Arno Hefer Administrator Posts: 18 10/15/2020

Arno Hefer Administrator Posts: 18 The AASHTO Structural Number (SN) makes provision for the effects of certain levels of drainage on predicted pavement performance. This is achieved using a drainage coefficient (mi) to modify the layer coefficient (ai) integrated into the SN equation along with the layer thickness (Di) input: SN = a1ˑD1 + a2ˑm2ˑD2 + a3ˑm3ˑD3 The drainage coefficient only applies to the effects of drainage on untreated/natural materials. The designer should assess the level or quality of drainage achieved considering average rainfall and prevailing drainage conditions. The following table contains definitions of drainage levels associated with typical mi ranges as a function of percentage time during a year the pavement structure would approach saturating moisture levels. For comparison purposes, the conditions at the AASHTO Road Test site are considered “Fair” (free water removed within 1 week) with a drainage coefficient of 1.0. Recommended drainage coefficient values for modifying the layer structural layer coefficients of untreated layers in flexible pavements (adapted from AASHTO, 1993) Reference: AASHTO Guide for Design of Pavement Structures (1993). American Association of State Highway and Transportation Officials, Washington, D.C. edited by on 10/21/2020 edited by on 10/17/2023

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