Guide to rapid road inspection

 

 

 

 

Rapid inspection methods

Definition of road improvement tasks

Information required on  road geometry

Definition of types of structural damlage

Calculation of the Structural Quality Index (SQI)

Types of damage to drainage structures

Factors to be assessed during drainage inspection

Using rapid inspection data

 

Overview

Rapid assessment methods are a vital to planning rural road networks. Planning improvements requires as a first step that information be gathered rapidly on the condition of every link in the network. This information, once transformed into work activities, and unit costs applied, provides a basis for preliminary costing, which can be used, together with socio-economic data, to apply ranking tools such as cost-effectiveness or cost-benefit indices   on a link by link basis. Sets of links can then be combined into coherent core networks. These may than be exposed to numerous filters, such as local needs for mobility, and constraints upon it, budgetary constraints, both on rehabilitation and future maintenance, and inevitably political expediency, leading, after a hopefully healthy debate at the community level, to a coherent and feasible programme.

This programme must then  be subjected to a more detailed and slower inspection procedure to provide the basis for a more detailed analysis  requirements and costs. This may result in modifications to the programme: costs of certain links may be higher than expected and their rank lowered, perhaps due to the inaccessibility of gravel or unforeseen problems of drainage; or efficient use of resources for rehabilitation may  require rescheduling certain links.

A typical programme usually consists of restoration of badly deteriorated roads, often constructed many years ago, but allowed to deteriorate through lack of maintenance to the point. New construction is rare. Sometimes rehabilitation may simply require an immediate and accelerated robust maintenance programme. More often, partial or spot reconstruction is needed, or complete rehabilitation.

There are a number of tools to facilitate rapid assessment, which an be used both for planning rehabilitation programmes and for annual maintenance inspections. They can be visual or computerized. Visual methods are described here where an experienced engineer or technician, adhering closely to a protocol, notes down, while being driven along the road, those aspects of the road that are most pertinent to rapid cost estimation. Subsequently, at the end of the day, they are summarised and classified. Computerised methods record automatically road surface and alignment characteristics using sensors and video, integrating and summarising the quantifiable ones and recording others for subsequent office evaluation.

A method taking account of low-volume road conditions, adapted from the Viziret approach, some of whose photos are reproduced here, where speeds are and comfort levels are low, and budgets do not allow more precise methods, is described below. It provides an excellent example of a procedure for rapid  visual assessment.

Definition of improvement tasks

This method  provides a structural quality index (SQI) for unpaved roads. This index is directly related to the tasks below listed below. It should be noted that rehabilitation of low-volume roads usually consists of combinations of these tasks, followed, hopefully, by annual labour-based maintenance, using cantonniers at 1-2 km intervals, augmented by an a programme of periodic maintenance.

  • routine maintenance: operations which are mainly done manually or with light equipment. They cover spot regravelling, cleaning ditches and culverts, and brush clearing
  • Blading/ grading : regular periodic mechanised operations consisting of grading the surface of the road in order to restore the original roughness index (IRI) of the surface but without addition of material and compaction;
  • Gravel resurfacing : consisting of restoring when judged necessary a deteriorated surface, adding gravel, and compacting after watering so as to get a smooth well bonded surface. Addition of material is for reinforcement and is not intended to restore the initial thickness nor the cross-section. In general, gravel resurfacing is performed at the same time as ditch cleaning and reshaping as well as culvert cleaning and repairs.
  • Regravelling: basically  the same task as resurfacing except that  material added is intended to restore the initial thickness of the road surface. It is normally carried out when gravel thickness has fallen to a critical level over the entire road.
  • Spot regravelling; calls for regravelling as needed on one or more sections. It is often necessary on slopes or in areas where water and traffic action have destroyed the road surface;
  • Reconstruction means complete rebuilding of the road, possibly with alterations of the alignment and cross-section. These should be minor, intended to reduce future maintenance problems, since low-volume rural roads do not justify extensive works to improve traffic speeds and comfort.

There are other tasks whose need should be noted during inspection such as repairing drainage structures: constructing new pipe or box culverts, repairing bridge parapets, removing fallen trees obstructing the road, and repairing land slides and other unforeseeable events.

Information to be collected on  road geometry

The geometry of the road is defined by its horizontal and vertical alignment  and cross-section :

Road Alignment: only very rough information about the alignment of a road can be obtained during a rapid inspection. It is obviously essential for  analysing drainage needs but to be useful  it must be supplemented by information about the level of the surrounding land. This information must be obtained at the detailed design stage

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.

 

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.

 

Traffic

(vpd)

Normal gravel loss (mm per year)
0 – 50 15 mm
50 – 100 20 mm
100- 200 25 mm
200 – 400 30 mm
> 400 35 mm

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

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 4×4 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.

Computerized 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, 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 simulation models such 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. The tables below give show expected gravel loss as a function of traffic and terrain.

Traffic

(vpd)

Normal gravel loss (mm per year)
0 – 50 15 mm
50 – 100 20 mm
100- 200 25 mm
200 – 400 30 mm
> 400 35 mm
Rainfall (mm per year)

 

Type of Terrain
< 1250 > 2500
+0 % +15% Flat
+15 % +30% Hilly
+30 % +45% Mountainous
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)      Muddy areas or soft spots in general result from destruction of the pavement due to standing water at locations where the slope changes. Damage will be aggravated by 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

 

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