Proceedings of the International Symposium on Weak Rock /Tokyo /21-24 September 1981

On shear behaviours of rock containing weak planes RYOJI KOBAYASHI & FUMIO SUGIMOTO Tohoku University, Sendai, Japan

1. INTRODUCTION

In designing a dam abutment, a tunnel or an underground structure in rock, it is necessary to evaluate the mechanical properties of rock. However, the rock is not uniform body, but contains joints, bedding planes or other weak planes, and these weak planes in rock reflect on the mechanical behaviours of rock. Rock specimens are almost taken from rock between these weak planes, and the laboratory tests using the specimens invariably lead to an overestimate of the strength of rock.

In order to investigate the mechanical behaviours of rock, the block shear tests are performed in field. But, the great expense and efforts are required to per-form the insitu tests. In recent years, the researches(e.g. Krsmanovi 1967) in respect to the direct shear test, using the relatively large specimen which contains weak planes, have been done. Nevertheless, it is difficult to in-

vestigate the mechanical behaviours of specimens collected from an arbitrary portion in rock, because the specimen is the large rectangular prism. In view of this point, it seems that the direct shear test using the boring cores collected easily from an arbitrary portion in rock is a rational method. This study is undertaken for the purpose

.of estimating the shear stiffness of rock containing weak planes. As basic research for the above study, the shear behaviours of rock specimens containing artificial discontinuous planes are investigated. Considering the effect of weak planes to the shear stiffness of the specimen, more-over, the direct shear test is conducted on the boring cores collected from rock, and the results of laboratory test make

comparison with the results of insitu test on rock mass. The failure lines in residual sliding are then obtained on the boring cores collected from rock mass of a lot of working places, and the relations between the friction angles measured by these failure lines and rock types are investigated.

2. TESTING MACHINE AND TESTING PROCEDURE

The schematic diagram of a direct shear testing machine which is developed in this study is shown in Fig.l. In this figure, the names of main parts of the machine are as follows ; is a ram which applies normal load to specimen, is a load cell for measuring normal load, is a rollar bearing, is an upper shear box, is a ram which applies shear load to specimen, O is a load cell for measuring shear load,

Fig. 1 Schematic diagram of direct shear testing machine.

15

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x

I f + l r l I r I / r l { / f

x xk

i l l l// // x l l l l l l l/////// xY

0//////1/////////%

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r / / r ^ / I ^

+ + f }+ i r i 1

QG is a lower shear box, is a ram for lifting the upper shear box QD , and are the linear variable differential transformers for measurin normal and shear displacements, and @I is a steel frame which supports the main parts of the machine.

The procedure in the test is as follows ; At first, a specimen is set into the lower shear box G , and the upper shear box is descented by operating the ramie

as the specimen fits into the upper shear box . When the direct shear test is conducted, a desired normal load is applied to the specimen by the ram and the shear load is then applied by the ram . During the shear test, the normal and the shear displacements are measured by the linear variable differential transformers .

The boring cores collected from rock mass contain generally weak planes and in case that the core separates into several pieces along the weak planes, their weak planes are bonded by a weak adhesiveagent. The core formed in a rod like shape puts into a cylindrical mold of 0IOOmm or 055mm, and an equal mixture of rosin and limestone powder is pured into a space of about 3mm between core and mold. In this procedure, the'lateral of core is covered with rosin mixture and its core is shaped into a cylindrical specimen of 0100mm x 200mm or 055mm x 110mm. After the rosin mixture of the lateral of core is removed with the width of about 5mm along the shear plane, a process of making shear specimen is completed, and the specimen is put into wet condition. The specimen containing the artificial

discontinuous planes as shown in Fig.2 is made in following procedure ; The inclin-ed angle ( 6) between the direction of shear load and the discontinious plane of specimen is taken from 22.5 to 157.5 at every angle of 22.5 . . The discontinuities in specimen are then bonded by the weak adhesiveagent, and the specimen is covered with rosin mixture as already stated method.

Fig. 2 Size of specimen and direction of discontinuities-

3. FUNDAMENTAL RESEARCH

Before carrying out the direct shear test, a distribution of the principal stresses in a specimen, which receives the normal and the shear stresses, is analyzed in condition of plane strain by F.E.M., using the physical values of OGINO tuff. Fig.3 shows a distribution of the principal stresses in specimen at normal stress of 71.2 kg/cm' and shear stress 31.8 kg/cm 2 .

Twtl. stress 1004q /c mr

Compressive ,tress

Fig. 3 F.E.M. analysis in condition of direct shear test.

In this analysis, Young s moduli of the specimen use the values which change on the basis of the differential stressstrain curves obtained by the triaxial compressive test and Poisson s ratio takes 0.2, as a rough estimate. As shown in Fig.3, the tensile stresses

in the specimen appear near the shear edges. From this stress distribution, it is thought that a crack initiation in specimen is originated by the tensile stress and the shear stress is con-centrated at the tip of the tension crack. Consequently, the crack may make progress along the direction of shear load, and the failure surface becomes undulate.

In order to investigate the effects of the direction of discontinuties on the shear behaviours, the direct shear test is performed on the specimen shown in Fig.2.

3.1 The direct shear test at the controll-ed normal stress

The shear tests are carried out at normal stresses of 27.1 kg/cm2 and 40.6 kg/cm2 . The shear stressdisplacement curves obtained by those tests are shown in Fig. 4. In this figure, it seems that the modes of the curves are different with the direction of discontinuities in the specimen. And also, the peak shear strengths

16

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sun

/ow. /^:^` eix ^^^^ -- e157.

?n.27.1 ke/crrJ ..22.6

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st-ar

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200 100 0 100 200 Peak shear st meth (ke,cmt)

Fig. 6 Anisotropy of peak shear strength in controlling normal displacement (OGINO tuff)

s ' fe D. : 0

9 Dn: 0.57.

100 0 100 NormM stress (kgkm, )

Fig. 7 Anisotropy of normal stress needed to control normal displacement (OGINO tuff)

1 2 3 4 5 6 7 8 9 01sp1ac.m.M (mm)

Fig. 4 Shear stress displacement curves (OGINO tuff)

obtained in the above shear tests are expressed in the polar coordinates as shown in Fig.5. From this figure, it is clear that the peak shear strength takes small value in the region of 9=45'. The results of these tests are in agreement yaiih the results of Hayashi(Hayashi 1966), who conducts the direct shear test on gypsum specimen containing artificial dis-continuous planes.

120 80 40 0 40 80

120

Pak shear st ength (kp/cm2 )

Fig. 5 Anisotropy of peak shear strength due to direction of discontinuities (OGINO tuff)

3.2 The direct shear test at the controll-ed normal displaciment

The direct shear tests at controlled normal displacement are carried out under following two conditions. One is a condi-tion that normal displacement is restrain-ed and the other is a condition that normal displacement of 0.5% of specimendiameter is permitted.

The relations between peak shear

strength and normal stress obtained by

these tests are shown in Fig.6 and Fig.7

respectively. It is apparent that both

peak shear strength and normal stress take

high values in the region of 9=45', In comparison between the shear strength at the controlled normal stress and that at the controlled normal displacement in the region of 9=45', the shear strength increases by about four times under the condition that the normal displacement is permitted to 0.5% of specimendiameter,

and by about five times under the condi-tion that normal displacement is restrain-ed.

On the above two methods of the direct shear test, it is found that the direction of discontinuities, existing in specimen, has a great effect on the shear behaviours. Therefore, it is necessary to take the direction of discontinuities into consideration in case of estimating the shear strength of rock mass. Since it is also considered that the normal displace-ment in the process of shear failure of rock mass is generally restrained in certain degree, the shear strength obtain-ed by insitu test permitting the normal displacement takes lower value than the true shear strength of rock mass. From this reason, it is a rational procedure that the shear strength obtained by insitu test is used in design of rock construction.

4. COMPARISON BETWEEN THE RESULTS OF INSITU BLOCK SHEAR TESTS AND THOSE OF DIRECT SHEAR TESTS IN LABORATORY

Since the number of testing blocks in the insitu test performed is generally several, it is not possible to evaluate the shear strength of rock mass from the results of insitu test when the strengths of testing blocks are scattered. Therefore, it has a significant meaning to conduct the direct shear test on the boring cores.

17

Fig. 10 Shear stressdisplacement curves of specimens collected from Shichigashuku dam 110

E 30

.1 20

toa

fio Y' ^

60 ^

40

collected from rock mass nearby insitu testing place for complementing the results of the insitu test.

In this study, the results of the insitu tests at the working places of Tamagawa dam(Tazawako town, Akita prefecture), Shichigashuku dam (Shichigashuku town, Miyagi prefecture), and Akagawa dam(Asahi village, Yamagata prefecture), are compared with the results of laboratory test using boring cores collected from rock mass nearby the insitu testing place.

4.1 Tamagawa dam

The shear stressdisplacement curves obtained by the direct shear test on the boring cores collected from rock mass (tuff breccia) nearby the insitu testing place are shown in Fig.8. And, the rela-tions between peak shear strengthnormal stress and between residual shear strength normal stress are shown in Fig.9. In Fig. 9, the residual friction angle is lower than the internal friction angle obtained by the peak shear strength. As shown in Fig.9, the failure line obtained by insitu block shear test lies between the failure lines of peak shear strength and residual shear strength, and the internal friction angle obtained by the insitu test is close to the residual friction angle.

2 3 4 5 6 7 Displacement (mm)

Fig. 8 Shear stress displacement curves of specimens collected from Tamagawa dam

Fig. 9 Relations between shear strength and normal stress in Pe,w

Residual direct shear test and a tn-,it in-situ shear test (Tamagawa dam)

60 40 x0 Norms) stress (kglcm 2 )

4.2 Shichigashuku dam

In the case of Tamagawa dam, it is possible to perform the direct shear test on the representative specimens in many boring cores. But, in the case of Shichigashuku dam and Akagawa dam, the number of boring cores collected from rock mass is not many. From this reason, the following procedure of the direct shear test is taken. ; Namely, at the desired normal load, the shear load is applied to the specimen. The failure in specimen takes place and the shear stress reaches to the residual shear strength. The normal load is then increased by the predetermined amount and the shear load, causing slide between the failure surfaces of specimen, is applied. After repeating such a procedure, a

failure line, which shows the relation between residual shear strength and normal stress, is obtained. The multistage direct shear test is one of advantageous methods by which the shear behaviours are studied on a few specimens. The direct shear test of boring cores collected from rock mass (tuff) of Shichigashuku dam is carried out by the above method, and the results of the test are shown in Fig.10. The relations between peak shear strengthnormal stress and between residual shear strengthnormal stress are shown in Fig.11.

Displacement (mm

3o m to Normal stress (kg lcmt )

In Fig.11, the internal friction angle obtained by peak shear strengths is nearly equal to the residual friction angle obtained by residual shear strengths. Besides, the measuring points of insitu test lie near the residual failure line, and the internal friction angle obtained by failure line of insitu block shear

Fig. 11 Relations between shear strength and normal aas stress in direct ""4'4 i

to-saw shear test and insitu shear test (Shichigashuku dam)40

18

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Normal stress (0g icm')

(a) Tuff breccia

ND M .o M Normalsln.s (aofem')

(c) Mudstone

MO 1

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Normal stress 0 ( Mount)

(b) Sandy tuff

0 .00 MB M0 1M NormY alnss (Ip(.mt)

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ltemrt eN.r laolue')

(e) Limestone

MO 1M t,0 a0 .0 Normal

Hm. (.olcm')

(d) Sandstone

test is nearly equal to the angles of residual failure lines.

4.3 Akagawa dam

The direct shear test is conducted on the boring cores collected from rock mass (tuff breccia ) of Akagawa dam, and some of the shear stressdisplacement curves are shown in Fig,12. The relations between peak shear strengthnormal stress and between residual shear strengthnormal stress are shown in Fig.13. It seems that the failure line obtained by insitu test lies near the failure line of the peak shear strengths as shown in Fig.13, and that the internal friction angle obtained from those results is nearly equal to the residual friction angle.

Displacement (mm )

Fig. 12 Shear stress displacement curves of specimens collected from Akagawa dam.

e0 60 00 20 Norm& stress (Nq/cm')

Fig. 13 Relations between shear strength and normal stress in direct shear test and insitu shear test (Akagawa dam).

As mentioned above, it is recognized that the insufficient data to design the rock construction are supplemented by the results of the direct shear test in laboratory, and it is possible to evaluate the internal friction angle of rock mass by the residual failure lines.

5. SHEAR CHARACTERISTICS ACCOMPANIED WITH RESIDUAL FRICTION

In order to evaluate the stability of rock construction, it is important to analyze the shear characteristics, namely, the residual shear strength, the residual friction angle etc.(Hoek 1977). Here, the direct shear tests using the

multistage method are performed on the specimens collected from rock mass of a lot of working places in Japan. The shear characteristics in residual sliding are moreover obtained in these tests, and the relations between the rock types and the residual friction angles are investigated. Jaeger(Jaeger 1971) has been proposed a

following equation in the relation between residual shear strength (Tr) and normal stress (on).

Tr = c (1e44" ) + ontanOr

In the above equation, Or is residual friction angle, c is cohesion, and b is constant. The specimens used in this study can be

classified into following rock types, namely, tuff breccia, sandy tuff, mudstone, sandstone, limestone, schalstein, granite, and crystalline schist. On each rock type, the relations between residual shear strength and normal stress are shown in Fig.14. Working places where the specimens are collected, Or, c, and b for each rock type are then summarized in Table 1.

Fig. 14 Relations between residual shear strength and normal stress for various rock types.

19

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(g) Granite Fig. 14 Relations between residual shear strength and normal stress for various rock types(continue).

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