 During the construction , the timbers that were used to support the fragile sections of the tunnel to be arched were left in place after the arch was completed , as it was thought too hazardous to remove them . When the earth slip occurred during the Civil War , these remaining timbers acted as levers , furthering the cracking and structural instability of the tunnel . As the new arch was constructed , the old timbers were removed , and the space in between the new and old walls was filled with brick and cement . 
