MECHANISED TUNNELLING | TECHNICAL
UCS [MPa] 0
AVN 400 AVN 800
AVN 1000 Left, figure 2:
AVN 1200* AVN 1600* AVN 1800* ≥ AVN 2000*
Recommended Feasible, with special design *Feasible, with special design
Recommended slurry MTBM application range in certain rock strengths
50 AVN 800 HR 100 150 200 250
However, going deeper increases the likelihood
of encountering hard rock and higher groundwater pressures for the MTBMs to handle. More variables in small diameter tunnelling mean
more data to record, manage and analyse. In that regard, digitalisation offers notable help. For example, digitalisation can help to evaluate key data to support wear prediction and tool change planning, and so help enable improved tunnelling performance. Design of technological systems can also find benefit
from digitalisation. Following several small-diameter hard rock MTBM projects, for example, Herrenknecht developed a device to control the roll of the MTBM in hard rock sections, as will be discussed below.
ROCK, SOIL, INVESTIGATIONS AND SELECTIONS As with all trenchless projects, geotechnical investigation and assessment form the basis of selecting the most appropriate no-dig methods and tools, and to establish the relevant parameters to support predictions on performance and wear. Yet such geotechnical values are often insufficiently considered in small diameter rock excavations.
Rock classification and weathering From the perspective of strictly geological classification, rocks are classified based on their origin and their stratigraphic and lithologic properties, respectively. As
an engineering material, rock can be defined as lithified or indurated, and crystalline or non-crystalline. Rock can be encountered in mass presence or large
and small fragments, all of which have consequences for design and construction options compared to those that are due to soil values. Classification of igneous, metamorphic, sedimentary, and pyroclastic rocks can be applied, but more important are the physical properties of the rock and the rock mass, and the engineering descriptive criteria. When subject to chemical or mechanical weathering,
rock generally decreases in strength. The weathering processes change the physical and chemical properties of the rock as, gradually, it is changed more toward soil – which is also called sediment and could be compacted to eventually form sedimentary rock. Many small-diameter utility tunnels, due to their
generally small overburden, are constructed in weathered rock. Rock, therefore, can be described based on its
petrographic, mineralogical, or geotechnical properties. For small-diameter tunnelling, geotechnical properties especially are important. After a successful borehole campaign, sampling at
the depth of the proposed tunnel or of characteristically representative rock, tests of the samples in laboratories establish geotechnical, such as: uniaxial or unconfined compressive strength (UCS – σc
); tensile strength (TS – σt); and, point load index (PLI). These values
Left, figure 3: Exemplary hard rock cutting wheel designs for AVN 400 up to AVN 2500
AVN 400 800 1200 1500 1800 2500
January 2024 | 23
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