MR MACGREGOR'S REPORT ON THE BREAKWATER.

South Canterbury Times, Issue 2274, 1 July 1880, Page 2

MR MACGREGOR'S REPORT ON THE BREAKWATER.

At a special meeting' oJ! the Harbor Board the following report from Air J. Hacgregur on the Timaru Breakwater was read and adopted :

Dunedin, loth June, 1880.

Sir, — I have the honor to inform yon that 1 visited Timaru on the 22nd May, inspected the Breakwater now being constructed, and received the necessary information from Air doodah to enable me to express an opinion on the questions submitted to me for consideration, and on which I beg to report as follows ;

1. The method of repairing the portion of the work where the blocks have slipped out.” —The outer row of foundation blocks at this point appears to have been laid on the edge of a sloping reef, with bags of concrete under the outer ends for the purpose of keeping the blocks level. The sea afterwards removed concrete bags, and the blocks slipped outwards on the shelving reef. The engineer very properly, I think, cut the monolith on the top, and allowed it to settle also.

In order to repair this portion of the Breakwater, I suggest that the three top monoliths already broken on the outside.should be cut and thrown over in front of the blocks that have slipped out; then lift the top row of blocks with the crane, and make special sloping blocks about 2ft. higher on the seaside as illustrated by cross sections, tig. 1 and 2 on accompanying tracing No. I, When these blocks that have been laid with the crane arc settled down on a solid foundation the monolith on the top can

be rebuilt, and the cross section of the .vail restored to its original form.

The Breakwater even at this point has not been injured right through the wall ; the two inner rows of blocks forming the foundation remaining intact apparently and in the same position as when they were laid. The damage done to the Breakwater by the outer row of blocks slipping seawards is more in appearance than reality and can easily ho repaired as above indicated. “ 2nd. How to proceed safely with the extension of the Breakwater on a bottom changing from sand to boulders and prevent the southern line of blocks from being drawn out?” In reply to this question I could have riven a more valuable opinion if I had scon the result of an actual boring shewing the material of which the bottom is composed, but in the absence of the information, I will assume that the natural bottom is hard enough to sustain the weight of the wall now being constructed, and on this assumption I have prepared cross sections of Timaru Breakwater Figs 1 and 2 tracing No 1, showing alterations on the sea side of the wall, designed with the object of overcoming difficulties that certain to be met with in extending the Breakwaters.

1 observe that although the outer row of foundation blocks settle down, and in some eases get drawn out of the Avail, the centre row, only 12 feet inwards, never get disturbed, and I am of opinion that if the bottom were protected for another 12 feet, seawards, in front of the

outer row, by triangular apron-blocks, as shown on sections, laid carefully with the crane at the foot of the wall, that tiic outer row would remain intact in the same way as the centre is now. A vertical sett wall can only he laid with safety on a rock foundation, because the action of the waves, after striking the face of the upright wall, exerts an enormous scouring power on the bottom, immediately at the foot of the wall, and if the material be not sufficiently hard to resist tins force, the sand or clay is drawn only from below (be outer edges of the blocks, and consequently the whole row of outer blocks settle seaward, leaving an opening in the Avail, the Avidtli of such opening being in proportion to the quantity of material scoured from under the blocks at the bottom.

To obviate this danger I propose to use those triangular apron-blocks, above referred to, and set all the blocks on the the sea side with a slope inwards, as shown on cross-sections. This slope provides for the blocks sinking through the sand or mud for a depth of two feet before the horizontal line is reached, and the apron-block increases the width of the wall at the base to 48 feet instead of dli feet as at present. Another precaution will have to be adopted with the sea side of the monolith that is now built on the lop of the blocks that arc supposed to sink, and that we know do sink a certain distance in the sand or mud.

You will observe on the sections Jigs. I and 2 red dotted lines drawn tlrough the monolith outside the crane road, this is intended to represent a joint as I think it will be found necessary to leave that portion of the monolith unbuilt until the sea lias thoroughly shaken the foundation hh >eks into a permanent position after which the capping can bo put on without any risk of further settlement. Unless this precaution is adopted the monolith will break on the line indicated, follow the settlement of the foundation blocks underneath, require to be cut up and removed, and a new block built in its i place, involving the cost of so many cubic yards of concrete to bo paid for twice, besides the loss of time occupied in performing the Work a second time, and bringing nil other operations to a standstill until it was iinished. If the overhanging of the crane is snflicient I would also suggest that these foundation blocks should bo set in advance of the last iinished section lon,gitndually, as shown on elevation I. The object of this is to prevent cho I rent blocks Horn sliding forward while the section is being carried up to low water-mark, and the monolith being linished on the top. The triangular blocks protect thc-south side, and those are intended to protect the seaward cud of the wall in a similar manner ; although the sinking longitudinally is not so serious as transversely, yet it is very important that the front row of blocks, at the bottom of each section, should bo kept from sliding forward and thus destroying the regularity of the work.

ord. The feasibility of providing immediate accommodation for shipping. I am of opinion that a timber wharf,'say JOOft. long, should be immediately constructed inside the Breakwater, and connected to the railway lino, by means of a covered approach from the north side. The cost should exceed £3OOO. and I consider this amount would bo well expended in providing accommodation lor small steamers to load and discharge cargo during due weather. I need scarcely tell the Board that it would bo impossible to hold a vessel at the wharf during bad weather, with tho present length of the Breakwater uncompleted. But the position would be improving as the Breakwater extended, would be sheltered to a great extentwhen, it will begin to run northward. Tho numerous advantages gained by loading and discharging cargo alongside of a wharf instead of by surf boats, would justify the Board in expending £3OOO on its construction, although it could not be used every day in the year for some time to conic.

4th. The shelter that the Breakwater, when completed, Mull give to the foreshore to the north, and the protection thereby to the railway viaducts. When the Breakwater is constructed according to Mr doodall’s design, tho outer arm null protect tho foreshore of Caroline Bay from the action of the heavy south-east seas, but Mull aifor d little or no protection from M’aves coming from cast, or any direction north of cast.

The heaviest muivcs come on to the shore at Timaru from a south-easterly direction, and the Breakwater m'lioii completed M'ould direct the seas on tu tho beach some considerable distance, north of the railway viaducts—easterly and north-easterly seas still continuing to act upon the beach as if no Breakwater had been constructed, but I do not attach any great importance to ibis pro. tectum of the railway viaducts by tho construction of any particular design of harbor works at Timaru. It seems to me that the foundation of a harbor is of the first importance, and the-protection of the viaducts the second, ft would not be a very diflicult or expensive M’ork to completely protect the viaducts from the encroachment of any sea in whatever direction the Timaru Bi'eak water may be constructed, and 1 am sorry to see that such a trivial matter should he maguilled into such grout importance.”