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, [email protected]

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

Persistence on Rock Stability. Int. 1. Rock Mech. Min. Sci. Geomech. Abstr., 20: 227-237.Griffith, A. A. 1921. The phenomena of rupture and flow in solids. Philos. Trans. R. Soc. London Ser. A.

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

Nemat-Nasser, S. and HOITi,H. 1982. compression-induced nonplanar crack extension with application to

splitting, exfoliation and rockburst. 1. Geophy. Res. 87(B8): 6805-6821.Reyes, 0.,and Einstein, H. H. 1991. Stochastic and centrifuge modeling of jointed rock. Part I-Fracturing

of Jointed Rock. Final Report submitted to the Air Force Office Of Scientific Research and Air

Force Engineering Services Center.

Segall, P. And Pollar, D. 1980. Mechanics of discontinuous faults. 1. Geophy. Res. 85(B8): 4337-4350.

Shen, B. 1995. The mechanism of fractures coalescence in compression- experimental study and

numerical simulation. Eng. Frac. Mech. 51(I): 73-85

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