370 



THE CIVIL KNGINEER AND ARCHITECTS JOURNAL. 



[Dec. 



of these shoals consist of alluvial gravel, without 'flints, but principally of 

 quartiose and granitic pcbblo, varieties of porphyry and of compact and 

 granular sandstone. This material, altbcjugli vcryhaid in some places, 

 offered no engineering difficulties. Other shoals consist of denuded beds of 

 hard red marl; this material being found in every instance, when the river 

 impinges upon the eastern or western limits of its valley. In most places 

 it was so bard, as to render its removal by the dredging machine (juite im- 

 practicable; and it is the object of tljc present commimicaiion to describe 

 the method of blasting, or breaking up this material, witli gunpowder, so 

 as to render it capable of being dredged up with facility. 



That part of the river Severn, above described, traverses nearly north and 

 south the great plain of red mat! of the new red sandstone foriualion, the 

 bed of the river from Stourport to about a mile I elow Holt liridge, near 

 Onilietsley, being formed through the upper strata of the new red sandstone; 

 upon Ibis lies the great bed of red marl (in places saliferous), d)])ping at a 

 small angle, but irregularly, to the south east. The river traverses the 

 whole of this strata, which is prohalily more than 1000 feet in thickness, 

 passing through the upper strata, and entering the lias formation above 

 Gloucester. 



The led marl is generally considered by geologists to he formed from the 

 debris of older rocks, and it appears to be totally devoid of organic lemains. 

 It lies generally in beds, rarely exceeding 15 inches in depth, and often 

 much less. It is divided occasionally by strata of greenish grey marl, and 

 near the upper part of the formation by thin, but very hard, beds of shaly, 

 or imperfect lias. 



It is difficult to describe the comparative hardness of materials, but when 

 it is stated, that in many places it was impossible to cause a steel chisel- 

 pointed boring tool to enter it by any ordinary exertion, by bond, from a 

 boat, it will be conceived that it could not be readily raised' liy dredging. 

 After exposure to the action of the air it breaks up into small fragments, 

 almost like the slaking of lime, so that solid blocks, which could only be 

 broken by the application of considerable force into tbarp-edged fiagme'nts, 

 would, in the course of a few days, fall to pieces and afford no criterion of 

 its hardness in an undisturbed state. 



■When the dredging machine was tried upon one of these mail shoals, it 

 was found impossible to raise above 50 or CO tons per day, and that with 

 constant risk and repeated accidents to the machine ; but such rate of pro- 

 gress was totally incompatil)le with the required progress of the work. At- 

 tempts were first made to break it up by driving iron bars into it, and priz- 

 ing it up, but this plan did not answer. A second attempt was made to 

 loosen it with a very strong plough, something like a "subsoil" plough, 

 which was proposed to be pulled through the marl by a powerful crab fixed 

 on a barge, the plough being guided by a strong pole ; the effect produced 

 was, however, so superficial, and tb.e expense of labour was so great, that 

 this method was also abandoned, and experiments were made to ascertain 

 the effect and probable cost of using gunpowder. These were so satisfac- 

 tory, that it was determined to bla>t all the marl shoals, )>revious to dredg- 

 ing them. In January, 1B45, as soon as tlie requisite maieiials and estab- 

 lishment could be prepared, this operation was commenced, and has since 

 been carried on with no other interruptions than those occasioned by freshes 

 in the river; the total length of blasting required (about a mile and a half,) 

 being now nearly completed, and a considerable portion of the marl since 

 dredged up, at the rate of 200 or 300 tons per day with perfect facility. 



The most economical method of using powder, to break up a depth of 

 rock like that described, would pnibably be to obtain a face of the required 

 depth atone end cf the work, to put in a row of shots at the back of it, and 

 after each discharge to remove the loosened marl ; continually repeating the 

 process; but this method would have been open to many serious objections. 

 The dredging machine and the blasting gang would have been constantly 

 waiting for cacli other, and having but two dredging machines to perform 

 the work, it was of great importance to economise their time in every possi- 

 ble way. By such proceedings also, a constant obstruction to the navigation 

 would have been created, equal to the whole width of the new channel. 

 The number of men that could have been employed in blasting would also 

 have been very limited. These objections, in 'this particular instance, far 

 outbal.-inced any little saving of gunpinvder. It was therefore determined 

 to put in perpendicular shots, througbnut the site of the cbauuel, at such 

 distances as experience might prove to be best, and proceedings were com- 

 menced with spaces of C feet from centre to centre of the shot holes. 



Six rafts were used, as stages to work from : they were each formed of 

 four baulks of timber, aliout 10 feet long ; the baulks, placed in pairs, were 

 secured at a distance of 4 feet apart, by cross pieces, G inches squaie, well 

 .^piked to the baulks at intervals of feet ; these' were covered with deals 3 

 inches thick, laid lengthways of the raft, a space of 12 inches in width being 

 left open along the centre. The ends of the rafts were provided with strong 

 ring bolts to moor by. Tiiese rafts were confined to one bank of the river 

 by ropes, and retained at the required distance from it by a series of " sets," 

 or booms, abutting against the bank. At tlie up-strcam'end of the raft was 

 a large barge fitted up as a blacksmith's shop, for tlie necessary repair of the 

 tools, viith dwellings for the watcbman, the ganger, &c. The hows of the 

 barge were strongly fortified, and a strong oblique boom of large baulks 

 reached from it to the shore, so as to protect the whole fleet from the craft 

 onming down the river. At the duwii-slream end was another barge, fitted 

 up as a powder magazine and as a shop, furnished with every necessary (or 

 the ipanufacture of the cartridges and for the storing of their' material. The 



words " powder magazine" 

 this vessel. 



in large letters were painted on both sides of 



The first operation consisted in placing and securing in their proper posi- 

 tions, the pipes through which the holes were to be bored. Small stakes, 

 painted with a series of numbers, were first driven into the bank, parallel to' 

 the work, at distances of G feet apart ; as far behind them as the slope 

 would allow, was anotlier row of stakes parallel with the first, so that a line 

 drawn through two stakes would be at right angles with the river, and a person 

 standing behind the two stakes could readily direct the workmen when to 

 lower the charge-pipe, which was then secured in its place, in the opening 

 of the raft, by a " timber dog," driven into the raft on each side of it. The 

 pipes were of wrought iron, drawn for the purpose by Messrs. James Russell 

 and Sons, of Wednesbury : they were 3J inches in diameter, ^ylis inch 

 thick, and 9 feet long.* Two collars, half inch square, were shrunk on 

 them near the upper end, for the purpose of retaining a rope, bv which they 

 were secured when the charge was fired. When the depth of the water 

 increased, these pipes could be lengthened 4 feet by an additional piece, 

 prepared for that purpose ; this joint was made by shrinking on a collar, 6 

 inches long, over the joint. The pipe being in its place, was driven through 

 any gravel that might remain and a few incises into the marl. The gravel 

 was generally so thick upon the marl, that it was requisite first to remove 

 it by means of the dredging machine. To protect the thin edges of the 

 pipes whilst being driven, a cast iron cap. or plug, was used, which received 

 the blows from a heavy wooden beetle ; the interior of the pipe was next 

 cleared of any sand, or gravel, that might have entered while putting them 

 down. The principal tool used for this purpose was an iron bucket or 

 cylindrical tube, 2 feet in length, of as laige a diameter as would pass down 

 the wUdle ; it was furnished at the bottom with a valve opening inwards, 

 and was jointed to a round rod, of the requisite length, half an inch in dia- 

 meter, and when used with a pumping motion, tpiickly brought up whatever 

 could not enter at the valve. 



The boring then commenced; a gang of three men being stationed at 

 each pipe. The first operation was that of the jumper, which was made 

 with a single steel edge, a little rounded. The jumpers were of round iron, 

 IJ inch diameter, except 2 feet in length at the lower end, which was li 

 inch diaiueler. For general use they were 15 feet long, and weighed about 

 52 lb. each ; after working them till they were nearly set fast, an'aujer was 

 inserted to raise the plug of loosened marl and to render the hole true. 

 The shell of the auger was 20 inches long and nearly closed up, the better 

 to retain the loosened borings. 



Tlie shot holes were bored two feet below the proposed bottom of the 

 dredging, as it was expected that each shot would dislocate, or break into 

 small pieces, a mass of marl of a conical or parabolic form, of which the 



Fig. 1. 



A-,' - I- ; • <>^-': " ' V '" 



* Where tlie mnrl was bo iU*ep as to require 3 or more pounds of powder, It was found 

 that the cartridtjes of such diiimeter as could be used in tliese small bores were fo long 

 as to lose part ot the effect of the gunpowder j BuUequently, pipes of 4 Inches diameter 

 ucie used uith advauUtje, 



