216 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[JULT, 



cootraclor, slates, would cost twice as much as concrete, and which Mr. 

 Smirke says would be more expensive than stone, should be used. 



With respect to the adoption of bloclis of concrete ;— far " from standing 

 remarkably well in the breakwater at Algiers," the whole mass of the 

 breakwater has settled bodily, not from 'he effects of gales of wind, but 

 from defects in the material (which time will further show); this, there is 

 no doubt, is occasioned by the chemical action of the sea, " which in the 

 Mediterranean contains 702 per cent, of sulphate of magnesia, whereas the 

 water in the ocean contains only 229 per cent., consequently, of two moles 

 made of the same concrete, the one in the oce.in may last an indefinite 

 period, the other will dissolve in a few years; and even mixing puz/.olano 

 with the concrete will not guarantee the lime from solution " Nor can 

 that work, under any circumstances, be cited as an example for our imita- 

 tion on the coasts of (ireat Britain. There are no tides in the Mediter- 

 ranean, and the climate there is well suited to the drying and consolidation 

 of that material, which is not the case in mure northern regions. 



It may be added, that the form of the work at Algiers is not that of an 

 upright wall, for its face has a slope of 15 degrees ; the work, therefore, 

 cannot be adduced as an example in favour of tliat form of construction. 



In support of my dissent from the adoption of the upright wall, I appeal 

 to the debate which took place at the Institution ofCivil Engineers, in April 

 1842," on Colonel Jones's " Observations upon the Sections of Hreak- 

 waters as heretofore constructed, wiih Suggestions as to some modifications 

 of their Forms." This debate may be taken as a very fair exposiiion of 

 the opinions of practical engineers on the principle of the upright wall. 



The president, Mr. Walker, look an important part in that discussion. 

 He said, " It is evident that if the materials are deposited at an inclination, 

 any portion being displaced, is only carried down elsewhere. Although 

 strictly speaking it may not be wanted, it must nevertheless assist in con- 

 solidating the mass, and the vacant spaces can easily be filled up. Under 

 similar circumstances (to those which displaced some of the stones in Ply- 

 mouth Breakwater) a perpendicular wall would suffer more severely, and 

 probably would have fallen entirely. He th^l•efore considered that in 

 situations like that of the Plymouth Breakwater, which was exposed to 

 a heavier sea than Cherbourg, a long slope for the sea face was essential." 

 Mr. Palmer, vice-president, observed that the form suggested by Colonel 

 Jones for the faces of breakwaters, did not appear sufticieutly justified by 

 observed facts; that the idea was entirely of a speculative character, and 

 was contraiy to the laws of nature, which should be the engineer's chief 

 guide; and he attributed the failure alluded to by Colonel Jones, in the 

 harbours of Aidglass, Portrush, &c., more to defects in workmanship, than 

 to faults in the principle of the structure. 



General Pasley said he conceived that a perpendicular wall, constructed 

 of large ashlar work, well cemenied, would assume the character of a rock, 

 and all the prejudicial action of the receding wave would be avoided. 



Mr. Bull dillered entirely from Colonel Jones's opinion as to breakwaters 

 with vertical or nearly vertical faces, because any disturbance of the foot- 

 ing, however slight, must have a tendency to overthrow the wall. 



Mr. George Kennie deprecated in strong terms the upright wall, and 

 stated that the late Mr. Thomas Telford had abandoned that mode of cou- 

 slruclion. 



Mr. Vignoles only agreed to a certain extent, to the form proposed by 

 Colonel Jones, and recommended a combination of a slope below with a 

 vertical parapet above. 



Mr. Gordon was in favour of the slope ; and stated that a sloping break- 

 water, composed of ))ierre perdue, with a sloping face, had withstood undis- 

 turbed the surf at Madras. 



Mr. M'Neill adduced the long slopes of sand, at an inclination of 10 lol, 

 thatched with straw, v%hich resist the waves of the ocean ou the coast of 

 Holland. 



Thus we have in this discussion, a majority of speakers of seven to two, 

 in favour of the slope ; and of the raiuority, one was for a combination of 

 the slope with a vertical wall above. Even Colonel Jones suggested this 

 modification. See page 125, vol. 2, Proceedings of Institution of Civil 

 Engineers. Plan No. 5, proposed by him for Dover Bay, was of this 

 description. 



I object to Kilrush Pier being adduced as a test of the principle of the 

 upright wall sufficient to warrant its adoption in the construction of a har- 

 bour of Kefuge in Dover Bay. Kilrush is a small tidal harbour on the 

 coast of Ireland for coasting vessels. The piers are built in only 9 ft. 6 in. 

 depth of water, at low tide. The foundations were laid without difficulty 

 by the diving-bell, with large masses of stone, which were easily and 

 quickly deposited. The area of the section of the wall is considerably 

 greater than that of the old work ; but so far from the upright wall huving 

 been built in consequence of the sloping profile as originally proposed 

 having failed. Col. Jones expressly says ■' that the old work stood remark- 

 ably well." There is nothing in this, therefore, either practically con- 

 demnatory of the slope, or sufficient to warrant the adoption of the upright 

 face, on such a scale as to which these proceedings relate. There is no 

 doubt, as Mr. Palmer says, the piers of the small harbours which Captain 

 Washington reports to have been so much damaged by the sea, were con- 

 structed in a very defective manner, and with materials of dimensions that 

 ought not to have been put in, and it likewise appears that the damage 

 which tliese piers may have sustained might easily be repaired ; but cer- 

 tainly no such errors would be committed in any new work of this descrip- 

 tion, far less in that great national work now under consideration. 



ProceediiiBS of the Inslitulioii of Civil Engineers. See Journal, vol. 6, 1842, p. 31a. 



The recommendation of a majority of the Commission in favour of the 

 upright wall is staled, in the Report, to have been made on a summary, 



1. Of the conflicting opinions entertained by the eight engineers whose 

 plans fjr constructing a harbour of refuge in Dover Bay were submitted 

 to the Commission ; and 



2. With reference to the opinions of those persons who had been re- 

 quested tj give their evidence or advice upon this important question. 



The following is a list of the engineers whose plans for constructing a 

 harbour of refuge in Dover Bay were sent in to the Commission : 



1 . James Walker ; 2. George Rennie ; 3. Captain Denison ; 4. Sir John 

 Rennie ; 5. Lt.-Col. Jones ; 6. W. Cubitt ; 7. Charles Vignoles ; 8. J. M. 

 Rendcl. 



1. Mr. M'alker, civil engineer, is somewhat inconsistently adduced in 

 the Report as an advocate, in principle, for the construction of a nearly 

 upright wall. The project submitted by Mr. U'alker to the Commission, 

 is to build these walls in immense vessels, or as he calls them, " utensils" 

 (caissons), three or four hundred feet long and seventy feet wide, containing 

 two or three thousand tons of ready-made breakwaters, to be towed by 

 steam tugs and stranded in Dover Bay I But we have Air. Walker's 

 authority, from what he said at the meeiiug of the Institution of Civil En- 

 gineers, on April 12, 1842, that his reason for proposing nearly upright 

 walls in this case, was to avoid the extravagant width which must be given 

 to these huge utensils, if the walls have any considerable slope ; for, at the 

 discussion to which I refer, Mr. Walker stated, that in situations exposed, 

 like that of the Plymouth Breakwater, to a heavier sea than that which 

 rolls into Cherbourg Bay, a long slope for the sea face was essential ; and 

 that had a perpendicular wall been constructed in Plymouth Sound, instead 

 of a sloping breakwater, it would, in the storms which assailed it, have 

 suffered more severely than it did, and probably would have been entirely 

 overthrown. 



The dangerous instability of works executed in deep water, by a system 

 of caissoniug, such as that proposed by Mr. Walker, is very generally 

 acknowledged, and is sufficiently proved by the perilous state in which 

 Westminster Bridge now remains, notwithstanding the costly expedients 

 by which it has been attempted to remedy the defects of its original con- 

 struction. These expedients consist in forming a cofferdam about each 

 pier, pumping out the water, and then driving rows of sheet piling into the 

 blue clay, so as to form a girdle round the base of the original caisson, and 

 thus to prevent the materials of the natural bed of the river from being 

 underwashed by the current, or squeezed out by the weight of tiie bridge, 

 into the gradually deepening water-courses. But it does .seem very strange, 

 that these expedients having failed to arrest the subsidence v\hich is still 

 taking place in Westminster Bridge, the method employed in the construc- 

 tion of that work should be proposed for adoption, on an immense scale, in 

 the formation of a harbour of refuge in Dover Bay. 



2. Mr. George Rennie deprecates the upright wall as impracticable and 

 dangerous, and strongly recommends a sloping breakwater, as at Ply- 

 mouth. 



3. Captain Denison is for a vertical wall formed of hexagonal prisms of 

 concrete (proposed by Monsieur Emy, in 1831, but never adopted;, 10 feet 

 long, and about 23 tons weight, to be manufactured at Dungeuess, and 

 dragged by steam tugs to Dover Bay, by being suspended to rafis formed 

 of two cylindrical pontoons, and there sunk by mechanical means. The 

 wall to be upright from the bottom to about low-water mark, with a super- 

 structure of granite. 



4. Sir John Rennie, after deprecating in strong terms all systems of 

 caissoning, and some other expedients, particularly the adoption of up- 

 right walls ; and after urging the disastrous consequences that may attend 

 any mode of construction which is not recognised as certain of success, 

 priiposes the adoption of the principle observed in the breakwater at Ply- 

 mouth. This he considers as having completely succeeded, and therefore 

 he conceives that it fully justifies the adoption of the like mode of con- 

 struction for the proposed harbour of refuge in Dover Bay. 



5. Colonel Joues is in favour of a combination, of a sloping breakwater, 

 up to low-water mark, with an upright wall of stone erected on it. 



C. Mr. Cubitt, after having been a little taken with the theory of the 

 upright wall, and having since bestowed upon this subject the most careful 

 consideration, comes to the conclusion that any attempt to erect an upright 

 wall in Dover Bay would be an undertaking of great difficulty, and that 

 the only safe and practicable mode of execution is by depositing masses of 

 stone, to form a sloping breakwater, as at Plymouth, with stone brought 

 from the Channel Islands, or I'rcnn Portland. 



7. Mr. Viguole's plan is to form a sloping breakwater, by depositing 

 cubical blocks of concrete up to about low-water mark, and upon this to 

 erect a vertical wall. 



8. Mr. Rendel is next adduced as an advocate for the upright wall. 

 Now, with great respect for the practical opiuiou of this eminent engineer, 

 it is of importance to review in detail his several examinations before 

 the Harbour of Refuge Commission, previous to his conversion to, or 

 adoption of, the new theory, and to advert to the circumstances with 

 respect to material, which induce him uovv to recommend a wall of that 

 form. 



In his examination of the 19th of June 1844, Mr. Rendel told us, that 

 to construct a breakwater in seven fathoms water is a very formidable un- 

 dertaking, especially if caissons or other machines should be resorted to; 

 and that he doubled very much whether if a breakwater is to be con- 

 structed in seven fathoms water, tlie only safe plan would not be, to depo- 

 sit stones in the usual way from vessels ; br.ngiug up the mass to within, 



