Wedge Shear Testing of SoilsAn insufficiently utilized new technique
Prof. Dr. Türker [email protected]
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METU Campus, 20 July 1966
A slip occurred
at the side of an
excavation for the
central heating
utilities of the campus
soon after the
excavation.
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This raised the QUESTION
Can one not devise a relatively simple test for pedicting, with fair accuracy, the safety of such slopes in stiff, fissured, unsaturated clays like the Ankara Clay?
The following 24 years saw to the development of three different versions of a testing technique which not only answers the above but also facilitates the shear testing of gravels and crushed rock.
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1. The in situ wedge shear test (iswest)
Suitable especially for unsaturated, stiff fissured and/or stony clays
Tried successfully on three slips in the Ankara Clay
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Different modes of loading
Different ways of measuring shear strength on a given plane by iswests.
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Six iswests carried out at Site C/1
7-23 June 1971; iswests in pit 9m from 1968 slip
Note inclination of shear plane in different iswests, conforming with the failure surface in the actual slip.
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Seven iswests carried out at Site D
30 May - 23 June 1972; in
pit close to 1966 slip
Note inclination of shear plane in different iswests, conforming with the failure surface in the actual slip.
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Comparison of the shear plane areas
60 mm dia. shear box
360 mm x 250 mm iswest mould
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Earlier loading setup
Load applied on face of test mould through steel balls and a loading plate
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Improved loading setup
Load applied through two grooved plates with steel balls in between
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Shoring for vertical iswests & slip plane
A slip occurred during the rainy season following these iswests
Vertical loading applied at these points
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The shoring after the slip
This type of shoring proved unsuccessful
This showed it is safer to do such tests at the side of an adequately supported test pit.
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Gauging stool for checking size of loading pit
Note the graduations on the legs for different
test moulds
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The pivoting and jacking frame
Used for passing test mould over soil
wedge squarely
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Iswest in pit supported by expansible steel segments
Bent out of 3 mm thick
mild steel plate, eight
such 40 mm high
segments ranging
between 1035 and 1255
mm in dia. were used to
support a 4 m deep pit.
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Comparison of factors of safety for three slips in the stiff fissured Ankara Clay
1.00 1.20 1.40 1.60
1.80 2.00 2.20 2.40
Slip 1 Slip 2 Slip 3
Fa
cto
r o
f s
afe
ty
Effective stress analysis based on triaxial tests on 36mm dia. specimens
Total stress analysis based on iswestsEffective stress analysis based on triaxial tests on 102mm dia. specimens
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Comparison of the approximate time required for the different types of test
- 20 40 60 80
100 120
Slip 1 Slip 2 Slip 3Te
sti
ng
tim
e (
da
ys
)
Effective stress analysis based on triaxial tests on 36mm dia. specimens
Total stress analysis based on iswests
Effective stress analysis based on triaxial tests on 102mm dia. specimens
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2. Cylindrical wedge shear test (cylwest)
Suitable for testing clayey tube samples up to about 100 mm in diameter, recovered from boreholes.
Suitlable for compaction control of clay fills.
Can be performed on clays or sands compacted directly in the cylwest mould.
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Principle of the cylwest
The sample is introduced in a thin-walled steel tubecut at an angle between 30o and 45o to its axis. Upper half of mould is clampedin the cross-beam of an existing compressionmachine or portable frame. Spacers are removed. Loading is effected ina similar way to that in iswests.
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Cylwest perfomed using a triaxial compression machine
The only modification is
to replace the cross-beam
by one that will accom-
modate the jaws used
for clamping the upper
half of the cylwest mould.
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Cylwest performed using a portable frame
Easilycarried bytwo men,this frameenables on-site applica-tion of the test.
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Loading in progress using portable frame
Sample
preparation
and testing
can be
completed
within one
hour.
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Double-cut test mould
Used for testing compacted
soils in the same mould in
which they are compacted
One half of removed upper coupling
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Compacting the soil in the double-cut mould
The mould is
attached to the
base plate
used in the
Proctor
compaction
test.
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Cylwest in progress using the double-cut mould and a portable frame
One pair of couplings is
removed, and the sample
is sheared along the
exposed plane. The mould
is then re-assembled,
and the sample sheared
along the second plane.
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Shear strength envelopes from cylwests and shear box tests on clays compared
Segments
of peak
strength
envelopes
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Shear strength envelopes from cylwests and shear box tests on clays compared
Segments
of residual
strength
envelopes
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Peak strength envelopes for compacted plastic clay at different water contents
3 % below optimum
At optimum
3 % above optimum
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Peak strength envelopes for compacted sandy clay at different water contents
3 % below optimum
At optimum
3 % above optimum
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Peak envelopes for compacted plastic clay at different compactive efforts
25 blows per
Layer
20 blows per
Layer
15 blows per
layer
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Peak envelopes for compacted sandy clay at different compactive efforts
25 blows per
Layer
20 blows per
Layer
15 blows per
layer
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Iswest with sub-lateral loading applied (lead to the develpoment of the priswest)
for keepingthe normalstressconstantafter thepeakstrengthis reached.
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3. Prismatic wedge shear test (priswest)
Suitable for testing gravel, crushed rock, and clay with < 38 mm particles.
Enables static compaction to be applied for clayey samples.
Can be performed in the lab or on site within two hours.
Total weight of equipment needed is about 250 kg – ¼ of a conventional shear box for the same size of samples.
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Principle of the priswest
The lateral loading device used in iswests ledto the devel-opment ofthe priswest.
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Sample placement in priswest mould
The remov-
able lid is
temporarily
replaced by
a collar
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Sample compaction in priswest mould
Kango
vibrating
hammer
being
applied on
gravel
sample.
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Loading setup used for priswests (1)
The 5-toncapacityportableframe. Verticalloading is used for clay.
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Loading setup used for priswests (2)
20-ton
capacity
frame.
Horizontal
loading is
used for
gravel.
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Priswest in progress using 20-ton frame
Dial gauges
are used
to measure
displace-
ments.
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Alternative devices for applying the lateral load in priswests
Air piston
Spring loaded piston
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Static compaction of a gravelly clay
One of the advantages
of the priswest is that it
enables static compaction
to be applied using the
same 20-ton frame as
used during shear
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Shear strength envelopes from priswests
Drained strength envelopes for crushed rock at two degrees of compaction, using moulds with differentangles
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Comparison of priswests and triaxial tests on silty gravel (19 mm – 38 mm)
Drained triaxial tests
Priswests
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Comparison of priswests and triaxial tests on crushed rock (< 38 mm)
Drained triaxial
tests
Priswests
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Why should the results be so close?
The priswest, being close to plane strain conditions, would be expected to give a somewhat higher strength than the triaxial test.
Cylwests on sand do give a somewhat higher strength than the triaxial test.
The answer to this apparent anomaly is given in the next slide.
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Plot of peak friction angles vs. angle between shear plane and bedding planes for six different sands
Cylwestswith abt.same as triaxialtests give higher
(deg)
Priswestshave lowerthan triaxial tests Angle between shear plane and bedding planes, (degrees)
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Duration of priswests and triaxial tests
- 2 4 6 8
Gravel Crushed rock Gravelly clay(undrained
test)
Test
ing
time (
hour
s)
Triaxial tests on 190 mm samples
Priswests on 300 mm x 300 mm samples
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Those who have helped investigate the different aspects of the wedge shear test:
Aybak, Turgut (M.Sc, 1988). Köseoğlu, Rüştü (M.Sc, 1988). Azimli, Fuat (M.Sc, 1989). Varan, Mustafa (M.Sc, 1989). Çağnan, Şener (M.Sc, 1990). Seçkin, Altan (M.Sc, 1993). Turp, Engin Zülfükar (M.Sc, 1993).
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Those who have helped investigate the different aspects of the wedge shear test
(continued) Gökalp, Alp (M.Sc, 1994). Şakar, Mehmet (M.Sc, 1997). Gün, Fırat (M.Sc, 1997). Filiz, T. Emrin (M.Sc, 2000). Erzin, Yusuf (PhD, 2004).
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Special thanks are due to
The many METU technicians who have made the special equipment.
The METU technicians who have helped during the testing.
All colleagues who have helped in different ways.
Last but not least, Dr. Murat Mirata for help in preparing this PowerPoint show.
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THE END
Thanks for watching. For details see literature listed at http://www.metu.edu.tr/~mirata/
16 March 2007