Detours are pavements onto which traffic is temporarily diverted until a permanent (new construction, reconstruction, etc.) structure is provided to carry the traffic over the time frame of a conventional performance period.

Detours can generally be placed into two categories. The first category is the diversion of traffic onto an existing road or roads that parallel or skirt the location of the permanent facility to allow traffic around the construction site. Based on the engineer’s evaluation, the existing road(s):

The second category entails the construction of a temporary facility, usually parallel to and in the general right-of-way of the permanent facility. This type of detour is usually removed when no longer needed or may become integrated into the permanent structure.

Estimating traffic loading over the time frame for a detour is complicated, especially when multiple highway sections are involved. Traffic data for detours can be specifically requested from TPP. For typical detours, the traffic data requested for the parent project will suffice when used in conjunction with the following detour design procedures.

The falling weight deflectometer (FWD) shall be used to evaluate the adequacy of any existing structure (e.g., shoulders or bypass routes) to carry detour traffic.

FPS 21 and the Modified Texas Triaxial Class (TTC) design procedure (as a standalone design option) are the primary analytical methods for detour design. As with all FPS designs, a check using the modified TTC check procedure is required; an allowable alternate version of the standard TTC procedure is described later in this section. If the detour incorporates existing roads/highways, traffic load estimates must include traffic from both sources. Districts may employ proven design strategies for detours and can develop catalog designs based on traffic levels and subgrade support.

When using FPS 21, traffic loading must be entered as the 20-yr. cumulative ESALs. It is only the analysis period that is adjusted to reflect the expected duration of the detour. Set the analysis period and time to first overlay to one or two years. The version of the modified TTC check most commonly used (as in the post-design check built into FPS) assumes a design life of 20 yr. For detours expected to last less than 2 yr., an alternate version of the modified TTC check may be considered – a version sometimes used for evaluating load-zoned highways where the design life is considered to be no more than 10 yr.

In the alternate version, the allowable wheel load scale is essentially doubled, allowing for reduced cover over the standard procedure. The easiest way to perform this alternate version of the modified TTC check is to simply divide the ATHWLD reported from TPP by 2 on the Traffic Analysis for Highway Design report, and enter this wheel load value into the appropriate field of the FPS 21 Modified Texas Triaxial Design check. Depending on the priority of maintaining uninterrupted traffic flow, the tandem axle multiplier can further reduce the overall thickness requirement when set to less than 50%. (NOTE: The tandem axle multiplier is employed when the reported percent tandem axles in the ATHWLD is 50% or greater.) If this alternative option is used, an additional mechanistic check

is highly recommended

to double check fatigue and rut life.

When using the Mechanistic Check option in FPS 21 to evaluate detour sufficiency in fatigue cracking and subgrade failure (rutting), the routine will estimate the passes to failure in both these modes of a standard 18-kip axle and compare that against the cumulative 18-kip axles in the first performance period of the FPS design. A “check result” message will appear indicating whether the design passes or fails. However, the FPS-calculated performance period is often much longer than the intended detour life; therefore, estimating the cumulative ESALs to the end of the projected detour life may be desirable. One procedure the designer can use to estimate cumulative ESALs over this short detour life is by calculating ESALs in “TRAFFIC6.xls” (refer to MNT,

Engineering Applications

, to download the spreadsheet) or similar estimating routine, then compare this value against the predicted crack and rut life numbers generated in the FPS Mechanistic Design Check. Refer to the for more details on using the FPS Mechanistic Check feature.

Once a structural design is generated by using any of the cited procedures, other considerations must be made to ensure adequate performance over the expected life of the detour structure. Among these are:

Proof rolling should be conducted prior to applying surfacing to ensure adequate sheer strength in the unbound layers. Consider the more stringent requirements in the applicable specifications to ensure quality workmanship is used, especially where traffic volumes and level of loading are high.

When the detour structure is no longer needed, mill and recycle pavement layers used in the detour within the parent project to reduce costs.