Structures In The Sea.

methods , whicli was believed to be inapplicable where there were marine worms , were given . The second was most effectual , but was generally expensive , ancl often . attended with danger . The last was also costly , besides being subject to delay in the progress of the works . In bridge building , of late years , the jjlan of forming enclosures of close [ piling , of the shape of the pier , and filing in with hydraulic concrete , hod been pursued by French engineers ; and the substitute : . of iron for perishable timber piling , in the construction of thsi plan of the piers of the Chelsea and Westminster bridges , by Mr . Page ( M . Inst . C . E . ) , was considered to be a successful departure from stereotyped rules .

Although the value of beton , or hydraulic concrete , was ri 5 w appreciated in tin ' s country as a substitute for masonry , and had heen employed iu some important works , yet its use was chiefly confined to forming a homogenous and monolithic bearing stratum for foundations , aud not , properly speaking , as a constructive material . The modes in which concrete had been applied for constructive purposes were , building it dry in mass , and allowing it to set before being laced in the workas bad

p , been adopted in the construction of the walls of the Victoria ancl of the London locks ; preparing it first in blocks , and allowing it to harden before being used , as employed at the Dover breakwater , and for the new sea forts at Portsmouth and Plymouth ; and depositing it in a liquid state , and allowing it o set under water , as practised at the Government Graving o cks at Toulon . The facilities for making beton , which had

e invaluable property of setting under water , and of thus foring an artificial rook or , stone , were very groat , as it might be acle either from the naturally hydraulic limes , the artificially ydi-aulic limes , or cement , or from the rich or non-hydraulic limes , rendered hydraulic by the admixture of other substances , such as Puzzolana , minion , or iron miue-dusfc . Various exles were adduced of the lication of concrete large

amp aep , on a scale , prepared from these different materials , " especially at the Mole of Algiers , at the breakwater at Marseilles , and at other French ports , as well as in the Pont d'Alma over the Seine , in which case both the arches ancl the piers were formed of rubble concrete .

As Engineers-in-chief for the new harbour works for the port of Greenock , the author and his partner , Mr . Bell , had an opportunity of introducing a system of constructing sea walls ancl quays in deep water , without the aid of coffer dams , diving apparatus , or other means equally expensive . These works were situated on the west side of the town , and had been projected almost entirely beyond the high water line into the sea . The outer pier would ultimately be upwards of 3000 ft . in length ancl

about 60 ft . wide at the top , with quays on both sides . AVitbin this there would be space for two harbours , each 1000 ft . in length , 15 ft . deep at low water , ancl 25 ft . at high water , with entrances 100 ft . wide , ancl ample room for the construction of graving docks , for the storage of timber , and for the erection of sheds . At present it was only proposed to erect about one-half of the sea pier , and to form one harbour or tidal dock . In the design of these works , it was suggested that tho walls under

low water should consist of a combination of cast-iron guide piles in the front , with a continuous stone facing , slid down over and enclosing these piles , timber-bearing piles being used in the body ¦ of the walls where required , and concrete backing- being deposited in a soft state ; and that the upper part of the walls should be built of masonry in the usual manner . The first operation , when the water was not sufficientldeepwas to dredge two parallel trenches to tho

y , required depth , 17 feet below low water , for the foundations . A staging of timber piles was afterwards erected in the line of the pier over its whole breadth , for carrying | the tramways , travelling cranes , ancl piling engines . The cast-iron guide piles were then driven from the staging , with great precision , 7 feet apart in the line of the face ol each quay wall . These piles were driven until their heads were near to the low water line ,

hy pile engines , furnished with long arms projecting downwards , strongly stayed by diagonals , and forming a trough , into which the pile was placed , ancl from which it was shot , like an arrow from a cross-bow . The piles were connected at the top transversely by wrought iron tie-rods stretching through the pier . AVhen the piling was driven , a bed of . hydraulic concrete , 3 feet thick , ancl 20 feet wide , was deposited in the trenches to form a

base for the wall , and to give a large bearing surface . Into the groves formed by the flanges of the iron piles , large granite slabs from the . Ross of Mull , from 18 inches to feet thick , were slipped , the bottom one resting on the concrete base , and on a

projecting web cast on the piles . This constituted the face of the wall , and iu each compartment between the piles , 16 feet in height and 7 feet in width , there were only three stones . Behind this facing hydraulic concrete was lowered under low water in large boxes having moveable bottoms , and was discharged in mass to form the body of the wall . To confine this at the back before it had set , loose rubble stones were deposited . The hearting of the pier consisted of hard" till , stones ,

ancl granite up to the level of the low-water line . AVhen the whole of this mass was consolidated , the heads of the iron piles and the granite facing blocks were capped by a granite blocking or string-course , and the upper portion of the walls was built in freestone ashlar ancl rubble . The remainder of the hearting between the walls was then filled in , and the whole finished with a granite coping ancl causeway . The walls were 33 ft . in height from the foundations 11 J- feet thick at the concrete base ,

, diminished by 5 ft . at the top . Iu the part of the work already executed , the outer flange of the iron piles was exposed to the action of the salt water . In future it was intended to reverse this plan and to make grooves in the stone facing , so that it should overlap the iron piles , filling in the grooves from the top with cement . AVhen the whole extent of the seaward pier was completed , the interior operations for the harbour would be proceeded with ; this pier serving as the principal coffer clamancl

, a short clam , about 100 ft . in length , closing the entrance . Ib was stated that this mode of constructing walls in deep water without coffer dams had heen most successful , and that a seapier of great solidity and durability hud been formed in deep water at a comparatively moderate cost . The works of the Albert Harbour were being executed under the superintendence of Mr . John Thompson , as resident engineer , by Messrs . AV . ancl

J . York , contractors . The application of this system to the construction of breakwaters ancl harbours of refuge , was then noticed , reference being first made to the principal modes of construction hitherto adopted , and to the peculiar phenomena by which such structures were affected . The usual method of forming breakwaters used by the pierre perdue , or long slope system , as carried out at Plymouth , Cherbourg , and Holyhead . AVhere stone was most abundant , a vertical wall was built from the bottom by

means of diving apparatus , of winch the breakwater at Dover , now in course of construction , was the most prominent example . Besides these systems , which might be taken as the extremes , an intermediate form of section , combining both , that was to say , a rubble mound to a certain depth under low water , and a vertical wall above , had been carried out at Alderney . From an examination of the general principles which affected breakwaters , ancl the modes of construction usually adopted ,

the conclusion arrived at was , that the vertical system was that which had best resisted , or rather averted , the destructive . action of the sea ; and required the smallest amount of material . Both the long-slope and the vertical systems , as at present carried out , were expensive , from the quantity of material used in the one case , and the costliness of the material and the mode of construction in the other ; the former might be characterised as involving the maximum in quantity , and the

minimum in cost of material ; the latter , on the contrary , the mimimum in quantity and the maximum in cost of material . The object sought to be attained in the new system was to effect a minimum , as far as possible , both in the quantity and in the cost of the material . Breakwaters might be thus constructed , either wholly vertical from the bottom , or partially vertical , springing from a rubble mound . The principal feature of the new plan was a framework of iron pilesor standards

, , and ties , which would serve during the construction as the staging , and would afterwards form an essential portion of the structure , by binding together a strong casing of stone , or other sufficiently durable material , which would enclose and form the facing of the breakwater , the interior being filled up with loose rubble , cemented into a solid mass by liquid concrete . As soon as a pair of piles transversely hacl beeu fixed , rubble

would be deposited up to , say , 18 ft . under low water . Strong casing blocks , either of stone or of beton , made to enclose the iron standards , would then be lowered , the blocks being locked or arched into each other , so as to resist pressure from behind , and made to break bond , if thought desirable . The hearting of the work would be proceeded with simultaneously with the building of the easing , and would consist of rubble in the centre , and of hydraulic concrete behind the stone casing . It was believed that such a structure eould be erected in a depth of 6 fathoms , with a range of tide of 15 ft ., for £ 190 per lineal yard