2420.01 DESCRIPTION.
This work shall consist of furnishing and constructing structural plate pipes, pipe arches, or arches meeting requirements of these specifications and of the sizes and dimensions shown in the contract documents.

Plates for a pipe arch shall form a cross section made up of four circular arcs tangent to each other at their junctions and symmetrical about the vertical axis.

2420.02 MATERIALS.
Materials for structural plate pipe, pipe arches, and arches shall meet requirements of Section 4144. Unless otherwise specified, a galvanized corrugated steel structure shall be furnished.

2420.03 BEDDING.
When a pipe structure is to be erected in a trench, the width of trench must be sufficient to permit thorough tamping of the earth backfill against every plate except the bottom one.

The pipe shall be bedded in an earth foundation of uniform density, carefully shaped with a template or shaped by other means and checked with a template, shall be supported at the desired grade, and shall have the required camber to fit the lower plate of the pipe. When rock in either ledge or boulder formation is encountered, it shall be removed below grade and replaced with suitable materials in a manner to provide a compacted earth cushion having a thickness under the pipe not less than 1/2 inch per foot (40 mm/m) of fill over the pipe, with a minimum thickness of 8 inches (200 mm). When firm foundation is not encountered at the grade established, due to soft, spongy, or other unsuitable soil, unless other special construction methods are specified, all unsuitable soil for a maximum distance of one diameter under and on each side of the pipe shall be removed and replaced with suitable earth or granular material properly compacted to provide adequate support of the pipe. Excavation of unsuitable soil below grade and subsequent backfill shall be done only at the direction of the Engineer and will be paid for as extra work unless otherwise provided in the contract documents.

The preparation of the base shall be in accordance with Article 2416.04, B. The bedding shall provide camber to allow for settlement after placing the fill. The amount of camber shall be varied to suit the height of fill and nature of supporting soil. A minimum camber of 1% of the length of the pipe shall be provided.

2420.04 MULTIPLE STRUCTURES.
When multiple structures of pipe or pipe arches are used, they shall be spaced so that the adjacent sides of pipe are from 50% of the diameter to a maximum of 4 feet (1 m) apart to permit tamping of backfill material. The distance between plates at skewback of multiple arch spans shall be not less than 10% of the longer adjoining span.

2420.05 FIELD ERECTION, STRUCTURAL PLATE PIPE AND PIPE ARCHES.
Beginning at the downstream end, full sized bottom plates shall be placed along the center line of the structure lapping each plate one corrugation with the previous plate. Plates at longitudinal and circumferential seams shall be connected by fasteners. Joints shall be staggered so that not more than three plates come together at one point. After all plates have been placed, all bolts shall be thoroughly tightened to a minimum torque value of 100 foot pounds (135 Nm) and a maximum of 300 foot pounds (400 Nm).

When end treatment requires a rigid headwall, the plates shall be anchored to the headwall with not less than 3/4 inch (19 mm) anchor bolts spaced at not more than 19 inch (480 mm) centers.

2420.06 SKEWED ARCH SPANS.
The end skew of arches shall not exceed 45 degrees. When the skew is more than 15 degrees, the length of the structure shall be so that no portion of the live load will be carried by the cut portion of the end. Where right-of-way or other conditions do not permit the required length, the cut end shall be supported by rigid headwalls designed to meet the conditions. Plates shall be anchored to the headwall with 3/4 inch (19 mm) bolts at not over 19 inch (480 mm) centers.

2420.07 ARCH ANCHORAGE.
Each side of the arch shall be anchored to the foundation by means of a formed channel or a structural angle bolted to the bottom row of plates. The arch plates shall bear directly on the channel or angle. Channels or angles shall be made of the same material as the plates and shall be anchored to the foundation at not more than 24 inch (0.6 m) intervals.

2420.08 FIELD ERECTION, ARCHES.
Erection of arch plates shall begin at the downstream end by bolting the side plates at intervals not greater than 24 inches (0.6 m) to the angle or channel attached to the foundation. Plates shall be set inside the vertical leg of the angle or the longer leg of the channel and shall bear directly on the horizontal leg of the angle or the web of the channel. Succeeding plates shall be assembled so that joints at right angles to the center line of the arch are not continuous for more than the width of one plate. The upper edge of each plate of the first ring shall be supported in its proper place until the full number of plates for the ring are in position. Bolts in each section shall be tightened as required in Article 2420.05.

2420.09 SHOP FORMING.
When specified, structural plate pipes shall be shop formed to increase the vertical diameter approximately 5% out of round before placement of the fill. A tolerance of ± 2.5% of the nominal pipe diameter or 5 inches (125 mm), whichever is less, will be permitted.

2420.10 BACKFILLING.
After the structure has been assembled, backfill shall be placed in accordance with Articles 2402.08, 2402.09, and 2402.10. Each layer shall be thoroughly tamped between the structure and the sides of the trench or for a distance on each side of the structure equal to the diameter of the structure. After the fill over the structure has been completed to the full height, the struts, if any, shall be released and removed. Care shall be exercised by the Contractor in backfilling to prevent excessive distortion of the shape of the structure, either in peaking action or rolling action. Deflection in any direction greater than 2.5% from the original specified shape will not be allowed during the backfill operation.

In addition to the backfilling required above, when ordered by the Engineer, the Contractor shall build approach fills that will provide a roadway 10 feet (3 m) wide over the culvert, with grades not steeper than 10%.

2420.11 QUALITY OF WORK.
In addition to compliance with the details of construction, the completed structure shall show high quality work. Plates on which the galvanizing has been damaged or broken, either in the shop or in shipping, or which show defective work will be rejected. The requirement applies not only to the individual plates but to the shipment for any contract as a whole. Among others, the following defects are specified as constituting poor quality work and the presence of any or all of them in any individual culvert plate, in any shipment, or, in general, in the completed culvert, will constitute sufficient cause for rejection:

Uneven laps.
Elliptical shaping (unless specified), or otherwise excessive distortion.
Variations from a straight center line.
Ragged edges.
Loose, unevenly lined or spaced bolts.
Illegible brand.
Damaged, scaled, or broken galvanizing.
Dents or bends in the metal itself.

2420.12 METHOD OF MEASUREMENT.
The quantity of structural pipe culvert, in feet (meters), will be the quantity shown in the contract documents for each culvert to the nearest foot (0.1 m). The quantity of pipe will be determined as follows:

A. Pipes and arches with either square or skewed vertical ends; end to end of metal on center line of structure.

B. Pipes or pipe arches with square ends, beveled; average end to end at top and bottom of pipe.

C. Pipes or pipe arches with skewed ends, beveled; average end to end at top and bottom of pipe parallel to center line.

D. Arches with ends other than vertical; as noted in the contract documents.

E. Excavation for structural plate pipe will be measured as specified for culverts in Article 2402.12, and when the pipe is installed without change in location, dimensions, or elevation, the quantities of Class 20 or Class 23 and Class 24 excavation, as shown in the contract documents, will be the quantities for which payment is made.

F. Excavation for structural plate arches will be measured as specified for structures in Article 2402.12 for the respective classes of excavation performed.

G. Structural concrete and steel reinforcement required will be measured as provided in Articles 2403.22 and 2404.09, respectively.

2420.13 BASIS OF PAYMENT.
The Contractor will be paid the contract unit price for structural pipe culvert of type and size specified per linear foot (meter). This payment shall be full compensation for furnishing all materials, labor, and equipment necessary to complete the work.

Excavation for structures, structural concrete, and reinforcement will be paid for separately.