Version 2.0
Annex 3: WDM deterioration modelling using HMEP compliant methodology.
1.
INTRODUCTION
Northumberland County Council (NCC) commissioned WDM limited to undertake lifecycle modelling
on a selection of roads for the purposes of a bid from the DfT challenge fund. The network includes
a total of 160km of A, B, C and U class roads.
This has been done by using a deterministic deterioration model developed for use for the Scottish
Local authorities.
1.2
Condition data
The model uses the current condition data collected from SCANNER surveys which include full
network coverage on roads suitable for SCANNER in 2013 and 2014
The modelling is based on the A, B and C class roads for which there is SCANNER data. In addition
there is 18km of unclassified roads, approximately 11% of the overall length. Based on visual
surveys it is considered that the cost predictions can be factored up to include the unclassified
roads by around 11%.
SCANNER assesses road surface condition through a number of parameters. If the roads are
exhibiting structural failures which are not manifesting themselves in surface condition this will not
be included within the modelling. SCANNER is collected by lane, and the models use lane data for
the purposes of analysis. All treatment lengths are based on the lane lengths.
1.3
Model parameters
The modelling exercise has assessed the impact of the additional traffic on the routes over a period
of 35 years from 2015. It reports all costs in terms of 2014/15 prices, and doesn’t consider the
following:

Delay costs during roadworks due to the longer time require for closures due to the
increased requirement for reconstruction.

Other costs and benefits not directly related to carriageway condition

The data available did not include and structural surveys; i.e. Deflectograph. This can
assist is assessing the structure of the road, and identify the optimum time to treat before
damage manifests itself.
The available data has been summarised at 100m lengths for the purpose of modelling and as an
appropriate length for a meaningful maintenance scheme, and includes:

SCANNER – rut depth, 3 and 10m profile, cracking % and texture depth.
The model was built to include a calculation of the National Indicator Road Condition Indices using
SCANNER weighting (adjusted for the affect of averaging at 100m length).
The current condition on the prescribed network is:
SCANNER RCI > 100
6.9% (RED)
SCANNER RCI 40 - 100
27.8% (AMBER)
This applies the appropriate thresholds for the A, B and C class roads detailed in the National
Indicators 130-01 and 130-02.
2.
DETERIORATION MODELLING
In order to predict the need for treatment, the consequence of investment and the effects of
deterioration models were developed. This model enabled the assessment of cost associated with
different investment strategies.
2.1.
Carriageway deterioration (SCANNER)
The modelling uses individual parameter scores as a means to identify treatments and future
condition predictions. In order to support this analysis WDM® provided results from detailed studies
undertaken on condition data from Scotland and Cornwall to assess deterioration trends on their
respective networks.
These studies considered data from up to 10 years worth of surveys on the respective networks
and assessed the change in condition over time. Based on the results of these studies deterioration
rates for rutting, 3 and 10m profile, texture depth and SCRIM have been derived and applied in the
models. Cracking data has only been collected since 2004 and there have been developments in
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the algorithms used to process the data. It has therefore not been possible to derive relationships
for cracking data and an empirical relationship has been applied. Table 1 provides the deterioration
rates used for the parameters in the carriageway deterioration modelling without any additional
traffic.
Parameter
Annual deterioration rate
Rutting
+0.30mm
3m profile
+0.2mm2
10m profile
+1.0mm2
Texture depth
-0.03mm
Cracking
+10% (if WTRC = 0 +0.03% in first year)
Table 1: Standard deterioration rates
For the do ‘something’ the same data from the studies was used; however the deterioration rates
applied were at the higher values identified from the sections of road.
A scenario with ‘accelerated deterioration has been included. This applies a higher deterioration
rates to rutting and 3m profile during the timber extraction. These parameters have been selected
as those most closely aligned to the structural condition of the road pavement. The ‘accelerated
rates’ are the highest observed from the studies.
Parameter
Annual deterioration rate (normal)
Rutting
+0.50mm
3m profile
+0.25mm2
10m profile
+1.7mm2
Texture depth
-0.03mm
Cracking
+15% (if WTRC = 0 +0.03% in first year)
Table 2: Deterioration rates: Do Something
2.2.
Annual deterioration rate (accelerated)
+0.60mm
+0.40mm2
+1.7mm2
-0.03mm
+15% (if WTRC = 0 +0.03% in first year)
Treatments
The deterioration model includes the identification of treatments for each 100m sub section of road
in the model. Figure 1 illustrates the application of treatment through the model. This considers
the SCANNER defects present in terms of:
below lower threshold:
green
between lower and upper threshold:
amber
above upper threshold:
red
1) Micro-Asphalt / Surface Dressing
RUT
3m
10m
WC Crack
Texture
OR
OR
OR
OR
2) Thin Surfacing
RUT
3m
10m
WC Crack
Texture
OR
OR
OR
OR
OR
OR
3) Thin Overlay
RUT
3m
10m
WC Crack
Texture
OR
OR
OR
OR
OR
OR
OR
OR
10m
WC Crack
Texture
OR
OR
OR
OR
4) Reconstruction
RUT
3m
Or
OR
5) Moderate Overlay
RUT
3m
10m
WC Crack
Texture
OR
OR
OR
OR
OR
OR
OR
OR
OR
RUT
3m
10m
WC Crack
Texture
OR
OR
OR
OR
OR
OR
OR
OR
OR
OR
6) Thick Overlay
RUT - Rut Depth
3m - 3m Longitudinal Profile Variance
10m - 10m Longitudinal Profile Variance
WC Crack - Whole Carriageway Cracking
Texture - Texture Depth
Figure 1: Model treatments using SCANNER defects.
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For the Northumberland modelling these treatments have been applied as:
Resurfacing: Surface treatment and thin surfacing
Structural treatment: thin/ moderate overlay
Reconstruction: thick overlay/ reconstruction
2.3.
Reset Values
Following treatment reset values are applied to the SCANNER data. The reset values have been
derived from the Cornwall study which considered approximately 30 schemes treated in 2008, and
surveyed in 2009.
Figure 2 illustrates the theory used in determining reset values.
Figure 2: The calculation of reset values (rutting)
Using this methodology the reset values used in the deterioration modelling are shown in table 3.
Parameter
Reset
value
treatment)
Rutting
3m profile
10m profile
Texture depth
Cracking
Table 3: Reset values
n/a
n/a
n/a
1.0mm
0%
2.4.
(surface
Reset
value
treatments)
(other
2mm
0.4mm2
2.2mm2
1.0mm
0%
Assigning treatments within model
The treatments are applied to include ‘red treatment’ as well as ‘amber’ to model a combination of
worst first and preventative maintenance. The actual lengths treated are determined by the model
requirements. The following process is applied.
1. Assess starting condition based on survey by converting Raw data to equivalent RCI
2. Apply deterioration to Raw data and assess resulting scores after deterioration period
3. Identify 100m lengths that require treatment based on condition
4. Determine type and extent of treatment required using treatment matrix
5. Apply reset values to treated lengths
6. Calculate condition after treatment
7. Reassess condition after treatment and deterioration for subsequent period
2.4.1.
Calculating Treatment costs
For those 100m lengths that have been identified for treatment, a cost to treat the Monitoring
Length in accordance with the treatment matrix. A treatment is identified for every 100m length,
and treatment cost calculated.
3.
MODELLING SCENARIOS
The modelling has been run on 2 scenarios:
1. Current budget. No change to traffic
2. Steady state funding with accelerated deterioration from 2015 -2025, with the
challenge bid funding used to carry out treatments in 2015 – 2017.
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3.1.
Current budget: no change to traffic
This model applies a predicted expenditure of £350k per year to the network and forecasts the
condition.
The model shows that the condition; as reported by the national indicators increases rapidly from
6.9 in 2015 (current) to 29.8% by 2025 and 71.4% by 2035, and then begins to level out at this
value. The total investment is £12.7m over the 35 years.
3.2.
Challenge bid funding.
This model applies a £42m programme of works in 2015 – 2017 and applies a higher deterioration
rate to the rutting and 3m profile in years 2015 – 2025. These are the main structural parameters
included in the SCANNER National Indicator and it is considered that with the number and types of
vehicles anticipated are likely to result in high rates of deterioration. The initial treatments address
all of the red and amber on the network, and this is sustained for a period of 10 years. Following
this 10 year period parts of the network stat to become amber. Due to the ‘up front’ nature of the
treatment the deterioration assumptions tend to result in works identifed in groups later in the
lifecycle.
Between 2030 and 2033 the model predicts that a programme of surface treatment; i.e. surface
dressing would be required to act as a preventative treatment.
The model then predicts a thin overlay treatment from 2039 as the accelerated deterioration
applied to the rutting from 2015 – 2025 starts to trigger treatments to manage the amber.
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Scenario 2 Deterioration with £43m investment in 2015-2017 (TEMPRO
traffic growth with timber traffic at 2015 levels )
100%
90%
Road Condition Index
80%
70%
60%
50%
40%
30%
20%
10%
0%
2015 2017 2019 2021 2023 2025 2027 2029 2031 2033 2035 2037 2039 2041 2043 2045 2047 2049
Year
What it looks Like
RCI Red category :
severe deformation & structural failure
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RCI Amber category :
Deformation of road shape, Extensive fretting, increased reactive
maintenance needed
RCI Green category :
Generally good road shape, sound surface conditions, minor surface
irregularities
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