1815] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



287 



In Marcli, 1844, Mr. C. H. Gregory, the resiJiMit engineer of Ihe Lomlim 

 and Croydon R dlway, permitled an exiieriment to be tried upon a piece of 

 bank selceted as the worst part of (lie hne. The spot cliosen wns near tlie 

 Sydenham station, at the side of the up-hne. A longitudinal trench, 4 feet 

 deep, was dug on the erown of the bank at a few feet from the edge ; and 

 other trenches, abtut 30 feet apart, descended from it to the open drain l.y 

 the bide of the permament way. The drain pipes were then laid in, and the 

 clay, which had been dug out of the trenches, was laid over the pipes. As 

 great advantage is, in such cases, to be expected from ventilation, an occa- 

 sional upright pipe rises from the longitudinal line of pi |ies to the surface. 



An interval of dry weather causes the soil round the pipes to crack in every 

 direction, and thus opens rumerous fissures for Ihe passage of water to the 

 drainpipes. This is particularly the case with clay; which, thou, h gene- 

 rally presumed to be nnlavourab'e for drainage, is, in this way, as easily 

 managed as any oiher soil. This maybe readily tested by exposing clay, 

 nhieh has not been manipulated, to a drying aimosphere for a tew hours. 



This piece of bank has rot slipped since it was drained j but, as it Is only 

 120 feet in length, and 20 feet in height, it is wished more particularly to call 

 attention to a bank of } mile in length, and rising to upwards of GO feet in 

 height, having higher ground behind i'. This bank is on the down-siile o' 

 the line, between Chalk Karm bridge and Primrose Hill :unnet,near the Lon- 

 don terminus of the London and Birmingham Railway. It was in a very 

 precarious state, when placed in the hands of the author, in June, 1844. Thg 

 soil was the London clay, and the trenches to receive the pipes Varied in 

 depth from 3 feet to 6 feet ; the workmen being guided in this respect by llip 

 appearance of the ground, as it was opened. The descending trenches were 

 cut about 60 feet apart. The work h:is proved very saiisfactory, and no re- 

 pairs have been required since it was finished. It was contracted forat£200 

 and, as the length of pipe laid down was 2600 feet, it Is at the rate of Is. 6J. 

 per foot, including every charge. 



Walls of the Euslon Incline, Birminff/iam Railway . 



Retaining walls are frequently found to sutfer severely from want of drain- 

 age, and, perhaps, no one more so than that of the Euston incline, London 

 and Birmingham Uaihvay. The application of these drain pipis was per- 

 mitted on part of this wall, and every facility was given to the undertaken 

 by the resident and consulting engineers. 



It was necessary to bore through the wall, and several feet behind it, in 

 order to insert the pipes, which, for this purpose, were made of cast iron, in 

 lengths of four feet each, and were about 3 inches diameter. Boring through 

 the wall was accomplished by a machine (described in this Journal, Vol. VII. 

 18 U, p. 66) ; and, at some spots, considerable quantities of water issued in- 

 (tantly ; but, in other places, no water appeared for several days, or until 

 rain had fallen ; and some of these borings were observed to dry up wet 

 places, which showed in the viall at the distance of several feet. This result 

 was expected, from the admission of air causing the earth to dry, and to 

 crack, all round the pipes, thus opening channels for the admission of water 

 into them, and after a heavy rain all the borings may be seen to yield water. 

 This relieves the wall from the considerable pressure which would otherwise, 

 in time, have caused its destruction. 



In boring through the wall, it was found that the bricks, which had been 

 placed in contact with the wet soil, had become soft and decayed, hut these 

 bricks (or rather the portions of them brought out by the boring tool) were 

 found to become hard again, when they bad been exposed to the air for a 

 few days. 



Three portions of the wall were thus drained in March, 1843, by making 

 three borings in each panel. The success of this first attempt v^■as not quite 

 apparent; but Mr. R. Stephenson having approved of a further trial, with 

 five borings in each panel, the result was so satisfactory that thirty more 

 were thus drained, between October and December, 1843. These drains 

 have therefore been more than a year in action ; and it is worth noticing that 

 leveral of them yielded water freely throughout the drought of 1844. The 

 good effect upon that portion of the wall is very plain : though, since the 

 mortar has been washed out in many places, it would hardly be expected 

 that the success should be so complete as it is. This may be attributed to 

 the borings having been made 16 feet deep inwards, and thus collecting the 

 water before it reaches the wall. 



The panels operated upon in this way are 25 feet in height, and 20 feet 

 in width. Each of them contains five borings ; but, in a more favourable 

 •oil, a lets number would suffice. The charge for the work, including every 

 expense, was 3<. 6d. per foot for the length of drain pipe inserted. 



Remark's. 



Mr. DocKRAY said, that previous to the insertion of the drain pipes, the 

 walls of the Euston incline appeared to be saturated with water, hut that 

 they were now drying fast and the water no longer settled behind them, 

 being discharged by the pipes as fast as it percolated through the backing. 

 The slope of the cutting near the Chalk Faim bridge showed a tendency 

 to slip, liut the insertion of the pipes appeared, by draining the bank, to have 

 (topped all movement of the snil. 



Mr. R. Stepuenso.v said, that the retaining walls of the Euston incline 



were instructive examples of the discrepancy between theory and practice 

 under peculiar circumstances. They were designed several years since, before 

 he had attained his present experience of the action of the London clay. 

 The usual theory fur the amount of pressure against retaining walls was 

 that of I'rony,* which might he stated to he, that the pressure was equal to 

 the weight of a prism of earth, slipping upon the face of the natural slope 

 due to the character of the soil. 



This theory held good as long as the soil was dry, but when it became, ai 

 in the London clay, saturated with water, the position was no longer the 

 same and the pressure was that due to a column of dense semifluid acting 

 upon the back of the wall. In this particular case the inclination of the 

 strata required to be considered, to account for what had occurred. The 

 wall had been forced forward at the spot where, in plan, it formed a consider- 

 able curve, thus appearing to oppose the convex surface of the back of the 

 wall to the pressure; while on the other side, where the hack was concave, 

 the wall did not stir. On examination it was found, that from the inclina- 

 tion of the strata, the water percolating through the fissures, accumulated 

 against the convex ba( k of the wall, reduced the clay to a semifluid state, 

 and increasing its mobility, reducing the cohesion, until it forced forward the 

 foot of the retaining wall, while the opposite wall, which would have been 

 supposed to be weaker, remained firm ; evidently because the inclination of 

 the strata acted as natural drainage. The walls were 18 feet high, they were 

 originally built 5 feet in thickness, but had been increased on rebuilding to 

 6 feet thick, and their foundations were sunk 5 feet deep below the surface, 

 still they were forced bodily forward. It was natural to attribute the failure 

 to the pressure of a dense semifluid, of a weight due to the altitude of the 

 wall. The drain pipes had acted well in drawing oft" the water, but fearing 

 that they might not prove sufficient in a long duration of wet weather, the 

 cast iron truss beams were thrown across in order to lend the support of the 

 lower and drier side to the upper siile, which was more exposed to injury 

 from the percolated water. 



Retaining walls should be built nearly parallel from the bottom upwards. 

 That was the practice of the late Mr. Kennie, and was well exemplified in the 

 walls of the Sheerness Dock, lie allowed -Ith of the height for the batter 

 of the face and ^th for the thickness. Whether the thrust of the earth wa? 

 opposed by a wall with a straight or a curved batter, an equally effective re- 

 sistance was obtained, with a less t|uantity of material than in a wall with a 

 vertical back and an inclined face ; but the curved form was more convenient 

 for dock walls and was more elegant in appearance. The whole question of 

 retaining walls might he reduced to the simple mechanical principle of the 

 lever and the inclined plane, indicated by the angle of repose of the soil which 

 the wall was intended to resist, due allowance being made for an increase of 

 its natural density from the absorption of water. 



* Vide •Arclillecture Hydraullqua,' tome i., pp. 288, 289; also, Hutlon's 'Mathema- 

 tics,* prop, xfv., vol. li., p. 201. 



AL.\BASTER MINES OF CASTELLINA. 



The Geology of Tuscany. By \V. Hamilton, Esa.. M.P. 



(From Ihe Quarterly Journal of the Geological Society.) 



By far the most interesting and irrportant of tlie ditferent varieties of gyp-urn is tile 

 6iie wliite alabaster found in the neighbourhood of Castpllioa, where it is regularly strati- 

 fied, and is worlied in properly constructed mining galleries. The httle town of Castel- 

 lina, distant -bont twentv or Iwenty-foiir miles from Volterra, is reached by a rocky road 

 &ver wild rugged mountains. It is situated on the W. N. W. slope of the hills of Monte 

 Vaso, overloolting in that direction an extensive and slightly undulating plain of tertiary 

 marls, which there can be no doubt extend round the north point of the Monte Vaso 

 chain, and are connected with those of the Val d'Era and the Volterra district. 



We entered the mine by an inclined path, and, passing under gr und, s ion reached an 

 open well or large invert-il cone, round which the inclined path is carried, and where tlie 

 section of mails and gypsum Is well exposed. As the descending road passes through 

 the third and fou th gypsum beds, galleries are seen striking into the rock in all direc. 

 tions. The first and second gypsum berls are of a uni'orm character and grey cdour, and 

 do not contain any alabaster blocks. These are found principally in the third and fouth 

 beds, and occur as irregular isolated spherical masses imbedded In the gypsum, from 

 which they are, mineralogically sneaking, distinctly separated by a thin blatk crust, which 

 indicates to the vvorkmen the existence of the finer nodules. Tliese nodules are most 

 frequent in the lower part of the stratum, and occur In regulai I 'yers. never touching, 

 altnough varyinir much in their distances from each other. They vary much in lize, 

 neighing from 20 or 30 lb. to upwariis of 2.00O lb. When the workman discover! th« 

 black crust, he is at once aware that he is near a block of alabaster, and by following the 

 direction of the crust, he removes the gypsum all round until he has nearly detached tb* 

 whole nodule, which is at hist carctully separated from the parent rock. Gunpowder Is 

 occasionally usetl to blast the rock when no black crust Indica'ea the existence of the ala- 

 baster. This crust In connection with the pure alabaster Is perhaps one of the most 

 curious features of the mine. On close examination it appears to l>e laminar and concen. 

 trie, and to consist of layers of I lue clay and gypsum. Ni w the whole formaUon of gyp- 

 sum contains a small portion of clay which gives it the greyish colour, and it la probable 

 that, when that peculiar principle, whether cryatailization, uitraction or electricity, which 

 caused the aggregation of the partirl^s of gypsum in greater purity and in a more cryaUt- 

 line state was in operation, one of Us chief etfecta was to expel to the circumterence all 

 the panicles of argillaceous matter previously mixed up wiih the gypsum; a proceM 

 wMch would continue until either the crust itself opposed a resistance to the further ae- 

 tion of this principle, or until two opposing spheres nearly cnine in contact with each 

 other. Very fine crystals of seienite, and sometimvs of a larpe sire, are not unfrequently 

 found In the ti^sures of the gypsum- They ere used for the purpose of making the fine 

 Scagliola cement, .ind are eonseqiientiy sold at a much higher price than the more ordi- 

 nary gypaum. The price of the line alabaster Is 5 Tuscan lire the 100 lb. TuBcao at th« 

 quarry, or 8 if delivered in Leghorn. 



