ASSESSMENT, MAINTENANCE,AND REPAIR OF SRWsThis article provides general guidance for the assessment, maintenance, and repair of existing segmental retaining wall (SRW) structures. The images il-

lustrate common ‘problem’ structures showing examples of undermining, impacted walls, bending and bowing walls, and walls with vegetation on the face. For most structures, inspec-tion is simply an observation to make sure the geometry around the wall has not changed and maintenance is a simple cleaning, if desired. When post-construction wall monitoring is necessary for applications where poor wall performance can significantly impact safety, damage property or the utility of the site, it is recommended

that a licensed surveyor prepare a “wall as-built” that defines the existing con-ditions and can be compared to the progression of any settlement or displace-ment. A new technology that can be used is a Digital Elevation Model (DEM) that can be viable for more complicated walls and can also be a valuable fo-rensic tool to assess deformation and assist with the correct response (monitor, deploy monitoring devices, corrective actions, etc.) of the wall.

SRW units provide a low-maintenance, easily constructed retaining wall sys-tem. The structural facing for the systems are zero-slump segmental concrete units. Field performance issues can manifest as a result of improper design, poor workmanship, inferior quality products, or lack of maintenance. When properly executed, SRWs have been used successfully for decades. Figure 1 shows an SRW wall constructed in Georgia with a maximum height of 40 ft (12.2 m). The backfill behind the wall consisted of a ‘Georgia clay’ a silty-sand fill. A one-story post office is built over the reinforced zone and the parking lot abuts the top of the wall. After several decades of vehicular and pedestrian traffic, this wall shows no signs of deterioration.

IDENTIFYING SOURCES OF DAMAGE

SettlementSRWs are relatively flexible structures that are typically build on flexible ag-gregate pad bases. The units can tolerate movement and settlement without causing significant structural distress at the face because the SRW units move and adjust relative to each other. An evaluation of the face can show settlement and vertical distortion of the facing. When an SRW is built correctly, the units are level, the joints between the units are tight, and the batter is constant and leaning back toward the fill zone (as intended if the system has batter); there is little chance of differential settlement occurring when the foundation soils are good and homogeneous .

SRW HISTORY ARTICLE SERIES

This article is part of the series of technical articles on the history of segmental retaining walls.

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Figure 1. SRW Wall Constructed in 1988

Poor compaction in the reinforced or foundation zones play a big part in SRW settlement. Figure 2 shows an SRW wall constructed over a buried pipe that has set-tled into the base. The deflection gives the inspector an indication of a potential foundation compression issue that could warrant further investigation.

Figure 3 shows a tall SRW wall where the upper por-tion of the wall turns off the lower portion of the wall. To the right of the lower wall the upper section is supported on a ‘rigid’ foundation of concrete SRW units where to the left of the split, the upper sec-tion is supported on compacted fill. The compacted fill may settle over time due to self-weight where the concrete SRW units will not. Different support stiffnesses have led to differential movements that caused an opening in the block facing which have

caused some SRW units to crack due to concentrated stresses. While the prob-lem is not structural, it does present an undesirable visual effect.

Gapping and Cracking of Con-crete UnitsSmall amounts of movements in the face are not necessarily a sign of im-minent failure, especially if horizontal and vertical alignment is maintained. Precautions can be taken to help reduce cracking and gapping. The most effec-tive precaution is to provide a thicker gravel fill area directly behind the block in the area of the radius of up to H/2 (wall height).

When evaluating cracks and potential wall movement it is recommended to use manual or mechanical crack width gages. In general, if the wall alignment and cracks do not change they can be considered purely aesthetic. If the cracks increase in number or width the wall will need to be evaluated by an engineer.

Alignment and BulgingAlignment is easily verified by visual inspection along the face of the wall to determine if the wall is bulging or if the face alignment had changed from the time of installation. Alignment may change if the wall loading has changed or the wall has been impacted.

Bulging occurs when the facing units are pushed forward, out of alignment of the original face batter as designed. Visual clues to bulging are:

• Opening gaps between units in the vertical joints.• Gaps between units in the horizontal plane (one unit tilting up, the

lower tilting downward).

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Figure 3. Differential Movements

Figure 2. Base Settlement

the reinforced soil mass as shown in Figure 5. While inspecting the wall, it is important to view not only the face, but the conditions below and behind the wall structure as well.

Undermining and ScourSRW walls built along waterways are subjected to water erosion (scour) along the front of the wall and at the base of the wall. Drainage outlets coming out of the wall can also cause erosion and potential undermining of the wall structure as shown in Figure 6. In applications where scour may be a concern, the first course of units below grade can be a different color to provide a quick visual reference

where erosion is occurring.

Another case where erosion can be a con-cern is when runoff water erodes the top of the wall or the soil in front of the wall due to lack of drainage measures. For these cases it is recommended to add swales and erosion control vegetation in the problem areas.

Differential Face MovementDuring extreme earthquakes, the entire wall mass may move due to the vibrations and shaking. At the base of the wall where the foundation may be restraining the wall, the upper SRW units may move out with respect

Bulging is usually the result of the reinforcing being too short and a failure of the mass moving forward inter-nally (steep back slope, external hydrostatic pressures, poor soil backfill). Figure 4 shows a SRW face where the soil behind the wall has failed due to a slope failure (Figure 5) and the mass is moving outward. The internal reinforcing maintains stability, while the face movement yields important information to the wall inspectors.

Soil CrackingCracks or breaks in the ground behind the reinforced soil mass may indicate global stability or movement of

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Figure 6. Erosion and Undermining

Figure 5. Slope Failure

Figure 4. Bulging Failure

to the units below. Figure 7 shows the base of a 16 ft (5 m) wall in El Salvador after the 2001 earthquake. The base of the wall was restrained by a concrete slab. However, as referenced in the AASHTO Bridge Manual (Ref. 1), GRS-IBS Implementation Manual (Ref. 4), and NCMA SRW Re-search (Ref. 5), SRW systems perform very well under ex-treme events and damage is usually minimal.

Face Aesthetic ConcernsSometimes the face of the SRW units may show efflores-cence (Figure 8) or mold/mildew build up where water is present. While these issues do not deteriorate the wall structurally, they are aesthetically unappealing. Therefore, proper water drainage is necessary to avoid mold and ef-florescence from developing on the units.

Another aesthetic concern can be caused when in some applications, soil gets trapped and vegetation will take seed and grow on the face of the wall as shown

in Figure 9. The roots can also penetrate between the units, grow, and eventually separate the units. Another source of movement may be roots from trees and plants behind the wall penetrating the face, also potentially causing separation between units.

DamageWhile movements in the walls due to earth pressures, con-struction or design practices that did not fully take into ac-count project conditions, seismic events and aesthetic fea-tures of concrete products can compromise the integrity of the wall, all can be avoided with proper manufacturing, de-sign and construction techniques.

Structural damage of a wall is more difficult to control be-cause it often results from vehicles, barges, or floating debris as illustrated in Figure 10.

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Figure 7. Restraint/Seismic Movement

Figure 8. Efflorescence

Figure 9. Vegetation GrowthFigure 10. Impact Damage

Freeze/Thaw DamageFreeze-thaw damage can result when SRW units are ex-posed to repeated cycling of freezing and thawing temper-atures under saturated conditions. In the 1980s and early 1990s; the understanding and technology associated with manufacturing a durable SRW unit was in its infancy (Ref. 3). Zero-slump products manufactured since that time and to the requirements of ASTM C1372 have been shown to be very durable and able to withstand the freeze-thaw re-quirements set locally. The SRW assembly shown in Fig-ure 11 is located in Minneapolis, MN, and has been sub-jected to repeated exposure to deicing salts since the early 2000s and shows no signs of distress except for the cap units where water and deicing salts pond.

REPAIR OPTIONS

Impact DamageLike any structure along a roadway or waterway, walls are subjected to impact and damage from traffic, barges or floating debris. Figure 12 shows a wall that was impacted by an automobile that jumped the curb and struck the wall, pushing the facing units into the fill zone. The wall was

constructed in the early 1990s and was struck approximate-ly five years later. The damage to the assembly remained as shown for over 15 years before repair was implemented.

When assessing repair options for impact damage, the first question is whether the damage is structural, impacting the performance of the wall, or aesthetic, a visual impact. The damage shown in Figure 12 was aesthetic because the units above the damaged area are fully supported and there is no vertical displacement of the upper units.

If the damage is aesthetic, the repair option can be as simple as “do nothing” or replace the damaged units. Replacing the facing units could be as simple as removing the front six inches of the SRW units in the impacted area and bonding new face veneers in their place. Figure 14 illustrates ve-neer pieces saw-cut from SRW units identical to the ones in the project. On the back of the veneer unit two anchors are bonded to provide mechanical anchorage. Once the dam-aged area is removed and the veneer pieces placed, grout is then injected behind the new face to lock them into the structure. The finished repair is shown in Figure 13. Color

Figure 13. Repaired Wall

Figure 14. Repaired Wall

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Figure 11. Freeze-Thaw Scaling

Figure 12. Impact Damage

16 in.(406 mm)

12 in.(305 mm)

16 in.(406 mm) 4 in.

(102 mm)

and texture matching would need to be addressed, but over time the units will fade and blend in with the origi-nal units.

If the damage is structural, such as a loss of support of upper courses of units, the units can easily to be removed and the wall rebuilt. The wall, the units are removed from top to bottom of the damaged area and then replaced using the same techniques used to build the original wall, attaching the geogrid to the units and replacing the wall fill as the wall is reconstructed. If the soil mass behind the assembly is reinforced, the soil mass is internally and stable, but there may be some raveling as the face is removed. An example of this technique is shown in Figures 15 and 16. For relatively short walls (less than 6 feet (1.8 m)), remov-

ing and restacking the units even if the damage is only aesthetic, may be easier than replacing the facing with veneer units.

Sometimes, the wall is too tall or is supporting a struc-ture where removing the face is not an option. In such cases, earth anchors can be inserted into the stable banks and an SRW veneer can be attached to the wall. Another option is installing anchors and applying shot-crete to the wall surface.

Top of Wall DisplacementUnits along the top of a wall may be displaced by vehicu-lar impact or rock fall event. Figure 17 shows a rock fall event that damaged the top four rows of the wall and Fig-

ure 18 shows the top of wall displaced by vehicular impact. If only the face units have been moved they can be removed and replaced in the correct position. If the damage is more substantial, the soil reinforcing may be cut and the soils excavated down to the base of the damage. The reinforced zone and wall face are then rebuilt back to the original grade.

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Figure 17. Impact Damage

Figure 18. Impact Damage

Figure 15. Damaged SRW Wall

Figure 16. Repaired SRW Wall

Efflorescence/MoldEfflorescence is a deposit of soluble salts and alkalis, usu-ally white in color, that sometimes appear on the surfaces of cement-based materials as shown in Figure 19. Efflo-rescence is strictly an aesthetic issue and will not affect structural performance. SRW units are manufactured with relatively high content for higher durability for salt scaling and attack by deicing chemicals. With the higher cement content there are more free salts available to cause efflores-cence. Efflorescence is caused by leaching of the free salts by water. Because of their exposure, it is virtually impos-sible to remove SRW assemblies from moisture exposure, either in the form of precipitation or moisture in the soil behind the wall.

However, during the manufacture of SRW units, water re-pellants can be incorporated into the mix. These admix-tures have been shown to reduce the tendency for to ef-floresce. Water in the backfill soils can be directed away from the wall facing, reducing the flow of water through the face. While only an aesthetic concern, in cases where there is a desire to remove the efflorescence, additional in-formation is available in TEK 8-3A, Control and Removal of Efflorescence (Ref. 6).

Mold is not detrimental to the structural integrity of the wall. In areas where the wall face remains damp from groundwater seeping through the face or spray from lawn irrigation systems, mold can develop, but can easily be re-moved with over-the-counter commercial cleaners.

Undermining/ScourUndermining or scour is where the soil in front of the wall has been eroded away and the foundation soils (or Rein-forced Soil Foundation (RSF), stone) are being removed by erosion as shown in Figure 20. If left unchecked the wall face may become unsupported and there could be dif-ferential movement in the face. In cases where scouring

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may be a design concern, the crushed gravel leveling pad can be replaced with a 4 in. (102 mm), unreinforced con-crete slab.

In repairing an undermined wall, the first step is to grout the area below the wall facing to fill the void and provide support to the wall facing. The area in front of the wall should be protected by a filter fabric and non erodible fill (riprap or articulating concrete blocks) installed to a suf-ficient depth to protect the wall for the area with potential for erosion.

VegetationVegetation in contact with the face of the wall is not detri-mental to the wall, but the root structure can penetrate the wall joints and cause the opening to expand. It is suggested that vegetation on the face of the wall be treated with a weed killer to stop its growth. If the source of the vegeta-tion is roots coming from trees or bushes from behind the wall, an arborist can make recommendations to minimize the root growth through the wall face.

Freeze-Thaw DamageThe SRW Research series article titled “Durability of Seg-mental Retaining Wall Blocks,” (Ref. 7) discusses chemi-cal attack on concrete from freeze/thaw damage and the potential for surface scaling. Following the recommenda-tions in the article, will mitigate the potential for freeze/thaw damage.

In cases where repair of damaged units is desired, the first step should be to determine whether the damage is surface scaling or more. If only the surface of the units are scaling, one option would be to apply a surface sealer to minimize future degradation. If the damage is more substantial, the units should be replaced.

Figure 19. Efflorescence

Figure 20. Undermining/Scour

Cap units are often the first units to see damage due to freeze/thaw because they are on a horizontal surface, water and saline solutions can set on the units. Replacement of the cap units is relatively easy and straight forward.

MAINTENANCEProperly manufactured, designed and constructed SRW systems will re-quire little to no maintenance over its design life. Maintenance items to consider include:

• Control of Vegetation – At the end of construction the wall face should be washed to remove loose soil that may have accumulated during con-struction. If vegetation does grow, spraying with a weed killer will stop further growth.

• Erosion – Observation of the stream flow (if applicable) and water flow around the wall face will indicate when potential erosion is possible. Re-grading the front of the wall to move the water away from the face may be necessary. Placement of non-erodible fills (rip-rap or articulating concrete blocks) at the wall face will also help keep flowing water away from the wall foundation area.

• Efflorescence – Specify SRW units containing and integral water re-pellent. Minimizing water flowing through the units will minimize the formation of the white powder on the face.

• Freeze-Thaw Protection – Specify units that meet ASTM C1372 or higher if the site conditions require it. Consider the additional recom-mendations on the “Durability of SRWs” article (Ref. 7).

REFFERENCES

1. AASHTO LRFD Bridge Design Specification, 6th edition, American As-sociation of State Highway and Transportation Officials, Washington, D.C. 2012.

2. ASTM C1372, Standard Specification for Dry-Cast Segmental Retaining Wall Units, ASTM International, West Conshohocken, PA, 2014a.

3. Durability of Segmental Retaining Wall Blocks: Final Report, Publication No. FHWA HRT-07-021, Federal Highway Administration, Washington DC, November, 2007.

4. Geosynthetic Reinforced Soil Integrated Bridge System, Interim Imple-mentation Guide, Publication No. FHWAHRT- 11-026, Federal Highway Administration, Washington DC, January 2011.

5. “SRW Research,” SRW History Article Series, National Concrete Mason-ry Association, Herndon, VA, 2013.

6. TEK 8-3A, Control and Removal of Efflorescence, National Concrete Ma-sonry Association, Herndon, VA, 2003.

7. “The Durability of Segmental Retaining Wall (SRW) Units,” SRW His-tory Article Series, National Concrete Masonry Association.