RAILWAY SURVEY

Hokitika Guardian, 5 September 1929, Page 7

RAILWAY SURVEY

PREDICTING EARTHQUAKE!!

BENCH -MARKS EVERY MILE,

CHRISTCHURCH, Sept. 3

Rome geologists and surveyors consider that the South Island is tilting to' the west, as shown by recent earthquakes, and that an noaurace survey would enable the next seismic disturbance to be anticipated with some certainty. Hr* W. F. Robinson, of the School of Engineering, Cary'erbury College, in reading a paper last Tuesday evening at the annual conference of the New Zealand Institute of Surveyors, suggested that an ideal means for such a survey was the railway line from ilie East to the West Coast,

According to present British practice, he said, five or six first-class bench marks would be necessary from here to tho other coast. These would consist of underground chambers with permanent marks which would not ho disturbed for many years. The marks would consist of bronze plugs and flint cubes. On top, working bench marks for cfveryday use would be placed, and at shorter intervals a simpler form consisting of buried marks could bo used. These would be stones buried in the earth and protected Iby cover stones. A.t mile intervals say, third-class bench marks, consisting of bronze plugs let into vertical faces of .permanent structures, such as station platforms, bridges, etc., would complete the survey.

Mr J. McNair, District Railway Engineer, told a, representative of "The Press" yesterday that survey’ pegs, or rather, blocks were to be found now at intervals of about n mile along, all the railway lines in the Dominion. The blocks or bench marks were of wood, two inches by two; sunk well into solid ground. Resurveys were not made regularly, hut the Ibloeks were utilised when new work was being done in any’ particular locality. In addition, there were permanent concrete blocks opposite all railway platforms, to mark the centre, but these were out of sight, sunk in the ballast. These blocks, which were ten inches square indicated wiliere tliq Ichaina-ge had been taken to, and registered the mileage. They were firmly and permanently fixed in the ground.

On the assumption that the sea level did not. change, no matter what upheaval took place, it was used a. c the datum, and for the Midland railway the high water mark at Sum ner was the basis. On every bench mark the height above sea level was •indicated, and the marks appeared in the plans. On an old plan of the ChristchurchLyttelion railway, drawn in 1860, the first bench mark commencing at Lyttelton was shown as being in the steps of the Union Bank at a height of 48.35 feet above high water mark at the Sumner bar. Mean sea level was often taken as the datum, and nil heights were related to that. The Christchurch railway station yard was shown in the plan as being 16 feet above high water mark at spring tide at Sumner, to whih must hr added 3.56 feet for nienn sea lqvol. Most of the bench marks along the railway lines were placed in position by the Public Works Department during eonsti action, and not much /use for them is found by the railway authorities, but for :big tunnel or bridge works permanent marks havt been left. For instance, at Arthur’!Pass and Otira concrete “monuments,’’ about three feet in height, which were used for . “sighting” in connexion with tne tunnel, may stili be seen. The actual “sight” was taken through a pinhole in a brass plate let into the concrete. These “monuments’’ will probably bo undisturbed for a great number of years.