A RATIONAL APPROACH TO DRAINAGE OF A PERVIOUS SUB-BASE

WHEN A PERVIOUS SUBBASE IS PLACED IN A TRENCH SECTION, DRAINAGE IS ESSENTIAL. APPROXIMATE EQUATIONS FOR TIME OF DRAINAGE ARE SET UP IN TERMS OF PERMEABILITY, SLOPE, AND AMOUNT OF DRAINABLE WATER. WATER IS ASSUMED TO REACH THE SUBBASE THROUGH JOINTS OR CRACKS IN THE PAVEMENT, OR ALONG THE EDGE OF THE PAVEMENT. ON FLAT GRADES THE CAPACITY OF THE BASE TO CARRY WATER LONGITUDINALLY CONTROLS UNLESS DRAINAGE IS OBTAINED BY A CONTINUOUS LONGITUDINAL UNDERDRAIN AT THE EDGE. OTHERWISE DRAINAGE IS CONTROLLED BY THE CAPACITY OF THE "BLEEDERS" OR OTHER OUTLETS SPACED ALONG THE SHOULDER. THE REQUIRED SPACING OF THESE OUTLETS SPACED ALONG THE SHOULDER. THE REQUIRED SPACING OF THESE OUTLETS BECOMES LESS AND LESS AS THE GRADE OF THE ROADWAY BECOMES FLATTER, INDICATING THAT UNIFORM SPACING REGARDLESS OF GRADE IS POOR DESIGN. THE THEORY INDICATES THAT CONTINUOUS LONGITUDINAL UNDERDRAINS SHOULD BE PROVIDED ON FLAT GRADES, OR THE SUBBASE SHOULD BE EXTENDED ACROSS THE SHOULDER. /AUTHOR/