Text 3. Excavation and treatment of gravel

Excavation of gravel is usually done by a series of trenches or paddocks with suitable width and extending in length for the full width of the valley or river flat – that is, the major axis of the paddocks is at right angles to the general direction of the valley. Should the river flat be more than 300 feet in width, it may be necessary to work the payable ground in sections, each section having its own sets of sluice boxes. Paddocks should be staked out well in advance of exploitation, and the overburden from one paddock should be thrown into the excavation left by the previous paddock.

When the river valleys are narrow and have steep sides, the flats are so reduced in width that it is not possible to excavate the ground by means of paddocks as stated above - that is, at right angles to the general direction of the valleys. In this case the paddocks are staked out parallel to the general trend of the valley, and the boxes are placed in position on the flank of the hill. Transport of gravel, from the paddocks to the sluices, may be by head-pans or wheel barrows; in the latter case, haulage should not exceed 4-00 to 500 feet, and a barrow track is required, which usually consists of planks, winch thick, laid end to end (fig.28).

If transport can be avoided, more economical results are obtained, as direct leading of the gravel into the sluice boxes is much cheaper, besides which a greater yardage can be treated; however, for direct loading certain conditions must obtain, which may be summed up briefly as follows.

The river flat must not be too wide and the natural grade of the valley should be steep – 80% At least - to enable sluice boxes to be right up, resting on the ground, thus avoiding the use of the trestles; the depth of overburden and gravel should not be excessive and the gravel, moreover, should be fairly free wash (fig. 29).

Fig. 29. Sluicing by direct loading method

A fairly wide paddock is first excavated, transversally, across the river flat, for the impounding of tailings, and the sluice boxes are then rigged up, across the entire width of the valley, over the edge of the excavation and parallel to the general direction of the valley, and at distance of from 13 to 14 feet apart. A series of iron or wooden launders then lead back from the heads of each set of sluice boxes to a flume, which supplies water from a leat, or else to a water distributing box, which supplies wash water direct from a dam.

The overburden excavated is thrown in to the finished cuts, and the gravel is loaded directly, by shovel, into the iron or wooden launders, where it is washed down by the flow of water into the moving sluice boxes, as launders can be added to as desired, and it moreover obviates the employment of men for transport purposes.

By building a series of dams across the valley, it is possible to exploit various sections of the payable ground, at one and the same time, as the water caught behind one dam serves as wash water for the boxes rigged up immediately behind, and so on down the valley.

Should the quality of water available for washing purposes be inadequate, as is sometimes the case in very narrow valleys, it may be necessary to build a series of small dams across the valley to store sufficient water for supplying one set of boxes; as the water behind the dam supplying the sluice is consumed, so the water from the next dam above is allowed to bank up behind it, and the procedure repeated, thus ensuring a constant supply of wash water for the day’s operation.

When the paddock approaches the dam, it is destroyed and replaced by a new one, farther up the valley; these dams, it should be noted, are constructed as cheaply as possible.

In the case of a very steeply inclined valley, where the quantity of water for washing purposes is ample, dams are dispensed with as being unnecessary: a series of sluice boxes are rigged up, at intervals, down the valley and in the river itself. The water discharged from the first sluice box flows down the river bed and is subsequently used in the second set of sluice boxes, whence it flows downstream to the third set of boxes, and so on for the whole series of sluices that have been rigged up. It should be mentioned that after passing through various sluice boxes, the river water will carry a great deal of fine silt, in suspension, and this must be allowed to settle before the water can be used for another series of sluice boxes, as otherwise the duty of water is reduced considerably (fig. 30).

It sometimes happens that a small payable alluvial deposit is encountered at the head of a river, where sufficient water is not available for sluicing on a large scale; in a case of this kind, a long tom, is employed for treating the gravel excavation from the deposit, the general scheme of work being as follows (fig. 31, 32).