Guide to Design of Culway - WIM

UGUIDE TO THE DESIGN OF CULWAY
Document No. 67-08-114
Standards > Road and Traffic Engineering > Roadside Items >
The information below is intended to reflect the preferred practice of Main Roads
Western Australia ("Main Roads"). Main Roads reserves the right to update this
information at any time without notice. If you have any questions or comments please
contact Glenn Walker by e-mail or on (08) 9323 4267.
To the extent permitted by law, Main Roads, its employees, agents, authors and
contributors are not liable for any loss resulting from any action taken or reliance made
by you on the information below or changes to its preferred practice.
CONTENTS
1. GENERAL STANDARDS AND APPLICATION
2. SITE LOCATION
3. DESIGN CRITERIA
4. PAVEMENT PROFILE AND SHAPE
5. SURFACE TREATMENTs
6. OTHER CULWAY DESIGN CONSIDERATIONS
7. CONSTRUCTION HOLD POINTS
8. REFERENCES
APPENDIX A
PENTAGON SYSTEMS PROCEDURE FOR CULWAY MSA STRAIN GAUAGE
INSTALLATION
1. General Standards and Application
This guideline outlines Main Roads policy and standards with respect to Culway sites so
that they are provided consistently and appropriately throughout Western Australia.
Culway is a high-speed weigh in motion (WIM) system conceived and developed by
Main Roads Western Australia. Culway produces both WIM (Weigh In Motion) and
Class Data.
Culway weighs and classifies traffic by using Mechanical Strain Amplifiers deployed in
an under-road box culvert structure. Vehicles passing over the culvert cause the roof to
deflect producing strain counts that can be converted to weights. The Culway system
employs 2 Piezo Electric Sensors, placed 10 Meters apart in each instrumented lane to
detect vehicle presence and synchronise the strain measurement for each axle.
Culway Data is used to optimise traffic movement information gathered by other systems
such as Counters and Classifiers. These systems cannot record the weight of vehicles,
nor differentiate between loaded and unloaded vehicles.
Culway data is an important tool in the efficient management of the road asset.
The information from Culway can be used to assist in:
• Bridge fatigue assessments
• Pavement design
• Assessment of damage to the pavement made by overweight trucks
• Freight transport task management
• Compliance with axle mass limits
MAIN ROADS Western Australia
Guide to the Design of Culway.doc

Spectrum of Bridge fatigue assessments need continuous and cumulative data to
provide the whole operating traffic load; including truck weight, vehicular configuration
and frequency of the traffic that uses any given structure. Bridge engineering studies in
Europe have shown that the 50 to 75% quartile of stress range distribution on a bridge
has the highest impact on bridge fatigue, long term changes to this distribution only
affects future lifespan not the elapsed one. Overestimations or crude assumptions will
heavily penalise bridges sensitive to fatigue. Bridge lifetimes can be greatly extended
and significant cost savings may be made where the load limits and density can be
reduced.
Culway data can be used to assist in the design of pavements in particular repair layers
to existing pavements as the data provided by Culway can give the Designer an
accurate picture of historical and current pavement loadings, which should lead to an
adequate and cost effective design.
Grossly overloaded trucks cause accelerated pavement wear, Culway data allows the
Asset Manager to budget and plan for future pavement reconstruction. Knowing the level
and frequency of overloading on a road will provide a more accurate estimate of
pavement life, thereby providing, improved safety, serviceability and cost savings to the
road owner.
It should be noted that Culway data is not accurate enough for enforcing axle-loading
regulations.
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 2

2. Site Location
The selection of a suitable location for a Culway installation is critical to the accuracy of
the information collected. The following criteria are to be used when assessing the
suitability of a site for Culway:
• Straight alignment (A minimum of 120 Meters each side of the Culway)
• Longitudinal grade preferably < 1% but not > 2%
• Crossfall between 2% and 3%
• Limited overtaking opportunity
• Open speed section
• A section of fill providing adequate cover for the culvert after ensuring a culvert of
minimum height 600mm can be placed above natural surface.
• Culvert to be dry, i.e. one not intended for drainage purposes.
These criteria are discussed in detail below.
The Culway should be located on a straight and level road section; sight distance should
not be so good as to make it an attractive overtaking opportunity for motorists. This
encourages poor lane discipline resulting in poor detection rates and low levels of data
confidence. Avoid placing Culway too close to curves since vehicles travelling at speed
may swing wide taking the curve and consequently affect the culway results by invalid
vehicle detections.
The speed of the traffic may also impact Culway results, normal highway speeds are
desirable for optimal operation. Slow speed, in particular speeds of less than 70kph may
cause problems with accuracy. Vehicles travelling below 30 km / h will probably be
overweighed.The presence of nearby intersecting roads or any other road geometry that
causes traffic acceleration or deceleration over the Culway is highly undesirable.
Ideally the longitudinal gradient shall be less than 1%, however the gradient may exceed
this where considered appropriate but gradients of more than 2% are not suitable.
Ideally the crossfall shall be between 2% and 3% with a minimum of 2%. The Designer
needs to balance the needs of Culway (i.e. as flat and smooth as possible) against the
engineering necessity of adequately draining the road.
The Culway should be located in a section where the fill is sufficient to achieve
essentially what will be a dry culvert or at least will ensure that any water will drain away
quickly from the Culway.
3.0 Design Criteria
3.1 Culvert
The design of the culvert for Culway shall meet the following requirements:
• Perpendicular to the road centreline.
• The culvert shall be a single span reinforced concrete box culvert, precast,
‘uncracked’ with a desirable span of 1200mm to 1500mm. Absolute maximum
span shall be less than 2700mm. Spans that are greater than 2700mm will
create an undesirable situation where two or more axle groups may
simultaneously cause strain on the instrumented culvert.
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 3

• Multi cell culverts may be used however the culvert should not have a slab lid
that bridges more than one cell of the culvert. If this type of culvert is used,
instrumenting the first cell will produce better results because the steer axle will
be less influenced by the heavier drive axles.
• The height of the concrete box culvert can vary between 600mm to 1500mm.
(The desirable height being 1500 mm). The greater the height the easier it is to
maintain and house the Culway equipment.
• Pavement cover will ideally be 800mm on top of a culvert having a section length
of 1200mm. Table 1 below shows acceptable configurations. Note as the Culvert
span increases the acceptable cover decreases. Conversely maintenance and
locating the electrical enclosure within the culvert is made easier as the height
and span increases. The height of the culvert is only a consideration where the
intent is to house the equipment within the culvert, or where the road
embankment fill level is marginal for a dry culvert.
Table 1. Ratings of Culway Installation Culvert Size/Level of Cover Configuration
• Cover is normally measured from the top of the pavement at edge of shoulder,
however required cover for Culway is based on cover at edge of seal.
3.1.1 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. (See Figure 1.)
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Unit
Influence
Line
Percentage
Unit Influence
Line (UIL)
Acceptable
Configuration
Score 1 - 4
Culvert Section
Cover
Length (mm) Span (mm) (mm) (mm) Error
1200 1200 600 2400 0 0
1200 1200 700 2600 0.16 1
1200 1200 800 2800 0.27 4
1200 1200 900 3000 0.9 4
1200 1200 1000 3200 2.2 3
1200 1200 1100 3400 6.4 1
1200 1200 1200 3600 9.8 0
1200 1500 700 2900 0.68 2
1200 1500 800 3100 1.27 4
1200 1500 900 3300 3.85 3
1200 1500 1000 3500 8.48 0
Page 4

Figure 1. Culvert Response for a single axle without any mutual influence. (Peters
1986)
Effective Culvert Span = Culvert Span + (2 X Depth of fill)
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.
3.1.2 Culvert Requirements
The Culway is constructed as any normal drainage culvert except that instead of placing
standard RCBs on a concrete slab floor, as is the normal practice in Western Australia,
the RCB’s are inverted with simply supported box culvert lids to provide the weighing
platform required for Culway.
The culvert sections will be bedded on to at least 100mm of non plastic compacted sand.
Care shall be taken to ensure that the laying of the culvert sections is done as evenly as
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 5

the culvert sections allow and taking care to avoid any damage to the culvert sections.
Should any culvert section or lid 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
sections with cement stabilised material should be employed to the level of the top of the
culvert section lids. The material used as cover over the culvert shall not differ from the
material being used either side of the Culway; however where stabilization is being
employed the unconfined compressive strength of any layer shall not exceed 1 MPA.
The placement of the culvert units has a large influence on the performance of the strain
gauges. Given that all of the current Culways are located on rural 2 lane roads having a
lane width of 3500mm, the culverts units shall be placed starting at the road centreline.
The first culvert section should straddle the centreline and bisect the 1200mm section
length, creating an offset of 600mm from road centreline to where the next unit will join to
it. Placing the rest of the units next to one another until the design width of the culvert is
achieved, will allow for the even spacing of gauges across the lane width. This in turn
ensures optimum gauge placement with a good cone of influence overlap between the
gauges.
Refer to Section A of Drawing 200331-007 for typical set-up of a Culway installation.
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. Specially ordered shorter length culvert units will be required to get
the instrumented culvert units in the right place.
Consideration needs to be given to the ultimate configuration of the carriageway if
different from what is proposed to be initially constructed e.g. a three lane carriageway
often is initially constructed as a 2 lane carriageway, with the third lane constructed as
required. In this case it may be more cost effective to install the culvert for the ultimate
situation, as it would remove the need for headwalls for each end of the culvert ending in
the median.
The placement of culvert units for a multi lane carriageway is similar to a single lane rural
application in that the first 1200mm culvert unit placed in both carriageways needs to
straddle the edge line of the lane next to the median creating a 600mm offset from the
edge line. Two more 1200mm culvert units are placed adjacent to this followed by a
special shorter length unit of 1100mm. This pattern is then repeated for each
instrumented lane before reverting to normal culvert placement practice. Note a shorter
length cell may be required to join the two carriageways where the culvert is continuous
across the highway.
Refer to Section B of Drawing 200331-007 for typical set-up of a Culway installation for a
two lane dual carriageway.
3.1.3 Backfilling of Culvert
Backfill over the Culway slab should be as shown in Figures 2 & 3 depending on the
height between the bottom of the pavement and the top of the culvert (H in the figures).
Figure 2 Backfilling of Culvert where Cover at edge of seal is 600mm or less
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 6

600 < H
CULWAY
PAVEMENT
BASECOURSE
QUALITY MATERIALS
CEMENT STABILISED
BACKFILL
Figure 3 Backfilling of Culvert where Cover at edge of seal is more than 600mm
PAVEMENT
600 < H < 1100
CULWAY
3.1.4 Other Construction Considerations
600
BASECOURSE
QUALITY MATERIALS
SUB BASE QUALITY
MATERIALS
CEMENT STABILISED
BACKFILL
In the interest of a smooth finish and to avoid joins the 120 meters lead in to the culvert
and lead out from the culvert should be placed in the middle of a basecourse lot, not at
the ends or over a join.
The same applies to the surface treatments, there should be no bitumen joins within the
240 Meter section that contains the Culway.
3.2 Length and End Treatments
The length and end treatments required shall be designed in accordance with the Guide
to Culvert Design.
4. Pavement Profile and Shape
When specifying a Culway site a distance of 120 metres either side of the Culway
installation should be as smooth and flat as the road design will allow, ideally the culvert
would have been installed when the road was built.
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 7

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

Barrier Design.
6.2 Culway Layout
The strain gauges used for weighing vehicles are mounted inside the culvert as shown
on Drawing 200331-007. The gauges are connected to a Culway logger along with other
peripheral equipment such as power supply equipment; communications equipment;
detector interface and gauge self balancing buffer boxes. This equipment is housed in
an IP66 Rated Electrical Enclosure. It is important that the Culway’s electrical enclosure
is sited outside of the road’s clear zone, on higher ground away from any watercourse. In
certain circumstances it may be possible to house the enclosure inside the culvert (Refer
to Section 6.6) It is highly desirable that the enclosure is not visible from the road to
lessen the incidence of vandalism.
6.3 Cabling
Strain Gauges: The Strain Gauge Cables need 100mm Conduit to run in from the
Culvert to the enclosure. In some circumstances the Piezo cable can also run into the
enclosure through this conduit.
Power: The Culway site is powered either by Solar Panels or a 240AC Supply provided
by Western Power. A Western Power connection requires the use of 40mm conduit to
run the power supply cable to the enclosure. Since Solar Electricity is already low
voltage the cable can be run in 25mm conduit or even flexi conduit.
Piezo Electric Sensors: The Piezo Sensor cabling runs from roadside pits to the
enclosure using 25mm Conduit. The lightweight cable may also be run through the
100mm conduit where convenient.
6.4 Gauge Installation
The surface to which the strain gauges are attached (the underside of the lids of the
culvert) should be uncracked and prepared so that the surface is smooth and true in all
planes. Refinement of the Culvert lid surface may be optimized by the use of a filler
material such as plastibond whilst installing gauges.
Main Roads Western Australia has adopted Pentagon Systems “Procedure for Culway
MSA Strain Gauge Installation”. All installation of strain gauges shall be done in
accordance with this procedure. The procedure can be found at Appendix A.
6.5 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 cable will be run inside their supporting pole
structures and enter the Electrical Enclosure in conduit from the pole under the surface.
6.6 Security Considerations
Vandalism and theft are a potential problem at all Culway sites and it is advantageous
that where the culvert height is 1200mm or greater that the equipment be housed inside
the culvert as shown 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 now
considered desirable (See Figure 4).
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 9

Figure 4 Gate Secured CULWAY Installation and Equipment House within Culvert
at Norseman
7. Construction Hold Points
Once the culvert has been installed a hold point of 1 day will be required after the
Basecourse is finished and before the bitumen treatment is applied to allow the Culway
Technicians to cut 2 or 4 x 25mm cube section into the road surface to allow the
placement of Piezo Electric Sensors into the Basecourse.
Figure 5 Installation of Piezo Sensors prior to seal coats being applied.
A Urethane modified epoxy with high wetting properties such as Epirez D5-050 Saw Cut
Sealant will be used to stabilise the sensors and fill the voids created level with the
existing Basecourse. Surface treatment can then begin. This kind of sealant usually
requires a day to cure and develops to full strength in 7 days.
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 10

8. REFERENCES
R J Peters Main Roads WA
Culway Notes 1985 Incorporated into ARRB Documentation
Bernard Jacob
COST 323 Weigh in Motion of Road Vehicles Final Report 1993 – 1998
AASHTO Roadside Design Guide (2002)
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 11

UDefinition
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Appendix A
Pentagon Systems
Procedure for Culway MSA Strain Gauge Installation
This information note has been produced to describe a technique used by Pentagon
Systems for many years as a method for repeatable and proper strain gauge placement.
Strain gauges are used to measure the strain on the underside roof of a culvert as a
vehicle pass over.
The strain transducer and matched buffer amplifier assemblies are calibrated by ARRB
by applying known mechanical strains to the transducer and then adjusting the buffer
amplifier gain to obtain a correct electrical output. These matched assemblies are then
issued with a calibration certificate. In-field calibration of a buffer is possible (refer to
ATM 22).
The strain transducers are machined from mild steel. In operation a displacement
measured over the 300mm length of the gauge is focused on the 30mm ring. This
produces a mechanical amplification of about 10.
UHandling
Strain transducers or MSA’s are highly sensitive calibrated pieces of equipment that
must be handled with extreme care to ensure accurate continued operation.
MSA’s should always be picked up from the centre ring and never from the ends as their
own weight could cause permanent deformation. Padded instrument suitcases should
always be used when storing and transporting gauges to site. Gauges should never be
dropped or have external pressure applied to them as damage could result. Any
permanent deformation to the gauge will result in an electrical output offset that in some
causes cannot be balanced out with the associated buffer amplifier. The transducer or
MSA in these circumstances must be either repaired or replaced.
UInstallation
This technical note does not cover details in regards to placement of MSA’s as this
varies with each site. This document however attempts to explain a tried and proven
procedure which is repeatable for securing gauges to the roof of the culvert.
Poorly mounted and secured gauges can cause a number of problems in relation to site
calibration, site drift and maintaining a consistent performance from the site. Gauges
which are mounted or secured with some element of deformation due to the method they
were secured or the fixing points can produce inconsistent outputs when put to work.
The gauges, by design have been calibrated to operate within a range. When attempts
are made to operate these gauges outside this range, erroneous results can lead to the
problems that have been identified previously.
Before any gauge is secured to the roof of the culvert the following precautions should
be made to ensure the gauge is not damaged and that it can operate in a not deformed
or pre-stressed state.
• The use of a gauge template (a steel bar which is dimensionally identical to the
Page 12

MSA) for drilling of mounting points to the culvert roof. Template bars with
alignment points should be used to exaggerate any unevenness in the culvert
roof. This will assist the technician to take corrective measures to fix the problem.
• Mounting studs must be perpendicular to the culvert roof. The studs must be
aligned to the centre of each gauge-mounting hole. Out of alignment studs will
result in MSA’s being prestressed before they are secured.
• Spacers and washers with large surface areas must be used to disperse the load
to the gauge as it is tightened to the culvert roof.
UGauge tensioning
Tensioning a gauge to the roof of a culvert is in some way similar to tensioning a cylinder
head to an engine block. Extreme care must be taken to ensure the tensioning
procedure produces consistent results with little or no prestressing.
The gauges are much more fragile and must be secured carefully. A method which is
used by a number of technicians requires the use of an old style buffer amplifier which
gives the operator a visual real-time indication of what stresses are being applied to the
gauge while it is being tensioned to the culvert roof. The newer style buffer amplifiers are
all of the auto balance type that makes continuous changes to balance that is unsuitable
for this exercise.
There is no need to have a matched buffer amplifier for this exercise, as we are only
interested in the relative changes to output as displayed on the analogue meter on the
front of the unit.
Firstly, the buffer amplifier must be zeroed with a known good working MSA prior to
tensioning any gauges on site. This establishes a baseline for unknown damaged
gauges.
The first of the site gauges is connected to the manual buffer box before it is placed in
position and which should only require a small amount of adjustment to be zeroed. This
is usually carried out while the gauge is on the floor. Gauges that deviate dramatically
from the zeroed point have some mount of permanent deformation that results in the
electrical offset described earlier. Depending on the amount, the gauge should be
returned to ARRB for checking and possible recalibration.
The gauge should now be placed in position while still connected to the manual buffer
amplifier and the securing nuts should be tightened by hand while at the same time the
meter of the buffer amplifier is viewed to determine if any induced stresses are occurring.
Little to no movement to the needle should be observed during this process.
A ring spanner, torque wrench is used to tension each nut such that the needle moves
no more than a 1 – 2 mm in either direction. The pattern of tensioning each bolt should
occur until such time the desired tension is achieved. By the time the tensioning has
finished the needle should be as close to zero as possible.
If the needle fails to zero during the tensioning process a problem exists with the
mounting location or mounting points. Ensure the surface is flat and the bolts are aligned
centrally to the gauge holes and perpendicular to the culvert roof. A right-angle square
assists with this procedure.
Adopting this procedure ensures all gauges have been mounted in a manner which
ensures minimal to no pre-stressing and ensures that the re-mounting of a replacement
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 13

gauges can be achieved with minimal variation to output.
MAIN ROADS Western Australia
Guide to the Design of Culway.doc
Page 14