Cross-section:
when inspecting a road network the following data about the
cross-section must be collected:1) the distance between both left
and right ditches; 2) the width of the rolling surface. These
are necessary to determine quantities of work to be done to
restore and maintain the road. The width of the rolling surface is
that which can be driven on during the visual inspection.
Definition of types of structural damage
There
are four types of structural damage on which data must be obtained.
These are summarised on four tables which
provide guidelines for estimating the severity of each. Each table
provides, for each level of severity of surface damage, default values
for the extent, a visual aid to evaluation, effect on driving behaviour,
and the type of corrective works required.
Deformation
Deformation
can consist of simple gravel
loss
due to traffic or
rain, or in the more severe forms of rutting
or subsidence.
It indicates structural damage caused by drainage problems
together with gravel losses. It depends upon traffic,
topography (loss is high on slopes and bends) and rainfall
(see table below). In low-volume roads, where conventional vehicles
rarely exceed 10-20 vpd, traffic is a lesser factor, while topography
and rainfall become critical, since roads are build to simple standards,
with steep slopes and sharp bends, augmenting gravel loss. Severity
can be quantified, if necessary, by measuring the deformation depth
using as 1.50m straight edge, but during rapid inspection it is usually
assessed visually.
When
surveying a network, distinguishing between the three types of deformation
is not necessary as it would complicate data collection without providing
compensatory precision. It will be necessary to distinguish between
these three possible causes later at the
design stage since they influence rehabilitation needs in different
ways. In the case of gravel loss due to traffic, maintenance may be
sufficient (grading if the depth is shallow, and gravel resurfacing,
or spot regravelling if deformation is severe), while in the case
of rutting and subsidence, which can affect the structure, local reconstruction
may be required.
|
Traffic
|
Normal
gravel loss (mm per year)
|
|
Rainfall
(mm per year)
|
Type
of Terrain
|
|
0
- 50
|
15
mm
|
|
<
1250
|
>
2500
|
|
|
50
- 100
|
20
mm
|
|
+0
%
|
+15%
|
Flat
|
|
100-
200
|
25
mm
|
|
+15
%
|
+30%
|
Hilly
|
|
200
- 400
|
30
mm
|
|
+30
%
|
+45%
|
Mountainous
|
|
>
400
|
35
mm
|
|
|
|
Potholes
Although
on paved roads potholes indicate surface rather than structural damage,
on unpaved roads, they are considered as structural damage since the
surface course is also the base course, and potholes tend to grow
and reproduce rapidly. The impact of potholes on maintenance works
is related rather to their number than to their size or depth. A small
number of potholes along a given section may be repaired by gravel
resurfacing and local compacting, while a high density requires spot
regravelling.
Corrugation
Corrugation
results from separation of base course material due to oscillatory
wheel impacts, which tend to be self-reinforcing. Although related
to the type of material and not the thickness of the road structure,
corrugation is considered as structural
damage because it reaches the minimum
thickness of gravel cover at the low points of the corrugation. When
the depth of the corrugation is slight, grading may be sufficient.
If deep, gravel resurfacing is required.
Gullies
Longitudinal
gullies are channels scoured
by high-velocity water flow down slopes. They can be found in the
middle or on the sides of the road. They result from drainage problems
and not from structural weaknesses. However, they are seen as structural
damage because they may result in deep erosion of the base course
requiring reshaping ditches and cross-section.
Lateral
gullies result from localised deficiencies in transversal
water drainage causing the water to flow over rather than under the
road. They are discussed separately in the section
on drainage below.
Calculation
of the Structural Quality Index (SQI)
The
Structural Quality Index (SQI) can be assigned four levels (from 0
to 3) corresponding to the extent of each type of damage. The quality
index of a given section is assumed equal to the highest severity
level of the four different types of damage observed on that
section. The type of maintenance or rehabilitation work required on
a given stretch of road is derived from the damage having the highest
severity level. The type of task to be performed does not change if
there are one or more of the same severity level. In general, road
sections of levels 0 and 1 can be restored by immediate routine and
periodic maintenance while roads of SQI 2 or 3 will require rehabilitation
works to restore them to maintainable condition.
SQI
= Max Severity Level [Deformation, Corrugation, Potholes, Gullies]
The
following figure shows the calculation of the
SQI for a section of road. While the vehicle travels along the road,
the type of damage and its severity is noted manually on horizontal
linear diagrams, generally divided into columns representing 100m.
The vertical axis may be divided into rows, each corresponding to
a type of damage. This will allow the use of previously agreed upon
symbols rather than text. In cases
of severe damage, where extensive works may be necessary, a stop may
be necessary to allow a brief visual inspection. Such stops must be
limited since these surveys must cover between 50 and 100km per day.
It is recommended to use the hours of daylight for the survey and
to calculate the SQI at the end of the day.
Example
of calculation of SQI for a section of road
|
Damage
|
Severity
of the damage along the road (distance progressing from left
to right)
|
|
Deformation
|
0
|
3
|
1
|
2
|
1
|
3
|
1
|
|
Corrugation
|
0
|
1
|
0
|
|
Gullies
|
0
|
3
|
1
|
2
|
1
|
|
Potholes
|
0
|
1
|
2
|
|
Calculated
SQI
|
0
|
3
|
2
|
2
|
3
|
2
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
The notes above apply to visual inspections When surveying roads with
computerised condition measurement systems such as
ROMDAS, the quality index is automatically calculated by the software.
Visual
inspections cost less, since they only require a 4x4 vehicle and driver.
They are usually more appropriate for low-volume rural road inspection.
In addition, the work can be delegated to local technicians, allowing
a number of road links to be surveyed in parallel and at the same
time, providing a training ground for future technical personnel.
This training is essential, using visual aids to ensure that variations
in assessment among inspectors due to subjectivity are minimal, and
to maintain consistency by constant monitoring of results.
Computerised
measurement systems require expensive equipment and specialised personnel.
However, costs will probably diminish over time with technical progress.
They have the advantage of saving time and ensuring internally
consistent results. They are more or less mandatory for main and secondary
road inspections.
Finally,
it should be noted that dust
problems, generally proportional to the speed and weight of vehicles,
and thus encountered mainly on secondary gravel roads rather than
tertiary low-volume roads, should at least be noted since they will
probably be raised during negotiations about whether a road should
be surfaced or not. The nuisance factor of dust, which has been a
bone of contention for many years (see video:
dust), is more and more resented by intermediate
transport users (IMT) and people living close to the road.
Their grievances should be taken seriously. It can carry disease,
render houses difficult to keep clean and of course spoil the daily
wash. Frequently, the population, supported by their politicians,
insist on surfacing to eliminate dust, whether or not it is economically
viable. In some cases they will actually surface already rehabilitated
gravel roads at their own expense, particularly in villages and towns.
This is expensive.
In
general the point at which a road should be surfaced is determined
when the marginal operating cost savings to users (calculated normally
by HDM4
or RED) are greater than the marginal costs of surfacing.
It is now a general feeling that that lifetime costs of
gravel surfacing have been pitched
too low. Good gravel is scarce and often distant and for that reason
costly. As a result, the volume of traffic needed to justify resurfacing
has perhaps been set too high.
Types
of damage to drainage structures
Road
drainage inspection requires data about type of material, topography,
rainfall, and type and condition of structures encountered. During
visual inspection data collection must be simplified to keep the cost
and time needed by data collection within reasonable limits. In surveying
the type and extent of maintenance works, observations must be limited
to rainwater drainage. Drainage of underground water and dimensioning
the structure to ensure evacuation of water flowing from the catchment
area are included during the detailed design stage should full rehabilitation
be required.
Deficiencies
in the drainage system for rainwater can result in four different
types of damage:
1)
Longitudinal
Gullies are
channels in the road surface scoured by high-speed water flow on slopes
which has not been diverted to the side ditches due to a deteriorated
profile or inadequate longitudinal drainage. Due to ditches that are
blocked or undersized . As indicated above, these have been classified
as structural damage. The severity of this damage depends upon the
depth of the gully in cross section rather than upon its length. Blading
may be sufficient when the gully is shallow. Depths over ten centimetres
cannot be repaired and gravel resurfacing must be performed along
the entire width of the road.
2)
Lateral
gullies
are formed by water flowing across the road rather than under
it in a culvert. They may be found at
low points where a culvert is broken, blocked or missing. It may also
result from a blocked lateral drainage ditch. They result in localised
and often expensive repairs difficult to estimate during a rapid inspection.
Severity can be estimated approximately based upon length or width.
However, the type and extent of repairs necessary are not always related
to these simple indicators. Evaluation of the gravity of damage should
be left to the detailed design stage.
3)
Mud
or soft spots in general consists of localised destruction of the
pavement at locations of change of slope due to standing water and
traffic. Severity can be estimated based the difficulty encountered
to cross this stretch of road. However, as with lateral gullies, the
severity of this type of damage is not as important as its extent.
Mud extending only over a few meters (most frequently found) can be
dried out by draining the soil or building a culvert. If it extends
over significant distances, raising the level of the road or changing
the type of material used may be necessary.
4)
Ditch or side erosion
results from rapidly flowing water scouring the ditch or side of the
road, often carrying it away, due to lack of protection or poor materials.
During visual inspection it is possible to note the problems but not
to determine the solution which will be determined at the detailed
design stage. It will require local rebuilding.
Factors
to be assessed during drainage inspection
Some
factors to be noted during visual inspection of drainage structures,
which can contribute to the damage described above to varying extents,
and which can be corrected by maintenance or rehabilitation works
are:
1)
The Road
level compared with ground
level may be: flat (road at the same level as the ground level);
in fill (road above the ground level); in cut (road below the
ground level); or in cut and fill (road above the ground on
one side, below the ground level on the other side). This information
can be easily recorded during visual inspection. However, in general,
the level of the road will not affect on the maintenance policy. It
is necessary should rehabilitation be required
2)
Ditches
may be noted as being absent, in good condition, in bad condition,
or eroded. The type of
ditch should be recorded (earth, concrete or
masonry).
3)
Large structures
such as bridges are useful as reference points. However, their rehabilitation
can make up a large fraction of the cost of works. The following information
should be recorded at this stage: location (distance from the origin
of the surveyed itinerary), length, and condition of the surface of
the bridge and parapets.
4)
Small structures
like pipe, box, and slab culverts must be taken into account during
inspection. However, it is not possible to spend much time recording
detailed information about culverts during rapid visual inspections.
Generally, it is sufficient to note their number and location. When
a culvert is demolished or blocked, mud is likely to be present and
should be noted. Stopping at each culvert to
note information not visible from a moving car is unrealistic, since
an inspection takes in general about ten minutes per culvert (in general
there are at least two culverts per km). This is not compatible with
the required rate of visual inspection which should be about 50 to
100 km per day depending on the state of the road and the size of
the network.
Using
rapid inspection data
Rapid
inspection data provides the basis for estimating the volume of works.
Using standard costs, usually derived from past contracts in the same
region, we can get a preliminary estimate of the expenditure by road
link on the different types of works necessary to restore the at least
the core network to good-enough
status. This, combined with socio-economic information, will provide
estimates of cost-effectiveness or cost–benefit of improvement
by link, and a basis for the iterative
consultations between the various stakeholders, different levels of
government, financing agency, and users. These lead to agreement on
a long-term prioritised and coherent programme framework and a short
or medium term action plan for the core
network, together with the nature and costs of rehabilitation
and maintenance works by link and in total (return
to costing page).
An
example of the procedure is provided here
Any comments
or suggestions?
Links
