Foundation Selection Process

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January 15, 2014

IEEE PES Winter MeetingNew OrleansOverhead Lines Subcommittee

Foundation Selection Process

By: Paul G Cass, P. E.


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STRUCTURE FOUNDATION

THE FOUNDATION SELECTION

PROCESS SHOULD BE PART OF THE

STRUCTURE SELECTION PROCESS


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FOUNDATION/STRUCTURE SELECTION IS

INFLUNCED BY:

ELECTRICAL CRITERIA AND EFFECTS TOPOGRAPHY/GEOGRAPHY

SITING/ROW LIMITATIONS

ENVIRONMENTAL ISSUES

GEOLOGY


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TOPOGRAPHY/GEOGRAPHY

STRUCTURE SPACING /TYPE

ACCESS

CONSTRUCTABILITY

STREAMS/FLOODING

GEOLOGIC HAZARDS


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SITING/ROW LIMITATIONS

ROW WIDTHBLOWOUTSTRUCTURE

SPACING

LOCATION AND QUANTITY OF ANGLE

STRUCTURES

AESTHETICS

RESTRICTIONS ON TYPE OF CONSTRUCTION


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ENVIRONMENTAL ISSUES

Wetland avoidance/restrictions

Construction noise

Endangered species restrictions Time of construction restrictions

Type of construction limitations


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GEOLOGY

WHERE IS TOP OF ROCK?

HARD ROCK ISSUES

WATER ISSUES SOFT/USSTABLE SOIL ISSUES

LANDSLIDES/KARST/MINING/SUBSIDENCE


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STRUCTURE CHOICES (TYP)

SELF SUPPORTING LATTICE STEEL TOWER

GUYED-V LATTICE STRUCTURE

STEEL POLE CONCRETE POLE

WOOD POLE

STEEL POLE H-FRAME WOOD H-FRAME


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SELF SUPPORTING LATTICE

STEEL TOWER

Best For

High Voltage, large or bundled

conductor

Long Span Construction, hilltop to hill

top

Rigid termination or dead-end

structures

Low material cost

Worst For High structure erection cost

Aesthetics


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GUYED-V LATTICE STRUCTURE

Best For

High Voltage, large or bundled

conductor

Long Span Construction, hilltop to hill

top

Low material and erection cost

Worst For

Limited right-of-way width


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STEEL, CONCRETE & WOOD POLES

Best For

General purpose structure for most

voltages and varied site conditions

Limited right-of-way widths

Worst For

Sites with shallow hard rock

Sites with deep soft soil


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STEEL & WOOD POLE H-FRAMES

Best For

69 to 230kV lines

Medium span construction (400 to

800 feet)

Low cost materials and

construction

Can be guyed for longitudinal

capacity

Worst For


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FOUNDATION TYPES

CHOICES

CYLINDRICAL FOUNDATIONS

SHALLOW FOUNDATIONS

GROUP ACTION FOUNDATIONS

ANCHOR TYPE FOUNDATIONS


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CYLINDRICAL FOUNDATIONS

DRILLED PIER

DIRECT EMBEDMENT

VIBRATORY POLE OR CAISSON HYBRID POLES


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DRILLED PIER

Best For Flexibility in designing

and constructing

foundation

Generally cost

effective for poles

Worst For

Sites with equipment

Access issues

Sites with shallowhard rock

Sites with deep soft or

loose soil


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DIRECT EMBEDMENT

Best For Generally more cost effective

than drilled piers

Tangent poles

Worst For

Large angle or dead-end poles

Groundline corrosion

Sites with high water table or

unstable soils

Sites with shallow hard rock Sites with deep soft or loose soil


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VIBRATED POLES OR CAISSON

Best For Sites with granular soils

Sites with high water table

Worst For

Stiff clays, shallow rock

Deep foundations



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SHALLOW FOUNDATIONS

GRILLAGE OR BASKET

CONCRETE SPREAD FOOTING

CONCRETE MATS CONCRETE RING


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GRILLAGE OR BASKET

Best For Sites with normal soil

conditions

Low cost using small

common equipment

Worst For


Sites with deep soft

or loose soil

Sites with shallowhard rock

Structures with heavy

loads


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CONCRETE SPREAD FOOTING

Best For Flexible foundation sizing

Heavy loads supported by soil

Potential to incorporate rock

anchors to improve uplift

capacity.

Worst For

Generally higher costs than

other foundation types

Sites with shallow hard rock


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CONCRETE MATSBest For

Soft, loose and varied soil conditions

Uplift resistance provided by foundation weight

Reduced bearing pressure on soil

Shallow water

Preventing differential settlement problems

Worst For

Relatively high cost

Normal soil conditions


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GROUP ACTION FOUNDATIONS

PILE GROUP

MICROPOLE GROUP

HELICAL PULLDOWN MICROPILE GROUP ROCK ANCHOR GROUP


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PILE GROUPSBest For Sites with expected

construction difficulties

associated with water

and side wall caving

Granular soil with

increasing density withdepth

Battered piles to

improve lateral capacity

Worst For

Limited uplift capacityin stiff clays

Relatively high cost


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MICROPILE GROUPSBest For Sites with thick layer of

soft or loose soil over

rock or dense soil.

May be cost effective

over drilled piers if rock

excavation is difficult Battered piles to

improve lateral capacity

Worst For Where drilled piers are

cost effective Relatively high cost


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HELICAL PULLDOWN MICROPILE GROUP

Best For Granular soils below the water table. Use cased helical micropile where

difficulty in maintaining an open hole

is anticipated.

Battered piles to improve lateral

capacity. Potential for significant cost savings

over drilled piers at difficult sites.

Worst For Where drilled piers are cost effective

PILE GROUPHELICAL PULLDOWN MICROPILE

Ref: AB Chance


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ROCK ANCHOR GROUPSBest For

Shallow hard rock May be cost

effective over

drilled piers if rock

excavation isdifficult

Worst For

Deep rock

Where drilled piers

are cost effective


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ANCHOR TYPE FOUNDATIONS

HELICAL ANCHORS

DEAD-MAN ANCHORS

EXPANDING ANCHORS ROCK ANCHORS

MALONE FOUNDATION

HELICAL ANCHOR FOUNDATION

ROCK EMBEDDMENT OF TOWER STUB ANGLE


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HELICAL ANCHORS

Best For Normal Soil

Flexible depth and helix

configurations.

Worst For

Boulders & cobbles

Deep soft soils

Shallow rock


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DEAD-MAN ANCHORSBest For

Anchor capacity in poor soil Improved anchor capacity in

normal soils

Flexible design

Worst For Relative high cost compared

with helix anchors


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EXPANDING ANCHORS

Best For

Normal Soil

Flexible depth and expanding

anchors.

Sites with cobbles Poor access sites - jackhammer

installation

Worst For

Shallow rock


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ROCK ANCHORS

Best For

Shallow rock

Worst For

Unstable overlying soils

Deep rock


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MALONE FOUNDATION

Design and construction similar toFranki foundation

Best For

Sites with loose to medium dense

granularimproves soil

Potential for significant costsavings

Worst For

Non-standard construction

Difficult QC

Sites with stiff clays or shallow

rock


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HELICAL ANCHOR FOUNDATION

Best For Towers with stub angles

Granular soils below the water

table

Sites with expected construction

difficulties associated with waterand side wall caving

Potential for significant cost

savings over drilled piers

Worst For Shallow rock

Sites with cobbles

Ref: AB Chance


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ROCK EMBEDMENT OF STUB ANGLE

Best For Shallow rock

Towers with stub angles

Low cost

Worst For

Deep rock

Limited flexibility in

foundation depth


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Case Study 1ELECTRICAL CRITERIA AND EFFECTS 345kV Single Circuit Line EMF controls structure height , conductor configuration and

phasing

TOPOGRAPHY/GEOGRAPHY Mountainous

SITING/ROW LIMITATIONS 250 ROW shared with existing 345kV tower line

ENVIRONMENTAL Wetland avoidance, Endangered Species

GEOLOGY Shallow Igneous Rocks or Soft Alluvial Deposits over rock


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Case Study 2ELECTRICAL CRITERIA AND EFFECTS 230kV single circuit replacing 138kV double circuit on towers

TOPOGRAPHY/GEOGRAPHY Flat

SITING/ROW LIMITATIONS Access limited to only the existing access road (10 except atstructures)

ENVIRONMENTAL Extreme limits on disturbance and time of construction

GEOLOGY High Value Wetlands, 4 organics overlying 30 of loose to

medium dense sand over 100 dense sand


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SUMMARY

STRUCTURE AND FOUNDATION SELECTIONS

SHOULD BE MADE TOGETHER.

FOUNDATIONS COST A LOT - 40% TO 100% OF

STRUCTURE COST CONSIDER OTHER INFLUENCES IN

STRUCTURE/FOUNDATION SELECTION PROCESS

Soft unstable soils and hard rock usually causethe greatest problems if the wrong foundation is

chosen

Foundation Selection Process

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