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8/12/2019 ISRM-EUROCK-2002-076_Coalescence of Offset Rock Joints
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COALESCENCE OF OFFSET ROCK JOINTS
M UG HIE DA , O ME R Jordan University of Science and Technology, Irbid, Jordan, Mughieda@just.edu.jo
A LZO'U Bf, A BD EL K AREEM
Jordan university of Science and Technology, Irbid, Jordan
ABSTRACT
T o s tudy the failure m echanism s of joints and rock bridges in jointed rock m ass a series of
uniaxial com pression tests were perform ed on specim ens m ade of rock-like m aterial.
Specimen s o f size 63 .5 ern x 27 .9 ern x 2 0.3 ern , mad e o f 72 % s ilica sand , 1 6 % cement and 1 2
% water by weight were tested. The joint inclination angle kept constant and equal to 45, while
offset angle i. e. ang le b etw een th e p lane o f th e jo in t and th e lin e conn ects th e tw o in ner tips o f
the joints, changed from 0 to 120 with an increment of 15. The failure mechanism monitored
by visual inspection and magnifier to detect cracks initia tion. In all of the tested sam ples
Curv il in ear crack s called w in g crack s in itia ted a t th e jo in ts tip s d ue to high tensi le s tressesconcentration. This wing cracks directed along the direction of the uniaxial load.
T he coa lesce nce m echanism of tw o cracks w as investigate d. R esults show ed that open
crack s can coalesce b y sh ear fa ilu re or tensi le fa ilure . The coalescen ce path man ly d ep end s on
the inclination of the rock bridge between cracks.
1. lNTRODUCTION
Rock masses are usually discontinuous in nature as a result of various geological processes they
have subjected to. Consequently, joints and rock brides formed in the rock mass. The initiation,
pro pagat ion and coalescence of rock cracks are important factors in controll ing the mechanical
behavio w' of brittle rocks. Crack propagation and coalescence processes primarily cause rock
failure in slopes, foundations and tunnels. Many studies have been performed on the initiation,
propagation and coalescence of cracks since Griffith (1921) have stu died the growth of pre-
existing two-dimensional crack. The studies that perform on jointed rock can help to explain the
joint (crack) propagation mechanism and serve as mo dels for the behaviour of jointed rock
m asses. Joint propagation and coalescence can reduce the stiffness of jointed rock m asses
causing the shear failure of rock slopes. (Einstein et. al. 1983). Also, i t can induce earthquakes
by forming shear faults . (Deng and Zhang, 198 9).A number o f stu dies h av e b een p erfo rmed o n crack p ro pagation in d ifferent materia ls
under uniaxial com pression. Lajtai (1969) perform ed direct shear test on natural rock
~pecimens with two parallel slots, Segall and Pollard (1980) studied analytically the stress field
In ro ck b rid ges betw een two s tepp ed cracks , N ern at-N aser and H OITi (1 98 2) in vestig ated the
coale scence be haviour of m ulti-cracks in polym er specim ens, R eyes and E instein (1991)
performed uniax ial tests on gyp sum specimens with two inclined flaws and Shen (19 95)
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conducted uniaxial tests on gypsum specimen with two cracks. However, all experimental
studies have been conducted on a small size sample and on limited test material.
In this current study we conducted a series of uniaxial loading tests on rock-like material
specimens of size 63.5 ern x 27.9 em x 20.3 cm, made of 72 % silica sand, 16 % cement (Type
I) and 12 % water by weight. Each specimen contains two pre-existing open cracks arranged in
different geometries.
2. SPCIMEN PREPERA TION AND TESTING
2.1 Specimen preparation
In the present study a model material was used. This model material was made of mixing
silicasand, ordinary Portland cement type I and water in 72%, 16%, and 12% proportion by
weight respectively. To obtain grain to grain contact which cause low tensile strength and highfriction angle, high sand content and low cement content was used.
The friction angle determinated from direct shear tests is 33. The ratio of unconfined
compressive strength to the tensile strength for material was 12. The ratio of secant modulus to
the unconfined compressive strength is 1100.
The formation of jointed rock mass from individual block elements has the following
shortcomings:
I. Imperfect matching
2. Imperfect closure
3. Imperfect matching or improper fitting of individual elements loads to concentration of
stresses.4. Rotation
5. Non-uniformity of the individual elements.
Due to the above-mentioned reasons Jamil (1992) developed a new method to form blocks
with closed persistent and non-persistent joints during casting. This method is used in the
present research.
The procedure used by Jamil (1992) and Mughieda (1997) have been used with some
modification for preparing open non-persistent joints. Following is a description of the
procedure of making open non-persistent joints with different configuration. One joint
orientation (~), 45 was used. The aluminwn angle frame supported by four posts was placed as
described earlier. This made the height of the frame ten inches from the base of the frame. Twosteel plates 0.06 in. (0.152 ern) and 0.02 in. (0.052 ern) thick respectively, 14 in. (35.56 crn)
high and varying in width according to the position and orientation of the joint cut in a manner
to form a shape of T. The arms of the T-shaped plates were made to rest on the aluminum
frame. Glass was used to form open joints. The glass was cut according to the width of the joint
and it is equal to 4.72 in. (12 ern) and installed between the steel plates. This glass was cut in
such a manner to extend about three inches above the plate. The stainless steel plate was placed
on the other side of the class so as to sandwich it. All the plates were arranged in accordance
with this procedure. To maintain proper spacing 3.94 in. (10 ern), two wooden spacers placed
between two joint rows. To maintain the orientation, the plate was made to touch the two sides
of the mold.Figure I shows sample geometry. Figures 2 shows mold arrangements.
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28 em
, . .
/
----~. joint
~ Bridge
/ #
20cm
Figure 1. Geometry of the specimens and pre-existing cracks
Figure 2. The mold before casting concrete.
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2.2. Test machine, instrumentation and recording
In order to perform the tests, the 2000 kN Universal Compression Machine was used. Load
cells installed on the testing machine were used to measure the load applied to the sample.
To measure the total deformation and the displacements in the vicinity of the joint, Linear
Variable Differential Transducers (LVDT) with maximum range 5mrn and .001mrn wereused.
Three to five LVDTs were used in each sample, two of them to measure the normal
displacement at joint and bridge segments, and two for measuring the shear displacement at
both joint and bridge. The last one was placed at the side of the sample to measure the total
deformation of the sample during the test. By using the Hewlett Packard data acquisition
system, load and LVDTs readings were recorded in the computer.
3.TEST RESULTS
The effect of bridge inclination on the failure mechanism of specimens made of rock like
material was investigated. The inclination angle of the joints (P) remained constant at 45 for all
specimens and the inclination angle of the bridge (a) was changed from 0 to 120 with an
increment of 15. The following results were found:
3.1 Non-overlapped joints
Specimens with bridge inclination, a = 0, were failed along the joint plane. Wing cracksinitiated at first then the secondary cracks initiated at the internal tips of the joints segments and
then propagated to meet each other at the a point in the bridge between the two inner tips of the
joints (cracks). The characteristics of the failure surface were investigated, there were a
significant amount of pulverized and crushed material and traces of shear displacement.
Specimens with a= 30 have failed as follows: the wing cracks initiated and propagated at
first and then the secondary cracks initiated at the tips of the joints and propagated to coalesce
at a point in the bridge caused the sample to fail. Crushed and pulverized materials, have been
noticed. At the ligament between the joints outer tips and the edge of the traces of shear
displacement can be seen, also these traces can be detected near the inner tips of the jointS.
About the surface of the other parts of the bridge away from the tips, there were no noticeable
pulverized, crushed materials or traces of shear movement, and so this surface most likely
created by tension stresses. Figure 3 shows the failure surface ofODJ4.
According to the characteristic of the failure surface, it seems that at first high tensile
stresses concentrated at the tips of the joints and this initiated the wing cracks. The inner wing
cracks stopped and the outer ones propagated, this can be explained by that the tensile stresses
eliminated at the inner wing cracks and persisted at the outer wing cracks. In all samples the
outer wing crack at the right joint extended more the other one at the left joint and in two ofthe
sample it extends to the edge of the sample. At the internal tips high concentration of shear
stresses initiated secondary cracks, but inside the bridge there were tension stresses initiate~
tension cracks. The tension cracks and that produced by shear coalesced with each other an
caused the failure. At the outer ligament there were evidences of shear failure produced by
shear stresses.
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WingCraCk~
Opencracks
Coalescence
~
Figure 3. Typical crack growth and coalescence of 0014
Specimens with a=45 have failed as follow: the wing cracks initiated and propagated
toward the direction of the applied load. The two inner wing cracks stopped, while the two
Outer wing cracks continued to propagate tell the end of the test. At the end of the test a
secondary crack initiated at the inner tip of either the left or the right joint, this secondary crack
propagated and coalesced with the inner wing crack at the other joint. Also secondary cracks
initiated at the outer tips and propagated. After the test the plan of the failure surface was
examined, it was found that near the outer tips of the joints small traces of shear displacement
could be seen, while less shear traces could be noticed at the inner tip of the joint at which the
secondary crack initiated. And at the other tip a very little pulverized and crushed materials
could be found. The surface of the failure plane away from the tips (bridge area) could be
characterized as tension surface, no crushed or pulverized material and traces of sheardisplacement. Figure 4 shows the failure surface ofODJ6.
Secondary Crack
initiation
Wing cracks Growth
Figure 4. Typical crack growth and coalescence of 0016, a =45, p =45.
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3.2 Overlapped joints
Thirteen samples with a =60, 75, 90 were tested.
Specimens with a = 60 have failed in the following sequence: after wing cracks initiated they
grow stably and then a secondary tensile crack initiated in the bridge, this crack could be easily seen.
The tensile secondary crack propagated stably and coalesced with the internal joint segments tips.The outer wing cracks extend to the edge of the sample. Other cracks initiated at the outer
tips of the joints and propagated unstably to coalesce with the edges of the sample. failure
surface and wing cracks were created by tensile stresses.
Samples with a =75 The wing cracks initiated and propagated stably in curvilinear path,
each inner wing crack initiated at the inner tip of one joint and finally coalesced with the inner
tip of the other joint. This coalescence leaves an elliptical core of intact material completely
separated from the sample. The outer wing cracks grow and extend to the edge of the sample.
The surface of failure at the bridge area is tensile surface (Type I), in such away that, no
crushed or pulverized materials and no evidence of shear movement. The wing cracks surfaces
also had the same characteristics of tension surface.At the time of failure secondary crack initiated at the outer tip of the left joint, it was
found that there were crushed and pulverized materials and traces of shear displacement, next to
the shear zone there were tension zone. Figure 5 shows the failure surfaces ofODJ13.
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4. CONCLUSIONS
The following conclusions can be withdraws from the results of the current work:
I. Incoplanar joints secondary cracks initiated at the inner tips of the joints caused the
failure. In fact these secondary cracks were shear cracks and the failure was shear
failure.
2. In specimens with bridge inclination angle of 30, secondary cracks initiated at the
inner tips of the joints and coalesced at the bridge. Shear and tensile stresses at the
inner tips and the bridge caused the failure.
3. In specimens with bridge inclination angle of 45, combination of shear and tensile
stresses caused secondary cracks to initiate at the inner tip of the joint and propagate to
coalesce with the wing crack at the inner tip of the other joint.
4. For bridge inclination angle of 60, tensile stresses at the bridge area initiated a
secondary crack. This crack propagated toward the inner tips of the joints and caused
the failure.
5. In specimens with bridge inclination angle of 75, tensile stresses caused the inner
wing cracks to propagate forming a separated parabolic core of intact material, this
coalescence caused the failure.
6. For bridge inclination angle of 90, the inner wing cracks coalesced but that did not
cause the failure and shear-tensile stresses at the lower joint cause the failure.
7. For overlapped joint the strength increase as the bridge inclination angle increase.
REFERENCES
Deng, Q. and Zhang, P. 1989. Research on the geometry of shear fracture zones. Journal of Geophys.,
Res., 89: 5669-5710.
Einstein, H. H., Veneziano, D., Baecher, G. B., and O'Reilly, K. 1. 1983. The Effect of Discontinuity
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221: 163-198.
Jamil, S. M. 1992. Strength of non-persistent rock joints. Ph.D. thesis, University of Illinois at Urbana-
Champaign, Urbana, IL, USA.
Lajtai, E. Z. 1969. Strength of discontinuous rock in direct shearing. Geotechnique 19: 218-233.Mughieda, O. S. 1997. Failure mechanisms and strength of non-persistent rock joints. Ph.D. Thesis,
University of lIlinois at Urbana-Champaign. Urbana, IL, USA
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