44 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[Fkbruary, 



the roof sunk down, causing the centre of the floor to form a ' horse-back,' 

 as it was termed by the miners. Wlien some old mines, in which this had 

 occiirreil, were entered and worked after a lapse of years, the indurated clay 

 of the floor had supported the roof, while the coal, which had been formerly 

 left as pillars, was subsequently cut away. 



It was true, that when the lloor was soft, it would swell. In the Primrose 

 Ilill, and the Kilsby tunnels, if the cutting was left for a few days, without 

 completing the brick arching, the timbers were broken. The expansion ap- 

 ])eared to be nearly the same, whether it was caused by the air, as in the 

 former case, or by the water, as in the latter instance. 



Mr. Thomson remarked, that in the Box Tunnel, it was usual to allow 6 

 inches for expansion, between the face of the work and the timbers, and that 

 space was scarcely sufficient. 



Mr. lircK said, that in the Heaton Norris cutting, which was chiefly 

 through sand, containing much water, he had completely drained the slopes, 

 and had stopped the running of the sand, by building at the foot, a retaining 

 wall about 4 feet in height and from 2 feet to 3 feet thick, with a hacking 

 of 2 feet in thickness of cinders. He was induced to do this by observing, 

 that cinders were constantly used in the neighbourhood, for forming drains, 

 and he had generally found, that from careful observation of local habits, 

 valuable hints might be gained. 



Mr. SiMPsox, had devoted much attention to embankments and cuttings 

 in the London clay, and had found it very treacherous and dithcult to man- 

 age ; he liclieved that an inclination of 4 to 1 was not too much for a slope 

 of any considerable height. 



He remembered the embankments of a reservoir near London, which had 

 been originally constructed with insufficient slopes ; within a few months 

 after tliey were finished large masses slipped down, and it was feared, that 

 the whole must have been destroyed. At first, attempts were made to repair 

 the slips with mingled gravel and sand, but although the slopes were then 

 foimed, at an inclination of about 3 to 1, they did not stand ; after 3 years, 

 they were made up with gravel and clay, mixed with materials from the 

 dust-yards of the metropolis, containing a mixture of all kinds of substances ; 

 this was of a dry, "porous nature, and the slopes had stood well since, al- 

 though they were subjected to very variable pressure, sometimes having a 

 bead of water of 20 feet upon them, and the next day much less. 



In constructing embankments, it was his custom to have a footing of 

 brickwork, resting against a toe of concrete, and with careful attention to 

 the drainage, he found this plan always successful. 



He attrihute<l the first motion of slips in railway cuttings, to the action of 

 water, and unless the water was diverted by complete I)ack drainage, and 

 that on the surface and within the slopes, was carried lapidly away, the 

 slopes would never stand, even at the inclinations which had been mentioned. 



The expansion of the London clay was certainly very remarkable ; he had 

 seen at Richmond, a well of 4 feet diameter, completely closed in one n'ght, 

 by the swelling up of the bottom, althougli there was not any water in it. 



Mr. Cluttekbuck observed, that slips in railway cuttings appeared to be 

 caused, sometimes by the geological condition of the soil, when acted upon 

 by water, and sometimes by what might be termed its chemical condition ; 

 the latter was produced by the air, causing such a disintegration, as rendered 

 it more pervious to water, and consequently more liable to be acted upon by 

 its mechanical force.- — All railways passing from Loudon must, more or less, 

 intersect the London and plastic clays ; the sand beds of the latter formation 

 rested upon the chalk, and if those sands were washed away or shaken, a 

 slip of the superstratum would necessarily fidlow. The plastic day above 

 the sand beds, was deposited in layers, in which a certain order of super- 

 position might be traced ; be bad recognized a striking similarity between 

 those beds under London, and at the outcrop near Watford ; the distinct 

 layers were known, and names were given to them, by those persons who 

 sunk shafts for getting the sand ; it was understood, that their security in 

 working the sand, depended on the thickness and strength of some of the 

 beds; to their inequality of strength, might probably be attributed some of 

 the slips that occurred in that formation. 



The plastic clays were usually covered by a stratum or bed of silt, con- 

 taining shells, sharks' teeth, ac, and upon the silt rested the strong blue 

 London clay ; the most inveterate slips that he had observed on the London 

 and Birmingham Railway, occurred in those localities, where the silt was 

 covered by a thin outcropping bed of the London clay, not sufficiently thick 

 to resist the infiltration of water; the silt thus became saturated with water, 

 and slid from the surface of the tenacious plastic clay lying beneath it. 



The cutting at Brentwood passed through strata, which he believed to be 

 silt, covered by beds, or layers of loam, sand, and gravel, all more or less 

 pervious to water; thus causing slips, which were attributable to the geolo- 

 gical condition of the soil. 



In the London clay, where there was no superficial deposit of gravel, the 

 slips, he thought, might be traced to its chemical condition. A distinction 

 was often made, between the yellow clay on the surface, and the blue clay 

 beneath ; this difference of colour, was caused by the state in which the iron 

 existed in the soil ; when excluiled from the action of the air, it was found 

 as a protoxide; and in the upper beds, when subject to that action, it be- 

 came a peroxide; hence, the difference of the colour and, as he conceived, 

 the cause of a certain amount of disintegration. The air was admitted, by 

 the cracks formed in the clay in drying, or by the roots of trees or plants 

 (whose course might be traced by the difference of colour in the clay) ; by 



the working of the earthworm, and by other causes. The water which fell 

 on the surface, carried particles of sand and other substances into the fis- 

 sures, rendering the clay, in some measure, permanently pervious to water : 

 it was to this percolation of water, through the upper or yellow beds of the 

 London clay, that slips, such as that as New Cross might, he conceived, be 

 attributed. 



It was the practice, on many railways, to cut a back ditch between the 

 boundary railing and the quickset fence ; this appeared to have caused many 

 slips. The bottom of the ditches being exposed to the action of the air 

 would, when they received a flush of water, permit its infiltration below the 

 top of the slope ; he bad remarked, that many slips occurred, about 1 foot 

 or 2 feet below the bottom of these ditches, which was about the angle, at 

 which the water would drain towards the face of the cuttings. 



In some cuttings, apparently with the object of economizing space, the 

 slope was carried to the edge of the quickset fence; where that was done, 

 the slips seemed more frequent, than where the ditch was further removed 

 from the edge of the slope. 



Sir Henry Delabeche said, that he viewed railway works with great in- 

 terest, as opening a large field for the economic geologist. The causes of 

 the slips, which had so frequently occurred in cuttings, deserved careful in- 

 vestigation, and great benefits would result, not only to the scientific world, 

 but in the practice of engineering, if those who had charge of such works, 

 carefully watched and recorded every event connected with their progress ; 

 such as the nature and position of the strata, their amount of natural drain- 

 age, the eflect of weather, and all other points calculated to produce any 

 changes. 



With respect to the origin of slips in general, but more particularly of 

 those in the London clay, Mr. Clutterbuck had treated the subject so well, 

 and his remarks contained so much truth, that there remained hut little to 

 be said. 



Whether the attention was turned to cliffs on the sea coast, to mountain 

 cuttings, or to artificial embankments, it would be seen, that in the majority 

 of cases, the slips were caused by the action of water. Wherever theie ex- 

 isted a soft vein, beneath strata with fissures which enabled the water to 

 percolate, the substratum became mud, and being squeezed out by the super- 

 posed weight, caused the whole mass to slip. 



There were many instances of this kind in the oolitic escarpments near 

 Bath. They were extremely interesting, from being the scene of the labours 

 of Mr. Smith, who had justly been styled the father of geology in England. 

 In that district, Mr. Smith had cured and also prevented many threatening 

 slips, by introducing a system of surface drainage, at the same time tunnel- 

 ling into the face of the escarpment, to drain the beds, and to prevent the 

 water from reaching the softer strata beneath. 



.Vt Lyme Regis, the strata, having a certain degree of inclination, became 

 saturated with water, the softened mass was forced out from the lower parts, 

 and caused the slips which so frequently occurred on that coast. 



It was evident, that the various angles at which difl^erent earths would 

 stand, depended, in a great measure, upon the relative tendency of the mate- 

 rials to form mud. 



The Directors of the Eastern Counties Railway had requested him to visit 

 the Brentwood cutting with Mr. John Braithwaite, their engineer. The 

 strata in that locality were nearly horizontal, and although the material cut 

 through, would have been easily set in motion on an inclination, he was of 

 opinion, that the banks would stand well, if they were perfectly drained. 

 The ground was, however, very full of water ; it was also of a very tenacious 

 nature ; but he had oliserved much water running out from beneath the up- 

 per and dryer beds. A good system of drainage was the only preventive or 

 cure ; Mr. Braithwaite was so well convinced of that fact, that be planned 

 and executed the dry shafts which had been mentioned : the credit of all the 

 good they had produced must be given to Mr. Braithwaite. 



Sir Henry Delabeche did not attribute much advantage to the friction of 

 the gravel buttresses; their weight, force, and friction, might retard a slip for 

 a time, hut unless the buttresses entered the water-bearing strata, and served 

 as perpetual drains, they would not be eflicacious. 



The London clay was not homogeneous ia its nature; it was more or less 

 pervious, and abounded with fissures in all directions ; many of these were 

 filled with a slimy substance, which was easily converted into mud by the 

 percolation of water, and hence slips so frequently occurred, where cuttings 

 were made througli the dip of the London clay, as at New Cross. Surface- 

 drainage was nut sufficient for such strata; the main springs must be tapped, 

 and regular drainage be established, otherwise slips would be of constant 

 occurrence. 



The rocks of Rossberg (Switzerland) and the Undercliffat the back of the 

 Isle of Wight, might also be quoted, as instances of the same action of water, 

 in converting the lower beds into mud, upon which the superstratum slipped, 

 in spite of all attempts to restrain it. 



Captain Moorsom had seen several instances where, in forming embank- 

 ments of gravel upon a clay bottom, the wet substratum had been squeezed 

 out, and had caused the foot to spread, until it was stopped by weighting it, 

 and thus re-establishing the equilibrium. 



He thought, that back-drainage was essential, and he had rarely found 

 it unsuccessful, if it was commenced far enough from the edge of the cut- 

 ting. It should be so contrived, so as to allow the surface-«ater to flow 

 rapidly and freely away. He had repeatedly found tapping and under-drain- 

 ing iueft'ectual, unless the surface-drainage was tboroughlycompletcd. 



