Civil Engineering Reference
In-Depth Information
A1 “stable”:
RMR 81-100
A2 “liable to rockfall”:
RMR 61-80
B1 “brittle”:
RMR 51-60
B2 “very brittle”:
RMR 41-50
B3 “non-cohesive”:
RMR 21-40
C squeezing rock”: RMR < 20
For tunnels under built-up areas, a statement should also be made about the expected
ground deformation or surface settlement with appropriate verification through calcula-
tions.
11.6.3 Excavation
The advance rate depends not only on the characteristic values of the ground but also on
the selection of excavation tools, the geometry and design of the cutterhead/cutting wheel
and the operating parameters of the machine. Changes in the geotechnical parameters can
be unfavourable but also favourable for the advance. Because of the extremely compli-
cated interactions between ground and tunnelling process, detailed analyses should be
performed to clarify the causes.
Sticking in the excavation chamber and increased wear on the excavation tools in particu-
lar are the most frequent causes of disappointing progress and increased costs. These are
now described in detail.
11.6.3.1 Sticking
The inclination of the soil to stick can have a decisive effect on the advance rate in mecha-
nised tunnelling. Sticking reduces the advance rate because, for example, the excavation
chamber of slurry-support machines has to be flushed or time-consuming manual cleaning
leads to unplanned stoppages. In addition, sticking in combination with a high content of
minerals liable to cause wear can lead to heavy wear on the cutting wheel and excavation
chamber. Any propensity of the ground to stick should therefore always be described in
geotechnical reports.
Soft ground with clay content, but also solid rock containing clay minerals, can result in
considerable delays through sticking. Clays with pronounced plasticity and sedimentary
rocks containing clay, like for example conglomerates/breccias with clay mineral con-
tent, siltstones and particularly claystones have proved particularly susceptible to sticking.
Sticking often occurs in combination with water, which can come from natural formation
water with open and earth pressure balance machines or process water (support suspen-
sion, soil conditioning, cutterhead jetting in hard rock).
The hindrance of progress through sticking can best be countered by recognising a poten-
tial sticking problem before the start of construction and appropriately adapting the equip-
ment of the machine and the planned advance rate to take the problem into consideration.
Geotechnical reports should provide the following information in this regard:
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