By Michael R. Simac, P. E., and Blaise J. Fitzpatrick, P. E.
While segmental retaining walls (SRWs) have been routinely used for more than 15 years now, there are still three challenging issues facing owners considering their use. What is the best way to procure, design, and then build these structures to minimize short-term problems and ensure long service life?
This three-part series will examine each of those issues, in an attempt to provide guidance for owners, designers and contractors that balances each of their prospective risks and rewards. Although these articles will focus specifically on SRWs, other reinforced-soil structures, such as MSEWs, reinforced soil slopes (RSSs), and basket walls, face these same issues, so the information presented is equally applicable.
Change-in-grade structures such as SRWs have revolutionized land development strategies for residential, commercial, and industrial sites as every project attempts to maximize the usable land area. This quest for usable space has led to taller and longer SRWs, making the structures a more significant engineering, construction, and cost component to these projects. An owner’s decisions on how to procure, design, and construct SRWs are critical to the overall success of the project, due to the SRW’s importance to the project and, usually, the construction schedule.
The landowners/developers must understand the options and how their decisions on these three key challenges affect the quality, usefulness, and long-term performance of the structure. When the landowner/developer is unaware of the options, it benefits both the designer and installer to review these options with the owner/developer to agree on the best approach for the project. The objective is to have similar and reasonable expectations on SRW performance and how to best achieve them.
Without this discussion before the project, unrealistic expectations and/or poor performance can lead to serious disagreements on whose responsibility it was to ensure a better end result. This scenario is occurring often enough that professional liability insurance companies have begun redflagging professionals practicing in retaining wall design.
This series presents our proactive options for addressing these challenges in ways that can benefit all stakeholders.
Part 1: Options for buying the SRW (October/November 2007, Geosynthetics, Vol. 25, No. 5)
Part 2A: Options for designing the SRW (February/March 2008, Geosynthetics, Vol. 26, No. 1)
Part 2B: Options for designing the SRW-continued (April/ May 2008, Geosynthetics, Vol. 26, No. 2)
Part 3A: Options for building the SRW (June/July 2008, Geosynthetics, Vol. 26, No. 3)
Part 3B: Options for building the SRW-continued (August/ September 2008, Geosynthetics, Vol. 26, No. 4)
Proper construction of the MSE wall/slope requires the owner to rely on skilled professionals performing their responsibilities correctly. The owner must be able to distinctly assign specific contractual responsibilities to each skilled professional to have the best chance for trouble free construction, and optimal retaining structure performance throughout the intended service life. By understanding the responsibilities of these professionals, the owner is in a better position to accomplish that objective.
3.1) Professionals constructing SRW or MSE walls and slopes
Contractor. The owner should contract with a licensed construction company responsible for executing the work that assembles the MSE wall/slope components and integrates the MSE structure into the overall project, according the design presented in the construction documents. The contractor may be a single entity, but in most cases consists of several independent companies (sub-contractors) that are coordinated by an overall entity (general contractor or construction manager). Specifically, the retaining structure is usually erected by an entity with specialized training in assembling the MSE wall/ slope components, referenced herein as, the wall installer. The contractor is responsible for supplying all materials to construct the wall, and ensuring all their work meets the contract specification requirements. This is usually accomplished by executing a quality control (QC) program, either internally or with assistance from a third party. The contractor should correct any deficient work identified by the quality control program before proceeding. The fill soil, reinforced with the geosynthetic material (geogrid or geotextile) is usually placed and compacted by the wall installer, but routinely supplied by the general or earthwork contractor from earth materials located elsewhere on site. Supplying fill soils consistent with the specified physical and design strength parameters is critical to successful construction of the MSE wall or slope.
Construction materials testing. The owner should retain a third-party construction materials testing professional, to observe all the work associated with the MSE wall/slope construction to assure compliance with the construction documents. This quality assurance (QA) program and testing is to ensure the owner is receiving the work contracted for and in some locals these are the designated “special inspectors.” The project geotechnical engineer may also function in this role. In either case, the QA testing professional must ensure the recommendations of the geotechnical engineer are being followed, as well as the MSE designer’s plans and specifications. This is a common requirement when the prevailing building code designates these structures for “special” inspection, dictating installation verification of wall/slope facing elements, geosynthetic reinforcement (as well as the fill type), compaction, and allowable foundation pressures. The materials testing engineer also identifies conditions inconsistent with the MSE designer’s assumptions (as identified on plans) that may require design modifications, such as unsuitable foundation soils, groundwater flow into the reinforced soil mass, or substitution of available fill materials. Work that fails to comply with those project requirements should be brought to the attention of the contractor and, if not corrected, the owner, MSE designer, and project geotechnical engineer should decide how to address the non-compliant work.
MSE designer. The MSE wall/slope construction documents should specify the testing and inspection requirements to ensure compliance of the contractor’s work, especially if the owner or local building code has designated the structure for “special inspection.” The MSE designer should receive timely construction materials testing reports, to identify any concerns or non-compliant work as it proceeds. Ideally, the MSE designer would make occasional site visits, to observe the work and testing. Such firsthand knowledge is valuable when preparing a statement of “substantial completion” or “compliance” from the “Design Professional In Responsible Charge (DPIRC),” a requirement under most “special inspection” provisions of local building codes.
Project geotechnical engineer. The project geotechnical engineer should be available to investigate conditions exposed during construction, to ensure consistency with their recommendations and analysis, especially if identified by the contractor or special inspector as questionable relative to their understanding of anticipated conditions. This is particularly important for foundation bearing materials and groundwater conditions encountered. The knowledge of soil conditions attained during site investigation makes the project geotechnical engineer uniquely qualified to perform the construction materials testing.
Site designer. The site designer should be available to investigate conditions encountered that would affect site design elements, such as topography, boundaries/easements, utilities, site storm drainage design, and/or landscaping. The special inspector would identify conflicts where the site designer’s expertise was needed to attain a satisfactory solution.
The coordination of these professionals presents the owner with both practical and contractual challenges relative to construction, depending on the method selected to procure the retaining wall system (see Geosynthetics, Vol. 25, No. 5).
- In do-it-yourself (DIY) projects, the owner has opted to forego professional assistance and build the wall personally, assuming the liability for performance.
- Contractor-supplied designs can present difficult challenges for owners because the construction specifications and testing requirements are developed by the wall installer’s designer. Therefore, owners should proceed cautiously to ensure that their actions, or the work of their representatives (specifically, the construction materials testing engineer), does not transfer some design responsibilities to them. The most common form of transfer is making the owner responsible for ensuring that the soils on site meet or exceed the design parameters assumed by the wall installer’s designer. Unreasonable or incorrect soil strength parameter assumptions by the wall installer’s designer then places owners, or their representatives, in the middle of verifying or justifying a design that they did not create. Those decisions should be contractually kept with the wall installer’s designer.
The project owner and even the general or earthwork contractor can manage this challenge by insisting on and signing contracts in which wall installers provides their own soils testing, including verification of soils consistent with design assumptions, and observations of work product. If local building codes require special inspection, make the wall installer provide it, along with the certification at the end by the MSE designer, the design professional in responsible charge. These functions are just an enhancement of the QC testing program that all wall installers should execute in their normal procedures of constructing MSE structures. This is best accomplished by placing these requirements in the original plans, specifications, and contract, minimizing the possibility for disputes on soil type availability and cost, which greatly affects the total price and wall installer selection.
Independent of the wall installer requirements, the owner should engage a construction materials testing engineer to perform a QA program for wall installation that notifies the contractor, wall installer, project geotechnical engineer, site designer, and owner of non-conforming work as it occurs. The owner should also have the materials testing engineer review the wall installer-supplied MSE design for compliance to project specifications or prevailing design practice, developing independently the testing and observations necessary for the owner to ensure compliance. The review should also address conflicts with the grading plan and/or utilities, prior to approving for construction, including an assessment as to whether global stability has been properly addressed.
- With a design-build approach, the owner should engage a construction materials testing engineer to perform a QA program for wall installation that notifies the contractor of nonconforming work as it occurs. The owner should also have the materials testing engineer review the MSE/SRW design for conflicts with the grading plan and/or utilities, prior to approving for construction. The design/ build contractor would be contractually responsible for performing all internal quality control testing and the special inspection necessary to document compliance of the field construction to their design documents. The design/build contractor would provide special inspection documentation, as-built drawings, and a warranty to the owner.
- In an owner-provided design situation, the owner should engage a construction materials testing engineer to perform a QA program for wall installation that notifies the contractor, MSE designer, project geotechnical engineer (if different party), and owner of nonconforming work as it occurs. The contractor should develop and execute a QC testing program independent of the owner’s QA program. Should local building codes require special inspection, the MSE designer should work with the construction materials testing engineer to develop the testing and observation requirements necessary to ensure the design professional in responsible charge, typically the MSE designer, has sufficient information to verify completion of the work according to the plans and specifications.
All 3 of these approaches provide a checks and balance system on the MSE wall/slope construction that ensures the owner that the specified materials were installed using appropriate techniques. This provides the best assurance that the project objectives and performance requirements will be realized throughout the intended service life. The specific issues and options within each of the contractual approaches above may vary, but in general the following should be addressed on each project.
3.2) Construction techniques and installation issues
Although the specific sequence and components in MSE wall/slope systems can vary, following are the general construction sequence and installation issues routinely encountered on most projects.
3.2.1 Typical construction sequence
The basics steps in the construction sequence are:
a) General excavation to foundation elevation for entire reinforced soil mass. Verification of suitable foundation conditions. Layout and installation of drainage collection systems, with a suitable gravity flow outlet.
b) Installation of the leveling pad to facilitate placing the first course of facing system (block, panels, baskets, etc.). The first course must be properly located (horizontally and vertically) on site, and then leveled to ensure proper alignment, which generally takes a significant portion of the construction time. Then the first course of facing system is filled and the reinforced zone filled behind it, before compaction begins.
c) The placement of facing units with filling and compaction is repeated, up to the first (and then next) geosynthetic reinforcement layer. Each facing unit should be filled and the corresponding reinforced soil volume behind it. The filling should occur in increments of the facing height or 9in. (3.5cm), whichever is less. Compaction should occur immediately after fill placement. The compacted lift thickness should not exceed 8in. (3.1cm) for SRW systems and 9in. (3.5cm) for MSE walls faced with welded wire mesh baskets.
d) Geosynthetic reinforcement shall be installed at the plan designated elevations and locations. Connect the assigned geosynthetic reinforcement type to the facing system according to the detail shown in the plans. Pull the geosynthetic reinforcement taut, no wrinkles, then fill over with soil. Care should be taken not damage the reinforcement during the installation process.
e) Repeat steps “c” and “d” as required to reach the top of wall. Top off the facing system per the plans and specifications. Ensure surface water is directed around the completed structure with a drainage swale or diversion berms, like curb and gutter. Protect the reinforced soil volume from surface water infiltration by placing an impervious surface (asphalt parking lot) or 8-12in. (3.1-4.7cm) of low-permeability soil before reaching finish grade, then complete the surface treatment above the wall as specified.
3.2.2 Construction procedures, options
Following are some of the more important construction techniques necessary to ensure good long-term performance:
a) Wall alignment and location. Prior to starting construction, ensure the wall location and alignment is consistent with the construction documents and property boundaries. Consider the effect of wall batter on the starting position of the bottom course and the finished position of the top course, to ensure there will be sufficient space at the top of wall to construct the planned improvements. Obtain any easements, such as permission to access adjacent property to facilitate construction or a permanent easement if any portion of the wall construction, like buried geosynthetic reinforcement, will remain on the adjacent property
b) Approved foundation bearing surface. Upon completion of general excavation for the SRW/MSE structure’s foundation bearing surface, have it tested, observed, and approved as suitable, prior to proceeding with construction. Unsuitable foundation conditions should be improved to acceptable, as directed by the MSE designer (see 3.3.1.b). Photo 1.
c) Leveling pad. Usually the construction drawings allow the contractor to select the type leveling pad. Since the leveling pad is not a footing, the purpose of the leveling pad is solely to start the wall on the proper alignment, at the proper elevation. Generally, installing the leveling pad and first course of block are the most time consuming stage of construction.
The 3 most common leveling pad materials are:
- unit fill the coarse-graded processed washed stone (#57–#67) that promotes good drainage, is easy to compact, and provides the stiffest filled facing system.
- well/dense-graded aggregate such as the local aggregate base course, which is easier to compact to a uniform level surface but limits options to drain the reinforced mass through the wall face above finish grade at toe.
- thin 2-3in. (0.8-1.2cm) and weak (< 1,000 psi) concrete topping slabs may provide an installation advantage when the first course is at a constant elevation for long distances.
d) Outlet the wall drainage system. The construction documents should define the required drainage components, including wall face drain, blanket drain, chimney drain, piping, etc. Ideally, the construction documents have planned and shown the type, location, and manner in which the drainage system is outlet. However, if the drawings just indicate in a cryptic note “gravity flow to daylight” or “outlet every 50 feet” the contractor must carefully plan the location of each outlet to ensure proper drainage. Drainage outlets through the front face above finish grade may appear to be a straightforward solution, but require drainage retarding material beneath finish grade, which may leave water trapped within the structure. The preferred drainage outlet is to slope (min. 2%) a solid pipe from the reinforced mass drainage collection system low point(s) beneath the leveling pad to a storm drain, manhole, or a slope face in front of and below the wall. The solid outlet pipe should be directly connected to the perforated collection pipes within the reinforced soil mass. To be effective, the perforated pipes of the reinforced soil mass drainage collection system should be located near the lowest point of the free-draining soil at any point along the wall and sloped (min. 1%) to funnel water to the outlet pipe. Photo 2.
e) Place and fill one course at a time. SRW and MSE structures should be built 1 course of facing system at a time because the most important structural component is strong soil attained through good compaction. Facing units that are larger than 12in. (4.7cm) should be filled in multiple lifts of soil, each not exceeding 9in. (3.5cm) in compacted thickness. The facing units should be backfilled first, then the reinforced soil, followed by the retained soil. Propagate fill placement from the front face back into the retained soil or parallel with the wall face (from the side). Compact the unit/facing fill first to ensure a stiff wall facing system (form) that resists lateral movement and provides sufficient restraint to compact the reinforced zone soil. This is usually best accomplished by vibration, and a vibrating plate tamp is easily maneuvered behind the facing system.
The reinforced zone soil may then be compacted with mechanized large compaction equipment, keeping this equipment a minimum of 3ft (0.9m) from the wall face. Good compaction is critical to good long-term performance, and imparting compaction energy every 9-in. (3.5-cm) lift makes that easier. The type and method of compaction should be chosen to best attain the target density for the soils being compacted. Photo 3.
Most SRW facing systems consist of a 12-in.-deep block unit plus 12in. of clean stone (into soil, horizontally, Figure 2 from Part 2B). The fill soil behind the clean stone of the facing system must be compacted using hand-operated or walkbehind compaction equipment to ensure that larger compaction equipment stays more than 3ft from the wall face. This hand compaction zone is problematic and requires special attention, such as more passes or even thinner lifts to achieve the required in-place density, depending on the soils that are compacted. One solution used by the authors is to use clean coarse-graded stone that compacts easily with vibration, to a minimum distance of 3ft. behind the wall face, thus eliminating problematic soils from the hand compaction zone. Attention to these compaction procedures in this zone is important for good performance and should be a part of the wall installer’s QC program.
f) Placement and compaction of unit fill. The coarse clean stone (#57 or #67) used to fill the SRW facing units, or welded wire basket forms, and just behind the facing is less about drainage and more related to having a soil material that is easily compacted within, between, and behind the wall facing units, so even solid units with mechanical connections still require use of the coarse clean stone, both for construction and drainage. Easy compaction creates a stiff face form, critical to attaining compaction in the soils immediately behind the facing system. End dumping of the coarse stone is insufficient to attain the required density in the facing system that influences its overturning capacity, sliding resistance, and connection strength to the geosynthetic reinforcement. Photo 4.
Overfill the SRW units with coarse clean stone by end dumping, then run a vibrating plate tamp just behind (not on top of) the SRW facing units, to ensure attaining proper density. Without overfilling and compaction, proper density cannot be attained, making the common installation practice of pre-stacking multiple courses of facing system prior to filling counter-productive to attaining a quality installation.
g) Check alignment of the facing system. The facing system, in particular SRW blocks, should be delivered to the job site free from physical defects. Check the delivery receipts and manufacturer’s certified properties to ensure consistency with the project specification. The facing system should be leveled, front to back and side to side on the first course, with the horizontal alignment, batter, and vertical coursing checked a minimum of every other course. These checks are necessary to ensure wall alignment stays within a tolerable and correctable range. Batter may be added to maintain horizontal alignment provided the facing system remains level and an alternate method of shear connection formed (like coarse gravel in block). Facing systems that rotate forward or backward on an individual course indicate problems with installation procedures or dimensional tolerance. Dry stack facing systems rely on tight dimensional tolerances, + 1/16in. (0.16cm), to maintain horizontal coursing and prevent rocking. The installer should send back blocks that fail to meet the specified manufacturing tolerances. The authors strongly discourage the use of shims, typically asphalt shingles, to correct a manufacturing tolerance or installation procedure issue.
h) Selection, approval, and documentation of reinforced soil. Prior to proceeding with SRW/MSE construction, the wall installer should designate the fill source for the reinforced soil volume. That fill source should be tested by both the wall installer’s own internal QC program and the owner’s QA program to verify the index and strength properties are consistent with the project specifications. Soil not meeting the project specifications should be rejected. The MSE designer then has the option to redesign to the available fill sources or require the wall installer to provide soil that meets the project specifications. The owner’s QA testing program should provide consistent, confirmative results on the fill source. When it doesn’t, the wall installer, MSE designer, and QC program should be notified. The fill source should be sampled periodically throughout construction to ensure the index properties have not changed, that may indicate a change in strength. Typical reinforced soil specifications are presented in previous sections (see 2.3.3 and 2.4.4).
i) Geogrid installation. The wall installer is responsible for ensuring the geogrid delivered to the site meets the project specifications and the material supplier provides a certification of the product properties. For smaller projects the certification is probably sufficient, but for larger projects the wall installer’s QC program and the owner’s QA program should obtain samples and test the key index properties of weight, aperture size, and wide strip tensile strength. The wall installer is responsible for placing the correct geogrid strength and length at the appropriate horizontal (Sta) and vertical (elev.) location within the MSE structure. The wall installer must perform the connection of the reinforcement to the facing system as shown on the drawings. The geogrid should be pulled taut (no wrinkles) prior to being covered with soil. Consistent tension in the geogrid will lead to a more uniform, as-built wall alignment. Photo 5.
j) Curves and corners Any that form 90° or more-acute angles tend to be problematic because the retaining walls move in orthogonal directions causing gaps to form between the vertical joints in each horizontal course of facing units. These detrimental movements can be mitigated by using high-quality reinforced zone soils to minimize movements or the largest diameter radius permitted by site geometric design constraints, diffusing the movement over a larger distance with more joints. The placement of geosynthetic reinforcement in corners can be difficult due to overlapping geosynthetics. Follow the details in the project drawings, or the standard details presented in most design guides or manufacturer’s installation instructions. Photo 6.
k) Protect the work area from water. The wall installer needs to ensure that the reinforced soil zone work area is protected from stormwater during initial installation. It is also prudent for the wall installer to work with the earthwork contractor to ensure the completed MSE structure is also protected during the remainder of construction, until the permanent storm water controls are in place and effective. MSE structures are quite vulnerable to erosion, loss of ground, and excessive saturation until the permanent erosion control, parking surfaces, and water diversions are functioning properly. The wall installer should ensure the reinforced soil volume, and in particular the wall face drain, is sealed off with impermeable soil to prevent water infiltration. The wall installer should attempt to contractually transfer responsibility for maintenance and storm water diversion to the general or earthwork contractor as soon after completion of the MSE structure as possible.
l) Fill between reinforced volume and excavation. The earthwork responsibility for fill placement and compaction behind the MSE structure reinforced zone and the excavation limits, or the general embankment fill should be clearly defined in the subcontracts. Without clear definition of those responsibilities, a wedge of loosely placed or poorly compacted soil behind the MSE structure could be falsely assessed as poor retaining structure performance. Placement and compaction of this soil is best accomplished when done in conjunction with placement of the reinforced soil volume.
m) Executing details around structures and utilities. Every project requires placement of wall facing and/or geosynthetic reinforcement around existing or permanent utilities or structures. The MSE construction drawings should contain details as to how the MSE designer has specified these connections, penetrations, and/or intersections be made. Follow the details in the project drawings, or the standard details presented in most design guides or manufacturer’s installation instructions. The guiding principal should be to minimize the cutting and/ or interruption of the strands, yarns or ribs running perpendicular to the MSE structure face.
Mike Simac is principal engineer at Earth Improvement Technologies Inc., based in Fort Mill, S.C.; email@example.com. Blaise Fitzpatrick, Fitzpatrick Engineering Associates P.C., is based in Lawrenceville, Ga.; firstname.lastname@example.org.
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