Director’s of Operations & Project Dev.: Approved 07/24/03 Team Leader: David Koenig
Chief Engineer: ________________ 01/14/03
Team Members and Resources
(Rationale: Section 720 will be a new section in the standard specification manual. This section was created by taking the current bridge special provision for Mechanically Stabilized Earth Wall Systems and incorporating it into a standard specification. This review involved an extensive rewrite and reworking of the previous special provision. All material references have been removed for the proposed Section 720. Two additional new sections have been proposed and written as part of the review of the MSE special provision. These additional new sections are Section 1010—Select Granular Backfill for Structural Systems, and Section 1052—Mechanically Stabilized Earth Wall System Components. All references to approved manufacturers have been removed from the proposed Section 720 because they will be available as a prequalified product listing on MoDOT’s public website.) 3/24/03 – EPC tabled this item pending results of field evaluations of small block walls. 4/28/03 – Specs are ok. We need to develop the PDM to give better guidance on when/where to use them.
Section 720 Mechanically stabilized earth wall systems
720.1 Description. This work shall consist of thespecification covers work shall consist of furnishing and constructing of mechanically stabilized earth wall systems in accordance with these specifications, and in accordance with the lines and grades as shown on the plans or as directed by the engineer.
720.2.1 All material shall conform tobe in accordance with Division 1000, Materials Details, and specifically as follows:
Material Requirements for Mechanically Stabilized Earth Wall System Components
702.2.2 All components of mechanically stabilized earth wall systems shall be tested in accordance with the applicable specification for that component. The engineer reserves the right to sample and testfor sampling and testing of components as deemed necessary to ensure ensure compliance with the applicable specifications.
720.2.2 720.2.3 Whenever a wall system is located adjacent to and within 10 feet (3000 mm) of the limits of a permanent roadway, the steel reinforcement used in the concrete elements of the wall system shall be epoxy coated. This requirement does shallwill not apply to the soil reinforcement and the corresponding attachments that are used for connecting the reinforcement to the wall system units.
720.2.3 720.2.4 Reinforcement for wall system units shall consist ofbe either Grade 60 (Grade 420) deformed bars or an equivalent welded steel wire fabric. Reinforcement for coping or top cap units shall consist ofbe Grade 60 (Grade 420) deformed bars.
720.2.4 720.2.5 Joint material for large block wall systems shall be used per in accordance with the wall manufacturer’s recommendations and specifications.
720.2.5 720.2.6 The unit fill that is used for small block wall systems shall consist of a granular backfill complying in complianceaccordance with the requirements for gradation Gradation D or E of Sec 1005.
720.2.6 720.2.7 Class B or B-1 air entrained concrete should shall be used for the coping or top cap units that are used for the wall system. Class B or B-1 concrete shall be used and for all of the cast-in-place concrete leveling pads that are used for the wall system.
720.3 Design Requirements.
720.3.1 Only the mechanically stabilized earth wall systems that are shown in the bridge prequalified products listing will be available for use by the contractor. The bridge prequalified products listing may be obtained through Bridge or on MoDOT’s web site. Any deviations from the prequalified wall system details that were previously submitted to Bridge shall be specifically outlined in the cover letter that is submitted with the design plans, details and computations.
720.3.2 Small block wall systems will be permitted for uses where the wall height does not exceed 10 feet (3000 mm). This limit can may be exceeded up to a maximum height of 12 feet (3.600 mm) to accommodate peaks in the wall or to accommodate lengths of the wall that do not exceed more than ten percent of the total wall length. The height of the wall shall be determined by measuring from the top of the concrete leveling pad to the top of the cap on the wall (top cap included).
720.3.3 The contractor shall be responsible for ensuring that The wall manufacturer will shall be responsible for submittingsubmits six complete sets of the manufacturer’s design plans, details and computations for each individual wall structure to the engineer. The contractor shall be responsible for any delay in submission of complete and acceptable wall design documents to the engineer. All submitted information should shall be clear and complete, and should be thoroughly checked before it the information is submitted. All submitted information should shall be legible and of sufficient contrast to be suitable for archiving in accordance with MoDOT’s current practice for archiving. Submitted information that is determined to not be unsuitable for archiving purposes will be returned for corrective action.
720.3.4 The contractor shall be The wall manufacturer shall be solely responsible for the content of the design plans, details and computations that are submitted by the manufacturer. The contractor The wall manufacturer will shall also be solely responsible for ensuring that the information submitted by the manufacturer submitted information conforms to all of the requirements of the contract plans and specifications and all of the requirements for the wall system that is used. Completed design plans shall contain all material requirements, fabrication requirements and construction requirements for erecting the wall system complete in place. The completed design plans shall also show the longitudinal and lateral layout of the drainage systems used for the wall system.
720.3.5 All of the design plans, details and computations that are submitted for distribution shall be signed, sealed and stamped in accordance with the laws relating to architects and professional engineers (Chapter 327, RSMo).
720.3.6 Mechanically stabilized earth wall systems shall be designed in accordance with the AASHTO specifications shown on the design plans and in accordance with additional publications or specifications referenced within the AASHTO specifications. The seismic performance category, angle of internal friction for the selected granular backfill for structural systems and other design requirements as shown on the design plans shall be followed when designingincorporated into the design of the wall system.
720.3.7 The manufacturercontractor is shall be responsible for the internal and external stability of the structure. Mass slope stability is not the responsibility of the manufacturer.
720.4 Construction Requirements.
720.4.1 Small Block Wall Systems.
7188.8.131.52 The contractor shall use a unit fill to fill the voids of the blocks for the wall system. This unit fill shall extend a minimum distance of 12 inches (300 mm) beyond the extreme back face of the wall system. Each course of the wall system shall have the unit fill in place before the next course of the wall system is placed.
7184.108.40.206 7220.127.116.11 Precast top cap units shall be used on small block wall systems. The top cap units shall be permanently attached as shown on the design plans utilizing either a resin anchor system or an equivalent prequalified detail.
718.104.22.168 722.214.171.124 Only the resin anchor systems that are shown in the bridge prequalified products listing will be available for use by the contractor. Any equivalent details that are used must shall be part of the prequalified mechanically stabilized earth wall system details that are on file for the manufacturer of the wall system. The bridge prequalified products listing is available through Bridge or on MoDOT’s web site.
720.4.2 Large Block Wall Systems.
7126.96.36.199 A Class 1 geotextile filter cloth shall be placed between the wall and in front of the select granular backfill for structural systems at all joints between the individual wall system units.
7188.8.131.52 Precast or cast-in-place coping should shall be placed on large block wall systems in accordance with the design plans. Capstone may be used in lieu of coping whenever it coping is specified on the design plans. When coping is used, the maximum distance between construction joints shall be 30 feet (9.100 mm).
720.4.3 Drainage Requirements. A drainage system shall be provided at the base of the wall system near the facing elements and at the interface of the select granular backfill for structural systems and the retained backfill. The drainage system shall consist of a perforated pipe wrapped in a Class 2 geotextile to prevent clogging of the perforations. The pipe shall be placed in a manner such that water drains freely from the pipe. When wall length is such that the slope of the pipe becomes excessive in the engineer’s judgment, lateral drain pipes shall be installed underneath the concrete leveling pad.
720.4.4 Foundation Preparation. The foundation for the wall system shall be graded level for a width equal to or exceeding the length of the reinforcing strips, or as shown on the plans. Prior to wall construction, the foundation, if not on rock, shall be compacted as directed by the engineer. Any foundation soils found to be unsuitable shall be removed and replaced, as directed by the engineer.
720.4.5 Leveling Pad. An unreinforced cast-in-place concrete leveling pad shall be provided at the foundation level for each base unit of the wall system. The leveling pad shall be built to the elevations as specified shown on the plans and shall not be raised in elevation to allow for the use of a particular wall system. The leveling pad shall be built a minimum width of 12 inches (300 mm) wide and a minimum depth of 6 inches (150 mm). The concrete on the leveling pad shall be cured a minimum of twelve 12 hours before any of the wall system panels are placed.
720.4.6 Batter Requirements. Wall systems shall be built with some inward batter, as determined by the wall system manufacturer, to accommodate the horizontal movement created by the placement and compaction of the selected granular backfill for structural systems. Facing elements that are out of alignment shall not be pulled or pushed into proper place as that may cause damage to the facing element or soil reinforcement strips. If misalignment occurs, then the select granular backfill for structural systems and the soil reinforcement strips shall be removed and the facing elements reset to the proper alignment.
720.4.7 Select Granular Backfill for Structural Systems Placement.
7184.108.40.206 Select granular backfill for structural systems shall be placed in conjunction concurrently with the placement of the retained backfill. The placement of the select granular backfill for structural systems shall closely follow the erection of each course of the wall system and shall be placed in a manner so as to avoid any damage or disturbance to the wall materials or any misalignment of the facing elements of the wall system. Any wall system materials which becomes damaged or disturbed during the installation of the wall system shall be removed, replaced or corrected at the contractor's expense, as directed by the engineer. Whenever placement of the select granular backfill for structural systems results in the wall facing system being misaligned or distorted outside the limits of this specification, the contractor shall correct the misalignment or distortion as directed by the engineer.
7220.127.116.11 The select granular backfill for structural systems shall be compacted in accordance with Sec 203 with the following exceptions:
(b) When the material used contains more than 30 percent retained on the 3/4 inch (19 mm) sieve, a method of compaction consisting of at least four passes by a heavy roller shall be used.
(c) The moisture content of the material prior to and during compaction shall be uniformly distributed throughout each layer. The placement moisture content shall not be no lower than three percentage points less than the optimum moisture content and shall not be no more than the optimum moisture content.
(d) Compaction within 3 feet (900 mm) of the back face of the wall system shall be achieved by at least three passes of a lightweight mechanical tamper, roller or vibratory system.
(e) At the end of each day's operation, the contractor shall slope the last level of the backfill away from the wall facing to rapidly direct runoff away from the wall face. In addition, tThe contractor shall not allow surface runoff from adjacent areas to enter the wall construction site.
(f) Class I 1 geotextile material shall be placed behind between the select granular backfill for structural systems and the retained backfill and over the top of the select granular backfill for structural systems to prevent piping of in-situ soil into the wall system.
(g) Tamping-type (sheepsfoot) rollers shall not be used for compaction of the select granular backfill for structural systems.
718.104.22.168 The select granular backfill for structural systems shall be initially placed parallel to the wall system and at the rear and middle of the soil reinforcement strips and then moved toward the facing elements of the wall system. Construction equipment shall at no time come in direct contact with the soil reinforcement strips. Each course or layer shall be compacted up to or slightly above the location of the next connection for the reinforcement strips prior to placing the next layer of reinforcement strips as designated in the erection sequence provided by the manufacturer of the wall system.
720.4.8 Construction Tolerances.
722.214.171.124 Wall systems shall be built in accordance with the dimensions and elevations specified on the plans and in accordance with the requirements of the system manufacturer. Alignments shall be maintained within the dimensional tolerances shown in Table II.
7126.96.36.199 Vertical alignments shall be measured along a theoretical vertical line established from the top of the wall system to the base of the wall system. For small block walls that have a built-in setback, the alignment shall be measured along the theoretical vertical line as mentioned above and the straight line that describes the horizontal setback.
720.4.9 Technical Assistance. The contractor shall obtain the services of a technical advisor from the wall supplier to provide on--site advice to the engineer. This advisor shall be a manufacturer qualified representative that is certified in product installation, and is shall be acceptable to the engineer. At the engineer's option, the on site technical support may be waived. However, the technical advisor shall still be available to provide a timely response if questions arise. The contractor shall be responsible for informing the technical advisor prior to the date of installation whether or not the technical services will be required on site.
720.5 Method of Measurement.
720.5.1 Measurement of mechanically stabilized earth wall systems will be made to the nearest square foot (0.1 m2). The quantity to be paid will be measured from the wall outline as shown on the plans. No adjustments in the measured quantity will be allowed for additional wall area that is required to meet the minimum wall elevations as shown on the plans for a particular wall system.
720.5.2 Final measurement will not be made except for authorized changes during construction, or where appreciable errors are found in the contract quantity. The revision or correction will be computed and added to or deducted from the contract quantity.
720.5.3 No measurement will be made for excavation that is required for the placement of the leveling pad for the wall system. All other excavation required for the construction of the wall system will be included in roadway items.
720.6 Basis of Payment. The accepted quantity of mechanically stabilized earth wall systems, complete in place, will be paid for at the contract unit price for each of the pay items included in the contract. Payment will be considered full compensation for all labor, equipment, and material and incidental work to complete the described work.