SERVICE LIMIT STATE DESIGN AND ANALYSIS OF ENGINEERED FILLS FOR BRIDGE SUPPORT

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SERVICE LIMIT STATE DESIGN AND ANALYSIS OF ENGINEERED FILLS FOR BRIDGE SUPPORT

ABSTRACT

    Engineered fills, including compacted granular fill and reinforced soil, are a cost-effective alternative to conventional bridge foundation systems. The Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS) is a fast, sustainable and cost-effective method for bridge support. The in-service performance of this innovative bridge support system is largely evaluated through the vertical and lateral deformations of the GRS abutments and the settlements of reinforced soil foundations (RSF) during their service life. While it is a common assumption that granular or engineered fills do not exhibit secondary deformation, it has been observed in inservice bridge abutment applications and large-scale laboratory tests. Evaluation of the secondary, or post-construction, deformation of engineered fills is therefore also needed. The aim of this study is to analyze and quantify the maximum deformations of GRS abutment and RSF under service loads, evaluate the stress distributions within the engineered fills of the GRS abutment and RSF, and investigate the time-dependent behavior of engineered fills for bridge support. The ultimate goal is to provide accurate yet easy-to-use analysis-based design tools that can be used in the performance assessment of GRS abutments and RSF under service loads. It is anticipated that the research performed within the scope of this dissertation will eventually help promote sustainable and efficient design practice of these structures. The research objective was achieved through development of numerical models that employed finite difference solution scheme and simulated the performance of granular backfill and reinforcement material. The backfill soil was simulated using three different constitutive models. Comparison of the simulation results with case studies showed that the behavior of GRS structures under service loads is accurately predicted by the Plastic Hardening model. The developed models were validated through comparison of model predictions with laboratory and field test data reported in the literature. A comprehensive parametric study was conducted to evaluate the effects of backfill soil’s properties (friction angle and cohesion), reinforcement characteristics (stiffness, spacing, and length), and structure geometry (abutment height and facing batter and foundation width) on the deformations of GRS abutments and RSF. The results of the parametric study were used to conduct a nonlinear regression analysis to develop equations for predicting the maximum lateral deformation and settlement of GRS abutments and maximum settlement of RSF under service loads. The accuracy of the proposed prediction equations was evaluated based on the results of experimental case studies. The developed prediction equations may contribute to better understanding and enable simple calculations in designing these structures. To investigate the time-dependent deformations of GRS abutment and RSF, a numerical model was developed. The time-dependent deformations are also known as secondary deformations and creep. To model the creep behavior of the backfill material, the Burgers creep viscoplastic model that combines the Burgers model and the Mohr-Coulomb model was used in the simulations. To model the creep behavior of geosynthetics, the model proposed by Karpurapu and Bathurst (1995) was used; this model uses a hyperbolic load-strain function to calculate the stiffness of the reinforcement. Results indicated that engineered fills can exhibit noticeable secondary deformation.       TABLE OF CONTENTS  

List of Figures ……………………………………………………………………………………………………………. VII List of Tables ……………………………………………………………………………………………………………. XIII List of Abbreviations and Symbols ……………………………………………………………………………. XIV Acknowledgements …………………………………………………………………………………………………… XVI Chapter 1. Introduction ………………………………………………………………………………………………… 1

1.1 Background in Deformation Analysis of Engineered Fills for Bridge Support……………… 1 1.2 Summary of Engineered Fills ………………………………………………………………………………… 2 1.2.1 Bridge Supports Using MSE ……………………………………………………………………………….. 3 1.2.2 Bridge Support Using GRS …………………………………………………………………………………. 5 1.2.3 Factors Affecting the Behavior of Engineered Fill for Bridge Support ……………………… 7 1.3 Research Motivation …………………………………………………………………………………………….. 8 1.4 Objective …………………………………………………………………………………………………………….. 9 1.5 Organization of the Dissertation …………………………………………………………………………….. 9

Chapter 2. Literature Review: Numerical and Constitutive Models for Compacted Fill and Reinforced Soil for Bridge Support………………………………………………………………………………. 11

2.1 Modeling of Compacted Soils ……………………………………………………………………………… 11 2.2 Modeling Reinforced Soil as a Single Composite Material………………………………………. 14 2.3 Modeling of Geosynthetic Reinforcements ……………………………………………………………. 15 2.4 Modeling of Soil-Reinforcement Interactions ………………………………………………………… 17 2.5 Numerical Modeling of Structures Supported By Engineered Fills …………………………… 19 2.6 Numerical Modeling of Long-Term Behavior of GRS Structures …………………………….. 26 2.7 Summary …………………………………………………………………………………………………………… 29 Chapter 3. Numerical Model Methodology …………………………………………………………………… 30 3.1 Model Development……………………………………………………………………………………………. 30 3.1.1 Overview of Full-Scale GRS Pier Testing Used for Model Calibration ………………….. 30 3.1.2 Numerical Model and Material Properties …………………………………………………………… 32 3.1.3 Results of Load-Deformation Behavior for GRS Piers …………………………………………. 42 3.2 Model Validations ………………………………………………………………………………………………. 44 3.2.1 Case Study of Bathurst et al. (2000) Experiments – GRS Retaining Walls ……………… 44 3.2.2 Case Study of Adams and Collin (1997) Experiment – Large-Scale Shallow Foundation on Unreinforced and Reinforced Sand ………………………………………………………………………. 51

Chapter 4. Design Tools Development to Evaluate Immediate Post-Construction Settlement and Lateral Deformation of GRS Abutments ……………………………………………………………….. 56

4.1 General Approach ………………………………………………………………………………………………. 56 4.2 Parametric Study ………………………………………………………………………………………………… 59 4.2.1 Phase 1 of Parametric Study ……………………………………………………………………………… 59 4.2.2 Phase 2 of Parametric Study ……………………………………………………………………………… 63 4.3 Prediction Equations for Estimating Maximum Lateral Deformation and Settlement ….. 66 4.3.1 Nonlinear Regression Analysis ………………………………………………………………………….. 66 4.3.2 Developing Prediction Equation ………………………………………………………………………… 68 4.4 Evaluation of GRS Abutment Prediction Equations Using Case Studies……………………. 71 4.5 Sensitivity Analysis ……………………………………………………………………………………………. 74 4.6 Distribution of Displacements and Stresses of GRS Abutments ……………………………….. 76  

Chapter 5. Design Tool Development to Evaluate Immediate Settlement of Reinforced Soil

Foundation ………………………………………………………………………………………………………………….. 99 5.1 General Approach ………………………………………………………………………………………………. 99 5.2 Parametric Study ………………………………………………………………………………………………. 100 5.3 Prediction Equations for Estimating Settlement ……………………………………………………. 106 5.3.1 Nonlinear Regression Analysis ………………………………………………………………………… 106 5.3.2 Developing Prediction Equation ………………………………………………………………………. 107 5.4 Evaluation of RSF Settlement Prediction Equation Using Case Studies …………………… 109 5.5 Sensitivity Analysis ………………………………………………………………………………………….. 110 5.6 Distribution of Stress Distribution and Settlement of RSF ……………………………………… 112

Chapter 6. Evaluating Secondary Deformations of GRS Abutment and RSF ………………. 139

6.1 Model Development for Long-Term Behaviors of GRS Abutment and RSF ……………. 139 6.1.1 Creep Behavior of Backfill Soil ……………………………………………………………………….. 140 6.1.2 Creep Behavior of Geosynthetic Reinforcement ………………………………………………… 141 6.1.3 Model Calibration ………………………………………………………………………………………….. 142 6.2. Long-Term Behavior of GRS Abutment …………………………………………………………….. 144 6.2.1 Benchmark Model ………………………………………………………………………………………….. 145 6.2.2 Effect of Reinforcement Spacing ……………………………………………………………………… 151 6.2.3 Effect of Reinforcement Length ………………………………………………………………………. 152 6.2.4 Effect of Reinforcement Stiffness …………………………………………………………………….. 154 6.2.5 Effect of Abutment Height………………………………………………………………………………. 156 6.2.6 Effect of Facing Batter ……………………………………………………………………………………. 159 6.3. Long-Term Behavior of RSF …………………………………………………………………………….. 162 6.3.1 Benchmark Model ………………………………………………………………………………………….. 162 6.3.2 Effect of Reinforcement Stiffness …………………………………………………………………….. 166 6.3.3 Effect of Number of Reinforcement Layers ………………………………………………………. 168

Chapter 7. Summary and Conclusions ……………………………………………………………………….. 172

7.1 Summary …………………………………………………………………………………………………………. 172 7.2 Conclusions ……………………………………………………………………………………………………… 173 7.3 Suggestions for Future Research Needs ………………………………………………………………. 177

References …………………………………………………………………………………………………………………. 179

Chapter 1. Introduction

   

1.1 Background in Deformation Analysis of Engineered Fills for Bridge Support

The use of engineered fills, with and without layered reinforced soil systems, is an economical solution to reduce deformations and improve bearing resistance of shallow foundations for bridge support. Notable studies of spread footings on engineered fills published by the Federal Highway Administration (FHWA) concluded that this technique was a suitable alternative to deep foundations (e.g., DiMillio 1982; Gifford et al. 1987). Engineered fills can be used to support bridge abutments and piers with various configurations. For bridge abutments, the engineered fills can be compacted granular fills or compacted granular fills with metallic or geosynthetic reinforcements, while for bridge piers, the engineered fills can be compacted granular fills or compacted granular fills with geosynthetic reinforcement. Bridge support using reinforced engineered fills contribute to better compatibility of deformation between the components of bridge systems, thus minimizing the effects of differential settlements and the occurrence of undesirable “bumps” between the bridge deck and the approach embankment transitions (Zevgolis and Bourdeau 2007). Abu-Hejleh et al. (2014) noted that state transportation departments have safely and economically constructed highway bridges supported on spread footings bearing on competent and improved natural soils as well as engineered granular and mechanically stabilized earth (MSE) fills. Despite these advantages, many transportation agencies do not consider shallow foundation alternatives, even when appropriate, for a variety of reasons, including concerns related to meeting serviceability requirements (e.g., vertical and lateral deformations). Due to the large size of spread footings for highway bridges, soil bearing failure is not likely (Samtani and Nowatzki 2006a). Therefore, the performance of spread footings in highway bridge design is evaluated primarily on the basis of vertical displacement (i.e., settlement and how differential settlements affect angular distortion) (Samtani et al. 2010). The Service Limit State (SLS) for shallow foundations often controls the design of bridge foundations; however, little guidance on the SLS has been provided for engineered fills (AASHTO 2014). SLS relates to stress, deformation, and cracking (AASHTO 2008). Existing limit states and tolerances of bridge components that are set forth by various agencies in the United States and internationally were presented by the Strategic Highway Research Program 2 (SHRP2) report, Bridges for Service Life beyond 100 Years: Service Limit State Design (Modjeski and Masters 2015).

1.2 Summary of Engineered Fills

FHWA defines engineered granular fill as high-quality granular soil selected and constructed to meet certain material and construction specifications (also called “compacted structural fill” and “compacted granular soil”) (Abu-Hejleh et al. 2014). Engineered fill may be reinforced with geosynthetics or metal strips. The high quality refers to gradation, soundness, compaction level, durability, and compatibility. FHWA provides gradation requirements for engineered granular fills (Kimmerling 2002), and FHWA’s Soils and Foundations Reference Manual: Volume I (Samtani et al. 2010) provides general considerations in selecting structural backfills. A number of State transportation departments, including the Washington State Department of Transportation (WSDOT), the New Mexico Department of Transportation, and the Minnesota Department of Transportation, have successfully utilized compacted engineered granular fills (Abu-Hejleh et al. 2014). For example, based on a survey of 148 bridges in Washington, FHWA concluded that spread footings on engineered fill can provide a satisfactory alternative to deep foundations, especially if high-quality fill materials are constructed over competent foundation soil (DiMillio 1982). National Cooperative Highway Research Program Report No. 651 reported higher resistance factors for the compacted granular fill than natural granular soil because of better control for compacted fill (Paikowsky et al. 2010). Nevertheless, concerns exist regarding the use of spread footing bearing on engineered granular and MSE fills. A number of State transportation departments have allowed and constructed spread footings on natural soils but not on engineered granular and MSE fills due to the concerns related to the quality and uniformity of compacted fill materials as well as costly design and construction of bridge footings on MSE walls (Abu-Hejleh et al. 2014). The FHWA report, Soils and Foundations Reference Manual: Volume II, recommends that compacted structural fills used for supporting spread footings should be a select and specified material that includes sand- and gravel-sized particles (Samtani and Nowatzki 2006b). Furthermore, the fill should be compacted to a minimum relative compaction of 95 percent based on the modified Proctor compaction energy, and structural fill should extend for the entire embankment below the footing.

1.2.1 Bridge Supports Using MSE

Since the first MSE abutment was constructed in the United States in 1974, MSE technology has been used in bridge-supporting structures such as bridge abutments, and both metallic and geosynthetic reinforcements have been used (Anderson and Brabant 2010). MSE abutments are MSE retaining walls subjected to much higher area loads that are located close to the wall face. Using MSE structures as direct support for bridge abutments can be a significant simplification in the design and construction of current bridge abutment systems and may lead to faster construction of highway bridge infrastructure. When a bridge beam is supported on a spread footing that bears directly on top of an MSE structure, this configuration is known as true MSE abutment, as shown in figure 1. To prevent overstressing the soil from the excess load exerted on a true MSE abutment, the beam seat is sized so that the centerline of bearing is at least 3.05 ft (1 m) behind the MSE wall face, and the service bearing pressure on the reinforced soil is no more than 4 kip/ft2 (192 kPa) (Anderson and Brabant 2010). Anderson and Brabant (2010) also reported that there are approximately 600 MSE abutments (300 bridges) built annually in the United States, of which 25 percent are true MSE abutments. MSE abutments may result in construction cost savings where deep foundations are not needed. Additionally, the use of true MSE abutments can result in significant cost savings (Anderson and Brabant 2010). True bridge abutments also have significant advantages over conventional abutments. The proverbial bump at the end of the bridge is alleviated because the footing settles along with the MSE wall in contrast to a deep foundation that does not settle at the same rate. Additionally, approach slabs are not necessary because of the elimination of conditions that would lead to the bump at the end of the bridge, and the elimination of approach slabs results in significant cost savings (Samtani et al. 2010). While there are proven advantages of MSE abutments, there are some limits for their applicability, as with any technology. A study by Purdue University and the Indiana Department of Transportation revealed that MSE structures on shallow foundations should not be used as direct bridge abutments when soft soil layers, such as normally consolidated clays, are present near the surface where significant deformation and differential settlement are expected (Zevgolis and Bourdeau 2007). In such conditions, a design configuration including piles should be used. In more competent foundation profiles, MSE walls can be used for direct support of bridge abutments.     Figure 1-1. True MSE abutment types (after Anderson and Brabant 2010).    

1.2.2 Bridge Support using GRS

Geosynthetically reinforced soil (GRS) technology consists of closely-spaced layers of geosynthetic reinforcement and compacted granular fill material. GRS has been used for a variety of earthwork applications since the U.S. Forest Service first used it to build walls for roads in steep mountain terrain in the 1970s. The spacing of GRS reinforcement should not exceed 12 in. (300 mm) and is typically 8 in. (200 mm) (Adams et al., 2011). As shown in Figure 1-2, geosynthetic reinforced soil – integrated bridge system (GRS-IBS) typically includes a reinforced soil foundation (RSF), a GRS abutment, and a GRS integrated approach to transition to the superstructure. The RSF is composed of granular fill material that is compacted and encapsulated with a geotextile fabric. The application of GRS has several advantages: the system is easy to design and economically construct; it can be built in variable weather conditions with readily available labor, materials, and equipment; and it can be easily modified in the field (Adams et al. 2011).   1 inch = 25.4 mm Figure 1-2. Typical cross-section of GRS-IBS (Adams et al. 2011)     Figure 1-3. Annotations of parameters of a shallow foundation on reinforced soil   where B = Width of foundation; b = Length of reinforcement layers below foundation; N = Number of reinforcement layers; u = Embedment depth of top geogrid layer; h = Spacing of reinforcing layers; d = Depth of bearing bed reinforcement; Df = Depth of embedment of foundation.

1.2.3 Factors Affecting the Behavior of Engineered Fill for Bridge Support

Various factors may affect the deformations of engineered fill for bridge support. For a reinforced soil abutment, these factors include:

  • Engineered soil’s characteristics: unit weight, strength parameters (frictional and cohesion), bulk modulus, and level of compaction
  • Abutment geometry: height, length, batter (i.e., inclination of facing)
  • Reinforcement stiffness
  • Reinforcement geometry: spacing, horizontal length (extent)
  • Service load
  • Temperature

For an RSF, these factors include:

  • Engineered soil’s characteristics: unit weight, strength parameters (friction and cohesion), bulk modulus, and level of compaction
  • RSF shape and dimensions
  • Reinforcement stiffness
  • Reinforcement geometry: spacing, total depth
  • Service load on RSF
  • Native (in-situ) soil type, unit weight, and strength parameters beneath RSF

 

1.3 Research Motivation

The GRS bridge abutment system is more sustainable than the pile supported abutment system for the bridge support. The GRS system is less expensive to construct and results in lower CO2 emissions and therefore less potential impact on climate change than the alternative pile supported abutment system. Accordingly, GRS structures have gained increasing popularity in the world. Basic design guidelines for GRS abutments are available that outline recommended soil type, gradation and level of compaction of the structural and backfill soil, along with the vertical spacing, strength, stiffness, and length of reinforcement layers (Adams et al. 2011b; Nicks et al. 2013). Although these design guidelines are reasonably well established, the prediction of GRS walls and abutments deformations under applied service loads requires further investigation. A realistic estimation for deformations of GRS abutments is important because differential movements of bridge substructures can negatively affect the ride quality, deck drainage, and safety of the traveling public as well as the structural integrity and aesthetics of the bridge which can lead to costly maintenance and repair measures (Modjeski and Masters 2015). Regardless of settlement uniformity, ensuring adequate clearance for bridge elevations is dependent on the total movement. Based on these reasons, the service limit state (SLS) often controls the design of shallow bridge foundations (AASHTO 2014; FHWA 2006). The SLS ensures the durability and serviceability of a bridge and its components under typical everyday loads, termed “service loads” (Mertz 2012). In SLS design, failure is often defined as exceeding tolerable displacements. Therefore, there is a need for a model which can accurately predict the settlement and lateral deformation of GRS abutment and the settlement of RSF. While it is a common assumption that granular or engineered fills do not exhibit secondary deformation, large-scale field tests showed that in in-service bridge abutment applications and piers experience long-term deformations. Evaluation the secondary, or postconstruction, deformation of engineered fills is therefore also needed.  

1.4 Objective

The key objective of this study is to analyze and quantify the maximum deformations of GRS abutment and RSF under service loads, evaluate the stress distributions within the engineered fills of the GRS abutment and RSF, and investigate the time-dependent behavior of engineered fills for bridge support. The ultimate goal is to provide the precise yet easy-to-use analysis-based design tools that can be used in performance assessment of GRS abutments and RSF under service loads. It is anticipated that research performed within the scope of this dissertation will eventually help in promoting sustainable and efficient design practice of these structures.  

1.5 Organization of the dissertation

This dissertation consists of seven chapters. Following the motivation and objective presented in this chapter, Chapter 2 presents the literature review of numerical and constitutive models for compacted fill and reinforced soil for bridge support. Chapter 3 presents the numerical model methodology and model calibration and validation using case studies to evaluate the performance of the prediction models for GRS piers, abutment and RSF. Chapter 4 presents the development of prediction tools for immediate lateral and horizontal deformations of bridge abutment with reinforced engineered soil at the end of construction and with different service loads. Chapter 5 presents the development of prediction tools for immediate settlement of RSF at the end of construction and with different service loads. Chapter 6 presents the development of prediction tools for secondary deformations of GRS abutment and secondary settlement of RSF due to creep. Chapter 7 of this dissertation presents the summary and conclusions derived from this study; this chapter also provides some recommendations for future research. SERVICE LIMIT STATE DESIGN AND ANALYSIS OF ENGINEERED FILLS FOR BRIDGE SUPPORT

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