Dr. Brendan C. O’Kelly
Trinity College Dublin
Dr. Patrick J. Naughton
Institute of Technology, Sligo.
Visiting Academic Presentation, Urban Institute Ireland, University College Dublin, 27th July 2007.
Development and uses of the
Background
• In 1995, Geotechnics Research Group (headed by Dr. Tom Widdis) began HCA project at Department of Civil Engineering, UCD.
• Identify and develop niche area in geotechnical laboratory research
• Decision to develop state-of-the-art hollow cylinder apparatus (HCA)
• Personnel: Brendan O’Kelly (PhD 1995–2000); Patrick Naughton (PhD 1998–2002); Frank Dillon and George Cosgrave.
• Presentation will include overview of HCA development,
– its versatility in simulating complex stress conditions in ground foundations – experimental studies undertaken,
– key publications.
Why need for HCA testing
(i)
Example: Yielding of ground foundation (plain strain)
Stress axes rotate to vertical direction (
)
Values of three perpendicular stresses change
independently (generalized stress conditions)
• Ground is anisotropic; mechanical response depends on stress
magnitude and direction
Principle of HCA testing
• Test specimen: hollow cylinder,
100mm outer diameter, 71mm
inner diameter, 200mm in
length
• Apply system of axial and
torsional loads, inner and outer
confining pressures
• Within specimen wall
thickness, control rotation of
stress axes (
), and magnitude
of 3D stresses (
,
,
)
• Measure 3D deformational
response, and
• The UCD HCA
Layout of apparatus
Pressure cell
Test specimen
Loading mechanisms
Screw-spline shaft actuated by stepper motors
Pressure actuators
Reaction frame
Instrumentation
Local to specimen (submerged) Outside pressure cell
• O’Kelly B.C. and Naughton P.J., 2003. Development of a new hollow
cylinder apparatus for generalised stress path testing, Ground Engineering Journal, 36(7): 26–28.
• O’Kelly B.C. and Naughton P.J., 2005. Development of a new hollow
Thrust-torque transducer Reaction platen Proximity transducer Test specimen Annular, porous disc with protruding blades
Inner bore cavity Loading platen Locating dowels Loading piston Outer cell chamber
Set up of specimen inside pressure cell
Sealing specimen from pressurized water in cell chamber and inner bore cavity of specimen
Mechanism to measure radial displacement of inner wall surface
Proximity transducer
Positioned remotely using gear mechanism
Transducer measures axial load and torque applied along length of specimen
Strain measurement capabilities of
10-3 to 10%strain (pseudo-elastic to failure
strain levels for geomaterials) covering
full range of engineering interest
• O’Kelly B.C. and Naughton P.J., 2003. Development of the University College Dublin hollow cylinder apparatus,
Automatic closed-loop control of apparatus
• O’Kelly B.C. and Naughton P.J., 2005. Closed-loop control of a hollow cylinder apparatus, Proceedings Symposium on
Stress distribution within HCA specimen
• Stress non-uniformity arises due to curvature of specimen wall
– variation in torsional stress across wall thickness, etc.
• Require near uniform stress distribution for accurate interpretation of experimental data
• Studied degree of stress non-uniformity
– that occurs across specimen wall thickness when probing different regions of 3D stress space
– for test specimen of sand material
•Naughton P.J. and O’Kelly B.C., 2005. Stress non-uniformity in hollow
cylindrical test specimens, Proceedings Symposium on Innovative Experimental Techniques, Joint ASCE/ASME/SES Conference on Mechanics and Materials (McMat2005), Baton Rouge, Louisiana, 1–3 June.
•Naughton P.J. and O'Kelly B.C., 2007. Stress non-uniformity in a hollow
Method of preparing hollow cylinder specimens of sand
Extension collar
Bottom loading platen
Specimen base pedestal Inner specimen mould
Outer specimen mould Deposited sand forming hollow cylindrical specimen Apply suction
Annular overflow container
Alignment pins
Loading piston
Geotextile between outer membrane and outer mould Overflow pipe
Deposit sand grains into water
contained between inner and
outer specimen moulds
Apply suction to specimen
so that free standing
Remove specimen moulds
Assemble pressure cell
Apply inner and outer
hydrostatic confining pressures
to specimen walls
Study 1:
Yielding of sand under generalized stress
Stress probing to determine points on yield surfaces
Probing 3D stress space
3D strain response Varying stress components
Naughton P.J. and O’Kelly B.C., 2005. The yield behavior of
Leighton Buzzard sand in a hollow cylinder apparatus, Proceedings ASCE Geo-Frontiers Conference, Austin, Texas, 24–26 January, Geotechnical Special Publication 138.
Identify series of points on yield surface
Map yield surface in generalized stress space
Study 2:
Validate whether existing yield criteria can
be extended for generalized stress conditions
• Matsuoka-Nakai (1985) and Lade (1975) yield criteria developed for 2D stress conditions
• Used experimental HCA data to show these criteria can be used to adequately predict onset of yielding in sedimentary sand deposits 0.8 0.9 1 1.1 1.2
0 0.25 0.5 0.75 1 Intermediate principal stress parameter, b
M a tsu o ka -N a ka i N o rm a lize d C o n st a n t
= 300 = 600 = 00 = 900
0.8 0.9 1 1.1 1.2
0 0.25 0.5 0.75 1 Intermediate principal stress parameter, b
L a d e N o rm a lize d C o n st a n
t = 300
= 600 = 00 = 900
Naughton P.J. and O’Kelly B.C., 2005. Yield behavior of sand under generalized stress conditions,
Study 3:
Inherent anisotropy and mechanical behavior
• Many sedimentary sand deposits have inherent cross-anisotropic
soil fabric due to mode of deposition through water
• Deposits are densified to different levels (very loose to very
dense)
• Study effect of soil fabric on subsequent mechanical behavior
under generalized stress conditions
Naughton P.J. and O’Kelly B.C., 2003. The anisotropy of Leighton Buzzard sand under general stress
conditions, Proceedings 3rd International Symposium on Deformation Characteristics of Geomaterials, Lyon, 22– 24 September, 1: 285–292.
Summary
• Versatile UCD hollow cylinder apparatus is state-of-the-art in
geotechnical laboratory testing
– facilitates generalized stress path testing, including rotation of stress axes
• Many unique features
– can measure mechanical response at pseudo-elastic strain levels – automatic closed-loop control to target a stress path
• Can simulate complex field loading (stress) conditions
determine more reliable stiffness and strength values from measured strain response; e.g. for use in numerical analysis
• Used to study fundamental behavior of sedimentary sand deposits
– validated extension of existing yield criteria to generalised (3D) stress conditions
Thank you
Acknowledgements:
Dr. Tom Widdis (PhD supervisor); Professor Eugene O’Brien; Aodh Dowley, and the support of all of the academic and technical staff at the Department of Civil Engineering, UCD, is kindly acknowledged.
Funding by University College Dublin through the Pierse Newman Scholarship in Civil Engineering, and a Research Fellowship, are also gratefully
