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Three challenges in using SRWs and other reinforced-soil structures: Part 3B

August 1st, 2008 / By: / Feature, Geogrids, Geotextiles, Reinforcement

Introduction

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 an MSE wall or slope requires the owner to rely on skilled professionals performing their responsibilities correctly. To have the best chance for trouble-free construction and optimal retaining structure performance throughout the intended service life, the owner must be able to distinctly assign specific contractual responsibilities to each skilled professional. By understanding the responsibilities of these professionals, the owner is in a better position to accomplish that objective. Part 3A (June/July 2008, Geosynthetics, Vol. 26, No. 4) examined construction aspects of building SRWs and MSEW walls/slopes, while this last article in the series addresses the critical role construction monitoring plays in ensuring these structures are built in accordance with the design plans and specifications (parts 2A and 2B).

3.3 Construction observation options and issues

Although MSE wall/slope system components vary slightly, the following descriptions are general construction observation and testing issues routinely encountered on most projects. Testing and observation methods for the various MSE components and installation techniques do not vary much between the wall installer’s quality control (QC) program and the owner’s quality assurance (QA) program. Their responsibilities vary depending on how the MSE structure was contracted for and should be clearly defined in the construction documents (Part 1). However, in most scenarios it benefits everyone if the testing and observation information is shared and noncompliant work is identified quickly to all parties, minimizing the effects, procedures, and costs for correcting the noncompliant work.

3.3.1 Soils and engineering options

Following are some of the more important soils and civil engineering observations that should be included in a QC (contractor) or QA (owner) program:

a) Wall alignment and location. Quality checks should include verifying the placement of the bottom of wall according to the installer’s control survey staked on-site. The QA program should perform an independent survey if the wall location is close to property lines or if it will intersect with the building plan location (Photo 1). Photo 1a |  Checking the wall location by surveying methods indicates SRW installed near (+ 0.25in.) the plan location. Photo 1b |  Checking the wall location by surveying methods indicates SRW installed, unfortunately, sometimes more than a foot off. The QA program should ensure all easements are secured prior to beginning construction.

b) Approved foundation bearing surface. The wall installer’s own internal QC program and the owner’s QA program should observe, probe, and test the entire foundation bearing surface (full geogrid length) and approve it before wall construction begins. Approval should be based on encountering the anticipated foundation conditions identified in the subsurface investigation conducted for preparing the design and construction documents, another important benefit an owner attains by conducting such studies for design.

If a preconstruction investigation was not performed, the authors recommend a thorough examination of foundation conditions during construction, to a minimum depth equivalent to the geogrid length. The foundation soils may be investigated by hand probing, hand auger probes, test pits, and/or conventional soil drilling equipment, whichever is practically cost effective. Quantitative field testing such as torvane, pocket penetrometer, and static or dynamic cone penetrometer testing, including SPTs to document the foundation soil strength is preferred over qualitative examination (soil description and consistency). However, remember that settlement constraints typically control allowable soil bearing pressures established by the MSE and geotechnical designers, so ensure those criteria are met.

Unsuitable foundation conditions should be corrected according to the MSE/SRW design plans and specification or as directed by the MSE designer, based on recommendations or alternatives provided by the project geotechnical engineer (owner). The contractor’s input and suggestions may be needed to develop a workable plan within the overall construction sequence, plans, and operational constraints (Photo 2). Photo 2 | Aggregate pier foundation improvement below SRW for settlement control of foundation soils with organic material.

c) Outlet the wall drainage system. The wall installer’s internal QC program and the owner’s QA program should document that the internal drainage system was installed according to plan. Physically mark on-site (e.g., stake and flagging) and map onto as-built drawings the exact location for outlet pipes and the approximate pipe slope.

Marking outlets provide the finish grading and landscaping contractor, coming on-site after wall erection, the visual identification necessary to avoid damaging the outlets and ensuring a clear path for water to escape the structure. This also provides the owner with the information necessary to maintain the outlet. Any free water encountered in the excavation bottom or backslope during wall construction should be noted as to location, elevation, and quantity or flow rate. QA and QC personnel should notify the MSE designer immediately upon encountering the free water to ensure these conditions were accounted for in the design plans.

d) Wall erection. Verify that the wall erection is according to the construction drawings. Note each day’s progress as to what station and elevation wall placement begins and ends. Ensure the facing units meet the project specifications. Obtain copies of the manufacturer’s material certifications and check against project specifications. Sample the SRW units and caps once every 5,000ft2 for weight, height, absorption, and compressive strength. Ensure no visibly damage SRW units are used for construction. Check that the installed wall batter and all horizontal restraints are being used and installed correctly. Observe how the SRW units are filled with clean coarse stone and compacted for each course/lift. Identify any changes in wall alignment as it occurs, and check for forward or rearward tilting of the facing units that may indicate dimensional tolerance issues or poor erection techniques.

e) Approval and documentation of reinforced soil. Test the fill source designated by the wall installer for use in the reinforced soil volume. Test for grain size, Atterberg limits, pH, compaction control (i.e., Standard Proctor ASTM D698 or Modified Proctor ASTM D1557), and soil strength using ASTM D3080 Direct Shear Method or ASTM D2166 Triaxial Shear Test. The reinforced fill should be monitored for consistency using index property testing, (i.e., grain size analysis and Atterberg limits), any time the fill changes significantly in appearance (color, texture, top/bottom size) but at least once every 5,000yd3. The full set of tests to initially approve the soil, including the shear testing should be run a minimum of once every 20,000yd3 or when the soil changes significantly as identified by index testing.

These fill source test results should be constantly compared to the specification requirements established by the structure designers (wall and geotechnical engineer). Reinforced filling operations should be interrupted immediately by QA and QC personnel when identifying out-of-spec fill soil so the wall installer can examine the out-of-tolerance soil materials and determine an appropriate procedure to address the issue. How the SRW/MSE structure was contracted for, as defined in the contract documents, dictates the party responsible for solving the problem.

f) Soil compaction. Verify that a sufficient number of passes per the construction specifications are being imparted behind the facing system to compact the clean coarse stone used for the wall face drain. Test the relative compaction of the reinforced soil routinely using the maximum dry density and optimum moisture content identified in the proctor test (standard ASTM D698, modified ASTM D1557). The field density testing method (Photo 3) should be selected so that it provides consistent and reliable test results in the reinforced soil being used. Photo 3 | Field density testing by nuclear methods Compaction tests should be taken on the following schedule:

  • at least once every 2ft change in elevation for any wall length.
  • at least once every 75ft of wall for every 2ft change in elevation.
  • at least once every 200yd3 of soil placed and compacted

Every fourth test should be at a distance of 4ft from the wall face to examine the hand compaction zone. Each test is representative of the area around it and that area must be reworked to attain the required compaction requirements should any compaction test fail.

Ensure the soil behind the reinforced soil mass is properly compacted as the MSE structure increases in height. One test every 200yd3 should be sufficient.

g) Protect structure from water. As work nears completion ensure the wall face will be protected permanently by a planned berm (curb and gutter) or drainage swale. Should these appear to be deleted from the plans, notify the MSE designer. Upon completion of the structure observe that the reinforced soil volume and in particular the wall face drain has been sealed off with a low permeability soil. Ensure that some type of temporary storm drain diversion is in place to route water around the MSE structure until the permanent site drainage features are operational.

h) Observe details around structures and utilities. Some projects require placement of wall facing and/or geosynthetic reinforcement around existing or permanent utilities or structures (Photo 4). Photo 4 | In this example, a downspout outlet left short of the wall face has directed water behind the SRW, into the hand-compaction zone, causing erosion, loss of ground, settlement behind the SRW facing, and backward rotation of the SRW facing system The MSE construction drawings should contain details on how the MSE designer has specified making these connections, penetrations, or intersections. 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 or interruption of the strands, yarns, or ribs of the geosynthetic reinforcement running perpendicular to the MSE structure face.

Additionally, there may be site improvements installed by others that affect MSE structure performance after the wall installer has left the site. These site improvements should be monitored by the owner’s QA provider to ensure a properly coordinated integration of these improvements with the MSE structure, to eliminate conditions that adversely affect performance.

3.3.2 Geosynthetic reinforcement options

Following are some of the more important geosynthetic reinforcement observations that should be included in a quality control (QC) and/or quality assurance (QA) program:

a) MSE design (QA only). Prior to starting construction the MSE design should be checked for compliance to the project specifications and the prevailing local design standard, whichever is more stringent. The MSE design check should include determining whether there are conflicts with existing or proposed utilities or structures. The QA program should also ensure that global stability has been assessed by the project geotechnical engineer and that the MSE design has followed all recommendations provided by the project geotechnical engineer.

b) Geosynthetic installation. Check the delivery receipts and manufacturer certifications for the geosynthetic reinforcement delivered to the project site to ensure it meets or exceeds the project specifications. For larger projects, the owner’s QA program and wall installer’s QC program should sample and test each geosynthetic reinforcement type every 10,000yd2 of delivered material. The key index properties to test for each geosynthetic reinforcement type are weight, aperture size, and wide strip tensile strength.

Although the wall installer is responsible for placing the correct geosynthetic strength and length at the appropriate horizontal (station) and vertical (elevation) locations within the MSE structure, both quality programs should verify and document that this is accomplished (Photo 5). Photo 5 | Failure to install the geogrid layer, or in this case failure to unroll the layer after it was installed, led to poor performance (collapse) of a section of this SRW, that was otherwise well-built. Confirmation that the wall installer is making the structural connection of the geosynthetic reinforcement to the facing system as shown on the drawings should be verified by the quality programs. That verification should continue through the tensioning and covering of the geosynthetic with soil fill.

c) Observe details. Placement of geosynthetic reinforcement on curves and corners, at structures and around obstructions should be observed to ensure the details on the construction plans are followed. Document the installation.

3.3.3 Design professional in responsible charge and special inspection options

Local building code requirements for special inspection of retaining structures will dictate more formal procedures for QA and QC reporting, as outlined below:

a) MSE designer. As the design professional in responsible charge (DPIRC), the MSE designer must receive QA and QC reporting in a timely manner to make adjustments in the design, testing, or material properties, if needed. A listing of the special inspection requirements by the DPIRC are provided on the plans and specifications, so the local building inspector and the QA/QC team know exactly what critical materials and procedures will be tested and observed throughout the project. At the end of the project, the DPIRC will be required to issue a substantial completion letter indicating the MSE structure was built according to the plans and specifications based on personal knowledge through site visits and the QA/QC observations and testing. Good cooperation and communication between the DPIRC and the QA/QC personnel is crucial.

b) Quality control program. The QC program conducted by the wall installer and contractor are relatively unaffected, with the exception of additional reporting requirements to the MSE designer.

c) Quality assurance program. The owner’s QA program is relatively unaffected in terms of the type of testing, observations, and reporting necessary. However, special inspection reporting requires greater interaction with the MSE designer, rather than just the owner. Additionally, reporting requirements are more time sensitive in providing QA/QC information to both the MSE designer and local building inspector. Many local building authorities have created Web sites for uploading testing and observation reports in a timely and organized manner, with penalties for failing to do so.

Discussion

Parts 3A and 3B have presented the critical construction issues facing SRWs and other reinforced-soil structures. Since the technology and theory of reinforced soil structures is well-proven and accepted throughout the civil construction industry, the implementation of that technology is what needs improvement.

Previous articles (parts 2A and 2B) highlighted the challenges of integrating good design principles among the various design disciplines involved, and how this has created some of the performance issues with MSE structures. However, construction- related issues also account for a significant portion of the performance problems with SRWs and MSE structures. By focusing on the critical construction issues that have been responsible for the majority of the construction-related performance problems, parts 3A and 3B of this series provide the authors’ suggestions for the best practices to ensure overall MSE structure performance.

The method of contracting for the MSE structure (see Part 1) affects how the construction plans and specifications are prepared and then implemented on the site. Although it affects many aspects of construction, selection of soil for use in the reinforced zone is directly influenced by the method of contracting. In all cases the owner must find a way to contractually bind the MSE designer’s soil strength assumptions during design to soil utilized by the wall installer. This article provides those options for the owner to consider.

In general, these construction issues stem from wall installers not following the guidelines for materials or directions for installation of the materials, guidelines that are fairly consistent for many MSE structures. In addition, many of the construction procedural problems, such as multistaking courses of the facing system and poor compaction near the wall facing have been promulgated as “acceptable” procedures by some material suppliers in the interest of boosting daily production rates. With good education and better enforcement of good construction practices these construction procedural problems can be alleviated to improve performance.

The authors suggest that a well-executed construction QC and QA testing and observation program can ensure that the owner attains a well-performing MSE structure. Ensuring that the appropriate materials are installed properly is the best way to maximize MSE structure performance. Successful QA and QC programs require good cooperation and communication among all the parties involved: contractor, designer, and testing agency. In the experience of the authors,owner-provided designs are the best way to achieve this communication and cooperation, with all contributing parties under direct contractual control of the owner.

Summary of this series

This series was written for project owners to better understand how to interact with, and contractually control, the interdisciplinary endeavor that building an MSE wall or slope has become. Owners have realized for some time that great rewards (cost savings) accompany this technology, without fully realizing what was necessary to make that technology perform well for them long-term. Those are not mutually exclusive propositions, especially since all the technical components and expertise are already involved with the project. The owner only has to realize how to contractually coordinate the efforts of the professionals involved and provide a clear definition of their responsibilities.

These articles have reviewed all these responsibilities and the options available to address them, in the context of the method of procuring (buying) the MSE structure selected by the owner. The owner has three main options to procure the MSE structure. In order of increasing control for the owner, these are: 1) Contractor-supplied design integrated into the standard design-build-bid approach for the main project; 2) Designbuild approach; and 3) Owner-provided MSE design, the conventional designbid- build approach used for most civil construction. While there are advantages and disadvantages for each of these approaches, as explained in the articles, all of them can be used to successfully design and build an MSE structure that performs well during a long service life (Photos 6 & 7). Photo 6 | Welded-wire mesh MSE basket wall for the Corporate Ridge project in Brentwood, Tenn., produced using a contractor-supplied design. Photo 7 | SRW for the Lost Mountain Middle School in Cobb County, Ga. Civil and wall design criteria were developed by the owner’s design team with a review of the contractor-supplied design.

Throughout this series the authors have presented favorable reasons for owners to move toward using the “ownerprovided design” approach. The authors believe this is the best and most direct approach by the owner to harness the design professional’s expertise on their behalf when building an MSE structure. The owner-provided design has the clearest and most direct contractual definition of responsibility among design professionals, while fostering more and better communication on design and construction issues when it can best benefit the owner from a cost and performance perspective.

The authors prepared this series of articles for design and construction professionals involved with MSE structures to better educate owners on their capabilities and how they integrate together. This series of articles summarizes our proactive suggestions to accomplish this dialogue and ensure the best possible performing MSE structure at an effective installed cost. The authors foresee many successful future MSE structures and hope the performance meets the expectations of all involved. We trust this series of articles can assist in creating a productive dialogue between owners, design professionals, and contractors on exactly what reasonable expectations of performance are for MSE structures. Good luck.

Mike Simac is principal engineer at Earth Improvement Technologies Inc., based in Fort Mill, S.C.; mike@earthimprovement.com. Blaise Fitzpatrick, Fitzpatrick Engineering Associates P.C., is based in Lawrenceville, Ga.; fitzwall@bellsouth.net.

References

AASHTO (2002). “Standard Specifications for Highway Bridges,” 17th Edition.

Collin, J., et. al. (1997). “Design Manual for Segmental Retaining Walls,” National Concrete Masonry Association, 2nd edition. Herndon, Va., TR-127A.

Elias, V.E., Christopher, B.R. and Perkins, S. (1997). “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,” prepared for Federal Highway Administration, Demonstration Project 82, Contract No.: DTFH61-93-C-000145, 371 p.

Elias, V.E., Christopher, B.R. and Berg, R.R. (2001). “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,” National Highway Institute Course No. 132042 prepared for Federal Highway Administration, Contract No.: DTFH61-99-T- 25041, 394 p.

Van Kavelaar, D. P. and Leshchinsky, D., 2002, “Inspection Guidelines for Construction and Post- Construction of Mechanically Stabilized Earth Walls,” Report DCT 143, developed for Delaware Department of Transportation, 49 p.

Simac, M.R., Fitzpatrick, B., (2007). “Part 1—Three challenges in using SRWs and other reinforced-soil structures,” Geosynthetics, Vol. 25, No. 5, October/November 2007.

Simac, M.R., Fitzpatrick, B., (2008). “Part 2A— Three challenges in using SRWs and other reinforced-soil structures, Design steps and options,” Geosynthetics, Vol. 26, No. 1, February/March 2008.

Simac, M.R., Fitzpatrick, B., (2008). “Part 2B—Three challenges in using SRWs and other reinforced-soil structures, Design steps and options-continued,” Geosynthetics, Vol. 26, No. 2, April/May 2008.

Simac, M.R., Fitzpatrick, B., (2008). “Part 3A—Three challenges in using SRWs and other reinforced-soil structures, Construction options,” Geosynthetics, Vol. 26, No. 3, June/July 2008.

Simac, M.R., Fitzpatrick, B., (2008). “Part 3B—Three challenges in using SRWs and other reinforced-soil structures, Construction options-continued,” Geosynthetics, Vol. 26, No. 4, August/September 2008.

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