1 843.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL 



.-wo 



the ebb tide, would be directed by the jetties towards the north wall, 

 and thus materially tend to deepen the river. It would also be 

 attended with the advantage of keeping up for a considerable time a 

 depth of water in the channel, which could not otherwise be main- 

 tained. The effect upon the channel of such an operation constantly 

 going on would speedily be felt in the improvement of the river. It 

 was also proposed to make a wet dock of the old channel for a space 

 of 600 or 700 yards, in the direction of DH, capable of affording ac- 

 commodation for ltlO sail of vessels. This could have been done at a 

 comparatively trifling expense. The space of ground, H, before 

 referred to, between the new and old channels, would be peculiarly 

 suitable for ballast quavs, affording opportunities for supplying ballast 

 in the wet docks, or to vessels lying in the channel. Having now 

 taken a short but accurate review of what has been proposed as im- 

 provements for the navigation of this river, it may be well, before 

 bringing these remarks to a close, to notice the effect of the dredging 

 operations constantly going on. We find, at the time Mr. Nimmo 

 made his report, that vessels drawing more than ten or eleven feet 

 water were not able to go up to the quays of the city during spring 

 tides, and with neap tides not higher than the King's Quay. From 

 constant dredging the bed of the channel has been very much deepened, 

 for vessels drawing eighteen feet water are now enabled to come up 

 to the quay during ordinary springs in one tide, which they were 

 before unable to do in consequence of being obliged to wait below the 

 Flats (a shallow some distance from the Meelagh Bank), until nearly 

 the top of high water, the consequence of which was that, by the time 

 they arrived near the King's Quay, the tide was too low for them to 

 proceed further, and were consequently obliged to wait until next tide, 

 a circumstance in many cases attended with no small inconvenience. 

 A channel having been now cut through the Flats, this inconvenience 

 has, in a great measure, been remedied. These have always been con- 

 sidered as the most formidable obstacle to the improvement of the 

 navigation. The channel, since it has been cut, has at this place 

 undergone very little change, which is so far encouraging. The 

 dredge-boats are at present employed in cutting off an angle of the 

 Meelagh Bank which projects much into the channel : when this is 

 accomplished, the greater part of the impediments to the navigation 

 below Black-rock Castle may be said to be removed. The straight- 

 ening of the channel as far as practicable has been at all times a desi- 

 deratum. The most obstinate shoal in the river above Black-rock 

 Castle is in a part of the channel a little below the end of the embank- 

 ment, H E, before referred to : this it has been found necessary to 

 dredge away several limes since the building of the wall; and it is 

 probable, from its so quickly accumulating, that it would before long 

 become a field of corn, were it nut for dredging, which will, as long as 

 the wall remains, be continually necessary, as the cause must be 

 removed before the effect can cease. 



The slab behind the embankment, DHE, is, as may be expected, 

 constantly filling up with mud, which is occasionally again dislodged 

 after heavy rains, and deposited in the channel. It will be evident, 

 from what has been already said, that dredging can never be entirely 

 dispensed with in this river under existing circumstances. 



The commissioners do not appear to exert themselves in this 

 matter as much as they ought, probably owing to the fact, that a por- 

 tion of the material raised is very suitable for ballast, which, being in 

 great demand, pays a great portion of the expenses incurred in raising 

 it. From the returns made bv the Ballast Office it appears that from 

 17,000 to 18,000 tons of this river clearance is annually supplied to 

 vessels leaving the port, which is paid for at the rate of a shilling per 

 ton, leaving a clear profit, after paying dredging, lighterage, and other 

 expenses, of about sixpence per ton. 



The quantity of material which the boats on this river are capable 

 of raising is very variable, being entirely dependent on the nature and 

 depth of the cutting, it being possible to raise a much greater quan- 

 tity of gravel than clay in the same time. The largest boat, which is 

 about lti-horse power, has raised as much as 60 tons of mud and 

 gravel in 26 minutes; this, however, must be. considered as a maxi- 

 mum, and worked under the most favourable circumstances. The 

 average expense of dredging may be estimated at from %£d. to 3d. 

 per ton. 



Hoping these remarks have not been extended to too great a 

 length, we must now bring them to a conclusion: before doing so, 

 however, we cannot help remarking that, had the various sums ex- 

 pended from time to time (in building embankments and other useless 

 works) been judiciously spent under competent advice, the necessity 

 of dredging would most probably be altogether done away with after 

 the channel had been once formed. On few rivers has so much been 

 done by nature, and so little, by art. 



SUGGESTIONS FOR THE MORE EXTENSIVE EMPLOYMENT 



OF CONCRETE. 



( Concluded from page 158.,) 



In a former paper on this subject will be found the particulars of a 

 dwelling house constructed entirely of concrete. As this is only an 

 example out of a great many others in which this substance has been 

 employed in France, for general building purposes, it cannot fail to 

 excite astonishment that it has never yet been tried in this country 

 for building cottages and dwelling houses of the humbler class. The 

 price of concrete is less than one-third that of either brick or stone : 

 so that the proprietor of the soil or the speculating builder would 

 effect a great economy by introducing it. It is deplorable to see in 

 many parts of this rich and highly favoured country — to say nothing 

 of the unhappy sister kingdom — -those miserable mud hovels in which 

 the honest labourer is compelled to rear his family. And yet there is 

 no hope of amending such a condition of things unless some material 

 be substituted, which possesses the solidity and durability of brick or 

 stone, without at the same time being nearly so expensive as either of 

 these. Such a material is concrete with respect to the two former 

 qualifications, while in point of expense it can probably be built at 

 least as cheap as a good wall of mud. The construction of mud 

 walls, in fact, although common in many of the northern counties, in 

 Scotland and in Ireland, has never been practised amongst us with the 

 same degree of skill, nor with the same success as in France, in 

 southern Russia, and in many parts of Asia. In those countries the 

 method of building en phi', as it is called by the French, has been 

 carried to considerable perfection, by skill in the tempering and pud- 

 dling of the clay, and by mixing with it various ingredients, such as 

 chopped straw, hair, &c, which increase its strength and cohesion. 

 It is important to observe, however, that wherever building en pise 

 has been successfully practised, there has been the advantage of a 

 nearly tropical sun to bake and indurate the newly formed walls. On 

 the other hand, in this country, the walls of mud, however well formed, 

 are subject to destructive atmospheric influence, before they can well 

 be hardened, and it is therefore no wonder that they are far inferior to 

 the pise walls of warmer climates. 



Concrete as a foundation for Roads. 



The first account we have met with of the use of concrete for this 

 purpose, occurs in Hughes' Treatise on Roads. ' It appears to have 

 been used by him on the Highgate archway road, under Mr. Telford's 

 direction, as long ago as the year 1828. Mr. Hughes' practical ex- 

 perience on this subject is extremely valuable, as he tried on the same 

 road another kind of concrete foundation, made of Roman cement and 

 gravel, and although the latter is of course far more expensive, he 

 decidedly gives the preference to lime concrete, owing to its greater 

 toughness and its consequent capacity of resisting fracture. His pro- 

 posal for the use of concrete in the roads round London is worth 

 quoting, and we entirely agree in the propriety of it: — "I should re- 

 commend for all the principal roads round London — after all the sup- 

 plies of water from the sides as well as that falling on the road have 

 been properly intercepted by longitudinal side drains and transverse 

 ones leading" into them, and occurring as often as the nature of the 

 subsoil may require — that a bed of lime concrete six inches in thick- 

 ness, be laid all over the breadth of the road, and that this bed be 

 afterwards covered with six inches of the best flint or pit gravel that 

 can be procured, in two courses of three inches at a time ; or with 

 what in my opinion would be a much more lasting and serviceable 

 material, four inches of broken granite stone: and I am convinced 

 that a road so constructed, however bad the under stratum may be, 

 will prove one of the hardest, most durable, and at the same time one 

 of the cheapest roads ever formed in the neighbourhood of London." 



Of late years concrete has been occasionally employed as a founda- 

 tion for the street pavements of London, and in some few instances, 

 road surveyors of more than ordinary intelligence, have introduced it 

 into their practice with very great success. All those who have tried 

 it for keeping down the subsoil of the London clay, have been highly 

 gratified with its success, wherever the proper precautions have been 

 taken to prevent traffic until the concrete has completely set. We 

 have heard of one instance where a concrete foundation was laid 

 down for a road near London, and a few inches of broken stone being 

 placed on it, carriages were immediately allowed to pass over it. 

 The wheels of these vehicles were actually in contact with the raw 

 unhardened concrete, and it is scarcely to be wondered at that it wore 

 into holes and became almost impassable. In this case the concrete 

 was pronounced a failure ; but it is scarcely necessary to observe how 

 unfair were such a trial and such a condemnation. We could mention 



1 The Practice of making eind repairing roads. London : Weale, 1838. 



50* 



