Geosynthetic Interface Shear Testing

R. Swan - Webinar: Geosynthetic Interface Shear Testing © 2019

Fabricated Geomembrane Institute - Webinar

Geosynthetic Interface Shear Testing

Robert H. Swan, Jr.Drexel University

[email protected]

Presented on 22 January 2019

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Outline of Presentation

• Methods of Measuring Interface Shear Strength

• Test Standards of the Industry

• Who, Who & When Regarding Tests

• Accreditation of Laboratories

• Details of the Interface Direct Shear Test Concept

• Details of the Interface Direct Shear Test Equipment

• Selection of Interface Direct Shear Test Conditions

• Data Reporting

• Development of Interface Shear Testing Work Plan

• Questions

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• Direct Shear

• Torsional Ring Shear

• Cylinder Direct Shear

• Tilt Table

• Pullout

• Triaxial Shear


Methods of Measuring Interface Shear Strength

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Direct Shear Method

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Direct Shear Method – Cont.

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Direct Shear Method – Cont.

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Torsional Ring Shear Method

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Torsional Ring Shear Method – Cont.

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Torsional Ring Shear Method – Cont.

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Cylinder Direct Shear

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Tilt Table Method

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Tilt Table Method – Cont.

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Tilt Table Method – Cont.

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Pullout Interface Method

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Triaxial Shear

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Test Standards of the Industry

• ASTM D5321/D5321-17 – Standard Test Method for Determining the Shear Strength of Soil-Geosynthetic and Geosynthetic-Geosynthetic Interfaces by Direct Shear. ASTM D 5321 was originally approved in 1992.

• ASTM D6243/D6243M-16 – Standard Test Method for Determining the Internal and Interface Shear Resistance of Geosynthetic Clay Liner by the Direct Shear Method. Was originally approved in 1998.

• ASTM D7702/D7702M-14 – Standard Guide for Considerations When Evaluating Direct Shear Results Involving Geosynthetics

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Who Requests Tests to be Conducted?

• Design Engineers (Design Phase)

• Contractors and Installers (Construction Phase (MQA/CQC))

• Manufacturers (R & D, Product Development , (MQC/MQA))

• Owners/Owners Reps. (CQA/CQC)

• Regulators (R & D, CQA, Failure Analysis, etc.)

• Lawyers (Failure Analysis)


Who, Who & When Regarding Tests

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Who Performs the Tests to be Conducted?

• Commercial Laboratories (Third Party, IndependentLabs)

• Institutional Laboratories (University , GRI)

• Manufacturers (R & D, Product Development, (MQC/MQA))


Who, Who & When Regarding Tests (Cont.)

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When should the Tests to be Conducted?

• During Design Phase (Site Specific Testing)

• During Material Selection / Qualification (Pre Construction)

• During Manufacturing Of Materials (MQC/MQA)

• During Construction (CQC/CQA)

• During Failure Analysis (Hopefully, this will not be needed, butfailures do happen. At this stage in a project, this should not bethe first time tests are conducted for a project!!!)


Who, Who & When Regarding Tests (Cont.)

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• There are various forms of Accreditation (ISO, GAI-LAP, UL, CE,Army Corp, DOT, etc.).

• The Geosynthetics’ Industry relies on GAI-LAP.

• GAI-LAP represents the Geosynthetic Accreditation Institute –Laboratory Accreditation Program which is part of the GeosyntheticInstitute under the direction of Dr. George Koerner, P.E. & CQA.

• GAI-LAP is an Accreditation by Test and requires annual audits withan on-site audit every 5 years. Laboratories must have a QP consistingof a QM and SOPs.

• There are over 75 laboratories involved ranging from one test to 155tests.


Accreditation of Laboratories

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Details of the Interface Direct Shear Test Concept

After Koerner (1998)

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Details of the Interface Direct Shear Test Concept – Cont.

After Koerner (1998)

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DIRECT SHEAR TESTING AND INTERFACE DIRECT SHEAR TESTING

CAN ONLY MEASURE TOTAL STRESS SHEAR STRENGTH

• Equipment Type

• Shear Box Size

• Normal Stress Loading

• Shear Force Loading

• Equipment Calibration


Details of Interface Direct Shear Test Equipment

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Equipment Type

GeoComp

GeoTest

Durham

Geo24/59

• The standardized size of the shear box has minimum plane dimensions which are thegreater of 12 in. by 12 in. (300 mm by 300 mm), 15 times the d85 of the coarser soil used in the test, or a minimum of 5 times the maximum opening size (in plan) of the geosynthetic tested.

• The depth of each container that contains soil must be a minimum of 2 in. (50 mm) or6 times the maximum particle size of the coarser soil tested, whichever is greater.

• The guidance from ASTM allows the use of smaller shear boxes if it can be demonstrated that the data generated from the smaller devices contain no bias when compared to data generated by the standard minimum box size of 12 in. by 12 in.


Shear Box Sizes

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• Typically it is believed that the larger the shear box the better the test results are, due to the reduction of boundary effects caused by using too small of a device.

• Shear box sizes up to 30 in. by 30 in. (762 mm by 762 mm) have been used to conduct interface testing.

• Shear box sizes of 6 in. by 6 in. (150 mm by 150 mm) and 8 in. by 8 in. (200 mm by200 mm) are used by some when conducting very high normal stress interface testing.


Shear Box Sizes – Cont.

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Normal stresses are typically applied using various applications:

• Dead Weight (Steel, lead, concrete plates/blocks) – Perfect for Very Low normal stress conditions from 50 up to 800 - 1000 psf (2.4 up to 38 – 48 kPa) on a 12in. by 12 in. (300 mm by 300 mm) test specimen.

• Pneumatic Cylinders – good for normal stress conditions between 500 to 3000 psf(24 to 144 kPa) on a 12 in. by 12 in. (300 mm by 300 mm) test specimen.

• Pneumatic Bladders – good for normal stress conditions between 800 to 21600 psf(38 to 1035 kPa) on a 12 in. by 12 in. (300 mm by 300 mm) test specimen.


Normal Stress Loading

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• Hydraulic Cylinders – OK for applying very high normal stress conditions.However the loading system must be able to relive itself during load application.

• Electric Stepper Motors with servo-controls requires continuous feed back to control to prevent overloading of test specimen. OK for full range of loadingconditions, still would use dead weight for very low normal stress conditions.


Normal Stress Loading – Cont.

All normal stress loading systems should include a method to verify the applied

normal load on the test specimen (i.e., load cell and/or pressure transducer).

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Shear Forces are typically applied using various applications:

• Hydraulic Cylinders – using a constant rate flow pump to maintain a constant displacementloading rate applied to the test specimen. Typically, unable to maintain very slow shear rate(Drained) conditions.

• Screw-drive (ACME or Ball Screw) connected to constant rate/constant torque electric motor or electric stepper motor with servo-controls allowing for continuous feed back tocontrol the constant displacement loading rate applied to the test specimen. Electric steppermotors allow for very accurate shear rate control at extremely slow shear displacement rates.


Shear Force Loading

The shear force loading system must be able to maintain a constant rate of displacement in a direction

parallel to the direction of travel of the shear boxes. The point of force application to the traveling

shear box must be in the plane of the shearing interface and remain the same for all tests.

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Shear Force Loading – Cont.

Most of the currently used shear boxes measure the applied shear force as it is applied to the traveling shear box. By doing so the shear loading system must be calibrated to account for friction that develops within the loading system as the test specimen is sheared. These additional frictional forces must be removed from the actual measured shear force during the test. A more traditional design of a shear loading system incorporates the measurement of the shear force from the stationary shear box.

Fixed Box Fixed Box

Traveling Box Traveling Box

Shear

ForceShear

Force

Typical Shear Box Design

Where shear force is measured

in the loading harness via

pushing or pulling.

Durham Geo, GeoTest,

GeoComp

Traditional Shear Box Design

Where shear force is measured

as a reacting force from the

stationary box. The moving box

is either pushed or pulled.

Presenter’s design

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Equipment Calibration

All Test Equipment Must Be Calibrated On A Regular Basis

What needs Calibration and When?

• Internal Friction Of Direct Shear Device – Minimum Every 6 Months

• Normal Stress Distribution – Minimum Every 6 Months

• Electronic Load Cells – Minimum Every 2 Years (Typically Yearly)

• Electronic Displacement Devices - Minimum Every 2 Years (Typically Yearly)

• Computer Data Acquisitions – Minimum Every 2 Years. The DA systems

should be used when the load cells and displacement devices are calibrated.

• Other support equipment – Scales, Balances, Ovens, Soil Test Equipment, etc.

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• Determine configuration of test setup.

• Sample Preparation.

• Soil Compaction Controls.

• Wetting, Saturation, Submerged, and Consolidation Controls.

• Shear Rate / Loading Controls.


Selection of Interface Direct Shear Test Conditions

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• Determine what you want to have tested.

• Should you include the soils and other geosynthetics above and below theinterface of interest?

• Doing so tends to give a more realistic representation of the field conditions.

• Using rigid sub/super stratums tends to give a more conservative result.


Determine Configuration of Test Setup

• Should you conduct single interface tests or multi-layer interface system (aka

a “sandwich” type) tests? This will be discussed in more detail in an

upcoming webinar by Dr. Tim Stark.

• Do you need to maintain the same orientation of the sample materials as

they are placed in the field?

• Should the soil materials always be compacted directly on the geosynthetics

that they come in contact with in the field?

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• Sample preparation is usually left up to the test laboratory/staff.

• Do not be afraid to give specific guidance to the laboratory if you want to model and/orevaluate a specific set of conditions or specific locations on a roll of material.

• Typically, test specimen are removed randomly on a diagonal method. Make sure the laboratory receives plenty of material for testing. Don’t be shy unless material is very limited for some reason (i.e., a failure investigation, etc.)

• Be clear in your specification or guidance given to the laboratory on how the testing should be conducted: if hydration, saturation, and/or consolidation are required; specify, applied normal load and duration, etc, and if additional wetting or other test conditioning is required.

• Remember, the responsibility of the laboratory is to conduct the requested test as accurately as possible under the requested conditions. If your conditions are not properlyconveyed to the laboratory they may not conduct the test as expected. The laboratory is not responsible for the engineering of the project. (See Appendix of ASTM D6243 fora handy check list for specifying tests)


Sample Preparation

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• Soil Moisture and Compaction Methodology (Remold Criteria).

• Soil compaction can typically be controlled to within +/- 0.5 pcf• Soil moisture can typically be controlled to within +/- 0.5 %

• Compaction directly on test interface.

• Compaction away from test interface.

• Development of over consolidated condition at interface.

• Think about what you are trying to model.


Soil Compaction Controls

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Soil Compaction Controls – Cont.

From RHS Data Base

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• Wetting at interface

• Soaking of materials / interface

• Are you saturated?

• Consolidation

• Submerged Interface

• Measurement of sample and interface moisture.


Wetting, Saturation, Submerged & Consolidation

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• Simulates condensation that can develop during a warm day anda cool night.

• By spraying the interface with a controlled amount of moistureallows the moisture to be placed where you want it.

• When coupled with consolidation and other soil conditioning , asaturated state can be achieved.


Wetting at Interface

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• Free Swell

• Under Load

• Duration

• Typical wetting front from the top down

• Water has access to all sides of sample except the interfacedirectly.

• May not model reality


Soaking of Materials / Interface

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Soaking of Materials / Interface – Cont.

Dry

Soaked

From RHS Data Base

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• In the Laboratory probably YES

• In the Field probably NOT?

• What is the correct situation to model? That’s an engineering decision.

• Some states like California allow an averaging of dry shear strength andhydrated/soaked shear strength.

• 1/3 dry strength + 2/3 hydrated strength

• 1/4 dry strength + 3/4 hydrated strength


Are You Saturated?

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• One Time Loading

• Incremental Loading

• Duration

• Submerged?

• Dissipation of pore pressure

• Important for Clays (fat & lean) and GCLs

• Monitoring of Consolidation

• Log of Time

• Square Root of Time

• Can be used to demonstrate if soil is in a saturated statethrough compression by squeezing air voids and allowingthe soil to approach the zero air voids curve (saturation).


Consolidation

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Consolidation – Cont.

From RHS Data Base

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• When should it be used?

• What does it model?

• Should lab account for soil buoyancy?


Submerged Interface

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• Measuring initial moisture of soil sample for remolding

• Measuring final moisture from interface surface

• Measuring the moisture content of GCLs

• What will it show?

• Effect of spraying (wetting)

• Effect of consolidation

• Remember shearing is occurring within the immediate soil atthe surface of the interface.


Measurement of Sample & Interface Moisture

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• Discussion on various shear rates in test standards

• Default shear rates in ASTM D 5321 – 0.2 ipm (geosynthetic-geosynthetic)and 0.04 ipm (soil-geosynthetic)

• Default shear rates in ASTM D 6243 – 0.04 ipm (GCL-geosynthetic) and0.004 ipm (internal strength of GCL)

• Both standards recommend a constant rate of displacement that is slowenough to dissipate soil pore water pressures unless the test conditions thatare to be modeled are for rapid loading to simulate field conditions.

• What is appropriate?

• How to determine the best shear rate?


Shear Rate / Loading Controls

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• The appropriate rate of shearing depends on several factors, including the geosynthetic,the materials on both sides of the geosynthetic, the soil, the normal stress level, thehydrating conditions, and the drainage conditions.

• For drained shearing the following equation can be used for guidance:

R = df / 50 * t50 * Θ

where:

R = rate of horizontal displacement

df = estimated horizontal displacement at peak shear stress

t50 = time required for specimen to reach 50% consolidation under the current normal stress increment. This time is typically determined from ASTMD 2435 One-dimensional consolidation test.

Θ = factor to account for drainage conditions on the shear plane.


Shear Rate / Loading Controls – Cont.

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Data Collection, Handling, and Interruption

Well Behaved

Not Well Behaved

Refer to ASTM D7702/D7702M-14

Standard Guide for Evaluating Direct

Shear Results Involving Geosynthetics

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Final Test Report

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• A good laboratory report starts with good quality laboratory testing.

• It is important to have multiple levels of review for an interface testing program.

• From the beginning when the samples arrive at the laboratory to when the reportleaves the laboratory the person reviewing the test results for technical accuracy should be involved in the project.

• All work should be checked for accuracy and correctness via a peer review process.

• The person running the test should not be the person reviewing the report, unless that person is a Geotechnical Engineer with extensive experience in conducting testing and understanding material behavior.

• By taking the time to check the results for technical soundness helps eliminate project delays, embarrassment to client and laboratory, expensive retesting, etc.


Level of Review

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When contracting for interface shear tests, a user should require the following:1. Regular calibration of shear testing device for accuracy of normal stress and shearing force

(minimum once per year recommended)

2. Specimen gripping surfaces that can impart uniform shearing to the test specimen

without slippage

3. Full GCL hydration is achieved (if applicable) before consolidation of the GCL to the desired

shearing normal stress (if applicable)

4. Consolidation of a GCL in small increments to minimize bentonite extrusion

5. Measurement of specimen volume change during hydration, consolidation, and shearing

6. Thorough inspection of failed specimen(s)

7. Measurement of initial and final GCL water contents and sub grade soil water contents (if

applicable).


Development of Interface Shear Testing Work Plan

Fox and Stark 2004

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When contracting for interface shear tests, a user should provide the following:

1. GCL material(s) (from actual project site or designated for actual project site if

possible),

2. Subgrade soil(s) (if applicable),

3. Geosynthetic interface material(s) (if applicable), and

4. Hydration liquid (if different from tap water).

When contracting for interface shear tests, a user should specify the following:

1. Specimen selection, trimming, and archiving procedures,

2. Number and type of tests,

3. Specimen configuration (bottom to top),

4. Soil compaction criteria (if applicable),

5. Number of interfaces (single or multiple) to be tested at the same time,

6. Orientation of GCL or GCL interface (machine or transverse direction),

7. Hydration normal stress and hydration time duration (or termination criterion),

8. Consolidation procedure, including load increments (or load-increment-ratio) and load

increment duration (or termination criterion), and

9. Shearing procedure, including shearing normal stress levels, magnitude of shear

displacement, and shear displacement rate.



Fox and Stark 200452/59

When receiving the results of interface shear tests, a user should expect the following:

1. Description of specimen selection, trimming, and archiving procedures,

2. Description of testing equipment,

3. Description of specimen configuration and preparation conditions,

4. Description of test conditions (hydration, consolidation, shearing),

5. Shear stress–displacement relationships,

6. Specimen volume change data during hydration, consolidation, and shearing,

7. Peak and large displacement shear strengths,

8. Location and condition of failure surface(s) within test specimens, and

9. Initial and final GCL water contents and subgrade soil water contents (if applicable).



Fox and Stark 2004

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• Interface Direct Shear Testing is not as simple as it sounds.

• Remember you just can’t specify ASTM D 5321 or ASTM D 6243 and expectto get accurate test results.

• Interface Direct Shear Testing is a performance test whose results are dependent on the test conditions that are applied to the test specimen.

• Be careful for what you ask for and take the time to properly specify the testconditions which are needed for the testing.

• Remember failure to specify correctly may lead to the failure of the project.


Final Conclusions

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Geosynthetic Interface Shear Testing

Robert H. Swan, Jr.Drexel University

[email protected]

Presented on 22 January 2019

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Contact Information

Robert H. Swan, Jr.

Associate Teaching Professor

Department of Civil, Architectural and

Environmental Engineering

Drexel University

[email protected]

Andy Durham

FGI Member

GMA Executive Council

Owens Corning

[email protected]

Timothy D. Stark

Professor of Civil & Environmental Engineering

University of Illinois at Urbana-Champaign

Technical Director

Fabricated Geomembrane Institute

[email protected]

Jen Miller

Coordinator

Fabricated Geomembrane Institute

University of Illinois at Urbana-Champaign

[email protected]


mailto:[email protected]

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Geosynthetic Interface Shear Testing

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