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Cut or Fill
Determination | Constructibility | Aesthetics
| Alternate Walls
Once a difference in grade has been identified in the design process, the
decision must be made to construct a slope or a retaining wall. If adequate
space exists, consider a slope. A retaining wall is required if adequate space
is not available. Maximum slope steepness is dictated by the quality of fill
soil available and whether or not the slope will be protected with riprap to
eliminate the need for mowing and other maintenance.
Consider the following criteria in choosing a retaining wall: cut or fill
determination, constructibility, and aesthetics.
Cut or Fill Determination
The first step in wall selection is to determine whether a wall will be built in
a cut or fill situation. Use fill type walls in fill situations. While fill
walls can be built in cuts, the opposite is not true for all cut walls. The
construction of fill walls in cuts requires additional excavation behind the
face of wall and, possibly, temporary shoring. For fill walls built in cuts, the cost of the wall, excavation,
and shoring can exceed the cost of a more suitable cut wall. Wall conditions
that determine wall type selection are fill condition, cut/fill condition, and
cut condition.
Fill Condition – Two common fill conditions are:
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Fill walls on level ground. This condition is best
represented by at-grade crossings that are upgraded to grade separations by
raising one roadway above the other. This is accomplished by placing fill
for the approach to the new, elevated structure. Approach retaining walls
are commonly needed in urban areas due to the lack of available right-of-way
for side slopes. The most common fill walls in this situation are
mechanically stabilized earth (MSE) or concrete block.
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Fill walls on slopes. Fill walls placed on slopes require
special consideration. Typical fill walls, such as MSE or concrete block
built on slopes, require that a bench be cut into the slope for wall
construction. The back of the bench may need to be supported with temporary
shoring. Consider other wall types if the fill
will extend into water. MSE and concrete block walls can be built if the
water can temporarily be lowered or a cofferdam easily and economically
constructed. This assumes that shoring will not be needed for the excavation
back into the slope for the wall construction. Consider the costs of other
wall types, such as sheet piling, if cofferdams or temporary shoring are
required for construction. See the following diagram of fill on slope.
Cut/Fill Condition – This condition consists of placing fill on the
upper portion of a slope and removing the lower portion of the slope. This
condition is typically encountered when upgrading controlled access facilities
when both the main lanes and frontage roads are widened. See the following
diagram of a cut/fill condition.
Consider the following wall types for this situation:
- L-shaped spread footing
- MSE or Concrete block walls. These wall types require that adequate space
be available to excavate into the slope. The back of the bench must either
be shored or sloped the same as for fill walls on slopes. Other wall types
may be more economical if temporary shoring is necessary.
- Drilled shaft walls. Depending on the location of the wall on the slope,
the drilled shafts may be constructed in one or two stages. If the wall is
closer to the top of the slope, temporary fill may be placed to allow the
shafts to be constructed in one stage. If temporary fill is not used, the
portion of the shaft below the existing ground line is constructed first,
and then the portion above ground is formed and poured as a column. In firm
soil or rock, drilled shaft walls can be an economical alternative.
- Tied-back walls. Use these walls only in a cut/fill situation when the
existing ground line is closer to the top of the wall (located in the upper
half of the wall) than the bottom. Place and compact
any fill before installing soldier piling. Typically tied-back walls are
economical only when significant quantities are used on a project.
- Sheet pile walls. Sheet pile walls have occasionally been used in a
cut/fill situation. The ground must be soft enough to a depth of one to two
times the wall height to allow the piling to be driven. It is difficult to
advance sheet piling in material stiffer than 12 in./100 blows.
- L-shaped spread footing. This wall type is commonly used when a small cut
is made at the base of a slope. The lack of a heel minimizes the excavation
required behind the wall.
Cut Condition – In this condition, the primary operation is removing
ground with little or no fill placed. The wall choices for this condition are
similar to those for the cut/fill condition. The same considerations apply,
except that tied-back and drilled shaft walls are easier to construct. See the
following diagram of a cut condition. Other cut wall types to consider here are
soil or rock nailed walls.
Soil and rock nailed walls may be constructed in any cut
situation but are best suited for low headroom situations under structures. This
is the wall of choice for turn-around wall construction under bridges. The top
of wall should be no more than 2 ft. above the existing grade.
Constructibility
Drilled shaft and tied-back walls require drilling a vertical hole in the
ground. This dictates that adequate overhead clearance be available for drilling
equipment. If clearance is not available, low headroom drilling equipment may be
used and shaft reinforcement or soldier piling members spliced as they are
inserted in the hole. These operations increase costs considerably. In a low
headroom situation, a nailed wall is the first choice.
Horizontal clearance is a consideration for tied-back and nailed
walls. Tie-backs are often installed with a continuous flight auger somewhat
longer than the depth of the hole, which means 50+ ft. of horizontal clearance
is desired. Sectional augers may be used in limited clearance areas. Nails,
being shorter, typically need around 20 ft. of clearance for installation.
Because of the minimum size of common drilling equipment used, 20-ft. horizontal
and 6-ft. vertical clearances should be considered minimum clearances.
Aesthetics
The final criterion is aesthetics, a difficult area because opinions vary
widely. Within reason, most aesthetic treatments can be accomplished
independently of wall type. Some walls such as concrete block walls, however,
have an appearance so unique that it cannot be duplicated by another wall type.
However, concrete block facing elements can be used with another type wall to
accomplish the aesthetic goal. Contact the Bridge Division for assistance
designing aesthetic treatments for walls. The aesthetic treatment of retaining
walls may involve items such as:
- Form liners to produce various surface finishes
- Paints, stains, or colored concrete to color surfaces
- Various wall geometries to accommodate landscaping
Depending on the treatment selected, cost may not be significantly affected.
The use of simple form liners can be economical, and colored concrete can be
expensive. Normal field surface finishing of colored concrete can yield variable
colors.
Consider also the amount of interaction that will occur between the motoring
public and the aesthetic treatment. A complicated graphic next to a high-speed
roadway is a blur to most passing motorists, who might view the graphic for only
tenths of a second. In this case, a simple form liner might be a more
appropriate treatment. If a wall faces a park or other public area, more
elaborate treatments may be warranted.
Potential wall distortions during construction or after construction may
significantly affect the appearance of the aesthetic. MSE walls, for example,
are flexible wall systems that experience some movement over the life of the
wall.
Aesthetic treatments with landscaping in conjunction with retaining walls
should be done carefully. If extensive watering of landscaping is anticipated,
additional drainage measures may be needed to ensure that excessive pressures do
not build up behind walls.
Alternate Walls
It is sometimes difficult to pick the most suitable wall for a cut or cut/fill
condition. The designer may not be able to evaluate factors that a contractor
considers important, such as equipment availability or haul cost for excavated
soil for MSE wall construction in a cut. In such instances, it is best to
include several wall types in the plans so that the contractor can determine the
most economical choice. When dissimilar wall types are included in the plans for
a single wall, present the wall types as alternates so that the appropriate bid
items may be included in each alternate. An MSE wall alternate in a cut must
include an item for temporary shoring, whereas the tied-back alternate would not
need a shoring item. See the following wall selection flow chart.
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