In sinking a pit through any cover, the nature of which is not known, the first thing to be done is to prove it by boring ; when that is done, the plan of operation most suitable can then be determined on.
Beds of quicksand are frequently found covering coal fields upon the margin of the ocean, or by the sides of rivers. One of the oldest plans for passing through them, is by what is termed casting out ; this is still prac tised successfully. If the sand lies close to the surface, and does not exceed sixteen feet in depth to the under lying bed of clay or the rock head, the plan of casting out is adopted ; if it is deeper than this, another process is required. When the depth is about sixteen feet, the first operation is to bring up a surface drain, as low as the situation will admit of, which, in most cases, from the flatness of the surface, is very little ; then from the point where the centre of the pit is to be, a circular area is marked off, of such a width, that if the pit is to be twelve feet diameter inside when finished, the sand may be excavated in the form of the frustrum of a cone, having the sides sloping at an angle of 45 degrees, which slope is set off at such width as to uncover the imper vious clay or rockhead to the width of twenty feet. In this case the mouth of the excavation would be about 52 feet in width. It is of great consequence to carry i on this operation very expeditiously ; in some instances from fifty to a hundred workmen are employed, and as much of the sand as possible is taken out by horses, carts, and barrows, and laid at a distance. As soon as the begins to gather, sloping pumps are used, and the water never allowed to accumulate. When the slope renders the using of carts and barrows no longer conve nient, then the sand is thrown out by various means, such as by men from one stage to another, by trams and tram rods laid along the slope, or by common jack-rolls or windlasses, with ropes and buckets erected upon fram ing supported by tresses. Horse-gins are also used, placed at a distance Irom the pit ; and when the water is very heavy, a small steam-engine is employed ; the framing over the excavation being either supported by tresses and stays, or by masts, from which the framing is suspended by tackles. No foundation can be got tor the main pumping engine till the pit is built up and the cover made quite secure. If towards the bottom of the
slope the water is likely to produce a running of the sand, the slope is carefully covered with compact turf, cut from a green sward, which, though a very simple remedy, is frequently quite effectual if applied in bate ; for it has been found from experience, that if once the bottom of the slope begins to run, it cannot be stopped in its progress, so that in a short time the whole labour of weeks may be lost. Plate CCCXC. Fig. 12. repre sents a bed of quicksand resting upon a bed of imper vious clay, which clay is the immediate cover of the rock. When in this case the clay is found, the first course of ashler stones, jointed to the radius of the pit, is laid in a true circular position in the centre of the area, and about a loot into the clay ; or in some cases where the clay is not very strong, a circular crib of wood is laid down ; upon this the ashler is built, and as each course of stones is laid, about a foot or fifteen inches of well-wrought clay is regularly beat all around the hack of the building, and whatever water comes from the sand, flows up at the back, and is allowed to run over the building into the area of the pit, in order to be pumped up. As the ashler and moating are pro gressively carried up, the space at the back is filled up with sand and rubbish, until the pit is completed at the top, and the surface made suitable for the operations of the colliery. By this plan none of the water found in the sand goes down the pit, but is kept back by the clay mowing till it finds a natural issue at the surface. A is the pit, a a the quicksand, b b the excavation afterwards filled up, c c the ashler building, d d the moating of clay. When this is done, the impervious clay is sunk through to the rock head, and secured with ashler in the manner mentioned for sinking through compact cover. If the sand rests upon the rock head, and the rock be of a kind which is impervious to water, then the ashler is laid upon it as soon as a level foundation is made ; hut if it is a porous or jointed stone, it must be sunk through until a stratum impervious to water is found, from which place the building and moating commence.