G d A h d A h d Ground Anchors and Anchored Structures

Presentation to:

WV Expo 2008 – Charleston Civic Center

Jonathan Bennett PE – Chief EngineerEarth Support Division

March 20, 2008

GeoStructures’ Earth Support DivisionGeoStructures Earth Support Divisionfocuses on the engineering and construction of

ground anchors and anchored structures.

Tieback Walls Soil Nailing

Underpinning Micropiles Tiedowns

Tieback Wall Soil Nailing

Underpinning Micropiles Tiedowns

Jon Bennett Speaker BioJon Bennett - Speaker BioChief Engineer – GeoStructures Earth Support Division (formerly TerraTech)

BS Civil Engineering (Structures) – West Virginia Institute of TechnologyMS Civil Engineering (Structures) West Virginia UniversityMS Civil Engineering (Structures) – West Virginia UniversityMEM Engineering & Technology Management – George Washington University

Professional Engineer in WV, VA, MD, and PA

Professional background in Structural and Foundation Engineering with CH2M Hill and the Parsons Corporation prior to joining TerraTech in 1993.

15 years involvement in the design, construction and management of design-build y g g gspecialty geotechnical construction projects throughout the mid Atlantic region of the United States.

Chairman of the DFI Tiebacks and Soil Nailing Committee. Heavily involved in g yindustry development efforts for Tieback Walls, Soil Nailing, and Micropilesthrough DFI, ADSC, and ASCE GEO Institute. Involved in AASHTO specification development for anchored walls and micropiles with T15 Committee.

Stayed at a Holiday Inn Express last night.

GROUND ANCHORSGROUND ANCHORS

• A Ground Anchor is a structural element installed in a grout-filled hole in gsoil or rock that is used to transmit an applied tensile force into the ground.

• Ground Anchors can be installed vertically, horizontally, or in inclined positions. A minimum inclination of 10 degrees below horizontal is desirable i d t ll f ll ti f th d h h lin order to allow full grouting of the ground anchor hole.

• Ground Anchors derive their load capacity from the bond stresses between the grout body and surrounding soil or rock. Hence, the load capacity of a ground anchor is the lesser of tendon strength or grout bond strengthground anchor is the lesser of tendon strength or grout bond strength.

• Grouted Ground Anchors used to offer lateral support are often referred to as “Tiebacks”. Soil Nails, Micropiles, and Tiedowns are also forms of Ground Anchors.

CComponents

• Anchorage

• TendonTendon

• Bonded Length

• Unbonded (Free) Length

• Corrosion Protection (for Permanent Applications)

Strand Tendon Bar Tendon

Picture courtesy of Lang Tendons Picture courtesy of Dywidag Systems International (DSI)y g y y g y ( )

SCOPE

• Materials / Corrosion Protection

• Tendon Design

G t G d B d G id li• Grout – Ground Bond Guidelines

• Handling and Installation

• Testing and Acceptanceg p

TIEBACK WALLSTIEBACK WALLS

• Definitions• Applicationspp• Components• Types and Facing Options• Construction Procedures / Sequence• Design Background

T ti• Testing• Case History

• As the name implies, Tieback Walls use “Tiebacks” or Ground Anchors for p ,lateral support.

• Tieback Wall construction typically is done from the top down. This is a departure from conventional retaining wall construction wherein the retained

t i l i d th ll i t t d d th th t i d t i lmaterial is removed, the wall is constructed, and then the retained material is put back behind the wall.

• Unlike conventional retaining wall construction, tieback wall construction does not substantially disturb the material or structures that the wall isdoes not substantially disturb the material or structures that the wall is retaining or supporting.

• Tieback Walls are often used to provide excavation support for the construction of conventional retaining walls. Using a permanent facing for the tieback wall often is more economical than constructing a conventional retaining wall.

• Because of the unique top-down construction approach, Tieback Walls can often be used where conventional retaining walls cannot be constructed oroften be used where conventional retaining walls cannot be constructed or are not an economically feasible option.

ApplicationsApplications• Permanent Earth Retention• Temporary Excavation Supportp y pp• Slope Stabilization / Landslide Stabilization or Repair• Repair or Rehabilitation of Existing Retaining Walls• Tieback Bridge Abutments• In-situ Hazardous Material Containment

ComponentsComponents1. Soldier Piles

2. Lagging

3. Tiebacks (Ground Anchors)

4. Wales or Through-Beam Connections

5 Permanent Facing System (if required)5. Permanent Facing System (if required)

ComponentsComponents1. Soldier Piles

2. Lagging

3. Tiebacks (Ground Anchors)

4. Wales or Through-Beam Connections

5 Permanent Facing System (if required)5. Permanent Facing System (if required)

Types• Temporary Excavation Supportp y pp• Permanent Earth Retention w/ CIP or Shotcrete• Permanent Earth retention w/ Segmental Precast Facing

Precast Facing vs. CIP or Shotcrete FacingPrecast Facing vs. CIP or Shotcrete Facing

Superior Drainage SystemFree-draining material and unrestricted outlet path for effective flow management.

Minimum Schedule ImpactPanels can be manufactured prior to or concurrently with pile installation.Rapid, productive installation.

Precision AlignmentAdjustable connections allow alignment to be adjusted independent of pile alignment.Interlocking panel joints maintain alignment.

High Quality ControlPlant manufactured.No Hot or cold weather placement concerns.

Superior AppearancePlant manufactured.Panel geometry easily accommodates form liners for a variety of finishes.

Other Precast Facing Architectural FinishesOther Precast Facing Architectural Finishes

Construction SequenceConstruction Sequence

Excavation Support Construction SequenceExcavation Support Construction Sequence1. Install soldier piles (by drilling or driving).

2. Excavate in safe lifts not to exceed five feet each and install lagging to 2 f t b l ti b k dfeet below tieback grade.

3. Install tiebacks. Allow 72 hour minimum grout cure time prior to testing.

4. Test tiebacks in accordance with PTI – Recommendations for PrestressedRock and Soil Anchors. Lock off tiebacks at specified load.

5. Continue excavation and lagging in accordance with Step 2 above to either 2 feet below tieback grade or construction subgrade, whichever comes first.g g

Excavation Support Construction SequenceExcavation Support Construction Sequence

Soldier Pile InstallationSoldier Pile Installation• Drilled and Set HP Section

• Driven HP Section

• Drilled Pipe

Lagging InstallationLagging Installation• Install Piles and Excavate in Safe Lifts not to Exceed 5’ in Soil.

• Install Lagging Boards on Exposed Soil Face.

• Repeat Excavation Lift and Board Installation as Required.

Tieback InstallationTieback Installation• Position Drill and Drill Hole for Ground Anchor.

• Insert Anchor in Drilled Hole and Tremie Grout.

• Post Grout as Required to Increase Bond Capacity.

Permanent Wall Construction Sequence (Precast)Permanent Wall Construction Sequence (Precast)1. Install soldier piles (drilling generally used for permanent walls).

2. Excavate in safe lifts not to exceed five feet each and install lagging to 2 f t b l ti b k dfeet below tieback grade.

3. Install tiebacks. Allow 72 hour minimum grout cure time prior to testing.

4. Test tiebacks in accordance with PTI – Recommendations for PrestressedRock and Soil Anchors. Lock off tiebacks at specified load.

5. Continue excavation and lagging in accordance with Step 2 above to either 2 feet below tieback grade or construction subgrade, whichever comes first.g g

6. Place filter fabric over timber lagging and install concrete leveling pad.

7. Layout and attach panel connections to soldier piles.

8. Set bottom course of precast facing panels, install drain pipe, and place drainage stone to top of precast course.

9. Install remaining precast facing panels and place drainage stone with each corresponding lift.

Permanent Wall Construction Sequence (Precast)Permanent Wall Construction Sequence (Precast)

Design BackgroundDesign Background

In 1939, Karl Terzaghi published the paper “A Fundamental Fallacy in Earth Pressure Computations” where he y precognized that the earth pressure distributions for braced or anchored cuts do not correspond to the traditional equivalent fluid pressure (triangular) diagrams derived from Rankineand Coulomb earth pressure theoryand Coulomb earth pressure theory.

The later works of Karl Terzaghi and Ralph Peck (1967) form the framework for earth pressure diagrams used in the design of modern braced and anchored cuts These worksKarl Terzaghi design of modern braced and anchored cuts. These works along with others since that time and actual field testing consistently point away from the classic triangular earth pressure diagram and toward a rectangular or trapezoidal apparent earth pressure distribution.

The most recent and definitive work in the design of Tieback walls with regard to apparent lateral earth pressure is the FHWA G t h i l E i i Ci l N 4 “G dFHWA Geotechnical Engineering Circular No. 4 “Ground Anchors and Anchored Structures” (1999) which recommends a trapezoidal earth pressure diagram.

Ralph Peck

Design BackgroundDesign Background

Tieback TestingTieback Testing

Ground Anchors are tested to verify load capacity.

• Tiebacks / Tiedowns

• Soil Nails

• Micropiles in Tension

• Micropiles in Compression

Ground Anchor TestingGround Anchor Testing

All “tieback” anchors are tested to erif load capacit• All “tieback” anchors are tested to verify load capacity.

• Tiebacks have an unbonded length or free stressing length to transfer the anchor load outside of the retained soil theoretical failure wedge as opposed to Soil Nails being fully bonded along their length.

• A sample of soil nails (typically 5%) are tested to verify bond transfer capacity assumptions used in the soil nail wall design.capacity assumptions used in the soil nail wall design.

• That is an important distinction when differentiating between tieback testing and soil nail testing.

• For this section, we are focusing on testing of “tieback” anchors.

Tieback Anchor Behavior

Load Elastic MaterialM

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Plastic Material

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Both the Post Tensioning Institute and AASHTO have published Ground Anchor Testing Specifications. The testing procedures are virtually identical with the AASHTO version being an adaptation of the PTI guidelines.

Proof / Performance TestingProof / Performance Testing

Incremental loading and nloading to a ma im m test load of 1 33DL• Incremental loading and unloading to a maximum test load of 1.33DL.

• The first two or three anchors shall be Performance Tested and a minimum of 2% thereafter. The remaining anchors shall be proof tested.

• Performance test utilizes cyclic loading in order to differentiate elastic movement from residual movement at each test increment.movement from residual movement at each test increment.

• In both types of tests, the maximum test load is held for a short term creep test to ensure the anchor will have acceptable long term creep behaviorbehavior.

• If the PTI Service Load design methodology is used, then a production anchor can be tested without increasing the bar size. If a maximum

l d f h 1 33DL i d h b b d i d ftest load of greater than 1.33DL is used, the bar must be designed for the maximum test load instead of the design service load.

P f T t P dProof Test ProcedureAL0.25DL0.50DL0.75DL1.00DL1.20DL1.33DL Max Test Load (10 minute hold)33 a es oad ( 0 u e o d)AL (optional)Adjust to Lock-Off LoadAdjust to Lock-Off Load

Performance Test Procedure

AL0.25DL

AL0.25DL

AL0.25DL0.50DL

0.50DL0.75DL1.00DL

AL0.25DL0.50DL

1.20DLAL0.25DL

0.75DLAL0.25DL

0.50DL0.75DL1.00DL

0.50DL0.75DL1.00DL

1.20DL1.33DL Max Test Load (10 min hold)ALAdjust to lock-off load

Acceptance CriteriaAcceptance Criteria• Creep

· Shall not exceed 0 040 inches at the maximum Test Load during· Shall not exceed 0.040 inches at the maximum Test Load during the load hold period of 1 to 10 minutes.

· If that value is exceeded, then the load hold period shall be d d 60 i d h l b 6extended to 60 minutes and the total creep movement between 6

and 60 minutes shall not exceed 0.080 inches.

• Movement

· Residual movement – no absolute criteria

· Minimum Apparent Free Tendon Length >= 0.80 (Lu + Lj)

· Maximum Apparent Free Tendon Length <= Lu + Lj + 0.5Lb

• Lock-Off Load

· Within 5% of designated Lock-off load as verified by lift-off.

Proof TestingProof Testing• Determines whether the anchor has sufficient load capacity.

• That the apparent free tendon length has been satisfactorily pp g yestablished.

• That the creep rate stabilizes within specified limits.

Performance TestingPerformance Testing• Determines whether the anchor has sufficient load capacity.

• That the apparent free tendon length has been satisfactorilyThat the apparent free tendon length has been satisfactorily established.

• The magnitude of residual movement at each load increment.

• That the creep rate stabilizes within specified limits.

SR 56 Section 021 Cambria County ~ Landslide StabilizationJohnstown, PA

SOIL NAILINGSOIL NAILING

• Definitions• Applicationspp• Components• Types and Facing Options• Construction Procedures / Sequence• Design

N il T ti• Nail Testing• Case History

• Soil Nailing uses an array of grouted ground anchors or “Soil Nails” to g y g gimprove the strength characteristics of a soil mass. This improvement in strength properties causes the soil to be self supporting and stable against lateral movement.Sh t t i t i ll d f i i d t t i th t i d il• Shotcrete is typically used as a facing in order to contain the retained soil.

• Like Tieback Wall construction, Soil Nailing is generally performed from the top down and as such offers many of the advantages that Tieback Wall construction does in addition to othersconstruction does in addition to others.

• Soil nailing does not require the installation of vertical pile elements nor does it require toe embedment. Because of this, it can be more economical than Tieback Walls in situations where it is appropriate.

• Because of the way in which Soil Nailing is installed, it is more sensitive to site soil conditions than Tieback Walls and may not be appropriate under certain circumstances.

ApplicationsApplications• Permanent Earth Retention• Temporary Excavation Supportp y pp• Slope Stabilization• Repair or Rehabilitation of Existing Retaining Walls

Componentsp

1. Shotcrete Facing System

2. Soil Nails

3. Permanent Facing System (if required)

Types• Temporary Excavation Supportp y pp• Permanent Earth Retention w/ CIP or Shotcrete• Permanent Earth retention w/ Segmental Precast Facing

Design ConsiderationsDesign Considerations

Because Soil Nailing is primarily a form of ground improvement, it is generally looked at in terms of soil-structure interaction as opposed to being designed strictly as a force resisting

Th f h d i f il ili i h d l ilstructure. Therefore, the design of a soil nailing system is approached as a slope or soil mass stability problem.

There are a number of methodologies used for the analysis and design of Soil Nailing systems.systems.

- Two part wedge limit equilibrium analysis – The Krantz Method- University of California Davis limit equilibrium model – The Davis Method- Modified Davis Method

L i l f i l di b di tiff Th Ki ti l M th d- Log-spiral surface including bending stiffness – The Kinematical Method- Methods outlined in FHWA Research Projects – Slope Stability / Soil-Structure Interaction

SOFTWARE- GoldNAIL by Golder Associates (Circular Slip Circles)GoldNAIL by Golder Associates (Circular Slip Circles)- SNAIL by Caltrans (Bi-Linear Wedge Analysis)- General Slope Stability Programs modified to consider the effects of Soil Nails

Similar results are obtained from all of these methods for normal design conditions that are vertical walls without slope surcharge. Some of the software on the market (i.e. GoldNAIL) tends to be quite conservative in terms of nail lengths and nail bond stresses required.

Design Guidelines and SpecificationsDesign Guidelines and Specifications

The FHWA took the lead in the 1990’s in developing design guidelines for Soil Nail Walls through a number of Demonstration Projects.

In 1996, FHWA produced the Manual for Design and Construction Monitoring of Soil Nail Walls with Golder Associates based on Demonstration Project 103. This document introduced the program GoldNAIL for the design of soil nail walls. GoldNAIL remains one of two dominant computer programs for soil nail wall design.two dominant computer programs for soil nail wall design.

In 2003, FHWA superseded the previous manual with Geotechnical Engineering Circular 7 – Soil Nail Walls. This document was developed by GeoSyntec Consultants and included CalTrans SNAIL in the software lineup. There is considerable debate in the industry

di th il t ti d i l d d i GEC 7 d t h b d tregarding the nail testing procedures included in GEC-7 and requests have been made to change those requirements. It is recommended to use the testing rpocedures included in the 1996 manual.

Later in 2008, it is expected that the Deep Foundations Institute will publish its GuideLater in 2008, it is expected that the Deep Foundations Institute will publish its Guide Specification for Temporary and Permanent Soil Nail Walls that will include provisions for using hollow bar (self drilling) soil nails. This specification was developed by the DFI Tiebacks and Soil Nailing Committee with input from consultants and design-builders throughout the United States. It is more of an industry consensus document than thethroughout the United States. It is more of an industry consensus document than the previous specifications included in the FHWA publications.

Soil Nail TestingSoil Nail Testing

In soil nail str ct re design e are more concerned ith bond transfer• In soil nail structure design, we are more concerned with bond transfer values along the nail than the overall pullout capacity.

• Tiebacks have an unbonded length or free stressing length to transfer the anchor load outside of the retained soil theoretical failure wedge as opposed to Soil Nails being fully bonded along their length.

• All “tieback” anchors are tested to verify load capacity. However, due toAll tieback anchors are tested to verify load capacity. However, due to the relatively large number and low capacity of soil nails on a typical job, a sample of soil nails (typically 5%) are tested to verify bond transfer capacity assumptions used in the soil nail wall design.

• Because we are not testing all of the nails, we use a higher test load (1.5DL vs. 1.33DL) to offset the reduction in number of tests.

S i h il ili lki b b d l ifi i• So, with soil nailing, we are talking about bond value verification testing and sampled proof testing to a higher test load.

BW Parkway / MD 197 Ramp B Southbound ~ Earth Retention SystemLaurel, MD

UNDERPINNINGUNDERPINNING

Underpinning is the addition of support to an existing foundation A foundationUnderpinning is the addition of support to an existing foundation. A foundation may require underpinning in order to transfer building loads to a lower elevation and permit adjacent excavation or to increase load carrying capacity. Normally, underpinning involves both vertical and lateral support. This process typically

i h d ti f ti l h ft ( it ) b l th i ti f tirequires hand excavation of vertical shafts (pits) below the existing footings. The pits are then filled with concrete thus creating new foundation elements for the building or structure.

MICROPILESMICROPILES

• DefinitionDefinition• History & Development• Materials• Installation Equipment• Structural Configurations• Typical Capacity Ranges• Applications

D i d C t ti S ifi ti• Design and Construction Specifications• Design Comparison• Testing• Testing• Q&A

• A Micropile is generally defined in the industry as a reinforced small p g y ydiameter (less than or equal to 12”) drilled and grouted replacement pile.

• A “replacement” pile refers to a foundation pile where soil or rock is removed during the installation process as opposed to a displacement pile th t di l il it i i t ll dthat displaces soil as it is installed.

• Micropiles can be installed at angles and are able to resist both axial and lateral loads.

• Micropiles develop their axial capacity primarily through the bond between• Micropiles develop their axial capacity primarily through the bond between grout and soil or rock in the bonded zone of the pile. Because of this, micropiles provide both tension and compression resistance thus making them useful in a variety of applications.

• They are installed using much of the same drilling and grouting equipment that is used for tiebacks and soil nailing.

• Because of specialized installation methods, micropiles can be used in soil and rock conditions where the use of conventional deep foundation systems are not a reasonable alternative (such as in Karst topography or low-headroom conditions).

History & DevelopmentHistory & Development

Which came first? The Wheel or the Pile?

History & DevelopmentHistory & Development• 1950’s – Post WWII Europe – Dr. Fernando Lizzi – Root Piles• Rapid Emergence in US following FHWA Research

1997 FHWA Mi il St t f P ti D t• 1997 FHWA Micropile State of Practice Document• 2000 FHWA Micropile Guidelines• 2003 DFI Guide Specification• 2005 NHI / FHWA Micropile Reference Manual• 2006 IBC Micropile Section Adoption• 2007 FHWA LRFD Design Specification Adoption2007 FHWA LRFD Design Specification Adoption• Currently estimated to be $300M US market

MaterialsMaterials• Pipe Casing (typically mill secondary oilfield casing)• Solid or Hollow Reinforcing Barsg• Neat Cement Grout

+

=

EquipmentEquipmentUse essentially the same or similar equipment used for installation of drilled and grouted ground anchors.

Typical Capacity RangesTypical Capacity Ranges• Over 500 Tons in rock• 20 to 200 Tons in soil (settlement more significant)( g )• In rock, structural capacity often governs• Most micropile lengths are less than 100 feet• Buckling is typically not an issue• Cost range from $50 to $200 / LF

ApplicationsApplications• Foundation Piles• Foundation Support through Sinkholes or Difficult Soilspp g• Foundation Underpinning• Slope Stabilization• Earth Retention (A-Frame & Reticulated Structures)

Design and Construction SpecificationsDesign and Construction Specifications• 1997 FHWA Micropile Guidelines• 2003 DFI Micropile Guide Specificationp p• 2005 FHWA / NHI Micropile Reference Manual• 2006 International Building Code• 2007 AASHTO LRFD Design Specification• Forthcoming AASHTO Construction Specification

GeoStructures can provide a recommended specification for Micropile Design and Construction.g

Design ComparisonDesign Comparison• FHWA Design Criteria

– Compression: Pa = 0.40fc’Ag + 0.47FyAb

– Tension: Pa = 0.55FyAb

DFI / IBC Design Criteria• DFI / IBC Design Criteria– Compression: Pa = 0.33fc’Ag + 0.40FyAb

– Tension: Pa = 0.60FyAby

• Imposed LimitationsFHWA Compression: F = 87 ksi max (strain compatibility )– FHWA Compression: Fy = 87 ksi max (strain compatibility )

– DFI Compression: Fy = 87 ksi max (strain compatibility )– IBC Compression: Fy = 80 ksi max– IBC Compression: 0.40FyAb >= 0.40Pa

TestingTesting• Generally based on ASTM D1143 Quick Test• The older FHWA specifications prescribed testing to 2.5 X Service

Design LoadDesign Load• Newer publications recommend 2.0 X DL. Terratech recommends

2.0 DL in most cases.T i t ti i ll id d t b ti• Tension testing is generally considered to be conservative compared to compression testing because it neglects any end bearing and is often more economical for checking capacity. However tension test results will not give representative movementHowever tension test results will not give representative movement results for compression case.

• Compression testing requires anchors to hold down testing apparatus adding to cost but gives representative results forapparatus adding to cost but gives representative results for compression loading.

• Some proposed testing procedures incorporate anchor testing procedures (cycles) but AASHTO is leaning toward testing just likeprocedures (cycles) but AASHTO is leaning toward testing just like other foundation piles.

TestingTesting• 1000k Maximum Test Load, Berryville, VA (0.5” total movement)

General Q & AGeneral Q & A

THANK YOUfor Your Time and Attentionfor Your Time and Attention

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