1845.] 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



43 



1 maas moving; down an indineiJ plane, by its gravity, onH he proposed to 

 counteract that tendency by friction. This was efTectcd by dividing tlie 



A Counterfort. 



The (lotted Una representa the litcit of the slij 

 peured before the ereetlon of tile couoiei 



Section of the Sk.pe in llie Biliivortli Cutting, London nud Birninlghum Ituilwuy. 



slipping mass into vertical sections, by excavating perpendicular cbases, 5 

 feet wide, and passing completely through the slip, down into ihe solid clay 

 below; these chases were 15 feet apart, and were filled lip with nililde 

 masonry, or with chalk or gravel, well rainnied down, so as to form a solid 

 immoveable mass. Thus the slip was divided into a number of isolated por- 

 tions, of comparatively small dimensions, each side of whicli came in contact 

 with the siile of a counterfort, and the friction lietween the masses had, in 

 every case, proved sutiicient to retain the slip from further movement. 



This mode of repair was first adopted about 5 years ago. and had since 

 been extensively userl, in repairing the numerous slips, in the slopes of the 

 cuttings of the London and Birmingham Railway. 



Mr. Gregory stated, that similar counterforts, with a revetment of gravel 

 along the foot, (I'igs 3 and i, January numlier) liad been tried with good 

 effect, on the Croydon Railway. lie had understood, from military engi- 

 neers, that thin revetments with long deep counterforts, bore a heavy fire 

 better than thick revetments without them, lie was well aware, that slips 

 were frequently occasioned by catch-water drains on the face, or at. the top 

 of slopes; on that account, his attention was constantly directed, to uniu- 

 terruptecNy keeping up the surface drainage of the earth-works under his 

 charge. 



Gtological, Chemical, and Mechanical Action of IValtr. 



Mr. Taylor believed, that the mechanical action of water, produced many 

 of the effects which had been mentioned, hut the chemical action upon clays, 

 and even upon solid rocks, must not he overlooked. lie would instance, 

 particularly, the well-known action of the air upon shale, which although so 

 tough and hard under ground, as to require the agency of gunjiowder for its 

 excavation, became, after a few weeks' exposure to the air, tlioroughly de- 

 composed. — Decomposed granite, called by miners 'pot grawen,' was ex- 

 tremely troublesome in mines ; it consisted principally of feltspar and potash, 

 and was the China clay (Kaolin) so much used in jiotteries. This suljstance 

 would appear to have been formed, by the decomposing action of the air, or 

 of chenucally-formed oxygen. — Pyrites, which appeared to have abounded 

 in the strata of the New Cross cutting, not only had a natural tendency to 

 decomposition, when exposed to the action of air, hut also affected every 

 thing with which it was in contact. 



It had become fashionable to account for all changes, by attributing them 

 to the agency of electricity, and since the interesting researches of Mr. Fox, 

 of Falmouth, there was much reason for believing, that electricity was capa- 

 ble of producing these wonderful changes. It was easy to understand, that 

 as soon as chemical action began, electricity might lie generated ; its flow 

 would be conducted through the fissures and veins of mineral substances ; 

 decomposition of the existing material proceeded, and other forms were as- 

 sumed ; this action could not he continued, williout a corresponding altera- 

 tion of the bulk of the mass, and when it reposed on an inclined bed, of 

 which the surface was covered with a semifluid film, such as the London clay 

 was described to be reduced to, by the solvent efTcets of water, the slightest 

 expansion or contraction would suflice to set the whole superstratum in mo- 

 tion, and to produce the slips. 



Primary rocks were subject to the same eflTects, and in sinking through 

 porphyritic rocks, fissures were frei|uently found, filled with foreign matter, 

 which swelled and forced in the sides of the shafts, when such an event was 

 least expectcil ; such occurrences could not he guarded against, as the direc 

 tion of these fissures was usually parallel willi that of the shaft. 



Mr. Taylor agreed in the neceskily for the precautions which had been 

 mentioned, in cuttings and other railway wiirks, and that It was generally 

 only in such strata as day-slate, granite, or other primary rocks, that works 

 could be left without artdicial protection. He had driven, on the line of the 

 Tavistock canal, a tunnel of 1 J mile liuig, through clay-slute, and granite, 

 which stood perfectly without any internal arching. 



Mr. Smith said, that although he had hut little experience in the forma- 

 tion of cuttings or embankmenls, he had devoted much attention to surface 

 drainage. He had been surprised at the visible want of Ihe precantinns, 

 which he conceived neces?ary, Ir prevent the saturation of the slopes, and 

 their consequent degradation. The liaek drains, which were frequently 

 carried along the top of cuttings, were ohjeciionahle, anil were likely to caufe 

 slips. — Slips were also, piobahly, laiised by the alteri'.ale contraction and 

 expansion of the clay, under e.\|Misure to changes of weather. From experi- 

 ment it was asei'rtained, that clay occupied fth or ^th less >paee when dried, 

 than when in situ. It could be iinagined, that during the summer, the com- 

 bined effect of the sun and tht- wind, formed cracks on the surface ; the 

 cnimbliug of the edges of these fissures partially filled tlicin : the rain whieli 

 fell in the winter, or was biought by the catch-drains from the neighbouring 

 land, tended to restore the clay to its original bulk, hut the fissures being 

 prevented from closing, by the crumbled clay within them, which also 

 swelled from the wet, the whole mass expanded in the line of least resist- 

 ance, which was towards the cutting. This process being repeated, during 

 several succeeding winters, would at length cause a slip. The best method 

 of prevention would, be thought, be a greater attention to surface drainage 

 in the line of the slojie, so as to carry off the water very rapidly. This had 

 been attempted, by working the surface into jiarallel furrows and ridges, 

 from the top to the bottom of the slopes ; hut neither those, nor the covered 

 drains were alone elfcctual. The latter were not deep enough, they should 

 be 5 feet or G feet beneath the surface at the bottom, and 3 feet at the top 

 of the slope, and not more than Ifi feet apart, so as to he suflieiently close 

 together, to collect and to carry off all the water that was nut conducted 

 down the slopes, by the furrows on the surface. — The gravel countcrforia 

 formed driiins, and thus, he conceived, were more beneficial, that by increas- 

 ing the friction between the masses, which could have but little eft'ect, when 

 once the mass of material was thoroughly satinalcd. — If einhankments eould 

 be formed in very thin 'concave layers, equally spread and hiaten riown, in 

 dry weather, while the clay was in hard lumps, leaving interstices, which, for 

 a long lime, would permit any water falling upon it to traver,se freely, until 

 the whole mass was consolidated, there would he liut little subsidence. He 

 was aware that this plan was too expensive, but the nearer it could he ap- 

 proached, Willi due regard to economy, the better would he the effect; 

 whereas, by the present system of maldng embankments in all weathers, 

 when frequently the whole mass was so thoroughly saturated, that it could 

 never dry, nothing hut failures could he expected. 



Gener.il I'aslkv said, that be always supposed the gravel counterforts 

 were intended to act as drains, at the same time that they gave increased 

 friction, and broke the continuity of the mass of earth, limiting any sUp that 

 might occur, to the extent of space between two counterforts. 



Mr. IIawkshaw thought, that arbitrary Ihnits could not be assigned for 

 slopes in given strata ; the different conditions under which the same strata 

 appeared, in different localities, precluded any general law. Clay, which in 

 a wet situation required a slope of 3 to I, would in another position, stand 

 well at 2 to 1. In the Andes (South America), he had seen gianite in such 

 a decomposed state, that it would have been very unsafe to have left perpen- 

 dicular sides in a cutting through it. The drift formation of Lancashire 

 might also be instanced. Sand was found, on the line of the Manchester 

 and Bolton Railway, which stood well iu slopes of 30 feet high, at an in- 

 clination of 2 to 1. These slopes were kept perfectly dry by drains, running 

 at intervals from the top, down to the bottom of their faces. — In mining 

 operations, the expansion of cliiy was well understood. The floors of old 

 mines were always expected to swell up. In the tunnel on the Manchester 

 and Bolton Railway, the timbers were frequently broken by the expansion of 

 the clay, although it appeared quite dry. 



Mr. Soi'wiTH instanced the 'creep' in collieries, which had been attri- 

 buted to this expansive action, but be rather thought, that the complete 

 closing of old mines, was owing to the weight of the superincumbent rocks, 

 which acting upon the pillars and walls, forced up the floor. 'Ihe subsiding 

 of the surface, which was so frequent iu mining districts, corroborated this 

 view. 



Beneath the village of Wallsend, there was a tract of coal, which the late 

 Mr. Buddie hesitated to get. but at last he decided upon continuing the 

 working in that direction, and the whole village had subsided nearly 2 feet 

 vertically ; but by care in the workings, it had occurred without materially 

 damaging any of the buildings. 



Mr. FonsTUR said, although it was well known, that in mines which were 

 carried to a considerable depth, the 'creep' would occur, and the floor of 

 undisturbed clay appeared to rise, he believed it to be an erroneous idea, and 

 that in consequence of the partially supported weight of the strata above, 



0* 



