Civil Engineering Reference
In-Depth Information
- Determination of the Atterberg limits and the consistency of the sol as an indication of
the sticking potential according to DIN 18 122 for soft ground,
- Clay mineralogical analyses for the determination of the content of the most significant
minerals (montmorillonite, kaolinite, illite, smectite, quartz etc.),
- Closer pattern of investigation in areas containing clay minerals for the more precise
determination of the affected sections and the content of clay constituents at the face.
11.6.3.2 Wear
The wear on excavation and mucking components depends on the abrasiveness of the
ground, the type of mechanical loading, the selection of tool materials and the operating
parameters of the machine.
In soft ground, the mineralogical composition and the strength are relevant for tool wear
but also the grading distribution, the grain shape and particularly the content of boulders
and blocks. The test of the Laboratoire Central des Ponts et Caussées (LCPC test) offers
one method of evaluating the abrasiveness of samples of soft ground with various miner-
alogical compositions and also takes the breakability of the grains into account. The verbal
classification based on the ABR value used in the tables in Appendix 3 was not intended
specifically for mechanised tunnelling and is currently being checked and revised in re-
search programmes. Wear forecasts should therefore not be based on a verbal description
of the abrasiveness shown by the values in the tables in Appendix 3, but use the index
value (ABR value). In addition, the mineralogical composition, cutting wheel design, type
of tool and process-related aspects of the excavation process should be taken into account.
In rock, wear can vary widely depending on rock strength, mineralogical composition,
jointing and tunnel orientation to the texture of the rock mass. The Cerchar Abrasiveness
Index (CAI) classifies the abrasiveness of rock. The most important parameters are the
equivalent quartz content and the rock strength. High rock strength and correspondingly
high CAI values lead to high primary wear in compact rock. In case abrasive, hard to
break rocks are loosened out of the rock mass in an uncontrolled manner, the wear can
increase over-proportionately due to impermissible shock loading. If the material flow is
poor due to sticking or the design of the cutting wheel is unfavourable to material flow, a
further increase of wear is likely (secondary wear). Further factors, which determine wear,
are: brittleness, ductility, grain size, texture, porosity, mineral hardness, any foliation, the
design of tools /cutting disc spacing, disc cutter diameter etc.), the materials used for tools,
mode of operation and tool management (checking and replacement cycle).
In coarse- and mixed-grained soils, the primary wear is mainly determined by the break-
ability and strength of the coarse-grained fraction, boulders and blocks. The secondary
wear increases with increasing equivalent quartz content and deterioration of the material
flow and ease of excavation, particularly in wide-graded grain mixtures. Depending on the
type of tunnelling machine used and the tools fitted, it is necessary to investigate whether
breaking and grinding processes will occur in order to estimate wear rates.
11.6.3.3 Soil conditioning
The addition of additives in liquid or powder form, suspensions or water can be used to
modify the properties of the excavated material. The concentration of the conditioning
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