 During the American Civil War ( 1861 – 1865 ) , a large crack was found within the tunnel 's arch , being in some locations as much as 4 inches ( 10 cm ) wide . Upon examination of the earth above the tunnel , it was found that a section of the earth had slipped , and was resting on the tunnel walls . With the tunnel in danger of collapsing , a plan to address the tunnel 's structural issues was developed that put in place a second arch below the original . The new arch was started 12 feet ( 3 @.@ 7 m ) above the rails and rose to a peak just above what was necessary to clear a brakeman riding atop a railroad car ( at that time , 17 feet or 5 @.@ 2 metres ) , around 3 feet ( 0 @.@ 91 m ) below the original arch . Because of the ongoing war and the lack of funds and resources , the new arching could only be completed in 10 @-@ foot ( 3 @.@ 0 m ) segments . It was deemed necessary to re @-@ arch a total of 450 feet ( 140 m ) of the tunnel , and as of 1864 , 260 feet ( 79 m ) had been completed at a cost of $ 23 @,@ 390 @.@ 73 ( Confederate dollars ) . 
