COMMENCEMENT OF WORKS. 33 



box coffer-dams (Figs. 20, 21, 22) sufficiently wide to admit of 

 the construction of the permanent works, and, after completing 

 such works, to utilize them as the means of excluding the 

 water from the site generally. 



On consideration, the disadvantages involved in the above 

 method of procedure were clearly seen to be as follows : 



The entire exclusion of the water from the site would have 

 been so long delayed as to have rendered the general progress 

 very slow, besides involving the necessity of providing an 

 immense quantity of very long timber to enable many isolated 

 parts of the work to be commenced simultaneously. 



Great risk would have been incurred in having the protect- 

 ing works in such close proximity to the disturbing influences 

 unavoidable in the operations of excavating and pile-driving 

 in the foundations for the basin walls. Extensive and costly 

 means of access would have been required to convey away the 

 excavated, and to supply building, material. 



Under these circumstances, it was proposed by the contractors 

 for the works to reclaim at once the whole of the site subject to 

 tidal influences. 



This object was accomplished by constructing first an inner 

 dam or shallow, a, on plan (Fig. 23), enclosing all the locks, 

 docks, and a large portion of the inner quay walls, and an outer 

 or main dam in deeper water, b on plan, enclosing the harbour 

 and outer quay walls. 



The inner dam, which was of a very simple construction, was 

 placed on the highest part of the mud, and was in no part less 

 than 100 feet from the site of any work, the excavation for 

 which was expected to cause its disturbance. 



The outer or main dam, b on plan (Fig. 23), enclosing 

 the second section being in deeper water, and in close 

 proximity to the sites occupied by permanent works, was of 

 a more substantial character although somewhat similar in 

 construction. 



During the construction of these dams, the ebb and flow of 

 the tide was provided for by sluices constructed across the 

 different channels, which, on the completion of the dams, were 

 closed at low water, and the tide excluded from the works. 



The completion of the inner dam, b} 7 which some 75 acres 

 of mud land was reclaimed, enabled the works generally to be 

 speedily opened up, for simultaneously with the construction 



