Actually gas storage does go on in Cheshire. An organisation to the south of
Middlewich buys natural gas when its 'cheap', stores it in specially formed
cavities in the rock salt and then sells it back as the market price rises.
These spherical chambers were formed by controlled brining at a depth of
200-300(?) metres.

The infilling contract to stabilise four of the Salt mines at Northwich has
just started recently and is going to continue until 2007. One of the major
concerns with this contract is that no unsaturated water/brine is allowed to
enter the workings during the process. The fact that fresh water floats on
the brine means that supporting pillars could be dissolved at roof level in
the fully flooded workings. In the mid 90s saturated brine was pumped into
the Barons Quay mine to help prevent this situation arising.

Further information on Vale Royal BCs web site. I think its
valeroyal.gov.uk.

Regards Alan Robinson


----- Original Message -----
From: "Tony Brewis" <[log in to unmask]>
To: "Alan Robinson" <[log in to unmask]>
Sent: 16 March 2004 19:23
Subject: Re: Salt Mines and Document Storage


Following the discussion on proposed document storage in a U.K. salt mine,
Alan Campbell says "Perhaps someone can explain why this mine has not
flooded already, as old workings below the water table inevitably do,
eventually. There are presumably some fairly impervious strata around, I
wonder how thick, and how reliable, they might be?"

He also adds "I believe that the Germans have found a better use for their
salt mines now, for energy storage. They pressurise some of them with air
during the night, using off-peak electricity, and release the stored energy
to
generate electricity at peak times, a bit like the pumped storage schemes in
the UK, which use water and surface dams. With an apparent need for gas
storage in the UK, I wonder if that also would be a possible and more
appropriate use?"

Taking the latter point first, please bear in mind that in the British Isles
there
are two salt mines, both in current production, one at Carrickfergus in
County
Antrim, and the other the Meadowbank mine north of Winsford in Cheshire. The
latter produces the salt we spread on our roads whenever there is the threat
of
icy conditions, so as long as this mine lasts and men have to work in it, I
guess
compressed air energy storage is not an option!

Going back to the question about the geology, in my time as editor of Mining
Magazine, I went to Meadowbank and wrote an article on the mine ("Winsford
Rock Salt Mine", Mining Magazine, July 1980, pp 12-23)  (1980??? -- if that
isn't mining history, what is?)

Giving some of the history, I noted that the original shafts had been sunk
in 1844,
and the mine had worked from then until 1892 when it was closed because of
overproduction of salt in the U.K. However, in 1928 the last underground
mine
near Northwich flooded, and Winsford was reopened, and has worked
continuously
since then.

In the article I described the geology as follows:  "The Cheshire plain is
underlain by
a basin-like structure of sedimentary rocks of teh Triassic Period bounded
to the east
and west by older rocks of the Carboniferous. They are for the most part
overlain by
varying thicknesses of glacial drift of Pleistocene age.

"The youngest of the Triassic rocks comprise the Keuper Marl, a series of
mudstones
and evaporites which reaches its maximum development within the U.K. in the
Cheshire/
Shropshire basin. There are in all  five disctinct zones within the Keuper
Marl, comprising
the Lower Keuper Marl (maximum thickness about 300m); the Lower Saliferous
Series
(max. 240m); the Middle Keuper Marl (max 300m); the Upper Saliferous Series
(max. 400m);
and the Upper Keuper Marl (max. 300m).  At Winsford, only the lower two of
these are present
and the glacial drift and clays of the Pleistocene lie unconformably
directly on the beds of the
Lower Saliferous Series.

"Some nine main beds can be differentiated in the latter in the area of the
mine workings [as
shown in a section in the article which, without scanner, I cannot show
here]. The lowest,
Zone 'A', consists of marls with some salt bands;  Zone 'B' is a bed of rock
salt 24m thick;
Zone 'C' 10m of marl; Zone 'D' contains rocksalt of erractic grade 27m
thick; Zone 'E', also 27m
thick, consists of rock salt heavily contaminated with marl bands; Zone 'F'
is 23 m of rock salt;
Zone 'G', 35m thick, has rock salt with massive bands of marl; Zone 'H' is a
further 16m of salt,
followed by Zone 'I' with 15m of salt with marl bands.  The highest grade
material and the zones
which have been worked so far [i.e. until 1980] are the bottom 7.6 m of Zone
'B' and the middle
7.6m of Zone 'F'.

"Despite their name, the "marls" of the Keuper are for the most part only
slightly calcareous and
consist largely of clay minerals.  Some of these are of wind-blown desert
origin, although the marl
mostly accumulated in shallow water.......The Keuper Marls are very
competent rocks in-situ, but
become friable and decompose rapidly on exposure to air and water."

The article goes on to describe the generally flat lying strata (a slight
dip of 3 degrees to the south)
with some structural complications arising from monoclines, where the beds
"bend", in one case
causing a vertical displacement in the beds of 85m. One such monocline
traversed the whole 3.5km
width of the then mined area. To trace the beds, boreholes had been drilled
from surface (typical
depths of working were 165m below surface, with future working planned on a
260m deep horizon).
Boreholes were typically 500m apart, with closer spacing being called for
when a monocline was
detected.  Borehole logging was by gamma-ray techniques, this corresponding
to the potassium
level in the rocks (gamma ray activity low in salt, high in the marls).

To quote once more: "The glacial drift is pervious, and groundwater has
penetrated through it as far
as the unconformity. Salt from the sub-outcrop of saliferous beds has been
dissolved, so that on the
sub-surface at the top of the Keuper deposits there are lakes of brine.  The
marls and rocksalt themselves
are impervious and prevent any further penetration of water to beds at
greater depths."

As noted above, exploration was by means of boreholes from surface so, to
quote once more:
" As the surface holes may pass through a brine lake on the sub-outcrop
horizon they represent a
potential hazard through the risk of flooding should an open hole be
encountered by the mine workings.
Two precautions are taken to prevent the ingress of water to the mine by
this means.  Firstly, once the
information has been obtained from any hole, it is completely sealed by
being grouted throughout its length,
thus forming a solid concrete plug. Secondly, a pillar of salt 75m in
diameter, centred on the location of the
borehole in the salt horizon as determined by survey, is left unworked, so
forming a barrier againest the inflow
of any brine should the concrete seal permit any to pass."

So, the short answer to Alan Campbell's question as to why the mine has not
flooded by now is that the workers
take great care that it doesn't!  The solution mining methods around
Northwich are not a threat as they are miles
away, but the fact that at Winsford they are working directly underneath a
brine lake at the unconformity overhead
is a threat, and is taken very seriously.  Rock mechanics experts from
Newcastle University were involved in strata control,
and in a follow-up article (Mining Magazine, January 1981, p12 et seq), Dr.
A. Szeki, a Lecturer in Mining Engineering
at Newcastle-upon-Tyne, wrote an article entitled "Rock Mechanics at
Winsford Rocksalt Mine".  He opened the article by saying "The rock salt
mine at Winsford is probably the most extensively instrumented mine in the
world from the point of view of ground control.  By this extensive
instrumentation a very close ground or strata control scheme is provided for
the management to monitor strata movement and, if it is necessary, to enable
them to implement preventive actions in time to maintain the stability of
old, current and future mine workings."

By 1993, a short article on the use of underground communication links at
Winsford gave the information that the workings by that time stretched over
an area 5km by 3km.  With only a few workers underground, I don't think the
moisture in their breath will pose much of a problem.

I hope the above is of interest!
Tony Brewis