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

By Michael R. Simac, P. E., and Blaise J. Fitzpatrick, P. E.

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:

2.1) Options and issues for professionals in SRW design and construction

2.1.a) Site/civil design

The site/civil designer (civil engineer or landscape architect) works with the owner to establish the site grading plan, based on existing topography, development objectives, and prevailing land use/environmental regulations. Using those constraints, the site designer determines whether a steep change in grade structure (retaining wall or slope) is necessary, and the height, length, and location of such structure, to make the site development plan feasible. The site designer is also responsible for the design and layout of all site utilities, including water supply, sanitary sewer, and surface water drainage. Site surface water drainage design includes the hydrologic analysis to size: inlets, pipes, and stormwater retention/detention ponds, along with their flow control (inlet/outlet). The site designer should follow the recommendations of the geotechnical engineer in establishing the site grading plan.

2.1.b) Geotechnical design and materials testing

The project geotechnical engineer investigates existing site conditions to determine the viability of the proposed site design, and makes recommendations on the suitability of existing site materials for use in construction. The geotechnical engineer should recommend suitable slopes for fills and temporary/permanent cuts, foundation type with allowable soil pressures, likelihood of encountering groundwater with drainage provisions to mitigate, earth pressures for retaining wall design, suitable fill types with compaction requirements, and specific recommendations to ensure global stability of slopes (and walls), or further examination of global stability of slopes, if warranted. The project geotechnical engineer is responsible for the MSE structure investigation, providing recommendations for soils properties, drainage systems, and analyzing and ensuring global stability of the MSE structure.

The materials testing engineer (who often is, but does not have to be, the project geotechnical engineer) is engaged to contemporaneously verify that construction is proceeding according to project plans and specifications. Likewise, a separate (or dual, i.e., contractor quality control/owner quality assurance) materials testing engineer may be engaged specifically for the MSE wall or slope. 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. Sometimes the owner engages the materials testing engineer to perform a third-party review of the MSE and/or site design to verify its consistency with standard design practices and make recommendations for any additional testing.

2.1.c) MSE design

The MSE wall/slope designer utilizes the site grading plan and geotechnical recommendations to prepare wall/slope profiles and cross sections. The MSE designer performs analyses to specify sufficient length, strength, and vertical spacing of geosynthetic reinforcement layers to ensure the external, internal, and local/facing stability of the reinforced soil mass for the wall geometry (height, length, and surcharge conditions) defined on the site grading plan and facing materials selected by the owner. The MSE wall/slope design package should consist of construction drawings showing the proposed walls/slopes profiles indicating the geosynthetic reinforcement layout (type, length and spacing) along the entire wall(s) length, typical wall/slope cross sections, cross sections for penetrations or conflicts with utilities, SRW or facing/basket details, and MSE construction specifications for materials, installation, and testing.

Consistent with building codes, design guidelines, and established professional practice, most MSE designers, as well as designers for other types of retaining walls, exclude global stability from their work and indicate so on their construction drawings, since the site designer and geotechnical engineer are responsible for addressing global stability. Some MSE designers, who are also qualified geotechnical engineers, may include global stability as part of their design services, which can benefit the owner when trying to coordinate global stability between the three engineering disciplines.

2.1.1

Coordination of these three disciplines presents the owner with both practical and contractual challenges relative to design. The practical challenge is identifying the design issues or conflicts and communicating those effectively to the parties involved. The contractual challenge is ensuring that the critical design responsibilities have been contractually assigned to a specific party. Outlined below are our suggestions on how to best accomplish this, depending on the method selected to procure the retaining wall system (see Geosynthetics, Vol. 25, No. 5, October/November 2007).

• In “Do-it-yourself” projects, the owner has opted to forego professional assistance relative to design and construction eliminating the need to coordinate these services, and assuming the liability for performance personally. The owner is entitled to execute this approach, within the limits of the prevailing building code, typically a maximum wall height above which professional services are required.
• For “Contractor-supplied” designs, the owner should engage a site designer to develop a grading plan and retaining wall specification that requires the contractor provide a “stamped” engineering design for the retaining wall meeting minimum design, installation, and performance criteria using the project geotechnical report. The owner should retain the geotechnical engineer to provide the data necessary for proper MSE structure design. The site designer should try to minimize conflicts and penetrations between utilities and the retaining wall. The owner should engage a materials testing engineer to review the contractor-supplied design for compliance to the specification and conflicts with the grading plan and/or utilities, refereeing those two parties for resolution of conflicts prior to approval for construction. The owner should also engage the materials testing engineer as the “special inspector” for wall installation, requiring correction of nonconforming work as it occurs.
• In the “Design-build” approach, the owner should engage a site designer to develop a grading plan and a design/build retaining wall specification that requires the design/build contractor provide a “stamped” engineering design for the retaining wall meeting minimum design, installation, and performance criteria using the project geotechnical report. The specification shall designate the design/build contractor as the solely responsible party to resolve conflicts, with site designer approval when grades or utilities are affected, and to obtain any additional geotechnical information or test data necessary to develop and/or verify the design. The design/build contractor would be contractually responsible for global stability and performing all the special inspection necessary to document compliance of the field construction to the design documents. The design/build contractor would provide special inspection documentation, as-built drawings, and a warranty to the owner.
• For “Owner-provided” designs, the owner should engage a site designer to develop a grading and drainage plan, a geotechnical/materials testing engineer to investigate the site with global stability evaluation, and an MSE designer to prepare the construction drawings for the MSE wall or slope. This team approach should be coordinated by the owner with each professional contractually obligated for the traditional duties as described above, with the site designer as the leader and clearinghouse for resolving design issues/conflicts. The materials testing engineer would issue daily construction reports to the owner, site designer, and MSE designer so each can verify compliance on their design component as the work progresses.

All three of these approaches provide a checks-and-balances system on the MSE wall/slope design and the overall design in general. The owner has a clear path of communication to define the project objectives with performance requirements and a definitive method to contractually divide the design responsibility appropriately for each project. The principal components of a wall/slope design are as follows:

2.2) Site design options and issues

While all three design areas influence work in the others, the general starting point for design is the grading plan, prepared by the site designer.

2.2.1) Grading plan

The grading plan establishes the height, length, and surcharge loadings applied to an MSE structure. Preliminary grading plans are usually based on the prevailing conventional site grades (i.e.,3H:1V to 5H:1V) known to be routinely stable in a given geology. When these conventional grades are insufficient to meet the owner’s development objectives for usable space, a change-in-grade retaining structure is utilized. The shallowest inclination necessary to meet the change-in-grade requirement is usually the most economical structurally, providing that the resulting aesthetics are acceptable to the owner. In order of increasing cost, roughly 25% per projected face area per increment below (based on the author’s experience), the inclination of the change-in-grade is usually steepened to:

1. Between 2.5H:1V or 1.5H:1V (22o to 33o) based on site-specific soils investigation, testing, and recommendations by the project geotechnical engineer, no reinforcement.
2. An MSE-reinforced soil slope between 1.5H:1V to 0.5H:1V (33o to 63o) based on site-specific soils investigation, testing, and recommendations by the project geotechnical engineer, utilizing the geosynthetic reinforcement strength, spacing, and length in native site soils, based on a design by geotechnical engineer or MSE slope designer. These slopes are usually vegetated with grasses and/or plants, presenting a “scruffy” look because of difficulty maintaining the slope. There are also “clean” hardened facing options available.
3. An MSE-reinforced soil slope between 0.5H:1V to 0.2H:1V (63o to 78o) can be designed based on site-specific soils investigation, testing, and recommendations by the project geotechnical engineer by the MSE slope designer. These slope faces must be formed temporarily during construction, generally consisting of galvanized welded wire form baskets, which can be used with a variety of facing options: vegetated, stone infill, rock covering, or a permanent form as the facing, such as manufactured blocks/panels.
4. An MSE wall design is required for slope faces between 0.2H:1V to vertical (78o to 90o). The MSE wall designer uses the project geotechnical engineer’s site-specific soils investigation, testing, and recommendations to determine the geosynthetic reinforcement strength, spacing, and length for the reinforced soil and facing option selected by the owner. The owner should select the facing appearance (e.g., color, texture, size), and not necessarily the specific facing system.

Selection of the lowest-cost MSE structure is difficult when preparing the initial site grading plan, due to the number of interdependent factors influencing the cost. For planning purposes, it is best to start with a full-height change-in-grade structure. Should slopes be necessary, above the crest or below the toe of the MSE structure, use a nominal/safe slope (e.g., 4H:1V) for sizing the MSE structure, or use a slope either above or below (preferred), not both. The lowest-cost change-in-grade structure does not result just from minimizing wall face area.

The grading plan and site layout must account for the slope inclination (face batter) of these change-in-grade structures. Grading plans based on vertical walls will require field modifications to successfully construct, since all these structures have some face batter. This is particularly important when the MSE structure is placed near property or environmental boundaries that support other site structures, such as buildings, roadways, or parking.

In an effort to coordinate the MSE wall alignment to the site/grading plan and communication between interested parties, it is strongly recommended that the civil engineer station the MSE wall alignment on the grading and drainage plans. Civil firms typically have a civil/site design module in CAD that allows greater accuracy in stationing the walls, compared to manually stationing the walls.

• The civil engineer should “station” each wall on the grading plan with a minor tick every 10ft and a major tick every 50ft with station numbers listed every 50ft—e.g., 0+00, 0+50, 1+00, 1+50, etc. The stationing must be along the outside front face alignment, and this is particularly important when going around curves and corners.
• Station 0+00 is located at the left end of the wall as referenced from viewing the front wall face from outside the structure.
• Each separate and unique wall location should be designated on the site/grading plan, such as Wall A and Wall B.

The site design must also provide pedestrian restraint (handrails or fences) above any MSE structure greater than 4ft tall. Likewise, if there is traffic near the MSE structure, vehicular restraints (guardrails) should be installed to prevent vehicles from overtopping the wall.

2.2.2) Surface water design

The collection, conveyance, and retention/detention of surface stormwater directly affects the site grading plan, and consequently the size and performance of MSE structures. Surface stormwater design also locates storm drain inlets and conveyance piping throughout the site, which affects the amount of surface water contacting the MSE structure.

Most MSE designs require that surface water not enter the reinforced soil mass. This is accomplished by directing surface water around the ends of the retaining structure. The site designer must consider how this is to be accomplished when slopes are above and graded toward the top of wall. The top of wall grades must be established to allow for positive surface water flow across the top of the wall and to exit at one or both ends of the wall. The site designer can rely on the minimum 8-in.-deep drainage swale required by MSE design guidelines, but the grading and drainage plan should reflect the presence of a swale, and provide the horizontal distance (3ft is typical) to install the swale. Failure to account for the swale requires the MSE designer to increase wall height to accommodate a functioning drainage swale. The site designer should perform wall area-specific hydrologic runoff studies to size the swale if it is to receive concentrated flows from other portions of the site and/or when the catchment area for the slope above the wall is capable of generating collected flows exceeding capacity of the standard 8-in.-minimum-depth swale.

Site designers should try to avoid lower top elevations in the middle of the wall than the ends, creating a valley. Such site drainage patterns will require the MSE designer to incorporate costly swale catch basins and drainage outlets within the reinforced soil mass to discharge the collected surface water. Minimizing and/or eliminating these low spots along the top of wall will reduce the cost of the MSE structure.

Although surface water should not be permitted to cascade over the wall face, in rare circumstances some MSE designers have allowed it to occur. These circumstances are: the catchment area above the wall extends < 10ft horizontally beyond the height and the structure has a good and thick (>12in.) sealing soil layer above the reinforced zone with a high-capacity wall face drain. This usually occurs when small walls are near buildings, and roof runoff is discharged through separate watertight piping below the wall.

Alternatively, the site designer may divert surface flow away from the wall with a berm, typically a concrete curb and gutter at the edge of pavement behind the crest of an MSE structure. If possible, it is desirable to direct pavement surface water flow away from the wall to collect in drop inlets located outside of the geosynthetic-reinforced zone.

MSE structures along waterways, lakes, and/or stormwater management ponds may become inundated as water rises in front of the structure. This condition requires special design analyses, as outlined in the MSE design guides. The site designer should be aware that this ebb and flow of water into and out of, or rising and falling beneath the structure, affects its performance, sometimes provoking settlement and tilting post construction. Therefore, it is recommended that all site improvements—buildings, roadways, parking, utilities, and all piping except stormsewer feeding the water impoundment—be kept a minimum distance of twice the wall height away from the wall face.

Lastly, surface water drainage design should also be evaluated for the construction phase, including erection of the MSE structure. The MSE structure excavation should be protected from overland flows, and any temporary detention ponds located well away from the MSE structure. The MSE structure is extremely vulnerable to surface water infiltration immediately after erection is complete until the finish-grade surface water controls are in place and operable. The duration of exposure to this hazard can be quite long, depending on the construction schedule, requiring that extra interim erosion control and/or diversion elements be installed to protect the MSE structure.

2.2.3) Buried utilities

If possible, locate all utilities outside of the geosynthetic-reinforced zone. The obvious advantage is that maintenance of the buried utility is simplified and has minimal effects on the MSE structure. Maintenance of utilities buried within or below the geosynthetic-reinforced mass will require removal and replacement of that portion of the structure affected by the repair.

If a utility must be located within the geosynthetic-reinforced zone, attempt to locate it perpendicular to the wall face rather than parallel, and beneath the reinforcement if possible. While utilities installed parallel to wall face in the reinforced soil mass are technically feasible, it makes construction of the wall and utility more difficult. Coordinating the efforts of two independent contractors, working in the same area, while trying to locate the utility between layers of geosynthetic reinforcement without damage, can be challenging.

With the location of liquid-bearing utilities within or near (10ft) the geosynthetic-reinforced zone, leaking should be considered cautiously. The consequences of leakage are grave, with the potential to cause excessive hydrostatic loads, loss of ground, surface settlement and/or collapse of the retaining structure. Stormwater pipes are subject to separation at joints and should consist of either continuous pipe sections or neoprene O-rings, which should be properly installed at the pipe joints. A double-lined pipe system or a leak management system could also be implemented into the stormwater design.

Manholes or vertical stormwater riser pipes placed within the geosynthetic-reinforced zone should be located entirely a minimum of 3ft behind the back of the MSE facing system. This space allows for proper compaction of soil and placement of the geosynthetic reinforcement. Additionally, if the top of the riser is also intended to function as an inlet, it should be constructed low, to allow for settlement of fill surrounding the manhole, or sufficient time allowed for the settlement to occur before final grading and paving.

Shallow utilities, such as electric, gas, telephone, and cable, could be placed in a specially located trench within the reinforced zone. Alternatively, an unreinforced fill slope placed above the MSE structure could accommodate these utilities, without affecting the geosynthetic reinforcement.

The dangers of subsurface irrigation systems within the geosynthetic-reinforced soil mass are obvious relative to leakage; however, overuse also has the potential to saturate the reinforced soil zone, generating detrimental hydrostatic pressures that may induce wall movement or failure. Therefore, the use of subsurface irrigation systems is strongly discouraged in slopes above or below the reinforced zone or within 20ft behind the geosynthetic-reinforced soil zone. The alternative landscape treatments to grasses are mulch and native shrubs that can survive on the normal rainfall.

2.3) Geotechnical design options and issues

Outlined below are the geotechnical design issues that should be addressed in the design phase of a project that includes an MSE structure. The construction materials testing and monitoring associated with geosynthetic-reinforced soil construction will be addressed in Part 3 of this series. Generally, the initial geotechnical site investigation is performed before, or in conjunction with, preparing the preliminary grading plan, so structure-specific investigation is usually not performed, and only general guidelines for site development are presented. Once the detailed grading plan has been established, budget-conscious owners tend to focus their geotechnical engineering resources on the building structures being planned. The issue for an owner is: When does an MSE structure shown on the project grading plan become of sufficient significance in construction cost or performance that it warrants its own site-specific investigation as any other structure?

2.3.1) Site-specific geotechnical investigation for MSE structures

While it can be extremely cost-effective to perform site-specific building/structure soil investigations with the initial site development investigation, often project planning has not proceeded sufficiently to identify the exact locations of those structures. Once the site designer establishes the preliminary grading plan, with buildings and retaining structures, the geotechnical engineer should identify for the owner all the geotechnical investigations necessary for site development. The geotechnical engineer should include investigation recommendations necessary for assessing slope stability and retaining structures, along with those routinely made for buildings, roadways, parking, and detention ponds. Direct input on investigation requirements from the MSE designer would be helpful, but the geotechnical engineer can proceed to convince the owner based on these general requirements established in national MSE design guidance documents that an MSE structure-specific subsurface exploration be done whenever the following conditions occur:

• For any publicly funded project (roads, waterways, etc.), per AASHTO and FHWA
• Any structure >15ft high or significant structure in size (area or length)
• Consequences of failure significant (potential loss of life or large property damage loss)
• Global stability concerns, or when steep slopes are present above/below the wall
• Site subject to seismic activity or groundwater within the structure
• Site close to known abandoned mines or Karst topography

2.3.2) Geotechnical investigation techniques for MSE structures

The subsurface exploration regarding MSE structures should consist of soil borings and test pits as performed by the project geotechnical engineer. The type and extent of the exploration should be decided after review of the preliminary data obtained from a field reconnaissance by the geotechnical engineer, and MSE designer if available. The exploration must be sufficient to evaluate the geologic and subsurface profile in the construction area of the MSE structure. The overall geotechnical objective is to define the groundwater and soil conditions within, behind, and beneath the proposed MSE structure.

National MSE design guidelines (AASHTO, NHI/FHWA, NCMA) provide detailed recommendations for subsurface exploration and laboratory-testing programs, generally summarized below:

• Soil borings and/or test pits should be performed at regular intervals (typically 100ft) along the wall/slope alignment, plus any geologically significant features, and any anticipated borrow areas, as determined by the geotechnical engineer.
• Soil borings should be advanced to a depth of at least twice the structure’s height using exploration techniques outlined in ASTM D1586 or AASHTO T-206 and T-207. The boring depth should be controlled by the general subsurface conditions, and should extend at least to a depth equal to twice the height of the MSE structure. If subsoil conditions within this depth are found to be weak and unsuitable for the anticipated bearing pressures of the MSE structure, then the borings should be extended until reasonably strong soils are encountered.
• Consolidated undrained triaxial shear tests, with back pore pressure readings, shall be performed to determine the effective friction angle, Φ’, and cohesion, c’, and total friction angle, Φ, and cohesion, c, for the critical soils to be used for design.
• Soil density tests to determine the unit weight for all soils to be used for design, and Proctor curves to be used for quality control testing of fills.
• Consolidation tests shall be required if it is determined that significant areas of soft soils are present in the foundation of the MSE structure. The site geotechnical engineer should provide an allowable soil-bearing pressure that precludes undesirable performance due to differential settlement.

2.3.3) Geotechnical recommendations for soils to be used with MSE structures

Based on the results of the subsurface geotechnical investigation and laboratory test results, the site geotechnical engineer can make recommendations related to soil shear strength, density, and consolidation of soils used in the wall system. The following information should be provided to the wall designer prior to construction of the MSE structure:

• Geosynthetic-reinforced soils and retained soils:
  • Allowable soil classifications and compaction requirements
  • Gradation limits and maximum percent passing the #200 sieve
  • Maximum liquid limit (LL) and maximum plasticity index (PI)
  • Effective friction angle, Φ’ and moist unit weight, γ_m
• Foundation soils:
  • Anticipated total/differential settlements for allowable bearing pressure recommended
  • Anticipated soil type and techniques to determine suitable conditions
  • Maximum liquid limit (LL) and maximum plasticity index (PI)
  • Effective friction angle, Φ’ , effective cohesion, c’, and moist unit weight, γ_m

The key geotechnical recommendation is the allowable soils for the reinforced zone. While it would be ideal to obtain input from the MSE designer, the project geotechnical engineer is responsible for providing the minimum requirements for construction based on the geotechnical design issues at the site. The MSE designer always has the option to make the requirements more stringent. The geotechnical engineer, after evaluating the site soil and groundwater conditions, imposed loadings, and performance criteria, should make recommendations on reinforced soil type. The national design guidelines offer a number of options, as listed below, with author comments on each:

• AASHTO–MSE Wall Specification for Geosynthetics (free-draining)
  • Max size 3/4in., 0%-60% passing #40, and max 15% passing #200, PI > 6.
  • Effective friction angle, Φ’ > 34o

The entire reinforced soil zone is a drain. The soil is easy to work with and compact within the structure, and has the least amount of post-construction movement. This is the best choice when deformation-sensitive structures are placed above reinforced-soil mass. The maximum size for geosynthetics could be raised to 1.5in. without significant site damage.

• NCMA–MSE Wall Example Specification for Geosynthetics
  • Max size 3/4in., 0%-60% passing #40, and max 35% passing #200, PI < 20.
  • Effective friction angle, defined by testing

This allows a wider range of granular soils to be used by relaxing the fines content and plasticity, but provides insufficient drainage capacity, so requires that the reinforced-soil volume be protected by a complete drainage system (Figure 1). The drainage system should be based on the groundwater conditions at the MSE wall location. The soil is relatively easy to work with and compact within the structure, but generates some post-construction movement. The maximum size for geosynthetics could be raised to 2in. without significant site damage to the material.

• FHWA/NHI RSS Specification for Geosynthetics
  • Max size 1”, 0%-60% passing #40, and max 50% passing #200, PI < 20.
  • Effective friction angle, defined by testing

This allows the full range of granular soils to be used by relaxing the fines content to the maximum 50% and uses plasticity to minimize detrimental effect (movements). These soils generally have insufficient drainage capacity, so require that the reinforced-soil volume be protected by a complete drainage system (see Figure 1). The drainage system should be based on the groundwater conditions at the MSE wall location. The workability and compaction of these soils is subject to its natural moisture content and generates moderate post-construction movement. The maximum size for geosynthetics can be raised to 2in. without significant site damage to geosynthetic materials.

• NCMA Allowable Range for SRW Construction using Geosynthetics
  • Max size 2in., 0%-100% passing #40, and max 100% passing #200, PI < 20.
  • Effective friction angle, defined by testing

NCMA allows fine-grain soils with low plasticity (SC, ML, CL) to be used in the reinforced soil volume, even though these low-plasticity soils are subject to moderate post-construction movements that can increase with poor quality construction. These soils require the complete internal drainage system (see Figure 1) and special attention to surface drainage. The drainage system should be based on the groundwater conditions at the MSE wall location. The workability and compaction of these soils is subject to its natural moisture content, making it sometimes difficult to achieve compaction specifications. Due to post-construction movements and internal settlement, the authors’ experience is that use of fine-grain soils should be limited to non-critical walls less than 15ft tall that can tolerate lateral movements.

2.3.4) Geotechnical recommendations for global stability

The project geotechnical engineer should evaluate the entire site for areas that may be subject to global instability, and provide recommendations to the owner, site designer, and MSE designer to ensure safe performance of slopes and walls throughout the project service life. Project geotechnical engineers are the most qualified professionals to perform these analyses, since they know more about the site conditions (soil and water) than any other party. The geotechnical engineer has the expertise to investigate, test, evaluate, and most importantly, interpret the soils information to identify potential unstable areas for direct analysis. This ability to identify potentially unstable areas (by soil stratigraphy, groundwater conditions, geologic features, topographic geometry, and surcharge loadings) is much more vital to ensuring an accurate global stability analysis, than the ability to operate and manipulate a computer model.

Since project geotechnical engineers have local knowledge of typical instability behavior, as well as the costs and capabilities for typical solutions for instability, they are best equipped to determine the most cost-effective method to achieve the desired stability. The project geotechnical engineer already understands the single solution that the MSE designer can provide, increasing the length and/or strength and number of layers of geosynthetic reinforcement necessary to ensure long-term global stability. The project geotechnical engineer should analyze stability around and through the MSE structure, adjusting the reinforcement length/strength, soil properties/materials, and surface geometry to obtain a stable configuration. Evaluating global stability around MSE structures is no different than any other road, embankment, earth dam or building on a hillside or, for that matter, any other type of retaining structure. The geotechnical engineer recommendations ensure the owner’s selected level of risk (safety factor) relative to the desired performance.

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

AASHTO (2007) “LRFD Bridge Design Specifications,” 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, Tenn., 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, 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.

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, pp. 14-20.

Simac, M.R., Fitzpatrick, B.J., (2007) “Part 1 – Three challenges in using SRWs and other reinforced-soil structures,” Geosynthetics magazine, Vol. 25, No. 5, October/November 2007, pp. 26-35.