Figure 3-9.-Subsurface drainage.
Final Cross Section
When the final vertical and horizontal alignments
of the road are fairly well fixed, final design is
commenced. During this phase, final construction
drawings are prepared and construction may begin.
Before actual construction starts, final cross sections
are prepared. From these final cross sections, slope
stakes are set as described in the EA3 TRAMAN. Final
cross sections are taken at each station along the center
line of the road. They show the actual shape of the road,
the horizontal width of components and their distances
from the center line, the finish elevations, and the
extremities of the cut and fill. They also show the slopes
of the roadbed surface, ditches, and shoulders. The term
final cross section is also applied to the as-built sections
that are taken after the road is completed.
The procedures used to plot cross sections are
discussed in chapter 14 of the EA3 TRAMAN. You
should review that chapter if you are unsure of the
procedures.
DRAINAGE
Drainage is a major problem in the location,
construction, and design of roads. A route should never
be located where the drainage presents a problem that
cannot be handled or would be too costly to handle. A
route may have to be relocated because there is not
enough material available to build a particular type of
road. It may also have to be relocated because of a
swamp or underground spring, high flood waters that
can cover the road, or flash floods that can completely
wash out the road. These are some of the reasons for
planning alternate routes. During construction, the
problem of drainage is mainly one of preventing
standing puddles on the roadway. This problem is solved
by slanting the worked surface of the road so that water
can run off quickly or by cutting ditches, called bleeders,
so that the water maybe carried away as it accumulates.
Subsurface drainage problems are solved by raising
the grade line of the road or lowering the water table. In
either case, the distance between the water table and the
top of the subgrade should be as great as possible. There
are several ways of lowering the water table. In one way,
deep, open ditches are set back beyond the roadway
limits. These ditches intercept the water table, allowing
groundwater to seep through the sides. The water then
flows along the bottom and out the end of each ditch
In another way of lowering the water table, a deep
trench is dug exactly where the finished roadway ditch
would be. The trench is then backfilled to a designated
depth with rocks or large gravel of varying size, with the
larger size at the bottom. The rocks are capped with a
layer of branches or straw and the remainder of the
trench backfilled with soil and compacted This trench
is called a french drain (fig. 3-9). A tile drain, also
shown in figure 3-9, is the same as a french drain except
that a perforated pipe or tile is placed in the bottom of
the trench. The trench is then backfilled with gravel to
the desired depth. The minimum pipe grade is 0.3
percent with the maximum varying to meet conditions.
Surface drainage involves water from direct
precipitation, surface runoff, rivers, and streams.
(Surface runoff is rainfall that is not absorbed by the soil
but runs off a surface in sheets or rivulets.) Rainfall has
an immediate effect upon a roadway. Obviously,
rainwater would be a safety hazard or cause weak spots
on the roadway if it were allowed to stand. Water that
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