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Three challenges in using SRWs and other reinforced-soil structures: Part 2B

April 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 integration of the MSE wall/slope design into the overall site design requires the owner to communicate with, and effectively manage, three overlapping engineering disciplines. The owner must be able to distinctly assign specific contractual responsibilities for the overall site design, which includes the MSE wall/slope design. By understanding the role, expertise, and standard design practice of each professional involved, the owner is in a better position to accomplish that objective. The three main areas / engineering disciplines of the design process can be defined as follows:

The site designer’s and geotechnical engineer’s responsibilities were described in Part 2A (February/March 2008, Geosynthetics, Vol. 26, No. 1). The remaining portion of Part 2 describes the MSE/SRW designer responsibilities and options, along with the author’s discussion of the design process.

2.4) MSE design options and issues

Outlined below are some of the MSE design issues/options that should be addressed in the design phase of an MSE structure. While the owner should have some input as to the minimum design standards, because it affects the initial cost of the structure and influences the performance risk level assumed by the owner, most of the following issues/options should be addressed by MSE designers in performance of their duties.

2.4.1) Selecting the design method

The owner and MSE designer by contract and/or project specification should agree to the method by which the structure will be designed and any deviations from the minimum design requirements. National design standards, as discussed below, which were developed through some consensual process, are the best way to define the project design requirements. The chosen design guideline should be implemented in its totality to the entire MSE structure. Mixing and matching parts of various methods violates the design assumptions and analytical approach used to develop specific individual methods. The available national design methods for practice in North American markets are:

  • AASHTO LRFD 4th Edition 2007: The Load Resistance Factor Design (LRFD) method for MSE walls was recently developed to accommodate the FHWA mandate to implement more rigorous LRFD procedures on all highway structures. Leshchinsky (Geosynthetics, Vol. 24, No. 4, 2006) provides an excellent definition of the LRFD method and comparison to AASHTO’s previous Allowable Strength Design (ASD) using limit equilibrium analysis, which LRFD was calibrated to replicate, providing the same benefits as its predecessor.
  • AASHTO Bridge Manual 17th Edition 2002: This ASD method provides for analysis of complex MSE walls, using geosynthetic reinforcement. The minimum design requirements for soil strength, and geosynthetic strength, length, anchorage length, and connection strength to facing elements results in the largest minimum reinforced soil volumes with the most geosynthetic layers. When combined with the freedraining fill requirement, the initial cost of the structure, while justifiable for long service life and heavily trafficked public use projects, may be beyond the budgetary means of many private sector projects.
  • NCMA SRW Design Manual 2nd Edition 1997: This ASD method provides for analysis of simple MSE walls with geosynthetic reinforcement, which has been used successfully for more than 15 years. Developed to accommodate the vast number of concrete block facing systems, it is applicable to other facing systems as well. Allowing a wider range of acceptable reinforced soil types and using their measured strength, the minimum design requirements for geosynthetic strength, length, anchorage length, and connection strength to facing elements results in cost-effective reinforced soil volumes with optimum geosynthetic layers. To improve performance or limit movement, the MSE designer may utilize higher-quality soils with this method, as determined to meet project objectives.
  • FHWA Wall Design–NHI 2001: This compilation / synthesis of previous ASD methods provides for analysis of simple MSE walls with geosynthetic reinforcement, which has been used successfully for more than 20 years. This method allows the use of granular soil with its measured strength, imposing minimum design requirements for geosynthetic strength, length, anchorage length, and vertical spacing, which results in cost-effective reinforced soil volumes with optimum geosynthetic layers.
  • FHWA Slope Design–NHI 2001: This compilation / synthesis of previous ASD methods provides for analysis of simple MSE slopes with geosynthetic reinforcement, which has been used successfully for more than 20 years. This method allows the use of any granular soil (< 50% fines) with its measured strength, imposing minimum design requirements for geosynthetic strength, length, anchorage length, and vertical spacing, which results in cost-effective reinforced soil volumes with optimum geosynthetic layers.
  • There are many other methods to design reinforced soil structures in the technical literature, promulgated by other national governments, and/or recommended by purveyors of facing and geosynthetic materials. However, due to either their limited use in North America and/or development without design professional peer review, these other methods should not be considered a design standard for professional practice in North America.

2.4.2) Engineering software for design methods

The MSE designer has a plethora of choices for engineering software to execute and implement the selected design method. The following software is identified for the benefit of MSE designers beginning their search for generic computer-aided design assistance. Other choices do exist, and apologies to those not listed here for space reasons:

  • MSEW (3.0) performs design and analysis for MSE walls using AASHTO LRFD 4th Edition 2007, AASHTO Bridge Manual 17th Edition 2002, and NCMA SRW Design Manual 2nd Edition 1997: The program accommodates analysis of complex structures (tiered walls, bridge abutments, back-to-back walls) with both seismic and global stability analysis for the AASHTO methods. MSEW also executes the NCMA method for simple structures. The large number of analytical options offered in MSEW requires that the user be wellversed in MSE design to take advantage of its computational power.
  • SRWall (3.01) performs MSE wall design and analysis for NCMA SRW Design Manual 2nd Edition 1997: The easy-to-use program guides the user through each analytical step for simple MSE structures, under both static and seismic conditions. The program provides the results of all intermediate calculations that facilitate manual checking of results. Using superposition techniques, more complex structures such as tiered walls and building surcharges can be addressed.
  • ReSlope (4.0) performs Reinforced Soil Slopes (RSS) design and analysis per FHWA Slope Design – NHI 2001. The program accommodates analysis of simple slope structures with multiple surcharges under both seismic and static loadings, with a variety of pore pressure options.
  • PCstabl6, Slope/W, UTEXAS4, G-Slope, ReSSA (2.0 and 3.0) are all generalized slope stability programs that accommodate geosynthetic reinforcement. The programs incorporate reinforcement as a stabilizing moment per the techniques outlined for analysis of RSSs per FHWA Slope Design – NHI 2001. These programs accommodate analysis of simple and complex slopes or walls, as well as embankment foundation reinforcement, with multiple surcharges under both seismic and static loadings, and with a variety of pore pressure options. Use these programs to analyze any reinforced or un-reinforced wall or slope for global stability.
  • Many MSE designers utilize more than one program for any particular project or design method as a manner to compare results and assess the level of conservatism involved.

Most of the programs listed above have default (typical) values for every input parameter necessary for the analysis. Every one of these parameters must be changed by the MSE designer to reflect the exact project conditions being analyzed, for the design to be correct. This is particularly important with product manufacturer-based software, to ensure that all the design parameters meet the site conditions, as well as the minimum design requirements for the chosen design method.

2.4.3) Geosynthetic reinforcement strength

There are two different reinforcement strengths used for the design of MSE structures which are defined very similarly in all the design guides, the only difference being the minimum specific reduction factors (RF) allowed, regardless of how much testing has been conducted. The MSE designer should utilize the correct strength based on the method implemented for the structure being analyzed:

  • Tal is the long-term material strength and is based solely on the degradation mechanisms. It is used directly in the design of RSSs and for all global stability:

    Tal = Tult / [ RFcr x RFd x RFid ]

    where: Tal = long term Material Strength

    Tult = ultimate Tensile Strength

    RFcr = Creep Reduction Factor

    RFd = Durability Reduction Factor

    RFid = Installation Damage Reduction Factor

    Ta is the long-term design strength or allowable strength, based on the material strength reduced by an overall safety factor FS. It is used directly in the design of MS

    Ta = Tal / FS

    where: FS = overall Safety Factor (typically 1.5 for walls )

2.4.4) Specifying the soil materials used for construction.

Fill material used to construct the reinforced and retained zones, or modify/ improve the foundation soils must be clearly defined in the specifications and based on the geotechnical engineer’s investigation/recommendations (see 2.3.3). Fill material containing any of the following shall be considered unsuitable: brush, sod, peat, roots, or other organic, perishable, or deleterious matter, including snow, ice, or frozen soils. The following properties should be specified for each unique fill material or zone where it is to be placed. The specified properties should be consistent with the design assumptions made by the MSE designer and consistent with the design method selected (see 2.3.3):

  • Acceptable Unified Soil Classifications
  • Gradation limits including maximum size, coefficients of uniformity and curvature
  • Maximum percent passing the #200 sieve
  • Maximum liquid limit (LL) and maximum plasticity index (PI)
  • Optimum moisture content or acceptable range for compaction
  • Maximum dry density per ASTM D698 or D1557.
  • Minimum dry density for compacted soil as % of maximum
  • Maximum lift thickness, loose laid and compacted
  • Minimum effective friction angle, Φ’
  • pH of the fill material

The MSE designer should specify how often fill compaction should be tested as a function of fill volume, change in elevation, and per length of structure per reinforcement layer, and the number of tests required per interval of compaction testing. The MSE designer should also specify how often the fill material should be tested for gradation, plasticity, classification, and strength.

2.4.5) Drainage

All the MSEW and RSS design methods assume that the reinforced soil volume will remain dry. The MSE designer is responsible for ensuring that this occurs by incorporating drainage features to intercept any water, convey it through or around the structure, and then discharge it at a location below and beyond the structure. The typical drainage features to protect the reinforced soil volume are:

  • Surface drainage: The top of the reinforced soil mass should be sealed off with a minimum 8in. compacted lowpermeability soil layer, or parking surface. A diversion berm (curb and gutter) near the wall facing sufficient to preclude water cascading over the facing is sufficient surface drainage protection; otherwise, a drainage swale should be incorporated just behind the wall facing system to convey surface water away from and off the reinforced soil volume.
  • Internal drainage: The AASHTO approach requires the entire reinforced soil volume to be free-draining, so specific drainage layers are not required. All other soils require special drainage layers to prevent the reinforced soil volume from becoming saturated. The drainage system (Figure 2) shows a face drain, a blanket (bottom) drain, and a chimney (back) drain. Figure 2 | Complete Drainage System for SRWs from NCMA’s Design Manual for SRWs 2nd Edition The chimney and blanket drains can be eliminated only under special circumstances, based on definitive knowledge of no groundwater.
  • Inundation: MSE structures along waterways, lakes, and/or storm water management ponds may become inundated as water rises in front of the structure. This condition requires special design analyses, as outlined in the design guides. Generally, the most cost-effective solution is to utilize clean free-draining fill soils (i.e., AASHTO #57 stone) for the entire reinforced soil volume to at least 1ft above the maximum predicted rise and let the water move in and out of the reinforced soil zone freely. The retained and foundation soil layers should be protected from loss of ground by placing a geotextile filter between those soils and the free-draining reinforced soil zone.

2.4.6) Surcharge loadings

The site designer must present the grading plan in sufficient clarity that the MSE designer may establish the surcharge loadings that the MSE structure will be required to support. The surcharge loadings are divided into two categories: dead loads (which remain constant in magnitude and location throughout the structure’s life) that can be used for resisting forces; and live loads (which can change in magnitude and/or location). All surcharges are used to calculate disturbing forces. The most common surcharges that the MSE designer should consider are:

  • Sloped backfills above the wall, either infinite or truncated after a certain height
  • Uniform surcharges for buildings, or vehicular loadings; cars only 100psf, tractor trailers 250psf
  • Lateral impact loads from guardrails, fences, or handrails
  • Line and point loads that represent strip or column footings
  • Tiered retaining walls: Walls stacked on top of each other with a setback (minimum 4ft) between them. The setbacks are usually used for landscape plantings. The lower wall needs to be increased in height, creating a parapet, which unreinforced, can be placed between the walls to receive the plantings.

2.4.7) Global stability by the MSE designer

The setback of one single-height retaining wall some distance (> 4ft, but < 2H) from another results in the MSE designer creating a reinforced slope, relative to the total change-ingrade. This type of configuration creates unique stress concentrations and a reduction in sliding resistance at the base of the structure. The influence of one or multiple walls above another in a tiered configuration must be examined using global stability analytical procedures by the MSE designer to ensure that the full height interconnected reinforced soil structure is stable globally for the total change-in-grade. This does not relieve geotechnical engineers (see 2.1 and 2.3.4) of their responsibility to examine overall global stability independently.

Many MSE designers perform their own independent global stability check to ensure the long-term performance of single-height MSE structures, even though the project geotechnical engineer (see 2.1 and 2.3.4) and site designer (see 2.1) are ultimately responsible. Should the MSE designer’s analysis indicate sub-standard safety factors for global stability, they should bring this deficiency to the attention of the owner, site designer, and project geotechnical engineer.

2.4.8) Anticipated movements during construction

All the design methods are based on the active state of stress in developing earth pressures for design, which requires lateral movement to occur. Therefore, some acceptable movements should be anticipated by the owner during construction and for some time thereafter depending on soils utilized. FHWA suggests predicting lateral movement based on the stiffness of the geosynthetic, whereas NCMA simply allows a rotation of 2° from the stacked batter. Analyzing data from some instrumented SRW, Bathurst (et.al., 1999) recommended predicting lateral movements as a small percentage of wall height: 1.0% for walls under 25ft, and 1.5% for walls taller than 25ft.

These movements are especially important near orthogonally positioned walls, which will be moving in diverging directions. Radius corners tend to tolerate this movement better, since movement can occur over the entire radius, where sharp angled corners tend to manifest the movement more dramatically through facing separation and cracking.

Post-construction movements can be minimized by using very good construction techniques: 8-in. soil lifts, good compaction technique, and tensioning the reinforcement. Movements can be minimized further by using high-quality granular soil in the reinforced zone. This is especially critical if building foundations will be located on/in the reinforced soil volume.

2.4.9) Plans and specification requirements

With the analysis complete, the MSE designer must convey the construction requirements to the contractor with construction drawings and specifications. The construction drawings should consist of a minimum of the following:

  • Plan location of the structures
  • Profile of the structure including:
    • Finish grade line at top and bottom of structure
    • Existing grade
    • Elevation of top and bottom of structure facing
    • Placement elevation, type (strength) and length of geosynthetic reinforcement
    • Applied bearing stress along length of structure
    • Internal drainage collection pipe profile and outlets
    • Drainage swale profile and outlets
  • Typical wall cross section
  • Any unique wall cross sections
  • Facing system details
  • Detail of connection of reinforcement to facing
  • Geosynthetic reinforcement installation details
  • Drainage swale details
  • Guardrail, handrail, or fence detail

The specifications should address the following items:

  • Define the material properties, installation requirements, and quality control testing for the construction materials to be used:
    • Facing system: SRW block, WWF face form, etc.
    • Soils: reinforced, retained, foundation
    • Foundation approval requirements
    • Drainage soil
    • Geosynthetic reinforcement
    • Filter fabric
    • Drainage pipe
    • Drainage swale materials
  • Field and laboratory testing requirements for the geotechnical consultant
  • Geosynthetic-reinforcement installation procedures
  • Requirements for site drainage during and after construction
  • Procedures when/if changing conditions are encountered
  • Frequency of observing construction methods, monitoring length, strength, and placement elevation of reinforcement, tensioning of reinforcement, etc.
  • List of the design parameters utilized in the design to facilitate design review and use by field construction personnel, including soil, reinforcement, surcharges applied, seismic coefficient, and minimum safety factors applied
  • Construction notes related to all underground utilities, irrigation systems, fences, guardrails, and erosion control.
  • Require the contractor to notify the MSE designer if any of the conditions encountered differ from those depicted on the plans

Discussion

This summary of the major design steps/options involved with SRWs and other reinforced-soil structures highlights the complexity of integrating three major design disciplines: site/civil, geotechnical, and MSE structure. Without good coordination and communication of these design disciplines, the likelihood of poor performance increases. Poor performance usually results when one or more design issues are left unaddressed. In many cases, it is a design service that either no party agreed to perform, or they all excluded it from their scope. This lack of coordination, plus the interdependency and overlap of the design disciplines, often makes it difficult to isolate specific responsibilities for the critical issue that precipitated a problem.

The likelihood of potential long-term performance problems is increased significantly by the prevalent use of assumed soil design parameters in current MSE design practice. These assumptions are made by MSE designers due to insufficient geotechnical data, lack of guidance from the owner and/or the project geotechnical engineer, or inappropriate interpretation of geotechnical information available. The MSE designer then attempts to force the owner, even when a contractual relationship may not exist (i.e., contractor-supplied design), through construction specifications to provide the soils assumed, whether readily and economically available or not. MSE designers using faulty or unrealistic soil design parameters really fail to meet the threshold of a site-specific design for a project. Furthermore, global stability analyses performed with assumed soil parameters provides the owner with little assurance of stability. The use of assumed soil parameters rarely favors the owner’s interest; an overly conservative assumption increases the cost of structures, and an overly liberal assumption leads to long-term performance problems.

The authors’ recommendation is for the owner to take control of both the MSE design and definition of the soil parameters. The authors recommend that MSE structure design is best accomplished when performed by a team approach coordinated by the site designer on the owner’s behalf. Having the three disciplines interact early and often through the design process streamlines design and construction costs for the project. Savings can not only be gained on the MSE structure itself, but also on the portions of the project affected by the MSE structure. Design changes during construction due to changed conditions can be handled more efficiently with all three design disciplines working for the owner.

Having the owner take control and provide the design up front represents the best approach for all stakeholders to achieve the best-performing SRW long-term at the lowest possible design and construction costs. At this stage of the market maturation process, ownerprovided designs appear to be the best way to improve both cost and quality of the finished product. The design-build approach would also work favorably for the owner to address these design issues and options.

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 Specifi cations for Highway Bridges,” 17th Edition,

AASHTO (2007) “LRFD Bridge Design Specifi cations,” 4th Edition, Nov. 2007 interims

Bathurst, R..J., Simac, M.R. and Sandri, D. (1995) “Lessons Learned from the Construction Performance of a 14m High Segmental Retaining Wall,” prepared for Geosynthetics: Lessons Learned from Failures short course at Nashville, TN 20 February 1995 published by IFAI, 311 p.

Bathurst, R.J.. (1998) “Segmental Retaining Walls – Seismic Design Manual,” National Concrete Masonry Association, 1st Edition, Herndon, VA, TR-160.

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, Demo 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.

Leshchinsky, D., (2006) “ASD and LRFD of reinforced SRW with the use of software Program MSEW 3.0” Geosynthetics magazine, Vol 24, No 4, August–September 2006

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

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