2(Jt 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL, 



[JULV, 



4-percIi drains next, and after these tlio lO-percli drains; the eross 

 drains in all cases were kept to tlie depth to which the parallel 

 drains had heen snnk. 



After some time, when the spoil of the sinkings of the centre 

 ilrain and of the verge drains accumulated to an inconvenient 

 height, and that the ramparts F G, hecame consididated, which 

 the pressure of tlie spoil, aided hy drainage, speedily effected, one 

 of tluise ranipaits, say F, was excavated and wlieelcd out beyond 

 the lO-perch longitudinal drain north, C; after which the centre 

 drain and verge drain were boldly snnk, and the spoil cast in upon 

 the excavated rampart. The i)arallel drains were then sunk; con- 

 currently with these the cross drains were also deepened; subse- 

 ipiently to these ojierations the ram))art G, was excavated and 

 wheeled out beyond the 10-percli drain, south side, and the cross 

 and parallel drains consecutively sunk, as on the north side. 



In this state of proceedings it hecame necessary to excavate and 

 wheel off the spoil of the cross drains, situate between the verge 

 drains and the 10-perch drains to a short distance beyond the 

 10-perch drains on each side, in order to admit of their being 

 widened and deepened boldly; the top openings increased to 6 feet, 

 and the depth to that of tjie parallel drains, the spoil of which 

 was cast back from the verges in order also to their being widened 

 and sunk, but in a less degree than those already mentioned. The 

 ])roxiniity of the river Suck, running nearly parallel with the canal 

 for its whole length, afforded opi)ortunities of floating off immense 

 <|uantitics of spoil in time of flood, which considerably accelerated 

 tlie iirogress of the work. 



Tlie effect of this system of drainage was to cause the whole 

 mass of bog, comjireliended between the 10-perch drain nortli and 

 the 10-perch drain south of the centre, to subside, subsidence com- 

 mencing at the centre and verge drains wliidi were unceasingly 

 operated upon daily, and decreasing gradually to the 10-perch 

 drains, which, with the intermediate drains, were less frequently 

 sunk. This rotation being performed, ramparts were prepared in 

 the manner before described, and when consolidated were exca- 

 vated and wheeled off to the prescribed distance from the centre 

 on each side, and the process of drainage between the verge drains 

 and the 10-perch drains proceeded with as before, all these opera- 

 tions being constantly repeated until the required depth was at- 

 tained. It was then found that the surface of the bog, originally 

 26 feet above bottom at the verges of the canal, had subsided on 

 an averge 20 feet, it having in many places sunk to within 4 feet, 

 and in some few places, where the bog was very soft, to the actual 

 level of bottom. 



The slopes were then ranged and di-essed — the centre drain 

 straightened and sunk throughout below the level of the bottom 

 of the canal, which was thus ke|it dry and firm to resist the col- 

 lapse of the sides — the bottom cleared up, the cross drains E and 

 D, from the canal to the t-perch drains on either side, flUed up to 

 the level of the bottom of these 4-perch drains H B, and when the 

 former were, at their entrance into tlie canal, below the top water 

 line, which occurred in many places, shoots of sufficient capacity 

 were inserted for the full breadth of the trackw ays, to ])reveiit any 

 accumulation of drainage or surface water in the parallel or cross 

 drains, which would, if this precaution were omitted, or the pas- 

 sage of the water through those shoots impeded, cause great de- 

 rangement by carrying in the sides, ])roducing the subsidence of 

 the trackways and forcing up the bottom, a common occurrence 

 from the cause here assigned, evidences of which are \isible in the 

 •work referred to. The sides or hanks w ere then slowly and care- 

 fully raised with the lightest and dryest of the spoil to the re- 

 quired height. In soft places, where the surface of the bog had 

 sunk to the level of the bottom of the canal, fascines of heath 

 were laid under the light dry spoil of which the bank was formed; 

 the tap-drains were then closed, and the water, for the purposes of 

 navigation, rapidly let in — a [irecautionary measure of much im- 

 portance, in order to [irevent the rising of the bottom and collapse 

 of the sides, which would have inevitably taken place if it had 

 been slowly admitted. A, soling of clay 10 feet broad and 1 foot 

 thick, with 6 inches of gravel, was then laid on to form the tow- 

 ing-path. 



\Ve may here observe, that throughout the whole of the execu- 

 tion of this reach of canal, 3,100 yards in length, none of the 

 numerous drains with which it was traversed attained a greater 

 depth, at any time, than about 4 feet, except at the margins of the 

 bog, terminating in the ravines at either ends; here the drains 

 were sunk to 6 feet and upwards; the work was three years in 

 hand, and had two years more been employed in the execution, 

 two-fifths of the cost would have been saved. 



It may reasonably be asked, vvhy a top opening of 54 feet was 

 assumed for the canal in 20 feet cutting, requiring, when complete, 



a bottom breadth of 24 feet, and slopes Ij to 1, which, hy these 

 data — viz., 20 X 3 4-24, would give a top ojiening of H 4 feet; and 

 why an extent of 10 perches on each side of the centre line of the 

 canal, should be adopted as the limit within which the drainage 

 was effected? To these queries we reply, that we had ascertained 

 previously, in the performance of similar undertakings, that sub- 

 sidence would result from drainage in jnoportion to the depth 

 and fluidity of tlie hog; and that if the tcqi opening were pre- 

 vented from narrowing by the closing in of the sides, the breadth 

 at top would be transferred to whatever point the subsidence of 

 the surface would rest at. We also had previously ascertained in 

 the instances which came under our observation, that the near de- 

 posit of the spoil to the opening, pushed in the sides by the in 

 cumbent weight, contracted the iqiening, closed tlie centre and 

 verge drains, forced u]) the bottom, and thus deranged the whole 

 system of drainage; hence it was, that distances of 10 perches 

 fi-om the centre line, on cither side, were fixed as the nearest 

 places of deposit of the spoil, as well as the least breadth to which 

 the drainage operations should extend; and the system adopted 

 fully answered our expectations. ^V'llere, for exam))le, the surface 

 had sunk to within 10 feet of the bottom of the canal; the height 

 at which the banks were formed (being 4 feet above water line, to 

 allow for ulterior subsidence), the opening at top was transferred 

 to tliat level where it became the proper breadth with slopes of IJ 

 to 1, and where the subsidence was greater or less, which necessa- 

 rily occurred from the unequal depths and composition of the bog, 

 and that the slopes were too prominent or too flat, or that contrac- 

 tion had ensued frcmi unskilful or negligent working; these irre- 

 gularities, from which slopes in clay cuttings and embankments 

 are not exempt, were easily rectified, and the saving was of great 

 value, both with respect to time and cost of execution. Had the 

 top opening been made in accordance with the specification — 

 namely 84 feet in breadth, the cubical quantity contained in a 

 lineal jierch would amount to 840 cubic yards; whereas with .i 

 54 feet top ojiening for the same length, and a subsidence to within 

 10 feet of bottom, the cubical contents would be 607 cubic yards, 

 making a saving of 233 cubic yards in the sectional area, which 

 upon the length of the lot, (486 perches) amounts to 113,238 cubic 

 yards, beingmore than 30 per cent., independently of the vast 

 reduction by drainage. 



The extent to which the system of drainage was carried — 

 namely, 10 perches on each side of the centre, was for the purpose 

 of consolidating a space through which the necessary sinking for 

 the formation of the canal could be made, which it would have been 

 scarcely possible to accomplisli, without first having brought a suf- 

 ficient breadth of deej) flow hog into a dry firm state, capable of 

 affording facility in the execution and the requisite security to the 

 na\iaation. 



Canals may be made through shallow and dry hogs with great 

 facility and cheapness by the ordinary process applicable to other 

 soils; care being taken in dry bogs, which may not be shallow, 

 that the bottom level of the canal be not remote from the gravel 

 line beneath the bog, which a judicious distribution of lockage 

 would enable the engineer to fix at a proper level; otherwise the 

 canal w ill be costly in its maintenance, and defective in its naviga- 

 tion, from the tendency of the banks to subside and the bottom to 

 collapse; and if by accident the water should run oft', serious de- 

 rangement would inevitably be the conseiiuence by the falling in 

 of the trackways and the swelling up of the bottom. Moreover, 

 fixing the bottom level of the canal considerably above the gravel 

 line would be greatly opposed to the reclamation of the bog. 



We have thus endeavoured with the aid of the accompanying 

 diagrams, to give practical information on subjects presenting dif- 

 ficulties which an engineer in this countiy is frequently called on to 

 contend with; and as the principles laid down are the results of 

 actual experience, we feel assured of their correctness, and do not 

 hesitate to recommend their adoption. 



Dover Breahcafer — The formation of this hieakwater, uiuler the super- 

 visiiiii of Messrs. WhIUci- and Biirgcs, is proceodiiig: satisfactorily. It begins 

 near the Look-out iiouso, ami is tu run out lo sea, in cants of about 8U0 ft., 

 to forai a harbour. Aliout 200 feet is executid, and 100 feet will shortly be 

 added. The outer faces are of Liramleigh Fall stone, backed up with Port- 

 land stone. Tbe stones measure about bO cubic feet each. The space be- 

 tween the two walls is filled in solid witb concrete. Large blocks of hard 

 and dry concrete are prepared, reaiiy to use as tliey get out more to sea, and 

 two diving-bells are in ilaily use. There is a stcam.crane for landing the 

 stone from the ships, and tbe mills for grinding lime and brick-earth are also 

 worked by steam-power. .Messrs. Lee, uf Cbiswill-street, are the contrac- 

 tors. 



