266 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



TAr 



GUST, 



thirty-four yards wide, measuring from the end of the mule on which 

 the Lazaretto stands, to that of the work in question. 



Under these circumstances, the engineer was obliged to resort to 

 other expedients, and he was thus led to form and execute a new plan 

 for establishing foundations at sea, which five years experience of the 

 works at Algiers has proved to be, according to all accounts, superior 

 to all those which have here'ofore been put in practice, and particu- 

 larly to those made of rubble work; a method much approved of 

 since the construction of the Cherbourg and Plymouth breakwaters, 

 the two most important maritime works executed in modern times. 



The principal feature of this plan is the \ise of blocks made of bt-ton. 

 These blocks are of two kinds ; one being constructed in the water at 

 the place it is intended to occupy, the other made on shore and 

 launched. 



The first is made by pouring the beton into cases without bottoms, 

 sunk on the place where the block is to rest. The frames of these 

 cases are made by putting together pieces of scantling in a rectangular 

 form, to which are nailed two courses of plank placed at right angles to 

 each other. The lower edges of the cases are cut out to fit the profile 

 of the surface on which they are to rest. They are lined with tarred 

 cloth, throughout the whole'extent of the inside up to the level of the 

 water. The cloth at the bottom is allowed sufficient fullness to ac- 

 commodate itself to the inequalities of the ground. The cases are 

 thus, in fact, converted into cloth sacks, the sides of which are strength- 

 ened by the timber work on which they are stretched and fastened. 

 The cloth sacks enable the mass of bJton to accommodate itself per- 

 fectly to the surface which receives it, the inequalities of which serve 

 to bind together the rock forming the bottom, and the beton. This is 

 a great advantage in the use of these cases, fur with the flat bottomed 

 ones generally used, it is necessary to level the surface to bo built upon, 

 which is a difficult and uncertain operation. 



The clcth bottomed cases are built upon stocks, laimched and floated 

 to the place they are to occupy. They are then sunk by means of 

 small wooden boxes, one foot square, filled with cannon balls or pig- 

 iron strung entirely round on the outside of the case, about one foot 

 and a half from the toj), by means of a cable passing through iron rings 

 fixed in the uprights. 



A similar use of beton was made by the Italians to prevent the dis- 

 integration of masonry immersed in water. They filled, with bOton, 

 bags similar to those used in fortification for making earth defences, 

 and placed them compactly, one iipon the other, and in such a manner 

 as to fill up the inequalities of the surface on which they rested. The 

 cement which oozed out through the interstices of the cloth, bound the 

 little rolls of K-ton together and soon formed a very compact and 

 durable mass. The cloth between the joints rotted and disappeared 

 in a few years. On one occasion, they filled a much larger sack with 

 beton than those above described, and" threw it into the sea in stormy 

 weather ; some days after the storm had subsided they found this 

 block very hard and strong. From the result of this experiment it 

 was natural that the adoption of very large blocks of beton should be 

 thought of, but the difficulty consisted in making bags of those dimen- 

 sions which would not burst, and fixing them in pusition, while being 

 tilled with beton. 



When the case is moored, the beton is lovi'ered and deposited in it 

 by means of a trough, which has a vertical and serai-rotary motion 

 communicated to it by a cylinder worked at each end by a crank. This 

 trough which contains a little more than a cubic yard, gives the ad- 

 vantage of putting in the case a large quantity at a time. The opera- 

 tion is thus made more rapid and there are fewer seams. 



The beton blocks made on shore are moulded in cases consisting of 

 four sides made of thick planks and lined on the inside with another 

 course of plank jointed together at the bottom and removable at plea- 

 sure. The bottom rests upon two large sills connected transversely, 

 forming an inclined plane which terminates at the point where tlie 

 block ;s to be launched. These cases like the others, are entirely 

 empty and without shores. When they are filled with beton, and it 

 becomes sufficiently hard, the sides are taken off and the block thus 

 stripped is launched into tlie sea. 



The mortar used in the large cases with cloth bottoms, is formed of 

 one part fat lime and two paj-ts of Italian puzzolana ; that used for 

 blocks on shore is composed of puzzolana and sand in equal propor- 

 tions. 



The lime should be made from the grey transition limestone, fine 

 grained and very hard; slaked in the ordinary way, and reduced to 

 the consistency of thick paste, it absorbs two and a half times its 

 weight of water. Its bulk is increased in the proportion of 1 to 1-S. 



The puzzolana is the same as that used along the Mediterranean 

 coast in the formation of hydraulic mortars. It is to be found in the 

 neighbourhood of Rome. The best comes from Saint Paul's cave, 

 near the church of that name. This puzzolana is brought by waggons 



to the Tiber, and thence by batlcaux to Civita Vecchia, whence it is 

 exported. It is sent abroad in the natural condition in which it is 

 found, the pieces varying in size from that of an egg to the smallest 

 grain of sand. JI. JuUien, the engineer, found by experiment that the 

 very finest grains were the only ones that could be used with effect ia 

 hydraulic mortars, and that when it was used in grains as large, for 

 example, as the largest grains of sea sand, it was as ineffectual as tne sea 

 sand itself. From tliis it appears to be necessary that the finest grained 

 puzzolana alone should l)e used in hydraulic works ; and as its efficacy 

 and quickness in hardening are in proportion to the fineness, too much 

 pains cannot be taken to pulverize if. 



Acting on this princi])le, the puzzolana brought from Italy and Africa 

 for the work on the mole was sifted at Algiers before" being used. 

 One half, forming the residue, was ground in a mortar mill and sifted 

 again, leaving a residue of one-tenth. 



That ground and sifted was of a quality inferior to that furnished 

 by the first sifting. The price of the puzzolana delivered at the work 

 was thirty-six francs per cubic yard, and the cost of sifting, grinding, 

 &c., twelve francs, making the total cost forty-eight francs. 



The cost of labour at Algiers, independently of the inferior quality 

 of the puzzolana obtained by trituration, and the consequent increase 

 of expense, made it desirable that if should be sifted at Rome and the 

 refuse left there. The soil on which this city and its environs stand, 

 is composed of this material, and is of course very cheap. The only 

 difference in the price would therefore arise from the cost of sifting, 

 which could be more than balanced by the freight saved in leaving the 

 refuse. 



Influenced by these considerations, the author, on the requisition of 

 the Governor General was authorized by the Secretary of War to re- 

 pair to Rome and superintend in person, the details of the operation. 

 He there fixed up a number of strong bolting cloths pierced with small 

 rectangular holes. The price of sifting one cubic yard of puzzolana 

 with labour hired of the pontifical government, was about twenty cents. 



The contractor who has leased from the Roman government, the 

 monopoly in the puzzolana trade, regarding the project as impracti- 

 cable, asked an exorbitant price for taking charge of it, but as soon as 

 he discovered it was both easy and cheap, he came forward and offered 

 for the future to send none but the sifted puzzolana to Algiers. It was 

 delivered there in 1S37, in this state, for forty-two francs the cubic 

 yard, and could, without doubt, be delivered for forty francs. By 

 adding one half sand, quite as good a commodity as the rough puzzo- 

 lana is produced, and you get for twenty francs what formerly cost 

 thirty-nine. At this price this material is likely to supersede all the 

 hydraulic lime and artificial cements made at the different localities. 

 It is easier worked, and the qualify is superior, or at least equal. 



Algiers is not the only place where this measure can be adopted ad- 

 vantageously; it can be practised with advantage on the whole Medi- 

 terranean coast and wherever the puzzolana of Italy is used. The 

 engineers of Toulon and Jlarseilles have already made arrangements 

 for the importation of the sifted puzzolana, and there is little doubt 

 but that it will become an extensive article of traffic. 



The mortar is made with one part lime in paste, and two parts 

 puzzolana. If the puzzolana is in the rough state the mortar becomes 

 hard in four days and resists the Vicat rod ; if it is sifted through the 

 bolting cloths it will become hard in two days, and if the puzzolana is 

 sifted through a fine hair sieve, it will become hard iu twenty-four 

 hours. 



It takes six days for mortar to become hard, which is made of one 

 part lime, one of bolted puzzolana, and one of sand. 



Beton is composed of one part mortar and two of stones broken to 

 the size of from one to two inches,* making two parts of beton. 



A cubic metre (3o-317 cubic feet) of beton weighs 568(j pounds. It 

 acquires in twenty-four hours, sufficient cohesion to withstand the 

 shock of a heavy sea without disintegration. In November 1S3D, a 

 block containing two hiuidred and fifty cubic yards, which had been 

 immersed only thirty-six hours was stripped of its enclosure, and re- 

 sisted the action of one of the most violent storms. M. Feueon, a 

 mining engineer, then at Algiers, was an eye witness of this remark- 

 able fact. 



The blocks made in the cases with cloth bottoms, measure generally 

 from one hundred and eighty to two hundred and fifty cubic yards ; 

 those made on shore, from fifteen to sixty cubic yards. When con- 

 structing the mole at Algiers, they placed first a set of the large b'.ocks, 

 and then, in advance of them, "to protect their bases, they placed a 

 number of the second size. The large cases serve as a platform from 

 which to launch the small blocks. The two lines of blocks are bound 

 together at intervals by large blocks of beton, and these intervals are 

 filled by stone measuring from five to eight cubic yards. 



* Whether cubic or superficial measure, is not stateJ, Tr. 



