306 — UNDERDRAINS, SUBGRADE DRAINS, AND SPRING CONTROL 2026 STANDARD SPECIFICATIONS FOR CONSTRUCTION AND MATERIALS CATEGORY 300 DRAINAGE SECTION 306 — UNDERDRAINS, SUBGRADE DRAINS, AND SPRING CONTROL
306.01 Description
Construct underdrains, subgrade drains, underdrain for spring control, underdrain pipe outlets, and blind drains using pipe, geotextile, and granular material. Clean existing underdrain outlets.
306.02 Materials
No. 57 Aggregate 901.01 Portland Cement Concrete 902.10, Mix No. 2 Pipe Section 905 Geotextile, Class as specified Section 919 Securing Pins or Staples 919.03 Flexible Delineator Post and Rodent Screens As approved by the Office of Materials Technology
306.03 Construction
Coordinate the field installation of traffic barrier, signs, lighting, and landscaping with the Engineer to avoid any damage to the underdrains, subgrade drains, or outlet pipes. Correct any damage to the underdrains, subgrade drains, or outlet pipes.
306.03.01 Excavation. Excavate trenches to the specified dimensions and grade. Ensure that the
sides and bottom of trenches are smooth and uniform to prevent tearing of the geotextile when backfilling. For excavated material, refer to 402.03.01.
306.03.02 Geotextile. Place geotextile when specified. Place it with the machine direction
parallel to the longitudinal direction of the trench. Ensure that it is of sufficient width to completely enclose the underdrain trench, including specified overlaps. Place the geotextile tightly against the underdrain trench to eliminate voids beneath the geotextile. Avoid wrinkles and folds. Maintain at least a 24 in. overlap at joint ends or breaks. Pin joints and overlaps to securely hold the geotextile in place until placement of the cover material. Pin longitudinal joints, overlaps, and edges no more than 50 ft on center. Replace or repair damaged geotextile. 306 — UNDERDRAINS, SUBGRADE DRAINS, AND SPRING CONTROL 2026 STANDARD SPECIFICATIONS FOR CONSTRUCTION AND MATERIALS
306.03.03 Pipe Placement. Slope the underdrain pipe to maintain positive drainage toward the
outlet. Place perforated pipes with the perforations down and arranged symmetrically about the vertical axis. Plug the ends of trunk lines, wyes, tees, or ells as directed. Make joints and connections in accordance with the manufacturer's recommendations.
306.03.04 Outlets. Outlet the underdrains into drainage structures whenever possible. Outlets
that empty into a drainage structure shall be at least 9 in. above the normal flow line in the structure and be constructed of normal underdrain outlet pipe. Maintain at least 18 in. of cover over the pipe. Rodent screens are not required when an underdrain outfalls into a drainage structure. When outfalled into a slope or ditch, slope the outlet pipe at least three percent. Use Type ‘S’ (smooth interior wall) polyethylene (PE), or smooth-wall polyvinyl chloride profile wall (PPWP) pipe as specified in Section 905. Construct a sloped concrete headwall with a removable rodent screen at the end of the outlet pipe. Place a flexible delineator post on the slope headwall. Space outlets for longitudinal underdrains at intervals no more than 250 ft and at the lowest elevation on all vertical curves. When changing the direction of the longitudinal underdrain or outlet pipe, ensure all bends have a radius of at least 3 ft.
306.03.05 Backfill. Backfill trenches to the specified dimensions and grades. Backfill underdrain
and outlet trenches as the work progresses.
a.Underdrain. Lightly tamp aggregate backfill, and screed or rake to provide proper thickness and grade.
b.Outlets. Backfill as specified in Section 210. Replace geotextile, underdrain pipe, and outlet pipe damaged by excessive tamping. Cover longitudinal underdrain with the next pavement layer within 72 hours. Cover all other underdrain within 48 hours. Protect underdrain, including the geotextile, from contamination by soil fines. Replace or repair clogged geotextile and any underdrain trench that becomes contaminated.
306.03.06 Video Inspection and Acceptance. Perform a video inspection of all new longitudinal
underdrain and outlets in the presence of the Engineer, as part of final acceptance. Correct all damage as directed.
306.03.07 Cleaning Existing Outlets. Clean existing underdrain pipe outlets and dispose of the
material. Remove and replace existing rodent screens. Where there are no screens, install them as directed.
306.03.08 Permanent Subgrade Drains. Construct permanent subgrade drains when specified.
Subgrade drains consist of trenches excavated through the shoulder and roadside grading from the edges of the road pavement to a side ditch, embankment slope, or other approved outlet and filled with aggregate. Locate subgrade drains at low points, and space them at 25 ft intervals for a 306 — UNDERDRAINS, SUBGRADE DRAINS, AND SPRING CONTROL 2026 STANDARD SPECIFICATIONS FOR CONSTRUCTION AND MATERIALS distance of 125 ft on each side of the low point, then at intervals of 100 ft to within 125 ft of the high point. Before placing the road pavement and before completion of the shoulder paving or final roadside grading areas, cut and shape trenches to a width of 24 in. Place No. 57 aggregate to the underside of the shoulder material and to the underside of the specified topsoil thickness in the roadside grading area. The portion of the trench within the roadside grading area shall be completely wrapped in geotextile. The bottom of the trench at the end adjacent to the road pavement shall be at least 2 in. below the subgrade.
306.04 Measurement and Payment
The payment will be full compensation for all excavation, pipe, coupling bands, aggregate, backfill, geotextile, video inspection, and all material, labor, equipment, tools, and incidentals necessary to complete the work.
306.04.01 Underdrains, Underdrain Pipe Outlets, Subgrade Drains, and Underdrain Pipe for
Spring Control will be measured and paid for at the Contract unit price per linear foot. Slope headwalls, rodent screens, and marker posts will not be measured but will be incidental to the cost of the Underdrain Outlet.
306.04.02 When an underdrain pipe is not used for spring control, all excavation and backfill for
spring control will be measured and paid for at the Contract unit price per cubic yard for Class 3 Excavation for Incidental Construction and Aggregate Backfill for Underdrain.
306.04.03 When directed by the Engineer, excavation for underdrains, subgrade drains, and
underdrain for spring control required to lower the trench to an elevation deeper than specified in the Contract Documents will be measured and paid for at the Contract unit price per cubic yard for Class 3 Excavation for Incidental Construction and Aggregate Backfill for Underdrain.
306.04.04 When measuring the length of a manufactured connection (tee, elbows, etc.) other than
coupling bands, each actual linear foot will be doubled and payment made at the Contract unit price per linear foot for the appropriate underdrain pipe item specified in the Contract Documents.
306.04.05 Cleaning Existing Underdrain Outlets will be measured and paid for at the Contract
unit price per each. The payment will be full compensation for locating outlets, removing and replacing the existing rodent screens, removal and disposal of material removed from the pipe, and for all material, labor, equipment, tools, and incidentals necessary to complete the work.
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Source: Standard Specifications for Construction and Materials, 2026 Edition. Pages 331–333 of 1,138. View official source ↗