SECTION 605 PAVEMENT AND SPECIAL UNDERDRAIN
604.02 Materials.
The geotextile used in this work shall be one constructed of non-woven synthetic fibers meeting the requirements of AASHTO M 288 for Separation Geotextile Class 3 and Section 810 of these specifications with the exception that geotextiles manufactured with polyamide will not be allowed. The geotextile filter shall be resistant to the heat and temperature range expected during placement of the Permeable Asphalt Treated Base. The geotextile shall be selected from List II-3, of the Department's manual titled "Materials, Sources, and Devices With Special Acceptance Requirements". Information concerning this list is given in Subarticle 106.01(f) and ALDOT-355.
604.03 Construction Requirements.
a.General. The surface area on which the geotextile fabric is to be placed shall be free of loose aggregate, foreign debris, and all sharp objects during placement of the fabric.
b.Installation. The geotextile filter shall be installed immediately prior to placement of the permeable asphalt treated base layer and extended full width between the inside and outside edge drains according to the manufacturer's specifications and as shown on the plans or in the proposal. Exposure of geotextile filter to the elements between lay down and cover shall not exceed three days. Adjacent geotextile rolls, seamed or unseamed, shall be overlapped in the direction of placement of the Permeable Asphalt Treated Base a minimum of 2 feet {600 mm} at all longitudinal and transverse filter fabric joints. The geotextile shall be held in place prior to Permeable Asphalt Treated Base placement by pins, staples, or other means as approved by the Engineer.
c.Seams. Both factory and field sewn or sealed seams, if applicable, shall conform to the strength requirements of Table 1 as outlined in AASHTO M 288 for separation applications. All seams shall be subject to the approval of the Engineer.
d.Damage Repair. Damaged geotextiles, as identified by the Engineer, shall be repaired immediately. Any geotextile filter which is ripped or torn during the construction process shall be replaced or repaired with a patch which extends 3 feet {1 m} beyond the perimeter of the tear or damage.
604.04 Method of Measurement.
The geotextile will be measured by the square yard {square meter} computed from the width (from pavement edge drain to pavement edge drain) and the length as shown on the plans, or established in writing by the Engineer. This excludes seam overlaps.
604.05 Basis of Payment.
a.General. The accepted quantities of geotextiles will be paid for at the contract unit price per square yard {square meter} in place.
b.Payment will be made under Item No.: 604-A Separative Geotextile - per square yard {square meter} SECTION 605 PAVEMENT AND SPECIAL UNDERDRAIN
605.01 Description.
This Section shall cover the work of constructing pavement and special underdrain. Pavement underdrain shall be drain lines in the median and at the edges of pavement, and shall include collection pipes, and outlets. Edge drains and median drains shall be aggregate filled underdrains. The details of the special underdrain will be shown on the plans. Shoulder drainage plane layers shall be constructed in accordance with the requirements given in Section 315. Geotextile filter fabric shall be placed to reduce the movement of soil into the drains. Pipes shall be installed to collect and provide outlets for the drainage. SECTION 605 PAVEMENT AND SPECIAL UNDERDRAIN
605.02 Materials.
a.Geotextile Filter. The geotextile filter for aggregate filled underdrain shall be a non-woven material meeting the requirements of AASHTO M 288 for Class 2 Subsurface Drainage Geotextile and Section 810 of these specifications. The geotextile shall be selected from List II-3, of the Department's manual titled "Materials, Sources, and Devices With Special Acceptance Requirements". Information concerning this list is given in Subarticle 106.01(f) and ALDOT-355.
b.Pipe. The required types and sizes of pipes will be shown on the plans. The pipe may be of any of the non-corrugated pipe underdrain or smooth lined corrugated pipe underdrain noted in Section 852, perforated or non-perforated as required. Pipe fittings shall be of the same material as the pipe or of a compatible material providing a crush strength equal to, or greater than, that of the type pipe being used.
c.Aggregate Filler. Aggregate filler shall meet the requirements for Coarse Aggregate, Section 801, for ALDOT size #4, #5, or #57 and the additional requirement that no more that 1% of the aggregate passes the No. 200 {75 m} sieve.
d.Slope Paving. Slope paving for outlet headwalls shall meet the requirements noted in Section 614.
605.03 Construction Requirements.
a.General. The installation of pavement and special underdrain shall be done in coordination with other work to prevent damage to the roadway. Materials from the excavation may be deposited outside the trench work area for reuse in the work. Excess material shall be removed and disposed of as shown on the plan details or as directed by the Engineer.
b.Aggregate Filled Underdrain. An aggregate filled underdrain shall be a trench that is lined with a geotextile filter fabric and then filled with aggregate. Pavement median drains and edge drains shall be an aggregate filled underdrain with pipes placed as shown on the plans to collect drainage and provide a connection to the outlets for the drainage. Exposure of geotextiles to the elements between laydown and cover shall be a maximum of 14 days to minimize the potential for damage. The geotextile filter shall be installed in such a manner that all splice joints are provided with a minimum overlap of 3 feet {1 m}. Securing pins shall be installed to anchor the filter fabric if the filter fabric does not remain in place during the construction of the drains. Where seams are required in the geotextile filter in the longitudinal trench direction, they shall be joined by either sewing or overlapping as outlined in Article 608.05 of these specifications. All seams shall be subject to the approval of the Engineer. Overlapped seams shall have a minimum overlap equal to the width of the trench and shall be anchored with securing pins as directed to ensure the required overlap is maintained. Where a filter repair is required, a piece of filter shall be placed over the damaged area and extend 3 feet {1 m} beyond the perimeter of the tear or damage. When an outlet pipe passes through the filter, a separate piece of filter of sufficient size to be wrapped around the pipe and flared against the side of the filled drain filter shall be used. Care shall be taken during the aggregate filler placement operation as well as the pipe installation, when required, to prevent damage to the filter. The aggregate filler shall be placed and compacted by methods acceptable to the Engineer.
c.Special Underdrain Outlets. Outlets may be classified and paid for as types of "Special Underdrain Outlets" as follows: Type A - Designated for connecting an aggregate filled or prefabricated drainage mat underdrain line to a roadway front slope and requires a slope paved headwall. Type B - Designated for connecting an aggregate filled or prefabricated drainage mat underdrain line to a drainage structure (inlet, junction box, etc.).
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Source: 2026 Standard Specifications for Highway Construction, 2026 Edition. Pages 483–484 of 826. View official source ↗