16. Maintenance of Roads and Streets
16.1 Introduction
For a road to be kept in first class travelling condition
it must be well constructed and well maintained. The amount of maintenance
necessary after construction can be reduced by a high standard of construction.
However, a road constructed to a high standard but neglected after construction
will soon fall into disrepair. Maintenance, to be successful, must be planned
and organised.
16.2 Maintenance of Road Drainage Structures
The drainage system is a most important part of any
road. Its functions include:
- the removal of water from the carriageway in
particular, and the road formation and road reserve in general;
- the interception of surface water flowing towards
the road formation; and
- the interception and disposal of ground water
that would otherwise penetrate the subgrade.
It is a principle of good drainage practice that the
concentration of water should be increased as little as possible. When, because
of road works, increased concentration is unavoidable the flow should be
so controlled that it does not cause scouring or other damage. As a general
rule it is good practice to interfere as little a possible with the natural
flow of water. Water flowing towards a road formation should be allowed to
return to its natural course as soon as possible after being passed under
the road.
Bituminous and Concrete Pavements
Bituminous and concrete pavements are relatively
impervious to water so that drainage is carried out by providing the surface
of the pavement with a crossfall normal to the centreline of the road. Most
pavements have crossfalls of 2 to 3 percent. The surface should be maintained
in a waterproof condition by the prompt repair of any cracks in the surface
and by filling minor depressions that would hold surface water.
Gravel Surfaces
As gravel surfaces are not waterproof the crossfall
is usually increased to 4 or 5 percent. Shoulders should be steeper depending
upon the nature of the shoulder material. Ponding of water in depressions
in the pavement can have serious results because of the consequent weakening
of the pavement material and its displacement under wheel loading. To prevent
serious damage from water and to maintain unsealed surfaces in reasonable
condition for traffic, scours and depressions must be promptly filled and
the whole surface kept in a smooth and self-draining condition by frequent
grading.
Unpaved Surfaces
As with sealed pavements the principal means of
draining an unpaved formation is to provide a crossfall normal to the centre
line that will remove surface water as quickly as possible. Grading to restore
the crossfall and to fill potholes and ruts is the major activity to maintain
riding conditions and surface drainage efficiency.
Drainage Structures
The function of a table drain is to collect water that
has fallen on the carriageway or the batters of a cutting and flowed to the
edge of the formation. Table drains are essential wherever the road is in
cutting and often desirable along the shoulders of embankments. Table drains
require frequent maintenance, such as grading of unlined drains to remove
silt and other debris that may easily block the flow, and to restore the
original shape. Scours in table drains need prompt attention to prevent serious
damage that may lead to undermining of the drain lining, the carriageway,
or the cutting batters.
It sometimes happens that water from table drains must
be discharged down the embankment batters. In such cases the slope of the
invert of the drain down the batter is too steep to allow the water to flow
over most natural materials because the high velocity induced would lead
to serious scouring. Batter drains are, therefore, generally lined with concrete,
metal, or some other hard material such as grouted stone. It is important
that batter drains should be inspected frequently and immediate steps taken
to make good any undermining and repair any breaks in the lining.
The primary function of a catch drain is to intercept
surface water flowing towards the road cutting or formation embankment. It
thus prevents the water flowing down the cut batter or along the toe of the
embankment, which may cause severe scouring. Maintenance involves periodic
inspection, clearing of obstructions, repairing of breached banks, and the
filling of scours.
Culverts are key structures in any road drainage system
and since they often provide the only passage for surface water from one
side of the road to the other failure usually has serious consequences. Maintenance
tasks include the constant clearing of debris and growth from the channel,
particularly after forest fires, or in seasons when trees shed their leaves.
The accumulation of silt or drift sand in the culvert barrel must also be
removed periodically by mechanical or hydraulic means. Scour in the vicinity
of culverts must be recognised in the early stages and repaired promptly
before the damage becomes extensive.
The two main purposes of subsoil drains are to lower
the level of the water table and to intercept or drain underground water trapped
or held by impervious material. To be effective subsoil drains need to be
not less than 500 mm below the subgrade level. Subsoil drains are constructed
near the outside edge of the pavement parallel to the centre line of the
road, but it is not uncommon for transverse drains to extend as branches
from longitudinal drains to at least the centre line of the pavement. Maintenance
action for buried drains consists primarily of inspection of outlets from
time to time to ensure that water is seeping from them.
- Underground Drains and Pits
Underground drainage systems, which are rare in a rural
situation but rather common where the road passes through urban or built-up
areas, consist of both longitudinal pipes or conduits running generally parallel
to the road centre line and transverse pipes passing under the road formation.
Frequent inspection and cleaning of the drainage system is essential because
serious damage can be caused if surface water bypasses inlets and overloads
the system at other locations leading to flooding of the road or adjoining
property.
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16.3 Maintenance of Earth and Gravel Roads
Earth roads are usually regarded as suitable for dry
weather use only. They may vary from wheel tracks on the natural surface to
formed but unpaved roads usually constructed out of the soil occurring along
the line of the road. Gravel surfaced roads may be defined as formed roads,
the surfaces of which are composed of gravel won from a selected deposit,
but which have not been given a bituminous surface treatment. They form a
large proportion of the total length of rural roads in Australia and their
adequate maintenance is of great importance to the rural community.
Grading
Grading is the most important operation in the maintenance
of earth and gravel roads. The basic purpose of grading is to keep the road
well drained to maintain a satisfactory running surface. The process of maintenance
grading consists of bringing material in from the sides or cutting down high
sections of the surface and filling the low spots with the surplus loose
material.
Scarifying and Reshaping
When a gravel road has become worn to a state where
the surface has hollows, potholes, and corrugations, it can sometimes be
restored by loosening and reshaping. The shoulders are normally built up
at the same time.
Gravel Resheeting
Gravel resheeting is the process of adding material
over the full width and length of a specified section of pavement that is
already gravelled. Resheeting is undertaken to restore the thickness of the
pavement to give adequate support to all vehicles using the road. The steps
in gravel resheeting are:
- Tyning (loosening the existing surface);
- Spreading the new gravel; and
- Compaction.
16.4 Maintenance of Bituminous Surfaced Pavements
A bituminous surface is applied to a pavement to
resist abrasion by traffic and to prevent penetration by moisture. Constant
surveillance of the bituminous surface is an important phase of road maintenance
because any failures may cause rapid deterioration of the pavement. Prompt
attention to minor faults may obviate major failures with consequent costly
repairs, and prevent the development of conditions likely to be hazardous
or inconvenient to the road user.
16.4.2 Repair Methods
If the bituminous surface of a pavement shows signs
of weakness it is essential to determine whether the fault is in the surface,
the pavement, or in the subgrade before any extensive repairs or resurfacing
works are begun. Correct diagnosis of the cause is often difficult (especially
in the early stages of distress), and it may be necessary to dig small inspection
pits to examine the various layers and arrange laboratory tests of the materials.
Proper investigation will guard against wrong treatment and waste of effort.
Defects such as those arising from a faulty subgrade, a soft area in the
pavement, faulty pavement material or poor drainage, will continue unless
the cause is removed.
Many failures result from the presence of excess
moisture in the pavement or subgrade, therefore, one of the most important
functions of surface maintenance is to prevent water reaching or remaining
in these layers.
If excavation of faulty pavement material is necessary
the sides of the hole must be trimmed vertical, the depth of the hole being
just sufficient to remove the poor material. Gravel or crushed rock used for
backfill must be moistened to facilitate compaction and all materials should
be compacted in layers not exceeding 100mm thick, preferably with mechanical
equipment. The amount of material used must be chosen to ensure that the
patch, when compacted, is flush with the surrounding pavement surface. On
no account must a patch be left higher for traffic to compact it.
Before applying the final bituminous surface the
patched area must be swept clean of dust and loose stones. The surfacing is
then applied using materials similar to those in the original surface. Premix
may be used for patching in some circumstances. Premix, made by mixing aggregate
(preferably dried) with fluxed and/or cutback bitumen by hand or in a mixing
plant, may be stored for several weeks or even months depending upon the
amount and type of flux and cutter used. A bitumen emulsion of a suitable
type may be used as the binder. Where available, hot mix is preferable to
cold mix provided it can be used satisfactorily before it sets.
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16.4.3 Particular Types of Defects and their
Repair
Ravelled Surface
Ravelling is a condition in which plant mix or
penetration macadam becomes rough in texture due to a loss of aggregate. The
causes could include insufficient or aged binder, excessively open-graded
mix, poor compatibility of aggregate and binder, or fracturing of aggregate.
Ravelled surfaces will require binder to be added, the rate of application
depending upon the amount and condition of the existing binder. If ravelling
has not developed too far the condition can be corrected by a seal, slurry
seal, enrichment seal, or fogcoat made by spraying a very light application
of binder with or without fine cover aggregate. Otherwise resurfacing with
plant mix will be necessary.
Stripped Surface
Stripping is a condition in which aggregate is
lost from a spray seal due to one or more of the following reasons:
- ageing and hardening of the binder;
- cold or wet weather before, during, or soon
after spraying;
- wet or dusty aggregate to which the binder
has not readily adhered;
- insufficient binder for the size of aggregate
used;
- aggregate of a rock type to which the binder
does not readily adhere;
- insufficient cutter in the binder when spraying,
preventing the binder from wetting the aggregate properly;
- excess cutter in the binder when spraying,
making the binder too soft to hold the aggregate under the action of traffic;
- insufficient rolling and/or excessive brooming
of the aggregate before allowing it to be traversed by fast traffic; and
- aggregate size incompatible with the aggregate
size of the previous seal.
If stripping is occurring due to aged and hardened
binder it should be treated without delay to avoid further loss of aggregate.
If there has been loss of scattered individual stones only the condition may
be corrected by an enrichment seal with or without fine cover aggregate. If
stripping has occurred over large isolated areas a reseal will be necessary
but care must be taken in selecting an aggregate similar to the existing.
Fatty or Slick Surface
A fatty surface is due to surplus binder on the
surface. Fatty surfaces become soft in hot weather (bleeding) and slippery
in wet or frosty weather. Slick surfaces are hard, smooth, and slippery. They
result from:
- loss of cover aggregate from sprayed seals
due to stripping;
- excessive application of binder during surfacing
operations or excess binder rising from the underlying surface, bad patches,
etc,.;
- breakdown of aggregate;
- poor spreading of cover aggregate in sprayed
seals;
- over-filled voids in plant mix;
- graded cover aggregate with particles so small
that they are covered by the binder; or
- aggregate penetrating the pavement surface
and sinking into the binder.
The successful treatment of this condition is difficult.
Possible methods of treatment are resurfacing with open-graded plant mix,
resealing, incorporation of additional aggregate into the existing binder,
and burning-off .
Polished Aggregate Surface
Exposed stones may become polished under the action
of traffic causing slippery conditions when wet even though the surface is
generally not smooth. A polished aggregate surface must be resurfaced with
either plant mix, sprayed seal, or slurry seal.
Shoving
Shoving is a fairly regular waviness (somewhat
resembling corrugations in a gravel road) which may develop in bituminous
surfaces due to movement under traffic. The deformations are usually shallow
and are not likely to be confused with larger depressions resulting from weaknesses
in the pavement or the subgrade. If the bituminous material has been compacted
by traffic to a stable condition it will be practicable to fill in the depressions
with premix. If it remains unstable it will be necessary to remove the unsound
material and replace it with a stable premix.
Rutting
Rutting usually takes the form of depressions in
the wheel tracks. If the transverse deformation is accompanied by adjacent
bulging of the pavement or shoulder surface it may be a sign of excessive
subgrade movement or weak pavement. The treatment is the same as that described
for shoving provided there is no subgrade movement. Any faulty subgrade material
must be replaced by suitable material.
Cracks
A bituminous surfacing may crack for a variety
of reasons and often, in the early stages, the crack pattern can indicate
the cause. When the cracks have developed over a large area and become sufficiently
wide and numerous to allow the entry of surface water or disturbance of the
surfacing by traffic it can be very difficult to determine the original cause
of the trouble. Cracks wide enough to be treated should be filled with a
binder having a viscosity low enough to enable it to be poured or worked
into the cracks. Wider cracks should be filled with fine premix or bituminous
slurry. Large areas with fine cracks should be spray sealed, slurry sealed,
or resurfaced with plant mix.
Pot Holes
Pot holes not accompanied by distortion of the
adjacent surface are usually due to a cracked bituminous surface allowing
moisture to enter the pavement. The repair of pot holes will involve the vertical
trimming of edges and removal of loose material prior to reinstatement.
Edge Failures
Fretting or breaking of the edge of a bituminous
surface may be caused by worn shoulders , inadequate strength at the edge
of the pavement, or entry of water through the shoulder. The failed area should
be boxed out and both pavement and shoulder material replaced.
Large Depressions
Large depressions occur in the pavement surface
when a fill has been inadequately compacted, for instance at bridge abutments.
Depressions caused by failure to compact the fill may continue to increase
in size and depth and deep seated correction may be necessary.
16.5 Maintenance of Road Shoulders
The road shoulder is that portion of the road adjacent
to the pavement. The net width of shoulder is that width available to traffic
inside guide posts or guard fencing or the edge of a table drain or fill slope
measured from the edge of the pavement.
Shoulders may be constructed of earth, gravel, or
bituminous concrete with surfaces unsealed, grassed, stabilised, or sealed.
A shoulder should have a smooth running surface, a minimum of loose material,
an adequate slope for drainage, sufficient strength to support wheel loads
and a surface flush with the pavement edge.
Maintenance of earth and gravel shoulders will normally
involve one or more of the following operations:
- smoothing and reshaping, usually with a motor
grader;
- adding new material to replace material lost
from the actions of traffic or water erosion(resheeting); and
- watering and rolling to compact new or existing
shoulder material.
Grass shoulders must be mown regularly.
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Page last modified 25 June 2002.