PROJECT FINAL.pdf

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EXCAVATION PROCESS

OF THE ICÔNE TOWER

CONSTRUCTION ENGINEERING PROCESS

RESEARCH PROJECT

Presented to:

Dr. Jian Hao

Prepared By:

Ali Lahlou

Omar Aoude

Jacob Peterson

Moamen Elgabry

Thomas Connolly

Mohamad Mashal

Mahmoud El-Koury

EXCAVATION PROCESS OF THE ICÔNE TOWER I



Certification of Originality

“We certify that this submission is the original work of members of the group and

meets the Faculty’s Expectations of Originality”

Name Student ID# Signature Date

Jacob Peterson 6319270 April 1, 2015

Thomas Connolly 6338828 April 1, 2015

Omar Aoude 9761373 April 1, 2015

Moamen Elgabry 6383033 April 1, 2015

Mahmoud El-Koury 1972367 April 1, 2015

Mohamad Mashal 1129163 April 1, 2015

Ali Lahlou 5993601 April 1, 2015

EXCAVATION PROCESS OF THE ICÔNE TOWER II



Table of Contents

Certification of Originality II

Table of Contents III

List of Figures VI 1 Introduction 1

2 Pilling Implementation 2

2.1 Introduction 2

2.2 Previous Site Investigation 2

2.2.1 Classification of Soils 2

2.2.2 Classification of Rock 3

2.2.2.1 Engineering properties of the rock masses 3

2.2.3 Tests for investigation site 3

2.2.3.1 Methods of In-situ tests for the exploration of soils 3

2.2.3.2 Laboratory tests for the exploration of soils 4

2.2.4 Classification of soils 5

2.3 The adequate piling 5

2.3.1 Definition and components of pile 5

2.3.2 Principle 6

2.3.3.1 Type 1. Displacement piles 6

2.3.3.2 Type 2. Replacement piles 7

2.4 Comparison of methods 9

3 Adjacent Buildings Retaining System 10

3.1 Introduction 10

3.2 Building A: (Concrete Wall Foundation) 11

3.2.1 Foundation Characteristics 11

3.2.2 Retaining System Description 12

3.2.3 Installation Process 13

EXCAVATION PROCESS OF THE ICÔNE TOWER III





5.3 Design loads and other considerations 27

5.3.1 Work Sequence: 27

5.3.2 Excavation 27

5.3.3 Installation of timber Lagging 28

5.3.4 Backfilling 28

5.4 Advantages and Drawbacks 28

5.4.1 Advantages 28

5.4.2 Drawbacks: 29

6 Rock Excavation 30

6.1 Introduction 30

6.2 Drilling 30 6.2.1 Drilling Equipment 30

6.2.2 Drilling Methods 32

6.2.3 Limitations of Drilling 32

6.3 Blasting 33

6.3.1 Fundamentals of Blasting 33

6.3.2 Preparation for Blasting 34

6.3.3 Blasting Method 34

6.3.4 Equipment 34

6.3.5 Limitations of Blasting 35

6.3.5.1 Air Shock Waves 35

6.3.5.2 Ground Vibrations 35

6.3.5.3 Fly Rock 35

6.4 Alternative to Blasting Method 36

7 Work Site Organization 37

8 Conclusion 38

9 List of References 39

EXCAVATION PROCESS OF THE ICÔNE TOWER V



List of Figures

Figure 1: Displacement Piles Process 7

Figure 2: Replacement Pile Process 8

Figure 3: Excavation Site-Google Map 11 Figure 4: Detail of retaining system 12

Figure 5: Building B Retaining System detail 15

Figure 6: Building B Retaining System details 15

Figure 7: On site 17

Figure 8: Components of Tiebacks 18

Figure 9: Continuous flight auger in action 22

Figure 10: Top hammer drilling mechanism 31

Figure 11: Top hammer drilling rig 31

Figure 12: Stages of rock blasting 33

Figure 13: Rock blasting hazards 36

EXCAVATION PROCESS OF THE ICÔNE TOWER VI





2 Pilling Implementation

2.1 Introduction

The Icône project is an enormous project and in such projects the foundation and

excavation are of immense importance in the early stage of the construction process.Consequently, the piling implementation is an essential part of that stage; therefore it requires

to be studied in full detail.

For this building, the piling will be implemented on the perimeter of the excavation

site. In order to transfer the vertical force due to the pressure of the soil around the site

during the excavation phase. Furthermore, the piles will serve as the foundation walls of the

structure. A total of five stories will be excavated in order to realize this project. This will be

achieved by removing about three stories worth of soil and the rest will be the bedrock,

2.2 Previous Site Investigation

As it is mentioned above, and extensive and accurate investigation of the site must be

done before the construction of any project. Every project differs from one another due to the

fact the composition and type of the soil will be different. This investigation is instrumental

in choosing the method of installing the piles that is appropriate for the specific excavation

site.

2.2.1 Classification of Soils

The first layers of soil consist of various minerals and organic material, and the

composition of the soil is different creating a number of soil types with their own

characteristics. In order to differentiate these soil types and classify them, we have to do

multiple tests. There are two main types of soil:

! Coarse – grained soils consists of particles that are visible to the naked eyed (gravel

and sands are referred as cohesionless and non-cohesive soils).

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! Standard penetration test (SPT)-This test investigates about the thickness of bearing

strata, which is a layer of rock or soil. Results give an empirical value that serves to know

the load capacity and estimation of the angle of friction.

! Static cone test-This test is used to determine the sleeve friction and point of

resistance for driven piles. Estimation of the shear strength of soils and reliefs the

production of more detailed soil profiles.

! Pressuremeter- This test is used to estimate the modulus of the soil.

! Plate-bearing test-This test aids to get the shear strength and modulus in all kind of

soils.

! Simple permeability test-This test is done with the interest of getting the estimation

of flow in permeable gravels and fissured rocks. This element, this information let to knowthe strength of the rock.

2.2.3.2 Laboratory tests for the exploration of soils

! Grain-Size tests: during this test, the coarse-grained soil with particles greater than

0.75 mm is the first to be sieved. Following that, a hydrometer test is carried out to analyze

the remaining fine-grained soil. Consequently, resulting in the classification of the soils

with respect to their weight.

! Atterberg Limits: This test classifies the soil with respect to its engineering behavior

such as the degree of plasticity, known as the plasticity index. To achieve this, the liquid

limit (wL) and the plastic limit (wp) must be determined. The plasticity index, which is the

factor that is used to classify the soils, is determined by taking into account the water

contained in the soil and the plasticity limit.

Ip= wL - wP.

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2.2.4 Classification of soils

The soils can be classified in various ways, which will depend on the characteristics of

the soils. Therefore, the more criteria we have of the soil, the better our understanding of the

soil will be.

! Classification by Compactness: A term describing the compactness condition of a

cohesionless soil in a qualitative manner. This is usually deduced from the results of a

Standard Penetration Test (SPT)

! Classification by Undrained Strength: This classification is achieved considering the

undrained strength. The results can vary from very soft to very hard consistency. In order

to obtain better results, we must have correlation between this undrained shear strength

and the SPT N-Index values. Quite simply, this is a classification for fine-grained soils.! Classification by sensitivity: Soil sensitivity can be defined as the ratio of intact to

remolded undrained shear strength. This test is measured by two ways. The first method is

conducted in the laboratory using the Swedish method of fall-cone. On the other hand the

second method is performed on site and uses the vane test which will aid in recording the

maximum torque applied to the soil before failure.

2.3 The adequate piling

2.3.1 Definition and components of pile

Piles are the foundation structures that transfer loads to different levels of the soil,

which have different mechanical characteristics. The latter must meet certain requirements in

quality to avoid any movement in the case of rupture or failure in the soil. Piles consist of

three parts: the end bearing, the pile and the pile cap. The length of anchor is defined by the

distance from the beginning of the rock layer down to the end of the pile. The anchor length

is in the resistant layers, which is the rock in our case, which are going to absorb the loads.

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2.3.2 Principle

The laws of equilibrium tell us that the sum of the forces acting on the structure must be

equal to zero. Ultimately, the load applied on the pile must be equal to the friction, whether

positive or negative, and the bearing at the end of the pile. In this case both will influence the

pile equilibrium.

Remarks about the friction created by the pile during the implementation:

When the pile goes into the soil, the displacement can occur in two ways. Either, the

pile moves faster than the soil displacement, or the soil moves faster than the pile

displacement. This results in two types of frictions: positive and negative friction. The friction

due to the soil against the pile goes upward means a positive friction. This can occur as the

pile enters the soil and the soil moves in the same direction as the pile movement. However,soil doesn’t move with the same speed as the pile, ultimately creating friction in opposite

direction, upward. On the other hand, negative friction is determined when the soil

displacement is in the same direction and faster than the insertion of the pile into the soil.

(Charlesrobert, 2006)

2.3.3.1 Type 1. Displacement pilesEquipment: the most used method is using the hammer impact. Usually the weight of

the hammer is about 50 KN, which is two times the weight of the pile. The diesel hammer lets

the hammer fall at different heights. There are singles- acting hammer and double-acting

hammers. The latter applies a force downward resulting in greater impact.

5 of 35

! Banut 850 piling rig, hydraulically operated machine with forward, backward and

side-to-side raking facilities. Overall height of rig 25m.

! Pile driving rig with hanging leaders

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Method: totally pre-formed displacement piles, with either tubular or solid sections, are

driven into the soil. The movement is by jacking, vibration and or driving.

Vibratory method is often used for installing sheet pilings and mostly achieved by

hammer impact. Three classes of piles are found for this type of piles: Auger screw piles,

totally preformed pile and driven cast-in-place. Furthermore, these different types of piles

can be made from various materials such as steel, concrete and timber or a combination of

two.

Limitation: the head of the pile can experience distortion or get damaged when the

mile is made of concrete. Noise and vibration may cause major problems. Additionally, the

effect of moving the soil can produce a major problem on adjacent structures.

Figure 1: Displacement Piles Process

2.3.3.2 Type 2. Replacement piles

Equipment: Track-mounted Drill, which consists of a mast and a boom, is considered asa light model of track-mounted drill and is up to 80-100 ft deep.

! Auger Rig with Kelly bar drive.

! Auger rig (spinning off) spoil.

7



Method: (Piles and Pile Foundations 2012) For this type of pile, an actual cylindrical

hole with a temporary boring wall support is created in the soil, and then is filled with pre-

cast concrete.The excavation part could then be completer by Percussion boring, rotary

boring, continuous flight auger piles and micropiles . This type of piles has three classes

consisting of; bored cast-in-place, grout-intruded CFA and concrete-intruded CFA. Sequence

for the installation for one of this type of piles:

! Driving the tube into the ground (using the hammer impact)

! Driving until the required depth is reached

! The reinforcement cage is place in the hole and then filled with concrete

!

Compact the concrete by vibrating as the tube is withdrawn! Complete concrete pile

Limitation: The concrete is not poured in the best conditions and it cannot be inspected.

This method also takes ground off the soil, leading to settlement of the structures.

Figure 2: Replacement Pile Process

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2.4 Comparison of methods

Piles are of immense importance for the foundation of the project as they support the

structure, retain walls during the excavation and being part of the structure form of the future

foundation walls. For the best method to be chosen, a thorough investigation of the soil must

be done. It is recommended that the depth of exploration should be at least one and a half

time the width of the loaded area. From a structural stand point, if the piles have a

considerable length, we must implement reinforcement in the form of steel rods in the

concrete piles. Consequently, this will increase the strength capacity of the pile. For the Icône

Project, the piles used are also subjected to bending due tothe the pressure of the surrounding

soil.

Furthermore, the groundwater conditions are of equal importance. It is essential togather a comprehensive profile of the soil in order to have a better understanding of the site

and be ready for any future complications. Other aspect that should be taken into account

during the site investigation:

! Seismic risk

! Aggressive soil conditions

! Possibility of aquifer pollution (If the ground is contaminated)

! Acceptable levels of noise

! Sensitivity of neighbouring structures

! Vibration and soil displacement

The selection of a suitable pile type is controlled by external factors such as; access

conditions, cost, vibration or noise level. Thus, there is no simple way to choose the best pile

for the prevailing soil with its different characteristics. (Viggiani, Mandolini, & Russo, 2012)

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3 Adjacent Buildings Retaining System

3.1 Introduction

New construction projects require the demolition of existing structures and excavation

for the new building. The construction activity with the highest potential impact on adjacent buildings happens during the excavation process for the new building. The excavation

process creates a weakness in the bearing capacity of the soil. Due to the removal of existing

soil, the adjacent buildings might be subjected to severe settlement or complete destruction.

As a result, it is required to install temporary earth supports such as cantilevered systems,

anchored systems strutted systems. A proper adjacent system is determined after making a

field investigation by the engineers working on the project. The Icône towers is a project

taking place in the downtown of Montréal and expected for completion in the spring 2016.

Two major buildings are located on the perimeter of the studied excavation site. Building A is

located on 1221 Blvd René-Lévesque West and building B is located on 1181 Rue de La

Montagne. For the Icône Towers, two different rating systems were determined for the

building. First, there is the concrete foundation system designed for building A. The retaining

system was easily stabilized by a standard retaining system due to the fact that the

foundation of the building was made of concrete. However, for building B, engineers were

obligated to come up with an alternative innovative system to fit the building’s

particularities. There was an old stone foundation where building B was planned to be built.

Under these circumstances, the engineers decided to design a system to save the old stone

and set the retaining system.

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3.2.2 Retaining System Description

Figure 4: Detail of retaining system

The retaining system consists of a 90° metal brackets, which are mounted on rails

connected to the piles. All piles exposed to the foundation wall of building A have their own

brackets, which will create an upward force on the foundation footing. The details of the

retaining system are shown in figure 2. It can be seen that the bracket is placed under the

footing to reduce settlement that could occur in the future. The mechanism of this bracketworks by calculating the soil bearing capacity under the footing and the same load will then

by applied on the bracket to recreate the equilibrium state. This kind of bracket is called

active. All calculation needed for the bracket are determined by engineers and regulated by

construction codes.

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3.2.3 Installation Process

This is considered a regular installation process for retaining systems in construction

sites. After the workers finish the general excavation phase and reach the planned depth of

the foundation footing, a temporary access hole is dug under the footing and around the pile.

Then, workers will be asked to weld rail on the pile and mount the bracket on the rail. At this

moment, the bracket is loose and doesn’t apply any force on the footing. After making sure of

the placement of the bracket, rail and of course the pile itself, a hydraulic jack will be placed

under the bracket, which will apply the necessary loads to prevent settlement of the footing.

The last step is to weld the brackets to rails to permanently fix the two components together.

Each of the brackets is done individually one by one. This way, the risk of settlements due to

the operation under the structure is minimized.

3.3 Building B: (Stone Wall Foundation)

3.3.1 Foundation Characteristics

As previously mentioned, there was some technical challenges faced by the engineers

and they had to plan an innovative design to overcome these problems. After the general

excavation was complete, engineers found that the foundation of the building was astonewall, which is unpredictable and could affect the work progress of any project. What

added more difficulties is the lack of information for the adjacent building and no details

were determined for the depth of this stone wall. This required more caution while placing

the retailing system for building B.

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3.3.2 Retaining System Description

The main components of the retaining system for building B are likely to be the same as

building A. The engineers used a 90" metal bracket mounted on rails, which are fixed on the

pile. However, there are some updates made on the regular system to fit the particularities of

the site for building B. First, two brackets are mounted on each pile. They are installed side

by side on a U shaped plate, which is connected to the pile. This provides a double surface

contact between the brackets and the stonewall, which will reduce the point stress at every

bracket. Moreover, there was more usage of the brackets as there was no single footing as in

building A. The workers had to add more brackets at multiple depths of the excavation. This

will provide more uniform stress distribution on fragile stonewall foundation. Furthermore,

the engineers couldn’t specify the total depth of the wall foundation, so the addition of brackets will ensure safety and secure the building more. This system is considered a passive

system as no upward forces were applied on the brackets. The reason for taking this path is to

reduce the total stress on the stonewall. Engineers couldn’t collect enough information about

the stonewall, such as the total depth and the strength of the stonewall and if it can resist the

applied force. As a result, no upward forces are required to provide the necessary retaining

capacity and this was the way the stress is distributed over the brackets grid. Finally, the

same way as for building A, every pile exposed to the foundation wall has brackets on it.

3.3.3 Installation Process

The installation process for building B was almost the same as building A.

Nevertheless; some changes were made on the system to fit the site particularities for

building B. First, the installation for the metal brackets on the piles didn’t change from

building A. On the other hand, connecting the piles to the foundation stone wall was

different than that of building A. The reason for that is the missing information about this

building and the bottom bracket won’t fit the whole foundation, which could cause some risk

on the adjacent building. To overcome this difficulty, engineers decided to remove one stone

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3.4 Conclusion

3.4.1 Comparison between the two methods

Engineers working on the Icône towers came up with two different retaining system

based on the site characteristics for the two buildings. First, an active system was used for

building A while a passive one was assigned for building B. The differences between the two

systems were explained in the previous sections. Building B consisted of an alternative

method that was designed just for this project to fit the special requirements of the building.

3.4.2 Risk Involved

Settlements can occur while installing such systems as buildings are subjected to

instabilities due to the excavation process. These settlements should be monitored using

settlement’s sensors placed at the critical locations of the structure. The values of settlements

of the adjacent building should be within the range specified by construction codes. The

settlement can be seen on buildings in the form of cracks around windows or openings in the

walls. If settlements weren’t monitored regularly, major collapses and the damage of the

buildings can occur and cause catastrophes to the contractor.

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4 Tiebacks

4.1 Introduction

According to (Allen and Iano, 2004), tiebacks are basically materials which are used in

the construction industry for the provision of horizontal resisting force for the restrainingwalls. The main reasons why tiebacks are always used is that there is a need to avoid

noticeable deflection of both the interior and finish walls and also reduction of earth

pressures experienced by the permanent walls which are constructed behind temporary

walls. Their use involves grouting stranded steel or steel bar of sufficient strength through a

rock or soil, which is behind a wall that is supposed to be restrained. What should be noted is

that their use is restricted to locations which have rocks and the ones whose soil is neither

soft clays nor silt. The other structures which can be used instead of tiebacks are the rakers

and braces. Tiebacks were majorly used in the construction of the ICÔNE project for retaining

walls around its perimeter except for a few areas where corner braces were used. Areas,

where corner braces were used, are in the adjacent building retaining walls whose location is

at René-Lévesque Boulevard West (Skyscrapercity.com, 2015). Because the tiebacks are the

ones which are mainly used for the support of the retaining walls at the construction site, this

section is intended for their discussion of their structure to their full installation procedure.

The figure below shows how tiebacks have been used in the Icône construction site.

Figure 7: On site

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4.2 Features of tiebacks

Tiebacks often pose various components which make it superior to its function. The

parts are as discussed below and also shown in the figure below (Duncan, 1992).

Figure 8: Components of Tiebacks

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4.2.1 Tendons

These are the steel wires or bars which transfer the horizontal forces from the structure

to the soil or rock. The most common tendon types are the threaded steel bars, cables, wires

or plain bars. The choice of the material being used for this part greatly depends on the

amount of tensile strength and flexibility required. During the Icône construction site tour,

the information provided was that steel wires were used as tendons and that their

incorporation into the rock was at 45 degrees. Features which are checked before a material is

used as a tendon includes its elastic properties, mechanical strength and response to creep.

Proper corrosion protection measures are always employed to the tendons because they are

always in constant contact with the ground. The various corrosion protection techniques

which are commonly employed includes, surrounding the tendons with a fluid such asgrease, coating of the tendons with substances which are non-corrosive such as plastic

sheaths or bitumen and using the tendon as a sacrificial anode.

4.2.2 Anchors

The tendons used to anchor the walls must always be connected to the soil. This is done

through the use of anchors. There are various types of anchors whose choice depends on theconstruction engineer. Anchors which may be used include, a simple cylinder which is

basically a hole drilled into a rock or soil, and then the hole is filled with grout, a cylinder

enlarged though grout pressure which entails drilling a hole as the one above, but the grout is

placed into the hole under high pressure resulting into a bell-shaped area and the final

method which involves enlargement of a cylinder through mechanical means. At the Icône

construction site, the type of anchor that was used was the cylindrical type which was drilled

six meters into the rock. It should be noted that the type of anchor used greatly depends on

the type of soil or whether a rock is used.

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4.2.3 Grout

This is just a mixture which has water, cement and other admixtures. It is very

important in tieback construction as it is the one which develops either the anchor-soil bond

or anchor-rock bond, hence securing the tieback. Its main functions include the transfer of

load from the anchor to the soil or rock as well as acting as a corrosion protector of the

tendons as it eliminates its direct contact with the ground. The composition of grout should

be made in such a way as to make the overall grout less corrosive and also exhibit sufficient

mechanical strength for holding the tendons in place without slipping. The factors greatly

depend on the choice of the mixture together with the contaminants which might be present

in the water used such as sugars, chlorides and organics which may end up accelerating

corrosion. The admixtures which may be added to the grout includes sand which is used as afiller and chemical agents which are used for hardening acceleration, flow improvement and

control of shrinking.

4.2.4 Anchor head

The major function of this part is the transfer of load from the tendon into the retaining

wall. This load transfer is done through structures which are used for holding the tendonsand the metal plates or rails which are used to distribute the loads evenly on the restraining

wall through timber lagging so as to prevent it from being destroyed. The most common

design of the anchor head is a cone or a wedge which is used to secure the tendons through a

tapered hole. This design is efficient because as tensioning takes place, the cones or the

wedges are always forced into the holes, thereby pinching them and locking them together in

place. A well designed and placed anchor head always allows simultaneous stressing of the

wires and individual locking off of the same wires. The anchor heads are always exposed to

the external environmental factors. Hence, special design is necessary so as to prevent them

from undergoing corrosion. The most common corrosion protection mechanism applied to

the anchor heads include the use of plastic caps which are filled with grease.

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4.3 Installation

Icône building is located in Montreal and before the use of tiebacks around most areas

of the construction perimeter, there were specific procedures which were followed to ensure

proper operation after their installation as discussed below (Chen and Duan, 2000).

4.3.1 Site Investigation

This is a mandatory and very important procedure as it leads to a lot of decisions being

made before the construction project commences. The information gathered from this

investigation is used to determine the method to be used for installation of the tiebacks, the

kind of corrosion protection to be employed, the materials to be used for the installation and

finally the capacity and the method of placing the tiebacks. It is at this stage that the rock and

soil samples are analysed, ground water is also analysed to determine the long-term effects of

corrosion and even the Geotechnical stability is investigated so as to determine the

magnitudes and intensity of the horizontal forces which will be exerted by the soil on the

wall. As is the case with the Icône construction site, the municipal utilities such as sewerage

systems and water lines were checked, underground structures such as the tiebacks from the

adjacent buildings and even the foundations of the adjacent buildings.

4.3.2 Drilling

After the investigation of the construction site by the construction specialists and

tieback construction is permitted, the drilling of the holes where the anchors are to be

mounted is done. Incompetency during drilling always leads to failures, and this is why a

competent contractor is always given the duty to carry out this task. The drilling alwaysranges from horizontal to nearly upward vertical holes. The majority of the tiebacks are

always installed in holes which are drilled at 45 degrees so as to ensure reaching of the hard

rocks which are located way below the ground surface. The drilling method being employed

always depends on the material being drilled as in if it is either a rock or soil. This can lead to

21



diamond drills being used or use of blasts which is occasionally used. Rotary drills are the

ones which are used when the drilling matter is soil. This process is always followed by

flushing which is intended to remove the drilled materials. The flushing process always

includes the use of water, air or bentonite slurry. The figure below shows a continuous flight

auger which is used for drilling of tieback boreholes.

Figure 9: Continuous flight auger in action

The first and the second level of tiebacks at the Icône construction site were through the

soil while the third and the final levels were on a rock.

4.3.3 Tendon incorporation

Just after the drilling of the holes has been carried out, the next procedure is inserting

the tendons in place in a process known as homing. A protective cone is always placed at the

end of the tendon to prevent it from damaging the sides of the borehole after which spacers

and centralizers are placed. The function of the centralizers is to keep the tendon wires at thecentre of the borehole while spacers are used to keep the tendons in a parallel position and at

the same time prevent them from tangling and coming into contact with each other as this

may result in high concentration of stress and subsequent failure. What should be noted is

22



that lubrication is provided at the points of contact between the centralizers, spacers and the

individual tendons so as to reduce friction build up during stress conditions.

4.3.4 Grouting

This is the process of incorporating grout into the borehole. It can be done as a single

stage or two stage. In single stage, the grout is placed after the tendon has already been put in

place inside the borehole while in two stages, the primary grout is first placed inside the

borehole then the tendons are inserted not later than 30 minutes after grouting then

afterwards, the secondary grouting is done.

4.3.5 Stressing

This is done about 24 hours after both the tendons and the grout has been put in place.

The main function of this is to ensure that the anchors provides a known load and also to

guarantee the development of the required capacity for testing of the anchors. This process is

done through the use of a hydraulic jack to pull up the tendons while at the same time

ensuring that they are not tangled. It should be noted that the end of stressing, all the forces

in the individual tendons should be equal. The tendons are then secured after stressing byuse of bolts in the anchor head.

4.4 Advantages and disadvantages of using tiebacks in a construction site

The use of tiebacks does exhibit both advantages and disadvantages especially when

compared to other methods such as the use of braces and rakers. They are illustrated below

(Winterkorn and Fang, 1975).

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grout which is the mixture used for keeping the tieback in place either in the soil or the rock,

anchor had which is the outer part of the tieback that that is always in contact with the

restraining wall through timber lagging for even force distribution and finally an anchor

which is the part that connects the grout to the tendon.

The installation process of the tiebacks is started by carrying out a site investigation so

that specific factors are considered for efficient and effective tieback construction, drilling of

the borehole is carried out afterwards using specialized equipment and then tendons are put

in place. Grout is then added to attach the tendons to the soil or rock and then after drying of

the grout which might be 2 days or more, stressing is carried out on the tendons and then

they are held back by the use of anchor heads which are meant to evenly distribute the

horizontal tension forces to the retaining wall though the timber lagging.

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5 Retaining Walls

5.1 Retaining Walls

To be certain that a deep excavation job is executed in a perfectly safe way, the earth or

soil around a work zone must be retained or held back to guarantee it does not collapse. Asmentioned before the method used for retaining was a tieback anchored vertical soldier piles

with timber lagging retaining wall. In this section the theory and application techniques of

the tieback anchors will be discussed.

5.2 Concept and Theory

The role of retaining walls, just like all other structural components that are part of agreater structure is to transfer loads and forces to the right elements, ultimately transferring

these loads to an external component, in our case the ground is the external component. With

timber lagging retaining walls, the lagging refers to the earth retaining element since it covers

most of the walls surface area. Hence, lagging is the section of the wall that transfers the

loads to the equidistant piles located on both sides and driven deep below the ground.

In conventional piles with timber lagging retaining walls, lateral forces acting on the

lagging y the surrounding earth is only transferred to the adjacent piles on both sides of the

wall. Because of this the piles must be placed deep enough below the surface, to insure the

loads are transferred properly, providing long term stability. With the case in hand, the piles

are only cantilevered on 1.5 m in the rock, and due to the importance factor of the deep

excavation, tieback anchors are placed to transfer additional loads from the piles to the

ground. It’s important to take note the significance of the tiebacks and there role in

distributing of forces.By definition, soldier piles with timber lagging retaining walls can be use for either

temporary of permanent purposes. The design process of each situation might differ

according to design guidelines, but the main idea remains almost identical. In our case, the

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retaining walls were permanently installed and used as formwork for upcoming concrete

foundations.

5.3 Design loads and other considerations

When a deep excavation is needed, like in this case, usually a specialized third party

would be brought in for the task; most likely a subcontractor specialized in deep excavations.

This is because design and building process of retaining walls requires a lot of hands on

experience from the contractor.

5.3.1 Work Sequence:

A typical sequence of construction of soldier piles and timber lagging consists of:

1. Perimeter excavation of a soil sample (around 4 ft)

2. Installation of timber lagging and connection to piling

3. Backfilling behind the timber lagging

4. Compacting the backfill.

In the next few paragraphs the construction of timber lagging retaining walls will bediscussed in detailed steps.

5.3.2 Excavation

Excavation for soldier piles is an incremental process done in small portions by

removing the soil in small laters, after which the timber is planted for support. Each removed

layer is around 4 to 5 ft deep, depending on cohesion factor of the soil. Because the face of the

excavated layer is kept unsupported until installation of the timber and the backfill is

compacted, it is very crucial to abide by the maximum height derived from the cohesion

tests.

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5.3.3 Installation of timber Lagging

After each layer of around 4 feet is excavated, piece of wood are installed while making

sure all voids are filled with dry straw. Installing straw between layers stops the backfilling

gravel from leaking or escaping in case of rain which could be common. The straw forms a

sort of paste when wet blocking the leaking. The size of the wood is decided on according to

the lateral loads such that they can support the loads without cracking or deflecting more

than the allowable limit. Type of wood or wood grade can greatly affect its performance.

5.3.4 Backfilling

Prior to placing the top timber lagging part, any voids behind the retaining wall is

backfilled with gravel and compacted to create further support for the soil. This step is done

manually by workers.

5.4 Advantages and Drawbacks

5.4.1 Advantages

With our case, the timber lagging wall was also used as formwork for the concrete

foundations, avoiding additional formwork. This results in a significant formwork savings.

Another advantage is that timber lagging walls are less costly to install relative to other

retaining systems.

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5.4.2 Drawbacks:

One of the differences between timber lagging and sheet piles soldier piles in the

installation method is that timber lagging does not prevent water from seeping into the

excavation. In this case the ground water was not an issue hence sheet piles were not needed.

Some other drawbacks to using timber lagging soldier piles are:

• Its limitation to compatible soils, meaning soil must be cohesive enough

to hold while lagging is under installation and backfilled.

• Deflections must be observed regularly, especially during cold weather

where freezing water can expand and cause an increase in pressure on the wall.

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6 Rock Excavation

6.1 Introduction

The Icône Tower will have five-level underground parking; therefore extensive

excavation of bedrock is necessary as approximately half of the underground levels are belowthe rock layer. To achieve the required excavation depth, blasting was used for this project,

because it is the most efficient method of achieving deep excavation into bedrock. The first

step in the blasting process is drilling. Holes are drilled into the rock in order to place the

explosives in a precise location, ensuring proper rock fracturing and safety. In this section, we

will discuss the methods, equipment and limitations involved with the blasting method of

excavation.

6.2 Drilling

As mentioned, drilling blast holes is the first step in rock blasting. Holes are drilled

deep into rock at various locations for the explosives to be placed in. These holes allow for

the explosives to break the rock apart after exploding. Proper drilling locations are integral to:

ensuring the safety of workers and the surrounding area as well as attaining the required

rock fracturing.

6.2.1 Drilling Equipment

The three main types of drills used for rock excavation are: hydraulic or pneumatic

drifters, down-the-hole drill, and hydraulic rotary drilling (Heiniö, 1999).

Drifters, also known as top-hammer drills, consist of a large drill bit attached to a large

boom. The steel-studded drill bit is driven down into the rock whilst striking and rotating

simultaneously. The percussive force ranges from 2000 to 5000 strikes per minutes while

rotating between 100 to 400 revolutions per minute. As the rock is broken up, the material is

pushed out of the hole using air or water pressure. It should be noted that the entire drill

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shaft is moving up and down during the percussive oscillations, limiting drill depth. These

types of drills are most often powered using large hydraulic pumps, however, some use

compressed air. This was the method used for the excavation of the Icône tower project.

Figure 10: Top hammer drilling mechanism

Figure 11: Top hammer drilling rig

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Down-the-hole drillers are essentially very large jackhammers that operate similarly to

drifters, except the drill bit does not rotate. Like drifters, down-the-hole drillers consist of a

drill bit attached to a boom, however, only the drill bit itself moves up and down. This

feature allows for very deep holes to be drilled, thus, down-the-hole drilling is most often

used for digging water wells and mining. This method was not used for the Icône Tower

project as it exceeded the required depths.

Hydraulic rotating drilling consists of a diamond studded drill bit that is pushed into

the rock while rotating. These drills excrete “drilling muds” to cool the drill bit at the point of

contact because the friction generated during drilling produces large amounts of heat. This

method allows for the greatest depth, capable of drilling kilometers into the earth’s surface.Due to its ability to reach great depths, this method of drilling is primarily used for oil and

gas extraction. It is infrequently used for drilling blast holes.

6.2.2 Drilling Methods

For the purpose of placing explosives, there are two basic methods of rock drilling,

vertical and horizontal drilling (Andrew, Bartingale, & Hume). Vertical drilling is most oftenused; it involves drilling holes from the top down into the bedrock. Horizontal drilling is

used when access to the top of the rock layer is limited. For the Icône tower, vertical drilling

is used because there is full access to surface of the rock.

6.2.3 Limitations of Drilling

The major limitation of rock drilling for the purpose of blasting is that all of the drilled

holes must be arranged in a very specific manner. This is not always possible due to the

nature of the terrain. If the holes are not drilled in the proper patterns with correct depths, it

can cause improper breakage of the rocks or unwanted damage to the bedrock. To combat

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this, the location and depth of holes must be precisely planned and measured (Atlas Corpco

RDE, 2002).

6.3 Blasting

6.3.1 Fundamentals of Blasting

In order to excavate rock using blasting, a hole is drilled in between the solid rock

mass and a free face and explosives placed inside the hole. Upon detonation, the explosives

create an outward compressive force inside the hole, on the free side of the explosion. The

force travels outward to the free face and rebounds back towards the hole, creating both

compressive and tensile stress in the rock section located between the hole and the free face.

These stresses cause the rock to fracture creating small voids and gases created during the

explosion rush into the voids and expand rapidly. This rapid expansion of gas causes the rock

section to break apart and collapse (Heiniö, 1999). This process is illustrated in the figure

below.

Figure 12: Stages of rock blasting

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6.3.2 Preparation for Blasting

A specialized contractor is often responsible for the blasting process as it is an intricate

and delicate operation. The subcontractor must meticulously every aspect of the blast.

Factors that are important to the outcome of a blast are: rock properties, explosive type, blast

geometry, surrounding environment, and safety. Planning is the most important part of the

blasting process because unlike other construction processes, after detonation of the

explosives, the affects are irreversible.

6.3.3 Blasting Method

The blasting method used in this project is known as controlled blasting. This method

uses large charges and tightly spaced holes in order to create smaller, more controllable rock

breakage. This method is slower than other blasting methods but is desirable because greatly

reduces the risk of over blasting (Andrew, Bartingale, & Hume, 2011). Over blasting is the

unintentional fracturing of nearby rocks, it is very important to avoid over blasting in an

urban area as it may affect the structural integrity of adjacent buildings. Another type of

blasting is called production blasting. This method uses large charges and widely spaced

holes in order to maximize rock breakage. This method is commonly used for mining and isnot suitable for usage in urban construction.

6.3.4 Equipment

In addition to the drilling equipment discussed earlier, the other main component of

blasting is the explosives. Explosives allow for rapid excavation using energy from chemical

reactions, rather than energy from man or machine power. The type of explosives used

depends on the rock: type, hardness and geometry (Andrew, Bartingale, & Hume, 2011). We

were unable to determine the exact explosive type used during the excavation of the Icône

tower, however, it is likely that the explosive used ammonium nitrate/fuel oil (ANFO) as it is

the most widely used explosive used for excavation purposes. ANFO emerged as a improved

34



alternative to traditional dynamite due to its low cost, water resistance and performance in

small diameters (Cook, 1974).

6.3.5 Limitations of Blasting

When conducting excavation using blasting in a dense, urban area, there are three

factors that need to be carefully controlled to ensure the safety of the public and surround

environment. These three factors are: shockwaves, ground vibrations, and fly rock (Heiniö,

1999).

6.3.5.1 Air Shock Waves

When an explosive is detonated it produces high-pressure, energy carrying wavesthrough the air. If at a close enough proximity, these waves can injure or even be fatal to

people. Calculations of the potential shock wave distance must be done in order to keep the

workers and the public at a safe distance when detonation occurs. Furthermore, studies of the

surrounding buildings must be done so that the shockwave does not cause structural damage

or break windows.

6.3.5.2 Ground Vibrations

In addition to shock waves, ground vibrations caused by explosions can also damage

adjacent buildings. Similarly to air shock waves, ground vibrations are energy-carrying

waves except they propagate through the bedrock as opposed to the air. It is necessary to

determine the wave velocity, wavelength and condition of surrounding buildings and roads

in order to avoid potential damage (Heiniö, 1999).

6.3.5.3 Fly Rock

Another hazard associated with the blasting method is known as fly rock. Due to the

massive forces produced by explosions can cause small broken off pieces of rock to travel at

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8 Conclusion

In this report we discussed in detail the steps involved in an excavation process for a

project located in a downtown location. When performing downtown excavations, particular

attention must be paid to the surrounding environment. Buildings, streets and sidewalkssurround the site; thus, the soil retention technique used must be chosen carefully to ensure

the safety of workers and the general public. Furthermore, vehicle circulation around the site

also has to be considered during the excavation.

The excavation process is complex and many techniques are available to contractors.

However, the techniques for piling implementation used were soldier piles with timber

lagging and tieback anchors. The excavation also required retaining the adjacent buildings as

well as rock excavation using blasting.

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