38S 



THE CIVIL ENGINEER AND ARCHITECrS JOURNAL. 



1SJ7. 



PNEUMATIC PILE-DRIVING. 



Pile-driving is a process of great importance to the hydraulic engineer, 

 and the means of facilitating it have engaged the attention of many. To 

 drive by the common monkey is a clumsy operation, because the power is 

 brought to hear on tiie substance of the pile rather than on the soil in which 

 it is to be driven, and because the depth to which a pile can be driven is 

 limited by the length of timber of which piles can be made. The elTeclive 

 power brought to bear has been increased by the American and oilier 

 steam pile-driving machines, but without materially reducing their cost. 

 AVithin the last two years a new process, called Dr. Potts's, has been intro- 

 duced, which has been already applied by Mr. Robert Stephenson and 

 other eminent engineers. 



Although Dr. Potts's process is very simple, it is so difl'ereut in its 

 effects from what is imagined, that it is necessary to speak of it rather 

 fully, in order to give a precise idea to those who, by forming quick pre- 

 conceptions, may miss the principle. We have already intimated that in 

 the common solid limber pile, power is applied to the head of the pile, and 

 not directly to its base, or the soil into which it is to be driven, whereas 

 Dr. Potts's pile is hollow, and the power is brought to bear immediately 

 on the soil in which the pile is to be tixed. This is done by making the 

 pile hollow, by exhausting the air from it, and so drawing up the soil from 

 below the pile, whereby it is made to sink. The pile is not driven down, 

 as most would think, by the sole pressure of the atmosphere on the top of 

 the pile, but the shingle, gravel, or sand being removed into the pile as the 

 air is exhausted, the soil is constantly excavated beneath the bottom of the 

 pile, and driving and excavating proceed at the same time. This we look 

 upon as the real distinction between the old and new process, and the 

 point in favour of the latter, while the power is further economised by 

 being applied direct to the true scene of action. 



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Our engraving represents the pier supported on pneumatic piles, laid 

 down by Mr. Robert Stephenson on the Chester and Holyhead railway 

 in the course of this summer, being one among the many novel and curious 

 works on that great public undertaking, and illustrating the enterprise of 

 its eminent engineer. The viaduct is skew, and is carried over a branch 

 of the sea, in the island of Anglesey, and consists of two land piers built 

 in the usual way, and of this centre pier, laid on a sand bank. It is 3(> 

 feet long and 3 feet wide, and is built on 19 cast-iron tubes, each 16 feet 

 long and 1 foot diameter. The tubes were sunk by means of a small 

 double air-pump, with cylinders X\ inches diameter, and IG inches length 

 of stroke, worked by four men ; the pumps were placed on the land pier, 

 and a half-inch lead pipe was carried from the pumps on to the water at 

 the place of driving. 



Each tube was placed perpendicular over the spot on which it was to 



be sunk, and then a square iron cap placed on the top, with the half-inch 

 leaden tube just described passing through it ; at every stroke of the pump 

 the air was exhausted from witbinside of the lube, and as the exiiaustiug 

 process proceeded, the pile or tube made its way downwards, and the soil 

 displaced at the bottom passed into the lower part of the tube, — and thus 

 the operation was continued until the pile was sunk to the required depth 

 \V'hen the whole of the nineteen piles were sunk to one level, as shown in 

 the annexed engraving, a cast-iron plate, weighing tons, was placed 

 over them, just on a level above the surface of the water, and formed the 

 foundation upon which the superstructure was built. 



The pumps were brought down by coach, put together, worked, unship- 

 ped, and sent back again, all within a few days, so that nothing cumbrous iu 

 the way of apparatus is involved in the application — and, indeed, the air- 

 pump can be carried where the pile-driving machine cannot. The piles 

 were driven at the rate of half-a-minute per foot for the first six feet, and 

 at about three minutes to the foot for the remainder. 



The arches are 20 feet wide on the square, and 26 feet on the skew ; and 

 the piers 3 feet wide on the square, and 3 ft. 10 in. on the skew. 



In July 184.'), a pile of cast-iron, of 2 ft. Gin. diameter, was driven into 

 the Goodwin Sands by the engineers of the Trinity House. The rise of 

 the tide and the state of the wealher prevented the uninterrupted progress 

 of the work, and it was unavoidably divided into three separate periods, 

 which gave the following results : — 



July 19 in 3 hours, driven 22 ft. in. 

 „ 21 1 „ „ 10 



„ 20 U „ „ 1 7 



The total depth driven below the surface of the sand was therefore 

 33 ft. 7 in. This is only one of many experiments performed by the 

 Trinity Board, who have a license for the applicatioo of the patent, and 

 have used it in many of their smaller works. 



In the autumn of this year, the Trinity Board erected a beacon, by the 

 pneumatic process, on the South Calliper of the Goodwin Sands — a very 

 dangerous spot. The centre column is a tube of cast-iron, 2 ft. 6 in. dia- 

 meter, put together in 10 and 20 feet lengths, and inserted 32 feet deep in 

 the sand. Around it are four other cast- iron tubes, each of 15 inches, 

 diameter, the whole braced together, and supporting a cage on the top, 

 which is 5C feet above the sand level. In the great storm in October, this 

 work was broken; but this failure had nothing to do with the pile-driving 

 process, which was efficiently carried out, the piles being driven 32 feet, — 

 whereas, in Admiral Beaufort's experiments on the Goodwin, he could 

 only drive a steel bar 8 feet with a sledge hammer; and Captain Bullock, 

 R.N., found that a pointed iron rod of 3 inches diameter, when driven 

 13 feet in the same sands, look 40 blows of a monkey weighing 1 cwt., and 

 with a lOfeet fall, to drive it one inch. It should be observed, the beacon 

 on the Goodwin was of cast-iron only. 



Dr. Potts's plan allows the application of cast-iron tubes of any diameter 

 and any length, whereas wood pile-driving is limited by the scantling of 

 the timber, and limber piles of a large scanting are very expensive. Two 

 feet six inches, used on the Goodwin, is an unexampled diameter for a pile, 

 but there are no such narrow limits to ihe new process. Metal or wood 

 may be used for the tubes, and they may be made of staves hooped. The 

 patentee offers to put down small fishing and bathing-houses and stations 

 in the sands, at very moderate rates, and the plan is likely to be applied 

 for columns for carrying electric telegraphs over rivers, and for piers or 

 towers of suspension bridges. 



It should be noticed that a cylindrical tnbe, placed vertically on a body, 

 of sand and water, cannot be made to descend without great pressure, and 

 then only a few inches ; but by exhausting the air from the tube and draw- 

 ing up the soil from the bottom it sinks most rapidly. 



It is found in practice that not merely will sand, shingle, mud, bog, and 

 clay be carried up the pile, but even large stones are carried in suspension, 

 so that every kind of soil can be mastered except rock — and there it is not 

 wanted, because there is a solid foundation. 



The hydraulic engineer will at once appreciate the utility of this inven- 

 tion for river and sea-walls, piers, and breakwaters ; but its applications 

 are very numerous, and, as it can be most economically used, it will lead 

 to many new classes of works, for it extends the range of engineering. 

 Mr. Alexander Gordon, in laying down one of his new colonial lighthouses, 

 proposes the application of this plan, of the practicability of which be 

 speaks frona experience, and in a paper published by him on the subject, 

 he writes warmly in favour of trying it in other situations. Mr, Robert 

 Stephenson has, however, been the lirst to apply the plan on any consider- 

 able scale, though what has been dune hitherto by all parties is far from 

 enough to make it generally known among the profession. Those of our 



