Abstract
As it is widely recognized that there is a large difference in strength between rock mass with fissures and intact test-pieces of rock, the author aimed to establish more useful equations that bind both strengths.
For this purpose, test-pieces with numerous micro cracks made by freezing and thawing cycles were prepared, and the relation between their uniaxial compressive strength and P-wave velocity was investigated.
The result showed that this relation is considerably similar to that of rock mass with fissures and indicates that this is a very useful method in laboratory.
Test-pieces of welded tuff were used for this experiment because of its uniform texture and appropriate hardness. On the basis of the results of laboratory experiments on the test-pieces of intact and cracy rock, an approximate value of shear strength of rock mass can be calculated by the following equation:
τm0=τp0 (Vpm/Vp0) n
where, in case of soft rock (uniaxial conpressive
strength qu0=10-100 kg/cm2), n=1.3-1.7
medium hard rock (qu0=100-500), n=1.6-2.8
hard rock class I (qu0=500-1, 000), n=2.7-4.0
hard rock class II (qu0≥1, 000), n=4.0-5.5
τm0: shear strength of rock mass (cohesion)
τp0: shear strength of intact test-pieces of rock (cohesion)
Vpm: P-wave velocity of rock mass
Vp0: P-wave velocity of intact test-pieces of rock.
Furthermore, the relation between shear strength of intact test-pieces of rock and that of rock mass obtained by blok shear tests at 10 localities of Hokkaido, can be approximately expressed by the following equation:
τm-b0=τp0 (Vpm/Vp0) N
where, in case of soft rock, N=3.0-3.5
medium hard rock, N=3.5-5.5
hard rock class I, N=5.5-8.0
hard rock class II, N≥8.0
τm-b0: shear strength of rock mass obtained by
in-situ block shear tests (cohesion, kg/cm2)
In planning of general construction the latter value (τm-b0) has been commonly used. However, the differerence of the two values (τm0 and τm-b0) must be recognized, and their application should be made according to the purpose of safety of dam, tunnel and slople stability of large cutting in rock mass, etc.
