210 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[June, 



and a considerable quantity of debris has fallen to the base of the 

 cliff. 



In the neighbourhood of Chatham are several chalk excavations, 

 but none so extensive as those at Dover. East of the town on the 

 road to IMaidstone, a cutting of about 30 feet in depth stands nearly 

 vertical. In the fortifications of Chatham, most of the chalk, as .-it 

 Dover, is faced with brickwork. One excavation, however, forming 

 a gorge from tlie i-iver to the crest of the works, has been cut foT- 

 some distance without facing, with a slope of i to 1, in depth about 

 40 feel ; another part of this gorge is faced ^^ ith brick, the same 

 slope being preserved. 



The princiiKil canal witli which I am acquainted passing to any 

 extent in the chalk formation, is the Thames and Medway, which is 

 formed from the village of Highara, to its termination at the Med- 

 way, a distance of 2i miles, by means of a tuiniel almost entirely 

 through chalk. In the open passing place on this canal, an excel- 

 lent example is seen of a chalk cutting, executed with a steep slope. 

 A goorl deal of peeling off has taken place here, and as this assertion 

 ^has been disputed, I beg to stale a fact coroborating its accuracy. 

 A small piece of brickwork, perhaps about 10 feet square, has been 

 built on one part of the face, and wdien I visited the place on two 

 occasions about two years ago, the brickwork projected at least 6 

 inches from, the face of the cut'ting, although when first built it had no 

 doubt been flush with this fece. Hence a very clear proof was obtained 

 that the chalk here had scaled off at least to the width of 6 inches. 

 I may add, that I have been informed by a gentleman wdio has seen the 

 place within the last four months that"the brickwork had then fallen 

 down, and that the chalk had very visibly wasted since onr iirst ex- 

 amination two years ago. Extensive ch.alk-eutlings are exposed at 

 each end of the tunnel, namely, at the Friendsbury entrance, where 

 the cutting is nearly upright," and at Higham, wliere the cutting is 

 al-o nearly vertical, but more detritus is seen at the base here than 

 at Frindsbury. All these cuttings are in the upper chalk, in wln'eh 

 plates of flint occur at short intervals. 



The flrmous quarries of Norlhfleet and Greenhithe are well known ; 

 in the former of these I witnessed the only instance that ever came 

 under my observation, of chalk standing perfectly upright without 

 anv appearance of scaling on the face, or of detritus at the base. 

 This, however, may certainly be seen on each side of a short road 

 leading from the London road into the Norlhfleet quarry, which 

 stretches down to the river. The depth is 30 feet, and the 'sides are 

 quite upright, with a greenish timcworn appearance. One place in 

 the quarry is 90 feet in depth ; the chalk has scaled and formed 

 debns at the base. This chalk has few fissures, and contains many 

 flints in the upper part, the lower being verv hard and fine. The 

 average depth of the Northfleet quarry is about 70 feet, standing 

 nearly upright, and upon the whole with little debris at the base, con"- 

 sidering its great depth. 



The Eastern quarry at Greenhithe is 8/ feet deep, nearly upright, 

 with rather more debris than at Northfleet. In some places round 

 the sides of this quarry, where ancient fissures have been filled in 

 with gravel, considerable fVills have been brought down, probably 

 by the well-known action of frost behind the mass. 



The Western quarry has a face of 80 feet, standing upright, but 

 much debris occasionally falls. It is diflneult in these quarries to 

 estimate the annual wasting of chalk by sealing or peeling off, be- 

 cause in many ca-cs it is frequently removed, and it becomes quite 

 uncerlam whether the observer is witnessing the accumulation of 

 only a few weeks or that of as many months. The top of this quarry 

 at Greenhithe is covered with a bed of loamy gravel, which has 

 filled up occasional cylindrical wells in the chalk of small bore to 

 the depth sometimes of 40 feet. 



The nature of the deconiposilion which operates so extensively on 

 many kinds of chalk, has been the subject of great variety 

 of opinion, and, as bearing on this question, I take the liberty of 

 pausing here to draw an important inference from the observations 

 above described on the chalk of this district. Excavations partly 

 composed of I lie upper chalk willi flints, and, towards their base, of 

 the lower or gray chalk, will best illustrate the position I am about 

 to advance, namely, that the decomposition almo^ entirely takes 

 jilace in the lower chalk, and not in the upper chalk, with flints. It 

 will be remembered that the only instance I have described of an 

 upright cutting, without detritus at the base, was at Northfleet, 

 where the depth was only .30 feel, while at other spots close by, 

 wnere the depth was SO or 00 feet ; considerable quantities of debris 

 have fallen down all along the face. From almost every instance I 

 have been able to observe, I think it clear that the ui)ner chalk with 

 tlints, where not much shaken, really will stand upright without 

 scaling. At the same time, it is no less certain lliat, below a particu- 

 lar c epth, the chalk undergoes a very apparent and extensive decom 

 posilion, and, in many cases, presents a base visibly hollowed out. 



It may be difficult to fix, in every chalk excavation, the line be- 

 neath which the decomposition is so evident; but, in the Northfleet 

 quarries, the line appears well defined by a curious and remarkably 

 regular plate of flint, which extends all round the faces of the 

 quarry, and which is considered identical with a similar plate or 

 layer, seen at both ends of the Thames and Medway Canal Tunnel, 

 and in the open cutting for the passing place. 



It is hardly in accordance with hitherto received opinions on the 

 subject of chalk cuttings, to say that these will stand better in their 

 upper than their lower beds, and yet the result of every observation 

 made on the chalk of Kent, leads positively to the conclusion that 

 the superior beds of chalk are less subject to decomposition than the 

 low'er. Instances in proof of this may be seen at Dover, jiarlieu- 

 larly on the eastern side of the town ; at Chatham, in the military 

 works ; at Rochester, and the other entrance of the Thames and Med- 

 way Tunnel ; and at the quarries of Norlhfleet and Greenhithe. 



In considering the nature of chalk dceomposition, it is necessary 

 to distinguish this from the falling of chalk which is shattered by 

 vertical fissures. In the latter ease, large masses fall down, when 

 loosened by high winds, or slightly burst by frost ; but the regular 

 decay of solid chalk arises from the peeling off or scaling wdiich 

 takes place on the face. This is almost invariably observed below 

 the flints, and is evident when very white spots are seen on the face : 

 from these spots large flakes or cakes of chalk have fallen down to the 

 base, and although these are inconsiderable in thickness, sometimes not 

 more than an inch, yet the frequency with which they fall renders 

 this a source of very extensive decay. 



Having mentioned the various railways intersecting the chalk in 

 other directions, as well as those in the south of England, I must 

 now explain that, with the former, my acquaintance is very limited ; 

 and I consider il more respectful at once to confess my inability to 

 say anything useful of them, than to attempt any review from such 

 iniperfeet means as those in my power. A few remarks, however, 

 on the chalk-cuttings of the London and Birmingham Railway, the 

 only other line of which I can speak from personal observation, may 

 not be entirely uninteresting. 



The Chalk of Watford Tunnel is very soft and white, with nume- 

 rous layers of flint, and much saturated with water, in which respect 

 it differs from all the fcirmer kinds of chalk I have described — these 

 being all remarkably dry, at least as deep as they have hitherto 

 been explored. At the northern end of Watford Tunnel is the same 

 soft, white chalk with flints. The sloi)e from the base is j to 1, until 

 within 1.5 feet of the surface, when the slope is increased to 

 Utol. 



The cutting near Cow Roost consists also of very white chalk, 

 much saturated, and the slopes are H to 1, in 25 feet cutting. 

 Further on the line, in a cutting near the road from Tring, the lower 

 gray chalk occurs in moderately sized blocks. The cutting at the 

 north end of the short Tring tunnel, consists of chalk, chalk marl, 

 and a little gravel. The slope, which is J to 1 in 33 feet cutting, 

 stands well, and tlie chalk appears drier than in .some of the cuttings 

 nearer London. 



The saturation of the chalk on this line is occasioned by the alti- 

 tude of the gault brick earth, which lies up very high in the neigh- 

 bourhood of Ivinghoe, and dams back the water, which cannot 

 eecapc from the chalk at a lower level than the top of the gault. 

 The Birmingham Railway, it is well known, passes through the 

 chalk in the same valley as the Grand .1 unction Canal. 



Having already submitted an opinion relative to the position of 

 the chalk which is most calculated to stand with au upright face, 

 I would, however, in this respect, make a great distinction between 

 the chalk of different places. The inferior beds of chalk are much 

 more shaken in some districts than in others, and even in composi- 

 tion the gray chalk comprises several varieties, as will readily be 

 seen on comparing the chalk of Shakespere's Cliff, at Dover, with 

 the chalk (of corresponding depth in the group) at Mersthani, and 

 elsewhere. When the lower chalk is much shattered, the face is 

 only prevented from giving way by the system of mutual dovetailing, 

 occasioned by the pyramidal form of the fractured blocks. The 

 angles of one block abut upon the angles of another, and, provided 

 the base be flrm, the whole w ill stand for some time. But it will be 

 evident that this support must fail when the angles become worn 

 down, as they constantly do become, by the process of real decomposi- 

 tion. Large falls may now be expected, and I believe that these are 

 the circumstances under which the lower chalk, when much shalfen, 

 invariably gives way. 



It seems very reasonable, therefore, that the system of facing the 

 chalk with a thin wall of brick, or of any kind of building or rubble 

 stone, should aftbrd protection against this danger. At present, 1 

 believe there are no examples of chalk-cutting with the sides upright 

 and faced in this way, but it will be highly useful, at some future 



