DAUB recommendations for the selection of tunnelling machines - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 388

11.9.1.3 Critical areas of application
The use of a tunnelling machine in the fields marked light grey (Symbol “-”) will prob-
ably require considerable additional measures or modification of the ground, otherwise
difficulties should be expected. The achievable advance rates and cost-effectiveness will
be considerably reduced in comparison to the core area. A founded analysis of the techni-
cal, economic and contractual risks and a comparison of variants with other tunnelling
processes are strongly recommended.
11.9.1.4 Classification in soft ground
The grading distribution represents the direct and indirect evaluation criteria for the sta-
bility and permeability of the ground. Based on the shear strength parameters and the
water pressure, and including consideration of the grading distribution, the stability of
the ground is evaluated and the required support pressure is determined. The technical
requirements placed on the machine increase with increasing ground and groundwater
pressure.
11.9.1.5 Classification in rock
The table recommendations serve primarily to select the tunnelling machine and not to as-
sess the cuttability. The rock mass classification and evaluation of stability are undertaken
based on the RMR system. It is recommended to analyse the tunnelling machine system
and all six project-specific parameters of the RMR system. Calculations to verify the sta-
bility and determine the support pressure are also recommended.
11.9.2 Notes about each type of tunnelling machine
11.9.2.1 TBM (Tunnel boring machine)
The main area of application is rock classed as stable to liable to rockfall, and water
ingress from strata and joints can be overcome. The uniaxial compressive strength σ D
should be between 25 and 250 MN/m 2 . Higher strengths, toughness of the rock and a
higher content of mineral resistant to wear represent economic limits to application.
A restricted ability of the machine to brace itself may also make the use of a machine
impractical. For the assessment of the rock, the tensile strength and the RQD value are
used. With a degree of fracturing of the rock mass with RQD from 50 to 100 % and a
joint spacing of > 0.6 m, the use of a TBM seems assured. If the fracturing is worse, the
stability should be checked. In soft ground or solid rock with similar properties to soil,
the use of a TBM is impossible.
11.9.2.2 DSM (Double shield machines)
Double shield machines are mainly used for tunnel projects with long stretches through
stable rock but also short stretches of rock classified as liable to rockfall to brittle. In a sta-
ble rock mass (see the requirements for the use of a TBM), the machine can work in con-
tinuous mode using the grippers for bracing. In fault zones or areas of lower rock strength,
where the grippers cannot be used, the shield joint is retracted and the machine pushes
itself using the auxiliary thrust cylinders against the last completed ring of segments.