Two common fill conditions are
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.
Fill walls placed on slopes require special consideration. Typical fill walls, such as MSE or concrete block built on slopes, require that a bench is 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 the 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
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 9 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, the 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 into 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.
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 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 the wall should be no more than 2 ft.
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
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 11 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, the 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 areas, 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.
Riprap Shoreline Installation & Restoration
Losing your shoreline to erosion? Riprap to the rescue!
When installed with care riprap can end your shoreline erosion.
What is riprap? It’s a barrier of stones (ideally fieldstone) that goes on a layer of tough aquatic filter-fabric. It prevents erosion and adds beauty to your shoreline and to your home. It also won’t disturb its environment, it deters muskrats and rodents, and it can increase your property value.