Page 69 - Bulletin 18 2014

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the surface of which would be 10 ft. above spring tide low water level. The 20° angle of rest

was established by the position of the lewis holes cast into each block. These were so
positioned that each block would hang from the 15 ton block-setting crane at the required

angle and be lowered to rest securely alongside or on top of those already laid. To this end
the topside of each block contained an upstand / saddle and the underside an indent / notch

that acted as a locking mechanism when the blocks were stacked one above the other. The
centre blocks in each slice of three were placed 9 inches ahead of the ones on either side, and

also slightly lower than them, to prevent the straight transverse joints that would otherwise

have arisen. The whole arrangement was known as inclined slice-block construction and it
was the first time this technique had been used in South Africa. (Figs. 2.23 – 2.25.) It had

been first used in dock construction at Karachi, India. (Simon’s Town’s east breakwater was

somewhat similar but constructed of vertical blocks resting on a rubble mound.)

Setting the foundation / bottom course of blocks accurately on the irregular seabed was

critical to achieving the slice angle of 20° and ensuring that the breakwater had vertical sides
and ran in a straight line. This work was done by two divers who used a special frame of the

same dimension as a block, and a spirit level, to build a foundation of bags of concrete to the

height required to hold the frame at the correct angle and level in the transverse direction.
(Fig. 2.23.) The frame was then removed and a block lowered into position on the concrete

bags. The space between the sea bed and the bottom of the block was filled up with hand-
picked stones. Five slices of bottom blocks were laid in this way; concrete was then placed on

either side and half way up the base courses, both sealing them and securing the toe of the
wall; finally, the space between the bottom of each block and the bed was grouted by

pumping concrete down the grouting cores that had been cast in each block. Thereafter four

of the set of five slices were built up to the required level above water. At this level the
advancing structure was cross-braced at every second slice by a 20 ft. long rod, 2.5 inch

diameter, secured to 2.5 inch diameter eye-bolts located centrally in the outer blocks.

The construction advanced into the sea in five-slice increments, but with the fifth slice

coming later so that a toe and blockface were always formed for positioning the next