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Three challenges in using SRWs and other reinforced-soil structures: Part 1

October 1st, 2007 / 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)

1.1) Do-it-yourself (DIY) projects

Unless the landowner has specialized training and/or experience in designing and constructing SRWs, this approach is usually limited to small homeowner projects. These projects are generally successful only when used for landscape definition walls. SRWs that support driveways, patios, or extend to greater than 3-ft. in height, tend to be problematic. Therefore, in most situations the owner benefits from having professionals involved to design and construct the SRW.

1.2) Contractor-supplied designs

This is the most widely used approach to procure SRWs, allowing the contractor to supply the design (drawings, calculations, and specifications) for the SRW to be built, based on limited information provided by the owner. This process starts with a general note on a grading plan indicating “wall design by others.” Sometimes there are specifications for approved wall systems, which rarely includes installation details or typical sections, and usually provides no guidance as to the design methodology or minimum design specifications.

Currently, the owner and site designer select this contracting approach in an effort to ensure competition among several proprietary SRW systems. There is significant marketplace momentum from materials suppliers who encourage and support this method, to foster business alliances with both wall contractors and SRW designers.

The material suppliers provide “preliminary designs” and/or “material quantity estimates” to the contractor for pricing to the owner. After the owner selects the SRW contractor, usually based on cost, the contractor commits to a material supplier, who in turn recommends or retains an SRW designer to produce a “final design” that includes construction drawings using only the material supplier’s products. In most cases, these construction drawings are sealed by a licensed professional, providing everyone some level of confidence.

This approach eliminates direct communication and a contractual relationship between the SRW designer and owner. The SRW designer is working in this method for the contractor, not the owner, and communication between the SRW designer and other design professionals (architect, civil engineer, and geotechnical engineer) is either limited or nonexistent. There is no contractual mechanism or means of communication that can resolve conflicts between various project design elements, and ultimately produce an SRW design that is totally integrated with the other project design elements.

Without owner-provided specifications and qualifications, it is difficult to compare the various SRW contractor proposals, other than price alone. Consequently, the most aggressive SRW design produces the lowest cost SRW to build. This aggressiveness may include: liberal soil strengths, optimistic loading conditions, and favorable groundwater conditions. Additionally, some proprietary design approaches are aggressive by eliminating or altering minimum standards-of-practice (e.g., facing connection, bearing capacity, internal failure surface orientation, and global stability).

This method tends to lead to confusion before, during, and after construction on exactly which party has engineering responsibility (and liability) for important design decisions. Foremost among the confusions:

  • Has the site (civil) designer adequately addressed wall batter and surface water diversion around the SRW in establishing the site grading plan?
  • Has the owner provided sufficient geotechnical information to perform the design? Who determines sufficiency and/or orders (and pays for) more testing?
  • Has the site (civil) designer adequately addressed global stability for the grades being established? Is the site (civil) designer and owner expecting the SRW designer to perform global stability?
  • Has the site geotechnical engineer provided specific allowable foundation bearing pressures for the proposed SRWs? Without specific foundation recommendations, the SRW designer may just designate the required foundation pressure, leading to significant foundation correction procedures that may negate the cost effectiveness of the entire SRW system.
  • Who is responsible for defining the strength properties, locating a suitable borrow source with those properties, and ensuring that the reinforced (infill) soil is properly compacted? Routinely, the SRW designer working for the contractor assigns that responsibility to the owner, a party whom the SRW designer has no authority to obligate, thus creating scenarios when the owner’s quality assurance testing professionals were unaware that those services were even needed.
  • Who is responsible for ensuring that the correct materials are installed in a proper manner for the entire structure? SRW installers should be responsible for their own quality control and document it. All too often, the only oversight is the owner’s paid quality assurance testing, which tends to be insufficient in scope.
  • Who is responsible for surface water design above and around the SRW? This is particularly important after SRW construction. But it is also important prior to completion of all storm drainage improvements and final grading, when temporary sediment and erosion control plans can unduly induce surface water intrusion not contemplated for final design.

While the SRW designer is clearly responsible for all subsurface drainage design, the designer is also responsible for ensuring that site surface water does not enter the reinforced soil volume. This is generally handled by a drainage swale, if not already included in the site (civil) drawings.

Alternatively, the site (civil) drawings may illustrate alternative means to channel water away from the SRW, such as a diversion berm or conventional curb and gutter for roadways or parking areas. In either case, the size and type of surface water diversion needs to be coordinated with the site (civil) designer to ensure that it is adequate.

Although this method is the most popular, and the path of least resistance for the site designer or owner, it can suffer greatly if the SRW is not properly defined. The owner can use standard specifications available through professional organizations or from material suppliers, provided that the following items are defined, which enhance the effectiveness of this method of procurement:

  • Method of analysis
  • Minimum design safety factors and material reduction factors
  • Define the external/live loading conditions
  • Assign specific design responsibility for global stability and foundation support
  • A finalized site plan design with good surface water drainage design
  • Require a quality control testing program by the SRW installation contractor
  • Have an independent third party provide a design review check

This contractor-supplied method also places design responsibilities on an SRW installation contractor who is neither equipped technically (by training), legally (by registration), or financially (by insurance) to accept the risks associated with design liabilities.

The advantage for the SRW contractor is a stream of business projects found by material suppliers who appear to be willing to work out all the technical details for little or no cost, in exchange for a loyalty to their products.

Inevitably, each stakeholder’s desire to expand its business creates further burdens on capacity, eventually leading to both parties looking outside this exclusive, loyalty-based relationship. This burden on capacity affects the owner, relative to the effort level, and quality of the SRW installed on the site, particularly regarding the SRW design.

1.3) Design-build approach

This is the method that the owner and site designer usually believe they are getting when option 1.2 (above)—“Contractor-supplied design”—is specified in the project documents. There are several procedural, contractual, and legal criteria necessary in the project specifications to invoke a true “design-build” scenario. The following criteria generally are necessary for the owner to ensure that a design-build contract is in place:

  • Design and construction is done by the same legal entity, licensed to provide and perform those combined services by the presiding governing authorities.
  • Total design/engineering responsibilities and control are given contractually to the design-build contractor. Responsibility for all aspects of engineering design include, but are not limited to: selection of materials, global stability, bearing capacity, and structural design and performance, including installation tolerances.
  • Design-build contractor is responsible for all pre-engineering testing, data collection, and quality control testing throughout the construction phase.
  • Design-build contractor is responsible for integration of the SRW design with other project design components, such as buildings, roadways, and utilities. Conflicts between design elements will be resolved by the design-build contractor.
  • The owner and design-build contractor agree on what defines the proposed SRW (i.e., plan location and change in grade) and the performance criteria for end-of-construction, and at the end of the agreed-upon performance period (i.e., 3, 10, 75, 125 years).

This approach is more easily integrated into a project when all other components are also procured via a “design-build” contract. However, with some care, it can be specified as a separate component in a conventional contract, with a pre-approved, experience-based bidder’s list. Even with a pre-approved bidder’s list, the qualifications for the design-builder should be listed in the contract documents, incorporating both experience and technical qualifications.

Finding qualified design-build contractors to ensure a competitive bid may be difficult. Currently, there are only a handful of companies nationwide that can provide these services. And there may be only one or two local providers of similar services.

Design-build entities tend to more closely follow standards-of-practice for design and construction because they are equally responsible for both, and thus, are unable to deflect criticisms or deficiencies as the other party’s fault. Consequently, design-build SRW firms with succesful project experience will reveal a company record that can be trusted by owners.

Both the “Design-build” and the “Contractor-supplied design” approaches allow for fair competition among various proprietary systems. Both of these approaches also have the added owner advantage of deferring the design costs to the construction stage.

1.4) Owner-provided design

This is the traditional method for almost all building and site development contracts in North America. The SRW design is easily incorporated into the site design drawings. This can aid (and sometimes is required by) the prevailing plan approval process of the local jurisdiction. Routinely, the owner retains a site (civil) designer who establishes site grades, stormwater management, and utilities.

The SRW designer just becomes part of the owner’s design team, usually through a subcontract with the site designer, such as how a structural engineer was retained when cast-in-place concrete retaining walls dominated the site development market.

The SRW designer develops design calculations, specifications, and drawings through direct communication with other members of the owner’s design team (i.e., architect, civil engineer, and geotechnical engineer). Issues related to geotechnical concerns and how the SRW is coordinated with the site grading can be addressed up front.

The key advantages of this approach are that the SRW design is integrated into the overall site design early in the site-planning process, providing the most cost-effective design alternatives, by adjusting all site design components to incorporate constraints that are interdependent on each other from all components, not just the SRW.

Geotechnical information required for SRW design can be obtained as part of the original site investigation, streamlining the geotechnical costs and design schedule for the SRW. The owner is assured that all retaining wall installation costs are based on the same design, formulated specifically for the site location and exact design requirements. This ensures a true installation cost comparison, based on the same design.

This can be accomplished by presenting the owner-provided design via two methods:

Base Contract Design

The SRW designer prepares a design with one unique set of SRW facing and geosynthetic reinforcement. All contractors are required to bid, installing the base contract design included in the project drawings.

A contractor can subsequently propose an alternative design using other SRW facings and geosynthetic reinforcement combinations as a value engineering proposal. The alternate design must be stamped by a different qualified design professional and approved by the original project SRW designer as an equivalent design.

Generally, equivalency will be based on: similar reinforcement strengths, vertical spacing, and length, particularly if global stability is controlling the reinforcement length, provided that all the required design safety factors are met using the same infill soil parameters. This approach ensures that all SRW installation contractors are competing on the same design.

If direct competition on SRW materials (facing and reinforcement) is desired, the owner may direct that a second unique design be placed in the project plans. When this is done, it is recommended that the contractor be required to declare in the bid proposal which system is to be built, so that the owner receives the best pricing at bid time, rather than the contractor obtaining it after the award.

Generic Design

The SRW designer prepares one design with a unique set of SRW facing and geosynthetic reinforcement properties that are appropriate for a variety of interchangeable materials. Contractors are required to bid and install the SRW using any combination of the specified materials included in the project drawings.

The owner can select two or three SRW facings with similar overall dimensions meeting their aesthetic appearance requirements. The SRW designer then selects two or three geosynthetic reinforcements for which the design properties are roughly similar. The unique generic SRW design is then based on the lowest performance property in each category for each of the combinations.

Although this approach fails to obtain the optimized design for each component part, it produces a conservative, cost-effective installation due to the cost competition created by interchangeable “generic” parts.

Advantages

Ironically, most of the contractor-supplied designs reviewed by the authors have been produced from the perspective of the SRW designer working for the owner. Obviously, SRW designers prefer the tighter controls and requirements that an owner-provided design places on the contractor.

The owner obtains much better control of the finished product by having an SRW design with known risks, by integration with other design disciplines, and creation of a quality assurance program that ensures compliance of construction to the design. The owner also obtains fair and even cost competition on his design.

The contractor also knows he is competing fairly on the same design, now being judged on his ability to build, without exposing himself to any design liability.

The material suppliers benefit because they can now compete solely on their ability to manufacture products, instead of how efficiently they can design SRWs, and can do so without any design liability.

Discussion

When SRWs entered the marketplace some 15-20 years ago, the owner and site (civil) designer were providing retaining wall designs with the project plans, usually cast-in-place, cantilever concrete walls designed by a structural engineer. Owners, quite naturally, did not want to pay for two designs, so SRWs were sold with a contractor/vendor supplied design.

Shortly thereafter, when the economic advantages of SRWs became so compelling, owners and site designers just dropped the cast-in-place concrete wall design completely and specified SRWs, as “Retaining Wall-Design by Others.” This seemed to make sense because the design appeared to be free.

But as we all know, nothing of value is ever really free, as the cost of the design was included in the materials or installation of the SRW. As cost competition moved from SRW vs. concrete wall, to SRW vs. SRW, design costs, material costs, and costs to build a design became significant issues in the bidding competition. The successful low bidder was the one who could reduce those cost components the most.

This cost reduction trend was initially welcomed by owners and developers. However, as time progressed, performance problems with SRW installations have increased, causing concern among some of the largest land developers, leading them to place constraints on SRW use or to enact strict design and construction standards, while still using the contractor-supplied design approach.

Some states, such as North Carolina, have passed building code amendments requiring that retaining walls higher than 4ft be designed at the time of zoning/site approval, instead of at the building permit stage. This amendment requires owners to develop the designs up front, at the project planning stage, as was originally done for cast-in-place concrete walls. North Carolina closes the design and construction loop by requiring special inspection of SRWs, with a compliance letter at the end of construction by the SRW designer of record, ensuring that the SRW was built in accordance with the plans and specifications.

While all methods described above are workable and offer certain advantages, the owner-provided design represents the best approach for all stakeholders to achieve the best performing SRW, long-term, at the lowest possible design and construction costs. Consequently, it appears to be time in the SRW marketplace maturation process for the owner to resume control of the design process. There are several key factors that support that approach:

  • Design methods are well established, with many qualified designers.
  • There are many proprietary systems, but their principles are the same.
  • The age of the market means that many patents are close to expiration.
  • There are numerous qualified SRW installers.
  • Contractors are less willing to be conduits for design services and their liabilities.
  • Design needs to be done in the owner’s best interest, with integration between the various design disciplines involved.
  • Design control will help to reduce the number of long-term performance problems.

This examination of procurement options provides owners, and their specifying site designers, with sufficient knowledge to select the approach that best matches their project objectives.

Although the authors believe that an owner-provided design is the best alternative in most situations, there are circumstances when other options are certainly appropriate. Choose wisely, because it affects the outcome of the project and SRW performance, more than you would initially believe.

Good luck.

Mike Simac is principal engineer at Earth Improvement Technologies Inc., based in Fort Mill, S.C. Blaise Fitzpatrick, Fitzpatrick Engineering Associates P.C., is based in Lawrenceville, Ga.

References

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

Elias, V.E., Christopher, B.R. and Berg, R.R. (2001) “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,” prepared for Federal Highway Administration, National Highway Institute, Contract No.: DTFH61-99-T-25041, 393 p

Elias, V.E. and Christopher, B.R. (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

North Carolina Building Code 2002 (2002) Sections 1610 “Soil Lateral Load” adding 1610.3 “Definition of Retaining Systems” and making Section 1704 “Special Inspectors” mandatory; based on International Building Code 2002.

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