THORNDON SEA-WALL

Evening Post, Volume CVI, Issue 16, 19 July 1923, Page 8

THORNDON SEA-WALL

BEGINNING A GREAT

WORK

METHOD OF CONSTRUCTION

ELABORATE PREPARATIONS.

Preparations for the construction of the Thorndon sea-wall, a work that will alter the aspect of the northern portion of the city, are well in hand. The contractor, Mr. C. F. Pulley, who has had a long and varied experience in work of this kind—this will be his tenth sea-wall for the Wellington Harbour Board—is making preparations in keeping with the magnitude of the task before him. The wall will be 3429 feet long and the depth of the water in stretches is over thirty feet. Some 68,000 cubic yards of concrete will be required for the wall, which will be a huge monolithic mass, rising in steps from the sea-floor to a considerable height above high water level. .

Xhe width of the wall at the base will be 21. feet and on the top three feet. The wall will be a counterpart of the existing sea-wall and will continue the direction of that across the Thorndon Bight, so that, if it touched land again along the line >of its present course, the point would be opposite the last of the oil stores on the other eide of the Hutt road. Before, however, it reaches this point, some distance off shore, it takes a turn more or less parallel with the beach, and runs in to land close to Kaiwarra, From the agle of the Pipitea breastwork, the wall has already been built in the direction which it will follow for, a distance of 931 feet. Anyone standing on this com-, pleted section of the wall and looking straight along it landward will be abl« to understand the plan. METHOD HALF THE BATTLE. In a work of this kind and size method of attack and preparation are half the battleahd the contractor'is too old a, campaigner in this field of engineering' not to make tho most complete preparations before he starts, so that, when one« tho work is commenced, it may proceed "according to plan." More than ever is it necessary to save useless labour, lo avoid excessive handling of material and waste of time, and to see to tho continuity, of construction that it. does not stop for lack of foresight and forethought. So Mr. Pulley has laid his plans carefully. with a view to smooth and efficient working. In a fairly extensive area— about 2 acres—oh the existing reclamation granted by the board for the purnose, he is completing tho assembly of his plant and equipment and accommodation for the personnel, machinery, aud material required. .'■'.•■. PROCESS OF BUILDING. Firstj as to material: Concrete consists of cement', sand, and gravel. in suitable proportions mixed with "water. The most bulky constituent is the gravel. This will be brought in scows from the Butt River and'landed at a.staging, built for the purpose, by the contractor, alongside tho existing .length of/ 1 seawall. The shingle or' gravel will be lifted ashore by a three-ton crane and deposited in a dump containing about 4000 cubic yards. From the dump it will be lifted by a powerful steam navvy'. riiito,'laSfg'eX hopper bins, whence it will run by gray- r ity to the measuring hopper, and on to the mixing chamber. _ The mixer is of extra large'size;'and is locally made by Messrs. Cable and Co.', of Kaiwarra, The cement will be housed in a shed of dimensions 90ft x 30ft, and here also gravity will assist in the assembly and mixing. But before the concrete is mixed much has to be done. The site, for instance, mußt be prepared for the seawall. This; is- done as- a preliminary by the dredge clearing a track'along the line of- the wall. Then divers go down and fix the stone and wood forms into which the* concrete is poured. The space enclosed by the forms, which are firmly, for the time, . attached to the soa bottom, is further cleansed of silt and loose mullock by means of an ingenious submarine pump or ejector. This is operated by a. c2j-h.p. motor, which drives the pump .and raises the silt by nieans of a jet playing up into a larger tube on tho ejector principle. This was used with conspicuous success in clearing the foundations for;.tho piers of a bridge over the Mississippi 'at St. Louis."

LABOUR -SAVING MACHINERY.

A further preliminary is necessary. This is the building of a sort of trestle or pier over the track of the wall, by means of piles driven on each side and joined by decking on top—a sort of long bridge.straddling the course of the wall. This bridge will carry a tramway track, and from its decking most of the operations 1 will be conducted. TKe bridge will be extended as the work proceeds. Once the preparations are complete and all the plant in place, the process will be roughly as follows :—The mixed concrete, will be raised in a lofty distributing tower about 40ft high, and from that will flow in a receiving hopper. It will then.be run out to where it is wanted along the bridge and poured into trip boxes and shot into the prepared forms to which it has been lowered by suitable apparatus. The hopper carries 4i yards of concrete, and it is hoped to nil the forms at tho rate of about .200 yards a day. The wall will be built in stages or steps, and will retain its position by its huge weight.

■ As the wall proceeds and rises above high-water level the reclamation may follow, silt being pumped by the dredge over behind the wall, which is vertical on its landward' face. The process will be qmte familiar to those who saw the iast reclamation work on the area, now covered by large warehouses and wool stores.

The area to.be reclaimed is over 70 acres, the largest addition, to Wellington s supreme site for many years. The seawall .will ultimately be faced by a breastwork, like the other seawalls which constitute the rim of Wellingtons working harbour to-day. The Harbour Board itself will,continue the breastwork round the angle where the Present seawall takes its turn obliquely i, •! ,/ roiA Pi Pitea iuay- Pi Pitea quay itself will be at a later day the starting point for wharves like the Pipitha I !■' but the new wall lies ™. direction of the prevailing winds, and mi Ye,V c, "sed with its, breastwork, when aetWl'klnid as.b6rth^ plaC6 ** W