Civil Engineering Reference
In-Depth Information
The excavation classes basically apply for drill and blast in solid rock (SIA 198, Section
5.2) and for roadheader drives in solid rock (SIA 198, Section 5.3) and for TBM drives
in solid rock (SIA 198, Section 5.4), but different evaluations are offered by the SIA 198.
Table 2-33
Matrix of excavation and boring classes, diagram from SIA 198.
Excavation class
Boring classes
X
Y
Z
I
I X
I Y
I Z
II
II X
II Y
II Z
III
III X
III Y
III Z
IV
IV X
IV Y
IV Z
V
V X
V Y
V Z
T
T X
T Y
T Z
For the application of the excavation classes, tunnels and inclined shafts are dealt with in
the same way. For vertical shafts, which are enlarged from bottom to top with a cutterhead,
there is no division into excavation classes.
The boring classes correspond to the cost of excavating solid rock with a TBM and take
into account both penetration and tool wear as decisive factors. Boring classes are speci-
fied by the employer from essential rock and rock mass characteristics. In this way, the
length of tunnel to be driven can be summarised as sections with the same class with
regard to the cuttability of comparable geological formations or rock types. The number
of boring classes is thus determined by the prevailing conditions and is specified in the
tender documents for each project. The decisive rock mass characteristics are to be given
as precisely as possible with their range of variation.
The boring classes X, Y, Z are to be understood as symbols for differently cuttable rocks
along the tunnel; X could stand for a gneiss defined in the longitudinal geological section,
Y for a granite. On another project, X could represent Schratten limestones, Y marl of the
Drusberg beds, Z a siliceous limestone.
Alternatively, the standard SIA 198 also permits the grading of boring classes from pen-
etration rates driven in-situ. The determination of the net boring speed is performed con-
tradictorily, daily on a stretch of tunnel that is representative for the drive, using a test
procedure specified in the contract (test stroke length, thrust force, condition of the cutting
tools and their state of wear). The procedure is however only regarded as suitable for spe-
cial cases, on the one hand as too many factors determine the penetration and on the other
as it is questionable whether the test stretch is representative.
The classification of the tunnel drive into individual classes is done by linking the rel-
evant excavation and boring classes. The combination of excavation and boring classes
is performed in a matrix; an example is shown in Table 2-33. Each entry in the matrix
corresponds to a stage of boring and the support work obstructing a stage of boring and is
assigned a unit price per metre of advance for the payment of this tunnelling class.
With this matrix, it can be that the advance achieved is determined solely by the boring
class. In rock mass that is hard to bore, it is possible to install the excavation support of
classes II, III or even IV in the time it takes to bore a stroke, unless the support is shotcrete.
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