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Construction Deep Foundations
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Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-1
CHAPTER 9: THE MANY WAYS TO CONSTRUCT DEEP FOUNDATIONS
But if a solid foundation is not found, and the site is loose earth right down, or marshy, then it is to be excavated and cleared and remade with piles of alder or of olive or charred oak, and the piles are to be driven close together by machinery, and the intervals between are to be filled with charcoal. Then the foundations are to be filled with very solid structures. Let double-walled formwork to be set up in the designated spot, held together by close set planks and tie beams, and between the anchoring supports have clay packed down baskets made of swamp reeds. When it has been well tamped down in this manner, and is as compact as possible, then have the area bounded by the cofferdam emptied and dried out by means of water-screw installations and water wheels with compartmented rims and bodies. The foundations are to be dug there, within the cofferdam.
From De Architectura written by the Roman Engineer Vitruvius, 100 BC
We learned in Chapter 8 that the objective of excavating deep to build a foundation is to remove
soft and weak material to a depth that has soil sufficiently strong to support the structure that will
eventually rest on it. The Romans already found, however, that many times the depth at which
appropriate soil can be found renders excavation technically infeasible or economically
impossible. As Vitruvius in 100 BC so eloquently wrote, there exists an alternative namely piles
standing on solid rock and carrying the stucture. In the Roman time, such piles were made of
alder or of olive or charred oak wood and driven into the ground by some kind of a machine.
Subsequently, the heads of the piles were covered with a solid structure thus serving as the base a
building such as a bridge column or a tower. While this technology has drastically evolved over
time the basic premise has not changed. Deep foundations offer an economical solution to build
in areas where the subsurface consists of soft soils to a large depth. This chapter will review the
many innovative approaches that contractors and geotechnical engineers have developed to
respond to the many different ground conditions that they face ini different parts of the world.
Table 9.1 Topics Covered in Chapter 9
CONSTRUCTION PLANNING & CONTROL
Equipment Methods Materials Managerial-
Engineering Tools Rules &
Standards Modern Tools
Trucks
Impact hammers
Pile driver
Vibratory drivers
Swinging lead
Auger drill
Trench Cutter
Hydrocylcone
Rakers
Mobile anchor drill
Kelly bar
CFA
Core barrels
Rock bucket
Rock auger
Progress. cavity pump
Pipe tremie
Franki Pile
Pressurized
Caisson
Cast-in-Place
piles
Deep Mixing
Method
Underreaming
Jet grouting
Grout batching
Open caisson
Bentonite
Slurry
Precast Piles
Cement Grout
Load bearing
piles
Fiber
Reinforced
Polymers
Colloidal
mixture
Sheet piles
Chemical
grouts
Navier-Stokes
equation
Anchor load
Safe pile load
Engineers News
Formula
Case Method
Capacity
Marsh Funnel
Viscosity
Pile toe resistance
Standard
penetration test
Pascal
Piezometer
ASTM A36
Steel
ASTM A82,
A615 and
A884
ASTM A416
A421, and
A882
FHWA-IF-
99-025
ASTM
D1586
PDA
CAPWAP
Accelerometer
Strain Gauge
Pile Integrity
Tester
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-2
9.1 From Wooden Stilts to Jetted Piles
For various reasons humans thought to build structures on grounds that were too weak to support
their weights. One of the most famous examples of what can happen in such cases is the 55.863
m (183 ft 3 in) high and 14,500 metric ton heavy bell tower of Pisa that is now leaning 3.9 m
(12 ft 10 in) at the top. Built off-and-on over 177, starting in 1173, it apparently allowed one of
his most famous citizens, Galileo Galilei, to conduct his famous drop experiments while
Professor at the University in 1590. While not with such dramatic results, building on weak
ground is still creating havoc when not treated with professional expertise and skill. The
following short review marks some of the key innovations that are allowing us today to create
massive supports never seen by the unsuspecting public.
4000 BC Houses Built on
Stilts
A Neolithic tribe called the "Swiss Lake Dwellers" built its houses on wooden piles in the lakes to be safe from attacks. Those piles are preserved still rammed into the bottom of some lakes in Switzerland.
500 BC Piles for
Pons Sublicius
Egyptians, Greeks and Romans built buildings, bridges, roads and viaducts on wooden piling. The Romans built the first bridge across the Tiber River, the Pons Sublicius, on
timber piles (around B.C. 500). Sublicius = "resting on pilings. Buildings in the cities of Venice and Ravenna were built on piles from B.C. 100 to A.D. 400 as well as the first bridge across the Thames River in London in A.D. 60.
500 BC Deep
Drilling
Chinese developed deep-drilling using oxen, wheels and ropes to power rotary and percussion drills. The goal was to mine rock salt from the surface.
230 BC Archimedes
Screw
Greek mathematician Archimedes invents the screw pump after visiting Egypt where he saw the compartmented rotating vertical wheel used to lift water.
1126 Deep Drills in France
French Carthusian monks in 1126 operated a mechanism with a thin rod and a hard cutting tip to drill a deep hole. The rod is struck with a hammer breaking up the bottom of the hole. Artesian wells are named after Artois, France where this took place.
1450 Piling Rigs Sophisticated drop-hammer piling rigs were invented. Francesco di Giorgio was an inventor.
1825 Erie Canal
Locks
Erie Canal locks in New York were constructed with one- and two-ton blocks on the floor (against uplift) supported on a system of 6-foot (1.8 meter) timber piles. Each lock was supported by 700 piles,
1839 Steam
Hammer
James Nasmyth born in Edinburgh, invented the steam hammer. This heavy machine allowed large steel pieces to be forged with great accuracy. A four-or five-ton hammer was lifted by one steam piston and than dropped accelerated by a second steam piston.
1851 Pressurized
Caisson
The first pressurized steel caisson was built for constructing the foundations for a bridge over the Medway at Rochester, in Kent, England,
1875 Steam
Hammer Pile Driver
The Vulcan steam hammer began when the company began to manufacture hammers under the patent of Thomas T. Loomis in 1875. This hammer used many of the main features of the Nasmyth hammer from 1839.
1887 (1925)
Chemical Soil Stabilization
Jeziorsky received a patent for a method to solidify soils with liquid glass (Sodium silicate) also known as water glass or liquid glass. Two holes had to be drilled one for Sodium silicate and the other for a coagulating agent. In 1925 von Joosten developed the concept for use in the field.
1908 Franki Pile
The Belgian Edgard Frankignoul invented an alternative to piling and drilled shafts that offered load capacities that surpassed traditional methods. He pressed the concrete at the bottom of a vertical shaft outwards, lowered a rebar cage, and filled it with concrete.
1939 Precast Piles 1st prestressed precast concrete piles used in Sweden.
1950 (1970)
In-Situ Soil Mixing
In the early 1950s a method was invented in the US of mixing soil using an auger driven down into the soil via a high torque turntable and pneumatically fed with a chemical. When the chemical is thoroughly mixed with the in-situ soil the auger is pulled back. On
30.11.1970 a first patent was initiated in Japan for a method to create subsurface piles/columns referred to as CCP (chemical churning pile.)
1972 PDA The Pile Capacity Computer was introduced (later renamed the Pile Driving Analyzer (PDA)) in 1972. Initially, these PDA units used analog computation with digital readout.
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-3
Figure 9.1: The
tower of Pisa today Figures 9.2: Stratified clays
under the Tower of Pisa
L=Low, M=Moderate, H=High
Rheology 5 m (18 ft)
1 m (3ft)
13m (43 ft)
Sand
Clay
Poor Clay Sand
Soft Clay
Rubble
7 m (23 ft) Sand
1976 WEAP In 1976 Goble and Rausche produced a wave equation analysis program (WEAP), a first public domain software to predict pile capacity modeling the dynamics of diesel hammers.
9.2 From the Accident File Case 1: A vibratory hammer rigged to the 70-ton lattice boom crawler crane was being used to install steel sheet
pilings. The hammer was powered by a power pack, which consisted of a diesel engine that provided the energy required by the hydraulic motors and hammer clamps. The victim's typical location during the steel sheet pile driving task was next to the power pack and outside of the crane's swing radius operating the vibratory hammer. There had been an extended delay in the steel sheet piling installation task due to an obstruction in the ground. During the general contractor's attempt to remove the obstruction, the pile driving crew's downtime led into their scheduled work break. The crane operator spent the downtime and the coffee break inside his cabin, and the victim went to his car but had walked over to the crane, which did not have a swing radius barricade around the rear, apparently to pick up a piece out of the toolbox. Outside the crane operator's line of sight he entered the crane's swing radius when the operator suddenly swung the superstructure without advanced notice crushing the hammer operator between the left rear track and the superstructure. Case 2: A construction worker was in a 15 foot deep trench setting the bottom of a 50 foot long pile weighing 1'500
pounds suspended within the leads of a pile driver. The crane used two hoist lines, one to lift the pile into position and a second to move the hammer in place. Only after a pile is set into its proper position should the hammer be lowered onto the pile held on top by a sleeve. A spotter outside the 15 ft deep trench signaled the operator to lower the pile into its final place by calling for line 1, which indicates the hoist line. Instead of lowering line 1, the operator lowered line 2 which lowered the hammer. While the pile was still suspended and manually pushed into place by the laborer in the trench the 5 tons hammer hit the top causing it to swing into the laborer pushing him into the wall of the steel trench box where he was crushed. Case 3: A specialty foundation company was contracted to drill and insert 63 pilings to underpin the basement and
foundation of a building. To perform the work the company employed a drilling machine which was powered by a diesel hydraulic system. The auger that was used to drill through the concrete floor and into the earth was approximately eight inches in diameter and spun at 200 revolutions per minute. A laborer was employed to assist the machine operator during drilling processes, and was using a shovel to clear the earth brought up by the auger. He had to work immediately next to (within feet of) the revolving auger in order to perform the job and had been warned of the danger of wearing loose clothing around the auger. The laborer had taken precautions that morning to tape his rain slicker close to his body. Nevertheless, as the 34th hole was being drilled an appendage on the auger caught the arm area of the rain slicker, pulled and spun him around the auger multiple times before the machine operator could disengage the machine. The machine operator was unable to immediately disengage the auger because he had walked away from it while it was running, and had to make his way back to the control panel in order to hit the emergency stop button.
9.3 Problems With Building on a Soft Ground
9.3.1 Disastrous Engineering That Became a World Heritage
Any man-made structure depends on a solid foundation to
rest on (Latin solidus = safe, sound, reliable). Not
surprisingly, early structures were built
not close to rivers and lakes but on hills
and mountains strong enough to carry the
load (weight) of the building. However,
commerce and industry depended on the
rivers for transport and power to drive
the many machines. Unfortunately, the
plains that had been created by rivers and
oceans, depositing layers of fine soil,
sand, gravel mixed in with organic
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-4
1904 Diploma as Mechanical Engineer 1906 Work for Austrian construction company 1910 PhD from Tech. Univ. Graz 1913-1916 Army engineer during WWI 1916-1925 Professor in Istanbul, Turkey 1925 Publication of Soil Mechanics based on Soil Physics 1925-29 Visiting Prof. at MIT 1929-1938 Prof. Tech. Univ. in Vienna 1939-1956 Prof. of Soil Mechanics at Harvard
Karl Terzaghi, 1883-1963
material does not provide the same sturdy ground as the mountains. The most renowned
building that depicts visually what can and has happened to many buildings built on the layered
deposits of rivers and oceans is the leaning Tower of Pisa its foundation constructed in 1173 on
the delta deposits of the river Po. Ignoring or not aware of Vitruvius book De Architectura the
foundation engineers did not us piles but a flat solid plate. Tower construction lasted 177 years
as it was repeatedly interrupted for long periods of time (up to 50 years) because of wars and a/or
a lack of money. Today we know that the long spans in between construction were the main
reasons why there is still a standing tower today. The person who helped the engineering
community in understanding the principles behind this phenomenon was Karl Therzagi, born
1883 to an Austrian military family. Through careful experiments with scientific tools that he
invented he singlehandedly established what is known today as Soil Mechanics. He found that
the settlement due to an added load consolidated the different soils in different amounts.
Every soil consists of small grains or, in case of clay, flakes that touch each other leaving some
empty voids in between. In case of the tower of Pisa, located in a river delta, the voids were
filled with ground water creating a saturated soil. While the water was relatively quickly
squeezed out of the sand layers during the construction of the tower the flakes of clay offered
only tiny pores for the water to leave leading to raising water pressure in the pores. As liquids
cant be compressed easily the water will take on the stresses added by the load thus reducing the
stress that would compact the flakes referred to as the effective vertical stress. (Only after water
is squeezed out of the clay will the effective stress that causes the settlement become active. The
consolidation, however, will be slowed down
immediately as the pore pressure will become active
again. As a result, the settlement of clay was a much
slower process than that settlement of sand underneath
the tower. Figure 9.2 shows that the clay had a total
height of 29 m (11 m + 5 m + 13 m) directly
underneath the tower.
Figure 9.3 presents graphical views of the behavior of
clay for the first two construction phases starting in
1173. The solid line shows the theoretical effect of
adding load L1 all at once while the dashed line represents the results of a slow construction
process of that time. It is apparent that the primary consolidation process slows down
significantly when compared to the settlement curve related to instantaneous loading. The long
interruption drawn logarithmically, leads to a secondary settlement of the clay as a result of
creeping clay and the decay of organic material. This long phase, of course, reduced the void
ratio, created a new balance between the soil and the tower, the creep widened the area of
consolidated soil reducing the maximum effective vertical stress before the second construction
phase began which repeated the slow process. On can now easily understand that the long
breaks between the construction reduced the speed consolidation and gave the clay the time to
achieving a new equilibrium. If this would not have happened the tower would most probably
not stand today. Later we will discuss what is being done now to actually reduce the leaning
which a method that we not available even 30 years ago.
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-5
9.3.2 Methods to Avoid Settlements of New Buildings
The consolidation function by Terzaghi taught us that settlement has four main causes:
1) compression index of the soil,
2) void ratio of the soil,
3) thickness or height of the stressed soil strata, and
4) ratio of effective stress caused by the new load.
As we will learn, modern deep foundation methods are geared towards improving one or more of
the root causes depending on the design parameters and the surrounding conditions. Lets see
how some of the most used approaches relate to Terzaghis function.
Figure 9.4 compares schematically four engineering designs that all have one desire in common:
Add little to no stress to the initial or natural stress that has been established over 100s or even
1,000s of years. This is achieved in different ways: b) Excavating a volume of soil that matches
the weight of the new building, d) driving piles, like the Romans, until the tips stand on stronger
material, c) drill shafts or caissons until solid ground is found and fill them with solid material, c)
excavate walls down to solid grounds and create a box on which the upper structure can rest.
c
Saturated Clay Flakes with Organic Material
and Sand
Primary Consolidation Due to Load (L1)
Slowly Adding Load (L2)
Secondary Consolidation of Clay
P1
P1
P0
1 2 3
Settle
me
nt
Load
L1
Settle
me
nt
Load
c
L1
Settle
me
nt
Construction Phase 1
1
a) Natural Soil State b) Building First 2 Floors c) Interruption Due to War d) Adding More Floors
P3
Time (T)
T
T
Load
c
Settle
me
nt T
2
Load
Construction Phase 2
3
L2
Figure 9.3 Consolidation mechanisms of clay under the Tower of Pisa
Figure 9.4 Main approaches to minimize consolidation
= Sandy Material
= Clayey Material
a) The Lucky Tower b) Soil Replacement c) Deep Walls d) Piling e) Shafts and Caissons
'zf > 'z0 'zf = 'z0 'zf >='z0 'zf = 'z0 'zf = 'z0
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-6
In the following sections we will study the main problems that each of the four approaches faces
during construction. As we learn, each requires a unique set of technologies and understanding
of engineering principles mainly related to Geotechnology and Hydraulics. But first, you will
be faced with a complex problem requiring you to comprehend the rich and advanced
technologies that have been developed over the last 3,000 years.
HEADER PROBLEM 9.1: Planning a Deep Excavation and Special Foundation
In Chapter 8, we studied methods that could be used to safely excavate a 60 ft (18 m) deep pit. One
geotechnical benefit of removing such a large amount of material, of course, that it will reduce or even
eliminate the consolidation caused by the weight of the building. In our case, however, the weight of the
new building is larger than the weight of the excavated material and thus will create a larger ground
pressure compared to the original status. Thus, a series of piles that support the building was found
necessary. Figure 9.5 provides the basic information about the subsurface structure.
9.4 Pile Driving Technology
If the architect of the Tower of Pisa had studied Vitruvius De Architectura before the
construction began, there would be no leaning tower and no UNESCO World Heritage site. In
fact, Vitruvius work had been lost but was resurrected by Leonardo da Vinci and Galileo in the
15-Hunderts. Since then, driven pile technology advanced not only in the materials used to make
Watertight Floor
Slab and Walls
Adjacent Building
Very Fine
Sand
New Building
Envelope
Fine
Sand
Clay
Clay
Silty Sand
Piles as Deep Foundations
Rock
Figure 9.5 Sketch of the engineers deep foundation concept
Adjacent Building
Load Bearing Diaphragm Wall
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-7
piles, but also in the use of sophisticated equipment, and lately in quality control devices such as
real-time monitoring of the bearing capacity.
9.4.1 Driving Equipment
In Figures 9.6 and 9.23 four different impact hammers were shown while in Figure 9.22
presented the most common system set-up to drive vertical piles. However, piles are also useful
to introduce horizontal forces, such as from a breaking train on a bridge, into the underground.
For this the lead needs to be positioned in an angle also referred to as batter. Figure 9.6
introduces at crawler crane manipulating a sliding lead thus adding the capability to reach
surfaces not in the same plane as the crane.
Figure 9.6 Flexible Pile Driving System Positions Lead in Various Batters (Angle)
Of special interest in Figure 9.6 is the sliding guide on the boom point that allows the linear
movement. This movement kicks in when the lead hoist rope, attached to the base of the lead, is
winched in or out. The angle or batter can be reached by activating hydraulic cylinder in the
telescopic brace that pushes or pulls the lead off its vertical position. As shown, the telescopic
spotter is able to create positive or negative batters sometimes calls fore or aft batters. In fact, a
so called moonbeam can be mounted on the end of the brace that allows side batters to the left
and the right of the plane made by the boom and crane. For this configurations, three separate
hoist lines are needed for lifting the: 1) Hammer, 2) pile, and 3) sliding lead.
9.4.2 Taxonomy of Driven Piles
Figure 9.7 presents a paradigm for organizing the many different load bearing pile types used in
construction.
Lead
Hoist Rope
Lead Sliding Guide
Telescopic
Spotter/Brace
Lead and Pile in Positive or Fore
Batter
Pile Gate
Impact Hammer
Top Plate, Hammer Cushion, Pile-Cap
(Helmet), Pile Cushion
Lattice
Boom
Truss Lead
Lead in Aft or
Negative Batter
Hoist Line for
Hammer
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-8
Figure 9.7 Overview of Common Foundation Piles
The main grouping is according to the material a pile is made of (steel, timber, concrete, or a
composite). In the following some of the types will be briefly introduced.
1. Steel Piles: Steel piles are produced in many forms but they predominately consist of H-
section or pipes sections that can be welded to reach 120 - 231 ft (36-70 m).
1.1 Steel Pipe Pile: The main specification for pipes is ASTM A252 - 98(2007) Standard
Specification for Welded and Seamless Steel Pipe Piles. Both, the open pipe and the H-section
are low displacement piles as they their cross-section does not require the soil to move aside
possibly causing a lifting on the top. Thus, they can be driven at close distance to each other.
Both can be designed as end or tip bearing piles working similar to free standing columns that
transfer the load from the top to the bottom or the tip resting on a solid stratum. A newly driven
pipe is either left as is or its center or core is excavated and filled with concrete possibly
combined with a steel beam. Of course, the acceptable design load varies widely. For example,
the Army Corps of Engineers specifies the design load for a pipe pile with a concrete core as
between 500 - 1500 tons (455 1,365 t) while without a core the allowable load is only 80-120
tons (73 - 109 t).
Alternatively, one end of hollow pipe can be welded shut with a flat or conical tip making them
closed end piles. A hardened steel tip is extremely helpful in soils that include boulders that
might damage the rim of a hollow pipe or, equally undesirable, deflect the tip from the intended
alignment. To improve the piercing capabilities further, some tips are shaped for particular
conditions. For example, welded on pointed teeth support driving through obstructions and keep
the pile on line when the tip reaches a sloped strata. Like every pile type, pipe piles have
advantages and disadvantages. For example, closed pipes will also result in soil displacement
which, of course, is minimal with open pipes. Also, steel pipes are costly compared to other
material. On the other hand, they can reach extremely deep strata when welded together and are
able to carry high loads when the core is full of concrete. Corrosion resistance can be obtained
LOAD BEARING PILES
3. 2 Precast
3.1 Cast-In-Place
Reinforced
Pre-stressed
Post-Tensioned
Pre-Tensioned
Uncased
Cased
Drilled
Pipe Cased W/ or W/out Mandrel
Shell Cased w/Mandrel
Monotube
4. COMPOSITE
FRP Shell-Concrete Fill
Steel Pipe w/Recycled Plastic
Filled
1.2 H-Beam
1.1 Pipe
Unfilled
1. STEEL
2. TIMBER
3. CONCRETE
S9.1 USACE Pile Driving Engineering Instructions S9.2 USACE Pile Foundations
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-9
with cathodic protection or the application of a coating made of coal-tar epoxy, metalized zinc or
phenolic mastics.
1.2 Steel H-Pile: This is probably the most used toe bearing pile in areas without large boulders.
With sufficient bearing capacity at the tip, Grade 50 steel allowing stresses up to 50 ksi (3.5
metric-tons/cm2) can be selected. H-piles will normally have to meet ASTM A36 with a strength
of 36 ksi (2.5 t/cm2). The design loads lay between 40 - 200 tons (36 -182 t). Like the hollow
pipes, they displace a small volume of soil and thus can be driven with relatively close spacing.
Sometimes, however, the soil can jam up between the flanges and the web. As a result, the
cross-section of the pile will mirror that of a closed end solid pile. Referred to as plugging, the
effect is increased driving resistance and possible soil heaving around the top of the pile.
As with steel pipe, H-piles can be easily spliced together to extend their reach. In the same vain,
piles that extend beyond the final grade can be easily cut with a torch. Naturally, the existence of
boulders and a sloped bedrock asks for special treatment of the tip that prevents damage and
deflection by a hard sloped surface. In fact, tip reinforcement is sometimes specified when it is
known that boulders or thin layers of rock will be met. Corrosion protection is similar to that of
steel pipe piles.
2. Timber Piles: More than 2,000 years before the Romans used timber piles to build solid
foundations in areas with soft underground, lake dwellers in other parts of Europe build their
houses on poles over water. Many of those driven piles have survived 4,300 years with little
damage. Thus, timber piles can be considered the oldest deep foundation structures. They are
made of round, undamaged and straight tree trunks trimmed of all its branches and its bark. Most
common trees used to make piles are Southern Pine and Douglas Fir with allowable stresses of
1.2 ksi. ASTM standard D25, Specification for Round Timber Piles, gives the minimum timber
dimension. Because the timber is slightly tapered, with the tip between 5 9-in (12 -23 cm) and
the butt 12 - 20-in (30-50 cm), these piles are extremely hard to splice. As a consequence,
timber piles are normally restricted to a depth of 66 ft (20 m) with the exception of Douglas Fir
reaching 120 ft (36 m).
Weak points are the soft tip and butt that will be impacted by
a heavy hammer. The potential problem is splitting and even
breaking of the pile body. Akin to a pipe end, the tip of a
timber pile can be reinforced with a pointed shoe or a boot,
shown in Figure 9.8. The use of heavier and more productive
hammers subjects the piles to higher compression forces when
the pile reaches obstructions. The metal point fits on the tip
and can be nailed and bent to fit the diameter of the pile.
Fitting the round boot may sometimes required some
small trimming. On the other hand, the point requires a
careful orientation so that the tip of the pyramid is
perfectly centered and aligned with the centerline of the pile. Otherwise, the pile will be easily
deflected away from its vertical axis. To protect the top of the pile from splitting in heavy driving,
dense subsurface, it is recommended that a metal band is applied at about 1.5 ft (45 cm) from the
top.
Figure 9.8 Timber Pile Point and Boot
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-10
Another method to reduce the driving forces needed is the use of a small water jetting nozzle at
the tip of the timber pile. We will learn later the, jetting will cause the liquefaction of the
surrounding soil and, as a consequence, its resistance to the advancing pile.
As we learned from historical artifacts, timber survives a long time if permanently covered with
water but decays easily when water and air can intermittently get access to the wood.
Naturally, dry wood can easily burn. While the pressure treatment with creosote will not preserve
the wood for ever from decay and wood borers, it will significantly extend its life.
3.1 Cast-in-Place Concrete Piles: Figure 9.7 lists two options for cast-in-place piles, cased and
uncased. The later was added just for completeness since it requires drilling, a topic covered in
the following section. That said, one recognizes the importance of a cylindrical shell that is
driven to a desired depth before it is filled with concrete. In that, the driving the shell that is
closed at the tip end is very much a displacement pile with the same potential for heaving.
Driving of the shell may be done with the help of a shaped mandrel that is inserted during and
removed when done with the driving. Shells driven with mandrel can be made of much thinner
steel (e.g. 1/8 -in) compared to the 1.0 2.5-in (25 63 mm). The mandrel driven shells are
sometimes corrugated which increases the frictional characteristic of the completed pile.
Of course, in soil that contains boulders, driving the shell will face problems that are akin to
driving pipes or H-piles such as deflection. On the other hand, the empty shell can be inspected
via a light and camera prior to filling it with concrete. The visual inspection will focus on
damage or distortion of the shell as well as the cleanliness and dryness of the inside especially
the bottom
3.2 Precast Concrete Piles: Again, Figure 9.7 shows two types, pre-stressed or plain reinforced.
On major advantage of precast concrete made in a plant is the consistency and quality of the
concrete in combination with a all-around perfect concrete cover not achievable when cast-in-
place. The final product is a high-strength pile that is corrosion resistance. With the help of
special cements and coatings these piles will also resist chemically or organically polluted water.
It is common that precast piles, prestressed or reinforced, are made with a hollow core. The
outside can be round, square, or octagonal. While the concrete has to meet ACI specification 318
and the rebar ASTM A82, A615 and A884 for reinforcing steel, pre- and post-tensioning cables
must conform to ASTM A416 A421, and A882.
In prestressed piles, (prestressing is extensively discussed in Chapter 11) the heavy longitudinal
bars are replaced with high tension rods or cables. Before or after the pouring of the concrete,
the tensioning steel is stressed causing the concrete to be in constant compression across the
entire cross-section. This in turn will lead to lower moment-deformations during transport to the
construction and thus avoiding cracking of the concrete. As a result, the use of high-stress
tension cables or rods allows that pre-stressed pile can be made with thinner walls leading to
lighter and longer piles. For example, piles with standard reinforcement reach a length of 50 ft
(16 m) while prestressed piles can be made with double that length. On the other hand, a pre-
tensioned pipe is very hard to shorten on site if they have been cast to long. Piles that use the
post-tensioning method are able to circumvent this major disadvantage. Instead of creating long
S9.3 USACE Driven Piles
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-11
a slender piles that have to be cast as one piece, post-tensioning allows that a pile can be cast in
segments to be assembled and stressed only when it is known how large one needs to be.
Another benefit of using precast concrete is the large 400 tons (364 t) of loads that can be put on
an end-bearing pile. On the other hand, they tension loading makes the tip, especially those of
prestressed piles more vulnerable to driving damage from boulders during driving. As a
protection a steel H-section or a stinger can be attached or cast into the tip. To improve the
footing of the end-bearing pipe in the rock surface of the solid underground, a special rock shoe
made of solid steel can be cast into the end disallowing any lateral movement in the future.
4. Composite Piles: The objective of composite piles is to allow the use of different materials
along the pipe in order to address conditions that differ from the top to the bottom. For example,
the lower end of a pile could consist of an H-pile, offering low soil displacement while being
protected against corrosion. This H-pile is then cast into a precast concrete pipe for the top part
providing the benefit of low corrosion. Obviously, the load capacity is tied to the lowest element
in the link.
More recently, the resin used in Fiber Reinforced Polymers (FRP) or recycled plastic has
attracted interest since it can be used to protect piles from decay and corrosion. For example,
the FHWA-HRT-04-043 report from 2006
(http://www.tfhrc.gov/structur/pubs/04043/index.htm#toc) presents three examples suitable for
load bearing. The first is a steel Pipe Core Pile where a normal steel pipe has been covered with
a thick shell of recycled plastic. While the steel core still provides the structural strength is the
plastic shell the function of a coating.
The second example mentioned in the FHWA report is the Structurally Reinforced Plastic
Matrix Pile where recycled plastic matrix takes over the place of concrete that is reinforced with
either FRP rods or a steel rebar cage. This composite pile type uses approximately 240 recycled
1-gallon (3.79-l) milk jugs per linear foot (0.305 m) of a 12-inch (0.305-m) nominal diameter
pile. The Concrete-Filled FRP Tube Pile simply replaces the steel of a pipe shell with RFP and
fills it with concrete with or without reinforcement. The RFP can be first filled with concrete and
driven after it is cured.
9.4.3 Pile Performance Evaluation Methods
How can a contractor know that a pile that is being driven 50 ft trough various layers of silt, sand,
clay, gravel etc. that it reached the capacity required by the design? How can he be assured that
the pile is still in good condition? For a long time the only true measure on could have were the
amount of settlement after a blow, established by the number of blows it took to advance a meter
or a foot, and the type and size of the hammer. One famous and long standing way was proposed
by Arthur Mellon Wellington on December 29, 1988 in a article in the Engineering News. It
became known as the Engineering News formula and had following form:
where
F = "constant determined from experience"
w = ram weight
h = drop height of ram (assumes single acting hammer)
Safe load L = F w * h s + c
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-12
s = penetration of pile per blow
c = "some constant in addition to s"
In addition to its extreme empirical nature of the formula its application was limited to timber,
used exclusively at that time, and a sandy and silty underground. Later following refinements to
the formula were made (for more information please download from the booksite a digital copy
of the Original book from 1893, Piles and Pile Driving, published by the Engineering New
Publishing Co.) New York.
w = weight of drop hammer or striking parts of the steam hammer (lb)
h = drop height of ram or striking parts (lb)
s = set of pile under last blow (in)
As a result of a scientific approach to soil behavior led by Terzaghi in the early 20 century, the
introduction of steel and concrete pipes, and the introduction of standard testing of the load
capacity after the completion of the drive led to the increasing dissatisfaction with the
Engineering News Formula. Today, the FHWA asserts that except where extensive data has been
collected to fit the empirical formula in an area with uniform conditions, this and other dynamic
formulas, as they are called, should not be used. New methods include many of the factors that
could not be measured at the time when the dynamic formulas propagated, such as the elastic
deformation of the pile under impact, the dynamic resistance of different soil conditions.
Let us jump from the past to today dominated by two science based tools: 1) Pile Driving
Analyzer with, 2) Case Method Capacity, and 3) CAse Pile Wave Analysis Program or
(CAPWAP).
Pile Driving Analyzer (PDA): This system is being used to collect and analyze data during the
driving of a pile that has been equipped with two types of sensors, an accelerometer and a strain
transducer. The data is processed real time to obtain velocity and force waves as they travel up
and down the length of the pile after the strike by a hammer. Today, the sensor output can be
transmitted wireless to the geotechnical office for immediate analysis with the CAPWAP
software presented below. In the field, the he PDA uses the Case Method to calculate the static
capacity of the pile and also evaluates pile integrity and establishes driving stresses and hammer
energy during pile installation.
The dynamic testing starts with the attachment of the four sensors three pile diameters bellow the
head of the pile, on opposite sides. The reusable gauges are bolted onto the pile to be removed
after the end of the test. The electronic cables are bundled, let hanging from the pile, and
connected to the PDA which collects the data and processes the analogue into digital data. The
result after each blow stored and presented on a screen for immediate review. Figure 9.9 offers
three different graphs that indicate different pile driving situations. Before we can interpret the
For drop hammers L (lb) = F 2 w * h s + 1
For steam hammers L (lb) = F 2 w * h s + 0.1
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-13
curves, it is necessary to understand the behavior of a wave, such as a stress wave creates by
hammer blow, and what an accelerometer will measure over time.
The speed of a wave speed = C, the cross-sectional area = A, and the elastic modulus = E.
When the hammer hits the head of the pile it creates a reaction force in the pile that compresses
the area around it and thus feeling an acceleration. The initial acceleration caused by the
hammer causes the neighboring material also to be accelerated at a velocity V, called a particle
velocity. In theory, the force pulse and the particle velocity in a pile that experiences no outside
resistance will follow the same wave pattern over time. The force at any time should equal the
particle velocity times a constant, E*A/C.
There are two important features how a wave behaves when it reaches the end of a solid rod,
such as a pile. At the two extremes, the end of a rod could be either totally free to move, such as
the prong of a tuning fork, or held rigidly fixed. When a wave reaches a free end, it returns in the
same phase as it arrived. When it is held, however, the force will cause phase change and the
wave returns back in the opposite phase. Figure 9.9 a) and b) show the results of those two
extreme situations.
Figure 9.9 Force and Particle Velocity Measurements for Various Piling Situations
The vertical axis shows that time in milliseconds after the hammer force has reached its
maximum (= 0). The first major spike after the initial blow indicates the return of the wave and
the force pulse that had traveled along the length L of the pile down and back up. The time is
when the first wave arrives lets us calculate the speed of the wave since the travel time of the
wave = 2*L/C. Naturally, the time 4*L/C is the time the first and largest wave traveled twice to
500 1000 kN 500 1000 kN
A = Cross-Sectional Area E = Elastic Modulus C = Wave Speed
a) Pile Without Toe Resistance b) Pile with Strong Toe Resistance c) Pile with Strong Shaft Resistance
Time in ms
0
10
20
30
0
2L/C
4L/C
Fat1
Fat2
Vat1
500 1000 kN
1.5 3.0 m/s
Vat2 10
20
30
Time in ms
Fbt1
Vbt2
Vbt1
1.5 3.0 m/s
0
Fbt22
= Particle Velocity = Force Pulse
Time in ms
10
20
30
Fct1
Fct2 Vct2
Vct1
1.5 3.0 m/s
0
0
2L/C
4L/C
0
2L/C
4L/C
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-14
the end of the pile and back. The size of the force measured by the strain gauge, measures a
complementary set of data. If the acceleration hits the free end there will be no action and not re-
action force created. Thus, the acceleration dies or attenuates. On the other hand, if the pile
feels resistance, the acceleration create a dynamic force which can be larger than the force
created by the hammer. Lets review what this theory means looking at the data.
Shown are two curves, one for the force pulse and one for the particle velocity. Initially, the
velocity is more interesting as it shows the phase-change indicated by dark gray. It is easy to
recognize that Figure 9.9 a) has no phase change, thus representing a pile with no resistance at
the end. This interpretation is supported by the fact that at 2*L/C no force pulse is coming back
from the pile end as the strain gauge measures even a negative force, meaning a stress wave
coming back. Figure 9.9 b), however, shows a very different situation. Not only can we
recognize that the returning waves changed their phase from a light to the dark gray but the
returning force pulse larger than the original at time 0. The particle velocity and the force pulse
are inverse, as one would expect. When the force reaches the free end it simply dissipates, as
there is not resistance, while the velocity doubles, as shown in Figure 9.40 a). On the opposite, if
a pile hits hard rock, the hammer force reaching the tip will be meet result in a force that is even
larger than the initial blow due to the dynamic response of the pile itself. As shown in Figure 9.9
b) the force Fbt2 returning to the strain gauge at 2*L/C is large then Fbt1, the initial blow, while the
velocity Vbt2 has turned to negative, indicating that the pile tip did bounded backwards. These
two simple cases are rare in the real world. Usually the resistance that a pile is experiences
comes from both, skin friction and toe. Let us look at a such a case
Figure 9.9 c) shows the recognizable wave forms of the force pulse and particle velocity have
disappeared. In other words, both the velocity and the force graphs dont show any oscillations.
This means, that at any time waves at various amplitudes arrive back at the sensor smoothening
each other out. The results are time-based measurements that are the result of averages from
many waves. The only possible interpretation of this pattern is a situation where the pile
experiences resistance all along its length, not just at its tip. Friction resistance at the
circumference of the pile also resist the acceleration and the force pulses, thus reflect waves that
overlap each other. Thus, the graph in Figure 9.9c) represents a case of a pile with strong
friction resistance along its skin.
Case Method Capacity: While the graphical representation of the waves help us understand the
conditions surrounding the pile a contractor needs more specific information about the pile
capacity, exactly, the static load that a specific pile would be able to carry if the driving
equipment would be turned off right now. The establishment of such an approach was the
longstanding topic of research a Case Western Reserve University in Cleveland, Ohio which
resulted in the Case Method Capacity method. It takes advantage of the PDA measurements and
a model of the pile as linearly elastic and a constant cross section to calculate the TotaL
Resistance RTL and the Static Resistance of a Pile, RSP:
RTL = (Ft1 + Ft2) + ( (Vt1 Vt2))* EA/C
RSP = RTL J*(Vt1 (EA/C) + Ft1 RTL)
Where: J is a dimensionless damping factor reflecting the soil type near the pile toe (0.1 for clean sand and 0.7 for clay)
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-15
The RSP function works best with piles experiencing a large shaft resistance. For piles with a
large toe resistance a different function should be used (see booksite FHWA Design and
Construction of Driven Pile Foundations Chapter 18)
From Figure 9.9 we learn that t1 refers to the time where the blow of the hammer reaches its
peak while t2 is the time when the first wave returns from the tip of the pile at 2*L/C. Let us
apply this function to the different cases in Figure 9.9.
Figure 9.9 a) Pile Without Toe Resistance: Fat1 = 1,100 kN, Fat2 = -100 kN, Vat1 = 3.3 m/s , Vat2 = 6.4 m/s, EA/C = 120 kNs/m, J = 0.7 RTL = (1,100 - 100)kN + ( (3.3 6.4)m/s) * 120 kNs/m = 500 kN (1.55 * 120)kN = 314 kN RSL = 314 kN 0.7 ((3.3 m/s * 120 kNs/m) + 1,100 kN - 314 kN) = (314 0.7(396+876) kN = - 576 kN
Figure 9.9 b) Pile With Toe Resistance: Fat1 = 800 kN, Fat2 = 1,500 kN, Vat1 = 2.7 m/s , Vat2 = -.3 m/s, EA/C = 120 kNs/m, J = 0.05 RTL = (800 + 1,500)kN + ( (2.7 + 0.3)m/s) * 120 kNs/m = 1,150 kN + (1.5 * 120)kN = 1,330 kN RSL = 1,330 kN 0.05 ((2.7 m/s * 120 kNs/m) + 800 kN 1,330 kN) = (1,330 0.05(324-530)) kN = 1,320 kN
Figure 9.9 c) Pile With Shaft Resistance: Fat1 = 900 kN, Fat2 = 700 kN, Vat1 = 2.9 m/s, Vat2 = -.2 m/s, EA/C = 120 kNs/m, J = 0.4 RTL = (900 + 700) kN + ( (2.9 + 0.2)m/s) * 120 kNs/m = 800 kN + (1.55 * 120) kN = 986 kN RSL = 986 kN 0.4 ((2.9 m/s * 120 kNs/m) + 900 kN 986kN) = (986 0.4(348-86) kN = 882 kN
It is interesting to see that the results of the Case Method Capacity calculations tell us that the
pile sitting on a tough layer has, with 1,320 kN, clearly the highest static capacity. It also shows
very plainly the small dynamic contribution to the RTL of only 10 kN. This stays in stark
contrast with the free end pile that shows a low RTL of 314 kN and a negative static capacity,
which is obviously not possible. This makes it apparent why the Case Method should only be
used on piles that have significant toe resistance such a example b) and c). In fact, the PDA
output for the last pile, shown in Figure 9.9 c), computes into a RTL of 986 kN and a RSL 882
kN, indicating a healthy dynamic contribution of 104 kN to the RTL.
Case Pile Wave Analysis Program (CAPWAP): This program takes PDA data collected on
site to conduct a more thorough analysis with the goal to refine the Case Method results. The
program is also based on the wave equation, the elastic pile and soil models. The final objective
of using this program is to match the collected data with that of a model for soil and pile. In this
iterative method, the factors representing possible soil conditions are changed until the match the
PDA data as close as possible representing the best estimate for the static pile capacity, soil
resistance on the shaft and its damping characteristic.
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-16
9.5 Non-Driven Load Carrying Piles, Columns and Caissons
This section will introduce a wide range of technologies that avoid the noises and vibrations
created by driving piles and sheets through boring, drilling, displacing , mixing, jetting,
vibrofloating and sinking of boxes and rings. As with piles, each method has its advantages and
disadvantages making it suitable in situations a where others are not. Figure 9.10 provides a
graphical product layout of those technologies that will be discussed bellow. Again, the
presentation follows that introduced sequence of first reviewing each process before studying the
mechanics of the main equipment and some unique tools.
Bored Piles
Figure 9.10 Overview of common deep foundation technologies
9.5.1 Drilled and Bored Cast-in-Place Concrete Piles
As with driven piles, the objective is translate the stresses created by the weight of a structure
built on the surface down into more solid soil layers or, if necessary, all the way to rock. Despite
this simple objective, the almost indefinite number of possible subsurface conditions, led to a
diverse set of equipment, tools and attachments.
We looked at the method to construction drilled cast-in place-piles for building vertical walls to
allow vertical excavation. While those were needed to carry horizontal forces coming from the
surrounding soil, the piles we are discussing now are built to transfer heavy vertical loads to a
lower strata or, if possible, down to solid rock. As a result, they are larger in diameter and
require much more care in terms of pile integrity and the quality of the contact between the
bottom/shoe of the pile in the soil/rock. While it is sometimes possible to deploy a continuous
auger, most often the size and to soil condition require much a shorter and more solid auger
including a set of additional tools not needed for wall piles. Figure 9.11 presents the more
common method without the use of a continuous long auger.
9.5.1.1 Drilling and Concreting Methods
a) Drilled b) Under- c) Franki d) Deep Mix e) Stone f) Jet Pile g) Jet/Grout h) Open i) Pressurized
Pile reamed Pile Pile Pile Column Underpinning Caisson Caisson
Existing
Building
S9.4 MoDOT Drilled Shafts S9.5 USACE Driven Piles
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-17
Figure 9.11 Basic steps for the construction of a large load carrying pile
1) Installation of surface steel casing to prevent collapse of top soil. Alignment of auger drill and Kelly bar powered by rotary drive and Kelly bushing
2) Auger drilling and spoil removal by lifting and cleaning the auger piece. Lowering of a casing if needed.
3) Removal of auger after desired depth is reached 4) Operation of cleaning bucket to remove loose soil at the bottom. Setting up of steel
bracket needed to position/suspend rebar cage inside drilled shaft until concrete has set
5) Pre-assembly of rebar cage and possibly the insertion of access tube (to be later used to test pile integrity) and grouting pipe
6) Lowering of prefabricated rebar cage using a spreader bar until cage collar sits on bracket 7) Placement of tremie pipe and pumping of concrete to fill the shaft from bottom up
without segregating the concrete. Tremie pipe is continuously raised as is the casing if
installed.
8) Integrity testing of pile to identify possible large holidays caused by soil collapses during concrete placement. Possibly grouting of area bellow the tip of the pile.
9) Load testing of pile if planned.
While the sequence of these 8 steps has gained wide acceptance, innovative contractors have
developed many modifications to improve their productivity where the job conditions allow it.
Modification A:
The casing is extended down into the shaft in order to support the walls or to cut off groundwater
from layers serving as aquifer.
Modification B:
5 Access Tube
Grouting Pipe
Rebar
Cage
Side Spacers
Pile Integrity
Tester
Pressure Grout
Grout Area 8
Possible Load Test
9
Kelly Bar
Surface
Casing
Down Force
Spoil Removal
Cleaning Bucket
Pumped Concrete
Telescoping
Kelly Bar Cage
Positioning
Bracket
6 4 7
Concrete
Concrete Tremie
Pipe
Spreader
Bar
Crane Hoist
Line
Auger
3
1
2
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-18
Side View
Kell
y B
ar
Kell
y B
us
hin
g
Le
ad
Ma
st
Kelly Drive
Hydraulic Motors, Gears, and
Sprockets Lead Mast
Top View
Figure 9.12 The Kelly Drill
Drive System
During the drilling bentonite is pumped into the shaft in order to keep groundwater out and the
support the walls of the shaft.
Modification C:
The short auger with Kelly bar extension was replaced with a continuous auger piece. Now the
screw-mechanism brings the soil directly to the surface.
Modification D:
The stem of the continuous auger is hollowed to serve also as concrete tremie pipe. After the
required depth is reached the auger is slightly raised and concrete is being pumped to the bottom.
Auger is lifted as the shaft fills with concrete interrupted by down-movements to compact the
concrete already cast. Rebar cage is being lowered or vibrated into the concrete of the already
filled shaft. Grouting of the area around the bottom of the pile is needed since the bottom had
not been cleaned prior to concreting.
Modification E:
Widening of the hollow stem of the continuous auger so a small diameter rebar cage will fit
through. After the auger reaches the desired depth, the rebar cage is lowered inside the auger
center pipe followed by a tremie to pump the concrete. Again, this pile bottom area should be
grouted.
Observing a large piece of equipment involved in drilling a deep shaft one can understand why
they are referred to as drilling rigs. Most probably, the term rig goes back to the Vikings who
raided England with rigged ships that included the mast, spars and sails. In ancient times, drill
rigs consisted of large timber towers and drilling tools to extract salt and other minerals before
the oil industry revolutionized the complexity and size or rigs and drilling platforms.
Todays drilling rigs for construction have to be mobile able to reach rugged environments or
low-ceiling spaces even inside existing structures. Thus, the
rigging of a modern drill consists of a: 1) carrier platform, 2)
plant for hydraulic power production, c) articulated mast to serve
as lead, and finally d) drive motors and winches. Figure 9.12
shows graphically that contractors and equipment manufacturers
have found ways to let the same carrier deploy various tools
based on the needs of the job. In general, one rig can drill using
a continuous flight auger (CFA) or a Kelly drill system. As the
insert shows, the key mechanism in the latter drill system is a
square or any non-round rod or bar that is being rotated by a
round drive wheel with an opening in the center through which
the bar can move freely up and down. During drilling, a drill
piece is attached to the bottom of the bar that is subsequently
turned by the Kelly drive powered by one or two hydraulic
motors. The drive mechanism includes also protective bushings
that can be easily adjusted to fit different bar sizes.
9.5.1.2 Drilling Equipment S9.6 USACE Drill Riggs
VG9.1 Drilling Equipment
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-19
Figure 9.13 Major drill rig configurations used in deep foundation construction
Naturally, the efficiency of drilling deep shafts is linked to several factors such as the
appropriateness of the auger head to extract the soil and rock, the available torque from the drill
rig, and lastly from the number of times an auger is being brought up to spin off the soil from the
screw. Should the length of the pile extend the height of the lead mast, extra non-productive time
has to spent on decoupling Kelly bars or continuous lift auger elements. Thus, it should not come
as a surprise to see drilling rigs that are 100 ft (33 m) high.
Crawler-mounted rigs offer more maneuverability and require less overhead clearance than the
other rigs, making them the rig of choice for restrictive work areas.
There are a variety of tools utilized by a contractor when drilling shafts. The wide assortment
includes drilling augers, for rock and soil, core barrels to casings and cleanout tools. Regardless
of how powerful the rig is, if the wrong tool or poor quality tool is used, the result can be costly
or even fatally.
Hydraulic
Power Unit
Kelly Bar
Rotary Drive with Kelly
Bushing
Auger
Guide
Hydraulic Motor and
Gear Sliding on Leads Up-Down
Continuous
Flight Auger
Hydraulic
Hose
Kelly
Rope
Casing
Driver Auger
Leader Positioning Cylinders
Rotary Drive
on Leader
Kelly
Rope
Leader Inclination
Cylinder
Leader
Hydraulic Motors, Gear
Box on
Leader
Continuous
Flight Auger
Kelly
Rope
Hydraulic
Drive Motor
Articulated Lead Mast
Tracked Drill Platform
Kelly Bar
Articulated Lead Mast
Hydraulic
Drive Motor
Continuous Lift Auger Drive after
Moving Motor to the
Top
a) Crane-Mounted Drilling Rigs b) Self-Deploying Drilling Rigs c) Crawler-Mounted Rigs
Casing
Twister
Leader Top with Multiple
Lead Pulleys
Articulated Lead Mast
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-20
Bottom Cleaning
Bucket
Rock
Auger
Core
Barrel
Rock
Bucket
Casing Head Auger
Rock
Head Auger
Soil
Single Flight Auger Screw Piece
Single Flight Auger Screw Piece
Earth augers, like the one shown in Figure 9.14, are typically used
in hard sands and cohesive materials. As the drill rig on the surface
turns the Kelly bar or the auger extension, the head teeth scratch
the soil and lift it onto the flight of the screw. The head is coupled
to the first auger piece pushing the loosened soil further up.
Rock augers are designed to overcome significant resistance to
cutting due to buried boulders or layers of harder material. Having
to take more abuse and exert more force, they are constructed of
heavier material than the earth augers. The flat teeth of an earth
auger are being replaced by ferocious looking conical hard-steel
dinosaur daggers. As these teeth wear more quickly than the
auger, they are set into sockets to make them replaceable.
Core Barrels Breaking or grinding through
hard rock that may be
encountered on the way to deeper
depths can slow down progress.
A more effective alternative to slowly fracturing the entire rock
is to cut out large pieces and retrieve them. Borrowing from
the concept of core drilling, where drills cut only along the
perimeter of the circle thus leaving the center intact for
investigation, shaft drilling contractors switch to core barrels.
After pulling up the auger piece, a hollow barrel is mounted
and lowered into the shaft. When the bottom edge of the barrel
hits the rock, its hardened teeth will cut along the perimeter
while leaving the core intact. When a joint or discontinuity is
encountered, the core breaks off and can now be removed as
one piece.
Buckets Buckets come in three types, earth, rock and cleanout bucket.
As the names imply, each has a designed use for either
advancing the shaft or for cleaning the bottom. Like the augers,
the buckets are attached to a Kelly bar and cut into the bottom
of the shaft. The cleanout bucket fulfills a critical need in that
it removes all the loose material that collects at the bottom of
the shaft before being filled with concrete. Excessive amount
of unconsolidated soil left at the bottom will cause the finished
concrete pile to drop if the friction force is insufficient to carry
the load. The cleanout bucket normally has a double bottom
allowing a cutting gate to open and close openings at the base.
When rotating the bucket in one direction the gate is open
9.5.1.3 Earth and Rock Augers
Figure 9.15Augers, Buckets,
and Barrels
Figure 9.14 Earth Auger
b) Hard Soil Auger Head
Hollow Stem
a) Single Flight Auger Pieces
Flight of Screw
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-21
Figure 9.16 Pile
Weakened by Soil
Intrusions
Side View
Soil
Failures
Cross Section
and the attached scraper picks up loose sediments from the bottom. When the direction is
reversed, the scraper gate closes the openings, trapping the material inside and allowing its safe
removal to the top.
The Casing This word is another example how construction adopted a Norman French, casse,
having its root in the Latin word capsa meaning case or box. Today, casing outside construction
can stand for many types of covers (e.g., computer casing) or materials that encase or enclose
(e.g., window casing). Casings used in drilled shafts do indeed fulfill the function of a protective
box as the Latin capsa in that it prevents the soil around from interfering with the open space
inside. Consisting of round steel pipes of a diameter that allows an auger
to fit trough, short pieces are used to protect the shaft rim from damage
and a possible flooding. Whenever the soil characteristics encourage the
break-off of chunks in the shaft wall or when the shaft should be kept dry
for groundwater longer casing pipes are lowered or drilled parallel to the
main operation.
We talk of temporary casings when they are pulled out as the concrete is
placed. Permanent casing are left in-place and become part of the cast-in-
place pile. A condition where such an expensive solution might warranted
is when concrete placement could not be done successfully without such
a protection. Since only an integrity test after curing will show if the pile
is acceptable, the contractor has no other option to fixing a weakened pile
as to replace it or to dig down to the area to be repaired. Figure 9.16
demonstrates that soil failures during concreting can cause significant
volumes of soil to fall onto the raising concrete surface and be
encapsulated thus leaving voids in the cross-section of the pile. On the
other hand, concrete will be lost as it fills in the space left by the
collapsed wall area.
While the rotary tools presented so far are most common used for drilling, in certain instances a
brute force technology is needed. One such example is a sloping hard-rock surface. While a
barrel tends to bind will the tungsten tips of a rock auger slide sideways and push the auger off
the vertical. Do avoid the costly consequences of loosing a tool at the bottom of the hole, the
contractor may opt to use a rock breaker that can be dropped inside the shaft serving the function
of a slow-moving pneumatic hammer as it is being raised up via a hoist line from the crane.
After exchanging the rock breaker with a clamshell or grab bucket, the broken-up rock pieces
can be retrieved before the drilling operation can continue. One can understand that the needed
time to switch from percussion to retrieval tools and back again slows down the drilling
operation.
Similar to the diaphragm wall construction discussed in Chapter 8, contractors were looking for a
method to protect the wall shafts of drilled piles without having to resort to installing a steel
casing. To no ones surprise, the identical problem set-up the solution was bentonite slurry.
9.5.1.5 Slurry to Support the Shaft Walls
9.5.1.4 Percussion Tools
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-22
where:
s = Density of bentonite slurry
w = Density of water
Hs = Height of slurry column
Hw = Height of water column and:
s > w and Hs > Hw
Figure 9.17 Wall support
mechanism of bentonite slurry
Hw
Hs
Liquid Gel
Pressure =
Hs * s = Hw * w
Bentonite
Slurry
Drilled
Shaft
Mud Cake
Marsh Funnel Viscosity (MV)
Viscosity is defined as the shear stress in the slurry liquid divided by the shearing rate. The viscosity of slurry (mud) can be measured with the Marsh funnel and translated into poise or centipoise. The modern funnel consists of a cone 6 inches (152 mm) across and 12 inches in height (305 mm) to the apex of which is fixed a tube 2 inches (50.8 mm) long and 3/16 inch (4.76 mm) internal diameter. While blocking the exit with one finger, the liquid to be measured is poured through a mesh into the cone holding about 1.5 liter. To take a measurement, the finger is released as a clock is started. The time in seconds is recorded as a measure of the viscosity (MV).
Marsh Funnel Viscosity (MV)
Even more important than in the construction of retaining
walls, which were eventually exposed during excavation,
contractors had to ensure that no cavities could develop
during both the excavation and concreting face. Thus, it
was critical to better understand the mechanics of the
interaction between the earth wall and the bentonite. Figure
9.17 highlights the effect of the bentonite slurry on the shaft
edge area. As shown, the slurry is changing from liquid
to a gel as it is pushed deeper into the soil due to the higher
pressure inside the shaft filled with slurry (H s > Hw
w). The reason for this soil clogging gel are the electrically
charged bentonite particles squaring up when left
undisturbed. Shown in Figure 9.17 is the gelling effect
causing the jelly zone, referred to as a filter or mud cake. It
is capable of sealing the shaft against an in- and out flux of
water weaken and eventually erode vertical wall areas
lacking the cohesion and shear strength necessary to
counteract the combined pressure from groundwater and
soil.
An alternative to the original Bentonite are polymers
consisting of chain-like hydrocarbon molecules. Like the
Bentonite plates the chains are electrical charged and act
similar, in particular, they can be pressured into sand or silty
soil where the long chains get lodged inside the pores of the soil. Eventually, the long polymer
strands are holding soil particles together and the large number begins to clog the flow of the
slurry resulting in the same sealing effect that bentonite exhibits.
Experience has shown that the prevalent soil
characteristics will have to be considered when mixing
the slurry. Most important is the viscosity of the
slurry measured in poise or centipoise named after the
French physician Jean Louis Marie Poiseuille (April
22, 1799 - December 26, 1869) who studied the
viscosity of blood inside the artery. A quicker but less
accurate measure is the Marsh funnel viscosity or MV
(see insert). Adopted from the oil industry, it was
found that clay, silt or sand need soil need a slurry
with a MV of 32 seconds (32 MV with a 946 ml
funnel volume) while gravel needs up to 50 MV to
create a sufficient filter cake. Slurry with larger MVs
are very hard to desand and can create problems
during concrete placement as it may stuck itself to the
rebar and thus create large caverns inside the
completed pile.
While drilling at the bottom of the shaft filled with bentonite, soil particles will enter the slurry
where it stays in suspension only to settle when excavation ends. Of course, the heavier sand
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-23
and fine gravel will accumulate at the bottom thus can create a problem when concreting starts.
A rule of thumb is that a slurry with a sand content exceeding 4% should be desilted and/or
desanded before concreting should begin. Of course, an alternative to desanding is removal and
replacement with fresh slurry with a sieve for larger objects and cyclones to remove the sand
particles.
Assume that you need to prepare a bentonite slurry inside a tank 24 hours before it is needed for
drilling a 24 m deep shaft with a diameter of 2 m (6.6 ft) and a water table at 1.5 (5 ft). Expect
an overbreak/added depth of 15%. For the existing silty-sandy soil conditions it is recommended
to use average values for viscosity and density.
How many 50 lb bags of bentonite and how
many gallons of water are needed to create
a workable slurry for the next day.
Assumptions: The desired density is 9.99
kN/m3 or 64.15 lb/ft
3. The density of water
is 998 kg/m3 at 20 degrees Celsius or 9.8 kN/m
3 (62.1 lb ft
3).
Calculation: The theoretical volume of the shaft = 24 m * 3.14 * 1 m = 75.4 m3
Volume that will fill overbreak and eventual holes = 1.15 * 75.4 = 87 m3
Water needed = 87 * 264.2 = 22,985 gallons In order to increase the density of water from 9.8 to 9.99 kN/m3 we need to add 0.19 kN/m
3 (1.2
lb/ft3) or 0.16 lb/gallon of Bentonite.
Amount of 50 lb bags = (22,985 *0.16)/50 = 74 bags
Discussion of Results: In order for the Bentonite to activate it is important to leave it 24 hours in
the mixing tank. It is not necessary to fill the shaft all the way since the water table is at 1.5 m.
However, the top of the slurry should not sink below 1 m. The viscosity should be measured
with the Mash funnel from time to time to make sure that it stays between 30 and 40 MV. Check
density and viscosity of samples from the bottom before concreting in order to avoid costly
repairs later due to sand and debris that settled and got encased.
Drilling into the multi-layered subsurface does not create the intended smooth cylindrical
opening. Drilling most often means breaking up and yanking large pieces out of the immediate
surroundings of the shaft wall. This can cause big surprises by the time that concrete is being
pumped into the cavity. For example, a shaft with a theoretical volume of 55 yd3 (42 m
3) may
need 132 yd3 (100 m
3) concrete to fill. Knowing the actual shape of the created concrete pile is
also important when interpreting the data of pile integrity and load testing. To solve this
problem, two methods have been developed. The first uses a mechanical or electronic caliper
device that is being lowered into the shaft (caliper = instrument having two adjustable arms or
Property Range During Drilling
Range Before Concreting
Density
9.95 - 10.3 kN/ m3
63.0 - 65.3 lb/ft3
9.95 - 10.4 kN/ m3
63.0 - 66.0 lb/ft3
Viscosity 28 50 MV for 0.946 ml 28 50 MV for 0.946 ml
These values apply to mineral bentonite only
9.5.1.6 Profiling the Shaft
Worked Out Example Problem 9.1: Bentonite Slurry Mixing
Table 9.2 Desired Slurry Properties
For the desanding of slurry, it can be
pumped a short distance to a set of special
equipment
S9.6 FHWA Drilled shaft installation plan S9.7 FHWA Drilled shaft log S9.8 FHWA Drilled shaft soil excavation log S9.9 FHWA Drilled shaft concrete log
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-24
Shaft off location or out of plumb
Base of shaft not in proper founding stratum
Crack in the shaft when hit by equipment early in curing process
Bulge or neck in the shaft (soft ground zones that were not cased)
Cave-in of the shaft walls
Excessive mud cake buildup
Temporary casing that cannot be removed
Horizontal separation or severe neck
Horizontal sand lens in concrete
Quarter-moon-shaped soil intrusion on the side of the shaft
Soft shaft bottom
Voids outside of rebar cage
Honeycombing, washout of fines or water channels in the concrete
Folded-in/encased debris
Potential Problems: FHWA-IF-99-025
jaws to measure the diameter or thickness of round objects.) Most recently, a 360 degree sonic
radar is being used to create a 3-D as built image of the drilled shaft showing the surface
relative to the vertical centerline in real time. This data, of course, allows the instant calculation
of the shaft volume as a basis for the contractor to order ready-made concrete from the batch
plant.
The second method uses data from a concrete flow meter or the hopper volume and the height of
the raising concrete column inside the shaft. The surface of the concrete can be easily measured
with a weighted tape lowered into the shaft and works even with slurry. When the weight at the
end of the lowered tape meets the concrete the tape slacks off telling the observer that it reached
the bottom who is able to read instantaneously the tape. By plotting the theoretical with the
actual volume of pumped concrete the concrete gets an immediate overview of the situation and
is able to predict the needed concrete the higher the column. Figure 9.18 presents the graph of a
hypothetical situation.
Figure 9.18 Concrete filling measurements for hypothetical shaft
The imaginary shaft is 24 m (80 ft) deep needing 20 m3 (26yd
3) of concrete resulting in the
theoretical linear fill line. One always needs to expect a modest overbreak of 3-6% but, as the
development of the actual fill line shows, three events result in the contractor needing almost 40%
more concrete as planned.
Event 1: When the concrete reaches a depth of 20 m
(66 ft), it stops to rise until 6 m3 (8yd
3) are added. At
11 m3 (14.3 yd
3) the top of the concrete is still at 19 m
(63 ft) instead of the expected 12 m (40 ft). As the
sketch indicates, one has to suspect that concrete
entered into a rather large side cavern that was created
or had existed there before drilling began.
Event 2: At 13 m (36 ft) the concrete surface
suddenly drops 2 m (7ft) even though more concrete is
pumped in. Different then in event 1, there was an
instant drop not just a smooth transition. This
indicates an abrupt creation of an opening in the shaft
leading to an empty cavern that was so far untouched.
ft m
3 3
2
1
0 5 10 15 20 25 30
Volume
yd3
m3
0 5 10 15 20 25 30 35
Actual
Fill Line
2
0
5
10
15
20
25
0
20
40
60
80
Dep
th
Theoretical
Fill Line
1
6% Overbreak
19 m
(63 ft)
8 m
(26 ft)
13 m
(43 ft)
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-25
- To enlarge, taper, shape, or smooth out (a hole) with a reamer
- To enlarge a bore or a shaft by removing material.
To ream:
The cause of such an event must have been the pressure of the concrete combined with a lack of
water inside the cavern.
Event 3: At a depth of 10 m (33 ft) the concrete start to rise quicker than predicted, meaning,
that the cross-sectional area got suddenly smaller. At a depth of 6 m (20 ft) the fill slope returns
what it should be. This can only mean that debris from cave-ins had accumulated at this depth
and was subsequently encased by the concrete thus reducing its needed volume.
While the vast majority of drilled shafts are being built without any difficulties the Federal
Highway Authority (FHWA) has put together a list of problem areas that a contractor has to
watch out for.
9.5.2 Underreamed Piles
The bearing capacity of most piles depends strongly on the amount of
load that the base or point of the pile can take over after the initial
settlement. The two main factors, of course, are the area and the soil
strength bellow the base of the pile. Widening or belling the base is one
method to take advantage of this equation since the area increases with
the square of the radius (area = r2)
Not surprisingly, foundation contractors did find a way to take advantage of the open shaft to
insert an expandable reaming tool similar to angioplasty balloon that opens up a clogged artery.
The tool is referred to as a belling bucket, underreamer or simply the reamer presented in Figure
9.19. After the drilling auger or bucket reaches a desired depth, it is replaced with the reamer and
lowered into the hole at the end of the Kelly as shown in Fig 9.19 b). A two link mechanism
forces the wings of the reamer outwards when the drilling rig exerts a downward force since the
round tip component is not turning with the Kelly and thus act as a the stable arm of a c-clamp
(Fig. 9.19 c). Rotating the Kelly will result in the cutting teeth of the reamer to dig into the soil
of the shaft wall which collects at the bottom of the shaft. Naturally, reversing the down-
pressure will cause the reamer wings to close again, trapping the loosened soil inside ready to me
removed by raising the reamer to the top through the shaft (see Figure 9.19 d). Repeating this
process will increase the underream angle until a mechanical stop which dictates the maximum
(see Figure 5.19 d). Finally, the bottom of the shaft can be cleaned with a separate bucket thus
making it ready for installing the rebar cage.
Figure 9.19 Process steps to ream out the bottom of the drilled shaft
Underream
Angle
a) Drilling to
Shaft Bottom
b) Lowering of
Belling Tool
d) Emptying of
Reamer
c) Opening by
Pressing Down
e) Full Swing Out
of Reamer Wings
f) Cleaning of the
Shaft Extension
Tw
o L
ink
Me
ch
an
ism
Chapter 9: Deep Foundation by L. Bernold June 2011 Construction Equipment and Methods
9-26
Drilling dry shaft to groundwater level
Advancing casing into the clay layer, creation of seal around casing
Drilling dry shaft to desired depth
Reaming bell, cleaning and installing rebar for bell and shaft
Concreting bell and shaft with a coordinated retrieval of casing
1
2
3
4
5
Steps to an Underreamed Pile
One necessary condition for the bellying operation to work as designed is a dry shaft. As we
learned earlier, a possible method to keep groundwater fr