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Going underground
Experts have been predicting for years that the nonmetallic
minerals industry would go underground. A few
pits did take this step, usually for environmental reasons,
i.e. to reduce the dust and noise nuisance for the local
residents. This included our client Märker Kalk GmbH,
whose resources at the Herrlingen quarry in Baden-
Wuerttemberg were nearing exhaustion.
There was no prospect of obtaining yet another approval
for surface quarrying close to a residential district. The
logical move was to consider using underground mining
methods and as a result the Mähringer Berg project was
put into action.
Planning considerations
Planning considerations were based market conditions
and the geology of the deposits ‘All is dark ahead of the
pick’- this old mining saying had twofold significance when
it came to the Märker project:
It applied firstly to the client’s general project for
developing the underground workings and building a
new kiln to process the high-grade white lime. This first
required a lengthy planning and clarification process that
included financing arrangements, technical equipment,
licences and partners. All these procedures were carried
out against a background of increasing demand for the end
product. But now that every - thing is ready for ‘harvesting’
the market appears to be much less favourable. The global
economic crisis, which has been accompanied by a fall in
demand, has left its traces. Nevertheless, as soon as the
markets recover this long-term investment project will be
delivering the expected returns.
And secondly to the geology in the strict sense of the
word. In 2001 an exploration roadway was driven and a
small-scale test extraction undertaken. And the geology, it
has to be said, was perfect.
However, after the contract to develop the Mähringer
Berg deposits was awarded in June 2007 to the Mähringer
Berg joint venture, comprising Schachtbau Nordhausen
GmbH and Thyssen Schachtbau GmbH, it was found that
the geological conditions encountered along the 1,350 m
of drivage differed somewhat from the original predictions
and findings.
Rockbolts, weldmesh panels and shotcrete had to be
employed on a large scale in the long-term excavations
Issue 04 | 2010
TRANSFER OF TECHNOLOGY
and roadways. However it is hoped that the poor geology
encountered in some of the roadway developments will not
be repeated to the same extent in the excavations made to
the north of the same extraction zone.
The ‘Mähringer Berg deposits’
development project
Excavation work on the conveyor road, parallel headings,
crosscuts and rooms for the crusher, transformer and mine
fan began on 09.09.2007. The highly karstified strata called
for the highest safety standards and required a lot more
support than had been planned. Roadways with profiles of
32 to 36 m 2 would have to serve as the main production
arteries for decades to come. The crusher room, for
example, was not spot-bolted but instead supported by a
30 cm-thick coating of shotcrete, two layers of weldmesh
and rockbolts up to 5 m in length. After each round of
shots the heading team had to reassess the condition of
the strata and determine in collaboration with client the
most appropriate measures to be used for support and
reinforcement.
In order not to contaminate the high-quality lime
shotcreting was only permitted in the cut-through road.
More than 3,000 tonnes of this material were eventually
used in all the infrastructure cavities.
The increased safety measures, additional length of
drivages required for the parallel heading and crosscuts
and the need for additional ripping at the roof and ancillary
support work all combined to delay the completion of the
project by nearly half a year.
The development work was completed at the end
of 2008. A total of 1,350 m of roadways and some
49,000 m 3 of cavities had been excavated at an average pull
of 3.14 m and with an explosives consumption rate of about
1.7 kg/m 3 . Each complete pull took 18.5 hours to execute,
including time for ancillary operations and unscheduled
support and reinforcement work.
The shotfiring pattern, length of pull and choice of
explosive all had to be coordinated so as to maximise the
heading performance, reduce the degradation rate and
keep the proportion of debris less than 20 mm in size below
35 %. Of course this could not always be achieved in those
karst joints that were filled with loam and silt. An upper
limit had also been placed on the vibration levels due to
blasting, with no firing permitted at all between ten in the
evening and six in the morning.
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