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
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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
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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
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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
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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
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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.
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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
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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
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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
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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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