Chp9 Deep Foundations Revision 2011

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


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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.


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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


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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.


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


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


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


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


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


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


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


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


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


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)


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.


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


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


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


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


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



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


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


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


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


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)


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


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

Documents

Chp9 Deep Foundations Revision 2011