PERFORMANCE AND COST EFFECTIVENESS OF MODULAR BLOCK RETAINING WALLS

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PERFORMANCE AND COST EFFECTIVENESS OF MODULAR BLOCK RETAINING WALLS

ABSTRACT

Modular block walls (MBWs), also known as segmental retaining walls (SRWs), continue to be a growing segment of the public retaining wall market.  While MBWs have long been popular in the private sector, estimates by the Federal Highway Administration show that

MBWs have had a distinct growth over the last ten years in the public sector. Still, many state Departments of Transportation do not permit the use of MBWs based on expectations of poor performance.  This thesis presents information obtained from available literature including published papers and reports in regards to cost and performance of MBWs.  In addition, a telephone survey was conducted of thirty-two departments of transportation to determine which states utilize modular block walls, how cost effective modular block walls have been, and how they have performed in comparison to more traditional wall types.  The effectiveness of new freeze-thaw requirements was evaluated.  States not currently using modular block walls revealed their current concerns that discourage these DOTs from utilizing MBWs.  Based on the completed analysis and evaluation, advantages and disadvantages of MBWs are presented, and recommendations for successful implementation of MBWs are offered.

 

 

 

 

TABLE OF CONTENTS

 

LIST OF FIGURES ………………………………………………………………………………………………….. vii

LIST OF TABLES …………………………………………………………………………………………………… ix

ACKNOWLEDGEMENTS ………………………………………………………………………………………. x

Chapter 1  Introduction …………………………………………………………………………………………….. 1

Background and Current Use of Modular Block Walls ………………………………………….. 2

Retained & Reinforced Earth Options for State Agencies ………………………………………. 6

Explanation of Survey ……………………………………………………………………………………….. 7

Survey Participation …………………………………………………………………………………… 9

States Using Modular Block Walls ………………………………………………………………. 10

Chapter 2  Design, Construction, and Composition of Modular Block Walls …………………… 12

Modular Block Wall Design ……………………………………………………………………………….. 12

Modular Block Wall General Construction Sequence ……………………………………………. 14Modular Block Units …………………………………………………………………………………………. 16

Geosynthetic Reinforcement ………………………………………………………………………………. 18

Backfill Material ……………………………………………………………………………………………….. 22

Chapter 3  Performance of Modular Block Walls …………………………………………………………. 26

Potential Performance Advantages ………………………………………………………………………. 26

Geosynthetic Reinforcement ……………………………………………………………………….. 27Alignment of Facing Units ………………………………………………………………………….. 28

Aesthetics and Adaptability …………………………………………………………………………. 28

Potential Performance Disadvantages ………………………………………………………………….. 29

Aesthetic Issues …………………………………………………………………………………………. 30

Mechanical Issues ………………………………………………………………………………………. 30

Other Issues ………………………………………………………………………………………………. 33

Performance History ………………………………………………………………………………………….. 34Minnesota Department of Transportation, 2001 Wall Review ………………………….. 34

Wisconsin Department of Transportation, 2000 Wall Review ………………………….. 36

Geosynthetic Reinforced Segmental Retaining Walls, Koerner and Soong, 2001 .. 37

A Database and Analysis of Geosynthetic Reinforced Wall Failures ………………… 38

Performance Related Survey Results …………………………………………………………………… 39

Performance Comparison of Modular Block Walls Versus Large Precast Panel

MSE Walls and Cast-in-place Walls ……………………………………………………… 39

Maintenance Comparison of Modular Block Walls Versus Precast Panel MSE

Walls and Standard Cast-in-place Walls ………………………………………………… 42

Freeze-Thaw Durability ………………………………………………………………………………. 44Limitations of Modular Block Wall Use ……………………………………………………….. 47

Chapter 4  Cost Effectiveness of Modular Block Walls…………………………………………………. 48

Potential Modular Block Wall Cost Advantages ……………………………………………………. 48

Speed of Construction ………………………………………………………………………………… 48

Labor Force ………………………………………………………………………………………………. 50

Modular Block Wall Cost History ……………………………………………………………………….. 51Federal Highway Administrations Relative Cost Data …………………………………….. 51

Geosynthetic Reinforced Segmental Retaining Walls, Koerner and Soong, 2001 .. 51

Cost Effectiveness Survey Results ………………………………………………………………………. 54

Cost Comparison of Modular Block Walls Versus Large Precast Panel MSE

Walls and Cast-in-place Walls ………………………………………………………………. 54

Maintenance Cost Comparison of Modular Block Walls Versus Precast Panel

MSE Walls and Standard Cast-in-place Walls ………………………………………… 58

Other Cost Survey Results …………………………………………………………………………… 59

Chapter 5  Summary, Recommendations and Conclusions …………………………………………….. 62

Summary …………………………………………………………………………………………………………. 62Conclusions ……………………………………………………………………………………………………… 64

Recommendations …………………………………………………………………………………………….. 65

Recommendations for Further Research ……………………………………………………….. 65

Recommendations for Successful Implementation………………………………………….. 66

References ………………………………………………………………………………………………………………. 67

Appendix A …………………………………………………………………………………………………………….. 69

Survey for States Using Modular Block Walls ……………………………………………………… 69

Survey for States Not Using Modular Block Walls ……………………………………………….. 77

Appendix B …………………………………………………………………………………………………………….. 82

Survey Result Comments for States Using Modular Block Walls ……………………………. 82

Cost Survey Comments ………………………………………………………………………………. 82Performance Survey Comments …………………………………………………………………… 95

General Survey Comments ………………………………………………………………………….. 100

Additional Survey Comments ……………………………………………………………………… 104

Survey Result Comments for States Not Using Modular Block Walls ……………………… 107

General Survey Comments ………………………………………………………………………….. 107

Additional Survey Comments ……………………………………………………………………… 110

Appendix C …………………………………………………………………………………………………………….. 111

Survey Results, Raw Data ………………………………………………………………………………….. 111

States Using MBWs, Cost Data ……………………………………………………………………. 111

States Using MBWs, Performance and General Data ……………………………………… 113

States Using MBWs, Performance and General Data for States Above Latitude

40 ……………………………………………………………………………………………………… 114

States Using MBWs, Performance and General Data for States Below Latitude

40 ……………………………………………………………………………………………………… 115

States Not Using MBWs, Cost, Performance and General Data ……………………….. 116

Chapter 1

 

Introduction

There are a variety of reinforced earth and retained earth structures available for use to departments of transportation.  Historically, cast-in-place concrete walls were employed, as they typically lived up to their 75-100 year design life; however, oftentimes they left much to be desired when it came to price.  Mechanically stabilized earth walls were later invented and rely on the mechanics of a stabilized soil mass rather than a large structure to retain soil loads.  One mechanically stabilized earth option, which has gained steam in the last thirty years particularly in the private sector, is the modular block wall (also known as the segmental retaining wall).  This wall type utilizes smaller concrete masonry units with geosynthetic reinforcement within a well compacted backfill.  Despite a period of relative growth, a stigma still exists in the public sector concerning the cost effectiveness and performance of modular block retaining walls.  In fact, many states still refuse to use this wall type for fear that they underperform.  A historical database exclusively focusing on modular block walls in regards to performance and cost does not currently exist to quell the states’ concerns.  Much of the stigma continues to focus on the freezethaw durability of the block units themselves.  Research funded by the Federal Highway Administration (FHWA) has been completed on freeze-thaw durability, and new specifications to manage freeze-thaw issues have been employed by a small number of states.  Again, the effectiveness of these new freeze-thaw specifications has not yet been explored.

This study explores cost and performance data from previous papers and reports while incorporating the results of a new survey.  The new survey, conducted via the telephone and answered by senior engineers at state DOTs, clearly gives insight into the cost effectiveness, performance, and maintenance requirements of MBWs in the public sector.  States not currently using modular block walls revealed their continuing performance concerns that discourage their states from utilizing MBWs.  Additionally, the success of new freeze-thaw specifications is explored.  Incorporating all survey comments and previous research, a summary of all possible

MBW advantages and disadvantages are presented along with recommendations for successful MBW implementation.

Background and Current Use of Modular Block Walls

Retaining walls are a vital component of highway design and construction.  These earth retaining structures can be used for a variety of purposes, from retaining wall to use as headwalls, wingwalls, and bridge abutments.  Historically, retaining wall structures were almost always rigid structures made of reinforced concrete.  While this construction was easy and performed well over time, it proved to be less desirable when it came to cost effectiveness.  Concerning performance, reinforced cast-in-place walls had difficulty dealing with differential settlement when placed in shallow foundations or when interacting with inadequate subsoil conditions (1).           The industry needed a more cost effective solution, and in the early 1960s, a French architect and engineer named Henri Vidal pioneered a new system of soil reinforcement.  Henri Vidal, referencing the knowledge that soil inclusions have been used for stabilization since prehistoric times, developed a company named Reinforced Earth® which used steel strip reinforcements to aid in soil retention.  Vidal’s company is still thriving today in the United States with a variety of soil stabilization and earth retaining structures.  The first wall to use this technology was built in 1972 on California State Highway 39.  Generally, this system utilizes large precast concrete panels (typically 20-25ft2) in conjunction with metallic reinforcement within a well compacted backfill.  By placing tensile reinforcing materials in the soil, the internal shear resistance of the soil can be significantly improved, leaving the facing unit system in essence self-supporting.  Since its inception, many other similar systems have been developed and used.  Overall, mechanically stabilized earth (MSE) walls have been a heavily used alternative to classic cast-in-place retaining walls.   Covering more than 750 million ft2 of wall facing, over 23,000 reinforced earth structures have been constructed worldwide.  In the United States, over 8,000 walls have been built since 1972 (1).

Currently, mechanically stabilized earth walls are documented to have been used in every state in America.  It is estimated that more than 9,000,000 ft2 of retaining walls with precast panel facings are constructed each year in the United States.  The majority of in-place and planned MSE walls use large precast panel facings with metallic reinforcement (2).  In fact, the longest mechanically stabilized earth wall in the United States and second largest in the world was just completed in Lewistown Pennsylvania on Route 22/322.

 

Figure 1-1: Precast concrete paneled, mechanically stabilized earth wall on Route 22/322.

This project, seen in Figure 1-1, cost around $142 million, has a 2.5 mile long wall with a maximum height of 30ft, and utilized 41,000 metallic strips for reinforcement.  Mechanically stabilized earth walls are frequently used at heights ranging greater than 40ft and can be feasible but are not often seen at heights of up to 100ft.  Very few of these walls, designed for a lifespan of 75-100 years, fail completely.  Yet, there is certainly a case history of these walls underperforming for their intent, which will be explored in later sections (3).

 

After the development of the first mechanically stabilized earth wall, the concept of using geosynthetics in MSE walls instead of metallic reinforcement soon emerged.  Geosynthetic is a general term that covers all flexible reinforcement options including geotextiles, geogrids and any other polymeric material generally consisting of polypropylene, polyethelyne, or polyester (4).  The first geosynthetic reinforced wall was built in France in 1971, and three years later one was built in the United States.  The use of geosynthetic reinforcement in large precast panel MSE walls has become more popular; however, the great majority of walls still use metallic reinforcement.

Within the last 20 to 30 years, dry cast modular block walls using geogrid reinforcement have gained acceptance in the private sector for their potential cost, construction, and performance advantages.  Acceptance in the public sector has been slow, but it has been reported that around 3,000,000ft2 of modular block walls have been built yearly in the United States.  This value is approximately one-third the area of large precast panel walls constructed in the United States each year.  According to the Federal Highway Administration (FHWA), as of 2010 approximately 100 projects per year used modular block walls in transportation applications (2).  This is a distinct growth from 2001, where it was reported that 2,000,000 ft2 of modular block walls were constructed yearly with about 50 projects per year (1).  A picture of a typical modular block wall can be seen below in Figure 1-2.  Even though modular block walls are used substantially in the United States, there is concern by DOTs that these walls also under-perform.

 

Figure 1-2: Typical dry cast modular block retaining wall.

While modular block wall construction has been approved for use by the Federal

Highway Administration, there is no comprehensive list of states utilizing modular block walls.   By researching and compiling several existing sources, a list of states utilizing MBW was created, as seen in Figure 1-3 below.  As the color-coded map conveys, there are inconclusive results for the majority of state DOTs based on currently published research.  The surveys completed by state DOTs for this study yielded a more accurate and comprehensive overview of MBW usage.

 

Figure 1-3: Map of known Departments of Transportation using modular block walls.

 

In the private sector, modular block walls are used regularly for a variety of purposes.  Modular block walls are most often used for landscaping purposes, and around parking lots, parks, and residential areas.  Also, a modular block wall’s low cost and aesthetic appeal often make it the wall of choice around commercial centers.

Retained & Reinforced Earth Options for State Agencies

This study focuses on modular block retaining walls with geosynthetic reinforcement as compared to cast-in-place walls and precast panel walls, though many other reinforced and retained-earth options exist.  In the past, traditional retaining structures such as gravity walls, cantilever walls, pile walls and anchored (tieback) walls have been used across the United States.  Over time, many alternatives such as soil nailing, gabion walls, T-Walls®, double walls, and a variety of mechanically stabilized earth walls have been used by United States agencies.  Table 1-

1 summarizes different retained and reinforced earth structures that are typically used by state DOTs.  DOTs are not limited to these wall choices; these are only the wall types most often used.

It is important to understand that each wall type has distinct advantages and disadvantages.  Overall, economics and site conditions are the two factors that most influence which wall type is chosen.

 

 

 

 

 

Table 1-1: Retained and reinforced earth structure types and description.

Structure Type Structure Description
   

Retained Earth Structures

 

Gravity Wall Typically concrete, earth retaining structure that uses its own self weight to resist soil pressures.
Cantilevered Wall Typically concrete, inverted T-shaped earth retaining structure that uses its own weight and configuration to resist soil pressures.
Pile Wall Steel, vinyl, or wood driven into the ground to resist soil pressures.  Often need tiebacks.
Anchored Wall Typically used with other systems to tie back/anchor a wall.
Soil Nailing A top down soil retaining system used on slopes, embankments, excavations, or retaining walls in which reinforcing bars are drilled, tensioned, and grouted to create a composite soil mass.
Gabion Wall Retained earth structure made of rectangular shaped wire mesh containers filled with stone or riprap stacked upon each other.

These generally act as a gravity wall.

T-Wall® Reinforced concrete module system that acts as a gravity wall.

The system stability is a function of the weight of units and their long concrete stems within a select backfill material.

Redi-Rock Wall® Dry placed massive precast concrete blocks stacked to create a gravity wall structure.  Most often do not need any type of geosynthetic reinforcement.
   

Reinforced Earth Structures

 

Segmental Precast Concrete Panel Wall System utilizes large precast concrete panels (typically 20-

25ft2) in conjunction with metallic or geosynthetic reinforcement within a well compacted backfill.

Modular Block Retaining Wall System utilizes small dry cast blocks in conjunction with geosynthetic reinforcement within a well compacted backfill.

 

Explanation of Survey

A review of published papers and reports indicated that a current compilation of states using modular block walls does not exist.  Two surveys were created to gather information on states using modular block walls and states not using modular block walls.  A list of senior geotechnical engineers at forty-one departments of transportation were gathered from the Federal Highway Administration.  Most surveys were conducted via telephone, while a few were received through email.  A script was followed to ensure consistency in the data collection.  The script, along with a copy of an unanswered survey of both types, can be seen in Appendix A.             There were two main goals of the survey.  For states not using modular block walls, the goal was to determine why the walls are not being used.  For states using modular block walls, the goal was to determine the modular block walls effectiveness in cost and performance when compared to its major competitors at the same wall heights.  From statistics discussed earlier, the most common wall types used in lieu of modular block walls are cast-in-place walls or precast panel MSE walls.  Therefore, most of the statements used in the survey compare the use of modular block walls to cast-in-place and precast panel MSE walls.  Important comments are summarized in the following chapters, with all complete comments seen in Appendix B.

When a state indicated that they currently use modular block walls, a survey with a variety of statements regarding cost and performance was conducted.  In addition to comparisons of cost and performance, general questions were asked regarding the estimation of how many modular block walls have been constructed, the design specifications were used, and the typical restrictions for use of modular block walls.

When a state indicated that they currently do not use modular block walls, a separate survey was administered.  The survey’s goal was to explore whether the state’s reason for not using modular block walls was cost related, performance related, or other.  Additionally, each state’s current satisfaction with cast-in-place walls and precast panel MSE walls were explored.

Both surveys were comprised of a series of affirmative statements in regards to modular block retaining walls.  Each statement, unless otherwise noted, could be answered on a one to ten scale.  On this scale, an answer of one indicated an opinion of “strongly disagree” while an answer of ten indicated an opinion of “strongly agree”.  After each answer, the survey taker had

the opportunity to add any additional comments pertaining to his/her answer.  In addition to the scaled statements, there were a variety of open ended questions that were not scaled.  All statements and questions can be seen in their full form in Appendix A and Appendix B.

Survey Participation

Of the forty-one states contacted by phone or email, thirty-two completed the survey.

Figure 1-4 below summarizes the states that participated.

 

 

Figure 1-4: Departments of Transportation that participated in survey.

 

States Using Modular Block Walls

As previously discussed, information regarding what states use modular block retaining walls is currently not available.  This information is very important when trying to assess why modular block walls are not being utilized and when determining if geography plays a role in this decision.  Of the thirty two states that completed the survey, twenty four indicated that they use modular block retaining walls, while eight indicated that they do not.  This information is summarized in Figure 1-5 below.  Florida is differentiated, as they are currently in the process of constructing their first modular block demonstration wall.

 

Figure 1-5: State responses for modular block wall use.

 

Participating state DOT representatives were given the opportunity to keep their names or their states’ names anonymous in the survey results.  Only a few state DOT representatives asked to remain anonymous, and these representatives were from states that do not use modular block

retaining walls.  Therefore, to maintain anonymity, the comments of all states not using modular block walls will remain unidentified.

PERFORMANCE AND COST EFFECTIVENESS OF MODULAR BLOCK RETAINING WALLS

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