- 603 - SECTION 715 -- MECHANICALLY STABILIZED EARTH (MSE) WALLS WITH MODULAR BLOCK FACING UNITS
715.01 Description
1.This work shall consist of designing, furnishing materials, and constructing mechanically stabilized earth (MSE) modular concrete block facing unit walls in accordance with these Specifications and with the lines, grades, dimensions, and details shown in the contract.
2.The excavation for the MSE wall shall consist of excavation for the leveling pads and “Excavation for Soil Reinforcement”. The excavation may consist of existing ground and/or ne wly constructed fill. The contractor shall excavate to the dimensions shown in the shop drawings. The contractor shall also follow the manufacturer’s recommendations with concurrence of the Department .
3.The MSE wall shall consist of a nonstructural leveling pad, modular block facing units, and soil reinforcement elements. Soil reinforcement shall have sufficient strength and frictional resistance and length as required by the design and as outlined in these Specifications.
4.Only the approved proprietary mechanically stabilized earth retaining wall systems are shown on the Department’s Approved Products List shall be used.
5.All appurtenances behind, in front of, under, mounted upon, or passing through the wall, such as drai nage structures, ut ilities, or other appurtenances shown in the contract, shall be accounted for in the stability design of the wall.
6.The MSE wall design shall follow the general dimensions of the wall envelope shown in the contract. The wall envelope will include the minimum 2 feet below ground or as shown in the contract. The contract will locate the theoretical leveling pad at or below th e theoretical leveling pad elevation. The minimum wall embedment below ground shall be 2 feet (600 mm) or as shown in the contract. The top of the block units shall be at or above the top of the wall elevation shown in the c ontract. The front face of walls may employ a setback or an incline from the vertical. The maximum slope on the front face of the wall shall be 11 degr ees unless otherwise required by the Engineer. Cast-in-place concrete will not be an acceptable replacement for block areas noted by the wall envelope, except for minor grouting of pipe penetrations and leveling required for traffic barriers. For walls with metallic reinforcement elements, the design height of the wall shall be from the top of the leveling pad to the top of the pot ential surface where the failure surface intercepts the ground surface. For walls with polymeric soil reinforcement elements in the design, wall height shall be the vertical distance from the top of the leveling pad to the point wher e the finish ground surface meets the back face of the wall. The failure plane for such a wall shall rise at an angle of 45 degrees + ø/2 from the intersection of the back face of the wall with the leveling pad.
7.The block units shall be designed to accommodate differential settlements along the length of the wall and transverse to the wall alignment. Differential settlements along the length of the wall shall not exceed 1 foot per 100 feet (1m per 100m) of wall length. When differential settlements transverse to the wall exceed 3 inches (75 mm), the lower level reinforcement connections shall be designed to accommodate the increased Mechanically Stabilized Earth (MSE) Walls
715.01 with Modular Block Facing Units
- 604 - tensile forces due to the settlement. Where shown in the contract or determined by the MSE wall supplier, vertical joints to accommodate excessive differential settlement shall be included.
8.Working Drawings : a. The Contractor shall subm it to the Engineer for review:
1.6 sets of working drawings. (2) 6 sets of design calculations.
3.Explanatory notes.
4.Specifications. (5) Proposed component materials for the wall system. b. The working drawings and design calculations shall be signed, sealed, and dated by a Professional Engineer registered in Nebraska.
c.These drawings shall include a block unit layout for fabrication and erection purposes, as well as for any required coping when it is prefabricated.
d.They shall further include the horizontal and vertical alignment of the walls as well as the existing and proposed ground lines, all as shown in the contract.
e.The drawings will also reflect: (1) All information needed to fabricate and erect the walls including the proposed leveling pad elevations.
2.The shape and dimensions of the modular blocks.
3.The size, number, and details of the reinforcing steel. (4) The number, size, type, and details of the soil reinforcing system and anchorage.
5.The size, details, and manufac turer of all fillers and filter cloth.
6.The size of leveling pad. (7) The dimensions of structural backfill required.
8.Limitations which shall be observed when extending the limits vertically and horizontally.
9.Any additional details pertaining to coping, railing, drainage, or electrical conduit required by the contract.
f.Leveling pad elevations ma y vary from footing elevations shown in the contract. However, the leveling pad elevations shall be such as to allow for transverse and longitudinal drainage structures shown in the contract and shall provide 2 feet (600 mm) minimum cover from the top of the leveling pad to finish grade.
g.The Contractor shall not start work on any earth retaining system until the working drawings are reviewed and returned by the Engineer. Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.02 - 605 - h. It is expressly understood that the review of the Contractor's drawings shall not relieve the Contra ctor of any responsibility under the contract for the successful completion of the work in conformity with the requirements of the contract.
i.The Contractor shall allow 30 calendar days for the review of the drawings and MSE wall design details by the Engineer.
j.Minor changes as defined in the working drawings may be allowed in the field after review by the Engineer without a revised Working Drawing or Shop Drawing.
715.02 Design Requirements
1.a. The design, by the wall system supplier, shall consider the internal stability of the wall’s retained mass. In conjunction with these Specifications , the following publications shall be used by the wall system supplier when designing the wall system:
1.FHWA NHI-10-024 Vol I and NHI-10-025 Vol II, “Design of MSE Walls and Reinforced Slopes,” (Berg et al., 2009);
2.Latest edition of AASHTO LRFD Bridge Design Specifications including Interims;
3.Design Manual for Segmental Retaining Walls National Concrete Masonry Association, Washington, D.C.
b.Design Height: The structure's design height, H, shall be from the top of the leveling pad to the t op of the wall where the ground surface intercepts the wall facing.
c.The Contractor is solely responsible for the satisfactory construction and performance of the wa ll and the internal stability of the wall’s retained mass. The Contractor shall submit a certification to the Engineer that the wall is designed in accordance with the current AASHTO Standard Specifications for Highway Bridges.
d.The Engineer shall indicate in the contract the "external site factors" which include:
1.Settlement both along and perpendicular to the MSE structure alignment;
2.Allowable bearing capa city of the foundation soil;
3.External drainage beneath and behind the MSE volume;
4.The design parameters for the foundation soils (cohesion and friction angle).
5.Global Stability e. Soil Reinforcement Length: The soil reinforcement length shall be the same length from top to bottom of the wall. The minimum soil reinforcement length shall be greater than or equal to 70% of the design height as measured from the front face of the wall to the end of the soil reinforcements.
f.Maximum reinforcement loads shall be calculated using the method presented in th e latest AASHTO and as per the requirements Mechanically Stabilized Earth (MSE) Walls
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- 606 - specified herein. No other design method will be allowed. Unless otherwise specified in the contract, all structur es shall be designed to conform to the requirements provided in the latest edition of AASHTO LRFD Bridge Design Specifications.
g.When a highwater surface elevat ion is shown in the contract at the wall face, the design stresses calc ulated from that elevation to the bottom of the wall must include a minimum differential hydrostatic pressure equal to 40 inches (1 m) of water. E ffective unit weights from the saturated soil condition shall be used to calculate external and internal stability.
h.The friction angle of the select backfill used in the reinforced fill zone for the internal stability design of the wall shall be 34 degrees unless shown otherwise in the contract. Before construction begins, the selected backfill shall be subject to approval to show conformance with this frictional requirement. The approval shall be based on the results of the standard direct shear test, AASHTO T 236, utilizing a sample of the material compacted to 95% maximum density as determined by NDOT T 99, at optimum moisture content. Compliance wi th the test requirements shall be the responsibility of the Contractor. The wall supplier shall be furnished a copy of the test results before cons truction. The friction angle of the foundation soils and random backfill shall be 28 degrees unless otherwise shown in the contract.
i.The optimum moisture co ntent shall be determined in accordance with NDOT T 99.
j.All structural connections shall be subject to the same metal loss rates and allowable tension requirements as outlined in this Subsection.
k.Steel Reinforcement: (1) Steel reinforcement shall be designed for a minimum service life of 75 years unless otherwise indicated in the contract. Time dependent corrosion rates for soil reinforcing and attachment services shall be considered. For steel reinforcements, including tie strips and loop inserts, the metal loss rates shown in Table 714.01 shall be assumed. Table 714.01 Metal Loss Rates Metal Rate Zinc (first 2 year s): ................................. 15 µm/year/side Zinc (subsequent years to de pletion): .... 4 µm/year/side Carbon Steel (after depletion of zinc): ... 12 µm/year/side Carbon Steel (75 to 100 years): ............. 7 µm/year/side
2.The allowable tensile stress in steel reinforcement and connections including tie strips and loop inserts, Ft, at the end of the service life, shall conform to the following:
i.Systems using linear reinforcement (strips): ( a ) F t = 0.55 F y at the reduced gross section (minimum cross section). ( b ) Ft = 0.50 F u at the net section at a bolt hole (applicable to bolted connections only). Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.03 - 607 - (ii) Systems with bar mats or welded wire mesh: Ft = 0.47 F y at all sections.
3.F y used for design shall not exceed 65,000 psi (445 MPa). Maximum allowable tension in reinforcements shall consider any reduction in cross sectional area of reinforcement due to punching and corrosion losses and shall not exceed 50% of pullout capacity of the connection devices embedded in facing panels.
l.The soil reinforcement lengths and percent coverage at a given level shall be in accordance with the contract. All soil re inforcement shall be positively connected to the modular bloc k facing units that is capable of resisting 100% of the maximum tensi on in the soil reinforcements at any level within the wall. Detailed document ation for connection strength shall be submitted as noted in Section 714.02 . The vertical spacing of the soil reinforcement for walls with modular block facing units shall be as follows:
1.The first (bottom) layer of soil reinforcement shall be no further than 16 inches above the top of the leveling pad.
2.The last (top) layer of soil reinforcement shall be no further than 20 inches on the average below the top of the uppermost unit.
3.The maximum vertical spacing between layers of adjacent soil reinforcement shall not exceed 32 inches. For walls deriving any part of their connection capacity by friction the maximum vertical spacing of the reinforcement should be limited to two times the block depth (front face to back face) to assure construction and long-term stability. The top row of reinforcement should be one-half the vertical spacing.
m.The initial batter of the wa ll, both during construction and upon completion, shall be within the vertic al and horizontal alignment tolerances included in this Specification. The in itial batter of the wa ll at the start of construction and the means and methods necessary to achieve the batter shall be provided in the working drawings. Subject to the Engineer’s approval, the initial batter may be modified at the start of construction by the manufacturer’s field representative bas ed on the evaluation of the backfill material selected by the contractor. Any such changes shall be documented in writing within 24 hours of the approved changes. This written document shall be sealed by the manufacturer’s design engineer who is a registered Civil Engineer in the State of Nebraska . Details of the wedges or shims or other devices such as clamps and external bracing used to achieve or maintain the wall batter, shall be as shown on the working drawings and/or accompanying the construction manual. Permanent shims shall comply with the design life criteria, and shall mainta in the design stress levels required for the walls.
715.03 Material Requirements
1.The Contractor shall provide a ll materials necessary to construct the specified walls. The Contractor sh all ensure that temporary slopes are safe during the period of wall construction, and shall adhere to all applicable local, state, and federal regulations . The Contractor shall furnish the Engineer a Certificate of Compliance cert ifying that all materials comply with Mechanically Stabilized Earth (MSE) Walls
715.03 with Modular Block Facing Units
- 608 - Section 715 of the Specifications . Materials not conforming to Section 715 of the Specifications shall not be used.
2.Modular Concrete Block Units:
a.Minimum face shell thickness of the modular concrete block units shall be 3 inches (75 mm) except at holes for connection pins. Minimum rear and side s hell thickness shall be 3 inches (75 mm). Block dimensions shall be within 0.125 inch (3 mm) except for height, which shall be within 0.0625 inch (1.5 mm).
b.Cement shall be Type I, II, or III in accordance with ASTM C 150.
c.Aggregates used in concre te blocks shall conform to ASTM C33 for normal weight concrete.
d.At the time of delivery to the work site, the modular block facing units shall conform to the following physical requirements:
1.Minimum required compressive strength of 4,000 psi (average 3 coupons).
2.Minimum required compressive strength of 3,500 psi (individual coupon).
3.Minimum oven dry unit weight of 125 pcf. (4) Minimum water absorption of 5% after 24 hours. (5) Maximum number of blocks per lot of 2,000. Tests on blocks shall be submitted at the frequency of one set per lot.
e.Acceptance of the concrete block, with respect to compressive strength, water absorption and unit weig ht will be determined on a lot basis. The lot shall be randomly sampled and tested in accordance with ASTM C 140. At no additional expense to the Department, the manufacturer shall perform the tests at an Agency approved laboratory and submit the results to the Engineer for approval. Compressive strength test specimens shall be cored or shall conform to the saw-cut provisions of ASTM C 140. Block lots represented by test c oupons that do not reach an average compressive strength of 4,000 psi will be rejected.
f.The units shall be tested to demonstrate freeze-thaw durability in accordance with Test Method ASTM C 1262. Freeze-thaw durability shall be based on tests from five specimens made with the same materials, concrete mix design, manufacturing pr ocess, and curing method, conducted not more than 18 months prior to de livery. Specimens used for absorption testing shall not subsequently be used for freeze-thaw testing. Specimens shall comply with either or both of the following acceptance criteria depending on the severity of the pr oject location as determined by the Engineer:
1.The weight loss of four out of five specimens at the conclusion of 150 cycles shall not exceed 1% of its initial weight when tested in water.
2.The weight loss of each of four out of five test specimens at the conclusion of 50 cycles shall not exceed 1.5% of its initial mass when tested in a saline (3% sodium chloride by weight) solution. Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.03 - 609 - g. Units that indicate flawed molding, honeycomb or open texture concrete and color variation on front face of block due to excess form oil or other reasons shall be rejected. All unit s shall be visibly efflorescence free. All units shall be sound and free of cracks or other defects that would interfere with the proper pl acing of the unit or significantly impair the strength or permanence of the construction. At the discretion of the Engineer, minor cracks no greater than 1/50 inch in widt h and no longer than 25% of the unit height are considered incidental to the manufacturing method or minor chipping resultant from shipping and de livery, are not grounds for rejection.
h.If pins are required to align modular block facing units, they shall consist of a non-degrading polymer or hot-dipped galvanized steel and be made for the express use with the modular block units supplied. Connecting pins shall be capable of ho lding the geogrid in the proper design position during backfilling.
i.The cap unit connection to the block unit immediately under it shall be of positive interlocking type and not frictional. Cap units shall be cast to or attached to the top of modular block facing units in strict accordance with the requirements of the manufac turer of the blocks and the adhesive. The surface of the block un its under the cap units shall be clear of all debris and standing water before the approved a dhesive is placed. The contractor shall provide a written 10-year warranty, acceptable to the Department, that the integrity of the materials used to attach the cap blocks will preclude separation and displaceme nt of the cap blocks.
3.Soil Reinforcements and Attachment Devices: a. Ladder and mesh reinforcing st rips shall be shop fabricated of cold drawn steel wire conforming to the minimum requirements of AASHTO M 32 and welded into the finished strip configuration in accordance with AASHTO M 55. Galvanizing shall confo rm to the minimum requirements of AASHTO M 111 (ASTM A 123).
b.Geostrap reinforcements shall be fabricated of high tenacity polyester yarns woven into a strap configuration and coated while under tension with polyvinylchloride (PVC).
c.Geogrid reinforcements shall be of the type and size designated in the approved contract. Polyester geogrids shall be fabricated of high tenacity polyester yarns woven into a geogrid structure. The geogrid shall be coated with either polyvinylchloride (PVC) or latex. Polyolefin geogrids shall be fabricated of high density polyethylene (HDPE) resin and shall be extruded into a geogrid configuration.
d.Connector pins shall be fabr icated from cold drawn steel wire conforming to the requirements of AASHTO M 322 and shall be galvanized in accordance with AASHTO M 111.
e.U-shape reinforcement connectors used as yokes to connect geostrap reinforcements to modular blocks shall be shop fabricated of cold drawn steel wire conforming to the minimum requirements of AASHTO M 32. Geostrap connector plates shall be sh op fabricated of hot rolled steel conforming to the minimum requiremen ts of ASTM A 1101, Grade 50, or equivalent. Galvanizing shall confo rm to the minimum requirements of AASHTO M 111. Mechanically Stabilized Earth (MSE) Walls
715.03 with Modular Block Facing Units
- 610 - f. Fiberglass alignment pins shall be fabricated of thermoset polyester resin and shall be of a di ameter and length recommended by the wall manufacturer.
4.Backfill:
a.Reinforced backfill and modular block drainage fill shall be select granular backfill materials free from organic and otherwise deleterious materials as defined by Section 1033.01, and shall conform to the gradation limits as determined by AASHTO T 27 and shown in Table 715.02. Table 715.02 Modular Block Select Granular Backfill Gradations Reinforced Backfill Modular Block Drainage Fill Sieve Size Percent Passing Sieve Size Percent Passing 3/4 inch No. 40 100 0 - 60 3/4 inch (19 mm) 100 No. 200 0 - 15 No. 200 (75 µm) 0 - 5
b.Unit (core) fill is defined as free-draining, coarse grained material that is placed within the empty cores of the modular block facing units. Unit (core) fill shall be a well graded crushed stone or granular fill meeting the gradation requirements show n in Table 715.03. Gradation for unit fill shall be tested at t he frequency of 1 test per 50 yd3 at the job site and for every change in the material source. Table 715.03 Gradation for Unit (Core) Fill
U.S. Sieve Size Percent Passing 1 1/2-inch 100 1-inch 75-100 3/4-inch 50-75 No. 4 0-60 No. 40 0-50 No. 200 0-5
c.In addition, the backfill material must conform to all of the following additional requirements:
1.The Plasticity Index (P.I.), as determined by AASHTO T 90, shall not exceed 6.
2.The material, when compacted to 95% of maximum density as determined by NDOT T 99 at optimum moisture content, shall exhibit an angle of internal friction of not less than 34 degrees as determined in a standard direct shear test (AASHTO T 236) or, if the contract allows, at the lowered minimum friction angle required by the wall system supplier.
d.Select granular backfill shall also conform to the following criteria:
1.Soundness -- The materials shall be free of shale or other soft, poor durability particles, and shall have a magnesium sulfate soundness loss of less than 30% after 4 cycles, as determined by Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.03 - 611 - AASHTO T 104. Alternatively, the material shall have a soundness loss of 15% or less when tested in accordance with AASHTO T 104 using a sodium sulfate solution with a test duration of five cycles.
2.The backfill material shall conform to the electrochemical requirements in Table 715.04 for use with metallic soil reinforcement and Table 715.05 when geosynthetic soil reinforcement is used. For all soil reinforcements, the organic content of backfill shall be less than 1%, determined in accordance with AASHTO T 267. Table 715.04 Electrochemical Requirements for Metallic Reinforcements Property Requirement Test Method Resistivity Minimum 3000 ohm-cm, at 100% saturation AASHTO T 288 ph Acceptable Range 5-10 AASHTO T 289 Chlorides Maximum 100 ppm AASHTO T 291 Sulfates Maximum 200 ppm AASHTO T 290 Table 715.05 Electrochemical Requirements for Geosynthetic Reinforcements Base Polyme r Property Requirement Test Method Polyolefin (PP and HDPE)* pH >3 AASHTO T 288 Polyester pH >3 and <9 AASHTO T 289 * PP: Polypropylene and HDPE: High Density Polyethylene
3.Electrochemical Requirements -- The backfill material shall have a minimum resistivity of 3000 ohm-cm at 100% saturation when tested in accordance with ASTM G 57. In addition, the pH of the backfill material shall be in the range of 5 to 10 as determined in accordance with ASTM G 51. The maximum soluble salt content of the reinforced backfill material shall not exceed 100 ppm chlorides and 200 ppm sulfates as determined in accordance with ASTM D 512 and ASTM D 516, respectively. For polymeric materials, the requiremen ts for resistivity, chlorides, and sulfates shall be waived. All other backf ill requirements of this Subsection shall be met.
4.Chloride and sulfate content shall be determined following the required ASTM test method. Howe ver, in each method, the select granular backfill material shall be prepared for testing by first accomplishing the following extraction procedure. Dry the sample material in an oven at 110°C for 8 hours. Measure 100 g of the material and transfer to a 500 mL Erlenmeyer flask. Add 300 mL of dist illed water and shake the mixture for 5 minutes. Repeat the shaking after 1 hour. Allow the mixture to settle for 8 hours. Vacuum filter the liquid layer through a filter apparatus containing a No. 42 Whatman filter paper. Pour the rema ining solid material into the filter paper without the use of an additional water rinse. Reserve the filtrate for testing.
5.The material, when compacted to 95% of maximum density, as determined by NDOT T 99, at optimum moisture content, shall Mechanically Stabilized Earth (MSE) Walls
715.03 with Modular Block Facing Units
- 612 - exhibit an angle of internal friction of not less than 34 degrees as determined in a standard direct shear test (AASHTO T 236).
e.Sampling and Testing:
1.The frequency of sampling of select granular backfill necessary to assure gradation control throughout construction shall be as directed by Department’s Materials and Sampling Guide or as directed by the Engineer. The Contractor shall furn ish to the Engineer a Certificate of Compliance certifying that the select granular backfill material complies with this Section of the Specifications .
2.A copy of all test resu lts performed by the Contractor, which includes: AASHTO T 27, AASHTO T 90, NDOT T-99, AASHTO T 104, AASHTO T 236, AASHTO T 288, AASHTO T 289, AASHTO T 290, and AASHTO T 291, shall also be furnish ed to the Engineer 30 days prior to beginning construction.
3.The construction of MSE Wall structures shall not begin until the Certifications of Compliance for Select Granular Backfills has been reviewed and verified by the Department.
4.Representative samples of select granular backfill shall be sampled in accordance with Department’s Materials Sampling Guide and submitted to the Department.
5.The direct shear test specimen shall then be fabricated and tested at 95% compaction of NDOT T-99 at optimum moisture based on the moisture density relationship. Adequat e moisture content of the backfill is necessary in the field to prevent pos t-construction settlements. The Select Granular Backfill Internal Friction Angle required is 34 degrees according to AASHTO T 236 on samples compacted to 100% of the maximum density according of NDOT T 99. If the contra ct allows, the vendor may utilize a friction angle less than 34 degrees, this lower value shall be taken as the minimum friction angle required for this specific wall location. The vendor shall provide to the Department, certif ied documentation stating the design parameters are acceptable for the in ternal stability of the MSE Wall.
6.The direct shear tests performed shall be based on wall heights for each specific project. All sele ct granular backfill submittals from the contractor shall be from tests made specifically for each project. No test data from previous or multiple projects will be allowed.
7.The direct shear test is a drained test and therefore shall be sheared at a rate between 0.004 to 0.008 in/min. The rate of shear shall also be noted on the direct shear test report. Testing procedures that do not follow these guidelines will not be accepted.
f.A minimum width of 1-ft of gravel fill shall be provided behind solid (non-hollow) modular block units. A minimum volume of 1-ft3/ft2 of drainage fill shall be provided. Gravel fi ll shall meet the requirements of the unit (core) fill. A suitable geotextile fabric between the gravel fill and reinforced fill shall be used to meet the f iltration requirements if the gravel fill does not meet the filtration criteria. The selection of a suitable geotextile for filtration purposes shall be supported by design computations taking in to account the actual gradations of the gravel fill and reinforced wall fill to be Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.04 - 613 - used on the project. Gradation for gravel fill shall be tested at the frequency of 50 yd3 at the job site and for every change in the material source.
g.A certification of analysis fo r modular block connection shall be provided with detailed calculations according to AASHTO (2007) and the results of laboratory test results performe d in accordance with Section C.3 in Appendix B of FHWA NHI-10-025, dat ed 2009 (“Mechanically Stabilized Earth Walls and Reinforced Slopes – Vo lume II”). Such ce rtification shall demonstrate that all connections, including block-to-reinforcement and block-to-block connections, and all re lated components meet or exceed the current AASHTO 75 year design life requirements and are capable of resisting 100% of the maximum tensi on in the soil reinforcements at any level within the wall. Long-term connection testing for extensible reinforcements is also required. The effect of wall batter and normal pressures representative of the full ra nge of wall configurations and heights shall be incorporated in the tests.
h.The Contractor shall furnish to the Engineer a Certificate of Compliance certifying that the select granular backfill materials comply with this section of the Specifications. A c opy of all test results performed by the Contractor which are necessary to assure compliance with the Specifications shall also be furnished to the Engineer. Backfill not conforming to this Specification shall not be used without the written cons ent of both the Engineer and the wall supplier. The frequ ency of sampling of select granular backfill necessary to assure gradation control throughout construction shall be as directed by the Engineer.
715.04 Construction Methods
1.Wall Excavation: Excavation shall be in accordance with the requirements of the specif ications and in reasonably close conformity with the limits shown in the contract.
2.Foundation Preparation: In t he absence of specific ground improvement requirements in the contract, the following applies:
a.The foundation for the reinforced fill and retained backfill shall be graded level for the entire area of the base of such backfills, plus an additional 12 inches on all sides, or to the limits as shown in the contract.
b.Before wall construction, the foundation, if not in rock, shall be compacted in accordance with Subsection 205.03, Paragraph 14.b.(3). Any foundation soils found to be unsuit able shall be removed and replaced as directed by the Engineer.
3.Proof Rolling a. The contractor shall perfo rm proof-rolling to evaluate the stability and uniformity of the subgrade on which the MSE structure will be constructed. Proof rolling shall be performed on the entire areas at the following locations:
1.At the bottom of the over excavation and recompaction zones; if specified in the contract.
2.At the bottom of the over excavation and replacement zones, if specified in the contract. Mechanically Stabilized Earth (MSE) Walls
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- 614 - (3) At the base of all walls or at the top of native soil.
b.Proof-rolling shall be done immediately after subgrade compaction while the moisture content of the subgrade soil is near optimum, or at the moisture content that was used to achieve the required compaction.
c.Proof-rolling shall not be used within 1 1/2 feet of any underdrains.
d.Proof-rolling shall be perfo rmed on the exposed surface with a loaded dump truck or similar piece of construction equipment. Proof-rolling shall be performed in the presence of the Engineer. Any unstable material should be a removed and replaced.
1.Proof-rolling equipment shall have an axle weight of 20,000 for a single axle or 34,000 for tandem axle.
2.Any deformations greater than 1 1/2 inches after one pass as measured with a 10 foot straig htedge will be considered unstable.
e.The contractor shall be responsible for maintaining the condition of the approved proof-rolled soils throughout t he duration of the MSE wall construction. Wall construction shall not commence until the foundation has been approved by the Engineer.
4.a. At each foundation level, an unreinforced concrete or a compacted granular leveling pad shall be provided as shown in the contract. The leveling pad shall be constructed to the design elevations shown on the working drawings. Allowable elevation tolerances are + 1/8 inch (3 mm) and - 1/4 inch (6 mm) from the design elevation.
b.The concrete leveling pad shall be cured a minimum of 24 hours before placement of wall panels. A geotextile shall be applied over the back of the area of any openings between the facing units and leveling pad steps. The geotextile shall extend a minimum of six (6) inches beyond the edges of the opening. The opening shall be filled with concrete, conforming to Subsection 4a, or sha ll be concurrently backfilled on both sides of the wall.
5.Prior to wall erection, the c ontractor shall install a subsurface drainage system as shown on the working drawings.
6.Erection of walls with Modular Block Units shall be as per the following requirements:
a.Vertical and horizontal alignment of the wall face shall not vary by more than 3/4-inch when measur ed along a 10-foot straightedge.
b.Overall vertical tolerance (plumbness) of the wall shall not exceed 1 1/4-inch per 10-ft of wall hei ght from the final wall batter. Negative (outward) batter is not acceptable.
c.The first row of units shall be level from unit-to-unit and from front to back. Use the tail of the units for alignment and measurement.
d.All units shall be laid snugly together and parallel to the straight or curved line of the wall spanning the joint in the row below (running bond). Shimming or grinding shall control the elevations of any two adjacent blocks within 1/16 inch. Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.04 - 615 - e. The top of blocks shall be che cked with a 3 foot long straight edge bubble level. Any high points identified by the straight edge shall be ground flat. Block front to back tilting shall be checked frequently. Correction by shimming shall be done no later than 3 completed courses.
f.Wall sections not conformi ng to these tolerances shall be reconstructed at no additional cost to the Department.
7.Placement of Metallic Reinforcement Elements: a. Before placing the first layer of reinforcements (strips or mats), backfill shall be placed and compacted.
b.Bending of reinforcements in the horizontal plane that results in a permanent deformation in their alignment shall not be allowed.
c.Connections of reinforcement to piles or bending of reinforcements around piles shall not be allowed. Cutting of longitudinal or transverse reinforcement bars to avoid conflicts with piles or utility obstructions shall not be allowed.
d.A structural connection (y oke) from the wall panel to the reinforcement shall be used whenever it is necessary to avoid cutting or excessive skewing of reinforcements due to pile or utility conflicts.
e.Soil reinforcements shall be placed normal to the face of the wall, unless otherwise shown in the contract or directed by the Engineer. If skewing of the soil reinforcements is required due to obstructions in the reinforced fill, rotatable bolted c onnections shall be used and the maximum skew angle shall not exceed 15 degrees from the normal position except in the case of acute corners where redundant reinforcements are used.
f.The contractor shall install joint pads and fillers as shown on the working drawings.
8.Placement of Geos ynthetic Reinforcement:
a.Geosynthetic reinforcement shall be installed in accordance with the manufacturer’s site-spec ific wall erection instructions.
b.Geosynthetic reinforcem ent shall be placed in continuous longitudinal rolls in the direction of t he main reinforcement. Joints parallel to the wall shall not be permitted, except as shown on the working drawings.
c.Reinforcement coverage shall be 100% of embedment area unless otherwise shown in the working drawings. Adjacent sections of geosynthetic reinforcement need not be overlapped.
d.Geosynthetic reinforcement shall be placed to lay flat and pulled tight prior to backfilling. After a layer of geosynthetic reinforcement has been placed, suitable means, such as pins or small piles of soil, shall be used to hold the geosynthetic reinforcement in place until the subsequent soil layer can be placed.
e.During construction, the su rface of the fill shall be kept approximately horizontal. Geosynthetic reinforcem ent shall be directly placed on the compacted horizontal fill su rface. The reinforcement shall bear uniformly on the compacted reinforced soil from the connection to the wall to the free end of the reinforcing el ements. The reinforcement placement Mechanically Stabilized Earth (MSE) Walls
715.04 with Modular Block Facing Units
- 616 - elevation shall be at the connection elevation to 2 inches higher than the connection elevation.
9.Backfill Placement:
a.Backfill placement shall closely follow erection of each course of facing blocks. Backfill material placed within the reinforced soil mass which does not meet the requiremen ts of this Specification shall be corrected or removed and replaced at no additional cost to the Department as directed by the Engineer. Backfill shall be placed in such a manner as to avoid any damage or disturbance to t he wall materials or misalignment of the facing panels. The contractor shal l place backfill to the level of the connection and in such a manner as to ensure to no voids exist directly beneath the reinforcing elements.
b.Any wall elements which become damaged during backfill placement shall be either repaired or replaced by the Contractor at no additional cost to the Department. Any wall elements which become disturbed during backfill placement sha ll be reset by the Contractor at no additional cost to the Department.
c.For metallic rein forcements, the fill shal l be spread by moving machinery parallel to or away from the wall facing and in such a manner that the steel reinforcement remains normal to the face of the wall. Construction equipment shall not operate directly on the steel reinforcement. A minimum fill thickness of 3 inches over the steel reinforcement shall be required prior to operation of vehicles. Sudden braking and sharp turning shall be avoided.
d.For geosynthetic reinforcements, the fill shall be spread by moving machinery parallel to or away from the wall facing and in such a manner that the geosynthetic reinforcement remains taut. Construction equipment shall not operate directly on the geosynthetic reinforcement. A minimum fill thickness of 6 inches over the geosynthetic reinforcement shall be required prior to operation of vehicles. Sudden braking and sharp turning shall be avoided.
e.Backfill shall be compacted to 95% of the maximum density as determined by NDOT T 99.
f.The moisture content of t he backfill material before and during compaction shall be uniform throughout each layer.
g.Backfill material shall have a placement moisture content less than or equal to the optimum moisture content.
h.Backfill material with a placement moisture content in excess of the optimum moisture content sha ll be removed and reworked until the moisture content is uniform and acce ptable throughout the entire lift.
i.The optimum moisture co ntent shall be determined in accordance with NDOT T 99.
j.The maximum lift thickness after compaction shall not exceed 10 inches (250 mm), regardless of the ve rtical spacing between layers of soil reinforcements. The Contractor shal l decrease this lift thickness, if necessary, to obtain the specified density. Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.05 - 617 - k. Before placement of the soil reinforcements, the reinforced backfill elevation, after compaction, shall be 2 inches (50 mm) above the connection elevation from a point approximately 1 foot (300 mm) behind the back face of the facing blocks to t he free end of the soil reinforcements, unless otherwise shown in the contract. Modular block drainage fill shall be placed and tamped directly behind, betw een, and within the cells of the facing units. Granular fill shall not be pl aced within the holes provided for the connection pins. Compaction of modular block fill shall be achieved by hand tamping or rodding.
l.Compaction within 40 inches (1 m) of the back face of the facing blocks shall be achieved by at least 3 passes of a lightweight mechanical tamper, roller, or vibratory system. The specified lift thickness shall be adjusted as warranted by the type of compaction equipment actually used, but no soil density tests need be taken in this area.
m.Care shall be exercised in the compaction process to avoid misalignment of t he facing blocks.
n.At the end of each day's ope ration, the Contractor shall slope the last level of backfill away from the wall facing to direct runoff of rainwater away from the wall face. In addition, the Contractor shall not allow surface runoff from adjacent areas to enter the wall construction site.
10.When shoring for phased construction is shown in the contract, the Contractor shall design and install t he shoring. The type of shoring used shall be determined by the Contractor. Design calculations for the shoring shall be prepared by a Professional E ngineer registered in the State of Nebraska. Before construction begins, 4 sets of the calculations and shoring plans shall be submitted to the Department for informational purposes only. The Contractor is solely responsible fo r the construction and performance of the shoring. All shoring shall remain in place and shall be cut off 2 feet (600 mm) below finish grade.
11.The wall shall be constructed according to the working drawings. Any field changes shall be reviewed by the engineer to determine if a revised working drawing is required.
715.05 Method of Measurement
1.The quantity of excavation for t he leveling pads is not measured for directly.
2.The quantity of “Excavation for Soil Reinforcement” is measured by the cubic yard (cubic meter) and computed using the contract dimensions. No adjustment in the pay quantity wi ll be made if the computed quantity, based on the working drawings, vari es from the contract quantity.
a.The quantity shown in the c ontract is calculated using the average end area method based on prel iminary cross sections of the existing ground and the calculated final cross sections. The excavation width limit is equal to 70% of the height of the wall plus the width required to step back the excavation as shown in Figure 715.01.
b.Additional “Excavation for Soil Reinforcement” may be needed to adjust the length of the wall to fit fi eld conditions. Overruns of “Excavation for Soil Reinforcement”, beyond that required from the original shop Mechanically Stabilized Earth (MSE) Walls
715.05 with Modular Block Facing Units
- 618 - drawings, will be measured by the cubic yard (cubic meter). The additional quantity is calculated using the meth od in Section 715.05 2. a. for the additional length.
c.Preliminary cross se ctions will include new fill if it is a required surcharge. EXCAVATION CALCULATION LIMITS EXCAVATION DEPTH 0.7 H. + 19 Ft (5.75 m) . 19.0 Feet or Greater (5.75 m) 0.7 H. + 14 Ft (4.25 m) . 14.0 Feet to 19.0 Feet (4.25 m to 5.75 m) 0.7 H. + 9 Ft (2.75 m) . 9.0 Feet to 14.0 Feet (2.75 m to 4.25 m) 0.7 H. + 4 Ft (1.2 m) . 4.0 Feet to 9.0 Feet (1.25 m to 2.75 m) 0 Feet to 4.0 Feet (0 m to 1.25 m) 0.7 H. of wall Figure 715.01
3.Dewatering, replacement of unsuitable soil or other unforeseen conditions will be paid as Extra Work described in SSHC Section 104.
4.The quantity of wall material s is measured by the square foot (square meter) and computed using the plan dimensions. The contract dimensions will include the envelope above the natural ground surface and a fixed area below the natural ground lin e a minimum of 2 foot (600 mm) for the length of the wall unless otherwise shown on the contract. No adjustment in the pay quantity will be made if the computed quantity, based on the working drawings, varies from the cont ract quantity. The wall surface area, as shown in the contract, includes wall penetrations such as pipes and other utilities.
a.Additional wall material may be needed to adjust the contract dimensions to fit field conditions. Overruns of wall material, beyond that required from the original shop drawin gs, will be measured by the square foot (square meter) of each additional block used.
5.The quantity of compacted earth le veling pads is measured by the linear foot (meter) and is computed using the contract dimensions. No adjustment in the pay quantity will be made if the computed quantity, based on the working drawings, varies from the contract quantity. Mechanically Stabilized Earth (MSE) Walls with Modular Block Facing Units 715.06 - 619 - a. Additional compacted earth leveling pads may be needed to adjust the contract dimensions to fit field conditions. Overruns of compacted earth leveling pads, beyond that required from the original shop drawings, will be measured by the linear foot (meter).
6.The quantity of granular backf ill is measured by the cubic yard (cubic meter) and is computed using the contract dimensions. The "Select Granular Backfill for Retained Earth Struct ures" shall include both reinforced backfill and modular block/drainage fill. No adjustment in the pay quantity will be made if the computed quantity, bas ed on the working drawings, varies from the contract quantity. The quantity shown in the contract is based on a volume equal to the height of the wall times the length of the wall times a width equal to 70% of the height.
a.Additional granular backfill may be needed to adjust the contract dimensions to fit field cond itions. Overruns of granular backfill, beyond that required from the original shop drawings, shall be measured by the cubic yard (cubic meter). The quantity is based on a volume equal to the area of the additional blocks time s 70% of the height of the wall.
715.06 Basis of Payment
1.P a y I t e m P a y U n i t Excavation for Soil Reinforcement Cubic Yard (CY) [ C u b i c M e t e r ( m3)] Wall Materials Square Foot (SF) [Square Meter (m2)] Compacted Earth Leveling Pad Linear Foot (LF) [Meter (m)] Select Granular Backfill for Cubic Yard (CY) Retained Earth Stru cture [Cubic Meter (m3)]
2.Excavation for the leveling pads will not be measured and paid for separately, but shall be subsidiary to the appropriate pay item requiring the excavation.
3.Revised Working Drawing or Shop Drawing will be paid for as extra work.
4.Additional quantities of Wall Materials, Compacted Earth Leveling Pad, and Select Granular Backfill for Re tained Earth Structure required due to field changes will be paid at contract unit prices.
5.Payment is full compensation for all work described in this Section. - 620 - SECTION 716 -- STEEL RAILINGS
716.01 Description
1.This work shall consist of fu rnishing, erecting and painting steel handrail, bridge rail, rail elements, rail posts, barrier rail, pedestrian railing, and all connecting bolts and anchors in accordance with the requirements of the contract.
2.The plans shall be supplemented by working drawings submitted in accordance with the requirem ents of Subsection 105.02.
716.02 Material Requirements
1.The materials shall conform to the requirements in Table 716.01. Table 716.01 Material Requirements Applicable Materials Section Structural Steel ........................................... 1045 Structural Threaded Fasteners ................... 1057, 1058 Steel Pipe for Rails and Posts .................... ASTM A 53, Grade B Chain Link Fence and Hardware ................ 1064 Steel Tubing ............................................... ASTM A 500, Grade B
2.Anchor bolts shall meet the r equirements of Section 1058 unless otherwise specified in the contract. The bolt projection shall be galvanized.
716.03 Construction Methods
1.The Contractor shall build raili ng to the lines, grades, and vertical and horizontal curves shown in the contract.
2.The Contractor shall set rail posts plumb. Concrete areas upon which railing posts are to be set shall be dressed to provide even bearing.
3.The Contractor shall paint a ll exposed metal except galvanized and weathering grade steel as described in Section 709.
4.The Contractor shall repair galvanized coating which has been scarred, burned, or otherwise damaged in accordance with Section 1061 at no additional expense to the Department.
716.04 Method of Measurement
1.Structural steel for handrails and bridge rails is measured by the pound (kilogram).
2.Pedestrian barrier rail, pedestr ian handrail, and pedestrian railing (chain link type) shall be measured by the foot (meter). The quantity to be paid will be the distance in feet (meters ) between centers of end posts, with no allowance for splices, joints, overhangs, or end sections.
3.The Department computes plan quantities from end post to end post, measured horizontally, with no adjustments made for changes in elevation.
Source: Nebraska Standard Specifications for Highway Construction, 2017 Edition. Pages 637–653 of 1,048.