How is a Trig Pillar Made?
The "History of the Retriangulation of Great Britain" by the OS. tells us that:
"The lower buried centre mark, consisting of a brass bolt set in concrete, is first inserted at a sufficient depth below ground level to be independant of the pillar of the pillar foundations. This depth naturally varies with the soil; on boggy ground, which is sometime encountered on hilltops, it was sometimes necessary to excavate as much as 15 feet before reaching rock or firm soil on which to emplace the lower mark. In such a case a correspondingly deep pillar foundation is necessary, whereas on out-cropping solid rock, a bolt is simply cemented in a hole drilled in the rock.
The lower centre mark and its concrete setting is covered with a small wooden box (which eventually distingtegrates) to prevent adherance to the pillar base, and so to prevent disturbance of the lower mark in case the pillar should be moved.
Concreting of the pillar base was commenced immediately over and around the box covering the lower mark, without interposing any loose earth or stones, which might weaken the pillar foundations. Concreting the pillar base is continued up to ground level where it is left rough to set. Four angle iron bars are set in the base to project well up into the corners of the pillar, as a means of preventing fracture between the base and the pillar. The pillar bolt (upper centre mark) is also set in the base; it is arranged vertically over the lower mark by means of a plummet and marked board resting between marked pegs, previously fixed in correct relation to the lower mark.
The pillar bolt is next covered with a small wooden box, which is provided with side holes (to take the inner ends of the four sighting and drainage pipes) and a top hole (to take the lower end of the galvanized pipe running down the centre of the pillar).
Wooden shuttering, which may be taken apart and used again, is next erected on the pillar base. This shuttering has four side holes to take the outer ends of the four sighting pipes, which are then inserted, and a wedge fillet to which the level flush bracket in one side of the pillar may be wired in a vertical position. It also carries wooden corner fillets to provide an automatic chamfering to the edges of the pillar.
The centre pipe, which serves as further reinforcement, is set in position and plumbed, the plumbing being continually being checked during concreting. A good 4:2:1 mix of concrete, containing sharp well-washed sand and crushed stone as aggregate, is then poured into the shutteringand rammed. Before the concrete sets, the brass spider, complete with holding down bolts, is set over the centre pipe and is carefully plumbed over the pillar bolt from a special temporary fitting to the spider, by sighting in both directions through the lower sighting tubes.
Concreting is then carried up to the top of the spider with an allowance of about 3/8 of and inch (a depth to which some pillars are suffering depletion from frost damage today) for later settlement of the concrete.
After a day to set, during and after which the green concrete may need protection from frost by a liberal covering of sacks, the shuttering is removed and the pillar is faced with cement plaster, to prevent possible disintegration by ice forming in cavities. Three of the sighting tube openings are plugged with paper and lightly cemented over to conceal their presence from visitors, who are often apparently unable to resist the temptation to stuff any 'foreign body' that comes to hand into the tubes. The fourth tube must be exposed to allow condensation to drain out of the pillar."