26 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[January, 



railway, and also toplace upon record, some account of the slips which have 

 occurred on that line. 



The formation of the London and Croydon railway, has laid open a com- 

 plete section of the London clay, down to its junction with the plastic clay 

 (Fig. 1). The line commences, at the Loudon end, upon the more recent 

 allurial formations, with a considerable depth of peat. The first embank- 

 ment, after leaving the Greenwich railway, lies upon the peat, which is there 

 about 10 feet id thickness, and beneath it are 4 leet of loose silt, and 4 feet 

 of shingle on tlie top of loose chalk, the whole full of water, to within 3 feet 

 of the surface. The greatest height of the cmljankmeiit is about 25 feet, and 

 in widening it during the past year, for the junction of the Bricklayers' 

 Arms branch railway, it was found necessary to cut tlie old bank into benches, 

 and to pound up a strong footing, to prevent lateral spreading of the new work. 

 Although the railway has been opened more than four years, this embank- 

 ment is continually subsiding, and the amount of ' packing' required, is about 

 double what is needed on any other equal length of the line. 



Passing the New Cross station, the line enters a cutting rather more than 

 two miles in length, tlirough clay, which material continues in variable qua- 

 lities, until a short distance from Croydon, where the gravel appears. 



Immediately beyond itie New Cross station, the cutting (Fig. 2) deepens, 

 and at a distance of about a quarter nf a mile, its depth, at the centre line, is 

 about 75 feet, giving, however, a depth of upwards of 80 feet to the western 

 lido, from the ground rising rapidly in that direction. 



The permanent way lies upon the top of the plastic clay, and is nearly level 

 with the bands of stone, sand, shells, and shingle, which immediately cover 

 that stratum. Above this, again, is the strong blue London clay for a depth 

 of 15 feet to 20 feet, and at the top is a yellow clay of a silty character, in- 

 tersectcd with strata of septaria, and abounding in soapy earths and various 

 mineral salts, rendering it extremely pervious to water, and more easily 

 affected by its action than almost any other soil. 



On Tuesday the 2nd of November, 1841, at about 8 o'clock, p.m., a move- 

 ment was observed in the highest or western slope of this cutting, which is 

 on the inside of a curve : some of the clay, washed by the continuous heavy 

 rain of that season into a semi-fluid, crept down towards the rails from the 

 lidc of the slope, which had previously been subject to frequent surface slip- 

 ping. It was soon seen that the stratum of yellow clay was in motion from 

 top to bottom, and in the course of four hours, a mass containing about 

 50,000 cubic yards of clay, had separated itself from the hill, and had sunk 

 down, so that the back of the slip rested in a self-formed basin in the slope, 

 while the front had slipped forward on the glass-like surface of the blue clay 

 and had overwhelmed the line of railway for a length of 120 yards and a 

 depth of 10 feet or 12 feet, while large detached blocks of blue and yellow 

 clay, had been forced up from below, and beetled over the lower part of the 

 slip, ready to follow it (Fig. 2). 



In the removal of this slip, the exigency of the case required that economy 

 of work should, in many respects, give way to expedition. Stages were 

 erected at each end of the slip, sufficiently high to allow the earth wagons 

 to run under them and to advance gradually into the slip, as a way was made 

 for them, by working a •' gullet" down to the rails. The wagons were then 

 filled, partly from the gullets, partly by casting from the slope, and partly by 

 barrow roads leading to the stages. Two sets were thus employed at the 

 north end of the slip, and the earth was conveyed by locomotives, to the 

 sides of the nearest embankment, while two other sets drawn by horses, 

 worked the clay from the south end of the slip, to a distance of about a 

 quarter of e mile, where it was wheeled out to the spoil-bank. The work of 

 removal was continued by a large force, day and night, and on November 

 18th, both lines of railway were cleared. On the night of the 22nd of 

 November, the large mass of slip, still remaining on the floor, travelled for- 

 ward and again covered both lines of rail. On the night of the 26th of 

 November, a heavy movement took place on the east slope, which had 

 meanwhile been subject to a considerable sloughing. The large mass in 

 motion came down over the rails, and the line was not cleared until Decem- 

 ber 23rd, when the trains again ran through. On the morning of the 7th 

 of January, 1842, the line was again obstructed by an extension of the slip 

 on the west side (in a southerly direction), and as it was found that a still 

 greater quantity of earth was pressing forward from behind, a temporary 

 bridge, of timber, was constructed across the line, at the south end of the 

 slip, in order to carry away the top weight to an adjoining spoil-bank. 

 During the whole of this time, relays of men had been unceasingly at work, 

 day and night, in the clearing of the railway, which was at lengtli effected, 

 and after the 10th of February, 1842, the trains ran through regularly. 



The sudden and extensive failure of so large a quantity of earth, in works 

 constructed with every appearance of solidity, and in which, previous slips 

 had been confimd to the surface, without giving any indication in the mass 

 itself, was naturally a subject of great interest ; and a careful study of the 

 character of the soil was necessary to throw a light upon the operations of 

 nature in this instance. 



'<"bp slopes having generally remained for some time in a stable condition, 

 at the inclination at which they were formed, it may be inferred, that some 

 fresh action must have been developed, by which the equilibrium was de- 

 stroyed. In most cases of slipping, this action is found to arise from the 

 solvent property or statical force of water; and to this cause principally 

 must, in tlie author's opinion, be attributed the change of form undergone 

 by the soil of the New Cross cutting. 



The main body of the excavation is in two different sorts of clay ; the blue 

 clay at the bottom is stiff and insoluble , homogeneous in its character , and / 



impenetrable by water ; the yellow clay above, is separated from the blue, 

 by a clearly defined line, excepting occasionally, where masses of the blue 

 clay of an imperfect and mixed character run up into it. The yellow clay is 

 extremely unequal in texture, mixed up in all directions with silt, ochre, 

 fuller's earth, iron, lime, and bauds of septaria ; it is also intercepted by 

 innumerable faults or breaks, running in all angles, from the top to tlie bot- 

 tom ; through these breaks, and the soluble particles of the earth, water 

 had gradually found its way, owing to the imperfect drainage of the adjoining 

 fields, and an unusually wet autumn and winter, and had saturated the whole 

 mass, until it became overcharged with moisture; the greatest quantity of 

 water accumulating on the top of the blue clay, through which it fouud no 

 outlet. 



This saturation had, doubtless, been much assisted by the alternating 

 effect of succeeding summers and winters ; the soil being charged with mois- 

 ture must, during each winter, have expanded materially in hulk, and this 

 expansion taking place laterally, had probably caused a gradual and indefi- 

 nitely small movement towards the unresisting opening left by the railway 

 cutting ; the warmth of the succeeding summer, while it dried much of the 

 saturated mass, could not bring the particles back into their former position, 

 and thus additional cracks would be formed, admitting greater quantities of 

 water, so that year by year the evil would become greater, and the tendency to 

 slip gradually increase. Again, the clay, which, in a dry state, is compa- 

 ratively tenacious, becomes when wet almost a semifluid, and would thus be 

 unable to support its own weight, or to maintain itself in slopes so steep and 

 so high as in its natural state. These two causes combined, would leave the 

 whole material in a very unstable equilibrium, shaking by every passing 

 train, until at length the balance was destroyed, perhaps, by the hydrostatic 

 pressure of the water, which percolated through cracks at the back, or under 

 some of the larger strata of septaria, and the mass, aided by its gravitating 

 powers, sank dowu and travelled forward on the top of the blue clay, where 

 the accumulation of water had formed a highly lubricated surface, and aided 

 its descent. 



In the case of the first slip at New Cross, this movement was materially 

 assisted by the natural dip of the strata from west to east, and by the fact of 

 the western slope being on the inside of the curve, thus leaving the slope less 

 supported laterally. 



This slip has been ascribed, in some degree, to chemical action, which, 

 although not the only cause, nor, it is presumed, the most eflfective one, may 

 probably have assisted the action of water. The lower part of the yellow 

 clay and the upper part of the blue clay abound in iron pyrites ; this is de- 

 composed by the action of the weather, and the sulphuric acid disengaged, 

 enters into combination with the carbonate of lime, which is found with sep- 

 taria, thus forming crystals of selenite which here exist in great quantities. 

 The act of crystallization, while it alters the bulk of the component ma- 

 terials, may be supposed, in a measure, to assist the separation of the clay j 

 in fact the innumerable breaks or faults in this soil are found to be covered 

 with the crystals, in minute flakes or spiculie, between which the water 

 would have a clear passage. 



From this consideration of the causes of the movement, and the account 

 of the manner and extent of the slips themselves, the author proceeds to 

 notice the various modes of treatment which have been adopted, both at 

 New Cross and elsewhere, under the instructions of Mr. C'ubitt (V.P. Inst. 

 C.E.), the consulting engineer of the Croydon railway. 



Since the slip of November, 1841, above 250,000 cubic yards of clay have 

 been removed from the two sides of the New Cross cutting, at the precise 

 place where the slips took place. By the aid of the Inspector-General of 

 Railways, the Company has been enabled, by obtaining possession of the 

 necessary ground above, to trim back the slopes to what is considered a safe 

 position, This has been done in the form of benches and intermediate 

 slopes ; on the west side the slope is divided into four parts by three benches, 

 and on the east, into three parts by two benches. These benches vary in 

 dimensions up to 65 feet in breadth, and are so arranged that they leave the 

 least heights of slope at the bottom, nearthe railway, and the greatest heights 

 near the top ; these slopes being generally at an inclination of about 2 to 1. 

 This arrangement, although not a curve, brings the general form of the 

 whole slope into accordance with the conditions previously alluded to. 

 Drains are formed in the benches and slopes to carry off the surface water, 

 and although some slight slips have taken place, there is now no further 

 symptom of greatly extended movements. 



The same nature of clay in which this slip took place is found in all the 

 cuttings on the Croydon railway ; in some places it is of a more solid cha- 

 racter, while in others, as at Forest Hill, it is very silty and untractable. 

 So pervious, indeed, is the soil at Forest Hill, that after much rain it runs 

 into mud ; and after in vain attempting a complete system of under-draiuage 

 or soughing, by means of horizontal drains round and through the bottom of 

 the slip to the face, and vertical drains from the top down to the back drains, 

 a wide bench was cleared, about 20 feet up the slope, and varying up to 70 

 feet wide by running upwards of 100,000 yards of clay to spoil ; and at the 

 back of the benching a retaining wall of gravel, varying from 5 feet or 6 feet, 

 up to 10 feet or 12 feet in height, and nearly double the same dimensions in 

 width was formed, with a good footing into the solid earth below, to sup- 

 port the slipping earth behind. The clay taken out to form this wall, was 

 thrown up in front of it for additional weight ; and although the earth is 

 still in some degree slipping away at the top, the foot has remained firm, 

 excepting once, when a slight movement was perceived and was immediately 



