AN IRISH DAM

Evening Post, Issue 29, 4 February 1935, Page 10

AN IRISH DAM

GREAT ENGINEERING

FIGHT AGAINST WATER

SIX YEARS' STRUGGLE

A monumental engineering work in Northern Ireland has been brought to mind recently by the presentation of a splendid paper on the subject of the construction of the Silent Valley Reservoir Belfast Water Supply, to the proceedings of the Institution of Civil Engineers by Mr. George Mclldowie, Associate Member of the Institution of Civil Engineers, who was resident engineer throughout the work, writes Mr. F. W. Furkert.

The inception of the. work dates back to the end of last century, when, in 1891, Mr. L. L. Macassey, M.lnst. C.E., advised the adoption of a catchment in the mountains of Mourne (famous in song and story). The first stage was the construction of a conduit 35 miles in length, "to bring in the ordinary flow of the Kilkeel and Annalong Rivers. It was finished in 1901. Storage was to be deferred until the rising demand exceeded the lowwater flows. In 1891 the demand was 9A million gallons per day; and in 1911 it had reached nearly 17 millions and storage became an imperative need Borings were made at a site in Silent Valley in 1913, but the advent of the war stayed proceedings until 1923, when a contract was let to Messrs S Pearson and Son, Ltd. The contract sum was £983,000 and the work involved was an earthern embankment 1500 feet long and 88 feet high, with puddle core and a concrete cut-off wall extending to solid rock very far below the Valley bottom. As the work of excavating the cut-off trench proceeded it became evident that the: depth to the rock had been much under-estimated. Later work showed that the material, of. glacial origin below the surface contained many very large boulders, some fifteen feet in diameter, and it seems definite that the original borings bottomed on such boulders and were thought to reach rock bottom. The contractors had already completed the valveshaft outlet tunnel,.-, screening chamber access roads, part of the spillway, and an access railway four and a half miles in length, of a total value of £500,000. Added to this, £824,500 previously spent on the conduit, etc., had to be considered, when debating the question whether the dam could be completed at a figure which could be justified—that is if it could be finished at.all, as to which there was grave doubt. A DIFFICULT MIXTURE. The character of the material lying .under the valley was most difficult, being glacial silt, gravel, sand and boulders, sometimes in layers sometimes mixed. The silt when wet was quite unworkable, and all the gravel contained enough silt to render it most difficult owing to the lubricating action of the silt. The engineer was taken seriously ill, retired, and shortly died. A.committee of famous engineers, Messrs. W. J. E. Binnie^ Ed Sajideman, and H. . Prescott Hill, M.M.Inst.CE., were called in, but before committing themselves to an °PT"? n«. that the work could be. completed they required much further exploratory work to be~ done. Additional borings were made, and indicated depth to rock approaching 200 feet below the valley floor,, while the permanent -ground water level ■ was 16 feet down. As the possibility of dewatering the ground through which the cut-off trench had to be sunk had a vital bearing on the subject, the committee decided to have three shafts sunk,.which could confirm boring information, and, serve: as pumping l a H? nt -for lowe"ng the water. These shafts had to be sunk under compressfh ffi a^ o an u-linediWith cast-iron, and the difficulties encountered were immense. It was necessary to. pump in some shafts while concentrating on sinking in another, and alternating Later the number of shafts was increased and in November 1928, they were actually pumping in.eight shafts, and. only sinking in two.. The river had been diverted from the site of thejorks and its., channel made watertight with clay covering, all swamps, 1h ?J he - locallty had been drained and the river channel straighten*! for over a mile below the site. Towards the.end: of 1927 the progress over a period of three months did not S% en feet per ™nth in two shafts. However some shafts were bottomed, and the engineers were tWnP/+£ ared t0 sa * that the complecl £ £ reservoir was feasible and should be .proceeded with. They outlined, their proposed methods and made an estimate of £1,350,000. It may. here be mentioned that the final cost did not exceed this estimate by more than 1 per cent. . _ The contractors were released from their original contract and engaged to carry on, on a cost plus percentage THE METHOD. OF CONQUEST. If it was slow! and difficult to sink a few shafts 12ft 6in in diameter evS witii the assistance of compressed air and pumps, some faint idea can be fanS.6^ 9 *: the .stupendous task which d t . the eni meers in devising and *^ gL 1 and strenuous and uncomfortable work called for from the s7x Or£rw?de Sinlr g ™ °pen « six-feet wide, at a minimum: 1500 iwW?* T t0212 deep, and keeping it m shape and clean enough to enable watertight concrete to^e #£ be excavated 'in • wet gf oundThe measured pressure of thl ™"which flowed like water, was more th^SO per cent greater than the water^ressur,% Ttabering became jJW. and for the central 500 feet, cast-iron segmental plates |in thick had to be used, arching on each side against special cast-iron thrust, blocks, whfch mL t n the f on J plates- 6ft 6in by lft 6in were placed, the ground-be-hind was cement-grouted. Though the joints were caulked until the grouting set, it was necessary then to ifainT-h >T a- er Pr«s^e behind the lining by_ boring out. the groutinH holes. This water was gathered ud in amprovised channels and long ca^ vas sheets were hung against the sides of the trench, but in spite of all efforts and complete oilskin suits the mS worked in very wet conditions. But fcf .th, e end of 1929 the task ■■■** Pleted and concrete placing coma£. one^bdng:com«^

+ Thus after six years of struggling *c work_was up .to ground level, and the building of the dam proper could be started. This took two years it does not call for very special mention exce?V? r the flat slopes adopted m view of the unsatisfactory nature of the foundation. These commence with. 1 in 3 at the top, then l m4;lin 5; and finally lin 6. - There was a considerable-peat area in the reservoir area, ranging up to 30ft deep. As its removal was impracticable, it was covered 2 to 3 feet with clean sand over an area of 342,000 square yards. OJ

During the whole time the works were in progress.the city's water had to be conveyed past and through the works, unpolluted and without interruption, while floods had also to be coped with. After, two. years in use.