If the quicksand is deeper than sixteen feet, it is thrown out in the first pace to that depth, in the manner before described, and the additional thickness of sand is passed through by means of strong frames of wood, at the back of which sheeting-piles are driven down pro gressively as the sand is taken out ; but this mode of passing through sand is uncertain of success, and is not in general practice.
The manner of passing through mud with tubbing having been before described, the same kind of opera tion is applicable to passing through quicksand ; and if the bed of sand is thick, it is paced through by what is termed a drop tub, that is, after a number of tubs have been sunk by great weights, and begin to get body fast, so that they will not sink any farther, another set of tubs is let down through these, and continued till they reach the rock. These tuts are constructed of such strength, as to resist the pressure and be water-tight. If the tubbing is judged to be too weak at the bottom for the pressure, an additional number of oak cribs are inserted. During the operation, the bottom of the tub which is sinking is kept slways a few feet lower than the spot where the men arc lifting the sand, in order to prevent the sand flowing in from the hack of the tubs ; and it is necessary to keep about a foot of water upon the top of the sand, and to hang the pumps in such a manner as only to be a few inches into it. When these tubs are completely secured and wedged tight at their junction with the rock head, and where they join each other, the whole internal face of the cribs is covered over with close jointed deals, an inch and a quarter thick, which gives the pit an appearance of a smooth wooden cylinder.
The most effectual method for passing through beds of quicksand is by means of iron cylinders, tetmed cast-iron tubbing. If the pit is of small diameter, these are made about four feet in length, with strong flanges, and bolt holes inside of the cylinder, with a counterfoil ring at the neck of the flange, with brackets; the end of the lowest cylinder has no flange, but is rounded to render its sinking through the sand more easy. if the pit is of a large diameter, then the cylinders are cast in segments of two, three, or more pieces, joined together with inside flanges, having a jointing of white lead and oakum. If the sand-bed is thick, suppose eighty feet for example, the practice now is, to divide that space into three sets of cylinders, thirty feet each in length, and so constructed as to pass through each other like the tubes of a telescope.
These cylinders are joined together, piece alter piece, and pressed down by great weights, using the same precautions in keeping the lower part always farther down than the top of the sand where the men are at work, and where the bottom of the pumps is for draw ing the water. When the first thirty feet of cylinders is put down, the lower part of the next set of cylindet s is passed through the first set, and the sinking carried on until the additional thirty feet of cylinders are sunk, after which the third set of cylinders is passed down through the two former sets, until the lower pats reaches the rock-head or impervious stratum ; the join ings of the three sets of cylinders are made tight by wedging with wood. It is proper to remark het e, that this mode of dividing the deep sand to be passed through into spaces, is an improvement but lately adopted at Newcastle. Before this improvement, almost every at tempt to pass through very thick beds of quicksand was rendered abortive.
The engine-pit being thus secured by one of the several ways before-mentioned, the process of sinking through the rock is now ready to be begun ; but, before commencing, the divisions of the pit formed of carpen tay, named brattices, are made ; tnis is done in several ways.
In common practice, and where great tightness of jointing is not required for ventilation, on account of inflammable air, bars of wood, named buntons, of about six inches thick and nine inches in depth, are fixed in a horizontal position across the pit; they are placed at different distances according to circumstances, some times at the length of a pump from each other, and at other times there are two or three in that space. These buntons, are all placed in the same perpendicular line, and upon these, deals of an inch and half thick are nailed, having their close; and each length of these deals takes in the half of the upper and under bunton to which it is nailed. In deep pits, when the ventilation is to be carried on by the brattice, the side of the bunions next the pumps is covered with deals in the same way, the joints made as tight as possible, and next the pit the irregular joining is caulked with oakum, and fillets of wood fixed all the way down on each side of the brattice ; this forms a double pit.