wall and (2) to accelerate the progress of the
work,
I have after careful consideration prepared the modified design of wall shown on the accompanying drawing. Section A shews the section submitted to Sir John Poode with the width of berm increased to 10 feet as recommended by him, while Section C shews the alterations I have made to submit for the consideration of Government.
6. As will be seen on the drawing, the chief alteration consists in the substitution of artificial blocks of Portland Cement concrete in the footings of the wall for the granite masonry footings of the original design.
Section C shows the blocks as 8 feet in length, 4 feet wide, and 3 feet in thickness; and would weigh about 6½ tons each. The surface of the rubble mound will be levelled truly by divers, and I propose to set the blocks by means of a floating steam derrick. On the lower course, a second course similar to the first will be laid. These two courses will bring the work to such a level as to allow the masonry of the superstructure of the wall to be carried on in the dry.
7. Before commencing to build the wall, however, I propose to lay similar concrete blocks for weighting the rubble-mound in the manner shown on the drawing (Figures D and E). Seven courses are shown, each two tiers in width. These blocks will be placed temporarily in lengths of about one hundred feet at a time, and when the mound has finally settled, and attained a condition of repose (to be ascertained by levels taken at regular intervals), the weighting blocks will be removed by the derrick and used in the same manner on another length of wall, with the exception of the blocks required for the lower courses of the wall as shown on Section C, which will be reset in their permanent position by the derrick.
J. The blocks as shewn will exercise pressure on the rubble mound of about...