Guide to Design of Culway - WIM

The shape or the completed pavement over the 240 Metre Culway section shall be
judged to be acceptable when the maximum deviation from a 5 metre straight edge
placed in any position on the surface does not exceed 4mm.
The pavement over the culvert should be well drained so as to maintain constant
strength properties.
5. Surface Treatments
The road surface should be very smooth. A person in a car driven over the site at the
speed limit should not feel any bumps on the road surface.
The preferred surface treatment above Culway is 30 mm of 10mm dense graded
asphalt. Ideally the road should be sealed with a two coat primer seal as specified in
Main Roads specifications prior to a surface treatment of 30mm of dense grade asphalt
for a minimum distance of 120m before and 120m after the culvert.
Typically dense graded asphalt is not used for speeds greater than 80km/hr since the
surface texture of dense graded asphalt does not conform to MRWA requirements.
However, small lengths of dense graded asphalt are only traversed by a vehicle for a
short period of time (3-4 seconds) and are unlikely to result in the driver losing control of
their vehicle in inclement weather conditions. Considering that the Culway site should
be on a straight section of road, the use of dense graded asphalt over small section of
road does not create unsafe driving environment for the motorists. Therefore it is
recommended that for this purpose that dense graded asphalt be applied.
Under exceptional circumstances it may be permissible to apply two lifts of either 5mm
or 7mm microsurfacing over a 10mm primer seal in lieu of asphalt. In the event that
microsurfacing is not available a Main Roads designed multiple seal treatment may be
applied.
A 10mm dense graded mix should only be placed in layers no greater than 30mm thick.
A 14mm mix would suffice in one layer up to 50mm thick. It is recommended that 30mm
of a 10mm mix or 40mm of a 14mm mix be used for Culway sites.
Shorter sections of asphalt (i.e.30 Metres) may be permitted if allowed for at road design
stage since altering the road profile to allow a smooth transition from chip seal to asphalt
and vice versa would not pose a problem if done at this juncture.
The lane over the culvert should be clearly marked for the traffic as this encourages the
vehicle to track down the centre of the lane. Where lane discipline is poor the installation
of raised pavement markers may be considered.
6.0 Other Culway Design Considerations
The preferred Culway layout is shown on Drawing 200331-007. (A link will be added to
this drawing).
6.1 Placement of Electrical Equipment
All electrical equipment associated with the Culway shall be installed outside the clear
zone as defined in Table 3.1 of the AASHTO Roadside Design Guide. If this is not
practical, consideration should be given to providing a safety barrier not only to protect
errant vehicles from collision with the equipment but also to protect the equipment.
Page 8
Any safety barriers required shall be designed in accordance with the Guide to Safety
MAIN ROADS Western Australia
Guide to the Design of Culway.doc