Wet sieving method

2 1.18 µm 600 425 300 212 150 75 (2 in) (11/2 in) (3/4 in) (3/8 in) (1/4 in) (4) (6) (10) (16) (30) (40) (50) (70) (100) (200) kg 10 8 6 4 3 2 1.5 1.0 - - - - - - - - - kg 4.5 3.5 2.5 2.0 1.5 1.0 0.75 0.5 - - - - - - - - - g - - - 1000* - 500* 350* - 300 200 100 75 75 50 50 40 30

Note 1. Numbers in brackets indicate equivalent ASTM sieve sizes or numbers. Note 2. *It may be more appropriate to use a larger diameter sieve for material of

this size, depending on the size of the fraction in the sample. 1 mm = 1000 microns (1000 µm)

b) The fine sieves must not be overloaded, because this not only leads to inaccuracy but also reduces the life of the sieve.

c) It is very difficult to prevent overloading, when using mechanical sieve shakers and mechanical sieve shakers are not recommended except for coarse grained materials.

d) Particles larger than 20 mm may be placed through the sieve by hand, but must not be forced through. All smaller sizes must be shaken through the sieves.

e) The sieves must be kept clean by brushing with a brass or camel hair brush and washing through all sieving. Fine sieves should be inspected for holes in the mesh before use. Care in the use of sieves and prevention of overloading will lead to longer lives.

3.3.3 Wet sieving method

3.3.3.1 Scope. When a perceptible amount of clay or silt or if fine particles are found

connected with the larger particles, then wet sieving must always be used.

3.3.3.2 Apparatus.

(1) A typical range of aperture or mesh sizes would be : 75 mm, 63 mm, 50 mm, 37.5 mm, 28 mm, 20 mm, 14 mm, 10 mm, 6.3 mm, 5 mm, 3.35 mm, 2 mm, 1.18 mm, 600 ± = µm, 425 µm, 300 µm, 212 µm, 150 µm, 75 µm. Lids and receives of appropriate size are required.

the necessary aperture sizes should be used, except that, for convenience or to prevent overloading, additional sieves may be used so that the requirements of Table 3.3.3 are complied with.

b) The defining size separating fine sand and silt grades is 60 µm. The aperture size normally found closest to this is 63 µm. However, in practice the 75 µm sieve is more commonly used because it is more robust and less time-consuming to use. This standard suggests the continued use of the 75 µm sieve as the washing sieve. Some manufacturers’ offer a special ‘washing’ sieve which is of 200 mm diameter and 200 mm deep with a 75 µm mesh.

c) It can be useful to have two sets of sieves, one for the wet sieving and one for the dry sieving processes.

(2) A balance readable to 1.0 g. (3) A balance readable to 0.1 g.

(4) Sample divider(s) of appropriate slot width (riffle boxes).

(5) Thermostatically controlled drying oven capable of maintaining 105±50C. (6) An evaporating dish about 150 mm diameter.

(7) A corrosion-resistant tray, a convenient size being about 300 mm square and 40 mm deep.

(8) Two or more large corrosion-resistant metal or plastics watertight trays with sides about 80 mm deep, or a bucket of about 12 L capacity.

(9) A scoop.

(10) Sieve brushes, and a wire brush or similar brush. (11) Sodium hexametaphosphate (dispersing agent). (12) A quantity of rubber tubing about 6 mm bore. (13) A sprayer such as a small watering can use. (14) Appropriate number of enamel or porcelain dishes. (15) A mechanical sieve shaker (optional).

3.3.3.3 Test procedure

(1) The representative riffled sample is oven-dried at 105±50C to give a minimum mass complying with Table 3.3.3. If separation of the silt and clay fractions is to be carried out, or if the particle size distribution is to be extended below 75 µm, a second riffled sample shall be obtained for a fine analysis.

(2) Weigh the cooled oven-dried sample to 0.1% of its total mass (m1).

(3) Sieve the sample through all required sieve sizes of 20 mm size and larger. The mass retained is recorded on the test sheet in each case. Any fine particles adhering to the retained material should be removed with a stiff brush during sieving. The brushing should be done carefully to avoid losing material. Take care with soft materials to ensure that the brushing does not remove parts of the large particles.

Note. If adhering fine material cannot be removed easily by brushing, the following procedure may be followed.

a) Remove the fine material from the coarse particles by washing. b) Dry and weigh the coarse particles to 0.1% of their mass.

c) Dry the washings, add them to the material passing the 20 mm test sieve, and mix thoroughly.

(5) The sample shall then be placed in a large tray, enamel or porcelain bowl or in the bucket, and covered with water. If the soil is cohesive add sodium hexametaphosphate first at the rate of 2 grams per litre of water and stir until dissolved. Sodium hexametaphosphate is a dispersing agent and helps to prevent fine particles sticking together.

(6) The sample should be soaked for a minimum of 1 hour and frequent stirring should be given during this time.

(7) The sample is then washed through the 75 µm (No. 200) sieve with a 2 mm mesh sieve placed on top of it to protect it. Washing is most easily done by the decantation method. In this method, water is slowly added to the bowl or tray and the contents are vigorously stirred. Allow the contents to settle for a few seconds before pouring. The excess water is decanted carefully over the side of the bowl through the 2 mm sieve and into the 75 µm sieve, making sure all the water passes through the 75 µm sieve before running to waste. This process is continued until the water leaving the bowl is perfectly clear and all clay and silt particles have been washed through the sieve. Make sure that the fine sieve does not become overloaded, either by retained soil or by water.

Note. During this process DO NOT rub the material on the 75 µm sieve with your fingers or otherwise. This is likely to damage the sieve and give errors in the test results.

(8) On completion of washing place the washed sample in a tray or evaporating dish and place in the oven to be dried at 105±50C.

(9) After drying and cooling, weigh the sample to 0.1% of its total mass before commencing sieving (m4).

(10) Fit the largest size test sieve appropriate to the maximum size of material present to the receiver and place the sample on the sieve. Fit the lid to the sieve. Note. If the sieve and receiver assembly is not too heavy to handle, several sieves, in order of size, may be fitted together and used at the same time.

(11) Agitate the test sieve so that the sample rolls about in an irregular motion over the sieve. Particles may be placed by hand to see if they will fall through but they must not be pushed through. Make sure that only individual particles are retained. Weigh the amount retained on the test sieve to 0.1% of its total mass. Keep each fraction separate so that check weighings may be carried out at a later date if required.

(12) Transfer the material retained in the receiver to a tray and fit the receiver to the next largest sized sieve. Place the contents of the tray on the sieve and repeat the operation in (11). Be careful not to lose fine material by using a brush to clean the sieve mesh and the receiver. Use of the lid helps to reduce loss of fines.

(13) Sieving is then continued through progressively smaller sizes until the sample has been passed through the 6.3 mm sieve. The mass of soil passing the 6.3 mm sieve is determined to 0.1% of its total mass (m5). If the mass of material

passing the 6.3 mm sieve is too big (i.e. substantially more than 150 grams), the actual mass passing should be recorded and the sample divided again by riffling to give a reduced sample of about 100 to 150 grams. The mass of the sub-sample is then determined to 0.1% of its total mass (m6).

Sieving is now continued through the remaining sieve sizes. The mass retained on each sieve is recorded to 0.1% of its total mass. The mass passing the 75

µm sieve should be determined (ME). This mass will be very small if washing

has been carried out thoroughly. If any of the sieves are in danger of becoming overloaded the sample should be sieved a little at a time and the material

weighing.

Note 1. If a mechanical sieve shaker is available this may be used to perform the sieving operation provided that all the sieves are the same diameter and that they are not overloaded during the process. A minimum shaking time of 10 minutes is required.

2. Sample dividing is carried out to prevent having to sieve large amounts of material through the fine sieve sizes with the consequent risk of overloading. If only one or two fine sieves are to be used it may be quicker not to divide the sample and to sieve the total sample through these sieves a little at a time. If 20 mm or 6.3 mm sieves are not being used, dividing may be carried out for convenience at the sieve closest to 20 mm and 6.3 mm.

3.3.3.4 Calculation and expression of results

(1) Summation Check. The first stage in the calculation is to check that all the

weights retained add up to those of the original sample or sub-samples making due allowance for the weights passing the smallest sieve and any sieve where the sample has been divided. If these weights are not close to the correct total (i.e. within 1%) it is then possible to re-weigh the containers and to locate any errors before the sample is discarded. If this check is left until a later date it will be necessary to repeat the complete test if any error is found.

(2) Calculation of correction factors

a) It is necessary to calculate the correction or riffle factor for the first sieve size where the sample has been divided:

Correction factor,

f =

Original mass passing sieve size

Mass of sub - sample after dividing

=

m

m

3

b) The correction factor is then applied to each sieve smaller than the one where the sample was divided until the sample is again sub-divided. Where a second sub-division takes place the new correction factor is given by :

New correction factor,

f = f x

Original mass passing sieve size

Mass of sub - sample after dividing

2 1

=

m

m

x

m

m

c) The adjusted mass retained MAR is then obtained for each sieve size by multiplying the actual mass retained MR by the respective correction factor.

% Retained =

AR

m

x 100%

1

e) The cumulative percentage passing is then obtained by deducting the percentage retained on the largest sieve size from 100% and then deducting the percentage retained for each smaller size from the previous cumulative percentage.

f) The percentages retained on each sieve and cumulative percentages passing each sieve should be calculated to the nearest 0.1%. The values can be expressed in tabular form and / or in graphical form.

An example of a sieve test calculation is shown in Form 3.3.1, and the results are shown plotted on a semi-logarithmic chart in Form 3.3.3.

3.3.3.5 Report. The report should include the tabulated results of the test calculated as

cumulative percentages passing to the nearest whole number. The results should be plotted on a semi-logarithmically form (see Form 3.3.3). The method of test should be reported and the operator should sign and date the test sheet.