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, constructability 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.
Two common fill conditions are:
1. 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.
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.
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:
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.
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.
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:
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.
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.
Wall Layout Considerations
Carefully consider the location of retaining walls. The location of a wall can affect the wall quantity significantly.
Embankment Side Slopes
Consider a typical grade separation where inadequate right of way requires retaining walls to be placed along the approach embankment. In these situations, the walls can be placed at the edge of the upper roadway with the top of wall coincident with the top of the embankment or at some distance from the edge of pavement with the slope extending from the edge of pavement to the top of wall. Placing the wall coincident with the edge of pavement requires an expensive concrete rail on top of the wall and eliminates any possibility for a future widening of the upper roadway; however, it improves the long-term serviceability of the wall. Placing the wall a distance from the edge of pavement requires the use of a guard fence or concrete barrier at the edge of the pavement. It also allows future widening of the upper roadway if the provisions are made in the design and detailing of the wall.
Widening Fill Sections
Fill sections that are being widened present special considerations. Typically some soil must be excavated to allow construction of an MSE wall. Placing the face of wall as close as possible to the toe of existing slope minimizes excavation and temporary shoring. Placing the wall close to the existing top of embankment requires use of a cut-type wall or a fill-type wall with extensive shoring.
In depressed sections, consider additional width for the lower roadway to allow for future lane additions. Once retaining walls are in place, they cannot be moved to accommodate future width requirements.
Place retaining walls a reasonable distance in front of bridge abutments to allow adequate clearance for wall construction. For most retaining walls, the face of the wall should be at least 1.5 to 3 ft. in front of the face of the abutment cap. For tied-back and MSE walls, this is especially critical because the tiebacks and wall reinforcements may need to be skewed around the abutment foundations. To improve the appearance of walls, control of the top of wall profile with vertical curves rather than discreet elevations at specific points results in a much smoother top of wall.
Structures behind Walls
Consider the proximity of a retaining wall to structures behind the wall. MSE walls are usually placed at least 1-3 ft. in front of foundation to allow space for attachment of the reinforcements to the facing panels and skewing of the reinforcements.
Unlike foundation failures, which can occur slowly over a period of years, retaining walls can fail rapidly in stability with catastrophic results. The failure of retaining walls can close a transportation facility just as quickly as a bridge failure. As a result, thoroughly investigate retaining wall stability. Stability analysis should be conducted for both short- and long-term conditions.
Sliding and Overturning
Sliding involves the lateral translation of a wall due to inadequate resistance to movement at the base of the wall. Past failures have involved marginal soil at the base of walls. Overturning does not involve the soil under the wall but only the mass of the wall to resist the soil driving forces behind the wall. Because the driving forces are applied to the wall at roughly two-thirds the wall height above the base, the wall has a tendency to overturn if the wall mass or geometry is inadequate. Consult the governing standard for minimum factors of safety for these two modes of failure.
The combination of vertical and horizontal loads on a wall combine to produce a resultant force at the base of a wall, which is not at the middle of the footing. The distance between the middle of the footing and the location of the resultant force is the eccentricity. The location of the resultant force is limited to the middle third of the footing to ensure that the rear part of the footing does not lift off the ground.
As a result of the weight of the wall mass and the active driving forces behind a wall, pressure is exerted on the foundation soil along the base of a wall. The pressure is greatest at the toe of the wall. If the ultimate bearing capacity of the soil under the toe of the wall is exceeded, the toe of the wall can plunge down into the foundation soil. The result is a local distortion of the wall face. A safety factor of 2.0 in bearing capacity is recommended.
Rotational failures of walls encompass the entire wall as well as a portion of the retained soil. This type of failure does not depend on the wall design specifically but more on the strength of the foundation and retained soil. Computer programs can evaluate rotational stability. A safety factor of 1.3 or higher is usually considered adequate.
Settlement can be significant when walls are constructed on soil softer than approximately 5/12 in. TCP. Settlement is mainly a problem in the coastal areas of the state where soil softer than 2/12 in. occurs to depths of 20 to 50 ft. If a bridge approach embankment is constructed over soil subject to significant settlement, try either to allow as much settlement to occur before completing the approach or to support the embankment with a foundation improvement such as stone columns.
Settlement can be accelerated by installing vertical drains through the compressible subsoil. Construction of embankments on very soft soil is also likely to result in rotational stability failures during construction if no precautions are taken. When encountering significant layers of soft soil, obtain samples for consolidation testing to determine potential settlement. Note that data obtained from consolidation testing is only approximate. Predictions of total settlement based on such data are commonly higher than observed in the field, and the time predicted for such settlement to occur can be incorrect by an order of magnitude.
Temper any values calculated for settlement with previous experience in the area. When significant settlement is anticipated, the best solution may be to lengthen the bridge and, thereby, reduce the height of the approach. This is often the most economical and practical solution.
The design of retaining walls requires a thorough knowledge of structural and geotechnical engineering. This does not mean that one person has to design every aspect of a retaining wall. Design loads and allowable pressures recommended by a geotechnical engineer are often later used by a structural engineer to design the wall. The following design procedures convey general methods and do not address every design situation.
Earth Pressure Distribution
Determine the pressure applied by soil on a retaining structure by different methods depending upon the wall type. The soil behind walls, which are free to deflect or move in response to the applied loads, is considered to achieve the active state. For this condition, calculate the earth pressure based on Rankine's or Coulomb's methods. The pressure distribution is triangular in shape with the maximum pressure occurring at the bottom of the wall. This is the case for spread footing, MSE, drilled shaft, and sheet pile walls. Usually soil pressure is assumed to increase downward at a rate of 40 psf per ft. of depth.
Structures such as tied-back walls or braced excavation shoring are more or less fixed and, therefore, unable to achieve the active state. For this condition, use an earth pressure distribution as proposed by Terzaghi and Peck. The pressure distribution is in the shape of a trapezoid.
Internal analysis refers to the design of the wall structure to resist the stresses induced by the earth pressure applied to the wall. This aspect of design comprises mostly structural engineering. The various elements of the wall must be designed to carry the stresses generated so that an adequate factor of safety is attained.
The external analysis of walls examines whether walls stay where built. A number of failures of walls and embankments prove that external stability is just as important as internal design. External stability is routinely evaluated for fill-type walls. Cut-type walls are not routinely checked for external stability due to the different approaches to their design. However, if exceptionally soft soil is present, check the various aspects of external stability for cut-type walls. As always, sound engineering judgment should prevail.
Recommended Construction and Maintenance System Selection
The project engineer must ensure that the retaining wall system selected for a given location is appropriate. MSE wall suppliers are only responsible for the internal stability of their walls. The overall (global) stability of an MSE wall system is the responsibility of the engineer who selects this type of wall for inclusion into the plans.
Location geometry most often dictates the selection of a retaining wall system. The Geotechnical Manual offers information regarding evaluation of geometry and selection of various wall types. MSE walls are commonly used on TxDOT projects; however, in many situations--especially cuts--MSE may not be the most appropriate wall type. Often the additional excavation and shoring required for installation of MSE walls in cut situations make them uneconomical and difficult to construct. Sometimes MSE walls are selected because only a geometric layout and a standard sheet are required in the plans (the final detailed drawings are produced as shop drawings). This minimal design effort up front makes MSE walls a popular choice among engineers with limited time and resources. Although tied-back, soil nailed, drilled shaft and spread footing walls all require considerably more design effort and time, they are preferable in some cut situations.
The stability of each proposed retaining wall installation must be evaluated. Usually this involves a simple review of the wall height, site geometry, and soil borings. Walls with heights of 20 feet or less, situated on level ground, with soils borings indicating Texas Cone Penetrometer (TCP) blow counts in excess of 20 blows per foot should not require a detailed analysis. Walls taller than 20 feet, situated on slopes, or on soils weaker than 20 blows per foot should be looked at more closely. In general, place walls on any slope steeper than 4:1 only with a careful review of both short and long-term stability. Of particular concern are walls placed on freshly cut slopes, where the soil data may indicate high strengths at the excavation level. Freshly exposed material will soften with time, and an assessment of long-term strengths must be made when analyzing walls in this situation. Local districts may want to modify these guidelines based on their experience with specific projects and local conditions.
The Texas Cone Penetrometer is poorly correlated for very low soil strengths and may yield overly conservative results. When evaluating stability of walls on soils weaker than 20 blows per foot, it may be appropriate to conduct laboratory or in-situ testing in addition to the TCP. Triaxial or direct shear laboratory tests will generally yield more accurate soil strengths for this type of analysis.
Engineers in the Geotechnical Branch of the Bridge Division are available to assist with the determination of testing for specific situations and with the slope stability analysis
Recommended Construction Practices
Actual Soil Conditions
Because soil borings are taken at discrete locations, it is difficult to determine what soils conditions will be experienced over a wider area. During construction of retaining walls, evaluate the proposed retaining wall location and notify the project designers of potential problems. Of concern are soils that are soft or wet, areas that are producing groundwater, and areas that exhibit slope failures during excavation. Each of these indicates potential stability problems and should be brought to the attention of the wall designer. It may be necessary to remove and replace poor soils, install drains, or modify the wall to address such field conditions.
Adherence to plans and specifications
Assure adherence to plans and specifications during construction, especially with respect to width of reinforced volume, length of straps, and type of backfill used. A number of the short and long-term retaining wall performance problems are the result of contractor failure to adhere to specification and plan requirements.
MSE walls require particularly close attention to placement and compaction of select fill. Monitor wall panels for verticality upon completion of the backfilling of each panel. Initial panel batter should be modified as required to achieve a plumb retaining wall. In many cases failure to evaluate panel plumbness throughout construction has resulted in walls that are significantly out of tolerance.
Make close observation of the retaining wall and backfill after heavy rainfall, particularly in areas with higher volumes of rainfall. Rain can soften or loosen the compacted backfill, and any rain that seeps into the backfill can increase pressures on the wall panels. Check the temporary surface cover for cracks and quickly seal any cracks to prevent seepage into the backfill.
Backfill the excavated area in the base of retaining walls as quickly as possible. Accumulation of groundwater or surface water in this area will soften the soils and reduce the stability of the walls. Excavation at the base of an existing wall for installation of storm sewer, roadway, of other structure should not proceed without a determination of wall stability in the excavated condition.
Cohesionless select fill is subject to erosion and piping if subjected to large quantities of water flowing into the wall. Filter fabric is required at each panel joint and is designed to retain wall backfill while allowing the water to pass. Gaps or voids in the filter fabric allow fill to escape from behind the wall.
Sealing of coping joints prevents excessive quantities of water from entering the top of the wall. The current RW(TRF) standard sheet requires all coping joints be sealed. This item of work should be required in the field and monitored for compliance.
Periodically inspect walls for evidence of backfill loss, loss of joint seals, or movement. Reseal joints, particularly those that may allow surface water to enter the wall backfill. If evidence of backfill loss is observed, backfill the effected area with select fill if the area is accessible, or use flowable fill if access is restricted. Water infiltration into voids in walls can cause excessive pressures within the wall and result in displaced panels and wall failures. Treat voided areas when they are small and manageable, as they will always increase in size with time.
MSE Walls have been the most common retaining wall type on TxDOT projects for the past two decades. The advantages of MSE walls include their low cost, low design effort, speed of construction, and attractive appearance. MSE walls will continue to be used in large quantities on TxDOT projects in the coming years. With this in mind, the Bridge Division recommends that the following be considered on upcoming projects utilizing MSE walls:
1. Selection of backfill for MSE walls
The 2004 Retaining Wall Standard Specification (Item 423) lists four types of select backfill for MSE walls. Type "B" is the default backfill for permanent MSE walls. It is a good quality backfill, and will result in acceptable wall performance. Type "A" is a coarser, higher quality material, exhibiting improved constructability and performance. It is generally a more expensive backfill material, but should be considered for projects where the enhanced performance would be desirable. With the introduction of the new Type "A" material in the 2004 Standards Specifications, it is no longer desirable for projects to include specific coarser backfill gradations in project general notes. Type "C" backfill is used only on temporary MSE walls, and is not appropriate for permanent walls. Type "D" backfill is a free-draining, rock backfill. Type "D" is intended for use in MSE walls that are subjected to inundation. Retaining walls subject to inundation should clearly state that Type "D" backfill will be required below the 100-year water elevation noted in the plans. Alternately, the entire wall volume may be specified as Type "D". For projects requiring Type "A" or "D" backfill in the MSE walls, either the general notes or the wall layouts themselves should clearly designate the required backfill type. If no backfill type is specified, the specification reverts to Type "B".
2. Increase Minimum Embedment
Consider increasing the minimum embedment of MSE walls from one foot to two feet below finished grade. On projects where a small amount of fill is to be placed below the wall, the designer may want to specify a minimum embedment of two feet below finished grade or natural ground, whichever is lower. The standard embedment of MSE walls is currently required to be one foot unless otherwise shown in the plans. Several Districts have begun requiring a minimum embed of two feet. Two feet gives a greater margin of error against inaccurate surveys or grading, and provides an additional measure of stability in soft soils. Projects over hard ground, or requiring excavation into rock may want to retain the one-foot embedment.
3. Steep Slopes
Discourage the placement of walls on slopes steeper than 4:1. Many soils in Texas exhibit marginal slope stability at 3:1 or even 4:1. The additional load of a wall on these slopes reduces their stability and may result in a failure. If project requirements dictate walls on slopes (perched walls), a detailed slope stability analysis should be performed, and measures should be taken to assure wall stability.
4. Avoid Using Cement-Stabilized Backfill
Although cement-stabilized backfill is an option allowed in our standard specifications and is an easy short-term solution, it compromises the long-term performance of the wall because it reduces the wall's flexibility and it does not allow drainage through the wall. On projects where settlement is anticipated due to soft soil, a general note should be added to the plans eliminating cement-stabilized backfill as an option.
Retaining walls serve well, but there are some key points for successful wall performance: the correct system must be chosen for each location, and proper construction practices must be employed. Also, as described above, there are a number of design and maintenance issues that are equally important. Feel free to call Mark McClelland, P.E., at (512) 416-2226.
Proprietary Retaining Wall System Review
Approved Concrete Block Retaining Wall Systems
Approved MSE Panel Systems
Loss of Backfill in Mechanically Stabilized Earth
Underwater Drilled Shaft Construction
Disregard Depth in Foundation Design
When performing the design of a drilled shaft or pile foundation for a bridge, an important parameter is the selection of the disregard depth. The disregard depth describes the amount of surface soil to be ignored in the design of the foundation due to potential erosion or scour, future excavation, soil shrinkage due to seasonal moisture variation, and other factors.
Grade separations not over water
Abutments - For abutments placed within embankments, the entire depth of the embankment is disregarded for foundation design. Foundation design initiates at the top of the existing ground. For abutments placed in natural ground, the disregard depth is generally taken as 5 feet based on the assumption that concrete or flexible slope protection of some type will be placed adjacent to the abutment to prevent erosion and moderate moisture loss. If no protection is placed adjacent to the abutment the disregard may be increased to 10 feet.
A special consideration for abutments at grade separations is the potential future construction of a turn-around lane with an associated retaining wall. If any possibility exists that a vertical wall will be constructed in front of the existing abutment in the future, the entire depth to the lower roadway grade should be disregarded. It is not recommended to tip an abutment foundation above the grade of the lower roadway in any case.
Interior Bents - The disregard depth for interior bents is generally taken as 10 feet. High plasticity clay soils common throughout Texas can contract to depths of up to 8 feet during periods of drought. The recommended 10-foot disregard accounts for this potential shrinkage, as well as potential future excavations. At the designer’s discretion, the disregard depth may be reduced to as little as 5’ in areas where rock or low-plasticity soils extend to near the surface.
Bridges over waterways
General - Design of foundations for bridges spanning waterways must take into account the potential for lateral migration of the waterway, long-term erosion, and short-term scour. The combination of all three of these mechanisms is often termed “scour”, but it is useful to discuss them as three different problems.
Most foundation problems in Texas are caused by lateral migration of rivers and streams. Bridges with foundations that are significantly shortened at bents away from the main channel have collapsed or required expensive underpinning when waterways have shifted laterally, relocating the main channel into the area with shortened foundations. In severe cases rivers have migrated such a distance laterally that bridges have had to be lengthened to accommodate the movements. It is prudent to evaluate the possibility that a waterway may migrate laterally throughout the entire length of a bridge. Waterways in beds of sand, clay and gravel can be expected to exhibit significant migration. Waterways that are well established into sound, non-erodible rock are typically not subject to this phenomenon.
Long-term erosion has been an issue in some regions of the state. Past river training and straightening projects have caused rivers to initiate downcutting in order to re-establish equilibrium. In early stages of downcutting the rate may exceed one foot per year. As downcutting continues the rate slows and eventually stops as equilibrium is established. Historical data from existing bridge structures in the region is the best indicator of history and current rate of downcutting. The Sulphur River in Northeast Texas is our best example of a river that has undergone long-term downcutting due to past river training projects.
Short-term scour describes changes that occur to a riverbed due to discrete flood events. Three components are generally considered. They are abutment scour, pier scour and contraction scour. Abutment scour describes the loss of ground that occurs because of turbulence and redirection of flow around bridge abutments, particularly those that project out into the channel. Pier scour occurs due to turbulence around the columns or piers in the channel. Contraction scour occurs as the channel flow is accelerated through a bridge opening that is restricted compared to the upstream channel. Although methods exist to predict the depth of scour for each of these components, they are based on very simple assumptions and generally lead to overly conservative predictions in Texas soils. Many predications have been found to be too deep by a factor of 3 or more. However, when a scour evaluation and prediction are provided for a structure, the prediction should be reviewed and considered by the designer. When available, historical data is the best indicator of potential scour. If an existing structure in the area has withstood several large storms with no significant scour documented, this information should be considered in determining the disregard depth. Also useful are the soil borings themselves. If borings show depths of soft or loose material overlying stiff or dense layers, it can be assumed that the upper materials are subject to being scoured and redeposited. These upper layers should be disregarded for design.
Lateral migration and long-term erosion are mechanisms that often result in permanent exposure of foundation elements. The disregard depth selected based on these factors should be considered in the initial design of the bridge foundations. Conversely, short-term scour is considered an extreme event, and loss of material due to a scour event is not assumed to be permanent. For this reason, additional disregard depth attributed to scour should be used only in evaluating a proposed foundation for a lower factor-of-safety in the fully scoured condition. The justification for this approach is that the structure will be inspected and repaired if necessary after the critical scour event. Design of structures to normal factors of safety is not justified for this extreme event. Discussion of this process can be found in Chapter 5 or the Geotechnical Manual.
Abutments - For abutments placed within embankments located well back from the channel, the entire depth of the embankment is disregarded and foundation design initiates at the top of the existing ground. For abutments placed in natural ground, the disregard depth is generally taken as 10 feet. If the possibility exists that lateral migration could eventually approach the abutment and require the bridge to be lengthened, the disregard should be the same as for the interior bents, and foundation tips should be based on the interior bent design.
Interior Bents - The disregard depth for interior bents is generally taken as a minimum of 10 feet. Factors noted above may require the disregard to be significantly higher for specific bents, especially those away from the main channel where the disregard may need to be taken as 10 feet below the flow line of the main channel.