%9 



sufficient to force an outlet outside the tunnel walls ; after which 

 it would meet with no resistance. 



The question of the repair of the dam, preser\dng the necessary 

 outlet for the water, is emphatically one which only a skilled and 

 experienced engineer should decide ; but it may be discussed from 

 a non-professional point of view. When a similar, though less 

 serious fracture occurred in the outlet pipe of the Yan Yean 

 Reservoir, it was ingeniously repaired by introducing rings of boiler- 

 plate securely connected with each other so as to form a continuous 

 lining ; the large diameter of the pipes (nearly 3ft. ), favouring 

 the adoption of a plan which would have been otherwise impracti- 

 cable. In the present case there seems to be no alternative but to 

 make an open cutting through the dam along the line of the pipes, 

 carrying it down until a solid bottom is reached. There would be 

 considerable difficulty in filling in such a cutting so as to make the 

 whole solid ; and the difficulty is, of course, much greater when an 

 outlet for the water has to be retained. But the geological condi- 

 tions do not greatly favour the construction of a new tunnel 

 through the adjacent bank, and the practical inconvenience attend- 

 ing the working of a syphon puts that method of discharge out 

 of the question. Supposing that a good bottom is reached, the 

 next thing would be to put in a foundation of concrete, on which to 

 build a strong culvert, extending from the base of the tower to the 

 mouth of the present tunnel. This culvert, in which the pipes would 

 be laid, should be built with wing walls or rings of solid masonry 

 at intervals along its whole length, and there ought also to be deep 

 lateral cuttings into the embankment on the right and left of the 

 culvert for the same reason ; the object being to prevent the leakage 

 of water, which always makes for the junction of old and new work 

 or of two difierent materials. The filling in would be the most 

 important part of the business, and would require the most careful 

 oversight ; no contract work would be admissible. 



Allowing that the dam thus repaired could never be quite safe 

 under the full vertical pressure which was originally contemplated, 

 it might yet be of considerable service. To relieve the pressure, 

 and still make provision for the storage of a considerable body of 

 water, it might be found desirable to form a second dam at the 

 point where the reservoir begins to contract in width, provision 

 being made for perfect control over the flow of water to the lower 

 part of the reservoir. The upper part might be easily enlarged 

 and improved to make up for the diminished vertical height of 

 water contemplated to be allowed in the lower ; but these additions 

 and extensions would entail considerable extra expense, and their 

 discussion is somewhat outside the proposed limits of this paper. 



In any case the old bye-wash would have to be greatly lowered, 

 and precautions taken to prevent the scour which has been so 

 destructive at its lower end, either by a series of steps, or by a paved 

 channel at a steep gradient, the former being the preferable course. 

 For such work only the hardest freestone should be employed : the 

 mudstone, which has been much used in some parts of the works is 

 quite unsuitable. 



The chief point remaining for consideration is the filtering of the 

 water. Fortunately the foreign elements are chiefly sand and mud, 

 which only require repose to precipitate them, and this may be easily 



