268 TUNNELING 



means a strutting with an insufficient factor of safety to meet 

 probable increases or variations in pressure. Insufficient strut- 

 ting may be due to the use of too light timbers, to the spacing 

 of the roof timbers too far apart, to the yielding of the founda- 

 tions, to insufficient bearing surface at the joints, etc. Collapse 

 is often caused by the premature removal of the strutting dur- 

 ing the construction of the masonry. The masons, to secure 

 more free space in which to work, are very likely, unless 

 watched, to remove too many of the timbers and seriously 

 weaken the strutting. 



(3) The third cause of collapse is badly built masonry. 

 Poor masonry may be due to the use of defective stone or brick, 

 to the thinness of the lining, to poor mortar, to weak centers 

 which allow the arch to become distorted during construction, 

 to poor bonding of the stone or bricks, to the premature 

 removal of the centers, to driving some of the roof timbers 

 inside it, etc. 



Prevention of Collapse. Tunnels very seldom collapse with- 

 out giving some previous warning of the possible failure, and 

 also of the manner in which the failure is likely to occur. 

 From these indications the engineer is often able to foresee the 

 nature of the danger and take steps to check it. The danger 

 may occur either during excavation or after the lining is built. 

 During excavation the danger of collapse is indicated before- 

 hand by the partial crushing or deflection of the strutting tim- 

 bers. If the timbers are too light or the bearing surfaces are 

 too small, crushing takes place where the pressures are the 

 greatest, and the timbers bend, burst, or crack in places, and the 

 joints opan in other places. The remedy in such cases is to in- 

 sert additional timbers to strengthen the weak points, or it may 

 be necessary to construct a double strutting throughout. 

 When the distance spanned by the roof timbers is too great, 

 failure is generally indicated by the excessive deflection of 

 these timbers, and this may often be remedied by inserting 

 intermediate struts or props. In some respects the best remedy 



