PULLOUT RESISTANCE OF SOIL ANCHORS IN COHESIONLESS SOIL UNDER VARYING VELOCITIES BY EXPERIMENTAL METHODS

ABSTRACT

 

An examination of the literature in respect to buried soil anchors shows an existing gap in parameters studied.  An in depth study on soil anchors subjected to varying pullout velocities has yet to be studied in detail.  This thesis outlines an experimental program to test anchors of two different diameters at various embedment depths and pullout velocities using symmetry.  The analyzed data suggests the ultimate pullout resistance is rate dependent and is further influenced by diameter size and embedment depth.  Anchors embedded at deeper depths with the larger diameter size showed to be more influenced by pullout velocities than those at shallower depths.  Therefore, anchors pulled at high velocities should be considered during design process.  Furthermore, results from this study can be used to update existing models to account for strain rate effects in addition to validate and calibrate new numerical models to include anchor foundations in potential applications that are subjected to dynamic loading.

 

 

 

 

 

 

TABLE OF CONTENTS

List of Figures ………………………………………………………………………………………………………….. v

List of Tables …………………………………………………………………………………………………………… vii

Acknowledgements …………………………………………………………………………………………………… viii

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

General ………………………………………………………………………………………………………… 1

Research Objective ……………………………………………………………………………………….. 2

Literature Review ………………………………………………………………………………………. 4

General ………………………………………………………………………………………………………… 4

Previous Experimental Studies ……………………………………………………………………….. 4

Previous Theoretical Studies…………………………………………………………………………… 6

Conclusions of Previous Studies ……………………………………………………………………… 8

Experimental Program ………………………………………………………………………………… 12

General ………………………………………………………………………………………………………… 12

Testing Matrix ………………………………………………………………………………………………. 12

Testing Apparatus …………………………………………………………………………………………. 13

Soil Preparation …………………………………………………………………………………………….. 17

Test Procedures …………………………………………………………………………………………….. 18

Symmetry Conditions ……………………………………………………………………………………. 21

Data Analysis ……………………………………………………………………………………………. 25

General ………………………………………………………………………………………………………… 25

Post-processing and Calibration of Data …………………………………………………………… 25

Results and Discussion ………………………………………………………………………………….. 32

4.3.1Load – Displacement Behavior ……………………………………………………………… 35

4.3.2Ultimate pullout resistance ……………………………………………………………………. 41

Summary ……………………………………………………………………………………………………… 48

Conclusions ………………………………………………………………………………………………. 49

Recommendations …………………………………………………………………………………………. 50

Appendix Raw time history plots of pullout tests ………………………………………………………….. 51

References ……………………………………………………………………………………………………………….. 65

  

 

Introduction

     General

Soil anchors are buried foundations that rely on the soil between the anchor and aboveground structure as the main form of resistance.  The anchor engages with the soil along its length to prohibit uplift and overturning of the anchored structure when subjected to loading such as wind or wave forces.  Soil anchors have historically found use as foundation systems for structures such as transmission towers, guyed towers, anchored bulkheads, offshore structures, and large fabric structures.

Extensive research has been done on the behavior of anchors during vertical pullout at quasi-static rates [1-5].  The majority of literature to date looks to predict, simulate, and/or observe the ultimate pullout resistance.  For this study the ultimate pullout resistance is defined as the peak point of resistance prior to a sustained, or plateaued, resistance.

This ultimate capacity is dictated by the shape of the failure surface during uplift which is further dependent on the relative density of the soil.  Theoretical formulations proposed to predict pullout resistance have been shown to yield widely varying estimates [1, 6, 7].  Meyerhof and Adams [6] recognized the inconsistencies in pullout capacity theories and suggested the inconsistencies could be due to inaccurately describing the failure shape as the anchor engages with the soil.  For example, shallow-embedded anchors in dense sand have shown to form a conical shape up to the surface, whereas deep-embedded anchors tend to form a ‘balloon’ shape.  Estimating the capacity of a deeply-embedded anchor using empirical equations derived from a shallow anchor can prove to be overly conservative and, in contrast, grossly underestimated for shallow anchors predicted with deep anchor theory [6].  It is therefore necessary to capture the failure surface when exploring new parameters to understand the behavior and capacity of embedded anchors.

The majority of research conducted on the pullout capacity of soil anchor foundations has been performed at the static or quasi-static loading rates.  At quasi-static rates, parameters such as soil density and dilatancy, embedment depth, plate roughness, initial stress state, anchor size and shape have all been studied in detail.  The most exhaustive study being that performed by Rowe and Davis [5].  Of these, initial stress state and plate roughness, when pulled perfectly vertically, were determined to have an inconsiderable effect on the pullout capacity.  While embedment depth, soil density and dilatancy, anchor size and shape all had a considerable effect on the buried anchor’s behavior.

     Research Objective

It is apparent from the current literature there exists a gap in data on the effect of different pullout velocities on the pullout resistance [1-14].  This study seeks to fill this gap by performing controlled experimental tests and analysis on soil anchors at various pullout velocities.  The experimental program will be conducted on embedded anchors to investigate the effect of anchor depth, diameter, and pullout velocity on the peak resistance of soil anchors.  A comprehensive analysis will be performed on the collected data, on the ultimate pullout resistance of different pullout velocities and characterizing the level of rate dependency on the size and embedment of the anchor.

An understanding of the resistance of soil anchors under high-velocity uplift could open opportunities for new applications such as anti-ram barriers, guardrails, etc., where investigating dynamic loadings on the component level is required prior to exploring a detailed design.

Experimental data will also provide a means to calibrate and validate strain-rate-dependent constitutive models to explore further applications via numerical models such as finite elements.

PULLOUT RESISTANCE OF SOIL ANCHORS IN COHESIONLESS SOIL UNDER VARYING VELOCITIES BY EXPERIMENTAL METHODS

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