Fig. 29.
After some 30 or 40 feet have been sunk the air at the bottom of the well may be very foul, especially in a well where blasting operations are being carried on, or where there is any great escape of noxious gases through fissures. Means must then be provided for applying at the surface a small exhaust fan to which is attached lengths of tubing extending down the well. Another good plan is to pass a 4 or 6 inch pipe down the well, bring it up with a long bend at surface, and insert a steam jet; a brick chimney is frequently built over the upper end of the pipe to increase the draught, and the lower end continued down with flexible tubing. With either fan or steam jet, the foul air being continuously withdrawn, fresh air will rush down in its place. This is far better than dashing lime-water down the well, using a long wooden pipe with a revolving caphead, or pouring down a vertical pipe water which escaped at right-angles, the old expedients for freshening the air in a well.
A means of increasing the yield of wells, which is frequently very successful, is to drive small tunnels or headings from the bottom of the well into the surrounding water-bearing stratum.
Fig. 30.
As an example, let [Fig. 30] represent a sectional plan of a portion of the water-bearing stratum at the bottom of the shaft. This stratum is underlaid by an impervious stratum, and, consequently, the water will flow continuously through the former in the direction of the dip, as shown by the arrow and the dotted lines. That portion of the stratum to the rise of the shaft, S, which is included within vertical lines tangent to the circle at the points m and n, will be drained by the shaft. The breadth of this portion will, however, be extended beyond these lines by the relief to the lateral pressure afforded by the shaft, which relief will cause the fillets of water to diverge from their original course towards the shaft, as shown in the figure. Hence the breadth of drainage ground will be a b, and it is evident that the shaft, S, can receive only that water which descends towards it through this space. But if tunnels be driven from the shaft along the strike of the stratum, as at m c, n d, these tunnels will obviously intercept the water which flows past the shaft. By this means the drainage ground is extended from a b to a´ b´, and the yield of the well proportionately increased.