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Materials Subsurface Pavement Drainage 550.02 Drainage Requirements by Roadway Type. 550.06 Drainage Collection System. Subsurface Pavement Drainage This section of the Materials Manual will systems to provide positive drainage for pavements. This section covers subs urface pavement drainage systems only and does not address surface drainage issu es such as open channels, culverts, storm drainage systems, or hydrology. For guidance on these topics, refer to Section 600 of the Design Manual. Pavement drainage can have an impact on pavement performance. Research indicates that drained and maintained pavements last longer than undrained pavement s. For this reason, an engineering analysis should Materials Subsurface Pavement Drainage Materials Subsurface Pavement Drainage A 0.15' (45 mm) average thickness binder course of asphalt pavement or perm eable asphalt pavement should be placed on top the rock cap to facilitate the paving operation. If the paveme 3" (75.0 mm) 1½" (37.5 mm) ¾" (19.0 mm) ½" (12.5 mm) No. 4 (2.36 mm) This gradation results in a permeability on the order of 30,000 ft/day (9000 m/day) (see fines into the rock cap, a filter layer or geotextile 2 in (50.0 mm) 1 1/2 in. (37.5 mm)) 3/4 in. (19.0 mm) 1/2 in. (12.5 mm) (No. 4 (4.75 mm) No. 8 (2.36 mm) Materials Subsurface Pavement Drainage The minimum layer thickness will be greater than 1.0' (300 mm), unless placed on existing pavement in which case the minimum thickness will be 0.6' (180 mm). The minimum thickness is based on particle size and geotextile protection rather than permeability. A subgrade separation geotextile must be placed between the subgrade and the angular rock base. A 0.15' (45 mm) average thickness binder course of asphalt pavement or perm eable asphalt pavement acilitate the paving operation. If the pavement is Materials Subsurface Pavement Drainage Stabilized Permeable Base should be open-graded asphalt treated aggregate conforming to the requirements of SSP 413, Asphalt Treated Permeable Base (ATPB). ATPB is stabilized with approximately 3.0% asphalt cement by weight. The minimum thickness of stabilized permeable base should be 0.3' (90 mm). Stabilized permeable bases should be placed over a filter consisting of at least 0.5' (150 mm) of crushed aggregate base. 550.06 Drainage Collection System. A conventional collection system c and aggregate backfill is recommended rather than ge ocomposite drains because of problems inherent in proper installation, blinding of the geotextile and the build-up of fines that pass through the geotextile and Materials Subsurface Pavement Drainage the width of the trench. Prior to placing the permeable layer, the excess geotextile sh lay flat outside the trench on each side. This will pe Materials Subsurface Pavement Drainage Materials Subsurface Pavement Drainage Materials Subsurface Pavement Drainage Materials Subsurface Pavement Drainage Subsurface Pavement Drainage Wc = Length of contributing transverse crack or joint (ft.) or (m) Cs = Spacing of contributing transverse cracks or joints (ft.) or (m) W = Width of proposed permeable layer (ft.) or (m) kp = Pavement coefficient of permeability (ft /day/ft /day/m It is recommended that a rate of Ic = 2.4 ft /day/ft. (0.223 m /day/m) be used for most design applications. However, if values of Ic by local observation of inf iltration and pavement performance are available, then these values should be used. Additionally, for new pavement kp is essentially 0. Secondly, the discharge of the permeable b ase can be calculated by Darcy's equation. Q = Flow capacity of the base (ft /day) or (m /day) K = Coefficient of permeability (ft./day) or (m/day) i = Slope of hydraulic gradient (ft./ft.) or (m/m) A = Cross sectional areas of flow (ft NOTE: The thickness of the permeable base may be estab lished by the time to drain analysis presented in Lastly, the pipe flow can be calculated by the equation Qp = Pipe flow (ft /day) or (m /day) qi = Infiltration rate (ft /day/ft /day/m