Page 302 - Introduction to Mineral Exploration

12: CLIFFE HILL QUARRY AGGREGATE RESERVES 285

10 and 50 m of overburden. Although the flight Between September 1981 and August 1982
augers could probably cope with this depth, the drilling was concentrated on two areas of
removing the clay from the auger and clearing the site:
the hole proved slow and difficult. The clay 1 Investigation of the proposed plant site.
was also expansive and, when wetted by the Some 10 boreholes were drilled where access
water flush from a diamond bit, the hole tended permitted.
to close and trap the core barrel. These related 2 Drilling of a newly acquired area to the
problems were ultimately solved by using a northeast of Stud Farm.
drag bit and water flush. A drag bit is a bladed The results confirmed the already familiar
bit which is used when the sticky material picture of highly variable overburden thick-
such as clay and marl would clog up air flush nesses, with the bedrock often “dipping away”
bits. The drag bit drilling technique produced beneath rapidly thickening Triassic Mercia
an open hole to bedrock almost as fast as the Mudstone overburden.
augers could when cleaning time was taken Between April 1983 and March 1985 drilling
into account. In addition, the clay had to some was carried out on a 50 m grid across the area of
extent expanded due to the water flush. By the proposed quarry (Fig. 12.2) with the aim of:
drilling with sufficient annulus size (the gap 1 confirming the geophysical interpretation;
between the drill rods and the sidewall up 2 identifying the volume of weathered
which the drilling medium (air, water, or foam) material;
carries the drill cuttings to the surface) and cas- 3 building up detailed knowledge of the faults,
ing placement to bedrock, a good core recovery rock quality, etc., to enable detailed quarry
was usually possible (Bell & Hopkins 1988). plans to be drawn up.
During the early drilling program the driller The drilling results were used to produce the
remarked on the apparent low penetration rate overburden isopach map (depth to bedrock)
of the core barrel. At the time this had been (Fig. 12.4). This map was initially drawn inde-
assumed to be the result of operating in an pendently of the geophysical data and clearly
uncased hole. Once the main program began it shows the success of the previous electromag-
became apparent that, although the high rota- netic and resistivity surveys in revealing the
tion speed improved penetration, the produc- distribution of the overburden (Figs 12.5 &
tivity was lower than expected. A series of 12.6). Although some inaccuracy is evident in
checks and experiments narrowed the problem the deeper areas, with the resistivity method
down to polishing of the impregnated core bits. underestimating thickness of overburden, the
After a series of trials, conducted in conjunc- two shallow ridges and steep sides were accur-
tion with one of the leading British bit manu- ately delineated. Drilling indicated that these
facturers, a bit matrix which gave the optimum features can be attributed to faulted blocks of
balance between bit wear and penetration was markfieldite alternating with late Proterozoic
developed. Once this problem had been solved metasediments, as seen in Cliffe Hill Quarry.
the drilling proceeded at a rapid rate (Bell &
Hopkins 1988). During the summer of 1981
a series of boreholes was drilled along a line 12.4.2 Geophysics
joining Stud Farm with Cliffe Hill Quarry As indicated previously, the planning applica-
(Fig. 12.2). The aim was to investigate the route tion was being prepared in parallel with the
of a proposed tunnel to link the two quarries. exploration work and the work was coordin-
The purpose of the boreholes was twofold: (i) to ated by a team of geologists, engineers, estates
delineate the bedrock–overburden interface, surveyors, and landscape architects. The con-
in order to ensure that the tunnel which was tinual acquisition of additional borehole data
planned to link Cliffe Hill Quarry to Stud Farm during the planning of the project meant that
(section 12.4.5) remained in solid rock through- occasionally parts of the scheme had to be
out its length, and (ii) to identify the engineer- redesigned. However, the boreholes revealed
ing properties of the bedrock with particular that the areal extent of the markfieldite
reference to jointing and faulting. Some heavy was somewhat larger than originally anticip-
faulting was identified in two boreholes. ated. It became apparent that a geophysical