282 



NATURE 



[July 19, 1900 



shafts of a tunnel, with heavy plumb-bobs suspended from them 

 in buckets of water, or of tar, to bring their oscillations to rest ; 

 the accurate direction being given by means of a theodolite or 

 transit instrument on the surface. 



The wires are capable of side movement by means of a delicate 

 instrument, and are gradually brought exactly into the same 

 vertical plane : hence, if they are correct at " bank," or surface, 

 they must also be correct below ground. The engineers below 

 have to drive the galleries or headings so that only one wire is 

 visible from their instrument : so long as one wire exactly 

 eclipses the other wire, the gallery is being driven in the right 

 direction. 



As regards accuracy in levels, this is done by ordinary level- 

 ling ; but it will be seen at once how much depends on care 

 being devoted to both these operations. 



tunnel, notwithstanding every precaution being taken, all the 

 men engaged in driving the drainage heading by means of a 

 tunnelling machine have died ; and in the case of the first 

 Vyrnwy tunnel crossing of the River Mersey— driving by Great- 

 head shield under pressure — the mortality was great. 



Having explained in very general terms some of the difficul- 

 ties of tunnel construction, we will proceed to the case of the 

 great tunnels through the Alps, and for the purpose of rendering 

 the subject more easily intelligible, the following particulars 

 may be given : — 



Fig. 3. 



Plan 



Mg^^ 



. , nf Tunnel as required 

 ItrUJ'L t- ~ 



Assume two shafts, iocxd yards apart, between which a gallery 

 has to be driven ; and, allowing a distance of lo feet between 

 the wires, which are fV'^'^ inch in diameter, an error of the 

 diameter of the wire at the shaft will cause a mistake of nearly 

 4 inches at the point of meeting, or of 7^ inches if a similar 

 error occurs at the other shaft in the opposite direction. The 

 trickling of water down the wires increases their diameter so 

 appreciably, and therefore conduces to further inaccuracy, that 

 it is found necessary to fix a small shield or umbrella on the 

 wire to deflect the water. 



Some years ago, a tunnel which had been commenced, but 

 not finished, had to be completed. The first thing to be done 

 by the engineers was to make an accurate survey of the then 

 condition of the work — this rough sketch (see Fig. 3) indicates 

 what was discovered. The explanation 

 given by the former "ganger" was, that 

 he found the rock too hard, and he thought 

 that by bearing round somewhat to the 

 right, he might get into more easily exca- 

 vated material ! 



When the wires are hung down the shaft 

 it'is sometimes almost impossible to prove 

 that they are not touching, and conse- 

 quently being deflected from the true 

 vertical line by some rope or pipe, staging 

 or timber in the shaft. To overcome this, 

 an electrical current was passed down the 

 wire — a galvanometer being in circuit. If 

 the wire proved absolutely silent, and no 

 deflection was obtained in the galvano- 

 meter, the conclusion could be safely drawn 

 that the wire was hanging freely and 

 ttuly. 



In driving the necessary adit or heading 

 for drainage purposes beneath a sub-aqueous 

 tunnel, a rising gradient from the shaft 

 bottom of I in 500 is allowed, to enable 

 the water at the " face" to flow away from 

 the workmen to the pumps in the " sump " 

 or shaft botom (see Fig. 4). 



When the heading is driven sufticiently forward to justify the 

 commencement of the main tunnel, a fresh difficulty presents 

 itself. This main tunnel has to be driven down hill, and con- 

 sequently the water collects at the working face A : the bottom 

 cannot therefore be removed until a bore-hole is put down from 

 A to a. When this is done the remaining excavation can be 

 taken out, and a further length of tunnel driven to B. A bore- 

 hole is now sunk from B to d, whilst that from A to a can be 

 plugged up : and thus the tunnel is gradually advanced. 



By the adoption of the Greathead shield much of this diffi- 

 culty can be avoided ; but one sub-aqueous tunnel through 

 water-bearing strata, at considerable depth, is sufficient for a 

 lifetime. 



As an illustration of the danger to which men are exposed in 

 such work, it is stated, with much regret, that in a certain 



NO. 1603. VOL. 62] 



Length of tunnel in 

 miles 



North or east portal 

 above sea-level, feet 



South or west portal 

 above sea-level, feet 



Highest level 



Maximum grade in 

 tunnel per icxx) ... 



Maximum height of 

 mountain above tun- 

 nel, feet 



Possible maximum 

 temperature of rock, 

 deg. Fahr 



9-3 



3639 



3757 

 3788 



5-82 



5598 



85° 



Simplon 



12*26 



2254 



2080 

 2314 



7005 



104 



MoNT Cenis Tunnel. 



The Mont Cenis, or as it is more accurately called, the 

 Frejus Tunnel, is nearly eight miles in length. It is for a 

 double line of way — width being 26 feet, and height above rails 

 20 feet 6 inches. 



In consequence of the gradients in the Mon^ Cenis ascending 



jDicLgTamTrLOtic Sectiorv to iliustrate method of construe tinq 

 Tunnel below River Bed . 



(Jfot to Sca.1 



from both ends, the smoke cannot get away, and it remains in a 

 dull, heavy cloud in the tunnel. It is worse during cold and 

 rainy weather, and particularly during the winter, when the 

 air is sometimes so deficient in oxygen that the plate-layers 

 cannot work. 



Trains coming from France with an ascending gradient of 

 I in 40 against them for a length of 7 kilometres, when followed 

 by a current of air in the same direction, produce a disastrous 

 state of things. In this, as in all other steep tunnels, engines 

 having a heavy load behind them go through with their regu- 

 lators full open, ejecting great volumes of smoke and steam which 

 travel concurrently with the train, and the inconvenience and 

 discomfort produced are very great. 



At each kilometre in the tunnel, a refuge or "grande chambre" 

 is provided for the men, and this is supplied with compressed 



