Embankment Side Slopes Consider a typical grade separation where inadequate right of way requires retaining walls to be placed along the approach embankment. Widening Fill Sections Fill sections that are being widened present special considerations.
Depressed Sections In depressed sections, consider additional width for the lower roadway to allow for future lane additions. Bridge Abutments Place retaining walls a reasonable distance in front of bridge abutments to allow adequate clearance for wall construction. Structures behind Walls Consider the proximity of a retaining wall to structures behind the wall. Stability Considerations. Sliding and Overturning Sliding involves the lateral translation of a wall due to inadequate resistance to movement at the base of the wall.
Consult the governing standard for minimum factors of safety for these two modes of failure. Eccentricity 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.
Bearing Pressure The weight of the wall mass and the active driving forces behind the wall exert pressure on the foundation soil along the wall base. A safety factor of 2. Global Stability Global failures of walls encompass the entire wall as well as a portion of the retained soil. Design Procedures. Earth Pressure Distribution Determine the pressure applied by soil on a retaining structure by different methods depending upon the wall type. Internal Analysis Internal analysis refers to the design of the wall structure to resist the stresses induced by the earth pressure applied to the wall.
Mechanically Stabilized Earth Walls: The internal design of MSE walls involves checking the earth reinforcements for allowable stresses and anchorage into the mass of select fill behind the face. Make allowances for metal section loss on the reinforcements when computing tensile stresses. Alter the reinforcement density and size to attain proper stresses and anchorage. The overall dimension of the reinforced mass is most often governed by external stability.
Tied-back Walls: The internal design of tied-back walls involves the analysis of a continuous beam soldier pile to determine the support reactions tied-back loads for an applied load diagram earth pressures.
Correct the tied-back loads determined by the continuous beam analysis to account for the anchor inclination. Select a soldier pile that will adequately resist the maximum bending moments from the continuous beam analysis. Then design the wall facing that spans between the soldier piling. Analyze this as a simple beam to support the maximum soil pressure. Then design the facing-soldier pile connection.
Drilled Shaft Walls: The design of these walls involves the analysis of a continuous beam on nonlinear supports. The nonlinear supports model the soil in which the beam is embedded. This approach accounts for the bending stiffness of the drilled shaft foundation unlike other methods, which consider the foundation to be infinitely stiff.
Use the program to determine the foundation response to the applied load for a range of embedment depths. Determine a foundation length by examining the embedment-deflection relationship for a suitable deflection either at the ground line or the top of wall. External Analysis The external analysis of walls examines whether walls will stay where built. Sliding and Overturning: Sliding of a retaining wall occurs when the active driving forces from the soil behind the wall exceed the frictional or cohesive forces along the base of the wall and the passive resisting force in front of the wall.
Whether to include passive forces in front of a wall depends on whether that soil will be present during construction or at some future date. Overturning occurs when the active driving forces exceed the gravitation resisting forces of the wall mass. The mass of the wall is considered the reinforced volume for an MSE wall or the weight of the concrete and soil above the heel for a spread footing wall. The safety factor is determined by adding moments about the toe of the wall.
Eccentricity: The eccentricity is the sum of the moments of the forces acting at the base of the wall divided by the sum of the vertical forces. The moments are normally calculated at the rear of the base of the wall. Bearing Pressure: Bearing capacity failures under walls involve the displacement of soil from under the wall. Use bearing capacity equations to determine the ultimate capacity of the foundation soil.
These equations require cohesion and friction values determined by triaxial testing. If this data is not available, use Texas cone penetration data to obtain allowable bearing pressures from the drilled shaft and spread footing design chart. The classical bearing capacity equation for the ultimate soil pressure is: N c , N q , N g are theoretical factors based on the geometry of the failing mass of soil beneath a footing, c is the soil cohesion, and g is the density of the soil.
Global Stability: Global stability of walls is a special case of slope stability. The limits of the wall affect where a potential failure surface can develop. The failure surface for a rotational failure can be either circular or noncircular depending on the stratification of the foundation soil.
For walls on uniform soft clay, the failure surfaces tend to be circular. If the soft zone is fairly thin, the failure surface tends to be noncircular following the soft zone. While the subsoil can be tested in advance to obtain strength data for analysis, the future embankment material properties are unknown.
An accurate answer is difficult to obtain because normally about half of the failure surface passes through the embankment behind a fill wall. Local experience may provide some insight into the strength of the proposed fill. While computer programs are used to evaluate wall stability, an approximate hand check of the results may be conducted by the method of slices.
Recommended Construction and Maintenance System Selection. Responsibility The project engineer must ensure that the retaining wall system selected for a given location is appropriate. Geometry Location geometry most often dictates the selection of a retaining wall system.
Soil Characteristics The Texas Cone Penetrometer is poorly correlated for very low soil strengths and may yield overly conservative results. Recommended Construction Practices. Actual Soil Conditions Because soil borings are taken at discrete locations, it is difficult to determine what soil conditions will be experienced over a wider area.
Potential stability problems include the following: Soft or wet soil. Areas that are producing groundwater. Areas that exhibit slope failures during excavation. 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.
Plumb MSE walls require particularly close attention to placement and compaction of select fill. Weather Make close observation of the retaining wall and backfill after heavy rainfall, particularly in areas with higher volumes of rainfall. Base Backfill Backfill the excavated area in the base of retaining walls as quickly as possible. Filter Fabric Cohesionless select fill is subject to erosion and piping if subjected to large quantities of water flowing into the wall.
Sealing Sealing of coping joints prevents excessive quantities of water from entering the top of the wall. Recommended Maintenance. Periodically inspect walls for evidence of backfill loss, loss of joint seals, or movement. It is a good quality backfill, and will result in acceptable wall performance. Type "AS" is a coarser, higher quality material, exhibiting improved constructibility and performance.
It is generally a more expensive backfill material, but should be considered for projects where the enhanced performance would be desirable. Type "DS" backfill is a freedraining, rock backfill. Retaining walls subject to inundation should clearly state that Type "DS" backfill will be required below the year water elevation noted in the plans.
Alternately, the entire wall volume may be specified as Type "DS". For projects requiring Type "AS" or "DS" 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 "BS. 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 2 feet below finished grade or natural ground, whichever is lower.
The standard embedment of MSE walls is currently required to be 1 foot unless otherwise shown in the plans. Several TxDOT districts require a minimum embed of 2 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 1-foot embedment. Steep Slopes : Discourage the placement of walls on slopes steeper than Many soils in Texas exhibit marginal slope stability at or even 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. 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 affects 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. Proprietary Retaining Wall System Review. DMS Concrete Block Retaining Wall Systems. Approved Systems.
Loss of Backfill in Mechanically Stabilized Earth. Loss of Backfill. 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.
Constructability Drilled shaft and tied-back walls require drilling a vertical hole in the ground. Aesthetics The final criterion is aesthetics, a difficult area because opinions vary widely. Wall Layout Considerations Carefully consider the location of retaining walls.
Stability Considerations Thoroughly investigate retaining wall stability. Design Procedures The design of retaining walls requires a thorough knowledge of structural and geotechnical engineering. Recommended Construction and Maintenance System Selection Responsibility The project engineer must ensure that the retaining wall system selected for a given location is appropriate. A mature Flex MSE installation requires very little maintenance and provides living stability for generations. Installing Flex MSE is uncomplicated and straight forward.
Traditional Approach — Successive layers are added in a standard brick and mortar running-bond pattern — This helps engage the Interlocking plate to provide the initial system strength.
Rapid Construction — Backfill and compact every two lifts — Compacted backfill and often geogrid reinforcement are added at regular intervals for retaining walls. All soil reinforcement options have unique characteristics for pullout and tensile capacity, corrosion, and durability. Select backfill allows for reliable construction and performance of the wall, in which the gradation, plasticity, electrochemical properties, and overall durability should be closely analyzed.
It can be obtained on site, or from a distributor. Choosing the right facing type depends on the application, aesthetics, differential settlement, service life, and other factors. They have been engineered flat to achieve superior and predictable bond with the compacted backfill, having no potential for voids between the soil and reinforcement.
Contact us today to discuss your geotechnical engineering construction project. View Our Featured Projects. Tap to Call Tap to Email.
0コメント