IMMEDIATE LOAD-SETTLEMENT RESPONSE OF STRIP FOOTINGS BEARING ON GEOGRID-REINFORCED CLAY

ABSTRACT

Footings on reinforced soil have long been considered as an effective solution to enhance foundation bearing capacity and reduce settlement. Notwithstanding the fact that footings bearing on reinforced clays can be an economic alternative to other expensive foundation solutions, studies focusing on such foundations are rather limited in number. In addition to consolidation and long-term creep settlement, which are most frequently quantified for footings on clay, immediate settlement (i.e., settlement at a time shortly after load application) of foundations bearing on reinforced clay should be

studied adequately to establish serviceability criteria that may govern the design.

The present research quantifies immediate settlement of strip footings bearing on geogrid-reinforced clay. Finite element analyses of a strip footing bearing on unreinforced and reinforced clay are performed to evaluate and quantify the effects of several material (e.g., soil and reinforcement properties) and geometric (i.e., those pertaining to reinforcement layout) parameters on load-settlement response of the footing. A nonlinear elastic, perfectly plastic constitutive model obeying a nonassociated flow rule is used to represent mechanical behavior of clay. geogrid

reinforcement is modeled as a linear elastic material. Soil-reinforcement interaction is modeled using cohesive interface elements above and below the reinforcement layers. The clay constitutive model is also used as the material model for the cohesive interface element and a contact interaction model is utilized to define the connection between the

cohesive interface and reinforcement layer.

Results show that settlement influence factor, which can be used for calculation

of immediate settlement under varying net stress applied at the footing base, reduces with increase in number of reinforcement layers and with increase in undrained shear strength of clay. An increase in plasticity index of clay causes an increase in immediate settlement and the same can be significant for foundations bearing on clays with high plasticity. Optimal values of different reinforcement arrangement factors obtained from this research are in agreement with those reported in literature.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TABLE OF CONTENTS

LIST OF FIGURES…………………………………………………………………………………………………… vi

LIST OF TABLES …………………………………………………………………………………………………… vii

ACKNOWLEDGEMENTS …………………………………………………………………………………….. viii

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

1.1. Background ……………………………………………………………………………………………. 1 1.2. Motivation and Research Objectives …………………………………………………………. 4

1.3. Thesis Outline ………………………………………………………………………………………… 5

Chapter 2 A Review of Existing Literature ……………………………………………………………………. 6

2.1.      Reinforcing Mechanisms and Bearing Capacity Equations for RSF ………………. 6

2.2.      Model-scale Experiments ………………………………………………………………………. 11

2.3.     Numerical Studies …………………………………………………………………………………. 17

Chapter 3 Finite Element Modeling ……………………………………………………………………………. 23

3.1. Clay Constitutive Model ………………………………………………………………………… 23 3.2. Geometric Model for the Soil Domain …………………………………………………….. 25 3.3. Modeling of the Reinforcement Layers ……………………………………………………. 28 3.4. Modeling of the Clay-Reinforcement Interface …………………………………………. 29 3.5. Convergence Study ……………………………………………………………………………….. 30

3.6.     Validation for the FE Model …………………………………………………………………… 32

Chapter 4 Analyses and Results …………………………………………………………………………………. 37

4.1.      Influence Factor I_q ………………………………………………………………………………… 37

4.2.        Effect of Number of Reinforcement Layers N on Influence Factor I_q …………… 39 4.3.            Effect of Undrained Shear Strength s_u on Settlement Influence Factor I_q ……… 45 4.4. Effect of Plasticity Index PI on Settlement Influence Factor I_q ……………………. 46 4.5.         Optimal Depth for Placement of the Top Reinforcement Layer …………………… 47 4.6.        Optimal Reinforcement Width L_R for a Single Layer of Reinforcement ……….. 48 4.7.        Optimal Vertical Spacing between Two Reinforcement Layers …………………… 50

4.8. Effective Total Reinforcement Depth d_e …………………………………………………… 51 4.9. Influence Zone Z_R below the Footing Base ………………………………………………. 52 4.10. Optimal Number of Reinforcement/Interspacing for Multi-layer …………………. 55 4.11. Influence of Reinforcement Bending Stiffness ………………………………………….. 56

4.12.   Effect of Clay Layer Thickness on Settlement Influence Factor I_q ……………….. 57

Chapter 5 Discussion and Conclusions ……………………………………………………………………….. 59

5.1. Comparison with Results Reported in Previous Numerical Studies on Footings on Reinforced Clay …………………………………………………………………………………………… 59 5.2. Comparison with Previous Experimental Studies ………………………………………. 61

5.3.     Conclusions ………………………………………………………………………………………….. 62

References ………………………………………………………………………………………………………………. 64

Chapter 1 Introduction

1.1.     Background

Geosynthetics are polymeric products, commonly available in the form of geogrids, geotextiles, geomembranes and geocells, which are frequently used in civil engineering practice. The polymeric nature of the material makes different geosynthetics products durable under different ground and environmental conditions. Common applications of geosynthetics in the field of geotechnical engineering include improving strength and stiffness of subsurface soil beneath shallow foundations and pavements, providing stability to earth retaining structures and slopes, ensuring dam safety, to name a few. Early applications of geosynthetics in 1960s were about their use as filters materials in

the United States and as soil reinforcement in Europe.

A geogrid is one of the most common geosynthetic products that are often used for improving mechanical performance of subsurface soil under external loadings. Geogrids are widely used as reinforcement layers in mechanically stabilized earth (MSE) and geosynthetic reinforced soil (GRS) walls, as a measure of slope

stabilization and as reinforcement in subsurface soil below pavements and footings. Soils are weak in tension; good tensile capacity of geogrids allows the reinforcement layers to take over a significant part of tensile stresses generated within a soil mass due to the action of external loading. Thus, geogrids act as “reinforcing” element and enhance load-deformation behavior of reinforced soil mass. Geogrids are commonly made of polymers; nowadays different a variety of geogrids are made of polypropylene or high density polypropylene (HDPP).

Based on manufacturing process, geogrids can mainly be categorized in three distinct types. The first type is commonly known as homogenous or punched geogrid. Bundles of polyethylene-coated polyester fibers contributing to the flexibility, are used

as reinforcing material in the second type. The third type is made by bonding

polypropylene rods together in a grid-like pattern using laser or ultrasonic technology. Geogrids can also be classified into three types according to grid structure: uniaxial, biaxial and triaxial. As the name suggests, while uniaxial geogrids are able to sustain mainly uniaxial stress (along the direction of longer gird dimension), biaxial and triaxial geogrids are capable of sustaining loads from two and three directions,

respectively.

Shallow foundations are often used in practice to transfer loads coming from the structure to the underlying ground at relatively shallow depth (usually less than five times the width of the foundation). Shallow foundations range from small isolated foundations, which support load from an individual column, to large foundation elements that support several columns, or even all the loads from structure. Shallow foundations are easy to build, requiring little to no specialized equipment. For shallow foundations, foundation-to-soil load transfer takes place predominantly through the base of the foundation element, and only a small fraction of the total load can be transferred through the sides of an embedded shallow foundation element (in most cases these are reinforced concrete blocks); however, such contribution is often neglected in design.

Shallow foundations are not suitable for subsurface in which weak load bearing strata exist at shallow depths. Highly compressible soils and uncontrolled fills are not ideal conditions to transfer structural load through shallow foundations. Under these conditions, adding reinforcements to the soil layer beneath the footing base is regarded

as a good choice to enhance foundation bearing capacity and reduce foundation

settlement. This type of foundation and reinforced-soil system is called reinforced soil foundation (RSF). Reinforced soil foundations could be an economical alternative to conventional shallow foundations with large footing dimensions which in turn increase foundation settlement due to an increase in the depth of influence zone below the foundation, or replacement of weak soil layers with competent materials. Geogrids are widely used to strengthen soil layers below footings because their performance is

generally better than geotextiles and geomembranes.

A schematic of RSF arrangement is shown in Figure 1-1. Several reinforcement layers can be laid within the soil under the footing base. Main design parameters of RSF includes: embedment depth of foundation D_f, depth of the top reinforcement layer or top layer spacing d₀ measured from the footing base, vertical spacing h between reinforcement layers, number of reinforcement layers N, total depth of reinforcement d=d₀+ (N-1)h, and width of reinforcement L_R.

 

Figure 1-1 Representative layout of a reinforced soil foundation

1.2.    Motivation and Research Objectives

With growing interest in employing shallow foundations to support bridges and other heavy structures, it is important to study and explore all potential combinations ground improvement and foundation solutions that would allow the use of shallow foundations even in conditions for which a deep foundation would otherwise be selected. Footing on reinforced clay is such an alternative foundation solution. Notwithstanding the fact that footings bearing on reinforced clays can be an effective and economic alternative to other expensive foundation solutions, studies focusing on such foundations are rather

limited in number. Moreover, in addition to consolidation and long-term creep settlement (which are most frequently quantified for footings on clay), immediate settlement (i.e., settlement at a time shortly after load application) of foundations bearing on reinforced clay should be studied adequately to establish serviceability criteria that may govern the design. Following such an objective, the present study aims to quantify immediate load-settlement behavior of a strip footing resting on geogridreinforced clay through the influence factor I_q (which will be discussed in chapter 4). Furthermore, effects of several parameters related to reinforcement arrangement and properties of soil and reinforcement on foundation performance (i.e., bearing capacity and settlement) are analyzed.

1.3.    Thesis Outline

The content of this thesis is presented in five chapters. Following the background and introduction to the problem, as presented in this chapter (Chapter 1), Chapter 2 provides a brief review of existing literature related to the topic. Pertinent details of finite element modeling including soil and reinforcement constitutive models, geometric model, interface condition and mesh convergence study are discussed in Chapter 3. Results from finite element analysis are presented and discussed in Chapter 4. Chapter

5 summarizes important conclusions and findings drawn from this research.

IMMEDIATE LOAD-SETTLEMENT RESPONSE OF STRIP FOOTINGS BEARING ON GEOGRID-REINFORCED CLAY

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