ACCIDKNTS AND REPAIRS IN TUNNELS lib"? 



Causes of Collapse. Collapse in tunneL* may be caused : (1) 

 by the weight of the earth overhead, which is left unsupported 

 by the excavation ; (2) by defective or insufficient strutting ; 

 and (3) by defective or weak masonry. % 



(1) The danger of collapse of the roof of the excavation is 

 influenced by several conditions. One of these is the method 

 of excavation adopted. It is obvious that the larger the 

 volume of the supporting earth is, which is removed, tl e 

 greater will be the tendency of the roof to fall, and the more 

 intense will be the pressures which the strutting will be called 

 upon to support. Thus the English and Austrian methods of 

 tunneling, where the full section is excavated before any of the 

 lining is placed, and where, as the consequence, the strutting 

 has to sustain all of the pressures, present more likelihood of 

 the roof caving in than any of the other common methods. 



The character 'and structure of the material penetrated also 

 influence the danger of a collapse. A loose soil with little 

 cohesion is of course more likely to cave than one which is 

 more stable. Rock where strata are horizontal, or which is 

 seamy and fissured, is more likely to break down under the roof 

 pressures than one with vertical strata and of homogeneous 

 structure. Soft sod containing boulders whose weight develops 

 local stresses in the roof timl>ering is likely to be more danger- 

 ous than one which is more homogeneous. A factor which 

 greatly increases the danger of collapse, especially in soft soils, 

 is the presence of water. This element often changes a soil 

 which is comparatively stable, when dry, into one which is 

 highly unstable and treacherous. The liability of the material 

 to disintegration by atmospheric influences and various other 

 conditions, which will occur to the reader, may influence its 

 stability to a dangerous extent, and result in collate. 



(2) Collapse is often the result of using defective or insuf- 

 ficient strutting. Of course, in one sense, any strutting which 

 fails under the pressures develojH-d, however enormous they 

 may be, can l>e said to be insufficient, but ais used here the term 



