When a rehabilitation investigation is being carried out, and associated designs are being developed, the effects of proposed treatments on the present road’s, or proposed road’s, geometry should be considered. Conversely the current or proposed road’s geometry may impact on what rehabilitation options are appropriate or feasible. Road geometry to be considered includes the horizontal and vertical alignments, cross fall, superelevation and the type cross section(s).
Opportunities to improve road geometry may arise when rehabilitation is planned. Examples would be designing and constructing road widenings so as to increase the (horizontal) radius of sharp bends/curves, and varying the transverse thickness of an overlay to correct crossfall/superelevation, application of superelevation or shape. (Related to this is correction of these and other aspects to address surface drainage, see Section 3.5.3.) Similarly works to correct the geometry of the road may present an opportunity to rehabilitate an exiting pavement.
If geometry is not considered, undesirable effects may result. An example would be increasing the carriageway width (through widening) on a narrow formation thereby increasing the slope of embankment or cut slopes to and undesirable level. Another is a geometric improvement that includes widening of the existing pavement by a (small) amount that is impractical to construct.
Chapter 2 also contains discussion about road geometry including guidance about when an evaluation of the road geometry is warranted. The Road Planning and Design Manual (RPDM) (QDMR Various) provides detailed guidance with respect to road geometry.
3.5.2.1 Restrictions due to vertical geometry
The limitations imposed by restrictions on altering the vertical alignment (e.g. raising the grade line) are often a major constraint. Situations likely to place such constraints on rehabilitation options include the following: • At intersections and some accesses (i.e. driveways).
• Where kerb or kerb and channel exists. This can be particular problem in urban areas and often in cuts.
• Where gullies that drain the surface exist.
• Where raising the grade line will cause problems associated with afflux (e.g. at flood ways in rural areas).
• Where road safety barriers exist, particularly if overlays have already been constructed since the barriers were first installed.
• Where other road furniture exists that would be adversely affected by a rise in levels (e.g. where slip bases on signs, street lighting and the like will no longer function correctly). This can be more of a problem where overlays have already been constructed since the road furniture was first installed. • Where vertical clearances under structures (e.g. sign or Intelligent Transport System [ITS] gantries
and pedestrian, road or rail overpasses) can not be altered. • At bridges which can not cope with increases in loading.
These limitations usually preclude the use of conventional overlay treatments and necessitate the use of alternative and frequently more costly solutions.
Such alternatives include:
• milling off or removing and replacing existing pavement layers.
• milling off or removing existing pavement layers immediately adjacent to the feature and overlaying. • in-situ stabilisation of existing materials.
• full reconstruction.
• use of asphalt with a polymer modified binder; • the use of warm mix asphalt; and
• combinations of the above.
In some cases it may be possible to adjust road furniture, etc (e.g. raising the kerb and channel to place an overlay, replacing road safety barrier) instead. However this option should be adopted with caution not only because of the additional expense, but also because it restricts choices in the future.
3.5.3 Drainage
Rehabilitation treatments also have the potential to affect the drainage of the road.
3.5.3.1 Pavement drainage
The effect a rehabilitation intervention has on pavement drainage must be considered. Drainage of the existing and proposed pavements must not be impeded. An example of this is ensuring a new widening does not impede drainage of the granular base and sub-base layers of an existing pavement.
3.5.3.2 Road drainage
Drainage can be affected by rehabilitation treatments. An example would be a new widening increasing surface flow paths leading to water film thicknesses greater than those recommended by the TMR Road
Pavement rehabilitation interventions may also present an opportunity to correct or improve any existing drainage issues.
Within the overall project therefore drainage needs to be considered. Ensuring it meets guidelines may however constrain which rehabilitation options can be chosen.
Chapter 2 also contains some discussion about road drainage. With the exception of pavement drainage the
RDM (TMR 2010) provides detailed guidance with respect to drainage.
3.5.3.3 Flooding
The level of flood immunity, and/or how flooding is catered for, along the road can also affect which rehabilitation options are viable. For example the use of floodways and their level of flood immunity may limit the options that can be used in these locations.
3.5.4 Safety
Safety requirements may also place constraints on the development of rehabilitation options. For instance: • if a road safety barrier exists and is to remain unmodified then thick overlays may not be possible; and • Open Graded Asphalt (OGA) surfacing may need to be used to address surface drainage concerns. Conversely pavement rehabilitation treatments have the potential to improve safety. For example:
• road geometry may be improved (refer to Section 3.5.2 and Chapter 2); • road drainage may be improved (refer to Section 3.5.3 and Chapter 2); • skid resistance may be improved (e.g. by providing a new surfacing);
• the provision of sealed shoulders (refer to Section 3.5.6) and/or how and when pavement widening is applied can improve safety (refer to Section 3.5.2).