360 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



[September, 



When the heading is driven, it is widened at 

 the top along one side, to form, as it were, a 

 shelf, \ipon which a crown bar may be laid 

 lengthways. When this is done, the centre 

 crown bar is placed along the top heading, and 

 supported against the roof, by an upright prop 

 at the remote end, and by resting it on the square 

 timbering of the shaft at the near end ; poling 

 boards are then arranged above the two bars to 

 carry the earth. This is shown in the annexed 

 section of the top heading. A similar excava- 

 tion or shelf is next made on the other side of 

 the centre crown bar, and a third bar placed 

 thereon, and poling boards inserted above, as 

 In the first instance ; a narrow slip of ground 

 is next removed from under the remote ends of 

 the two side crown bars, to the bottom of the 

 heading ; and rough props inserted to support 

 them in the same manner that the centre crown 

 bar is supported ; their other ends being in 

 like manner supported by the square timbers of the shaft. The earth may 

 next be removed from under the two side bars, which leaves the heading 

 much wider than before. 



Sometimes when the top heading is wide enough, two crown bars are in- 

 serted and poled above, and the insertion of the side bars (by excavating a 

 shelf to the right and left, as before described,) is then proceeded with, in 



the manner shown in the annexed en- 

 graving. The bars are kept at the proper 

 distance apart, by inserting five or six 

 struts between every t ivo bars, as shown 

 ) in the next engraving. The temporary 

 props, at the remote end of the bars, rest 

 upon flat foot-blocks, to prevent the super- 

 incumbent weight pressing them down. 

 ■ The foot-blocks are either placed at the 

 bottom of the heading, or the ground is 

 dug up to admit of their base standing 

 upon the intended level of the under-side 

 of the top sill. In either case, they are 

 placed far enough outwards to admit there- 

 after of the sill being placed in front of 

 them. — The dotted portion of the props 

 and foot-blocks, in the above cuts, shows 

 the end of the props so placed below the 

 bottom or floor of the beading : however, 

 it is not always that the ground will allow 

 of this being done in the first instance. The perpendicular face of the work 

 is secured from falling in, by the insertion of poling-boards across it, at the 

 back of the props, as shown in the last figure. 



In the manner above described, bar after bar is inserted to the right and 

 left of the top heading; propped and strutted from (lie ground and from 

 each other ; and the poling-boards inserted both in the roof and against the 

 face of the excavation, the bars being so arranged as to follow nearly the in- 

 tended figure of the tunnel : or rather, such an arrangement is preserved as 

 will be best suited for the subsequent insertion of the brickwork,— as will be 

 hereafter explained. 



The annexed engraving shews a section of the work in this stage of pro- 

 gress, which is technically called " getting in the top." 



'""•miJIiijjilii'illiiillii^'^nm 



It has been stated above, that the near end of the crown-bars is'at^first 

 temporarily supported or propped from the square timbers in the shafts ; it 

 must however be observed, that, by so doing, a great weight is thrown upon 

 the square timbers in addition to that of the brickwork of the shaft, which is 

 all that it is designed to carry, and in which it is materially assisted bv the 

 ianging-rods, or shaft-sills described in the preceding chapters ; and, for this 



reason, the square timbers should be as speedily as possible relieved from the 

 weight of tlie bars, and whatever pressure of earth they may be sustaining : 

 — this is finally done, when the the top sill next the shaft is inserted in its 

 ])lace, by propping every bar therefrom. When the ground is good, there is 

 no danger in temporarily supporting the near ends of the crown bars from 

 the square timbers; but where it is soft, or yielding, it is unsafe thus to load 

 them; for under such circumstances, the ground, instead of steadying the 

 square timbers, is liable to give under the pressure ; and when once the square 

 timbers get out of the perpendicular they would require no great additional 

 weight to force them in, and the yielding or soft ground which would thus 

 lead to the accident, would follow from behind the shaft, and in all proba- 

 bility bring the shaft down with it. * * * 



The cost to the Contractors for excavating the side lengths at Blechiugley 

 would, upon an average, be as follows : 



£ a. d. 

 Miners . . 96-2 days . at 6s. . 28 17 2 



Labourers, . . 95-0 . at 3s. 6d. . 16 12 6 



Horses . . 28-9 . at 7s. . 10 2 4 



Candles . . 4 dozen . at 6s. Gd. . 16 



Gunpowder . . IJ cwt . at 46s. . 2 17 6 



Tools, and sharpening picks, wedges, &c., . . . . 15 



Contractors' Superintendence, . 22 days . at 7s. . 7 14 



Clearing up the work when completed, . per length 5 



Total 



£68 19 6 



Thus the cost to the Contractor averaged £68 1 9s. Gd. per length of 12 

 feet. 



In making the engagement with the gangers, or subcontractors, a price per 

 lineal yard, for the side and shaft lengths taken together, was agreed upon, 

 which price was £\5, or .tiOO for each side length; they to find all manual 

 and horse labour, candles, gunpowder, working tools, &c. Mow it was well 

 known that at such a price no profit could be derived from the side lengths ; 

 as the working expenses would, upon an average, exceed such price, which 

 was proved by the result, as shewn above ; but taken together with the 

 shaft lengths, which, at the same time that they were longer than the side 

 lengths, required muc/i lexs lime and labovr to construct, they yielded a fair 

 amount of profit. When the particulars of the shaft lengths liave been 

 given, this subject will be recurred to, for comparison between the actual 

 average cost, and the price paid to the contractors. 



When the leading lengths were in progress, the miners obtained a bonus, 

 in a charge to the bricklayers of .£3 per length for lowering their materials, 

 .IS bricks, cement, &c., to the underground works; which was done by load- 

 ing the descending skip, at the time that the earth from the excavation was 

 being raised in the other. This yielded a profit of about £2 — the third 

 pound being paid for extra labour in loading bricks, &c., and the loss of time 

 occasioned to the miners' own work. But during the construction of the 

 side and shaft lengths no such profit could be obtained, because the excava- 

 tion was at a total stand, whilst the bricklayers were at work in each of 

 these three lengths ; whereas, during the progress of the leading work, the 

 bricklayers would be proceeding at one end, from the shaft, whilst the miners 

 would be progressing at the other, and vice versa, whereby the earth exca- 

 vated by the latter could be raised to the surface at the same time, and by 

 the same power, that the materials of the former were lowered. * * * 



The side lengths at Blechiugley and Saltwood were twelve feet long, and 

 so situated as to leave between them, upon an average, fourteen feet for the 

 shaft length ; the three lengths making together thirty-eight feet of tunnel 

 under every shaft, from which to carry on the work in both directions. 

 When this portion of the work is done, the difficulties of the tunnel may be 

 said to be over ; as the subsequent proceedings are comparatively straight- 

 forward and safe : at all events, there can be but few natural difiiculties that 

 cannot be foreseen, and consequently their effects provided for, or guarded 

 against; unless by injudicious proceedings, or absolute carelessness, difficul- 

 ties and dangers arise which otherwise would not have existed. 



Previously to commencing the leading lengths, it is requisite to construct a 

 platform over the invert of the lengths already completed, as shown in the 

 drawing : and wliich platform must be continued each way, as the work ad- 

 vances. It is made of planks, laid on sleepers or transverse timbers, placed 

 across the invert, so as to leave a free channel for the water to pass along the 

 invert, to be drained ott' through the heading; or, in cases where the water 

 is not abundant, it may hence be conducted to a proper receptacle or sumph, 

 convenient for the workmen to use it in mixing their cement or mortar ; for 

 wliere there is no water in the tunnel, the conveyance of that material to the 

 shafts for the bricklayers' use forms a considerable item of expenditure. 

 This was partly the case at Blechiugley ; the water, which was in abundance 

 at first, diminished in quantity as the work advanced, and towards the last, 

 (except at the west end) the land springs appeared to have been drained 

 nearly dry. 



The process of driving a leading length is nearly the same as that described 

 for a side length ; with this difference, that the bars in that case have to be 

 propped and supported, at both their ends, whereas, in the leading work, they 

 only require such assistance at their remote end, or against the face of the 

 excavation ; the near or back end of each bar, being left to rest behind, or 

 upon the biickvvork of the arch already turned. K'^Ci?*);; 



The work is commenced by getting in the top, in the manner described as 

 for the side length. A top iieading is driven in the middle liue of tlie tun- 



