Engineering geology aspects for design and classification - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 46

The quotient of the third, J r (joint roughness number), and the fourth factor, J a (joint
alteration number), can be considered an indicator of the shear strength at the interfaces
between the individual blocks of rock.

The fifth parameter, J w (reduction factor for the influence of joint water), is a measure of
the water pressure, while the sixth factor, SRF (stress reduction factor), permits various
interpretations:
- loosening pressure in the case of shear surfaces and rock containing clay,
- ground pressure in a stable rock mass and
- squeezing and swelling pressures in plastic, instable rock mass.
This sixth parameter is regarded as the overall pressure parameter. The quotient of the fifth
and sixth parameters describes the active stress.
Barton regards the parameters J n , J r and J a as more significant than joint orientation. This
is implicitly included in the parameters J r and J a , since these relate to the most unfavour-
able joint.
Using the Q value, it is indirectly possible to come to a conclusion about the necessary
support measures with reference to the actual size of the cross-section and the intended use
of the tunnel. The ratio for the support measures is a function of the constraints described
above. It is determined by dividing the opened length, the diameter or the height of the
excavated cross-section by a defined factor determined by the intended use of the struc-
ture, the Excavation Support Ratio ESR.

S ADHm
ESR

L ,

or []

= with

S equivalent support measures number
A L excavation length [m]
D tunnel diameter [m]
H cross-section height [m]
ESR Excavation Support Ratio