structures, zones in embankments, foundations, and select fill materials. The geotextiles shall be designed to allow passage of water while retaining in situ soil without clogging. When used as a soil stabilizer, a detailed design process shall be followed taking into consideration the separation function of the geotextile, along with its filtration and reinforcement functions.
608.02 Materials.
The geotextile shall meet the requirements of AASHTO M 288 for Separation Applications, Class 2, and Section 810 of these Specifications. The geotextile shall be selected from List II-3, of the Department's MSDSAR manual. Information concerning this list is given in Subarticle 106.01(f) and ALDOT- 355.
608.03 Construction Requirements.
The installation site shall be prepared by clearing and grading the area as required. All sharp objects and large stones shall be removed and trees and shrubs shall be cut flush with the subgrade. The removal of topsoil and vegetation is not necessary. When appropriate, soft spots and unsuitable areas shall be identified during site preparation or subsequent proof rolling. These areas shall be excavated and backfilled with select material which is compacted to a depth such that the area provides equal stability as the adjacent area. Stabilization of these areas may be enhanced by use of a geotextile at the bottom of the excavation before backfill.
608.04 Installation.
The geotextile shall be unrolled as smoothly as possible on the prepared subgrade in the direction of the construction traffic. Adjacent geotextile rolls shall be overlapped in the direction of subbase placement using the following guide: Required Overlaps Soil Strength (CBR) Unsewn Overlap (inches {mm}) Sewn Overlap (inches {mm}) < 1 - - 9 {225} 1 - 2 38 {950} 8 {200} 2 - 3 30 {750} 3 {75} > 3 24 {600} - - Unless shown otherwise on the plans, the required overlap shall be as shown for a CBR value of 1 - 2. The geotextile may be held in place prior to subbase placement by pins, staples, or piles of fill or rock. Geotextiles may be folded or cut to conform to any curvature of the area. A fold or overlap shall be in the direction of construction and held in place as prescribed above.
608.05 Seams.
Both factory and field sewn or sealed seams 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.
608.06 Subbase Placement.
The subbase shall be placed by end dumping onto the geotextile from the edge of the geotextile, or over previously placed subbase aggregate. On subgrades having a CBR of 1, the subbase aggregate shall be spread simultaneously with dumping to minimize the potential of a localized subgrade failure. Direct traffic on the geotextile, along with any sudden stops, starts, or turns on the subbase material by construction equipment, shall be avoided where possible. Using whatever means, the subbase material shall be spread from the back dumped pile. Except in the case of low volume roads (ADT < 200), a minimum thickness of 6 inches {150 mm} shall be maintained. In the case of low volume roads, this thickness shall be 4 inches {100 mm}. A smooth drum roller shall be used to achieve the specified density. Any ruts occurring during construction shall be filled with additional subbase material and compacted to the specified density. The use of vibratory compaction will not be allowed as it may cause damage to the geotextile.
608.07 Damage Repair.
Damaged geotextiles, as identified by the Engineer, shall be repaired immediately. The damaged area, plus an additional 3 feet {1 m} around the area, shall be cleared of all fill material. This area shall be covered with a geotextile patch extending 3 feet {1 m} beyond the perimeter of the damage. The removed subbase material shall be replaced and compacted to the specified density. SECTION 609 AGGREGATE SLOPE PROTECTION
608.08 Method of Measurement.
The geotextile will be measured by the square yard {square meter} from the payment lines shown on the plans, or from the payment lines established in writing by the Engineer. This excludes seam overlaps. Excavation, backfill, bedding, and cover material are separate pay items.
608.09 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.: 608-A Separation Geotextile - per square yard {square meter} SECTION 609 AGGREGATE SLOPE PROTECTION
609.01 Description.
This Section shall cover the work of furnishing and constructing a slope protection layer of graded aggregate on slopes as shown on the plans or directed in accordance with these Specifications. This Section shall also cover the work, if shown on the plans, of furnishing and installing a geotextile filter blanket on slopes which are to receive the aforementioned aggregate protection layer.
609.02 Materials.
Materials furnished for use shall conform to the appropriate requirements of Division 800, Materials, and the following:
a.Geotextile Filter.
1.General. The geotextile shall meet the requirements of AASHTO M 288 for Permanent Erosion Control Geotextile, Class 2, and Section 810 of these specifications. In addition, the fabric shall have a minimum coefficient of friction of the wet fabric on wet aggregate of 0.4 as determined by the Department's Central Lab. 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.
2.Securing Pins. Securing pins for anchoring the filter shall be 3/16 of an inch {5 mm} steel bars, pointed at one end and fabricated with a head to retain a steel washer having an outside diameter of not less than 1.5 inches {38 mm}. The length of the pin shall not be less than 18 inches {450 mm}.
b.Aggregate. The aggregate shall be crushed stone meeting the requirements given in Section 801. The aggregate gradation shall be: AGGREGATE GRADATION FOR AGGREGATE SLOPE PROTECTION Percent Passing by Weight {Mass} Sieve Size Minimum Maximum 7" {175 mm} 100 - 6" {150 mm} 90 100 4" {100 mm} 20 55 3" {75 mm} 0 15 2" {50 mm} 0 5
609.03 Construction Requirements.
a.Preparation of Surface. The slope to be treated shall be prepared by removing vegetation, topsoil, and dressing the slope to reasonable line and grade. The surface shall then be compacted, if necessary, by the use of mechanical tampers to the satisfaction of the Engineer before placement of the geotextile filter and
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Source: 2026 Standard Specifications for Highway Construction, 2026 Edition. Pages 491–492 of 826. View official source ↗