling pad concrete will be required.
Filter fabric shall be placed behind all wall joints, and at the intersection of retaining walls with other structures, including riprap. Filter fabric shall cover joints a minimum of six (6) inches on each side and shall be positively held in place.
As fill material is placed, the panels shall be maintained in position by methods acceptable to the Engineer. Care shall be exercised during lifting, setting, and alignment of panels to prevent damage to the panels. Any operation which results in chipping, spalling, or cracking of panels shall be discontinued. Damaged panels shall be removed and replaced, or repaired, as approved by the Engineer, at the Contractor's expense. External bracing may also be required for the initial lift.
Vertical tolerances and horizontal alignment tolerance shall not exceed three-quarter (3/4) inch when measured along a 10-foot straight edge. The maximum allowable offset in any panel joint shall be three-quarter (3/4) inch. The overall vertical tolerance of the wall (plumbness from top to bottom) shall not exceed one-half (1/2) inch per ten (10) feet of wall height.
Backfill placement shall closely follow the erection of each lift of panels. At each reinforcement level, backfill shall be leveled and compacted before placing the reinforcement. Reinforcements shall be placed perpendicular to the face of the wall, unless otherwise shown on the Plans. The normal lift thickness will be eight inches (loose measurement). The Contractor may increase or decrease the lift thickness if necessary to obtain the required compaction. Compaction shall be accomplished without distortion or damage to the reinforcement system. Compaction in a strip three feet wide adjacent to the backside of the wall shall be achieved in accordance with “Ordinary Compaction”, of Item 132 “Embankments”. Compaction in the three foot strip shall be accomplished with hand operated or walk-behind compaction equipment. If displacement of panels occurs during compaction, the Engineer may increase the width of the strip, or require modified procedures.
For systems utilizing nonmetallic earth reinforcements, each layer of reinforcement shall be pre- tensioned to remove slack before placement of backfill. The devices used shall be capable of mechanically applying and holding the required force. Before starting work, the Contractor shall demonstrate to the satisfaction of the Engineer the method proposed for pre-tensioning the reinforcement.
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The Contractor shall be responsible for maintaining the stability of the interface area between the embankment or natural ground and the select fill. Any loosening, caving, or other failures in this area shall be removed and replaced at the Contractor's expense.
Unless otherwise shown on the Plans, or when rock backfill as previously defined herein is used, each backfill layer shall be sprinkled as required and compacted to the extent necessary to provide the density specified below:
Location Density, Percent
Top three (3) feet under travel Not less than 95% way (roadway plus shoulders)
All other areas Not less than 90%
The laboratory density determination will be made in accordance with Test Method Tex-114-E. Field density determination will be made in accordance with Test Method Tex-115-E.
Rock backfill or material which in the opinion of the Engineer will make density testing impractical shall be compacted by the “Ordinary Compaction” method of Item 132, “Embankment”. Vibratory compaction may be required. The upper two (2) feet of the rock backfill shall contain no stones larger than three (3) inches in their greatest dimension and shall be composed of material with sufficient fines to fill the voids in a compacted state.
When rock backfill material is used, the same type of filter fabric used behind the panel joints shall be used to cover the rock backfill material prior to placing the upper two (2) feet. The fabric shall have a minimum overlap of at least 18 inches and extend past the edge of the rock backfill material at least 18 inches.
The Contractor shall be responsible for preventing surface water or rainwater from damaging the retaining walls during construction. This shall include shaping the backfill to prevent water from ponding or flowing on the backfill or against the wall face. Any damage or movement caused by erosion, sloughing, or saturation of the retaining wall or embankment backfill shall be repaired at the Contractor's expense.
423.5 MEASUREMENT.
Retaining walls will be measured by the square foot of the front surface area of the wall. Unless otherwise shown on the Plans, the area will be measured from one (1) foot below finished grade to the top of wall including any coping required.
This is a plans quantity measurement Item and the quantity to be paid for will be that quantity shown in the proposal and on the “Estimate and Quantity” sheet of the contract Plans, except as may be modified by Part I, General Provisions – Division I, Requirements and Covenants. If no adjustment of quantities is required, additional measurements or calculations will not be required.
423.6 PAYMENT.
The work performed and materials furnished in accordance with this Item and measured as provided under “Measurement” will be paid for at the unit price bid for “Retaining Walls”, of the type and/or special surface finish specified. This price shall be full compensation for excavation in back of retaining walls and for footings; furnishing and placing footings, leveling pads, copings and traffic railing foundations, when required; furnishing, placing and compacting backfill (except as shown below), including cement for stabilization; furnishing and placing concrete, reinforcing steel, waterproofing material, filter material and drain pipe, joint material, water stop, and filter fabric when required; fabricating, curing and finishing all panels; furnishing and
423-{PAGE } ITEM 423 RETAINING WALL placing earth reinforcement, anchorage systems and fasteners; for wall erection; and for all labor, tools, equipment and incidentals necessary for a complete retaining wall.
Retaining wall backfill areas, which are also in embankment areas, will be measured and paid for as specified in Item 132, “Embankment”, of the compaction method specified. Such backfill material shall meet the requirements of the retaining wall type used.
No payment will be made for piling or drilled shafts, except when used as foundations for spread footing or MSE walls.
When piling or drilled shafts are installed beyond limits shown on the Plans at the direction of the Engineer, the additional quantity will be paid for in accordance with the appropriate bid item or Part I, General Provisions – Division I, Requirements and Covenants.
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