EVALUATION OF BEARING CAPACITY DUE TO RADIAL...
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EVALUATION OF BEARING CAPACITY DUE TO RADIAL EXPANSION
USING EXPANDING PILES
POUYA MOLKARAIE
UNIVERSITI TEKNOLOGI MALAYSIA
EVALUATION OF BEARING CAPACITY DUE TO RADIAL EXPANSION
USING EXPANDING PILES
POUYA MOLKARAIE
A project report submitted in partial fulfilment of the
requirements for the award of the degree of
Master of Engineering (Civil - Geotechnics)
Faculty of Civil Engineering
Universiti Teknologi Malaysia
JUNE 2013
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ABSTRACT
The engineering community is familiar with different techniques to improve
the bearing capacity and settlement characteristics of weak cohesive soils. Preload
piers or expanding piles are newly invented method for improving soft soils by
means of radial compression. These piers are inserted in the soil and expanded about
three times their original cross-sectional area. Due to this expansion, the surrounding
soil will be radially stressed, which results in consolidation and general stiffening.
The main objective of this research was to investigate improvement in the bearing
capacity and settlement changes in a soft clay (Kaolinite) due to applying a preload
pier. To gain this goal, soil samples of Kaolinite with undrained shear strength of 15
kPa was prepared in cubic boxes. Then two concrete piles were casted in these soil
samples; first one in a cavity expanded by an expanding pile to reach 6 cm diameter
and the other one casted in a bored cavity with the same diameter(without
expansion). Then piles were tested by loading them until failure. Load testing of the
piles yielded a load settlement graph. Based on the graph applying a preload pier
improved bearing capacity of the soil up to 12 percent. Settlement of the pile
decreased by 11 percent.
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ABSTRAK
Komuniti kejuruteraan biasa dengan teknik-teknik yang berbeza untuk
meningkatkan keupayaan galas dan ciri-ciri pemendapan tanah jelekit yang lemah.
Tiang pra-muat atau cerucuk berkembang adalah kaedah yang baru dicipta untuk
menambah baik tanah lembut dengan cara mampatan jejarian. Tiang-tiang ini
dimasukkan ke dalam tanah dan mengembang kira-kira tiga kali ganda luas keratan
rentas asal. Disebabkan perkembangan itu, tanah sekitarnya akan tertekan berjejarian,
dimana ia mengakibatkan pengukuhan dan pengerasan umum. Objektif utama kajian
ini adalah untuk menyiasat penambahbaikan dari segi keupayaan galas dan
pemendapan tanah liat lembut (Kaolinit) disebabkan oleh penggunaan tiang pra-muat.
Untuk mencapai matlamat ini, sampel tanah Kaolinit dengan kekuatan ricih tak
tersalir, 15kPa telah disediakan dalam kotak kubik. Kemudian, dua cerucuk konkrit
di acukan ke dalam sampel tanah; yang pertama dalam ruang yang dikembangkan
dengan cerucuk berkembang untuk mencapai 6 cm garis pusat dan satu lagi di
acukan dalam ruang digerudi dengan garis pusat yang sama (tanpa pengembangan).
Kemudian cerucuk-cerucuk telah diuji dengan mengenakan beban sehingga gagal.
Ujian beban cerucuk menghasilkan graf pemendapan beban. Berdasarkan kepada
graf, penggunan tiang pra-muat, keupayaan galas tanah meningkat sehingga 12
peratus. Pemendapan cerucuk pula telah dikurangkan sebanyak 11 peratus.
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TABLE OF CONTENTS
CHAPTER TITLE PAGE
TITLE OF PROJECT
DECLARATION
DEDICATION
ACKNOWLEDGEMENT
ABSTRACT
ABSTRAK
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF ABBREVIATIONS
LIST OF SYMBOLS
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1 INTRODUCTION 1
1.1 Background of the Study
1.2 Mechanisms Of Improvement By Preload Piers
1.3 Research Motivation
1.4 Objectives
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2
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2 LITERATURE REVIEW 4
2.1 Background of Information 4
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2.2 Determination of bearing capacity based on load test
2.3 Soil improvement methods
2.3.1 Introduction
2.3.2 Prefabricated vertical drains
2.3.3 Stone columns
2.3.4 Deep soil mixing
2.3.5 Jet grouting
2.3.6 Soil nailing
2.3.7 Mini-piles
2.4 Influence of improvement methods on bearing capacity
2.5 Concept of preload piers or expanding piles
2.6 Installation process of preload piers
2.7 Preload pier improvement mechanism
2.8 Zone of influence during installation of preload piers
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8
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3 RESEARCH METHODOLOGY 26
3.1 Introduction 26
3.2 Physical modeling
3.2.1 Soil sample preparation
3.2.2 Expanding pile embedment and concrete pile
casting
3.2.3 Casting of concrete pile (in non-expanded soil)
3.2.4 Load testing of concrete piles
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4 RESULTS AND DISCUSSIONS
4.1 Introduction
4.2 Basic test results
4.2.1 Water content and unit weight
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4.2.2 Atterberg limits
4.2.3 Permeability test
4.3 Load test results
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4.4 Comparing load test results with a numerical modeling
results
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5 CONCLUSIONS 57
5.1 Summary of the work 57
5.2 Conclusions 58
5.3 Suggestion for future works 59
REFERENCES
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LIST OF TABLES
TABLE NO. TITLE PAGE
4.1
4.2
fall cone test results for liquid limit determination
Falling head permeability test results
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LIST OF FIGURES
FIGURE NO. TITLE PAGE
2.1
2.2
2.3
2.4
Plot of load vs. net settlement
Determination of load bearing capacity
Determination of load bearing capacity by Chin Fung
Kee (1977) method
Butler and Hoy’s Method (1977)
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2.5
3.1
3.2
Configuration of preload pier(Biringen, 2006)
Cubic boxes for filling soil
Mixing clay and water in the mixer
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3.3
3.4
3.5
3.6
3.7
3.8
3.9
3.10
3.11
Preloading soil samples
Determination of undrained shear strength by Vane shear
test
Maintenance of soil sample
Expanding pile (in expanded state)
Extracting soil by the T-shaped extractor
Extracted cylinder of soil for expanding pile embedment
Soil expansion due to inflation of the expanding pile
Monitoring the diameter of the expanding pile
Placing a plastic shield to prevent bulging of the soil
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3.12
a) The steel bar and the screw b) narrow rods welded to
the steel bar
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3.13 Concrete casting
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3.14 Casted concrete pile
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3.15
3.16
3.17
Load cell and hydraulic jack setting
Steel plate and steel pipe for loading (Loading is in
progress)
Load testing mechanism. Steel plate, steel pipe, load cell,
LVDT
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4.1
4.2
4.3
4.4
Liquid limit determination by Penetration-moisture
content graph
Load settlement graph for piles in expanded and
non-expanded soil
Load settlement graph. Settlement comparison
Load-settlement graphs for piles with and without
expansion, in different diameters
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4.5
Simulation of stone column expansion: (a) model of
improved soil; (b) modeling column expansion; (c)
discretized improved soil.
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LIST OF ABBREVIATIONS
FEM - Finite Element Method
ASCE - American Society of Civil Engineers
ASTM - American Society for Testing and Materials
BS - British Standard
LVDT - Linear Variable Differential Transformer
LL - Liquid Limit
PL - Plastic Limit
PI - Plastic Index
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LIST OF SYMBOLS
c’ - Effective cohesion
u Pore pressure
' - Effective friction angle
γ - Unit weight
γw - Unit weight of water
- Total normal stress
k - Permeability
D - Diameter
t - Time (second)
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CHAPTER 1
INTRODUCTION
1.1 Background of the Study
Construction on soft soils remains a trouble making foundation engineering
challenge as usually a significant load is imposed by the new structure and the
available shear strength of the soft clay is low thus applying a method for improving
of soil characteristics seems to be necessary. The engineering community is familiar
with different techniques to improve the bearing capacity and settlement
characteristics of weak cohesive soils. Some of the in situ techniques to improve the
properties of soft ground (e.g. soft clays) prior to placement of foundations are
preloading, deep cement/lime mixing, stone columns, grouting (Henn, 1996;
Moseley, 1993).
Besides these better known methods, a novel approach has recently been
proposed to achieve soft ground improvement by means of applying radial
compression using Preload Piers. Preload Piers can be performed as pre-treatment or
site improvement before a structure or embankment is built. They are intended
especially for soft ground construction and can be placed anywhere between ground
surface to 15 m depth at relatively close spacing (about 2 m on center). Preload piers
can be utilized to stiffen soil at a cost which is significantly less than that
encountered in using aggregate piers or other soil reinforcing systems.
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1.2 Mechanisms Of Improvement By Preload Piers
The idea behind the Preload Piers is densification in the radial direction due
to consolidation of the soil. The improvement would result in an increase in the
stiffness of the soil which enables to solve the problems raised by soft soils. The
preload pier method includes the boring a cavity in the section of ground and
applying at least one elongate expandable member which can be distended from
normal to expanded configuration, which will provide densified soil adjacent to
expandable member.
Preload pier technique could be applied in soft clays in two main functions.
First method is to place piers directly under foundation, thus each of them assist load
bearing capacity duo to an increment in side friction. Second, is to install preload
piers as a group, so they improve the surrounding soil by densification and radial
consolidation and will result in a pier-soil composite with enhanced stiffness and
strength, which is capable of bearing loads without exceeding settlement limitations.
1.3 Research Motivation
A search of the literature review has indicated that researches have done
mostly on load capacity of vertically loaded piles. Seed and Reese (1957)
investigated the influence of adjacent soil on the load capacity of a pile by focusing
on the improved skin friction, but not on the stiffness improvement of the soil.
Moseley (1993) studied influences of preloading on the stiffness of the soil by
focusing on soils which are preloaded vertically but a theoretical approach which
provides prediction in vertical stiffness changes in response to radial preloading is
not investigated so much. One of the few researches around vertical stress and strain
changes in soils due to preloading has done by Biringen (2006). This research
investigated improvement in vertical stress-strain-strength properties of a normally
consolidated soft clay in response to preloading in radial direction.
3
In summary, going through literature has shown that an investigating of
changes in bearing capacity of a soft soil due to Preload Pier application and
consequent improvement in bearing capacity adjacent to pile is a soil mechanics
problem has received little or no attention. Also, it is not clear how much bearing
capacity improvement can be expected due to stress changes caused by a radially
expanding pier. Thus my research will focus on investigating influences of installing
expanding piers on bearing capacity of adjacent soft clay.
1.4 Objectives
In this study, I will investigate influence of applying preload pier in a soft
soil. Focus of this study will be on monitoring changes in the soil duo to radial
expansion of a pier. The main objectives of this study are:
i. To determine ultimate bearing capacity of the soft soil adjacent preload piers
ii. To investigate settlement in the soft soil due to installation of expanding piers
iii. Modeling a preload pier in expanded and non-expanded status, and
comparing the results.
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