Passive Force on Bridged Abutments and Geofoam Inclusions with Large-Scale Test. Eric Glenn Scott

Transcription

1 Passive Force on Bridged Abutments and Geofoam Inclusions with Large-Scale Test Eric Glenn Scott A project submitted to the faculty of Brigham Young University In partial fulfillment of the requirements for the degree of Master of Engineering Kyle M. Rollins, Chair Michael A. Scott Paul W. Richards Department of Civil Engineering Brigham Young University July 2015 Copyright 2015 Eric Glenn Scott All Rights Reserved

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3 ABSTRACT Passive Force on Bridge Abutments with Geofoam Inclusions from Large-Scale Test Eric G. Scott Department of Civil and Environmental Engineering, BYU Master of Engineering A vertical zone of compressive material or an inclusion can be used as a barrier to decrease lateral earth pressures placed on structures. Such load reduction can be of particular importance when determining possible by either the structure or the soil backfill that is being separated. The compressible material is typically expanded polystyrene (EPS), or geofoam. Geofoam inclusions are often considered in reducing reduced active earth pressures felt by backfill structures. However, in bridge abutments it is also necessary to understand the passive force versus backwall deflection relationship when geofoam inclusions are in place. Neither current design codes nor available field tests provide any understanding regarding the ability of a geofoam inclusion to reduce passive lateral loads felt by a structure and the corresponding soil backfill resistance. To explore this issue, large-scale field tests were conducted with a geofoam inclusion acting as the barrier between the backfill soil and the simulated abutment backwall. By inducing lateral forces to displace the backfill wall, it was possible to measure passive force-displacement curves as well as geofoam and soil backfill compression behavior. These tests are the first of their kind to investigate the behavior of geofoam inclusions on passive lateral soil resistance. Keywords: passive force, EPS geofoam, bridge abutment, deflection curves

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5 TABLE OF CONTENTS 1 Introduction Objectives Scope of Work Organization of Report Literature Review Current Understanding of Geofoam Inclusions Horvath (1997) Ertugurl and Trandafir (2012) Ertugurl and Trandafir (2011) Bathurst, Zarnani, and Gaskin (2007) Bathurst and Zarnani (2013) Geofoam Properties Passive Force-Deflection Relationship for Sand Duncan and Mokwa (2001) Cole and Rollins (2006) Ultimate Passive Force Theories Rankine Earth Pressure Theory Coulomb Earth Pressure Theory Log-Spiral Theory Test Layout and Procedures Test Layout The Reaction Foundation Loading Apparatus Pile Cap Geofoam Inclusion Soil Backfill Zone Testing Instrumentation String Potentiometers and Reference Frame Geofoam Compression Heave and Vector Displacement iii

6 3.2.4 Red Sand Columns and Internal Failure Surface Test Procedure Geofoam Placement Backfill Placement and Test Preparation Pile Cap Displacement Final Measurements Test Results Passive Force-Deflection Curves Overall Failure Patterns Backfill Heave and Surface Cracking Longitudinal Displacement and Strain of Geofoam and Backfill Backfill Strain Backfill Soil Displacement Vectors Compression of Geofoam Analysis Prediction of Passive Force PYCAP Performance Prediction Observed Pile Cap Performance Conclusions iv

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8 LIST OF FIGURES Figure 2-1: Geometry of the Log-Spiral Failure Plane (Marsh 2013) Figure 3-1: Plan View and Cross Section of Testing Layout Figure 3-2: Plan View and Cross Section of Geofoam Inclusion Figure 3-3: Backfill Soil Gradation Curve Figure 3-4: String potentiometer locations on the pile cap (Marsh 2013) Figure 3-5: Geofoam and Backfill Grid System Figure 3-6: Red Sand Columns to determine Failure Plane Figure 4-1: Baseline for 0 Skew Test Figure 4-2: Total Load, Baseline Load, Passive Force Comparison Figure 4-3: Passive Force versus Deflection Figure 4-4: Passive Force versus Displacement for 0 Skew Test and Geofoam Inclusion Figure 4-5: Internal Failure Plane Figure 4-6: Failure Plane at Backfill Surface Figure 4-7: Geofoam Shear Cracking at the Corners of the Pile Cap Figure 4-8: Geofoam Inclusion Bending at the End of the Second Loading Figure 4-9: Backfill Heave Contours and Cracks on a 2-ft grid for the Sand Backfill Test (Marsh 2013) Figure 4-10: Backfill Heave Contours and Cracks for the Geofoam Inclusion Test Figure 4-11: Total Backfill Displacement versus Distance from Backwall Face at Selected Cap Displacements for 1st Loading of the Geofoam Inclusion Test Figure 4-12: Total Backfill Displacement versus Distance from Backwall Face at Selected Cap Displacements for 2nd Loading of the Geofoam Inclusion Test vi

9 Figure 4-13: Total backfill displacement versus distance from backwall face at selected cap displacement intervals for the sand backfill test (Marsh 2013) Figure 4-14: Backfill Compressive Strain versus Original Distance from Pile Cap Face Figure 4-15: Backfill Compressive Strain versus Original Distance from Pile Cap Face for Second Push Figure 4-16: Backfill compressive strain versus original distance from backwall face at selected displacement intervals for the sand backfill test (Marsh 2013) Figure 4-17: Soil Displacement for Geofoam Inclusion Testing Figure 4-18: Grid System on Geofoam Figure 4-19: Geofoam Compression versus Distance from Pile Cap for Specified Passive Forces Figure 5-1: Comparison of Low Bound and High Bound PYCAP Passive Force versus Geofoam and Sand Backfill Deflection Curves Figure 5-2: Comparison of Low Bound and High Bound PYCAP Passive Force versus Geofoam and 0 Skew Deflection Curves Figure 5-3: Rankine Failure Zone vii

10 1 Introduction A vertical zone of compressive material or an inclusion can be used as a barrier to decrease lateral earth pressures placed on structures. Such load reduction can be of particular importance when determining possible failure by either the structure or the soil backfill that is being separated. The typical material of choice for the inclusion is expanded polystyrene (EPS), or geofoam. Research indicates that EPS geofoam panels of low stiffness installed against rigid retaining structures will readily compress under the lateral earth pressures exerted by the retained soil mass since geofoam is intentionally the least stiff component (Ertugrul and Trandafir 2011). Although the influence of geofoam inclusions has been investigated for the case of active earth pressure(ertugrul and Trandafir 2011; Ertugurl and Trandafir 2012; Horvath 1997), very few tests have previously been conducted to examine the effect of geofoam inclusions on passive earth pressure (Bathurst and Zarnani 2013; Horvath 1997). In some cases, it might be desirable to isolate the bridge structure and abutment walls from the passive backfill force. For example, in the event of liquefaction in an underlying sand layer, lateral spread displacements could cause passive force to develop against the abutment as the overlying backfill soil slides towards the bridge abutment. Alternatively, dynamic forces from inertial earthquake loading could cause structure movement towards a soil backfill leading to large passive pressures on the backwall. 1

11 By performing large scale tests that use geofoam as a buffer between a bridge structure and backfill soil we will be able to draw conclusions as to the effectiveness of including geofoam as a compressible inclusion. Comparisons will be made with a previous test involving the same backfill soil but without a geofoam inclusion. This report describes the properties of the backfill and geofoam materials, describes the testing procedures employed, and provides results from the tests. Test results include passive force-deflection curves, lateral and vertical deformation of the geofoam and backfill soil, shear pane formation and surface cracking patterns, and backfill strain. 1.1 Objectives The research and presentation of the corresponding results will try to meet the following objectives: 1. Identify the overall reduction in peak passive force as a result of a geofoam inclusion 2. Compare the changes in the passive force- deflection curves between tests with and without a geofoam inclusion. 3. Identify the effect of geofoam inclusions on the backfill deformation, crack and internal shear patterns, and the backfill strain. 1.2 Scope of Work Large-scale lateral passive force tests for this study involved a 5.5 ft high pile cap used to simulate a bridge abutment along with transverse wingwall. A compressible inclusion consisting of geofoam blocks was constructed adjacent to the pile cap with that was 3 ft deep in the direction of loading, 16 ft transverse to the loading and 6 ft tall. Clean sand was compacted to a depth of 6 feet behind the geofoam inclusion. Load was applied incrementally until cumulative deflection of the pile cap reached about 6 inches so that the influence of the inclusion could be 2

12 examined at large deflection levels. Because the testing of the geofoam block included using dense, clean sand with a 0 skew angle similar to testing done by Marsh (2013), such test results serve as a baseline comparison for the testing with the geofoam inclusion conducted in this study. Therefore, comparisons between the results from this study will be made with previous tests conducted by Marsh (2013) to highlight the influence of the geofoam inclusion on passive force behavior. 1.3 Organization of Report This report will begin with a literature review that addresses the current knowledge of geofoam and how inclusions specifically change surrounding soil behavior. Attention will be given to the passive force-deflection relationships for backfills involving soil only. Additionally, a discussion will be presented with regards to the need for further research on geofoam inclusions. Next, the testing layout, setup, and site features will be presented. The tests results will then be compared against the current methods used for predicting the passive force. Lastly, appropriate conclusions will be presented along with recommendations. 3

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14 2 Literature Review The concept of including a compressible inclusion for geotechnical uses is not new to today s society. Materials such as cardboard, glass-fiber insulation, or even bales of hay have been used with the intent to reduce stress felt by surrounding soil (Horvath 1997). While the fore-mentioned materials have provided solutions to stress problems for soils, they have their short-comings; materials that have organic properties often display unpredictable stress-strain relationships (Horvath 1997). Even though there is not a lot of information present from largescale testing on geofoam inclusions subject to dynamic loading, there is a basic understanding of being able to reduce forces felt by both structures and surrounding soil through the use of a compressible system. 2.1 Current Understanding of Geofoam Inclusions Various applications of compressible inclusions in geotechnical settings are currently in use. While proper design and use must be determined by an engineer who understands geofoam properties, there are various categories in which geofoam inclusions have been implemented. These include the use of geofoam behind earth-retaining structures, beneath foundation elements, or above pipes, culverts, and tunnels (Horvath 1997). Additionally, Athanasopoulus et. al (1999) suggest EPS geofoam blocks for use in scenarios involving thermal insulation, lightweight fills, compressible inclusion, and vibration damping. The selection of a material to be used as the inclusion requires a product that can be purposefully designed as the weakest link in a scenario. 5

15 Thus, the geofoam must readily compress before the failure of surrounding soil or an adjacent structure. Additionally, as the compressible inclusion is in direct contact with soil, it must be able to withstand the demands of biodegradation. Consequently, because geofoam readily compresses under loading, and can endure the stresses of an organic material, it is widely selected as the material of choice for earth-fill scenarios. Furthermore, geofoam proves to be advantageous because of the ability to accurately measure and predict a stress-strain relationship for the material whereas other inclusions have fallen short in this regard Horvath (1997) Horvath (1997) described the early state of understanding regarding geofoam inclusions and presented potential uses of geofoam inclusions. In particular, he hypothesizes the idea of a Reduced Earth Pressure (REP) Wall concept along with a Zero Earth Pressure (ZEP) Wall idea when referring specifically to retaining wall design. The major difference between the ability to obtain a ZEP wall versus that of a REP wall is whether a designer decides to include layers of tensile reinforcement in the soil backfill, such as a geogrid or geotextile product. Thus, by implementing a correct combination of stiffness from geofoam and a horizontal reinforcement in the soil backfill, designers can decrease the lateral earth pressures to zero. The author further contends that the seismic stresses on soil backfills caused by shaking can be significantly reduced through use of a compressible inclusion. He mentions that the concept of using a compressible inclusion to reduce the seismic earth pressure can be beneficial for both new construction and existing structures. Retrofitting existing structures with a geofoam inclusion can help to satisfy the current code requirements for seismic resistance. Numerous finite element models have been run to try and predict the behavior of a seismic loading scenario where geofoam was used to attenuate the lateral forces. However, the author does mention that most of 6

16 the research and experimentation (both computer models and large-scale testing) have been done with coarse-grained soils. Thus, more attention is necessary for fine-grained soils Ertugurl and Trandafir (2012) Ertugurl and Trandafir conducted studies having to do with yielding flexible retaining walls that have geofoam inclusions and the corresponding reduction in static lateral loads. In so doing the authors worked with two different geofoam inclusions of varying thickness. The authors used a stiff sand box and performed three tests to measure the lateral loads and the wall stem (retaining wall) deflections. The first test (used as a baseline) did not include a geofoam inclusion but rather had a dry clean sand in direct contact with the yielding wall. In the second and third tests, EPS geofoam panels of 50mm and 100mm thick were used, respectively. The dry sand backfill was placed in four lifts of approximately 180mm thick. After backfill placement was completed, the lateral stresses were recorded at four different locations along the retaining wall and the lateral wall displacement was measured at the top and mid-height of the wall. Testing was done by releasing hydraulics jacks that restricted the lateral movement of the yielding wall, thereby stimulating behavior expected of a flexible or yielding retaining wall. Data acquisition occurred until the yielding wall saw no more lateral deflection. Thus, testing was able to find correlation between the wall fixidity and wall deflection. Furthermore, by comparing the results from the three tests, the authors were able to draw conclusions as to the effectiveness of a geofoam inclusion at reducing wall deflections. They concluded that the presence of a geofoam inclusion reduced the wall deflection depending on the wall flexibility and the inclusion thickness. This reduction in wall deflection and lateral forces was attributed to the arching effect in the backfill soil by the lateral compression of panels of geofoam inclusions. 7

17 2.1.3 Ertugurl and Trandafir (2011) With the intent of understanding how geofoam inclusions can be used to reduce lateral loads in a static setting, Ertugurl and Trandafir (2011) performed testing to stimulate using a rigid, non-yielding retaining wall. Additionally, a finite-element model was developed to estimate the lateral earth pressures against various retaining walls that both included and disregarded a compressible inclusion. Small-scale testing was done by using a 0.7 meter high wall. Four different model wall configurations were tested in the physical study. In the first configuration, the lateral stresses on the wall were measured without a geofoam inclusion. This served as the control test for the other three test set-ups that included geofoam panels of three thicknesses characterized by a thickness to height ratio. Thickness to height ratios of 0.07, 0.14, and 0.28 were tested by installing the appropriate geofoam inclusion behind the rigid retaining wall. Four earth pressure cells were installed with a 200-mm spacing along the height of the wall to measure the lateral earth pressures. These pressure cells were installed in small recesses in the wall to ensure that the sensor was flush with the face of the wall that sat against the geofoam inclusion. Finite-element modeling was performed to be able to accurately predict the lateral earth pressures that could be experienced by the rigid retaining wall. Boundary conditions for the model included horizontal and vertical displacement restrictions on bottom horizontal boundary along with a horizontal displacement boundary along vertical extension of the wall. Furthermore, to produce reasonably accurate results, an elastoplastic model was used for the soil backfill whereas a linear-elastic model was implemented for the geofoam. The results from the pressure cells used in the physical model were compared against the finite-element model. 8

18 From the physical and finite-element models, the authors were able to draw conclusions as to the effectiveness of a geofoam inclusion at reducing lateral earth pressures on a rigid, nonyielding retaining wall. The relative thickness and stiffness of the geofoam inclusion appeared to be the biggest factor in determining a lateral earth pressure reduction. Both models indicated a significant reduction in the earth thrust when the set-up included a compressible inclusion installed behind the wall (Ertugrul and Trandafir 2011) Bathurst, Zarnani, and Gaskin (2007) Large-scale testing for geofoam inclusions subjected to dynamic forces was stimulated by Bathurst, Zarnani, and Gaskin (Bathurst et al. 2007). Testing was done using a shake-table with the intent to understand geofoam s ability at reducing dynamic forces that would be present from earthquake or other types of lateral loading. The test set-up included a test without a geofoam inclusion (base-line) and three other tests with geofoam that had varying densities. By merely changing the geofoam density between the different tests, authors were able to draw conclusions to the effectiveness of a geofoam inclusion at attenuating dynamic forces felt by a rigid retaining wall. The instrumentation for the test set-up included the use of pressure cells placed against the boundary of the retaining wall and the geofoam inclusion. Also, displacement potentiometers were placed at the boundary between the soil backfill and the geofoam to show the lateral deformations in the geofoam during loading. Embedded in the soil backfill were four accelerometers that allowed for the recording of the soil backfill movement. In order to stimulate earth-quake loading, the models were excited by using an acceleration history that increased in 0.05g increments and held each amplitude for 5 seconds. The maximum base acceleration from the shake tables was 0.8g. The model was excited only in the horizontal cross-plane direction in order to be consistent with the critical orientation for the seismic design of retaining walls. 9

19 In looking at the results, researchers used hysteric response curves developed from the recorded forces of the pressure cells and strain values computed from the displacement as measured by potentiometers. However, of greatest importance were the results found from plotting the force felt by the pressure cells located at the retaining wall against the base acceleration from the shake-table. The authors show that structures that included geofoam against the retaining wall generated less horizontal load than the rigid wall without an inclusion during dynamic shaking. Furthermore, a reduction was shown by using a geofoam inclusion with an initial stiffness of 12 kg/m 3, and then removing 50% of the foam via coring to produce a new density of 6 kg/m 3. Researchers were able to measure a 31% reduction in lateral forces when comparing the above mentioned geofoam inclusion against the rigid retaining wall at a common peak base acceleration of 0.7g. Such results prove geofoam s ability to attenuate horizontal forces when a system is subject to dynamic loading Bathurst and Zarnani (2013) In order to justify their work on force attenuation of dynamic loads by geofoam inclusions, Bathurst and Zarnani used several numerical models for comparison against large-scale testing scenarios. The authors tried to produce computer models that closely displayed results measured from their shake-table testing (Bathurst et al. 2007). The first of such models was a displacement model that used springs to measure the forces acting on the various components of the test set up. A linear failure plane was assumed to propagate through the backfill soil from the heel of the buffer at an angle to the horizontal. Thus, spring components were used to determine the force present in both the geofoam-soil backfill boundary and the boundary created by the failure plane. Multiple springs were used at each boundary condition to model both the normal forces and the interface shear forces. Using a dynamic loading pattern on the shake-table as described in section 10

20 2.1.4, the authors found that the numerical displacement model was a very accurate fit for determining the peak force on the backfill wall. There were various discrepancies in the model up the excitation acceleration of 0.7g. They estimated that at higher accelerations there are more complex system responses that cannot be accurately captured by the simple displacement model that was employed. Authors additionally used the finite difference method computer program FLAC to estimate the peak forces felt by the test set-up. The first of such models employed a linear-elastic plastic geofoam material subjected to a Mohr-Coulomb failure criterion and Rayleigh damping. A second model used the equivalent linear method to also estimate the peak force felt by the backfill wall during dynamic loading. Models were found to show that there is reasonably good agreement between measured and predicted results regardless of the model type. Through the process of developing numerical models to estimate the peak backwall forces that were reduced by geofoam inclusions, the authors aimed to provide useful design information to the engineering community through preliminary design charts. Through the numerical models that essentially confirmed the physical test models, design charts were created to quantify the isolation efficiency of geofoam inclusions based on their values for Young s modulus, the type of geofoam used, and thickness of the inclusion. 2.1 Geofoam Properties As shown by various studies on geofoam inclusions, the most important properties when selecting an inclusion are the geofoam density and the thickness to be used in the set-up. Horvath speculates that the minimum density of geofoam that can be manufactured is 10 kg/m 3 or slightly less (Horvath 1997). Manufacturing geofoam inclusions with density lower than 10 kg/m 3 would 11

21 result in an insufficient fusion between the individual expanded polystyrene beads. Insufficient bonding between these beads would lead to a material that would readily break apart. Thus, selecting a material that has a specific density that allows for ready compressive but still provides adequate strength becomes the challenge. Experience indicates that the minimum EPS density that strikes an economical balance between stiffness and durability is approximately 12 kg/m 3 (Horvath 1997). Using a high density of geofoam can lead to strength benefits for system as long as the geofoam inclusion is the weakest component of a set-up. By being weaker than the other various components, one can assure that a geofoam inclusion will readily compress under static or dynamic loads. Hesitation can arise from the inclusion of allowing something overly soft in a system. Thus, mid-range density foams are often used because they are lower in density yet still strong, or stiff, enough for operation purposes. Selecting a thickness of geofoam requires the ability for soil to mobilize resistance. Traditional approaches for designing a retaining structure would require that sufficient strength be provided to resist at-rest earth pressure or the pressure from a dynamic loading scenario. Using a geofoam inclusion in the system would allow for a structure to be designed with lower requirements for forces yet still have an ability to withstand the stresses. Designing a geofoam inclusion for thickness requires for arching to occur within the soil. Thus, it is desired for the compression of a compressible inclusion to be sufficient as to allow soil to strain and mobilize its strength. If a small thickness inclusion were used, it would compress easily but not allow for soil arching to occur and would be useless to the system. While geofoam thickness is an interesting parameter in design, internal strain of geofoam blocks is only partially dependent on thickness. Internal strain is also a function of foam density and strain rate (loading rate) of geofoam inclusions (Bret Lingwall, correspondence, May 07, 2013). 12

22 A basic method for selecting the proper thickness, as described by Horvath, requires matching the displacement of the retained soil necessary to mobilize the active state with the stress-displacement characteristics of the compressible inclusion (Horvath 1997). Since the stress-displacements of geofoam inclusions are a function of its stress-strain behavior and thickness, designers can appropriately select the required thickness to be used in specific applications. Other approximations for determining the thickness to be included are frequently used throughout the industries that are based on intuition and experience. Another important property of geofoam inclusions is Young s modulus of elasticity. According to Athanasopoulus et al (2009), in geofoam with low compressive strains (up to approximately 1%), the geofoam appears to behave linearly and an initial tangent Young s modulus of elasticity, Eti, can be defined. He suggests the following empirical equation for estimating the modulus of elasticity as seen by equation 2-1 E ti = 0.45 ρ 3 (2-1) Where Eti is in units of MPa and ρ is the geofoam inclusion density in units of kg/m 3 (Athanasopoulos et al. 1999). When the compressive strain placed on geofoam is greater than 1%, the material behaves nonlinearly and the tangent value for Young s modulus will decrease with increasing strain values. There are only a few instances of experimental data reported that shows the varying effects of loading strain on geofoam response. However, authors agree that on the nonlinear behavior of EPS geofoam for strain rates greater than 1%. 2.3 Passive Force-Deflection Relationship for Sand Numerous factors affect how passive force pressures are developed. Among those include structure movement, structure shape, soil strength parameters, and the soil-structure interaction parameters (Duncan and Mokwa 2001). Most theories used for calculating the ultimate passive 13

23 force independently do so of movement from the structure. Thus, in order to determine the relationship between structure movement and developed passive forces researchers have performed tests and numerical analyses. Researchers agree that a passive force versus backwall deflection relationship can be approximated with a hyperbola. However, the magnitude of such a backwall deflection require to develop a peak passive for is still unclear. Numerous large-scale tests were performed with the intent of showing that for dense sand the peak passive force occurred at deflections between 3% and 5.2% of the backwall height (Cole and Rollins 2006). The equation for determining the passive force with respect to wall displacement is approximated by a spreadsheet program developed by Duncan and Mokwa (2001). PYCAP is used to provide a quick and easy method for determining the passive force-displacement relationship for a given backfill material. This sheet includes the Brinch-Hansen correct factor for three-dimensional effects and is shown in Equation 2-2. P = y 1 y (2-2) K + R f max P ult where P = passive resistance P ult = ultimate passive resistance as calcualted by the log-spiral method y = wall deflection K max = initial slope of the load-deflection curve R f = P ult hyperbolic asymptote (dimensionless) 14

24 Various researchers have prepared laboratory and field tests with the intent of showing a relationship between the passive force and displacement for pile caps or bridge abutment backwalls. The following sections will discuss such research Duncan and Mokwa (2001) Authors of this paper discussed the resistance to structure movement as provided by the passive soil pressure. In specifically gauging the resistance to pile cap movement the authors used two test set ups where they tried to measure the passive soil pressure that was developed as a pile cap was pushed into a soil backfill. In the first test set up a natural soil existing at the site was used. This soil was classified as a desiccated hard sandy silt (ML) and sandy clay (CL). The pile cap was loaded horizontally as to push it into the natural soil with incrementing loads ranging from 12.5 to 15 kips until a peak load of 138 kips was applied. Load increments were maintained for about 1 minute before applying the next load. Deflections were observed to about 1.6 inches. Cracks were then observed off of the corners of the pile cap extending parallel to the direction of loading. At the same time, the passive resistance dropped off indicating that a failure surface had developed in the soil. A similar procedure was done for the second soil testing set up. However, prior to the second test, the existing soil was excavated and replaced with a gravel backfill classified as a crusher run aggregate (GW-GM and SW-SM). Load increments ranged from 10 to 15 kips until a maximum load of 91.7 kips was applied resulting in a deflection of 1.5 inches. During the second test, a bulge was observed in the backfill a distance of 7.5 feet from the pile cap face. Cracks were observed extending from the pile cap out towards the bulge. The authors and those conducting this experiment wanted to use their data to compare the deflection of the pile cap and soil backfill against the applied load. Secondly, they were interested in showing how the predominate passive soil pressure theories compare against their 15

25 measured values. The most correct values were found from using the log spiral method combined with the Ovesen-Brinch Hansen correction factor for 3D effects (Duncan and Mokwa 2001) Cole and Rollins (2006) Testing was conducted in the Salt Lake City area to investigate the relationship between pile cap deflection and lateral resistance (provided by passive forces). Tests were performed with a pile cap supported by 12 steel piles oriented in a 4 x 3 pattern. Loads were applied by hydraulic jacks with the application of load placed 0.36 meters above the bottom of the pile cap. String potentiometers (SP) and linear variable differential transducers (LVDT) were used to measure the displacement of the pile cap in both the horizontal and vertical direction. Loads were applied to result in a targeted displacement and held for 5-10 minutes. Then the load was reapplied and cycled for times to the same approximate displacement. The final load cycle was then held for minutes while inclinometer and elevation readings were taken. For the next loading sequence the process was repeated and set to a different displacement. Several tests were run with no backfill soil in place behind the cap, four different tests using varying soil types for a backfill, and a test using a 0.3 meter trench separating the backfill and pile cap. Tests that did not include a backfill along with the trenched tests served as baseline tests for comparison against testing that include soil backfills. The authors observed that the peak passive force occurred for normalized deflections between 3.0 and 3.5% of the pile cap height for the clean sand, fine gravel, and coarse gravel tests. The silty sand test required a wall movement of 5.2% of the wall height to develop the full passive force resistance. The authors attributed this higher amount to a higher fines content present than in the other soil backfills. Additionally, testing results showed 16

26 that the log spiral method can be used with accuracy to estimate the length of the failure surface beyond the face of the pile cap (Cole and Rollins 2006). 2.4 Ultimate Passive Force Theories Several theories are commonly used to estimate the ultimate passive for earth retaining structures. Passive pressures arise when a wall that is used to retain soil masses are pushed into the soil. With sufficient wall movement, a soil wedge will fail and thus the lateral pressure that occurs from this condition is called the passive earth pressure. The varying methods are commonly used for estimating passive forces are the Coulomb (1776), Rankine (1857), and Log- Spiral (1943) theories. These theories each have their own set of assumptions and advantages when compared against each other. The Coulomb and Rankine pressure theories are used more commonly because of their ease to effectively use. Whereas, the Log-Spiral method is used less frequently due to complex calculations. The three theories reduce to the general form shown in Equation (2-3) for calculating the passive pressure, p. σ p = 1 2 K PγH K P c H (2-3) where K P = σ`p = passive earth pressure coefficient σ`0 γ = moist unit weight of the soil H = backfill height c = soil cohesion 17

27 The major difference between the three theories is the method of determining the passive earth pressure coefficient. Methods of determining the passive earth pressure coefficient, Kp, according to the Coulomb, Rankine, and Log-Spiral methods are explained below Rankine Earth Pressure Theory The Rankine earth pressure theory assumes a linear failure surface place that begins at the bottom of the wall. When working with passive soil pressures the Rankine method sees the failure plane extending from the bottom of the wall through the soil backfill at an angle equal to 45 - ϕ /2, where ϕ is the angle of internal friction of the soil used for the backfill. Even though the method assumes a linear failure plane, it will not produce accurate results where the wall friction angle is greater than approximately 40% of the soil friction angle (Duncan and Mokwa 2001). Equation (2-4) shows the passive earth pressure coefficient as determined by Rankine. K P = tan 2 (45 + φ 2 ) (2-4) where φ = effective soil friction angle Coulomb Earth Pressure Theory The Coulomb earth pressure theory takes into account the Mohr-Coulomb failure criterion. Similar to the Rankine earth pressure theory, Coulomb s theory assume a linear failure plane that develops at the base of a backfill wall and extends into the soil. The angle of rise from the backfill wall through the soil is determined iteratively either with the help of graphical solutions or by computer computations. Equation (2-5) shows the passive earth pressure coefficient as determined by the Coulomb theory. 18

28 K P = sin 2 sin 2 ( β φ ) 2 sin 2 β sin(β + δ ) [1 sin(φ + δ ) sin(φ + α) sin(β + δ ) sin(β + α) ] (2-5) where β = angle of backwall from horizontal φ = effective soil friction angle δ = wall friction angle α = angle of the backfill from horizontal In the same fashion as the Rankine earth pressure theory, the Coulomb method will not produce accurate results where the wall friction angle is greater than approximately 40% of the soil friction angle (Duncan and Mokwa 2001). Other researchers have modified the Coulomb equation allowing it to take in mind both wall friction and non-horizontal backfills Log-Spiral Theory The Log-Spiral method is widely considered to be the most accurate at solving for the passive earth pressure coefficient, Kp, when realistic wall friction is considered. Unlike the Rankine and Coulomb theories, the log-spiral approach does not assume a linear failure plane extending from the backfill wall. Rather, the log-spiral method assumes a curved portion of a failure zone that is most accurately modeled by a log-spiral. The curved portion of the graph is found to be a part of the Prandtl zone, and connects to a linear failure surface named the Rankine zone. A smooth connection is found between the two zones as shown by figure

29 Figure 2-1: Geometry of the Log-Spiral Failure Plane (Marsh 2013) There are various methods that allow for the calculation of the passive force using the Log-Spiral method. The most robust method includes the use of a graphical solution which has been implemented into an excel spreadsheet call PYCAP (Mokwa and Duncan, 2001). Although this method for solving the Log-Spiral theory may become the most difficult to solve, it can account for soil cohesion and complex backfill geometries. 20

30 3 Test Layout and Procedures Large-scale lateral load tests with a geofoam inclusion were performed at a site located near the control tower at the Salt Lake International Airport. A variety of in-situ and laboratory tests have been done on the subsurface soil in the area to understand the specific interactions between the piles and the soil (Christensen 2006). However, the information available with respect to these subsurface soil properties will not be discussed here because they have little relevance to the testing performed. 3.1 Test Layout As indicated previously, in this study a large-scale lateral load test was performed to define the passive force-deflection curve for a sand backfill with a geofoam block inclusion. Previous tests conducted with the same test layout and backfill but without an inclusion had previously been performed as reported by Marsh (2013). Tests performed by Marsh (2013) and others included the use of angle wedges, concrete wingwalls, and varying backfill heights that allowed an investigation into the results of skew angles and other variables on the passive soil pressures developed by the backfill soil. With the geofoam testing, skew angles were not investigated, thus results will be compared against the 0 skew testing done by Marsh. The layout for the test consisted of several components: the geofoam block inclusion, the backfill zone, loading apparatus, pile cap and the reaction foundation. Figure 1-1 shows plan and profile 21

31 views of the test set-up with the geofoam inclusion. Additionally, Figure 1-2 shows a schematic of the geofoam inclusion used in the testing layout along with the joints between the geofoam blocks. The test layout used by Marsh (2013) was identical; however, the inclusion was not present The Reaction Foundation The reaction foundation was used to create a sturdy structure against which the actuators could push. This reaction foundation was created by installing two 4-foot diameter drilled shafts spaced 12 feet apart center to center. The east and west shafts extend to depths of 70.5 feet and 55.2 feet, respectively. Each shaft was capped with a four-foot square by two-foot thick cap. Reinforcement for the shafts was provided by 18 #11 vertical bars with a #5 bar spiral at a pitch of 3 inches for the top 35 feet of the shaft. Below the 35 feet mark, vertical reinforcement consisted of 9 #11 bars with a #5 bar spiral at a pitch of 12 inches. The concrete cover over the reinforcement was approximately 4.75 inches throughout the length of the shaft. Concrete throughout the length of the shaft had a compressive strength of 6,000 psi. Additionally, two large I-beams were placed directly south of the drilled shafts and north of a sheet pile wall. The I-beams had an approximate depth of 64 inches and were 16 inches high while spanning 28 feet in the east-west direction. Both I-beams had their strong axis oriented in the north-south direction. I-beams were used with the intention to provide lateral rigidity to the foundation reaction system. Additional stiffeners were installed between the flanges to prevent the beam from buckling during loading. Furthermore, a sheet-pile wall spanned the north side of the drilled shafts. AZ-18 sheet piling was used for the sheet wall. This was made from ASTM A-572 Grade 50 steel. Also, a vibratory hammer was used to install the wall as close as possible to the drilled 22

32 shafts. The sheet piling wall extended to depths ranging from 33.6 feet to 35.6 feet below the ground surface Loading Apparatus The loading apparatus used for testing included two MTS hydraulic actuators capable of pushing against the reaction foundation. Figure 3-1 shows the actuators placed north of the reaction foundation. Also, as seen in Figure 3-1, the actuators had a capacity of providing 600 kips of force in extension and 450 kips of force in contraction. In order to tie the system together, DYWIDAGs served as the connection between the actuators and the reaction foundation. Similarly, eight DYWIDAGs were embedded into the pile cap to function as the connection between the pile cap and the actuators. These actuators were centered 2.75 feet above the base of the pile cap. In the loading scenario, the actuators were used to displace the pile cap to certain pre-determined displacements and then the corresponding actuator load for the desired displacement was recorded. Careful consideration was given to the possibility of rotating the pile cap rather than directing it with a straight displacement into the soil backfill. Thus, by using the string pots connected to the pile cap, loads were able to be increased in the actuators in order to ensure a balance between the east-west sides of the pile cap and their respective displacement. 23

33 24 Figure 3-1: Plan View and Cross Section of Testing Layout 24

34 Figure 3-2: Plan View and Cross Section of Geofoam Inclusion Pile Cap Concrete that had a compressive strength of 6,000 psi was used to construct the pile cap and to fill the steel piles. The top of each pile was embedded a minimum of 6 inches into the base of the pile cap. Furthermore, in order to provide a sufficient connection between the piles and the pile cap, 18-foot long reinforcing bar cages were extended into the steel piles. These cages consisted of 6 #8 vertical bars and a #4 bar that spiraled at a pitch of 6 inches. 4.8 feet of the cages extended upwards into the pile cap and the remaining 13.2 feet was lowered into the piles. The steel piles had an outside wall thickness of inches with a wall thickness of 0.75 inches. ASTM A252 Grade 3 steel was used for the piles. The piles had an average yield strength of 57 ksi and were driven approximately 43 feet below the ground surface. The piles were placed in two rows of three piles oriented in the east-west direction. Additionally, reinforcement was provided for the pile caps with upper and lower mats that had #5 bars placed in the longitudinal and transverse direction at 8 inches on center. The pile cap dimensions were 15 feet long (north- 25

35 south direction), 11 feet wide (east-west direction) and 5.75 feet high. The south edge of the pile cap was located 16.4 feet to the north of the reaction foundation Geofoam Inclusion The geofoam barrier that was placed between the pile cap and the soil backfill consisted of four blocks of EPS19 geofoam. EPS19 is a medium density geofoam that provides strength in application situations but is also readily compressible when subjected to various loads. Four geofoam blocks were placed on the existing soil. The bottom two blocks were 4 feet tall while the upper blocks were 2 feet tall as seen in Figure 3-2. These dimensions allowed the geofoam inclusion to extend beneath the pile cap while the top surface of the geofoam remained relatively level to the top of the pile cap. All blocks were 3 feet thick in the direction of loading, and 8 feet wide. Thus, the blocks spanned 16 feet on the face of the backfill zone. The selected geofoam inclusion was an EPS19 type geofoam with a density of 1.15 lb/ft 3 or about 1/90 th of the dry unit weight of the backfill sand. EPS19 type geofoam is a medium density geofoam. The 19 describes that the geofoam has a density of 19 kg/m 3. The minimum density EPS that can be manufactured is 10 kg/m 3, however experience shows that the minimum density of EPS that has an economical balance between stiffness and durability is approximately 12 kg/m 3 (Horvath 1997). Various densities of EPS geofoam is used depending on the application. Projects that require higher strength geofoam inclusions can find EPS46 (46kg/m 3 ) available. The elastic modulus of the selected geofoam was 580 psi. These geofoam blocks offer 13.1 psi of compressive resistance at 5% deformation or 16.0 psi of compressive resistance at 10% deformation (EPS Geofoam 2012). Geofoam interestingly has a negative Poisson s ratio, and will therefore contract rather than expand when subject to a compressive load. Normal polymer foams have a positive Poisson s ratio, whereas re-entrant polymer foams (geofoam included) 26

36 have a negative Poisson s ratio (Hazarika 2005). Although the Poisson s ratio of EPS19 is not supplied directly by the manufacturer, we do know it to be a negative value. The pieces of geofoam were relatively light and lifted into place by a two man crew during the test set-up. The blocks were simply stacked on top of each other and not connected using any mechanical or geometric interlocking system. The upper blocks weighed approximately 55 lbs. while the lower blocks weighed approximately 110 lbs Soil Backfill Zone The backfill zone used for both the 0 skew test and the geofoam inclusion test was approximately 24 feet wide and 24 feet long. The zone was placed directly north of the pile cap and geofoam inclusions. With the intent of accommodating the predicted failure zone modeled by the log-spiral method, the soil backfill extended 1 foot below the pile cap for the first 8 feet of backfill. Such accommodations hoped to allow for the formation of Prandtl zone failure surface as described in section During placement of the backfill zone, two nuclear density gauge measurements were taken for each lift of soil placed as to ensure compaction and to determine the moisture content and unit weight of the soil used. Soil for all of the tests was imported to the site. The backfill soil was poorly graded sand and classified as SP soil type according to the Unified Soil Classification System. The maximum density of the soil according to the modified Proctor compaction test (ASTM D1557) was lbf/ft 3 with an optimum moisture content of 7.1%. Figure 3.3 shows the soil gradation curve for the imported soil. 27

37 Percent Passing Pre-Test Particle Size [mm] Figure 3-3: Backfill Soil Gradation Curve 3.2 Testing Instrumentation The test set-up required a variety of monitoring systems for measurement of how the pile cap, geofoam inclusion and backfill soil acted under loading. The following sections will describe the instrumentation used to measure this response String Potentiometers and Reference Frame String Potentiometers (string pots) were used to measure the displacement of the pile cap and the reaction foundation. By attaching them to an independent reference frame we were able to accurately determine the displacement of the pile cap and the reaction foundation during testing. Two of these six string pots were attached to the reaction beam while four of the string pots were attached to the corners of pile cap at 3 inches and 51 inches from the top of the pile cap, and 3 inches and 129 inches from the west side of the pile cap. These locations are shown in Figure 3-4. The other two string pots that were attached to the reference frame also attached to the large I-beam found to the north of the drilled shafts. They were placed in line to be at the 28

38 same level as the line of action of the actuators and the approximate middle of the drilled shafts. Thus, the two additional string pots were able to measure the north/south movement of the foundation reaction during the loading process. Furthermore, additional string pots were placed in the soil backfill and on the geofoam inclusion. By including string pots on the backfill and the geofoam we were able to measure the movement in these components during testing and compare them against the pile cap displacement. Seven additional string pots were mounted to the top of the pile cap 10 inches from north face of the cap. Stakes were driven into the soil backfill and the geofoam inclusion. String pots were attached to the surface of the soil and geofoam at distances from the pile cap as shown in Table 3-4. String pots were located at 0.5 ft longitudinal intervals in the geofoam and at 2 ft intervals in the backfill soil behind the geofoam. Ideally we would have been able to place all of the string pots along the centerline of the pile cap and the soil backfill, but this was not possible because of the width of the instruments used. Figure 3-4: String potentiometer locations on the pile cap (Marsh 2013) 29

39 Table 3-1: String Potentiometers Distances from the Pile Cap Face Measured Parallel to the Direction of Pile Cap Movement Geofoam Inclusion Test String Pot ID Dist. from Face Dist. from Center ft ft SP SP SP SP SP SP SP SP SP SP SP SP NOTE: Negative distances from center of pile cap indicates the string pot is located to west of backfill centerline The use of string potentiometers allowed us to measure the compressive strain,, in the backfill soil. This strain was calculated using Equation (3-1). ε = ΔL/L (3-1) where ΔL = change in distance between stringpots L = original north south distance between adjacent stakes Geofoam Compression In order to understand how the geofoam would act under compressive loading, a 2 inch square grid pattern was drawn on top of the geofoam block. The 2 inch squares were measured at 30

40 several loading intervals in order to see how sections of the geofoam would compress under the loads provided by the actuators. Measurements provided insight into what sections of the inclusion experienced more compressive displacement than others and whether or not the inclusion responded in a linear fashion or not. Figure 3-5 shows the grid pattern on the geofoam inclusion Heave and Vector Displacement In order to understand the heave and displacement of both the geofoam and the soil backfill, a total station surveying system was used to measure the changes seen in the system. To facilitate these measurements, a two ft square grid pattern was spray painted on the backfill soil to complement the grid pattern on the geofoam. In some cases behind the geofoam a one foot grid was employed to increase resolution. Between the load sets, the total station was used to measure the horizontal displacement and vertical heave of both the geofoam inclusion and the sand backfill. In measuring the heave, the total station was used to determine the relative elevation change of each grid intersection point. In so doing we were able to produce contour drawing showing heave behind the pile cap which facilitates understanding of the governing failure mechanisms. Using the displacements from the total station at the end of the test, vectors of horizontal displacement were subsequently computed at each grid point to understand how the soil backfill had displaced. Figure 3-5 shows the grid system that was used to determine the heave and vector displacement from both the soil backfill and the geofoam inclusion. The corresponding images are shown in the heave and vector displacement results section. The circular targets shown in the photo at each grid point were used in connection with a video displacement measurement system which was not successful and will not be discussed further. Lastly, surface cracks that developed throughout the load testing were mapped against the grid 31

41 pattern to provide an indication of the failure surface that developed in the backfill soil. These cracks are shown on the heave contour plots. Figure 3-5: Geofoam and Backfill Grid System Red Sand Columns and Internal Failure Surface In an attempt to locate the internal failure surface as characterized by the passive force analysis methods, vertical sand columns were excavated and fill with red sand.. A 2-inch diameter hand auger was used to core holes in the soil backfill. These cored holes were then refilled and compacted with red-dyed soil. After the completion of testing a trench was excavated next to these holes and the offsets in the sand columns produced by shearing on the failure surface were used to identify the failure plane. Sand columns were located on the longitudinal centerline at 2, 4, 6, and 8 feet from the backwall face. Figure 3-6 shows an example 32

42 of two of these sand columns. Their corresponding failure plane locations will be discussed in the test results section.. Figure 3-6: Red Sand Columns to determine Failure Plane 3.3 Test Procedure A series of lateral passive force tests were performed in the summer of 2013 at the Salt Lake City Airport. These tests included baseline tests that were run without soil backfill and other backfilled tests that experimented with various design parameters. As described before, only the 0 skew unconfined test reported by Marsh (2013) and the geofoam inclusion test are described in the current report. Such test comparisons will allow us to draw conclusions as to the effectiveness of including geofoam from compressible inclusions. The passive force on the wall was obtained by subtracting the baseline resistance from the total force measured by the actuators. 33

43 3.3.1 Geofoam Placement The geofoam inclusions were hand lifted into place by two men crews. Section lists the weights of the various geofoam inclusions. They were simply stacked on each other and did not include any sort of connections between the blocks. Typical geofoam applications may include mechanical or geometric interlocking devices, but such connections were not used in this test as discussed in section Backfill Placement and Test Preparation Backfill soil was placed in soil lifts approximately 6 to 8 inches thick. Each lift was compacted with a drum roller and a walk-behind vibratory plate compactor, as described in section Backfill soil was compacted to a density greater than or equal to 95% of the modified proctor maximum value, which was pcf for the soil backfill used. Water was added to the soil in order to help it reach the desired compaction while the target optimum moisture content of 7.1% for the soil was used. After backfill placement, a 2 foot grid was painted on the backfill surface and the relative elevations of each grid point were measured using a surface level. As previously mentioned in section 3.2.4, a 2-inch diameter hand auger was used to install the red-dyed sand columns Pile Cap Displacement Following the backfill placement and test preparation, actuators were used to displace the pile cap into the backfill zone. The actuators pushed the pile cap to target displacement intervals of approximately 0.25 inches. Furthermore, they were used to displace the pile cap at a velocity of 0.25 inches per minute. At the end of each displacement increment, the actuators were held in place for approximately 2 minutes before additional load was applied so that manual readings of 34

44 the load and displacements could be obtained. This load-displacement sequence was repeated various times until the pile cap had displaced about 3.5 inches into the geofoam-soil backfill. Further pile cap displacements could not be used in order to prevent failure of the driven piles used to support the pile cap. However, the passive force-deflection behavior of the geofoam-soil backfill at higher displacement levels was important to the study. Therefore, at the end of this loading sequence the actuators were used to bring the pile cap back to its original starting position and plywood sheets were inserted into the resulting gap between the pile cap and the geofoam inclusion. With these plywood shims in place, the actuators could push the pile cap an additional 2.25 inches and extend the passive force-deflection curve for the geofoambackfill soil system. This procedure allowed for a further displacement into the soil backfill and the geofoam without jeopardizing the integrity of the piles supporting the pile cap. Load values used to displace the pile cap were recorded in order to understand the passive force resisting movement and the corresponding displacement of the pile cap. Results are presented in the section discussing the passive force-displacement relationship for the geofoam inclusion test in comparison with sand backfill only Final Measurements Following the final displacement of the pile cap into the soil backfill zone the final measurements were taken on the geofoam grid pattern, the cracks in the backfill surface were mapped with paint and then recorded, and the total station measurements were on each grid point while the actuator load was still applied. After the actuator load was released, the soil adjacent to the red-dye sand columns was excavated and photographs and measurements were taken in order to identify the position of the failure plane. 35

45 36

46 4 Test Results This section will describe the observed backfill failure mechanisms by considering backfill lateral movement, lateral compressive strain, and internal and external failure configurations. With the end goal of understanding the prevailing failure mechanisms the test data will be presented as followed: 1) Overall failure patterns, 2) passive force-deflection curves, 3) backfill and geofoam heave, 4) Longitudinal displacement and strain of geofoam and backfill and 5) compression of geofoam. 4.1 Passive Force-Deflection Curves A baseline test was performed to define the ability of the pile cap to resist lateral movement without the help of backfill materials. The baseline test was conducted in the summer of In this test the actuators were used to displace the pile cap in absence of a soil backfill. Using the actuators and the string pots, we were able to measure the deflection seen by the pile cap and the corresponding loads required to displace the cap so that a baseline force-deflection curve could be developed as plotted in Figure 4-1. The load in this figure was obtained from actuator and the displacement was the average of the four string pots on the back side of the pile cap. Next, the displacement test involving the geofoam inclusion was conducted in June of This test included both the geofoam inclusion against the pile cap and backfill soil directly north of the inclusion. Similarly to the baseline test, loads from the actuators were measured in 37

47 Longitudinal Force [kips] order to understand the required forces to displace the pile cap into the backfill a given displacement. In order to determine the passive force supplied by the soil backfill, we recognize that the difference between the forces required to achieve displacement in the baseline and traditional test is what must be supplied by the soil backfill. Figure 4-2 then shows the total load, the baseline load, and the baseline resistance. Figure 4-3 shows the deflection versus passive force comparison. Lastly, Figure 4-4 compares the passive force from the two tests Pile Cap Deflection [in] Figure 4-1: Baseline for 0 Skew Test 38

48 Passive Force [kip] Longitudinal Force [kips] Total Load Baseline Load Passive Force Lateral Backfill Resistance Pile Cap Deflection [in] Figure 4-2: Total Load, Baseline Load, Passive Force Comparison Backwall Displacement [in] Figure 4-3: Passive Force versus Deflection 39

49 Passive Force [kips] Skew Test Geofoam Backwall Displacement [in] Figure 4-4: Passive Force versus Displacement for 0 Skew Test and Geofoam Inclusion As can be seen in Figure 4-3, the peak passive force of kips occurs at the point of maximum displacement of 3.78 inches for the first push. Both pushes are represented in Figure 4-3 with the second push including the 2.25 inches of plywood sheathing inserted between the pile cap and the geofoam inclusion. Additionally, Figure 4-3 uses the baseline testing information from two baseline tests. The first baseline test occurred when the pile cap had not been displaced in some time, thus the area next to the piles had filled with soil. Whereas the second baseline test was run immediately after the first, thus the surrounding soil had not had the chance to fill into the areas around the piles. This is similar to the testing done on the geofoam inclusion where the 1 st push moved the pile cap while soil was around the piles. The 2 nd push occurred immediately after the 1 st, and thus gave little time for the soil to settle around the piles. Therefore, corresponding baseline test results were used to generate Figure 4-3 depending on whether the data presented was from the 1 st or 2 nd push. Figure 4-4 shows the comparison of the 40

50 development of the passive force resistance in the geofoam testing against the 0 skew angle testing involving a clean sand backfill. As seen in Figure 4-4, the peak passive is 481 kips at 2.97 inches of deflection for the 0 skew test. Whereas the geofoam testing has a peak passive for of 187 kips at 3.76 inches (Figure 4-8). A more reasonable comparison could be made by seeing that the geofoam test had a passive force of 148 kips at 2.99 inches of deflection from the pile cap. Based on these contrasts we can stipulate that the geofoam inclusion was very effective at reducing the passive forces due to lateral backwall movement. As seen in Figure 4-4, the peak force from the geofoam inclusion test is only about 30% of the peak passive force the sand passive force. This 70% reduction is fairly uniform across the board when comparing the two tests. Additionally, Figure 4-4 shows the initial stiffness in the geofoam inclusion as compared to the backfill sand, as measured by the passive force. Obviously the sand test provides stiffer results. However, Figure 4-4 shows that the geofoam stiffness behaves non-linearly until the backwall has displaced around 0.75 inches, where the geofoam then begins to lose its stiffness. After 1.0 inches of backwall displacement, the geofoam begins to translate in a linear fashion with the soil backfill. 4.2 Overall Failure Patterns Failure in the soil backfill occurs when the soil is no longer able to sustain displacement and loads from the pile cap. Consequently, the soil backfill exhibits the formation of a failure surface as characterized by the ultimate passive force theories (Rankine, Coulomb, Log-Spiral). As mentioned in the test layout and procedures section, core holes were drilled and filled with red sand in order to help locate the failure plane. The inability to sustain loads past the full development of the failure plane is characteristic of an overall failure pattern. Figure 4-5 shows the failure plane as exhibited by the red-dyed soil. The failure plane initiates from the 41

51 intersection of the two blocks of geofoam and extends upward towards the surface. As seen in Figure 4-6, the failure plane extends just beyond 4 feet in the direction of loading from the face of the geofoam and soil backfill, or just beyond 6 feet from the face of the pile cap. The angle of inclination of the failure plane,, is given by the equation = 45º - /2 where is the internal friction angle of the sand. Assuming the failure surface daylights between 4.1 and 4.5 feet behind the wall leads to a friction angle between 38º and 42º. The failure surface as seen by Figure 4-5 and Figure 4-6 showed a linear failure geometry. In contrast, the failure zone observed by Marsh (2013) with the sand only scenario showed more of a log-spiral shape with the Prandtl zone extending blow the base of the pile cap. The failure zone of the geofoam inclusion test was relatively shallow when compared against the other tests that were run. While the soil backfill showed signs of failure, distress was also seen in the geofoam inclusion during testing. As seen in Figure 4-7, portions of the geofoam inclusion started to exhibit shear cracks at the edges of the pile cap. Figure 4-7 shows the interface between the pile cap and the geofoam inclusion. At the stage of testing shown in the figure, loading had compressed the geofoam inclusion in the center portions where there was direct contact with the pile cap face. The crack shown comes from the corner of contact with the pile cap. Figure 4-8 shows the bowing of the geofoam inclusion when subjected to loading. 42

52 Figure 4-5: Internal Failure Plane Figure 4-6: Failure Plane at Backfill Surface 43

53 Figure 4-7: Geofoam Shear Cracking at the Corners of the Pile Cap Figure 4-8: Geofoam Inclusion Bending at the End of the Second Loading 44

54 4.3 Backfill Heave and Surface Cracking Figure 4-9 and Figure 4-10 show the backfill heave contours and cracking patterns for the sand backfill test and the geofoam inclusion test, respectively. For the sand test, heaving occurred in relatively symmetric semi-circular shapes extending from the ends of the pile cap. The maximum heave (>1.8 inch) occurred in half-elliptical strips stretching parallel to the direction of loading. Heaving was seen to have affected the backfill material as far as 20 feet from the face of the pile cap. The shear crack patterns radiated out from the corners of the pile cap and extended to a distance of about 4.5 to 5 ft beyond the edge of the pile cap increasing the effective width of the cap to about 20 ft. Based on other tests at the site where a complete failure surface developed at the ground surface, the failure surface generally manifested itself at locations where the heave was between 0.5 and 0.75 inch. Based on this criterion, the failure surface would likely have day-lighted at about 17 ft behind the face of the pile cap. Similarly, heave contours for the geofoam test also produced elliptical strips. However, these elliptical strips stretch extending from the geofoam inclusion perpendicular to the direction of loading. In comparing the two tests, it is seen that the geofoam mitigated the backfill heave to 12 feet north of the pile cap in contrast to 20 ft for the sand backfill test. Less heave was measured in the sand backfill test than that seen in the geofoam inclusion test. However, heave from the geofoam testing is more localized near the abutment than heave from the sand test. Assuming that the ultimate failure surface would be located where heave was around 0.5 inches, the failure surface may have ultimately developed at a distance of 8 to 10 ft from the back of the geofoam block wall. Shear cracks initiated from the corners of geofoam block wall and extended outward about 1 ft increasing the effective width of the shear zone to 18 ft. Shear zones clearly daylighted at about 4 ft behind the geofoam wall; however, as indicated previously, this failure 45

55 wedge appears to be associated with the upper 2 ft block. The heave contours behind the shear cracks at 4 ft suggests that a failure surface associated with the full block height might have developed with greater displacement. Figure 4-9: Backfill Heave Contours and Cracks on a 2-ft grid for the Sand Backfill Test (Marsh 2013) 46

56 3 ft Figure 4-10: Backfill Heave Contours and Cracks for the Geofoam Inclusion Test Both tests show a maximum heave near the edge of the soil-structure boundary (counting the geofoam as the structure). This would be consistent with the assumption that the peak passive forces would occur at the edge of a wall face. 47

57 Backfill Displacement [in] 4.4 Longitudinal Displacement and Strain of Geofoam and Backfill Obviously the loading scenarios caused displacement in the geofoam and the soil backfill. As described in section 3.2.1, string pots were mounted to the top of the pile cap and were located at various distances in soil backfill and the geofoam. The data from the string pots allowed for measurement of the backfill movement and strain. Figure 4-11 and Figure 4-12 show the deflection of the geofoam inclusion and the soil backfill during the first and second pushes as measured by various string pots. Figure 4-13 shows the deflection from the soil backfill during the sand backfill test as performed by Marsh (2013) in 1.01 in 1.99 in 2.99 in 3.78 in Initial Distance from Backwall Face [ft] Figure 4-11: Total Backfill Displacement versus Distance from Backwall Face at Selected Cap Displacements for 1st Loading of the Geofoam Inclusion Test. 48

58 Backfill Displacement [in] Backfill Displacement [cm] Backfill Displacement (in) in 1.01 in 1.99 in 2.98 in 3.76 in Distance from Pile Cap (ft) Figure 4-12: Total Backfill Displacement versus Distance from Backwall Face at Selected Cap Displacements for 2nd Loading of the Geofoam Inclusion Test Initial Distance from Backwall Face [m] in in in in in Surface Crack (Est.) Initial Distance from Backwall Face [ft] Figure 4-13: Total backfill displacement versus distance from backwall face at selected cap displacement intervals for the sand backfill test (Marsh 2013) 49

59 Figures 4-11 and 4-12 show that the majority of the movement occurred within the first 7 feet behind the pile cap. Beyond this zone the backfill displacement decreased roughly linearly with respect to the original distance from the face of the backwall. Figure 4-12 also shows the inability of the geofoam to compress during the second loading cycle. This points to some obvious plastic deformation from the geofoam inclusion Backfill Strain The backfill displacement information presented previous allows us to calculate the backfill strain using Equation (3-3). Figure 4-14 shows the backfill strain of the soil with respect to the distance from the backwall face for the geofoam inclusion test. This figure illustrates the strain curves as shown at selected pile cap displacement intervals. Very low strain values were seen in the range of 2 feet through 4 feet in the soil backfill. Figure 4-15 is showing the compressive soil strain values in the soil backfill for the second loading push. The negative compressive strain values seen are characteristic of tension effects in the soil. This corresponds with the heave values of the geofoam inclusion and the soil backfill as discussed in section 4.3. The larger strain values in the range from 4 to 8 feet in the soil backfill are consistent with the observed shear zone failure as described in section 4.1. Figure 4-15 shows the second push data after the plywood inclusions were stuck between the pile cap face and the geofoam inclusion. The values shown in this figure are similar to those seen in Figure 4-14 with the exception of the higher strain values seen 4 feet past the geofoam inclusion, or 6 feet north of the pile cap face. These larger strain values illustrate the formation of a failure plane and are consistent with the heave and cracking data shown in section 4.3 along with the overall failure data from section 4.1. Figure 4-16 then shows the compressive soil strain data for the sand backfill skew test with pure sand. 50

60 Compressive Soil Strain (%) Compressive Soil Strain (%) in 1.01 in 1.99 in 2.99 in 3.78 in Initial Distance from Pile Cap Face (ft) Figure 4-14: Backfill Compressive Strain versus Original Distance from Pile Cap Face in 1.01 in 1.99 in 2.98 in 3.76 in Initial Distance from Pile Cap Face (ft) Figure 4-15: Backfill Compressive Strain versus Original Distance from Pile Cap Face for Second Push 51

61 Compressive Soil Strain [%] in 1.12 in 1.99 in 2.97 in 3.21 in Surface Crack (Est.) Initial Distance from Backwall Face [ft] Figure 4-16: Backfill compressive strain versus original distance from backwall face at selected displacement intervals for the sand backfill test (Marsh 2013) As seen in Figure 4-16, an estimate was made as to where the surface crack should have appeared. However, due to the limitations of the system set-up, such a failure plane did not fully develop and was not visible at the conclusion of testing. Such is commonly the case as the peak passive pressure forms and mobilization of the soil occurs, however additional displacement is required for the full formation of the failure plane. 4.5 Backfill Soil Displacement Vectors Figure 4-17 shows the displacement vector plot for the geofoam inclusion and the soil backfill as recorded by the total station measurements at the end of testing. As seen in the figure, the length of the arrows represents the amount of geofoam or backfill displacement. In addition, the arrows are color-coded. Red arrows indicate a movement of more than two inches, green arrows show movement between one to two inches, and the blue arrows show displacement that is less than one inch. Thus, as seen in Figure 4-17, most of the major backfill movement occurred 52

62 in the geofoam and the soil backfill that is in the 8 feet directly north of the pile cap. Figure 4-17 also shows that the geofoam inclusion compressed in the east-west direction when subjected to north-south compressive loads. This can be attributed to the negative Poisson s ratio characteristic of geofoam inclusions as discussed in section Displacement vectors were not obtained for the companion test with the sand backfill reported by Marsh (2013). However, vector plots for other similar tests generally indicated a more gradual reduction in backfill displacement with the distance from the pile cap due to the soil sliding up the failure plane. In contrast for the geofoam inclusion test the majority of the movement occurred in the geofoam inclusion and the backfill area directly next to the geofoam. This goes to show the effectiveness of geofoam inclusions at attenuating the loads placed on the backfill wall. 53

63 Geofoam Inclusion Figure 4-17: Soil Displacement for Geofoam Inclusion Testing 4.6 Compression of Geofoam To understand how the geofoam inclusion performed under compressive loads, a 2 inch square grid was drawn onto the top of the geofoam inclusion as discussed in section Monitoring this grid allowed us to measure the compression strain to the passive force on the geofoam block wall. Figure 4-18 shows the geofoam inclusion with the grid system drawn on the top face. Figure 4-19 shows curves that depict the compressive strain of the geofoam squares as a function of the passive force. 54

64 Compressive Strain [%] Figure 4-18: Grid System on Geofoam kips kips kips kips 57.3 kips Distance from Pile Cap [in] Figure 4-19: Geofoam Compression versus Distance from Pile Cap for Specified Passive Forces 55