ETL 1110-1-189
14 Feb 03
unreinforced base thickness for a given traffic level. The required depth of reinforced aggregate
base is then computed using Equation 1:
t(reinforced base) = t(unreinforced base) x BCR
(Equation 1)
This BCR factor should be determined using the materials and construction conditions that will
be used during the actual pavement construction.
3.2 Reinforced Flexible Road Design Procedure
3.2.1 Determine the Pavement Layer Properties. The first step in designing a reinforced flexible
pavement is to determine the design subgrade conditions as noted under Section 2.0. Once the
design subgrade conditions have been determined, an assessment of the applicability of
geosynthetics should be conducted using the guidance presented in Section 3.1 and Table 6. If
the use of a geotextile and/or geogrid is warranted based upon the applicability assessment, the
following procedure can be used to design the reinforced flexible pavement, otherwise the
procedures described in Technical Manual TM 5-822-5 should be used to design an unreinforced
flexible pavement. The subgrade soil CBR can be determined using Figure 4 based upon either
cone index or shear strength (C). The shear strength (C) can be directly measured using vane
shear devices.
Each aggregate layer, base and subbase (if used), in the flexible pavement system must meet
strength and gradation requirements defined in TM 5-822-5. The material requirements for the
bituminous surface course layer are defined in TM 5-822-8.
3.2.2 Determine Design Traffic. The next step is to determine the design traffic. The design
traffic should be determined according to TM 5-822-5, which results in a design index (DI). The
design index combines the effect of average vehicle axle loadings and expected traffic volume as
expressed by road classification. The design index ranges from 1 to 10 in order of increasing
traffic loading and volumes. The design index is selected using Table 3-1 of TM 5-822-5,
presented here as Table 7 for clarity.
3.2.3 Determine the Required Layer Thicknesses. The next step is to design an unreinforced
flexible pavement for the given subgrade conditions. The design subgrade CBR strength is
determined according to the methods presented previously. Figure 8-1 of TM 5-822-5 (Figure 9
in this document) is entered with the appropriate CBR of the supporting layer, either the
subgrade strength or the subbase strength. The required pavement thickness above the
supporting layer is determined by drawing a vertical line from the strength on the x-axis to the
intersection of the design index. A horizontal line is then projected from the intersection to the
required pavement thickness on the y-axis. The required pavement thickness should be rounded
up to the nearest 0.5 inches for surfaced flexible pavements. Figure 9 was adapted from
Figure 8-1 of TM 5-822-5 for clarity. Minimum thickness values for the surface AC and base
courses are provided in Table 6-1 of Technical Manual 5-822-5, and are presented in Table 8 of
this document. Generally, an asphalt thickness of 3 in. or less is appropriate for design index
values of 7 or less. The minimum required base thickness is 4 in. The final pavement structure
is determined by using the minimum AC thickness for the available aggregate as noted in Table
8. The difference between the total required pavement thickness above the subgrade/subbase
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