Two methods of estimating the cost of tunnel work are employed. The first is to calculate the probable expense of the various items of work, based upon the available data, per unit of length, and then add to this a margin of at least 10% to allow for contingencies; the second is to apply to the new work the unit cost of some previous tunnel built under substantially the same conditions. In the first method it is usual to consider the strutting and hauling as constituting a part of the work of excavation. To estimate the cost of excavation involves the consideration of three general items, viz., the excavation proper, the strutting of the walls of the excavation, and the hauling of the excavated materials and the materials of construction. The cost of excavating the preliminary headings or drifts is greater per unit of material removed than that of excavating the enlargement of the section. The cost of bottom drifts is also always greater than that of top headings, the material penetrated remaining the same. Mr. Rziha gives the comparative unit costs of excavating drifts, headings, and enlargement of the profile as follows:-- Bottom drifts $9.20 per cu. yd. Top headings 4.80 „ „ „ Enlargement of profile 2.84 „ „ „ The cost of hauling increases with the length of the tunnel. This fact and amount of this increase are indicated by the following actual prices for the Arlberg tunnel:-- Top heading $6.76 per cu. yd., increasing 37 cts. per mile Bottom drift 7.40 „ „ „ „ 26 „ „ „ Enlargement of profile 2.70 „ „ „ „ 10 „ „ „ In all the prices given above, the cost of strutting and hauling is included in the cost of excavation. The cost of excavation is not always the same for the same character of materials in different tunnels. The following figures show the prices paid for the excavation of calcareous rock in four different German tunnels:-- Berliner Nordhausen Wetzler R.R. $1.24 per cu. yd. Ofen 1.30 „ „ „ Stafflach 2.76 „ „ „ Gries 1.92 „ „ „ The method of tunneling has little influence upon the cost of the work, as shown by the following figures from tunnels excavated through calcareous rock by different methods:-- Ofen tunnel Austrian method $93.19 per lin. ft. Dorremberg tunnel Belgian method 86.08 „ „ „ Stafflach tunnel English method 91.69 „ „ „ The Martha and Merten tunnels, excavated through soft ground by the Austrian and German methods respectively, cost $87.95 and $87.55 per lin. ft. respectively. In the excavation of the various sections of the tunnel for the new Croton Aqueduct in America, the following prices were paid:--