4S 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[February, 



reservoir, wliicli was formed by an escarpment of clay, skirting the river 

 Brent ; part of the waste water, together witli the percolation from the re- 

 servoir, was permitted to traverse the site of the embankment, rendering the 

 ground marshy, even in the driest seasons. The late Mr. M'Intosh had 

 frequently told him, that a larger quantity of material was used in maintain- 

 ing, than in constructing the Hanwell embankment. 



Wr. Coi-THDRST, in answer to questions from members, explained, that the 

 fissures shown in the clay, beneath the embankment, were assumed from the 

 form of the depressions of the surface. The sections of the ground were 

 taken weekly, during the whole time of the susidence, so that he contended, 

 the form of the substratum might be assumed as being correct. The spread- 

 ing of the lower side of the embankment, displaced the bank of the river 

 lirtnt for some distance. 



Sir IIknby Delabeche remarked, that if the sections (Figs. 8 and 9), 

 which were exhibited, approximated to truth, it would appear, that the em- 

 bankment was formed upon a fault of greater magnitude than usual. The 

 consequences were inevitable; when the fault yielded, the embankment sunk, 

 and continued to subside, until the mass was stopped by weighting the foot, 

 and thus restoring the equilibrium. 



Mr. CoLTiiuRST said, that th.e slimy beds, and the fissures, which ran in 

 all directions in the clay, were most difficult to be guarded against, and they 

 were, he believed, the principal causes of slips and subsidences. 



Mr. Rraithwaite said, that from the observations of Sir Henry Dela- 

 beche, it might be inferred, that slips and other movements of earth, were 

 more frequently due to mechanical, than to chemical action, although in the 

 case of the New Cross slip, the latter cause had been much insisted upon. 



Mr. John Braithwaite gladly availed himself of the geological know 

 ledge of Sir Henry Delabeche, and his approbation of the measures pursued 

 was highly gratifying to him. 



With respect to the Brentwood cutting, although the strata were nearly 

 horizontal, and it might have been imagined, that there would be little 

 tendency to slip, yet from the ground being so full of water, more than ordi- 

 nary attention to its drainage was required, fur it was so retentive of mois- 

 ture, that a drain had but little influence at a few yards from it. 



The draining shafts which were sunk, had operated well, to the extent to 

 which they were carried, and he believed that generally, the mode of treat- 

 ing the Brentwood cutting was considered successful. 



He bad understood, that the trenches which bad been alluded to, had not 

 been extensively used. 



Mr. Phii'ps explained, that the trenches and the wall with dry backing, 

 were tried under his direction merely as an experiment, prior to the examina- 

 tion of the ground by Sir Henry Delabeche. The dry shafts were subse- 

 quently sunk, and the only doubt he entertained was, whether there was a 

 sufficient number of them to drain the bank effectually. 



Sir Henry Delabeche said, there could not be any doubt of the ground 

 being completely drained, if a sufficient number of shafts were sunk to inter- 

 cept the water, but then the question of their cost must be considered. 



Mr. J. Green, in answer to questions from the President, stated, that his 

 experiencs did not enable him to lay down any rule for the prevention of 

 slips in cuttings or embankments. They were generally to be attributed to 

 tlie presence and pressure of water, acting upon the substratum; the method 

 of discharging the water must depend on the direction and the nature of the 

 strata; in all ordinary cases, he conceived, that with proper application of 

 the known methods of drainage, successful results might be attained. — He 

 had not made any particular observations, as to the relative duration of the 

 tendency to slip, exhibited by the slopes of embankments and cuttings, in 

 canals and railways ; but he conceived, that in a canal, the weight of the 

 water acted as a support to the internal slopes, and tended also to counteract 

 the upward pressure of water in the substrata. He had frequently observed 

 this in cuttings, with embankments on the sides ; while the canal was full of 

 water, the banks stood well, but when the water was drawn off, the banks 

 subsided, and the bottom of the canal rose up. — A curious instance occurred, 

 in forming part of the Exeter sliip-canal, through mud lands in the estuary 

 of the Exe. The embankments on the sides of the cutting, remained firm, 

 so long as their weight only just balanced tlie upward tendency of the water, 

 in the substratum of the bed of the canal ; but when the increased weight 

 of the mass, destroyed the equilibrium, the embankments sunk down, and 

 the bottom of the canal was forced up in proportion. — This occurred in seve- 

 ral places, even after the works had preserved a perfect section for some 

 months, but the canal had not then been filled with water. It was found on 

 examination, that at a few feet only below the bottom of the canal, there 

 existed a bed of peat, which, although capable of resisting the weight of the 

 banks for a considerable time, at length gave way ; thus the embankments 

 sunk down, the bottom of the canal rose up, and it became necessarv to 

 drive strong piles in the line of the bottom of the canal, on each side, in a 

 lateral direction, and to support these piles by rough inverted arches of 

 stone, at intervals of about 20 feet, for a considerable distance, after which 

 the banks being slowly raised, stood well. 



The President said, it must have been observed by all engineers, that 

 in the embankments and cuttings of canals, the slips generally occurred, 

 within the first 6 or 8 months after the works were completed ; but in rail- 

 way works, the slips constantly occurred even after years had elapsed. He 



observed on many of the railways, upon which he travelled habitually, that 

 the slopes were almost as frequently under repair, after being open for many 

 years, as they were within a few months of the first opening. He was de- 

 cidedly of opinion, that although water might be the primary cause of the 

 slips, the vibration caused by the passage of the trains, was the more imme- 

 diate cause. 



When, as had been so ably explained, the lower beds became converted 

 into mud, and the adhesion of the particles was destroyed, the mass only re- 

 quired a slight impulsive force, such as the vibration consequent on the pas- 

 sage of an unusually fast or a very heavy train, to set it all in motion and to 

 cause a slip. 



Some of the methods proposed for the formation of embankments, such 

 as only constructing them during suitable weather, and with thin layers of 

 material, regularly laid and pounded, &c., might be used in the construction 

 of reservoirs for retaining water ; but they were not compatible with the 

 manner in which extensive works required to be carried on, independent of 

 the extra cost they would occasion. Experience bad shown him, that the 

 best method of constructing a heavy embankment was, to run forward two 

 tips, parallel with each other, forming the outsides of the bank, and leaving 

 a void in the centre, which was subsequently filled up. The greatest amount 

 of pressure, was thus brought to act vertically upon the material, and the 

 two sides having become somewhat consolidated, were better able to resist 

 the pressure, and they had not any tendency to slip away. This method 

 bad been ably treated by Mr. J. B. Hartley, in a paper read before the In- 

 stitution in 1841. 



He had not found any difficulty in inducing contractors to adopt that 

 method ; when proper precautions were taken to insure thorough drainage, 

 he believed, that embankments would generally stand well, although made 

 in the wettest weatht-r. Moisture would only cause the mass to become 

 more consolidated, and when once that was the case, but little water would 

 subsequently percolate. 



He concurred in the opinion, that the gravel counterforts acted rather as 

 drains, than as supporting buttresses ; for he believed, that they stood gene- 

 rally at a steeper angle, than the slopes which they v,-ere supposed to sup- 

 port. 



Jlr. Clutteubuck said, in confirmation of the President's opinion, he had 

 been told by the persons who worked in the sand-pits, under the plastic 

 clay, near the London and Birmingham Railway, that they were afraid to 

 remain under ground, during the passage of the heavy luggage trains, on ac- 

 count of the extreme vibration of the earth. 



Mr. Green was convinced of the correctness of the President's opinion, 

 as to the effect of vibration upon banks saturated with water. He had seen 

 instances even in canal embankments, where, at the head of locks, the vibra- 

 tion arising from the sudden and careless closing of the lock-gates had pro- 

 duced slips. 



He did not think any commensurate benefit would result, from the extra 

 expense of pounding the earth in embankments, as had been suggested. The 

 degree to which earth might be safely consolidated by pounding, could only 

 be determined by great attention to the nature of the material, and to the 

 circumstances under which it was used. He had known much injury caused 

 by the earth-backing for walls, being too much pounded, when, from defec- 

 tive drainage, the expansion of the earth had subsequently thrown the walls 

 down. 



Mr. Hughes presented a specimen of Watson's drain pipes, described in 

 the Journal, Vol. VII., page 49. 



RAISING SHIPS. 

 " .Jn Account of the Plan employed for raising the ' Innisfail' Steamer, sjink 



in the river Lee, near Cork." By Georoe Preston White, Assoc. Inst. 



C. E. (From the Minutes of the Proceedinys of the Institution of Civil 



Enrjiueers.) 



The " Innisfail," a steamer of 400 tons burthen, and 180 h, p., sunk iu the 

 river Lee, in consequence of having run foul of an anchor, with such force 

 as to tear a plank of C4 feet in length, and varying from 8 inches to 10 inches 

 in breadth, out of the bottom close to the keel. As the vessel lay right 

 athwart a narrow part of the channel, it was necessary to take immediate 

 steps for its removal. The Directors of the St. George Steam Packet Com- 

 pany being aware that Mr. William Preston White, the Harbour Master of 

 Cork, had succeeded in raising several large vessels, solicited his assistance 

 to remove their steamer. The method he had adopted on former occasions, 

 was that of slinging or weighing, which is done in the following manner: — 

 After the position of the sunken ship has been ascertained, a chain cable is 

 passed round it by means of two vessels, which are placed near the bow, with 

 the cable suspended between them, so that its centre shall sweep the ground, 

 and it is moved to and fro until it comes in contact with the stem of the 

 su]d<en ship. The two vessels are then moved astern, the ends of the chain 

 are brought together, and passed through an elliptical ring, which is loaded, 

 in order that it may fall close to the stern, and the cuds are secured to the 

 vessels. The main chain being secured in its position, bridle chains are 

 affixed to it at intermediate distances ; these being attached to other vessels 

 alongside, all the chains are strained at low water and as the tide flows, the 

 sunken vessel is raised from its bed, and is brought to shore. Jf the vessel 

 cannot be left high and dry at low water, the operation is repeated as often 

 as circuiustaaces may require. 



