DEEP FOUNDATIONSTHE MAGAZINE OF THE DEEP FOUNDATIONS INSTITUTE MAR/APR 2020

DFIIOT NA SD INN

SU TO IF T

UPE T

E E

D

®

Bolivian Cable Car Micropiles

Reducing Excavation-

Adjacent Risks

Micropiles Stabilize Factory

Machines

Helical Pile-Foundation

Load Tests

Active Soil Nail Wall Shores Up Historical Building

CONTENTS FEATURES

DEEP FOUNDATIONS • MAR/APR 2020 • 5

Departments

PRESIDENT’S MESSAGE Why Are DFI Events and Conferences So Good? . . . . . . . . . . . . . . . . . . . . . . . . . . 7

EXECUTIVE DIRECTOR UPDATE SuperPile – A Must-Attend 2020 Event . . 9

DFI ACTIVITIESSuperPile ’20, upcoming events, new publications, new DFI team members, a new magazine editor and more. . . . . . . 25

REGIONAL REPORTDFI of India . . . . . . . . . . . . . . . . . . . . . 61

EDUCATIONAL TRUST REPORTUpdates on the new Terracon Scholarship, new trustee, Fine and Langan Legacy scholarship recipients, at-large scholarship applications, WiDF professional development grant applications, and upcoming events. . . . . . . . . . . . . . . . . 65

TECHNICAL ACTIVITIES UPDATENew committee co-chair, update from the Augered Cast-in-Place and Drilled Displace-ment Pile Committee, and an FHWA National Geotechnical Team Update . . . 69

LEGALLY SPEAKINGPotential Reduced Construction Impact of Endangered Species Act . . . . . . . . . . . 111

DFI PEOPLE AND COMPANIESNews about people and companies . . 115

CALENDAR & AD INDEX . . . . . . . . 122

103 Improving Helical Pile-to-Foundation ConnectionsSerhan Guner, Ph.D., P.Eng., and Sundar Chiluwal

Many structures are subjected to sequential applications of compression and tension

loads due to events such as windstorms. Tall and light structures that are particularly

vulnerable to the tensile components of cyclic loads can be developed with helical pile

foundations as part of addressing this. DFI’s Helical Pile and Tiebacks Committee

funded a research study to help evaluate current helical pile designs and inform the

development of more robust helical pile-to-foundation connections.

95 Micropiles to Improve Stability of Factory Machining CentersGlenn Patterson

Modern manufacturing facilities employ a quick production

pace and precision machines that work with few inter-

ruptions. The large machining centers involved require an

independent foundation that is free from vibrations of all

other shop machinery and equipment. As a result, micropiles

can improve installation of machinery due to their minimal

impact on factory schedules, speed of installation, small

footprint of installation equipment, and ability to create

lasting, competent foundations for machinery.

DEEP FOUNDATIONSThe Magazine of the Deep Foundations Institute (DFI) is published bimonthly by DFI.

326 Lafayette Avenue Hawthorne, NJ 07506, USAT: +1 (973) 423-4030 | F: +1 (973) 423-4031Email: staff@dfi.org

87 Adjacent Structure Technical and Legal RisksScott J. DiFiore, P.E., Gregory R. Meeder, James P. Chivilo and Matthew H. Johnson, P.E.

Heavy civil construction in urban

environments can damage existing,

adjacent structures through ground

movement and vibration. Owners of

new construction projects and their

team must be aware of legal and

technical risks posed by excavation,

dewatering and demolition to navigate

and allocate those risks. This article

discusses approaches to early and

frequent communication among project

participants that are necessary to

manage risks, regardless of local

regulations that may or may not exist to

drive risk-related processes.

DFI is monitoring recommendations from the World Health Organization (WHO), the US Centers for Disease Control and Prevention (CDC), and health authorities of those countries where we have scheduled conferences and other activities. Check www.dfi.org and event-specific web pages for updates.

COVID-19 (Coronavirus) Update

Lattice Towers Wind Turbine Towers Light-Frame Steel Buildings

DEEP FOUNDATIONS • MAR/APR 2020 • 103

AUTHORS Serhan Guner, Ph.D., P.Eng., and Sundar Chiluwal, The University of Toledo

FEATURE ARTICLE

Improving Helical Pile-to-Foundation Connections

Many structures are subjected to cyclic

loads, which involve sequential appli-

cations of compression and tension loads

due to events such as windstorms, earth-

quakes or heavy vehicular traffic. Tall and

light structures — such as telecom-

munication or transmission towers, wind

turbines, and light-frame steel buildings —

are particularly vulnerable to the tensile

components of cyclic loads. Owing to their

high-tension capacities, helical piles

present a significant potential to create

cost-effective, practical and robust foun-

dation systems for resisting significant

tension forces. To realize their full

potential, helical pile-to-foundation

connections must be properly designed.

Even a well-designed pile group and a

concrete foundation may fail if their

connection cannot resist the applied forces.

However, there is a lack of research,

knowledge and specific code provisions on

creating robust connections for resisting

tensile loads.

DFI’s Helical Piles and Tiebacks Com-

mittee funded a research study from May

2018 to September 2019 to advance

knowledge on the influence of the con-

nection zones of helical piles, to understand

the effectiveness of current connection

designs, and to publish recommendations

for improving robust connection design.

Conducting large-scale experimental tests

was neither practical nor cost-effective due

to the large foundation dimensions and

numerous design configurations that would

While geotechnical studies typically

focus on the behavior of isolated or grouped

piles without considering the influence of

the connection zones, structural studies

exclusively focus on the concrete foun-

dation response without considering the

influence of piles. The connection design is

also typically left unaddressed and is

commonly dealt with as an afterthought,

using general code equations not intended

for application to pile-to-foundation

connections nor tensile loads.

have been involved, and due to complex

loading protocols that would need to be

tested. To overcome these challenges, the

research team from The University of

Toledo, Ohio, used state-of-the-art high-

fidelity computational modeling along with

experimental verification studies as a first

step in developing design recommen-

dations for helical pile connection zones

subjected to tensile loads.

This article describes the use of this

approach to create 162 full-scale founda-

tion systems and to digitally examine the

influences of connection bracket types and

embedment depths, reinforcement per-

centages, shear span-to-depth ratios and

loading conditions. The copious simulation

data generated allowed the team to quantify

the influences of each variable, identify

undesirable design configurations and make

design recommendations. While this study

addresses helical piles, the research findings

are also applicable to micropiles because

they use the same termination brackets.

Tall, light structures impacted by tension forces

19 mm(0.75 in)thick

254 mm

254 mm(10 in)

Single Plate Bracket

Column

Base Plate

Anchor bolts

2100 mm (6.9 ft)

60

0 m

m(2

ft)

Tension

Pile Cap

ρx= 0.2%, 0.4% or 0.8%

he = 140, 300 or 460 mm(5.5, 12 or 18.4 in)

340 mm

(13.4 in)

51x51 mm(2x2 in)

160 mm

(6.3 in)

Studded Plate Bracket

he= 140 or 300 mm(5.5 or 11.8 in)

Double Plate Bracket

CL

d = 530 mm

a = 890, 750 or 590 mm(35, 30 or 23 in)

(20.9 in)

excellent accuracy for numerical simula-

tions at the ultimate collapse conditions.

The failure modes and crack patterns were

also simulated very well. These results

demonstrated the successful verification of

the numerical modeling approach.

The effect of the connection zone con-

figurations on the system-level load

capacities in the 162 numerical simulations

were systematically analyzed and com-

pared according to the load cases to which

they were subjected. As with typical cyclic

reversals, the reversed-cyclic-load results

are divided into two parts as cyclic tension

and cyclic compression.

Load-Displacement Response Simulation

• The load capacity increases by an

average of 30% when the embedment

depth (h ) is changed from bottom to e

middle. The further increase in h from e

middle to top does not appear to affect

the capacity (see overlapping red and

green lines in Figure 4a).

The peak load capacities were graphically

represented and contain 27, 9 and 18

simulation results, respectively. The results

demonstrated that:

System-Level Load Capacities

• The studded bracket has two h e

positions considered in this study. The

change in h did not influence the load e

capacity (see overlapping lines in Figure

4b), except for the higher reinforcement

Simulation Results

Figure 4. Load capacities at failure subjected to tension and compression load cases

104 • DEEP FOUNDATIONS • MAR/APR 2020 DEEP FOUNDATIONS • MAR/APR 2020 • 105

A concrete foundation system, repre-

sentative of a pile cap, was designed using

two helical piles. This configuration creates

a one-way stress flow and helps better

isolate the connection response. The

helical piles are terminated with one of

three bracket types, as shown in Figure 1: a

single plate, a studded plate or a double

plate. The influencing parameters inves-

tigated include three bracket embedment

depths ( ); three longitudinal rein-he

forcement percentages (also called rein-

forcement ratios, or ); three shear span-ρx

to-effective depth ratios (a/d ratios of 1.68,

1.42, and 1.11), which represent how thick

a concrete pile cap or footing is; and three

loading conditions: monotonic tension,

monotonic compression and reversed

cyclic.

Foundation System Details

Computational Modeling

Although 2D numerical models are

computationally more efficient and faster

than 3D counterparts, they commonly

assume that stresses and strains are

constant through the out-of-plane

thickness of the elements. This assumption

is not valid when modeling helical pile

connections where the stresses propagate

in a conical shape from the edges of the

termination plate towards the surface of the

concrete, as shown in Figure 2a. This stress

pattern results in a conical failure termed

Experimental Approach

A two-dimensional (2D), continuum-type,

nonlinear plane-stress element was used

for the computational models through the

computer program VecTor2. The formu-

lation employed is based on the Modified

Compression Field Theory (MCFT), which

has been adopted by several design

standards worldwide. The MCFT employs

a smeared, rotating crack approach within

a total-load, secant-stiffness solution

algorithm, and allows for the consideration

of the coupled flexure, axial and shear

effects. Although a 2D model is involved,

the triaxial concrete confinement is

accounted for by in- and out-of-plane

reinforcement components. Additionally,

VecTor2 incorporates a large number of

advanced material behavior models.

Figure 1. Pile termination bracket types studied

Experimental Verification

the concrete breakout (or cone) failure. To

capture this failure mode using 2D models,

the research team developed the Equiva-

lent Cone Method (ECM). This method

determines an equivalent concrete thick-

ness for use in 2D models to a yield

breakout failure load that is approximately

equal to the load which would be predicted

by a 3D model (see Figure 2b).

The numerical models were verified using

the experimental data generated by the

research team of Dr. M. Hesham El Naggar

from Western University, Canada. Their

experimental study tested nine specimens

consisting of a single plate bracket

embedded into foundation specimens of

500 mm by 500 mm by 1600 mm (20 in by

20 in by 64 in). Note that the test setup was

an inverted foundation where the pile shaft

at the top applies tension loads. These

specimens were modeled using the high-

fidelity modeling approach presented

above, and the simulated responses have

been compared to the experimental ones

(see Figure 3). The simulations captured the

experimental load capacities with a

simulated-to-experimental average of 1.01

with a coefficient of variation of 6% – an

Figure 2. The equivalent cone method

Figure 3. Verification of the failure mode predictions

19 mm(0.75 in)thick

254 mm

254 mm(10 in)

Single Plate Bracket

Column

Base Plate

Anchor bolts

2100 mm (6.9 ft)

600

mm

(2 f

t)

Tension

Pile Cap

ρx= 0.2%, 0.4% or 0.8%

he = 140, 300 or 460 mm(5.5, 12 or 18.4 in)

340 mm

(13.4 in)

51x51 mm(2x2 in)

160 mm

(6.3 in)

Studded Plate Bracket

he= 140 or 300 mm(5.5 or 11.8 in)

Double Plate Bracket

CL

d = 530 mm

a = 890, 750 or 590 mm(35, 30 or 23 in)

(20.9 in)

excellent accuracy for numerical simula-

tions at the ultimate collapse conditions.

The failure modes and crack patterns were

also simulated very well. These results

demonstrated the successful verification of

the numerical modeling approach.

The effect of the connection zone con-

figurations on the system-level load

capacities in the 162 numerical simulations

were systematically analyzed and com-

pared according to the load cases to which

they were subjected. As with typical cyclic

reversals, the reversed-cyclic-load results

are divided into two parts as cyclic tension

and cyclic compression.

Load-Displacement Response Simulation

• The load capacity increases by an

average of 30% when the embedment

depth (h ) is changed from bottom to e

middle. The further increase in h from e

middle to top does not appear to affect

the capacity (see overlapping red and

green lines in Figure 4a).

The peak load capacities were graphically

represented and contain 27, 9 and 18

simulation results, respectively. The results

demonstrated that:

System-Level Load Capacities

• The studded bracket has two h e

positions considered in this study. The

change in h did not influence the load e

capacity (see overlapping lines in Figure

4b), except for the higher reinforcement

Simulation Results

Figure 4. Load capacities at failure subjected to tension and compression load cases

104 • DEEP FOUNDATIONS • MAR/APR 2020 DEEP FOUNDATIONS • MAR/APR 2020 • 105

A concrete foundation system, repre-

sentative of a pile cap, was designed using

two helical piles. This configuration creates

a one-way stress flow and helps better

isolate the connection response. The

helical piles are terminated with one of

three bracket types, as shown in Figure 1: a

single plate, a studded plate or a double

plate. The influencing parameters inves-

tigated include three bracket embedment

depths ( ); three longitudinal rein-he

forcement percentages (also called rein-

forcement ratios, or ); three shear span-ρx

to-effective depth ratios (a/d ratios of 1.68,

1.42, and 1.11), which represent how thick

a concrete pile cap or footing is; and three

loading conditions: monotonic tension,

monotonic compression and reversed

cyclic.

Foundation System Details

Computational Modeling

Although 2D numerical models are

computationally more efficient and faster

than 3D counterparts, they commonly

assume that stresses and strains are

constant through the out-of-plane

thickness of the elements. This assumption

is not valid when modeling helical pile

connections where the stresses propagate

in a conical shape from the edges of the

termination plate towards the surface of the

concrete, as shown in Figure 2a. This stress

pattern results in a conical failure termed

Experimental Approach

A two-dimensional (2D), continuum-type,

nonlinear plane-stress element was used

for the computational models through the

computer program VecTor2. The formu-

lation employed is based on the Modified

Compression Field Theory (MCFT), which

has been adopted by several design

standards worldwide. The MCFT employs

a smeared, rotating crack approach within

a total-load, secant-stiffness solution

algorithm, and allows for the consideration

of the coupled flexure, axial and shear

effects. Although a 2D model is involved,

the triaxial concrete confinement is

accounted for by in- and out-of-plane

reinforcement components. Additionally,

VecTor2 incorporates a large number of

advanced material behavior models.

Figure 1. Pile termination bracket types studied

Experimental Verification

the concrete breakout (or cone) failure. To

capture this failure mode using 2D models,

the research team developed the Equiva-

lent Cone Method (ECM). This method

determines an equivalent concrete thick-

ness for use in 2D models to a yield

breakout failure load that is approximately

equal to the load which would be predicted

by a 3D model (see Figure 2b).

The numerical models were verified using

the experimental data generated by the

research team of Dr. M. Hesham El Naggar

from Western University, Canada. Their

experimental study tested nine specimens

consisting of a single plate bracket

embedded into foundation specimens of

500 mm by 500 mm by 1600 mm (20 in by

20 in by 64 in). Note that the test setup was

an inverted foundation where the pile shaft

at the top applies tension loads. These

specimens were modeled using the high-

fidelity modeling approach presented

above, and the simulated responses have

been compared to the experimental ones

(see Figure 3). The simulations captured the

experimental load capacities with a

simulated-to-experimental average of 1.01

with a coefficient of variation of 6% – an

Figure 2. The equivalent cone method

Figure 3. Verification of the failure mode predictions

• The helical pile-to-foundation con-

nections are found not to influence the

monotonic compression load capacities

of the foundation systems in any of the

termination bracket types examined;

no connection failures are predicted. To

• The results of this study clearly demon-

strate the need for performing an explicit

capacity and service-load cracking

analysis for the connection zones when

resisting tensile forces, in addition to the

global structural and geotechnical anal-

yses. The recommended design con-

figurations are not intended to replace

the explicit connection zone analyses.

• The full project report, an open-access

spreadsheet, YouTube demonstrations

of the research and latest journal

publications can be downloaded from

Dr. Guner’s research website at

www.utoledo.edu/engineering/facult

y/serhan-guner/Publications.html

and from the Helical Piles and Tiebacks

Committee website at www.dfi.org/

commhome.asp?HLPR

• Future research and large-scale

experimental tests are required to

further investigate the effects of critical

system variables and develop simple

design equations for use in practice.

maximize the compression load capa-

cities, high ρ and low a/d ratios are x

recommended for all bracket types.

AcknowledgementsThe authors would like to thank the DFI

Committee Project Fund and the DFI

Helical Piles and Tiebacks Committee for

supporting this study, Mary Ellen Large for

administering this project and Dr. M.

Hesham El Naggar for providing the

experimental results. The authors would

also like to acknowledge the contributions

of Rafael A. Salgado (Ph.D. candidate) for

the statistical analyses, and Sálvio A.

Almeida Júnior (graduated M.S. student)

for his review and valuable comments.

Serhan Guner, Ph.D., P.Eng., is a civil engineering faculty member at The University of Toledo, Ohio, where he

received an ExCellence in Civil Engineering EDucation (ExCEEd) Fellowship from ASCE. Prior to joining

Toledo, he worked as a consulting engineer and received the Carson Innovation Award for his retrofit design of a

foundation system. Guner has published 35 technical papers and contributes to 5 national committees.

Sundar Chiluwal is a senior graduate student at The University of Toledo. His research focuses on advancing

the understanding of the response and failure modes of helical pile-to-footing connections and creating analysis

methods to advance current design practices.

Helical Piles and Tiebacks Committee meeting

Figure 6. Capacity predictions and experimental results for three methods

106 • DEEP FOUNDATIONS • MAR/APR 2020

The simulated deflected shapes, crack

patterns and failure modes are presented in

Figure 5 for the selected, representative

specimens. Major connection zone

cracking and failures are predicted for

Crack Patterns and Failure Modes

• The compression load results are

presented in Figures 4d to 4f, where the

overlapping, colored lines demonstrate

that the compressive load resistance is

independent of h changes for all e

bracket types examined.

• The tensile capacity of all bracket types

increased significantly with higher ρ , x

except for the bottom h , where the e

connection zone failure prevents any

significant capacity increase with

higher ρ . x

ratio (ρ ) percentages where the x

increased system load capacity resulted

in connection zone cracking in the

bottom h position, such that the e

connection zone starts to influence the

response (see the deviation in dashed

and dotted lines of Figure 4b).

• The a/d ratio, which represents how

thick a concrete pile cap or footing is,

affects the system capacity significantly,

with lower ratios (i.e., thicker members)

providing exponentially higher system

capacities (see bi-linear nature of lines

in Figures 4a to 4c).

• The double plate bracket has only one

h position, which exhibits a good e

performance since the connection zone

does not influence the system capacity

(see Figure 4c).

Comparison with Sectional Analysis Methods

some of the specimens subjected to tensile

loads. The bottom of the single plate he

bracket exhibited the least favorable

performance by sustaining connection

zone failures, as shown in Figure 5a. While

performing better, the bottom of the he

s tudded bracket exhib i ted major

connection zone cracking (see Figure 5b),

which reduced, but did not govern, the

system capacity. This type of cracking may

be detrimental to the long-term durability

of helical pile foundations. The double

plate bracket (Figure 5c), the middle of he

the single plate bracket (Figure 5d) and the

middle of the studded bracket (Figure he

5e) performed satisfactorily with no major

connection zone cracking. The double

plate bracket may provide additional

advantages, such as resilience to other

types of loads and long-term durability, due

to the top and bottom plates confining the

entire depth of the pile cap. Subjected to

pure compression loads, all specimens

with all bracket types exhibited global

failure modes of either flexure or shear,

without exhibiting any major connection

zone cracking, as shown in Figure 5f.

Both the numerical simulations and the

experimental tests include the influence

and failure modes of connection zones. The

traditional sectional analysis methods, on

the other hand, consider the concrete

foundation globally while neglecting the

local influences such as how the load is

introduced or how the supports and

Figure 5. Representative failure modes and crack patterns

connection zones are detailed. To assess the

significance of considering or neglecting

the connection zone behavior, the experi-

mental specimens were also analyzed with

the sectional methods contained in the ACI

318-19 standard. It is clear from Figure 6

that the experimental capacities are much

smaller than those predicted by the

sectional analysis method. This result

confirms that the connection capacity may

govern the entire system response and that

the use of the sectional analysis may

overestimate the system capacity (by a

factor of 2.2 on average in this study).

• The tension load capacities of the

concrete pile caps (all of which are

doubly and symmetrically reinforced)

are found to be 54% of their compres-

sion load capacities. If analyzed with the

traditional sectional analysis methods,

which neglect the influence of the

connection zones, their load capacities

in tension and compression would be

incorrectly calculated as equal.

Conclusions and Recommendations• The results demonstrate that helical

pile-to-foundation connections may

govern the entire system capacity for

the load conditions incorporating net

tension components. The traditional

global analysis methods, which are not

intended for the connection zones, may

significantly overestimate the capacity

of the helical pile systems, especially for

the design configurations involving

single plate terminations at the bottom

embedment (h ) position. e

• Connection zone failures are predicted

for the bottom h of the single plate ter-e

mination, with a decrease in the global

load capacity by 25% on average. It is

recommended that the middle h be used e

if the single plate termination is used.

• While no connection zone failure is

predicted, major connection zone

cracking is observed for the bottom h of e

the studded plate termination. For the

configurations involving the bottom h , e

the change of the bracket type from

single to studded improves the system

capacity by an average of 22%;

consequently, the studded plate bracket

may be preferred over the single plate

bracket for the bottom h . For the most e

optimum results, however, the middle

h is recommended for both the single e

and studded plate terminations.

• Although the bottom of the single plate he

termination demonstrated the least-

favorable behavior, it can still be success-

fully used for resisting tensile forces if a

special connection zone detailing is dev-

eloped (e.g., sufficient amounts of verti-

cal ties in the connection zone). This rec-

ommendation is also applicable to the

bottom of the studded plate terminations.he

• The double plate termination is con-

sidered with one embedment depth

covering the entire pile cap section,

which performed very well with no con-

nection zone failure in all simulations

contained in this study.

DEEP FOUNDATIONS • MAR/APR 2020 • 107

• The helical pile-to-foundation con-

nections are found not to influence the

monotonic compression load capacities

of the foundation systems in any of the

termination bracket types examined;

no connection failures are predicted. To

• The results of this study clearly demon-

strate the need for performing an explicit

capacity and service-load cracking

analysis for the connection zones when

resisting tensile forces, in addition to the

global structural and geotechnical anal-

yses. The recommended design con-

figurations are not intended to replace

the explicit connection zone analyses.

• The full project report, an open-access

spreadsheet, YouTube demonstrations

of the research and latest journal

publications can be downloaded from

Dr. Guner’s research website at

www.utoledo.edu/engineering/facult

y/serhan-guner/Publications.html

and from the Helical Piles and Tiebacks

Committee website at www.dfi.org/

commhome.asp?HLPR

• Future research and large-scale

experimental tests are required to

further investigate the effects of critical

system variables and develop simple

design equations for use in practice.

maximize the compression load capa-

cities, high ρ and low a/d ratios are x

recommended for all bracket types.

AcknowledgementsThe authors would like to thank the DFI

Committee Project Fund and the DFI

Helical Piles and Tiebacks Committee for

supporting this study, Mary Ellen Large for

administering this project and Dr. M.

Hesham El Naggar for providing the

experimental results. The authors would

also like to acknowledge the contributions

of Rafael A. Salgado (Ph.D. candidate) for

the statistical analyses, and Sálvio A.

Almeida Júnior (graduated M.S. student)

for his review and valuable comments.

Serhan Guner, Ph.D., P.Eng., is a civil engineering faculty member at The University of Toledo, Ohio, where he

received an ExCellence in Civil Engineering EDucation (ExCEEd) Fellowship from ASCE. Prior to joining

Toledo, he worked as a consulting engineer and received the Carson Innovation Award for his retrofit design of a

foundation system. Guner has published 35 technical papers and contributes to 5 national committees.

Sundar Chiluwal is a senior graduate student at The University of Toledo. His research focuses on advancing

the understanding of the response and failure modes of helical pile-to-footing connections and creating analysis

methods to advance current design practices.

Helical Piles and Tiebacks Committee meeting

Figure 6. Capacity predictions and experimental results for three methods

106 • DEEP FOUNDATIONS • MAR/APR 2020

The simulated deflected shapes, crack

patterns and failure modes are presented in

Figure 5 for the selected, representative

specimens. Major connection zone

cracking and failures are predicted for

Crack Patterns and Failure Modes

• The compression load results are

presented in Figures 4d to 4f, where the

overlapping, colored lines demonstrate

that the compressive load resistance is

independent of h changes for all e

bracket types examined.

• The tensile capacity of all bracket types

increased significantly with higher ρ , x

except for the bottom h , where the e

connection zone failure prevents any

significant capacity increase with

higher ρ . x

ratio (ρ ) percentages where the x

increased system load capacity resulted

in connection zone cracking in the

bottom h position, such that the e

connection zone starts to influence the

response (see the deviation in dashed

and dotted lines of Figure 4b).

• The a/d ratio, which represents how

thick a concrete pile cap or footing is,

affects the system capacity significantly,

with lower ratios (i.e., thicker members)

providing exponentially higher system

capacities (see bi-linear nature of lines

in Figures 4a to 4c).

• The double plate bracket has only one

h position, which exhibits a good e

performance since the connection zone

does not influence the system capacity

(see Figure 4c).

Comparison with Sectional Analysis Methods

some of the specimens subjected to tensile

loads. The bottom of the single plate he

bracket exhibited the least favorable

performance by sustaining connection

zone failures, as shown in Figure 5a. While

performing better, the bottom of the he

s tudded bracket exhib i ted major

connection zone cracking (see Figure 5b),

which reduced, but did not govern, the

system capacity. This type of cracking may

be detrimental to the long-term durability

of helical pile foundations. The double

plate bracket (Figure 5c), the middle of he

the single plate bracket (Figure 5d) and the

middle of the studded bracket (Figure he

5e) performed satisfactorily with no major

connection zone cracking. The double

plate bracket may provide additional

advantages, such as resilience to other

types of loads and long-term durability, due

to the top and bottom plates confining the

entire depth of the pile cap. Subjected to

pure compression loads, all specimens

with all bracket types exhibited global

failure modes of either flexure or shear,

without exhibiting any major connection

zone cracking, as shown in Figure 5f.

Both the numerical simulations and the

experimental tests include the influence

and failure modes of connection zones. The

traditional sectional analysis methods, on

the other hand, consider the concrete

foundation globally while neglecting the

local influences such as how the load is

introduced or how the supports and

Figure 5. Representative failure modes and crack patterns

connection zones are detailed. To assess the

significance of considering or neglecting

the connection zone behavior, the experi-

mental specimens were also analyzed with

the sectional methods contained in the ACI

318-19 standard. It is clear from Figure 6

that the experimental capacities are much

smaller than those predicted by the

sectional analysis method. This result

confirms that the connection capacity may

govern the entire system response and that

the use of the sectional analysis may

overestimate the system capacity (by a

factor of 2.2 on average in this study).

• The tension load capacities of the

concrete pile caps (all of which are

doubly and symmetrically reinforced)

are found to be 54% of their compres-

sion load capacities. If analyzed with the

traditional sectional analysis methods,

which neglect the influence of the

connection zones, their load capacities

in tension and compression would be

incorrectly calculated as equal.

Conclusions and Recommendations• The results demonstrate that helical

pile-to-foundation connections may

govern the entire system capacity for

the load conditions incorporating net

tension components. The traditional

global analysis methods, which are not

intended for the connection zones, may

significantly overestimate the capacity

of the helical pile systems, especially for

the design configurations involving

single plate terminations at the bottom

embedment (h ) position. e

• Connection zone failures are predicted

for the bottom h of the single plate ter-e

mination, with a decrease in the global

load capacity by 25% on average. It is

recommended that the middle h be used e

if the single plate termination is used.

• While no connection zone failure is

predicted, major connection zone

cracking is observed for the bottom h of e

the studded plate termination. For the

configurations involving the bottom h , e

the change of the bracket type from

single to studded improves the system

capacity by an average of 22%;

consequently, the studded plate bracket

may be preferred over the single plate

bracket for the bottom h . For the most e

optimum results, however, the middle

h is recommended for both the single e

and studded plate terminations.

• Although the bottom of the single plate he

termination demonstrated the least-

favorable behavior, it can still be success-

fully used for resisting tensile forces if a

special connection zone detailing is dev-

eloped (e.g., sufficient amounts of verti-

cal ties in the connection zone). This rec-

ommendation is also applicable to the

bottom of the studded plate terminations.he

• The double plate termination is con-

sidered with one embedment depth

covering the entire pile cap section,

which performed very well with no con-

nection zone failure in all simulations

contained in this study.

DEEP FOUNDATIONS • MAR/APR 2020 • 107