of the excavation of a dry dock was usually a substantial part of the total cost. Obviously it was liable to vary considerably between one method and another, and also between different estimates for the same method. With regard to method, the contract documents usually confined themselves to saying that the contractor was deemed to know all sorts of things which in the time available for tender he usually had little opportunity to investigate fully. This combination of variables and unknowns was liable to result in the ultimate cost of the work to the client, to the contractor, or to both being greater than it need be. In this case, however, the Engineers had defined in detail in the contract documents an obviously economical method of carrying out the excavation. It was true that, from the design of the dock structure, it was reasonable to assume that some such method would be practicable, but even so, so clear a declaration of the Engineers' opinion must have been very encouraging to the estimators' morale, with a consequent benefit to the client.

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who has been closely associated with the.prestressing work, and Mr D. J. Coats, B.Sc., A.M.I.C.E., the Resident Engineer for the Allt-na-Lairige Scheme. DiscussionThe Author introduced the Paper with the aid of a series of lantern slides and a film. He described the method adopted to observe deflexions of the dam which had been referred to in $ 5 5 and gave particulars of records of deflexion during prestressing of the dam and reservoir impounding obtained since the Paper had been prepared.62. The method of observing deflexions was illustrated in Fig. 19. A telescope, removed eom a dumpy level, had been rigidly mounted at the top of the dam to sight on a viewing plate fixed to the downstream face of the dam near to the base. Since deflexion of the dam would be accompanied by a rotation movement, a sensitive spirit level, graduated to read 24 sec of angle to each division on the bubble scale, had been fixed adjacent to the telescope as a means of measuring the degree of rotation. A protective wooden box had been mounted over the telescope and level and removed only when readings were being taken.63. The graph plotted in Fig. 20 showed the actual deflexions after correction for rotation. The maximum deflexion observed as a result of the prestress load was just over 0.2 in. in an upstream direction. During a period of 11 weeks until impounding commenced the deflexion decreased to 0.1 in., but it had been observed that the deflexion was almost continually changing owing to atmospheric conditions and temperature and the recorded points marking observations subsequent to prestressing might not necessarily have covered all of the variations that occurred. Conditions with the dam empty were indicated by the broken line and conditions during impounding by the full line. The slight increase after impounding commenced could be attributed to temperature change. When the reservoir had been filled to overflow level the deflexion in an upstream direction due to the prestress load had been almost exactly counterbalanced and the overall results confirmed what was anticipated in prior calculations when the dam had been designed.

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