- 585 - SECTION 714 -- MECHANICALLY STABILIZED EARTH (MSE) WALLS WITH CONCRETE FACING PANELS
714.01 Description
1.This work shall consist of designing, furnishing materials, excavation, and constructing mechanically stabilized earth (MSE) walls with concrete facing panels in accordance wi th these Specifications and with the lines, grades, dimensions, and deta ils 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 newly constructed fill. The contractor shall excavate to the dimensions shown in the working drawings. The contractor shall also follow the manufacturer’s recommendations with concurrence of the Department .
3.The MSE walls shall consist of a nonstructural leveling pad, concrete face panels, and soil reinforcement elements mechanically connected to each facing panel. Soil reinforcement shall have sufficient strength, frictional resistance, and l ength as required by the design and as outlined in these Specifications .
4.Only the approved proprietary mechanically stabilized earth retaining wall systems that 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 c ontract will locate the theoretical leveling pad elevation. The minimum wall embedment below the finished ground surface shall be 2 feet (600 mm) or as shown in the contract. The top of the face panels shall be at or abov e the top of the p anel elevation shown in the contract. Where coping or barrier is used, the wall face panel shall extend up into the coping or barrier a minimum of 2 inches (50 mm). The top of the face panels may be level or slop ed to meet the t op of the wall line noted. Cast-in-place concrete will be allowed for minor grouting of pipe penetrations and leveling required for coping or traffic barrier.
7.Where walls or wall sections intersect at an angle of 130 degrees or less, a special vertical corner elem ent panel shall be used. The corner element panel shall cover the joint of the panels that abut the corner and allow for independent movement of the abutting panels.
8.The face panels shall be designed to accommodate differential settlements along the length of the wa ll and normal 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 the expected differential settlements normal to the wall exceed 3 inches (75 mm), the lower level reinforcement facing connections shall be designed to accommodate the increased 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. Mechanically Stabilized Earth (MSE) Walls
714.01 with Concrete Facing Panels
- 586 - 9. 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 numbered panel layout for fabrication and erection purposes, as we ll 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 panels.
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) 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 provi de 2 foot (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.
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 contractor’s MSE wall design details and drawings by the Engineer. Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.02 - 587 - 714.02. -- Design Requirements
1.The design, by the wall system supplier, shall include 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:
a.FHWA NHI-10-024 Vol I and NHI-10-025 Vol II, “Design of MSE Walls and Reinforced Slopes,” (Berg et al., 2009);
b.Latest edition of AASHTO LRFD Bridge Design Specifications including Interims.
1.The mechanical wall height for the purposes of design calculations shall be from the top of t he leveling pad to the top of the ground surface where the potential failure su rface intercepts the ground surface. The Contractor is solely responsible for the satisfactory construction and performance of the wall and the internal stability of the wall’s retained mass. The Contractor shall submit a certification to the D epartment that the wall is designed in accordance with the curre nt AASHTO LRFD Specifications for Highway Bridges.
2.The Engineer shall indicate in the contract the "external site factors" which include:
a.Settlement both along and perpendicular to the MSE structure alignment.
b.Allowable bearing capa city of the foundation soil.
c.External drainage beneath and behind the MSE volume. d. The design parameters for the foundation soils (cohesion and friction angle).
1.Global Stability
3.Maximum reinforcement loads shall be calculated using the method presented in the current AASHTO and as per the requirements specified herein. No other design method will be allowed. Unless otherwise specified in the contract, all structures s hall be designed to conform to the requirements provided in the current edition of AASHTO LRFD Bridge Design Specifications.
4.When a highwater surface elevatio n is shown in the contract at the wall face, the design stresses calculated from that elevation to the bottom of the wall must include a minimum differential hydrostatic pressure equal to 40 inches (1 m) of water. Effective unit weights from the saturated soil condition shall be used to calculate external and internal stability.
5.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 Mechanically Stabilized Earth (MSE) Walls
714.02 with Concrete Facing Panels
- 588 - 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.
6.The optimum moisture content shall be determined in accordance with NDOT T 99.
7.All structural connections shall be subject to the same metal loss rates and allowable tension requirement s as outlined in this Subsection.
8.The soil reinforcement shall be the same length from the bottom to the top of the wall. The reinforcement length defining the width of the entire reinforced soil mass may vary with wall height. The minimum length of the soil reinforcement shall be 0.7H for walls with level surcharges, or 0.7H 1 for walls with a sloped surcharge or supporting an abutment. The mechanical height, H 1, shall be the vertical difference between the leveling pad and the elevation at which the failure surface, as described above, intercepts the ground surface supported by the wall.
9.Steel Reinforcement: a. Steel reinforcement shall be designed for a minimum service life of 75 years unless otherwise indi cated 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
b.The allowable tensile st ress in steel reinforcement and connections including tie strips and loop inserts, Ft, at the end of the service life, shall conform to the following:
1.Systems using linear reinforcement (strips): ( i ) F t = 0.55 F y at the reduced gross section (minimum cross section). ( i i ) Ft = 0.50 F u at the net section at a bolt hole (applicable to bolted connections only).
2.Systems with bar mats or welded wire mesh: F t = 0.47 F y at all sections.
c.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. Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.03 - 589 - 714.03 -- Material Requirements
1.The Contractor shall make ar rangements to purchase the material covered by this Section of the Specifications , including concrete panels, reinforcing mesh or strips, attachment devices, fasteners, joint materials, and all necessary incidentals from t he wall system supplier or suppliers approved by the wall system supplier. The Contractor shall furnish the Engineer a Certificate of Compliance cert ifying that all materials comply with Section 714 of the Specifications . Materials not conforming to Section 714 of the Specifications shall not be used.
2.Concrete Facing Panels:
a.Concrete facing panels shall have a minimum thickness of 5 1/2 inches (140 mm) and a minimum concrete cover on reinforcing steel of 1 1/2 inches (38 mm). Ceme nt shall be Types I, II, or III and shall conform to the requirements of AASHTO M 85. C oncrete shall have a compressive strength at 28 days as described in this Subsection. Additives containing chloride shall not be used without the ap proval of the Engineer. Attachment devices, connecting pins, PVC pipe, and lifting devices shall be set in place to the dimensions and tolerances shown on the working drawings and called out in these Specificat ions before casting.
b.Testing and Inspection: Accept ability of the precast units shall be determined on the basis of comp ressive strength tests and visual inspection. The precast units shall be considered acceptable regardless of curing age when compressive strengt h test results indicate that the compressive strength will conform to the 28-day requirements. The Contractor or supplier shall furnish facilities and perform all necessary sampling and testing in an expeditious and satisfactory manner. Panels utilizing Type I or II cement shall be co nsidered acceptable for placement in the wall when the 7-day initial strengt h equals or exceeds 85% of the 28-day strength (4,000 psi) (27.6 MPa).
c.Casting: The panels shall be cast face down in level forms supported on a flat working surface. Guides shall be used to locate and support attachment devices set in the back face of the panel. The concrete in each panel unit shall be placed without interruption and shall be consolidated by the use of an approv ed vibrator, supplemented by such hand tamping as may be necessary to forc e the concrete into the corners of the forms and to prevent the formation of rock pockets or cleavage planes. The same type of clear form oil or release agent shall be used throughout the casting operation.
d.Curing: The units shall be cured for a sufficient length of time so that the concrete will develop t he specified compressive strength.
e.Removal of Forms: The forms shall remain in place until they can be removed without damage to the units.
f.Concrete Finish: Unless otherwise indicated in the contract or elsewhere in the specifications, the concrete surface for the front face shall have an ordinary steel form finish; and for the rear face, an unformed finish. The rear face of the panel shall be free of open pockets of aggregate and surface distortions in excess of 1/4 inch (6 mm). Mechanically Stabilized Earth (MSE) Walls
714.03 with Concrete Facing Panels
- 590 - g. Tolerances: All units shall be manufactured within the following tolerances with respect to the dimensions shown in the working drawings:
1.Attachment Device Locations and Alignment -- Lateral position of reinforcing strip attachment devices shall be within 1 inch (25 mm). Embedment measured from th e back face of the panel shall be within 1/4 inch (6 mm) and -1/2 inch (-0 .12 mm). Bearing surf aces of multiple attachment points for a single soil reinforcing element shall align within 1/16 inch (1.5 mm).
2.Panel Dimensions -- All panel dimensions shall be within 1/4 inch (6 mm). All hardware embedded in the panel, with the exception of attachment devices, shall be within 1/4 inch (6 mm).
3.Panel Squareness --, as determined by the difference between the 2 diagonal s, shall not exceed 1/2 inch (13 mm).
4.Panel Surface Finish -- Surface defects on smooth- formed surfaces, measured on a length of 5 feet (1.5 m), shall not exceed 1/4 inch (6 mm). Surface defects on te xtured-finished surfaces, measured on a length of 5 feet (1.5 m), s hall not exceed 5/16 inch (8 mm).
h.Compressive Strength: Acc eptance of the concrete panels, with respect to compressive strength, shall be determined on the basis of production lots. A production lot is defined as a group of panels that shall be represented by a single set of compressive strength samples and shall consist of not more than 40 panels or a single day's production, whichever is less.
i.Compressive strength test s shall be performed on prepared cylinders in accordance with AASHTO T 23. During the production of the concrete panels, the manufacturer shall randomly sample the concrete in accordance with AASHTO T 141. A single set of compressive strength samples, consisting of a minimum of 4 cylinders, shall be made for every production lot.
j.For every compressive strength sample, a minimum of 2 cylinders shall be cured in the same manner as the panels and tested at 7 days. The average compressive strength of these cylinders, when tested in accordance with AASHTO T 22, will dete rmine the initial strength of the concrete. In addition, a minimum of 2 cylinders shall be cured in accordance with AASHTO T 23 and tested at 28 days. The average compressive strength of these cylinders, when te sted in accordance with AASHTO T 22, will determine the compressive strength of the production lot.
k.If the initial strength test results indicates a compressive strength greater than or equal to 4,000 psi (27.6 MPa), then this test result will be utilized as the compressive streng th test result for that production lot and the requirement for testing at 28 days will be waived for that particular production lot. Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.03 - 591 - l. Acceptance of a production lot will be made if the 28-day compressive strength test result is greater than or equal to 4,000 psi (27.6 MPa). If the 28-day compressive strength test result is less than 4,000 psi (27.6 MPa), the acceptance of the production lot will be based on its meeting the following accept ance criteria in its entirety:
1.90% of the compressive st rength test results for all of the production lots shall exceed 3,815 psi (26.3 MPa).
2.The average of any 6 consecutive compressive strength test results, including the one in questi on, shall exceed 3910 psi (27.0 MPa).
3.No individual compressive strength test result shall fall below 3,600 psi (24.8 MPa).
m.The date of manufacture, the production lot number, and the structure component shall be clearly indicated on each cylinder.
n.All units shall be handled, stored, and shipped in such a manner as to minimize the danger of chipping, cracks, fractures, and excessive bending stresses.
1.Panels shall be stored and shipped in stacks, front face down.
2.Firm blocking of sufficient thickness to prevent damage to the stacked panels shall be provided.
3.Lifting inserts shall be installed on the top edge of the precast panels to allow lifting at the project site.
4.Reinforcement connection inserts (tie strips or loop inserts) shall not be used for lifting or handling the panel.
o.Acceptance Criteria: Units s hall be subject to rejection because of failure to meet any of the requirement s specified above. In addition, any or all of the following defects may be sufficient cause for rejection:
1.Defects that indi cate imperfect molding.
2.Defects indicating honeycombed or open-texture concrete.
3.Defects in the physical characteristics of the concrete, such as broken or chipped concrete.
p.The Engineer shall determi ne whether spalled, honeycombed, chipped, or otherwise defective concrete shall be repaired or be cause for rejection.
1.Repair of concrete, if allowed, shall be done in a manner satisfactory to the Engineer.
2.Repair to concrete surfaces which will be exposed to view after completion of construction must be approved by the Engineer.
3.Reinforcement and Attachments:
a.All reinforcing and attachment devices shall be carefully inspected to insure they are true to size and free from defects that may impair their strength and durability.
b.Reinforcing strips shall be hot rolled from bars to the required shape and dimensions. Their physical and mechanical properties shall Mechanically Stabilized Earth (MSE) Walls
714.03 with Concrete Facing Panels
- 592 - conform to ASTM A 572/A 572M Grade 65 minimum. Galvanization shall conform to the minimum requirements of AASHTO M 111 (ASTM A 123 and shall be applied after the strips are fabricated, inclusive of punch holes for bolts as shown on the working drawings. A minimum galvanization coating of 2.0 oz/ft2 (605 g/m2) or 3.4 miles (85µm) thickness is required.
c.Reinforcing mesh and bar mats shall be shop fabricated from cold drawn steel wire conforming to the minimum requirements of AASHTO M 32, and welded into the finished mesh fabric in accordance with AASHTO M 55. Galvanization shal l conform to the minimu m requirements of AASHTO M 111 (ASTM A 123) and shall be applied after the strips are fabricated, inclusive of punch holes for bolts as shown on the working drawings. A minimum galvanization coating of 2.0 oz/ft2 (605 g/m2) or 3.4 mils (85µm) thickness is required.
d.The tie strips shall be sh op fabricated from hot rolled steel conforming to the minimum requirements of ASTM A 1101, Grade 50 or equivalent. The minimum bending radius of the tie strips shall be 3/8 inch. Galvanization shall be applied after the strips are fabricated, inclusive of punch holes for bolts and shall conform to AASHTO M 111 (ASTM A 123).
e.Fasteners shall consist of hexagonal cap screw bolts and nuts which are galvanized and conform to the requirements of AASHTO M 164 or equivalent.
f.Connector bars and pins shall be fabricated from cold drawn steel wire conforming to the re quirements of AASHTO M 32 and be galvanized in accordance with AASHTO M 164.
g.Structural plate connecto rs and fasteners used for yokes to connect reinforcements to wall panels ar ound pile or utility conflicts shall conform to the material requirements fo r reinforcing strips and fasteners in Paragraphs 3.a., 3.d., and 3.e. of this Subsection.
h.Geostrap reinforcements shall be fabricated of high tenacity polyester yarns woven into a strap configuration and coated while under tension with polyvinylchloride (PVC).
i.Geogrid reinforcements shall be of the type and size designated in the approved plans. Poly ester 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.
4.Joints: a. Horizontal and vertical joints between panels shall be covered by a geotextile approved by the wall supplier and meeting the requirements for filtration applications as spec ified by AASHTO M 288. The minimum width of geotextile shall be 1.0 foot. Adhesive used to hold the geotextile filter fabric material to the rear of the facing panels before backfill placement shall be approved by the wall supplier.
b.The back face of vertical and horizontal joints shall be covered with geotextile filter. Joint filler, bearing pads, and geotextile filter shall be as Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.03 - 593 - recommended by the wall manufacturer and shall meet the requirements shown on the approved working drawings.
c.Where the wall wraps around an inside corner, a corner block panel shall be provided with flange extensions that will allow for differential movement without exposing the panel joints.
d.Bearing pads shall meet or exceed the following material requirements:
1.Preformed EPDM (Ethylene Propylene Diene Monomer) rubber pads conforming to ASTM D 2000 Grade 2, Type A, Class A with a Durometer hardness of 70.
2.Preformed HDPE (High Density Polyethylene) pads with a minimum density of 0.946 grams per cubic centimeter in accordance with ASTM D 1505.
3.The stiffness (axial and lateral) size, and number of bearing pads shall be determined such that the final joint opening shall be 3/4 + 1/8 inch unless otherwise shown on the contract. The MSE Wall designer shall submit supportive calculat ions that verify the stiffness (axial and lateral), size, and number of bearing pads assuming, as a minimum, a vertical loading at a given joint equal to 2 times the weight of facing panels directly above that level.
5.Backfill:
a.The select granular backfill material used in the MSE structure shall be free from organic and otherwise deleterious materials as defined by Section 1033 and shall conform to the gradation limits as determined by AASHTO T 27 and shown in Table 714.02. Table 714.02 Facing Panel Select Gra nular Backfill Gradation Sieve Size Percent Passing 4 inch (100 mm)(Note 1) 100 No. 40 (4.74 mm) No. 200 (75 µm) 0 to 60 0 to 15 Note 1: Maximum particle size shall be limited to 3/4 inch for geosynthetics and epoxy- or PVC-coated reinforcement s unless the contractor provides tests, acceptable to the Engineer, that have evaluated the extent of construction damage anticipated for the specific fill material and reinforcements combination. Construction damage testing shall be performed in accordance with the requir ements of Chapter 5 of Publication No. FHWA NHI-09-087, dated 2009 (“Corrosion/Degradation of Soil Reinforcements for Mechanically Stabilized Earth Walls and Reinforced Slopes.”)
b.In addition, the backfill shal l conform to all of the following requirements:
1.The Plasticity Index (P.I.), as determined by AASHTO T 90, shall not exceed 6.
2.The material shall be s ubstantially free of shale or other soft, poor durability particles. The material shall have a magnesium sulfate Mechanically Stabilized Earth (MSE) Walls
714.03 with Concrete Facing Panels
- 594 - soundness loss of less than 30% after 4 cycles, as determined by 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.
3.The backfill material shall conform to the electrochemical requirements in Table 714.03 for use with metallic soil reinforcement and Table 714.04 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 714.03 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 714.04 Electrochemical Requirements for Geosynthetic Reinforcements Base Polyme r Property Requirement Test Method Polyolefin (PP and HDPE)* pH >3 AASHTO T 289 Polyester pH >3 and <9 AASHTO T 289 * PP: Polypropylene and HDPE: High Density Polyethylene
4.Chloride and sulfate content shall be determined by the indicated ASTM test method. However, 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 212°F (100°C) for 8 hours. Measure 100 g of t he 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 exhibit an angle of internal friction of not less than 34 degrees, or if the contract allows a lower friction angle, at the lower friction angle required by the wall system supplier, as determine d in a standard direct shear test (AASHTO T 236).
c.The frequency of sampling of select granular backfill necessary to assure gradation control throughout construction shall be as directed by the Department’s Materials and Sampling Guide or as directed by the Engineer. The Contractor shall furnis h to the Engineer a Certificate of Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.04 - 595 - Compliance certifying that the select granular backfill material complies with this Section of the Specifications .
1.A copy of all test resu lts performed by the Contractor, which includes: AASHTO T 27, AASHTO T 90, AASHTO T 104, AASHTO T 236, AASHTO T 288, AASHTO T 289, AASHTO T 290, and AASHTO T 291, shall also be furnished to the Engineer 30 days prior to beginning construction.
2.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.
3.Representative samples of select granular backfill shall be sampled in accordance with Department’s Materials Sampling Guide and submitted to the Department.
4.For cohesionless soils such as Select Granular Backfill, if the moisture density curve shows a well-defined peak, the maximum dry density and optimum moisture content shall be determined based upon this peak. If no well-defined peak in the moisture density curve is observed, the soil shall be compacted at increasing in tervals of moisture content until the maximum water content that can be retained in the compaction mold is achieved (saturation). The maximum dry density and optimum moisture content shall then be determined based on the point on the curve 1% below the point of saturation. The direct shear test specimen shall then be fabricated and tested at 95% compacti on and at optimum moisture based on the moisture density relationship. Adequat e moisture content of the backfill is necessary in the field to prevent post-construction settlements.
5.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.
6.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.
7.In addition to the Certif ication of Compliance for Select Granular Backfill, a 60 lb sample of the proposed material shall be submitted to Department 30 days prior to construction of the MSE Wall.
8.If the Select Granular Backfill material changes or a new material is used, construction of the MSE Wall shall be halted until the material has been approved by Department.
714.04 Construction Methods
1.Excavation shall be in accordance with the requirements of the contract. Mechanically Stabilized Earth (MSE) Walls
714.04 with Concrete Facing Panels
- 596 - 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. Any foundation soils found to be unsuitable 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,
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 Leveling Pads:
a.At each panel foundation le vel, an unreinforced concrete leveling pad shall be provided as shown in the contract. The leveling pad shall have nominal dimensions of 6 inch (150 mm) thickness and 12 inch (300 mm) width and shall be cast using concrete which achieves a compressive strength of 2,175 psi (15 MPa) in 28 days. The leveling pad shall be cast to the design elevati ons shown on the working drawings. Allowable elevation tolerances are + 1/8 inch (+3 mm) and - 1/4 inch (-6 mm) from the design elevation. Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.04 - 597 - b. The 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 6 inches beyond the edges of the opening. The opening shall be filled with concrete, conforming to Subsection 4a, or shall be concurrently backfilled on both sides of the wall. 5 Erection:
a.For erection of walls with rigid facing, panels shall be handled by means of lifting devices set into the upper edge of the panels. Panels shall be placed in successive horizontal lifts in the sequence shown in the contract as backfill placem ent proceeds. As backfill material is placed behind the panels, the vertical panel joints shall be maintained in a plumb position by means of temporary wooden wedges, clamps, or bracing as recommended by the manufacturer. A minimum of two, but not more than three, rows of panel wedges shall remain in place at all times during wall erection. Wedges shall be removed from the lower rows as panel erection progresses. The contractor shall repa ir any damages to erected concrete panels as directed by the Engineer and to the Engineer’s satisfaction. No external wedges in front of the wall shall remain in place when the wall is complete. External bracing is required for the initial lift.
b.Erection of walls with panel facing shall be in accordance with the following tolerances at the time of wall construction:
1.Vertical and horizontal ali gnment of the wall face shall not vary by more than 3/4 inch when measured along a 10-foot straightedge.
2.The overall vertical tolerance (plumbness) shall not exceed 1/2 inch per 10 feet of wall height. Negative (outward) batter is not acceptable.
3.The maximum permissible out of plane offset at any panel joint shall not exceed 3/8 inch.
4.The final horizontal and vertical joint gaps between adjacent facing panel units shall be within 1/8 and 1/4 inch, respectively, of the design final joint opening per the approved calculations required in
714.02 Section 4.
5.Wall sections that do not conform to these tolerances shall be reconstructed at no additional cost to the Department.
6.Erection of permanent walls with flexible facing (such as welded wire mesh) shall be in accordance with the following tolerances:
a.Vertical and horizontal alignment of the wall face shall not very by more than 2 inches when measured along a 10-foot straightedge, or as shown in the contract.
b.The overall vertical toleranc e (plumbness) of the wall shall not exceed 1 inch per 10 feet of wall height. Negative (outward) batter is not acceptable.
c.The offset limit between c onsecutive rows of facing shall not exceed 1 inch from planned offset in the outward direction. Mechanically Stabilized Earth (MSE) Walls
714.04 with Concrete Facing Panels
- 598 - 7. Placement of Metallic Reinforcement:
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.All joints between precast concrete panels shall be covered with geotextile on the backside of the wall. Adhesive shall be applied to the panels only and not within 2 inches of a joint. The geotextile shall have a minimum width of 12 inches and shall ov erlap fabric a minimum of 4 inches.
g.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 except when exposed in a wrap-around face system, at which time the reinforcement rolls shall be overlapped or mechanically connected per the manufacturer’s requirements.
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 with Concrete Facing Panels 714.04 - 599 - elevation shall be at the connection elevation to 2 inches higher than the connection elevation.
9.Backfill Placement:
a.Backfill shall be placed in such a manner as to avoid any damage or disturbance to the wall materials or misalignment of the facing panels. The contractor shall place backfil l 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.Any backfill material placed within the reinforced soil mass which does not meet these Specifications shall be corrected or removed and replaced by the Contractor at no add itional cost to the Department.
f.Backfill shall be compacted to 95% of the maximum density as determined by NDOT T 99.
g.The moisture content of t he backfill material before and during compaction shall be uniform throughout each layer.
h.Backfill material shall have a placement moisture content less than or equal to the optimum moisture content.
i.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.
j.The optimum moisture co ntent shall be determined in accordance with NDOT T 99.
k.The frequency of sampling of t he backfill material necessary to assure gradation control throughout construction shall be as directed by the Engineer.
l.The maximum lift thickness after compaction shall not exceed 10 inches (250 mm), regardless of the ve rtical spacing between layers of soil Mechanically Stabilized Earth (MSE) Walls
714.04 with Concrete Facing Panels
- 600 - reinforcements. The Contractor shal l decrease this lift thickness, if necessary, to obtain the specified density.
m.Before placement of the soil reinforcements, the backfill elevation, after compaction, shall be 2 inches (50 mm) above the attachment device elevation from a point approxim ately 1 foot (300 mm) behind the back face of the panels to the free end of the soil reinforcements, unless otherwise shown in the contract.
n.Compaction within 40 inches (1 m) of the back face of the panels 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.
o.Care shall be exercised in the compaction process to avoid misalignment of the panels or damage to the attachment devices.
p.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.
6.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.
714.05 Method of Measurement
1.The quantity of excavation for the leveling pads is not measured directly.
2.The quantity of “Excavation for MSE Wall” is measured by the cubic yard (cubic meter) and 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 plan 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 714.01.
b.Preliminary cross se ctions will include new fill if it is a required surcharge. Mechanically Stabilized Earth (MSE) Walls with Concrete Facing Panels 714.05 - 601 - 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 714.01
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 concrete face panels is measured by the square foot (square meter) and is computed using the plan dimensions. No adjustment in the pay quantity will be made if the computed quantity, based on the working drawings, varies from t he contract quantity. The wall surface area, as shown on the contract, includes the surface area of nominal panel joint openings and wall penetrations such as pipes and other utilities.
5.The quantity of concrete leveling pads shall be measured by the 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.
6.Backfill: a. The quantity of “Select Gra nular Backfill for MSE Walls" is measured by the cubic yard (cubic meter) and computed using the contract dimensions. No adjustment in the pa y quantity will be made if the computed quantity, based 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 lengt h of the wall times a width equal to 70% of the height.
b.The Random Backfill shown in the contract will not be measured for payment, but shall be considered subsidiary to the item “Select Granular Backfill for MSE Walls”.
7.The quantity of "Coping" is m easured by the linear foot (meter) and is computed using the contract di mensions. No adjustment in the pay Mechanically Stabilized Earth (MSE) Walls
714.05 with Concrete Facing Panels
- 602 - quantity will be made if the com puted quantity, based on the working drawings, varies from the contract quantity. The coping quantity shall be based on the coping cross-sect ion shown in the contract.
8."Shoring For Mechanically Stab ilized Earth Structures" is measured by the lump sum. Items that are in cluded in the lump sum price are the shoring design, plan preparation and submittals, and all labor, materials, and equipment necessary to construct the shoring.
714.06 Basis of Payment
1.P a y I t e m P a y U n i t Excavation for MSE Wall Cubic Yard (CY) [ C u b i c M e t e r ( m3)] Concrete Face Panels Square Foot (SF) [Square Meter (m2)] Concrete Leveling Pads Linear Foot (LF) [Meter (m)] Select Granular Backfill Cubic Yard (CY) for MSE Walls [Cubic Meter (m3)] Coping Linear Foot (LF) [Meter (m)] Shoring For Mechanically Lump Sum (LS) Stabilized Earth Structures
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.Construction of the V-ditch shown in the contract at the top of wall is subsidiary to the pay item “Coping”.
4.Payment is full compensation for all work described in this Section. - 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
Source: Nebraska Standard Specifications for Highway Construction, 2017 Edition. Pages 619–636 of 1,048.