Culway Installation

The design of the culvert for Culway should meet the following requirements:

  • The culvert should be as perpendicular to the road centreline as possible (with a skew tolerance of less than 5 degrees).
  • The culvert should be a single span reinforced concrete box culvert, precast, 'uncracked' with a desirable span of 1 800 mm to 2 400 mm. Spans that are ≥ 2 700 mm will create an undesirable situation where two or more axle groups may simultaneously cause strain on the instrumented culvert.
  • The internal height of the concrete box culvert should ideally be between 1 200 mm to 2 100 mm. The greater the height (but < 2 400 mm) the easier it is to maintain and house the Culway equipment (strain gauges and conduits).
  • The required cover for the culvert is based on cover at edge of seal. Pavement cover will ideally be between 600 mm and 1 000 mm on top of a culvert having a unit length of 1 200 mm (all culvert units that will be used to measure strain must be the same length). Note, as the culvert span increases the acceptable cover decreases.
  • The length and end treatments required should be designed in accordance with AS1597.2.

Culvert Selection

  • Do not use cast-in-place culverts if possible.
  • Use pre cast simply supported lid culvert cells if possible.
  • Inverted U culvert cells work OK but not as well as the pre cast.
  • New dry culvert installations must be based on a compacted crushed rock base.
  • Normal cells are 1.2 m width.
  • One MSA per cell is ideal (centred).
  • For new dry culvert installations line up the cell joint to the centreline of the lane.

Unit Influence Line (UIL)

When a wheel approaches and passes over a culvert, the response from beginning to end is described as the influence line. This is the effective longitudinal culvert length.

The UIL is important when considering the depth of cover required over the culvert; this is because Culway relies on the cone of influence from a passing vehicle influencing more than one gauge as it passes. This overlapping influence means that the strain counts from all the gauges are averaged allowing Culway to negate the effect of transverse positioning of the vehicle as it passes over the Culway.

Effective culvert span = Culvert span + (2 X depth of fill)

Figure: Culvert response for a single axle without any mutual influence

Fig 7.1

Source: Peters 1986

Construction Criteria

The Culway culvert is constructed as any normal drainage culvert by placing the standard RCB unit on a concrete slab floor (see figure above).

The culvert must be constructed to the same high standard as would normally be expected for drainage purposes (with a concrete floor and wing-walls, even if it is not intended to carry water). Care must be taken to ensure that the laying of the culvert units is done as evenly as the culvert units allow and avoids any damage to the culvert units. Should any culvert unit be damaged or if it sustains damage during construction, it must be replaced. Upon completion of the culvert the normal method of backfilling around the culvert units should be employed.

The placement of the culvert units has a large influence on the performance of the strain gauges. On a two-lane road the first culvert unit should straddle the centreline and bisect the 1 200 mm unit length, creating an offset of 600 mm from road centreline to where the next unit will join to it. The rest of the units must be placed next to one another until the design width of the culvert is achieved. This will allow for the even spacing of strain gauges across the lane width. This in turn ensures optimum gauge placement with a good cone of influence overlap between the gauges.

A Culway that is to be installed on a multi-lane carriageway must be designed with the ultimate lane configuration in mind. The placing of culvert units in this instance needs to be treated differently as the placement of culvert units must always start from the lane next to the median. Ideally the culvert units should straddle the lane and the edge lines should bisect the 1 200 mm culvert units.

Pavement Considerations

In the interests of a smooth finish and to avoid joins the 120 m lead-in to the culvert and lead-out from the culvert should be placed in the middle of a base-course lot, not at the ends or over a join. Ideally the culvert would have been installed when the road was built.

The same applies to the surface treatments; there should be no bitumen joins within the 240 m section that contains the Culway. The shape of the completed pavement over the 240 m Culway section must be judged to be acceptable when the maximum deviation from a 5 m straight edge placed in any position on the surface does not exceed 4 mm.

The pavement over the culvert should be well drained so as to maintain constant strength properties.

Road Surface Treatment

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 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.

Installation Guidelines

Placement of electrical equipment

All electrical equipment associated with the Culway must be installed outside the clear zone as defined in AASHTO-2006. 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.

Culway layout

The strain gauges used for weighing vehicles are mounted inside the culvert. The gauges are connected to a Culway logger along with other peripheral equipment such as power supply equipment, communications equipment, piezo axle sensors and gauge self-balancing buffer boxes. This equipment is housed in secure concrete or an IP66 rated electrical enclosure.

It is important that the electrical enclosure is located outside of the road's clear zone, on higher ground away from any watercourse. In certain circumstances it may be possible to house all the Culway equipment inside a dry purpose-built culvert.

It is highly desirable that the enclosure is not visible from the road to lessen the incidence of vandalism.


Strain gauges: The strain gauge cables typically use 100 mm conduit to run in from the culvert to the enclosure. In some circumstances the piezoelectric feeder cable can also run into the enclosure through this conduit.

Power: The Culway site is powered either by solar panels or a 240 AC mains or street lighting. A 240 AC mains connection requires the use of 40 mm conduit to run the power supply cable to the enclosure. Since solar electricity is already low voltage the cable can be run in 25 mm conduit or even flexi conduit.

Piezoelectric sensors: The piezoelectric sensor cabling runs from roadside pits to the enclosure using 40 mm conduit. The lightweight cable may also be run through the 100 mm conduit where convenient.

Strain gauge installation

The surface to which the strain gauges are attached (the underside of the roof of the culvert) should be uncracked and prepared so that the surface is smooth and true in all planes. Refinement of the culvert roof surface may be optimised by the use of a filler material such as plastic-bond whilst installing gauges.

Piezoelectric sensor installation

Installation of piezoelectric sensors must be done in accordance with instructions supplied by vendors (Refer to Piezoelectric WIM Installation and Central Weighing-2005).

Solar power and communications

Poles used for communications and solar power should be sited in a location where they are as inconspicuous as possible. This is not always possible.

To reduce the risk of vandalism, power and communications cables should be run inside their supporting pole structures and enter the electrical enclosure in conduit from the pole under the surface.

Security considerations

Vandalism and theft are potential problems at all Culway sites and it is advantageous that where the culvert height is 1 200 mm or greater that the equipment be housed inside the culvert as shown in the figure below. Not only will this assist security but also keep the equipment cool. Note the gates on this particular Culway are not as heavy duty as is considered desirable for a dry purpose-built culvert; a solid steel door would be more suitable.

Figure: Gate secured Culway installation and equipment housed within culvert

Fig 7.2

For some jurisdictions, such as DTEI, the preferred enclosure for all newly installed Culway WIM loggers and ancillary equipment is a concrete hut with steel door (for added security of equipment and protection of staff.

Figure: Concrete hut used to house Culway equipment (used for all new sites by DTEI)

Fig 7.3 

Occupational health & safety (OH&S) requirements

All field and site work for the installation and maintenance of Culway systems should be undertaken in accordance with the relevant OH&S policies and procedures. In particular 'Work Zone Traffic Management' procedures must be strictly followed (as per AS1742.3).


 Further Reading

Australian Standard, AS 1597.2 2013, Precast reinforced concrete box culverts: large culverts (exceeding 1200 mm span or 1200 mm height and up to and including 4200 mm span and 4200 mm height)

Australian Standard, AS1742.3-2009, Manual of uniform traffic control devices, part 3: traffic control for works on roads.

Main Roads Western Australia, 2003, Guide to the Design of Culway, Main Roads Western Australia, Perth, WA.

Central Weighing 2005, ‘Roadtrax BL piezoelectric axle sensor’, Central Weighing Australasia, Mooloolaba, Qld.

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