. Shield and compressed air tunneling . omething which will bend and give warninglong Ijefore it will give way. When such warning is given putin special reinforcing timbers. Hardwood should be used forwedges and it is often advisable to use steel plates on the tops ofposts and to use steel wedges for tightening up between caps andposts. The effort should be toward a general distribution of pres-sure and not toward a concentration at one point. 24. Timber Used.—It will be realized that the quantity oftimber used is large, when the tunnel is large and the ground that used for blocking,
. Shield and compressed air tunneling . omething which will bend and give warninglong Ijefore it will give way. When such warning is given putin special reinforcing timbers. Hardwood should be used forwedges and it is often advisable to use steel plates on the tops ofposts and to use steel wedges for tightening up between caps andposts. The effort should be toward a general distribution of pres-sure and not toward a concentration at one point. 24. Timber Used.—It will be realized that the quantity oftimber used is large, when the tunnel is large and the ground that used for blocking, packing and wedging is enormous,and cases are plentiful where the timber used for these purposescomes to about the same volume as that of the main carryingmembers. Drinker (p. 626) gives the following tal)le wiiichillustrates this well. 22 SHIELD TUNNELING Table I.—Quantity of Timbeu Used in Tunneling by Mining Opera-tions IN Various Ground(From Drinker) Character of rock Timber used in cubic feet per 100cu. ft. of ground excavated. Firm and self-supporting Firm Liable to fall Soft ground Pebbly ground Running ground If we take the figures given here as the mean and suppose atunnel having a cross-sectional area of 625 sq. ft., equivalent to acircle of 28 ft. diameter, then for each mile of such a tunnel thequantities of timber and its cost at an assumed price of $ thousand feet B. M. for the various kinds of ground listedwill be as shown in Table II. Table II.—Tunnel of Cross-section 625 Sq. Ft., One Mile Long. Quantity and Cost of Timber at $ per M. B. M. for VariousKinds of Ground Character of rock Timber required per mileof tunnel Cu. ft. M. B. M. Cost of tim-ber at $ M. B. M. 125,000218,750500,000781,0001,125,000Running ground 1, 656, 250 Firm and self-sustaining. Firm Liable to fall Soft ground Pebbly ground 1,5002,6256,0009,37513,50019,875 i 75,,,, This table shows how expensive a matter the v
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