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Drainage
Proper drainage is important to ensure a high quality long lived pavement;
moisture accumulation in any pavement structural layer can cause problems.
Moisture in the subgrade and aggregate
base layer can weaken these materials by
increasing pore pressure and reducing the materials' resistance to shear.
Additionally, some soils expand when moist, causing differential heaving.
Moisture in the HMA layers can cause
stripping because it, instead of the
asphalt binder, will adhere to aggregate particles.
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Figure 1: Stagnant Roadside Ditch Caused by Clogged
Pipes and Shallow Grades |
Figure 2: Storm Drain Pipe
Awaits Installation in a Road Reconstruction Project |
Moisture sources are typically rainwater, runoff and high groundwater.
These sources are prevented from entering the pavement structure or accumulating
in the subgrade through surface drainage and subsurface drainage. Usually,
it is more cost effective and less risky to prevent moisture entry and
accumulation using surface drainage than to effect moisture removal using
subsurface drainage.
Surface drainage is concerned with removing all water that is present on the
pavement surface, shoulder surface or any other surface from which it may flow
onto the pavement. If not systematically removed, this water can
accumulate underneath and weaken the pavement structure. There are three
primary means used to prevent water infiltration and accumulation:
- Impermeable HMA. HMA tends to be impermeable below about 8
percent air voids, therefore proper
compaction practices should be followed to
ensure an impermeable pavement. Also, minor cracks in the HMA should be
promptly sealed.
- Slope. The pavement section should be sloped to allow
rainwater to sheet flow quickly to the edge where it is typically collected in a
curb and gutter system or a roadside ditch. A generally accepted
standard is a 2 percent cross slope.
- Grade. The curb and gutter or roadside ditch must be properly
graded to allow flow to central collection points such as catch basins or
detention ponds. A generally accepted standard is a grade of 0.5 percent
or more although lesser grades have been used effectively.
Subsurface drainage is concerned with removing
water that percolates through or is contained in the underlying subgrade.
This water, typically the result of a high water table or exceptionally wet
weather, can accumulate under the pavement structure by two chief means:
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Gravity flow. Water from surrounding
areas can be absorbed by the soil then flow by gravity to areas underneath the
pavement structure. In pavement with high air voids (above 8 - 9
percent), water can percolate down through the pavement structure itself.
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Capillary rise. Capillary rise is
the rise in a liquid above the level of zero pressure due to a net upward
force produced by the attraction of the water molecules to a solid surface
(e.g., soil). Capillary rise can be substantial, up to 20 ft. or
more. In general, the smaller the soil grain size, the greater the potential
for capillary rise. Often, capillary rise is a problem in areas of high
groundwater tables.
Most pavements have performed adequately
without considering these effects.
However, HMA
pavements can fail because of subgrade support deterioration as a result of excessive
moisture or other water-related problems. These issues can be addressed in
two manners:
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Minimize water infiltration into the pavement
structure. In most cases, the accumulated water in the underlying
subgrade that causes the damage comes from surface infiltration. This
infiltration can be minimized by providing proper roadside drainage and
minimizing air voids within the HMA.
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Provide subsurface drainage. This needs
to be done judiciously, because it may be somewhat akin to treating the
symptom rather than the problem. Subsurface drainage consists of three basic
elements (see Figure 4):
-
A
permeable base to provide for rapid removal
of water which enters the pavement structure. Based on recent research
from California, permeable base layers may
strip and become clogged with
fines thus weakening the overall pavement structure.
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A method of conveying the removed water away
from the pavement structure. At the least, this may consist of a base
sloped towards a drainage ditch. At the most, this may consist of a pipe
collector system.
- A filter layer (such as a geotextile, graded aggregate layer or
ATB) to
prevent the migration of fines into the permeable base from the
subgrade,
subbase or shoulder base material. Excess
fines in the permeable base will clog its drainage routes and render it
ineffective. Depending upon the subgrade and pavement structure a filter
layer may not be used.
Figure 4: Subsurface Drainage
Example
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