SIMPLE DRY SIEVING

Dry sieving is the simplest of all methods of particle size analysis. According to the British Standard dry sieving may be carried out only on materials for which this procedure gives the same results as the wet seiving procedure. This means that is applicable only to clean gramiliar materials, which usually imples clean sandy or gravely soils that is, soils containing negligible amouns of particle of silt or clay sixe. If in doubt about the validity of the dry-sieving procedure should be followed instead.

If particles of medium gravel size or larger are present in significant amounts, the initial size of the sample required may be such that riffling is necessary at some stage to reduce the sample to a manageable size for fine sieving. The procedure is then referred to as “composite sieving”.

SAMPLE PREPARATION

The specimen to be used for the test is obtained from the original sample by riffling, or by subdivision using the cone-and-quarter method. The appropirate minimum quantity of material depends upon the maximum size of particles present, and is indicated in Table 1.

·         The specimen is placed on a tray is allowed to dry, preferably overnight, in an oven maintance at 105-110 ⁰C.

·         After drying to constant weight, the whole specimen is allowed to cool, and is weighted to an accuracy within 0,1 % or less of it is total mass.

Maximum size of material present in substantial proportion retained on BS sieve (mm).	Minimum mass of sample to be taken for seiving
Pass 2 mm or smaller	100 g
6,3	200 g
10	500 g
14	1 Kg
20	2 Kg
28	6 Kg
37,5	15 Kg
50	35 Kg
63	50 Kg
75	70 Kg
100	150 Kg
150	500 Kg
200	1000 Kg

Table 1 Minimum quantities for particle size test

EXECUITION OF THE TEST

Selection of sieves.

The complete range of sieves specified by the British Standard is given in table 2. It is not necessary to use all sieves for every test, but the sieves used should adequately cover the range of aperture size for each particular soil. For classification purpose we can use a shot set. The sieves to be used are selected to suit the size of sample and type of material.

Sieve frames must not be out of true, and should fit snugly one inside the other, to prevent escape of dust. Sieves are nested together with the largest aperture sieve at the top, and a receiving pan under the smallest aperture sieve at the bottom.

Construction	Aperture size full set	Standard set	Short set	Suitable sieve diameters
	A	B	C	450 mm	300 mm	200 mm
Preforated Steel Plated ( Square Holes)	75 mm 63 50 37,5 28 20 14 10 6,3 5	+ + + + + +	+ + +	+ + + + + +	+ + + + + + +
Woves Wire	3,35 2 1,18 600µm 425 300 212 150 63	+ + + + + + +	+ + + +	+ + +	+ + + +	+ + + + + + + + +
Lid and Receiver	+	+	+
	19 sieves	13 sieves	7 sieves

Table 2

Test Procedure

·         The dried soil sample is placed in the topmost sieve and is shaken long enough that all particles smaller than each aperture size can pass through. This can be achieved most conveniently by using a mechanical sieve shaker.

·         The shaker, he whole nest of sieves with receiving pan is placed in the dried soil is placed in the top sieve, which is then fitted with the lid, and the sieves are securely fastened down in the machine.

·         Agitation in the shaker should be for a minimum period of 10 min. Some shaker have a built-in timing device which can be pre-set to switch off the motor automatically after the desired period.

·         The maximum mass of sample, which can be sieved in one cycle, is depending on the used sieves and the particle size of the sample.

·         Weighting, the material retained on each sieve is tranferred to a container. Any particles lodged in the apertures of the sieve should be carefully removed with a sieve brush, the sieve being first placed upside-down an a tray or a clean sheet of paper. These particles are added to those retained on the sieve. Weighting of each size fraction should be to an accuracy of at least 0,1 % of the total initial test sample mass. The masses retained ( Ms^­­­­₁, Ms₂,--) are recorded.

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VALVE

These are devices incorporated in pipelines to control the flow into, through, anf from them. Valves are also known as faucets, cocks, bibs, stops, and plugs. The term cock is generally used with an adjective indicating its use. For example, a sill cock ( also called a hose bib ) is a faucet used on the outside of a building for connection with a garden hose. A faucet is a valve installed on the end of a pipe to permit or stop withdrawal of water from the pipe.

Valves usually are made of cast or malleable iron, brass, or bronze. Faucets in bathroom or kitchens are usually faced with nickle-plated brass.

The type of valves generally used in water-supply systems are gate, globe, angle, ball, and check valves.

Gate valves control flow by sliding a disk perpendicular to the water flow to fit tightly against seat rings when a handwheel is turned. The type of valve is usually used in locations where it can be lrft complectly open or closed for long periods of time.

Globe valves control the flow by changing the size of the passage through which water can flow past the valves. Turning a handwheel moves a disk attached at the and of the valve stem to vary the passage area. When the valve is open, the water turns 90% to pass through an orifice endclosed by the seat and then turns 90% again past the disk, to continue in the original direction. Flow can be completely stopped by turning the handwheel to compress the disk or a gasket on it against the seat. This type of valve usually is used in faucets.

Angle valves are similiar to globe valves but eliminate one 90^o  turn of the water flow. Water is discharged from the valves perpendicular to the inflow direction.

Check valves are used to prevent reversal of flow in a pipe. In the valves. Water must flow through an opening with which is associated a moveable plug ( or flapper ). When water flow in the desired direction, the plug automatically moves out of the way, however, a reverse flow forces the plug into the opening to seal it.

Ball valves are quick-closing ( ¼ turn to close ) valves, which consist of a drilled ball that swivels on its vertical axis. This type of valve creates little water turbulence owning to its straight-through flow design.