The blasting was done in the winter of 1967/68 and cavers didn't know what was going on until it was well advanced. That was the year of a really bad Foot-and-Mouth outbreak so that all caving, walking, climbing, etc. was put on hold throughout the country for nearly 6 months. Hence no-one went near Giants, which is on farm land, and Mr. Watson, who owned Peakshill Farm, and Mr. Revell, who did the actual "mining" work, were able to get on undetected.
The idea was to turn it into a show cave but they could not get planning permission for the necessary car park so, after all the damage and expense, they had to give up the idea.
Garlands is gradually returning to its original depth as the rubble gets washed away down the Crabwalk. We were shocked when we first got in after the winter if 67/68 to find that Garlands was only 15 ft deep, instead of its original 30ft. It seemed that once the passage had been opened sufficiently at Pillar Crawl and the sump to allow someone to get through with tools, the rubble from the blasting was wheelbarrowed down the passage and tipped over the top of Garlands Pot. In the summer of 1967 I can remember seeing a caver right at the top of the old fixed ladder on the 30 ft deep Garlands Pot with water pouring over his shoulder as the cave was in fair flood.
in early 1967 Eldon P.C. had chiselled a shallow channel from the Backwash Pool downstream which allowed the static sump to drain sufficiently to leave a half-inch airspace, so the dams went out of use. However, the remaining duck was low enough to catch out someone who tried to go through with a rope round his shoulders and jammed solid, having to be hauled back out by his feet. There was at least 6ft of flat-out crawling in more-or-less flooded passage and, once you got into the water you raised its level so effectively cut of the air-space anyway and you had to treat it like a sump and dive through.
The cave suddenly became much more popular once you didn't have to bail the dams, and that's evidently what gave Mr. Watson the idea of turning it into a show cave. Also remember, no cave SSSIs at that time, so no worries about damaging a cave by blasting chunks of it away as there was no legislation to stop you doing whatever damage you wanted to.
The original breakthrough which passed the sump, was in May 1954, after 4 year's determined digging led by Ken Pearce and his helpers. It was Ken and Co. who constructed the dams, the 2nd. one further back up the passage towards Pillar Crawl being required because you couldn't get sufficient water into the 1st. dam immediately in front of the sump. The problem was that the sump was continually being filled, albeit slowly, by the backwards flow from the pool in Backwash Chamber so you had to bail the water back into the dams faster than the sump could refill and you also had to be out of the cave and back past the dams before the sump had filled sufficiently to cut you off.
That's one reason why the 25 hour Donna Carr rescue from Giants in 1965 was so epic: because the whole sump area had to be drained and kept clear for the stretcher to get through and then Pillar Crawl and the Curtain had to be negotiated.