Sedimentation Analysis of Soil: 3 Methods | Soil Science

The following points highlight the three main methods used for sedimentation analysis of soil. The methods are: 1. Pipette Method 2. Hydometer Method 3. Plummet Balance Method.

1. Pipette Method:

Calibration of Pipette:

The experimental setup for pipette method is shown in Fig. 6.8.

Calibration of pipette consists of determination of internal volume of pipette and tap 2 as explained in the following steps:

1. The pipette is thoroughly cleaned and dried, and its nozzle is immersed in distilled water.

2. Tap 1 (upper tap) is closed and tap 2 (lower tap) is opened. Water is sucked into the pipette by means of a rubber tube. Now, tap 2 is closed and the pipette is removed from the water.

3. Surplus water, drawn into the cavity above E, is removed into a small beaker by opening tap 2 in such a way as to connect D and F (refer to Fig. 6.8).

4. A weighing bottle is taken and its weight is determined. The water in the pipette and tap 2 is discharged into the weighing bottle and the water’s weight is determined again. The internal volume (V_P) of the pipette and the tap 2 is computed to the nearest 0.05 mL.

Pretreatment of Soil:

The process of removing organic matter and calcium compounds from the soil sample is known as pretreatment. Organic matter and calcium compounds act as cementing agents and cause aggregation of particles. Pretreatment is done to remove the organic matter and calcium compounds from the soil sample, so that it is ensured that only individual particles settle instead of particle aggregations.

This process is done as explained in the following steps:

1. If the soil contains > 1% soluble salts, determined as per IS – 2720 (Part 21) – 1977, the soil is washed with water for removal of soluble salts before further treatment is done, taking care to ensure that soil particles are not lost.

2. Two samples of this soil are taken, and the water content of one sample is determined.

3. About 20 g (clayey soil) to 50 g (sandy soil) of the second sample is taken and its weight is determined to the nearest 0.001 g and placed in a 650-mL conical beaker.

4. Fifty milliliters of distilled water is added to the soil and the soil suspension is boiled gently until the volume is reduced to about 40 mL.

5. Seventy-five milliliters of hydrogen peroxide is added to the soil-water mix after cooling and the mixture is allowed to stand overnight, covered with a glass cover.

6. The suspension is then gently heated and the contents are stirred frequently. After vigorous frothing has sub­sided and when there is no further reaction by the addition of fresh hydrogen peroxide, the contents are boiled to reduce the volume to about 30 mL.

7. In case the soil contains calcium compounds, 10 mL of hydrochloric acid (HCl) is added to the contents after cooling. The solution is stirred with a glass rod for a few minutes and allowed to stand for about 1 h. The treat­ment is continued till the solution gives acid reaction to litmus.

8. The pretreated soil is then transferred to a Buchner or Hirch funnel and washed through a filter paper with warm water until the filtrate shows no acid reaction to litmus.

9. A glass evaporating dish is taken and its weight is determined to the nearest 0.001 g. The wet soil, from the filter paper in the funnel, is transferred to the evaporating dish using a jet of water, using as little water as possible.

10. The dish and contents are dried in an oven at 105°C-110°C. The dish is cooled and the weight of the soil remain­ing after pretreatment is recorded as W_b.

Dispersion of Soil:

Dispersion of soil consists of preparing the dispersing agent, adding the dispersing agent and water to the soil sample, and mixing them thoroughly to get 500 ml of soil solution with the uniform distribution of soil particles throughout the soil solution.

This is done in the following steps:

1. Thirty-three grams of sodium hexametaphosphate and 7 g of sodium carbonate are placed in a 1000-mL meas­uring cylinder. Water is added up to the 1000-mL mark and the contents are mixed to make 1 L of the dispersing agent solution.

2. Twenty-five milliliters of the dispersing agent is added to the soil along with 25 mL of distilled water and the contents are mixed with a glass rod.

3. The mixture is warmed gently for about 10 min and then transferred to the cup of the mechanical stirrer, using a jet of distilled water. The amount of water used should not be more than 150 mL. The soil suspension is then stirred well for 15 min.

4. The suspension is then transferred to a 75-µm IS sieve, placed on a receiver, and the soil is washed on this sieve using a jet of distilled water, of not more than 150 mL, from a wash bottle. The suspension collected in the receiver is transferred to the 500-mL measuring cylinder. The measuring cylinder is filled with more water so that the total volume of soil suspension becomes 500 mL.

5. An identical 500-mL measuring cylinder is taken and 25 mL of the dispersing agent is added and then the cylin­der is filled with distilled water up to the 500-mL mark. This cylinder is known as the comparison cylinder and is used to determine the combined correction.

Sedimentation:

Sedimentation is the process of allowing individual soil particles to settle down in the suspension and recording the observations at regular time intervals.

It is done in the following steps (Refer to Fig. 6.8):

1. The contents of the measuring cylinder containing the soil are stirred thoroughly by inverting the cylinder sev­eral times, closing its opening, and then placing in the erect position, after which the stopwatch is started.

2. The pipette is inserted vertically into the soil suspension, with tap 2 in the closed position, to a depth of 100 mm below the surface of the suspension.

3. Taps 1 and 2 are opened and a sample of the soil (V_P) suspension is drawn into the pipette. Tap 2 is closed. The entire process is done in such a way that insertion, sampling, and withdrawal would each take about 10 s. No disturbance should be caused to the soil suspension during insertion and withdrawal of the pipette.

4. Tap 1 is closed and tap 2 is opened to remove the sample in bulb 3.

5. The sample in the pipette is now collected into a weighing bottle, by opening tap 1. Any suspension, adhering to the walls of the pipette, is washed into the weighing bottle by adding distilled water from the top of the pipette.

6. Sampling with the pipette is done at time intervals calculated using –

where D is the diameter of the soil particle in suspension (mm) with typical values of 0.02, 0.006, 0.002, and 0.001 mm, µ is the coefficient of dynamic viscosity of water (in poise), G is the specific gravity of the soil used in the analysis, H is the height of fall of the particle or the sampling depth (in cm), and t is the time of sampling in minutes.

7. Sampling is done in the comparison cylinder at any time during the test in a similar procedure.

8. The samples collected in the weighing bottles are dried in an oven at 105°C-110°C. After cooling in a desiccator, the weight of the weighing bottle and the dry contents is determined to the nearest 0.001 g.

9. The pipette method of sedimentation analysis takes 24-30 h.

10. The specific gravity of the soil passing through the 75-µ.m IS sieve is determined as per IS – 2720 (Part3/sec1) – 1980.

Calculations:

The particle size and cumulative % finer are calculated as follows:

1. Loss of soil during pretreatment is calculated using –

P = 100[W_b (100 + ω)]/W_a …(6.9)

where P is the percent loss in weight of soil during pretreatment, W_a is the weight of dry soil, W_b is the weight of soil after pretreatment, and ω is the water content of the air-dried soil sample taken for the analysis.

2. The diameter of the soil particle at any time during sedimentation is computed by Eq. (6.8).

3. Cumulative % finer corresponding to any particle diameter at any time (t) is calculated from –

where W₁ is the weight of dry soil in sample 1, V_P is the volume of the pipette, W_s is the weight of sodium hexametaphosphate in 500 ml of dispersing agent, and W_b is the weight of soil after pretreatment.

2. Hydometer Method:

Calibration of Hydrometer:

Calibration of a hydrometer involves preparing a calibration chart for the hydrometer, which is a graph between the hydrometer reading (R_h) and the effective depth (H_e). Once a calibration chart is prepared, it can be used for hydrometer method using the same hydrometer and 1000-mL measuring cylinders.

As the effective depth is a func­tion of volume of the hydrometer, the cross-sectional area of a 1000-mL measuring cylinder, and the hydrometer reading, these variables are determined for calibration of a hydrometer:

1. Volume of the Hydrometer:

The volume of the hydrometer is determined using one of the following methods:

A. Method 1:

(a) About 800 mL of water is taken in a 1000-mL graduated measuring cylinder. The reading of the water level in the cylinder is noted.

(b)The hydrometer is now inserted into the water in the measuring cylinder. The reading of the water level in the cylinder is again noted.

(c) The difference in the above two water levels gives the volume of the hydrometer bulb plus the volume of the immersed portion of the stem.

B. Method 2:

The weight of the hydrometer is determined to the nearest 0.1 g. The weight in grams gives the volume of the hydrometer in cubic centimeters, as the density of the hydrometer is 1 g/mL. This can be taken as the volume of the hydrometer, without any serious error, although it includes the volume of the stem.

2. Cross-Sectional Area of the Measuring Cylinder:

The vertical distance between any two graduations of the meas­uring cylinder, for example, the 100-mL mark and the 1000-mL mark, is measured accurately. The cross-sectional area of the measuring cylinder is given by –

A = V/h …(6.11)

where A is the cross-sectional area of the 1000-mL measuring cylinder, V is the volume between two graduations (say 1000 – 100 = 900 mL = 900 cc in this example), and h is the distance between two (say 100 and 1000 mL in this example) graduations.

3. Effective Depth:

The graduations on the stem of the hydrometer indicate the specific gravity (or density) at the mid-height of the bulb. The distance between any hydrometer reading graduation mark and the mid-height of the bulb is known as effective depth. Figure 6.10(b) shows a 1000-mL measuring cylinder containing a soil suspension for which the sedimentation analysis is to be carried out.

The hydrometer reading is taken at the surface of the soil suspension (level AA), which gives the density of the soil suspension at the mid-height of the bulb (level BB). However, due to insertion of the hydrometer in the soil suspension, the level A A rises to A₁A₁, as shown in Fig. 6.10(c); the difference in the levels being equal to the volume of the hydrometer divided by the area of the measuring cylinder (V_b/A). Similarly, the level BB rises to B₁B₁; the difference in the levels being half the volume of the hydrometer divided by the cross-sectional area of the measuring cylinder (V_h/2A). The point to be noted here is that soil particles at level B₁B₁ after immersion were at the same concentration as at level BB before immer­sion of the hydrometer. Hence, effective depth –

H_e = H + (h/2) + (V_h/2A) – (V_h/A )= H + ½ (h – V_h/A) …(6.12)

where H_e is the effective depth (sampling depth), H is the distance between the level of the hydrometer reading graduation and the top of the bulb, h is the height of the bulb, V_h is the volume of the hydrometer, and A is the cross-sectional area of the measuring cylinder.

In Eq. (6.12), the terms h, V_h, and A are constant. The depth H can be measured from different hydrometer read­ing graduations to the zero mark, and the effective depth can thus be computed for any hydrometer reading. The values are tabulated as shown in Table 6.3.

A graph is plotted between the hydrometer reading (R_h) on the x-axis and the effective depth (H_e) on the y-axis, as shown in Fig. 6.11. This graph is known as the calibration chart of the hydrometer. From the calibration chart, the effective depth (H_e) corresponding to any hydrometer reading R_h can be obtained.

Pretreatment of Soil:

The process of removing organic matter and calcium compounds from the soil sample is known as pretreatment. Organic matter and calcium compounds act as cementing agents and cause aggregation of par­ticles. Pretreatment is done to remove organic matter and calcium compounds from the soil sample, so that it is ensured that only individual particles settle instead of particle aggregations.

1. If the soluble salts present in the soil are more than 1%, the soil is washed with distilled water, taking care to see that the soil particles are not lost.

2. Two samples of about 50 g (for clayey soil) to 100 g (for sandy soil) of air-dried soil passing through the 4.75-mm IS sieve are taken. The water content (ω₁) of one sample is determined. The weight of the other sample (W₁) is determined accurately to the nearest 0.01 g. The sample is placed in a wide-mouth conical flask fitted with a filter paper.

3. About 150 mL of hydrogen peroxide is added to the soil in the conical flask and the mixture is stirred gently with a glass rod for a few minutes and then left to stand overnight after placing a cover.

4. The mixture in the conical flask is then gently heated and stirred periodically to avoid frothing over. After vigorous frothing has subsided, the contents are heated further to reduce the volume to about 50 mL. For organic soils, more quantity of hydrogen peroxide is required to completely oxidize the organic matter. If the soil is inorganic, this step may be omitted.

5. When the soil sample contains calcium compounds, about 50 mL of HCl is added to the sample after cooling. The solution is stirred with a glass rod for a few minutes and allowed to stand for minimum 1 h. More quantity of HCl may be added until the solution gives acid reaction to litmus. If the sample does not contain calcium compounds, this step may be omitted. The mixture is then filtered and washed with warm water until the filtrate shows no acid reaction to litmus.

6. An evaporating dish is taken and its weight is determined. The soil on the filter paper and the funnel is trans­ferred to the evaporating dish using a small quantity of jet of distilled water, taking care that soil particles are not lost. The contents are placed in an oven and dried at 105°C-110°C and then cooled in a desiccator. The weight of the dry soil and the evaporating dish is determined to the nearest 0.01 g.

7. Weight of soil remaining after pretreatment W_b = W₃ – W₂.

8. Weight of soil taken for hydrometer method before pretreatment is –

W_a = W₁/100 + ω₁ …(6.13)

Dispersion of Soil:

Dispersion of soil consists of preparing the dispersing agent, adding the dispersing agent and water to the soil sample, and mixing them thoroughly to get 1000 mL of soil solution with uniform distribution of soil particles throughout the soil solution.

This is explained in the following list:

1. About 100 mL of the dispersing agent is added to the soil in an evaporating dish and the mixture is warmed gently for about 10 min. The contents of the evaporating dish are transferred to the cup of a mechanical mixer using a small quantity of jet of distilled water, taking care to avoid loss of soil particles.

2. The soil suspension is stirred well in the mechanical mixer for about 15 min. The contents are then transferred to the 75-µm IS sieve placed on a receiver. Care is taken to transfer the entire soil from the cup of the mechanical mixer on to the sieve using a jet of distilled water and avoid loss of soil particles. The total quantity of water to be added during the procedure (where the soil is washed on the sieve) is about 500 mL.

3. The soil suspension, passing through the sieve and collected in the receiver, is transferred carefully to a 1000-mL measuring cylinder and made up to exactly the 1000-mL mark using distilled water. The soil suspen­sion in the 1000-mL cylinder is used for sedimentation analysis. The soil retained on the 75-µm IS sieve is oven dried and subjected to sieve analysis.

Sedimentation:

Sedimentation is the process of allowing individual soil particles to settle down in the suspension and recording the observations at regular time intervals and the process is explained in the following list:

1. A rubber bung is placed over the measuring cylinder, which is then shaken vigorously. Finally, the measur­ing cylinder is inverted end to end and then placed in the erect position, after which the stop watch is started immediately.

2. The hydrometer is immersed into the cylinder to a depth slightly below its floating position and then allowed to float freely. Hydrometer readings are taken after 0.5, 1, 2, and 4 min. The hydrometer is then slowly removed from the cylinder, rinsed in distilled water, and then placed in the comparison cylinder.

3. The comparison cylinder is a 1000-mL measuring cylinder in which 100 mL of the dispersion agent is added. The cylinder is then completely filled with distilled water up to the 1000-mL mark and the contents are mixed. Hydrometer readings are taken in the comparison cylinder, at regular intervals, to determine the combined correction.

4. Hydrometer readings in the measuring cylinder containing the soil suspension are then taken at 8, 15, and 30 min and then at 1, 2, and 4 h from the time of commencement of the test. After 4 h, hydrometer readings are taken once or twice in 24 h. The final reading is taken at 24 h time interval.

5. Hydrometer readings are taken by inserting and withdrawing the hydrometer gradually into the soil suspen­sion, taking 10 s for each operation, without disturbing the soil suspension.

6. The observations are presented in Table 6.4.

Corrections to the Hydrometer Reading:

The hydrometer readings taken during the test should be corrected for temperature, meniscus, and dispersing agent before using them for the calculation of particle size and cumulative % finer, as shown in the following list:

1. Temperature Correction (C_t):

The hydrometer is calibrated to read the density of the soil suspension at 27°C. If the temperature of the soil suspension is more than 27°C, the suspension will become lighter (less dense) than that at 27°C.

Hence, temperature corrections are as follows:

i. Positive when T > 27°C.

ii. Negative when T < 27°C.

2. Meniscus Correction (C_m):

When the hydrometer is inserted into the soil suspension, during sedimentation analysis, a meniscus is formed at the contact surface due to surface tension. As the soil suspension is opaque, the hydrometer reading is taken at the top of the meniscus, above the surface of the soil suspension. As the hydrometer reading increases in the downward direction, meniscus correction is always positive.

Meniscus correction can be found by inserting the hydrometer in a measuring cylinder containing distilled water and taking the hydrometer reading at the bottom and at the top of the meniscus. The difference between the two hydrometer readings gives the meniscus correction, C_m.

3. Dispersing Agent Correction (C_d):

The addition of a dispersing agent increases the density of the soil suspension. Dispersing agent correction is always negative.

4. Composite Correction:

The corrected hydrometer reading –

R_h = R’_h+ C_m – C_d ± C_t …(6.14)

or R_h = R’_h ± C …(6.15)

where C is the composite correction to hydrometer reading = C_m – C_d + C_t. The composite correction to hydrome­ter readings can be directly found by taking the hydrometer reading in a 1000-mL comparison cylinder containing distilled water. The dispersing agent is added to the distilled water in the same quantity as in the soil suspension.

The hydrometer reading is taken in the comparison cylinder at the top of the meniscus. The negative of this hydrometer reading gives the composite correction (C) to the hydrometer reading. The hydrometer reading is taken in the comparison cylinder along with the hydrometer reading in the soil suspension during sedimentation analysis to determine the corresponding combined correction to every hydrometer reading in the soil suspension.

Calculations:

The particle size and cumulative % finer are calculated from the corrected hydrometer readings and the results are then combined with those of sieve analysis.

The following procedure is followed for the calculations:

1. The combined correction to hydrometer readings is obtained from the hydrometer reading in the comparison cylinder (C). This correction is applied to the observed hydrometer reading (R’_h), at different time intervals, to determine the corrected hydrometer reading (R_h).

2. The effective depth (H_e), corresponding to each corrected hydrometer reading, is taken from the calibration chart (Fig. 6.11).

3. The particle size (equivalent diameter), corresponding to different time intervals, is determined from the effec­tive depth (H_e) using –

4. The cumulative % finer, corresponding to each time interval or particle size, is determined from –

N = [100G/W_b (G – 1)]R_h …(6.17)

3. Plummet Balance Method:

Procedure:

The procedure for sedimentation analysis using plummet balance consists of the following steps (refer to Fig. 6.12):

1. The instrument is leveled with the help of the leveling screws and the plumb bob.

2. The calibration of the instrument is checked by attaching the “zero” rider weight to the string in place of the plummet. The adjustment screw is used to set the indicator to read zero, if necessary. Similarly, the second rider weight “100” is attached to the string in place of the plummet and the adjustment screw is used to make any correction to the pointer reading to 100 on the graduated scale.

3. The plummet is then hooked to the indicator beam after lowering in a container filled with distilled water. The pinion knob on the vertical pillar is operated to bring the center of the plummet to a standard depth, usually 9 cm below the water surface. A mark is made on the string to ensure constant depth of immersion of the plummet. If the pointer does not read zero, it is adjusted to read zero with the help of the adjustment screw.

4. The pretreatment and dispersion of soil are similar to that in the pipette and hydrometer methods.

5. The measuring cylinder, containing the soil suspension and fitted with a rubber bung at the top, is shaken vigorously end over end, inverted once, and then placed in the erect position on the stand of the plummet balance. The stop watch is started immediately.

6. The plummet is then slowly lowered into the suspension and hooked on to the balance. The plummet is brought to the required depth, as indicated by the mark on the string, by turning the pinion. The percentage of soil at the center of the plummet is directly read by the pointer and the reading is noted at 0.5, 1, 2, 4, 8, 15, 30 min, and 1 h, and thereafter at hourly intervals.

Calculations:

The particle size (equivalent diameter) is determined using –

Here, D is the particle size in millimeters; H_e is the effective depth of the center of the plummet below the surface of the soil suspension, usually 9 cm; t is the time in minutes; and K is a coefficient which is a function of temperature (T) and specific gravity of the soil particles (G), as read from the calibration chart shown in Fig. 6.13.