Section 4: Pavement Detours and Pavement Widening
Anchor: #i1003896Pavement Detours
Detours as described in this manual are pavements on which traffic is temporarily diverted until such time that a permanent (new construction, reconstruction, etc.) structure is provided to carry the traffic over the timeframe of a conventionally designed performance period. Detours can generally be placed into two categories. The first category is that of an existing road or roads that parallel or skirt the location of the permanent facility to allow traffic a means around the construction site. Based upon the engineer’s evaluation, the existing road(s) may be used as is if little detrimental effect caused by the additional traffic loading is anticipated, or may need structural upgrading prior to opening of the detour, or rehabilitation once the detour is closed. 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 the base of the permanent structure. Estimating the traffic loading over the timeframe the detours will be used is one concern. But the temporary nature of the detour is not a license for ignoring other basic considerations such as drainage, underlying support and quality workmanship.
Structural Design of Detours
FPS-19W and the Modified Texas Triaxial Class (TTC) design procedure (as a stand alone design option) are the primary methods for detour design. As with any FPS design, a check using the modified TTC check procedure is required. If the detour incorporates existing roads/highways, traffic load estimates must include traffic from both sources. When using FPS-19W, 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. The version of the modified TTC check most commonly used 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 was considered only 10 yr. In the alternate version, the allowable wheel load scale is essentially doubled, allowing for reduced cover over the standard procedure. This option must be worked manually, by modifying the charts available in the Pavement Design Notes of “Tex-117-E.” If this option is used, an additional mechanistic check is highly recommended to double check fatigue and rut life. The mechanistic models used in the FPS-19W mechanistic design checks can be used, but must again be used outside the program in conjunction with a linear elastic modeling program that can generate the strain parameters at the bottom of the bituminous/HMA layers and at the top of the subgrade. Once a structural design is generated by using any of the cited procedures, other considerations must be made to insure adequate performance over the expected life of the detour structure. Among these are adequate pavement width to insure wheel loads do not encroach upon the pavement edge where lack of lateral support may result in shear failures at the edge, properly prepared subgrade (including investigation into soil properties), adequate drainage by insuring proper cross slope and ditch lines, properly compacted pavement lifts including the HMAC surfacing, and sufficient HMAC density at mat joints. Proof rolling should be conducted prior to applying surfacing to insure adequate shear strength in the unbound layers. Consider the more stringent requirements in the applicable specifications to insure quality workmanship is used, especially where traffic volumes and level of loading are high.
Anchor: #i1003921Pavement Widening
Widening is conducted to increase the capacity of the highway or to improve safety aspects such as the inclusion of shoulders, turning bays, etc. Consideration should always be given to maintaining the original cross section for the widened portion. This serves two purposes:
- it maintains uniformity in the section which facilitates future evaluation and rehabilitation options for the section as a whole, and
- it serves to maintain subsurface drainage patterns which are essential to preventing trapped moisture.
Exceptions to this philosophy are generally related to poor performance in the existing section, or the desire to expedite construction of the widened section to minimize interference with traffic. In addition to cross section considerations, widening will entail a full-depth joint that is inherently a weakened interface in the structure. Compaction against the vertical plane of the old structure will be more difficult to achieve than with full-width construction. Placement of this joint as far from the wheelpath as possible will improve performance. Also, applying a final HMAC overlay across the entire section with the overlay joint offset by 6-12 in. from the underlying vertical interface will improve the impermeability of the interface over the short term. Usually however the underlying vertical interface at the widening will cause reflective cracking through to the surface. An aggressive crack sealing program will limit the amount of precipitation runoff from entering into the structure. Consideration should also be given to using geotextiles/stress absorbing membrane interlayer (SAMI) over the widening joint prior to applying the full-width overlay as a means to delay reflective cracking.
Recommendations for Similar Cross Section Widening Strategies
The objective will be to match as close as practical the same cross section and materials in the widened section as were used in the original section.
Unbound Granular Base Sections
Other considerations are:
- Treatment of the subgrade under the widened location. This may serve to reduce moisture fluctuations at the new pavement edge which in turn should reduce potential for longitudinal edge cracking. An alternative may be to use geogrids at the subgrade/base interface. Treatment should be accomplished below the level of the old flex base.
- When milling any HMAC to the outside of the existing lane, also remove an extra 12.0 in. to the inside of the lane edge (stripe). The object is to offset the HMAC joints above the widening interface.
- Selection of the new flexible base material should be based on laboratory evaluation of both new and existing materials to compare the moisture susceptibility of each. Preferably these should be about the same. A material that is more highly moisture susceptible may draw moisture from both the original section and from outside the structure. A material that is less moisture susceptible may send moisture into the original base, particularly during the original curing process.
- Apply level-up to match original HMAC surface elevation. Make repairs to original surface as necessary. Apply reflective crack retardant (geotextile, SAMI) if desired.
- Apply overlay across full section. Longitudinal mat joints should be placed at lane stripes when used.
- Insure ditch lines are sufficient to prevent hydraulic backflow into the pavement structure.
Figure 5-2. Unbound base sections.
Bound Base Sections
Other considerations will closely parallel those cited for the unbound base situation. There are cases where it may be desirable to use full-depth HMAC for the widening to expedite construction, even though the original bound base was cement-treated material. This strategy should not cause subsurface moisture flow problems (“bath tub” effect) provided that the cement treated base is not itself moisture susceptible. Laboratory evaluation of core samples will divulge the degree of moisture susceptibility of the existing base.
Recommendations for Dissimilar Cross Section Widening Strategies
This widening strategy is not encouraged because it often results in lateral subsurface drainage restrictions (“bath tub” effect). Were it possible to guarantee that no moisture would accumulate beneath the surface of the original structure, this type of widening would be easier to endorse. Areas of the State that experience low rainfall and deep water tables may experience better performance with this type of widening strategy. If this type of widening is considered in wetter areas, consideration should be given to installing subsurface drainage at the widening interface, with laterals to the ditch line. Unfortunately, this remedy is generally counter to expediting construction. Sources of in-pavement moisture accumulation are shown below.
Figure 5-3. In-pavement moisture accumulation.