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Hybrid joint proposed by Hancock in 1985 belongs to a kind of extensional
-shear fracture which mechanism is between shear fracture governed by Coulomb'
stheory and tensile fracture by Griffith's theory. On the study of hybrid join
t, Kuo and Huang conducted a series of tensile-compression tests to simulateth
e mechanical conditions for hybrid joint in the laboratory, and proved itsexis
tency. Chen discussed the relationship between geometry of hybrid joint andstr
ess field induced by fold according to joint field survey as well as FEMnumeri
cal simulation result. This research is divided into three parts: physical
model test in thelaboratory, model analysis and field survey. Physical experim
ent-tensile-compression test- simulates the stress conditions for hybrid fract
ure usingservo-controlled stiff machine. Specimen size is also considered in o
rder toextend the range of tensile stresses hoping for the complete stress fai
lureenvelope in the tensile stress zone. A model is constructed for rock fract
ureunder three dimensional stress strain conditions according to polyaxialexpe
riment data in different stress zone by Kuo, Huang and Reches. Fieldsurvey foc
uses on the correspondence between hybrid joint and other geologicalstructures
. Experiment results show that sandstone is fractured into three patterns:c
onjugate hybrid fracture, vertical extension fracture and singly-inclinedfract
ure. Fracture is plotted as great circles and poles by equal areaprojection, a
nd can be recognized as the pattern of steep, small dihedralangle in the direc
tion of maximum principle stress. Dihedral angle approximatelydecreases as the
minimum principle stress also decreases. In two dimensionalstress space, Grif
fith failure criterion underestimates the strength butoverestimates the dihedr
al angle; regressed Hoek-Brown and Mohr-Coulomb failurecriterion parameter are
obviously deviated for tests under different stresspaths. In three dimensiona
l stress space, there is accorded trend for thetensile-compression test data a
nd thus three dimensional failure envelope isconstructed by method of normaliz
ation. Fracture patterns are also includedin this envelope to explain the stre
ss state inducing hybrid fracture andorthorhombic fracture. Field joint data r
eveal that strikes of joints are roughlyperpendicular to the strike of anticli
ne in both cases of study and areindicators of contemporary stress field cause
d by geological changes.
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