A non-profit organization focused on bringing together

A non-profit organization

focused on bringing together

Contractors, Engineers, Geologists,

Manufacturers and Public Agencies in the

pursuit of excellence

1

www.GeohazardAssociation.org – Est. 2013

Ground Anchoring 201Presented by:

Thomas Bird; VP Corporate Development

Peter Speier, VP Engineering

Williams Form Engineering Corporation

Pete & Tom

Pete Speier, P.E.

VP Engineering

Over 24 Years Experience in

Industry

Current Co-Chair for Joint

ADSC/DFI Micropile Committee

Champion for ADSC/DFI Research

of Metallurgical Creep at High

Stress in 150 ksi bars – ongoing

[email protected]

Tom Bird

VP Business Development

Over 25 Years Experience in

Industry

Past ADSC Anchored Earth

Retention Committee Chair

Championed FHWA Hollow Bar

Study

[email protected]

Today’s Agenda

Anchoring 101 Review

Applications

Equipment

Products

Anchoring 201

Material Property Considerations

Geotechnical Considerations

Specifications

Durability Overview

Economics of Ground Anchors

Q & A

Anchoring Applications

Rockfall Mitigation

“Rock Anchors & Rock Dowels are common terms for anchors to

actively or passively resist rockfall through facial stability, rockfall

netting drapes or impact barriers

Dowel vs Preloaded Anchor

A preloaded or prestressed anchor

will resist the applied uplift load

positively until the applied load

becomes greater than the preload

The preload is applied as part of the

testing & installation procedures,

and is usually performed via a

calibrated hydraulic test jack.


Soil Nail Walls

This common excavation support method is based on passive

dowel anchors called “Soil Nails”

Soil Nails are closely spaced, fully grouted anchors that resist

ground movement through mutual interaction.

Slight movement should be anticipated until the anchors

become “activated” by ground movement.


Tieback Walls

This picture shows what is commonly referred to as a “Tieback Wall”. Preloaded (post-tensioned) ground anchors are

installed and are designed to transfer load to a structural front wall face.

Less movement would be expected with this system given the use of Prestressed Anchors

Prestressed anchors used n this application have a Bond Zone & a Free Stressing Zone

Anchoring 101

Specialized Drilling Equipment

Specialized Drilling Equipment

Open Hole Cased Hole

Open hole drilling occurs when the ground does not collapse upon removal of drill string.

Open hole drilling is much more efficient than drilling with “casing” {pipe}.

Drill must be disengaged from casing, anchor placed inside, filled with grout and then the casing can be removed (typical)

Anchoring Applications Require

Specialized Grouting Equipment

A Grout Plant is a device that typically

includes a mixing tank, a hopper, and a

grout pump to deliver the grout under

pressure to fill the hole.

High shear grout mixers are important

to thoroughly produce properly mixed

grout with lower water to cement ratios

higher resultant compressive strengths

High pressure pumps can be utilized to

increase geotechnical bond & perform

secondary grout operations called

“fracking”

Anchor Testing &

Application of Preload

The testing and introduction of a “Preload” into an anchor is an extremely important step of the anchor installation & critical to the overall system design

Safety should be a primary concern during the operation!

Make sure to understand the hydraulic principals of the testing equipment

Testing to a “Load” {ex: 140,000 lbs}

Need to correlate to the test jack Pressure Gauge to Load

Linear Relationship

Calibration Curve often provided

Example: A 200,000 lb test setup may have 20 in2 of “ram area” (See stamp on side of cylinder)

140,000 lbs / 20in2 = 7,000 psi on gauge

Note past AGHP Webinar available for more details

Quick Review of Different Anchor Types

These are the most common type of anchor installed. They can be used

in a variety of temporary and permanent applications.

• Requires maintaining an open hole

• Requires grout cure prior to load testing or load application

• Relatively Inexpensive {$/load capacity)

• Threads on bars must meet/exceed rebar deformation characteristics

• Robust, “Knuckle” Threads. Durable for field use.

Ground Anchor Product TypesGrout Bonded Solid Bar Anchors (Most Common)

These anchors are increasingly becoming the choice of many

contractors and engineers as State and Federal Agency

specifications now allow for their use in all temporary and some

permanent applications.

They are ideal for use in collapsing ground conditions.

• Requires simultaneous drilling & grouting

• Eliminate the need for a cased hole

• Inexpensive installed price, especially in comparison to

cased hole installations

• Bars can be coupled to achieve long anchor lengths

• Variety of sacrificial drill bits available depending on ground

conditions

Ground Anchor Product TypesHollow Bar Self Drilling Anchors

These are very common anchors, especially for high

capacity and long anchor lengths. They can be used in a

variety of temporary and permanent applications.

• Requires maintaining an open hole

• Requires grout cure prior to testing or load application

• Load lock-off requires special techniques

• Anchor capacity increases by adding additional strands

• Inexpensive {$/load capacity)

• No coupling required for long anchor lengths

• Anchors packaged as coils. Optimal on narrow benches

or other tight access locations.

Ground Anchor Product TypesStrand Anchors

Ground Anchor Product TypesMechanical Rock Anchors

The main differences include the maximum load carrying capacity and the way the

cone is engaged into the expansion shell. Direct pull vs torque-set.

• Requires good RQD to properly install

• Minimizes Overall Anchor Length

• Immediate Load Carrying Capability

Resin Anchors can be a convenient anchor and are typically used for rockfall

mitigation projects. The resin comes prepackaged in 12” long “sausages”.

Resin sausages are placed in open hole and bar is rotated through the resin

and mixes the activator into the polyester resin.

• Requires drilling in Rock and maintaining an open hole

• Drill Hole Diameter must be as per manufacturer recommended

• Should not be used in water conditions or high preload

• Economical anchor

• Very little mess

Ground Anchor Product TypesResin Anchors

Fiberglass anchors are used in special applications where

permanent steel tendons are not preferred:

• Top row of anchors in walls to allow future utility installation

• Situations where stray currents may cause issues

• Highly corrosive applications

Ground Anchor Product TypesFiberglass Tendons

On to Anchoring 201

Anchoring 201

Understanding Tensile Strength Values

Technical Considerations

Understanding Tensile Strengths

(2) Tensile Strengths of Interest:

Yield Strength

Maximum Load where stress & strain are linear

0.2% Offset typically used

0.7% Elongation Under Load (EUL)

Ultimate Strength

Maximum Load prior to rupture

Modulus Value

Stress/Strain

29-30 Mpsi typical for steel

Manufacturer’s Tensile Strength Data

Charts from Anchor Manufacturers

Important Notes:

1. Nominal Bar Diameter (Bar

Designation) different than Max Outer

Diameter

2. Ultimate & Yield Strengths

1. Grade typ refers to yield stress

2. Gr150?

3. ASTM A615 now moving to 80ksi

1. Design manuals still use 75ksi

2. Price is about same

Technical Considerations

Other Mechanical Properties

Elongation

Measure of “ductility”

Expressed as % of gage length

Reported with Tensile Test Data

Brittle materials have low elongation values

Charpy Impact

“Toughness” measure

Cold weather impact load applications

Crack propagation test

ASTM A193 B7

20 ft-lbs @ -40 F (also -40 C)

Ground anchors require Deformation

Patterns for the steel bars to facilitate

tendon to grout bond, various

manufacturers have their own methods

to comply.

ASTM A615 rebar deformation pattern

as a minimum requirement

ACI provides guidelines for

“Development Length”

Minimum Embedded Length for bar to

grout interface

NC threads do not comply!

Deformation Patterns &

Load Transfer

Understanding Anchor Creep

Geotechnical or Mechanical?

For a Prestressed Anchor it is important that the anchor can “Hold”

the Applied Load

Common Specifications require holding a Load, greater than the

Design Load, for 10 Minutes and measuring anchor movement

PTI Acceptance: <0.04” (1mm) movement during 10 min load hold

For Extended Creep Testing Anchors are held for a 5 hour hold

ASTM A722 is a spec for prestressing steel

Geotechnical Capacity

Geotechnical strength is different

than anchor steel strength

Geotechnical strength derived from

grout-to-ground bond interface

Each subsurface condition has

unique geotechnical capacity

Tabled references provide

preliminary estimation of ground

strength

Things That Could Effect Geotechnical

Strength & Performance

Drilling Methods

Open Hole vs Cased vs Auger-cast

Hollow Bar (Simultaneous Drill/Grout)

Pressure Injection of Grout

Tremie (Gravity) Grout

Pressure Grout

Post-Grout

Hole Collapse/Voids/Fractured Rock

Grout Sock

Things That Could Effect Geotechnical

Strength & Performance

Governing Specifications

Engineers should reference appropriate

governing documents/references when

specifying ground anchors:

Selection of Documents Depends on

Application

Actual Specification should cover:

Anchor Type (materials specs)

Installation Methods

How specific do you need to be?

Testing Requirements

Reporting Requirements

Reference Documents for

Pre-Stressed Anchors

Post-Tensioning Institute (PTI) “Recommendations for Prestressed Rock and Soil Anchors”

FHWA Geotechnical Engineering Circular #4

Numerous Aspects:

Design

Materials

Corrosion Protection

Testing & Preload

Guidelines for Soil Nails

FHWA Geotechnical

Engineering Circular

#7

Rock Anchor/Rock Dowel Specifications

These may be from:

State DOT

Private Utility

Railroad

Mining Company

These specifications vary but commonly address:

Anchor Design/Materials


Testing

Instrumentation to Detect Load Loss

Load Cell

Great for detecting load loss.

Sensitive instrument, temperature changes

noticed

Correlation between load cells and calibrated

rams must be resolved

Strain Gages

Strain gages are attached to the tendon and

“stretch” along with the steel tendon

Device measures “strain” of the tendon, which can

be correlated to load

Various styles for bar & strand

Recommend duplicate strain gages

Lift-Off Tests

A method to verify the prestressing load in an

anchor is to perform a Lift-Off Test

PTI definition

Procedure to measure the actual load in a locked-

off anchor by reapplying force until initial

movement of the anchor head or wedges is

observed

Inexpensive test to perform. Requires planning.

Anchor must have details to be able to be

retensioned.

Enough bar length to install coupling

Enough strand tail or externally threaded anchor

head

Shorter unbonded (Free) lengths can be

troublesome

Jack length in comparison to unbonded length

Anchor Durability

i.e. Corrosion Protection

In permanent applications, and in

some temporary applications,

anchors must be protected from

the effects of corrosion.

Common Corrosion Protection

Approaches:

MCP/DCP Corrugation

Epoxy Coating

Galvanizing

Sacrificial Steel

This is the highest level of


Corrugated plastic sheathing

placed over tendon to provide

impervious barrier

Bar Anchors are pregrouted in

factory

Strand Anchors are supplied with

corrugation and grouted complete

in field

100+ Yr Life Span in virtually any

ground condition


MCP/DCP

This picture shows coiled strand anchors with factory applied corrugation

Epoxy Coating Very Common for Soil Nails and Rock Anchor/Dowels

Relies on complete encapsulation of steel to isolate from environment

Green Epoxy (ASTM A775)

Typical rebar epoxy

Purple Epoxy (ASTM A934)

Bars cannot be bent after coating

Coating Thickness should not exceed 16mil

(7-12 mil recommended)

Field Handling Important to Avoid Scratches

Field touch-up may be required


Epoxy Coating

This picture shows the in-line continuity detector common in epoxy

coating process. It will detect, and record small defects called “holidays”

for patching as required

Galvanizing involves a layer of

metal zinc applied to clean, bare

steel

Protection is achieved by the zinc

acting as a sacrificial anode, and

until the zinc is all gone, it protects

the steel

Common for end anchorage

{bearing plates and protective

covers}, and often used in

electrical or utility work.


Galvanizing: Sacrificial Zinc Layer

This picture shows a common procedure for galvanizing parts. Note that

with 150ksi bars, the “Pickling Operation” is avoided and a mechanical

sandblast is preferred to avoid hydrogen embrittlement

For some applications a protective

coating is likely going to be

damaged during anchor

installation.

Using a theoretical rate of cross

sectional loss over a projected

lifespan, you can oversize the steel

product to account for this loss.

Risky approach for anything except

non-aggressive soil conditions

Anchor Durability

The Idea behind “Sacrificial Steel”

Quality Assurance

Relevant Industry

Specifications

Job Specific

Specifications

Contractor PO & Anchor

Manufacturer Drawings,

Mill Specifications &

Certificate of

Compliances

Contractor QA

Procedures for Material,

Means & Methods, and

Reporting

Successful Project!

Design

Construction

Fabrication

Planning

Raw Material Manufacturing Process

• The initial process is the

melting of the steel tendon

• Steel mill metallurgists run

tests to make sure

mechanical properties are

as specfied.

• Mill publishes test results

as “Steel Certifications”

• Typically lot size is

180,000 lbs

• The initial process is the

melting of the steel into

“ingots”

• Ingots then processed into

final shape

• Metallurgists run tests on

steel lots to determine

mechanical properties

• Mill publishes test results

as “Steel Certifications”

• Typically lot size is

180,000 lbs

Material Certifications

Important aspects of the Material Certification

include:

Compliance to ASTM Spec

Tensile Strength (ultimate)

Yield Strength

Elongation

Chemical Composition

Anchor manufacturer should review certifications upon reception and

accept/reject shipment as required

Material Tracking

The steel mill attached steel tags to the bundles of bars that are traceable to the Material

Certifications

Anchor manufacturer is responsible to positively track material throughout the entire anchor

fabrication process.

Anchor Economics

Installed anchor cost can vary

greatly depending on Job Specific

circumstances

Installed Cost includes:

Material Costs

Temporary or Permanent?

Installation Costs

Mobilization

Equipment Req’d

“Risk” Assessment

It is important to thoroughly

research the parameters of the job

when selecting the most

economical approach:

Job Specifications

Geotechnical Information

Access & Work Sequence

Equipment Required


Anchor Economics

Grade 75/80ksi All-Thread Bars

Priced about $.80/lb

Fy=75ksi

Fu=100ksi

Grade 150ksi All-Thread Bars

Priced about $1.40/lb

Fu=150ksi

Relative Durability Costs:

Plain=1.0

Galvanizing= 1.6

Epoxy Coating = 1.5

Pregrouted Anchor=1.8

Above factors apply to coating

costs as an addition to steel costs

Thank You!Questions???