394 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[November, 



A drawing, and the specification of the cost of the work, with a schedule 

 of prices, accompanied it. 



'• Descriplmn. of a Ilritlye l/uill of lilue Lias Limestone, acroii the Uii-- 

 miiigham and Gloiicexter Jiailwai/ at Dunhampstend." By Captain James 

 Vetch, Assoc. Inst. C. E. 



The pccnliaritics in the construction of this bridge are, th.it the arch was 

 composed of very small stones of the hlue lias limestone, from three to five 

 inches thick, and squared to about nine inches long and broad ; that it was 

 erectod without the usnal timber centreing, and that the mode of removing 

 the earth centreing precluded any danger from unequal sinking in the arch. 

 The span of the bridge is 60 feet, with a rise of 10 feet. The material of 

 the cutting where the bridge is situated, consisted of weak slate and clay, 

 conscquoiitly the mode of construction was subjected to a severe test. The 

 ahntmcnls being completed to the springing beiglit, tlic ground was cut away 

 roughly to the form of the arch ; seven "iows of pegs were then inserted with 

 their upper cuds correctly designing the proper curve; a line of jilanks .3 

 inches thick was laid transversely beside each row of iiegs, and upon them 

 were jdaccd lines of battens on edge, gauged to tlie exact profile of the 

 bridge ; the caith was consolidated, and a flooring of battens laid over all to 

 form a true bed for the soffits to rest upon. From the absence of parallelism 

 in the lias stones, their varying thickness, and the difficult adhesion of the 

 mortar, it was deemed necessai y to introduce seven transverse bonds of free- 

 stone, which imparted to the whole structure a tendency to settle in the 

 lines of the radii of the arch, and also prevented any rent in the lias masonry 

 from proceeding to a dangerous extent ; these free-stone bonds were firmly 

 fastened witli iron cramps. The face bad a batter of 1 in 9, from the spring- 

 ing to the string course, in order to counteract any tendency to bulge to- 

 wards the fares, or in the line of the least resistance. The base was also 

 extended and the crown narrowed, which gave a concave form to the string 

 course. The whole arch being filled in with the full depth of stone work on 

 each springing, and the bonds of free-stone all placed, the lines of each be- 

 tween the second and third bonds were keyed up, and then those between 

 the third and the centre bond, which thus apparently formed the key stone. 

 The earth centre was removed by cutting a heading 4 feet 6 inches wide, 

 directly beneath the key stone, and then gradually excavating on either side 

 uniformly towards the abutments, stopping at certain intervals to allow any 

 settlement to take place. By proceeding thus, as successive portions of the 

 arch were left to their own bearings, regular compression ensued, and a small 

 portion only of the work was exposed to the risk of fracture from inequality 

 of pressure ; the rising of the haunches which generally accompanies any 

 undue depression of the crown, appeared by this method to be entirely 

 avoided. 



The author ascribes much merit to the careful manner of keying in the 

 courses, as no cracks occurred, and the settlement of the arch did not ex- 

 ceed 2.1 inches, lie conceives this experiment to have answered completely, 

 as there was a saving of time, the exjicnse of erecting the usual wooden cen- 

 tre was avoided, and the bridge was ready when the railway cutting reached 

 it. lie considers that this system may be advantageously used in many situ- 

 ations upon railways, and that the span may be at least double that of the 

 bridge now described. 



The communication was accompanied by three drawings, showing the 

 details and progress of the construction. 



" Dmcrij/lion of the great Aqueduct at Lisbon, over the Valley of 4lcantra." 

 By Samuel Clegg, Jun. 



This aqueduct was founded by king John the Fifth in 1713, and completed 

 by the Marquis of Pombal, 1755. It resisted uninjured the shocks of the 

 great earthquake in that year, although it was observed to oscillate con- 

 siderably. The most conspicuous part of the work is that which crosses the 

 Valley of Alcantra; it consists of 32 arches, with spans varying from 50 to 

 105 feet; the crown of the centre arch is 225 feet from the ground. The 

 length of this portion is 3000 feet. 



The sources from which the supply of water is derived, are situated in the high 

 ground in the neighbourhoods of Cintra and of Bellas— they are eighteen in 

 number; one of these tributaries is conveyed by a culvert from a distance of 

 15 miles. The main duct into which the tributary streams empty themselves, 

 forms a trmnel of 6 feet wide, and 7 feet high, ventilated by vertical shafts, 

 at distances of a quarter of a njile apart. The channels for the water are 

 made with " drain tiles," 12 inches wide and 9 inches deep, open at the top. 

 After passing over the great aqueduct, the main duct runs under ground for 

 half a ndle, is carried across the " Estrada do aico Cavalho" on seven arches 

 of 40 feet span each, on the south side of which it continues beneath the 

 surface until it reaches the aqueduct of " Agua Livres" in Lisbon, and emp- 

 ties itself into the reservoir at its ternnnation. 



This reservoir is 00 feet long, by 54 feet wide and 27 feet deep. The 

 quantity of water contained in it when the author took the measurements 

 ■was 64^800 cubic feet, lie was unable to obtain a section of the retaining 

 walls, but supposed them to be about 23 feet in thickness. 



The pipes through which the water is distributed to the neighbouring 

 fountains arc of earlhenware and stone set in mortar. The velocity of its 

 flow thrnupb the main duct is 75 feet per minute. The quantity discharged 

 is about 73,000 gallons in 24 hours during the winter months. 



Consttiiction. — Tl :e particulars relating to the construction of the aque- 

 duct, the author translated from the documents preserved at the office of 

 Public Works in Lisbon. The foundations were laid in May 1713, arul the 



piers, which in common with the rest of the work are of gray marble, carried 

 up without footings. They are faced with ashlar work in courses from 1 foot 

 G inches to 2 feet deep. The stones are dowclled together with bronze and 

 iron ; the centre portion of each pier is filled in with rubble masonry to 

 within 30 feet of the top, above which it is left hollow. The voussoirs of 

 tlie principal arch, to which the author more particularly refers, are carefully 

 jointed, their thickness being on an average 8 feet at the springing, and 5 

 feet on the square at the crown. The figure of the arches is pointed Gothic, 

 the rise being -jiy of the span. The spandrels are of closely joinled ashlar 

 work, about 2 feet C inches in thickness. The backings are filled in with 

 rubble quite solid; nor is there any provision made for the drainage. 



The mortar used was made with lime from the gray marble of the neigh- 

 bourhood, and sharp sea sand, in the proportions of one of the former to four 

 of the latter. 



Mode of raising the materials. — No mechanical contrivances were used for 

 hoisting the blocks of marble, but they were slung upon poles from men's 

 shouldeis, and carried up a series of inclined planes to the height required. 

 Some of these blocks weighed upwards of three tons. The scaffolding and 

 inclined planes erected round the piers were of a very substantial description. 

 The lower parts were trussed framings formed of double Riga or Dantzic 

 timbers 15 inches square, fastened together with trenails of teak and chesnut. 

 The inclined planes had a rise of about 1 foot in 6 feet, with a level space at 

 each end of the pier to serve as a resting place, where a separate gang of 

 men received the stone block, and relieved the others. The ends of the 

 upright timbers of the scaffolding were not suflTered to be surrounded by 

 earth or moisture, but were placed upon blocks of stone bedded firmly and 

 evenly upon the rock, and kept well tarred. The struts and braces retaining 

 them were also secured from decay in the same manner. These precautious 

 were necessary, not only from the great weight they had to support, but from 

 the length of time they remained in use — not less it is supposed than thirty 

 years. 



The centreing for the arches was constructed by an Italian architect named 

 " Antonio Davila." The arches were commenced from each side of the valley 

 at the same time, and a temporary gangway erected over them as they pro- 

 ceeded, so that the inconvenience of raising the material from the bed of the 

 valley was avoided. The centreings were framed in their places. The 

 crad'es which supported the bearing timbers of the lower truss, were morticed 

 into sleepers resting upon projecting stones left for the purpose; those on 

 the same pier were secured by cross timbers so as to balance each other. 

 The lower framings were first fixed and secured by straining pieces, and the 

 upper portion erected afterwards in the manner of a roof principal. All the 

 scarfs were cut vertically, fastened by trenails of leak, and but little iron 

 was used in any part of the struetuie. The striking wedges were placed 

 under each voussoir, as in the French centreings. As the arch rose from the 

 springing, the crown of the centreing was loaded with stones to prevent it 

 rising, and altering the shape of the arch. 



The cost of the entire aqueduct, which was about 21 miles long, with all 

 the immediate and collateral works, and including the reservoir', was two 

 millions and a half sterling. 



The communication was accompanied by three elaborate drawings of the 

 general construction and details of the aqueduct, with the manner of cann- 

 ing the stones. 



May 18. — The President in the Chair. 



Thomas Lloyd was balloted for and elected a member. 



" On Sea Defences constructed with Peat Moss." By the Hon. Wont- 

 gomei'y Stuart. 



In the commencement of this communication, the author refers to the 

 curly period at which the art of reclaiming land from the sea was practised, 

 and to the extensive districts both in Britain and on the continent, where 

 sea defences of various kinds are constantly in course of construction. He 

 then proceeds to detail the modes suggested by the experience of many years, 

 and practised by him in constructing sea defences in the Bay of Wigtown, 

 for tlie protection of the estate of his brother, the late Earl of Galloway. 

 The whole of the district abounded with peat moss, possessing many pro- 

 lierties which rendered it, independent of its cheapness, a peculiuily valuable 

 material for constructing embankments to resist the action of the sea. Its 

 tough fibrous nature, its elasticity, and at the same time, the rapidity with 

 which the mass became solid, were useful qualities which he sought to take 

 advantage of. He f<mnd also that it possessed advantages as a material for 

 puddling ; as from its absorbent nature it imbibed and retained all the mois- 

 ture that approachcil it, and never cracked from dryness, as occurs so fre- 

 quently with clay puddle. In case also of holes being made in the puddle 

 either by vermin or external injury, they soon closed again from the elastic 

 nature of the peat moss, and ils tendency to grow together. 



Uses of Peat Moss.— 'the aulhoT somclimes uses peat moss as a puddle 

 between two ranges of stone walls, and sometimes as a hacking instead of 

 clay sod ; hut he more particularlyreeomniends it as a hacking to a stone 

 def^ence parallel with the shore. For this purpose, the turf should be cut 

 thin, placed against the hank, and the stone-work built against it; he has 

 found this the most durable and effectual defence against the sea; the action 

 of the waves against it even adding to its security, as from ils fibrous nature 

 it retains the sdt thrown against the wall until all ilie interstices between the 

 stones are completely filled"", and a defence is thus formed for the wall itself 

 by the accumulation against it. The method he employs is to build the sea- 



