Simon Chapman has been kind enough to remind me that Cleveland Potash's
Boulby mine in North Yorkshire, U.K. also produces salt for a range of 
commercial uses, including spreading on roads as a de-icer.  I should
have remembered this, having visited Boulby underground while on a 
press visit to see some new road-header at work quite a few years ago.

Boulby's main product, as the company's name implies, is potash but, for
reasons of better ground stability, all main underground haulage roads are
now driven in a salt horizon below the potash.  

Thus in Mining Magazine, August 1983, p 78, the news-item "Highlights" section 
reported: "New tunnels for Boulby:  A new £400,000 drivage, aimed at further 
improving productivity, will shortly be completed at Cleveland Potash, Boulby, 
Saltburn by the Sea, Cleveland, U.K. The two tunnels are each 9m wide by 3.8m 
high and will run 1.2 km from the main shaft to the mine's north eastern production 
area. They run through salt, 6 m below the potash seam and approximately 1,100 m 
below the surface".

In MM, March 1984, p 266 it was reported: "Between January and August 1983, 
four crews of six men, working 4,000 ft below ground at Cleveland Potash Ltd.'s 
Boulby mine in north east England, completed the North Development drivage. 
Consisting of twin 3.8 m x 9 in roadways driven in salt with connecting cross cuts 
at 75 m intervals, the drivage was mined at a rate of 8000 t/w and a total of 1800 m 
of roadway were driven in 17 months."

Writing in the November 1982 issue of Mining Magazine the then General Manager of 
Boulby wrote about the stress problems encountered in the mine (MM, Nov 1982, p421): 
"Panel design
 "The potash seam and the strata immediately above it are variable in the extreme. Seam 
thickness varies from 0 to 22 m and the grade can vary from 20% to 60% KCl. The depth 
of the seam, combined with the plastic behaviour of the potash, has made strata control 
exceedingly difficult. Excessive convergence of the roof and side walls makes it almost 
impossible to keep roadways open in conventional room-and-pillar workings. Lack of 
confidence in the long term stability of potash roadways has caused the mine to place 
main development roadways in the footwall salt, which is a waste product.
"Neglecting such factors as shale intrusions, the potash beds are strong and can form a 
good roof beam provided that the beam is not destroyed by lateral pressure and/or punching 
of the pillar. A stress relief system of mining, whereby three parallel roadways are driven with 
very narrow pillars between each roadway has afforded a practical solution. The two outer 
roadways fail owing to the lateral movement of the sidewall, but the narrow pillars on either 
side of the centre roadway are unable to transmit lateral movement and gradually crush, 
allowing the roof beam to maintain its integrity. At the beginning of 1979, the first serious 
attempts at stress relief were made and it is interesting to trace the history. The area for 
the experiment was in thick primary ore, and this was chosen deliberately so that side 
effects due to marl down-warps, shale intrusions, etc., would not cause confusion in 
interpreting the results.
"It was decided to drive three roadways in close proximity (6 m wide roadways separated 
by 4 m pillars), the centre roadway being the conveyor road, and to work the remainder of 
the panel by normal room-and-pillar methods. The reasoning behind this was that all roads 
other than the conveyor road could be conveniently ripped and that, since the major part of 
the convergence occurred within 60 days of mining, a good stable roadway would be formed 
if the ripping took place after 60 days. Again, since the ore was thick, the ripping would be 
a useful source of additional potash. This was more than theory, because in similar conditions 
bad roof and sidewalls had been ripped using a Dosco Mark IIA roadheader, and the resultant 
roadways looked very good indeed. Alas, they did not remain good for very long, movement 
did not cease and this proved once and for all that big pillars were to be avoided at all costs. 
At the same time, the conveyor roads stood very well indeed, apart from the sidewalls, which 
was no more than could be expected with 4 m wide pillars."

[Note his remark at the end of the first paragraph (a bit of mining history!) "...salt, which is a
waste product."]

In summary, the primary output from Boulby is potash, while the comany now sells salt 
produced from the need to drive main raodways in salt.

Tony Brewis