The Griffith Tube: A Simple Settling Tube for the Measurement of Settling Velocity of Aggregates

Transcription

1 School of Australian Environmental Studies Griffith University AES Working Paper 3/86 May 1986 The Griffith Tube: A Simple Settling Tube for the Measurement of Settling Velocity of Aggregates By P. Hairsine and G. McTainsh 1. Introduction Data for the settling velocity distribution of soil aggregates ina fluid is fundamental to the study of erosion and deposition of soils. Recent process-based soil erosion models (eg. Knisel (198) and Rose, Williams, Sanders and Barry (1983)) describe the interactions of entrainment, transport and deposition processes for a runoff event. Data on aggregate settling characteristics is a primary input to such models. For example, the model of Rose et al (1983) describes the rate of deposition from flowing (or stationary) sediment as: di = vi ci where: d i = the deposition rate of sediment per unit area and time (kg/m/sec), for sediment with a settling velocity of v i. c = concentration of that settling velocity class i (kg/m) i = an integer labeling the particular settling velocity class By considering a given soil sample as composed of various fractions or classes, each with characteristics of that class, the total deposition rate of the soil can be obtained by summation. Settling tubes have been used in a number of fields to analyze two-phase systems for most of this century. Developments have led to the very accurate discrimination between particle sizes using the associated variation in settling velocity. Felix (1969), Emery (1983), Schlee (1966) and the Inter-agency Committee on Water Resources (1957) are examples of such studies for soil particles and sands. However, measurements of the size distributions of soil aggregates for use in soil erosion-deposition models, pose particular problems. Where the settling velocity characteristics of soils in a fluid is the required information, settling tubes provide the most direct method of measurement. The majority of the sediment from soil commonly exists in the form of aggregates which settle at a rate dependent upon the aggregate s size, shape, roughness and density relative to the fluid. Childs (1969) in his review of the assumptions behind Stoke s Law suggests that significant surface and form drag effects limit the use of Stoke s Law to particles to less than 60µm diameter. For diameters less than 60µm standard pipette or hydrometer methods (Black, 1965) are convenient to use. However, for sediment greater than 60µm the relationship between size and settling rate is complex and depends on the particular variations found in aggregate shape and density (eg. Gibbs et al, 1971, Watson, 1969). Thus measurements of aggregate size cannot be accurately converted into settling velocity or vice versa, without the complex task of obtaining the relationship between these two measurements for each particular soil type. The most common method used to measure aggregate size is wet sieving (Yoder, 1937). For the reasons given above, there is no universal relation between settling velocity and size, and under wet sieving any

2 such relationship will vary with the degree of abrasion, which depends on sieving technique. Settling tubes, in general, involve minimal abrasion and thus reduce this source of variation. The apparatus presented here is an adaptation of the Siltometer developed by Puri (1934) and is designed to provide accurate information on the fall velocity distribution of sediment outside the size range covered by Stoke s Law. The Griffith Tube provides this information for settling in clear water, without the interaction which would occur in a natural polydispersed system. This interaction is due to the fall velocity of faster sediment being inhibited by falling through slower (smaller) sediment. 2. Apparatus Description A full description of the apparatus and its operation are given in the appendices. The Griffith Tube, as shown in Figure 1, has three basic components: the settling tube, the sample introduction device and the collection turntable on which settled sediment samples are collected. 2.1 The Settling Tube The semi-fixed Perspex tube is 2.0 meters in length and 65 millimeters in diameter. A simple lever to raise and lower the tube 10 millimeters is mounted on the top of the apparatus. This lever permits sample trays to be interchanged at the tube s exit during the conduct of the trial. The base of the tube is beveled such that it seals on contact with a closed-cell foam pad which doe snot permit leakage while filling the tube. During the trial the top of the tube is sealed to maintain a static column of water through which the sediment passes.

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4 2.2 The Sample Introduction Device Trials were conducted using two types of sample introduction devices: an injection barrel and a selfinverting cup. Figure 2 shows an exploded view of the injection barrel device. The barrel is filled with the sample (pretreated as described later) which is initially supported by an aluminum flap. This hinged flap prevents the sample from prematurely falling from the Perspex barrel (see Figure 2). The seal on the top of the water column is maintained by both the o-ring between the sample introduction device and the tube and the dual seals on the syringe plunger. The plunger is automatically drawn into the tube when the tube is lifted from the foam pad as a result of the weight of the water column. The travel of the syringe plunger is limited by an end stop and the seal is maintained at the completion of sample injection. The injection barrel produces localized turbulence during the entry of the sample into the settling column because the soil enters with a small relative velocity which allows the sample to break down into its component aggregates. This turbulence may result in a loss in sensitivity in the discrimination of settling velocities; however, in such cases the style of device to be preferred is the self-inverting cup which is now described. The self-inverting cup was designed for granular materials where adhesion of the sample to the introduction device is not a problem. The cup, shown in Figure 3, uses a small battery-powered electromagnet to lift the locking bar which results in the unstable cub inverting so releasing the sample into the settling tube. A minimum of turbulence is produced by this inversion. Selection of the appropriate sample introduction device depends upon the nature of the sediment material. Where a sample is likely to adhere to a glass surface, the injector barrel is to be preferred. However, if a trial requires an absolute minimum of turbulence in the tube, the self-inverting cup is to be preferred.

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6 2.3 The Collection Turntable The turntable base consists of a 700 mm diameter by a 100 mm deep tray constructed of p.v.a. plastic. Figure 4 illustrates twenty copper sub-sampling trays submerged in the base tray. The sub-sampling trays are 10 mm deep and the tapered plan shape shown in Figure 4 allows them to fit closely together such that all settling sediment is collected.

7 The base tray, supported by a rigid turntable, turns freely in a bearing to enable sub-samples to be taken from the exit of the settling tube. Water in the collection tray provides a seal to the exit of tube to prevent air entering and displacing the hanging column of water in the settling tube. 3. Principle of Operation Settling is initiated by lifting the tube from the foam pad which activates the sample introduction device (in the case of the injection barrel). Alternatively, the self-inverting cup is triggered by powering the electro-magnet once the tube is positioned above the first sampling tray. As the sediment settles out the turntable is rotated so that the first sub-sampling tray is located below the tube exit. The turntable is turned at predetermined time intervals to divide the settling sediment into settling velocity classes. On completion of the experiment, the settling tube is returned to the foam pad and the introduction device removed for cleaning. Excess clear water can be decanted from the samples before oven-drying and weighing. The settling tube is filled with distilled water with a temperature in the range of 15 to 25 C. The air temperature surrounding the settling tube should be very similar to that of the water in the tube to prevent the formation of convection currents at the boundaries of the water column. A temperature difference of 2 C between surrounding air and tube water appeared to have no appreciable effect on measured settling velocity characteristics. Gibbs, Matthews and Link (1971) present tables describing the variation of settling velocities with water temperature. Where distilled water is in short supply (typically 12 liters are required per trial), experiments will be required to assess the suitability of the available water supply for the apparatus. A comparison of such test results with those for identical samples settling in distilled water will quantify the scale of error. Such comparisons will be specific to the water-soil combination. Gibbs et al 91971) conducted a limited number of such trials for glass beads settling through water of varying salinity. However, it must be pointed out that Gibb s work will be of limited utility in this application since no consideration is given to the effect of water quality on the degree of dispersion induced or suppressed.

8 4. Sample Pre-Treatment In discussing the pre-treatment of samples for wet sieving, Grieve (1979) comments that the initial condition of the sample is more significant than the choice of the test itself. Similarly, the appropriateness and consistency of sample pre-treatment is equally important for tests conducted with the Griffith Tube. Varying initial moisture contents and rates of wetting have been investigated by a number of workers (eg. Low 1954 and Russell and Feng 1947). Emerson and Grundy (1954) clearly demonstrated the effects of

9 varying rates of wetting on aggregate stability. Grieve (1979) present s a review of the appropriateness of such pre-treatment to different applications. The Griffith Tube permits a range of pre-treatments to be employed. Consideration of the intended use of the data will dictate the particular sample preparation adopted. For instance, flood wetting or immersion is generally appropriate where erodibility and deposition characteristics of an in situ soil are under consideration (Kemper, 1965). However, if the sample under test has already been eroded the sample should be more gently wetted (eg. under tension to prevent further aggregate breakdown). 5. Presentation of Data Table 1 shows the calculation procedure for obtaining settling velocity results and Figure 5 shows the settling velocity curve of the same sample, with percent slower than plotted versus settling velocity on a logarithmic scale. This technique permits comparison of the settling characteristics of the sample independent of the total initial sample size. Table 1. An Example of the Calculation Procedure Sample Identification: Greenmount Bay 2 Sample Date: Random Surface Sample Test Date: Initially Crumbly Moist (see ratio below) Pre-Treatment: Shock Wetted within Barrel Replicate No.: 1 Room Temperature: 17.4 degrees C Water Temperature: 16.2 degrees C Duplicate Identification No.: 43 Initial Total Moist Mass (TMM): grams Calculated Total Dry Mass = (TMM * Duplicate Dry Mass/Duplicate Moist Mass = * ( )/6.433 Sample Time (secs) Sub-Sample Number Jar Mass (grams) = grams Jar and Soil (grams) Difference (grams) Cumulative Read Down (grams) % Slower Than

10 6. Length of Trial As previously mentioned, the Griffith Tube may be used to complement the data for fine tractions provided by pipette or hydrometer analysis. From Stoke s equation, a particle of size 60µm and specific density of 2.65 would settle at the rate of 3.3 millimeters per second in water at 22 C. This corresponds to 605 seconds to settle 2 m. Therefore, where pipette or hydrometer analysis is being employed to measure the <60µm fraction of a sample, this time would correspond to the cut-off time of the last sample to be tested each twenty minutes including the time to refill the tube and remove the samples. The Griffith Tube is also capable of measuring settling velocity distributions of aggregates smaller than 60 microns; however, this is not recommended because of the long time periods for these aggregates to settle two meters. 7. Comparison with Bottom Withdrawal Tube Rose et al (1983) suggests that the Bottom Withdrawal Tube technique (Lovell and Rose, 1984) is appropriate in obtaining the settling velocity data required as an input to the deposition component of Rose s model. The Bottom Withdrawal Tube technique mixes the sample in the length of the tube in order to simulate the deposition characteristics of an initially uniform suspension of sediment in a body of water. Calculations involved in the processing of the Bottom Withdrawal Tube data accumulate more error than the Griffith Tube due to the use of a correction factor and the graphical interpretation of the Oden curve (Lovell and Rose, 1984). The Griffith Tube differs by allowing each of the aggregates to fall through

11 water with only sediment of the same fall velocity. That is, few individual aggregates pass slower-settling aggregates in their descent. In the Bottom Withdrawal Tube such overtakings and the associated interactions do occur and the frequency of these interactions is governed by the concentration within the tube. A complete experimental comparison of the results obtained with the Griffith Tube and the Bottom Withdrawal Tube is presented in Lovell and Rose (in prep.). 8. Summary The Griffith Tube provides a simplified and accurate technique for obtaining the settling velocity distributions of soil samples. It provides data, useful in applications such as deposition modeling, for soil in aggregated and non-aggregated states. Acknowledgements The authors gratefully acknowledge the assistance of Mr. Jim Mollison in the conduct of reproducibility trials and the skilled assistance of Mr. Norman Allaway in the construction of the apparatus. References Allen, J.R.L. (1982) Developments in sedimentology sedimentary structures their character and physic basis. Vol Elsevier Sc Pub Co. Black, C.A. et al. (1965) Methods of soil analysis. Volume 1 Physical and mineralogical. Amer Soc of Agronomy, Madison Wis. Bryan, R. (1968) Developments of laboratory instrumentation for the study of soil erodibility. Earth Sc Jnl 2(1), Childs, E.C. (1969) An introduction to the physical basis of soil water phenomena. Wiley and Sons, London. Clement, C.R. and Williams, T.E. (1958) An examination o the method of aggregate analysis by wet sieving in relation to the influence of diverse leys on arable soils. Jnl Soil Sc 9(2), Emerson, W.W. and Grundy, G.M.F. (1954) The effect of rate of wetting on water uptake and cohesion of crumbs. Jnl Agric Sci 44, Emery, K.O. (1938) Rapid method of mechanical analysis of sands. Jnl of Sed Petrol 8(3) pp Felix, D.W. (1969) An inexpensive settling tube for analysis of sands. Jnl Sed Petrol 36(2), Gibbs, R.J., Matthews, M.D. and Link, D.A. (1971) The relationship between sphere size and settling velocity. Jnl Sed Petrol 41(1), Grieve, I.C. (1979) Soil aggregate stability tests for the geomorphologist. British Geomorphological Research Group technical bulletin no. 25, 28 pp.

12 Inter-agency Committee on Water Resources (1957) The development and calibration of the visualaccumulatio tube. Report no. 11 St. Anthony Falls Hydraulics Laboratory Minnesota, 109 pp. Knisel, W.G. (1980) (Ed.) CREAMS: A field-scale model for chemical, runoff and erosion from agricultural management systems. USDA Conservation research Report no 26, 643 pp. Lovell, C.J. and Rose, C.W. (1984) Determination of soil aggregate fall velocities: effect of concentration. Paper presented at Conference on Agricultural Engineering, The Institution of Engineers Australia, Bundaberg, Lovell, C.J. and Rose, C.W. (in prep.) Measurement of the settling velocity of soil aggregates. Part 2: The effect of sample pre-treatment and experimental technique. Low, A.J. (1954) The study of soil structure in the field and in the laboratory. Jnl of Soil Sci 5(1), Puri, A.N. (1934) A siltometer for studying size distribution of silts and sands. Punjab Irrigation Institute Research Publication 2(7), 10 pp. Riley, S.J. and Bryant, T. (1979) The relationship between settling velocity and grainsize values. Jnl Geol Soc Aust 26 pp Rose, C.W., Williams, J.R., Sander, G.C., and Barry, D.A. (1983) A mathematical model of soil erosion and deposition processes: 1. Theory for a plain land element. Soil Sci Soc Am Jnl 47(5) Rose, C.W. (1985) Developments in soil erosion and deposition models. In: Advances in soil science Vol 2. Springer-Yerlag Berlin 63 pp. Russell, M.B. and Feng, C.L. (1947) Characterization of the stability of soil aggregates. Jnl paper J-870 Iowa Agric Experimental Stat, Ames, Iowa. Schlee, J. (1966) A modified Woods Hole rapid sediment analyzer. Jnl of Sed Petrol 36(2), Watson, R.L. (1969) Modified Rubey s Law accurately predicts sediment settling velocity. Water Resources Res. 5, Yoder, R.E. (1937) A direct method of aggregate analysis of soils, and a study of the physical nature of erosion losses. Jn Amer Soc of Agron 28 pp

13 Appendix 1: List of Equipment Required a. Settling tube (see Apparatus Description and Figure 1) b. Sample Introduction Device (see Apparatus Description and Figures 2 and 3) c. Collection Turntable (see Figures 1 and 4) free-turning on central vertical axis a rigid circular base 700 millimeters in diameter a p.v.a. circular tray of the same diameter and 70 mm deep. d. Oven capable of heating to 105 C, plus drying trays and gloves. e. Balance capable of weighing samples to three decimal places and a capacity of at least 100 grams f. 20 tapered sampling trays (see Figure 4) 15 millimeters deep with joining seals g. A closed-cell foam pad (to fit inside one of the trays) to seal the tube during filling h. Sets of twenty weighing beakers (optional). For multiple tests 5 sets of beakers per tube will usually suffice. i. A small quantity of vacuum grease for O-rings. Also useful for temporarily sealing the foil flap on injection barrel in place prior to the sample release at t=0 j. A riffle box to split initial sample to required size See notes on applicability in section a2.2.1 Sub sampling. k. A 63µm sieve (if sample is in suspension) l. A stopwatch m. A supply of distilled water to fill the tube If several runs are anticipated it is advisable to store sufficient water overnight to allow the water temperature to stabilize. n. A facility for emptying the tube o. A wash bottle used in preparing samples

14 Appendix 2: Trial Procedures The following pages outline the procedures used in operating the Griffith Tube. As discussed in the Apparatus Description, there are options in the pre-treatment and the sample introduction device employed. These options are presented below as numbered choices of a particular step; for example, steps d1 or d2 for the two types of sample introduction device. The step-by-step procedure presented here is intended as a guide only. A2.1 Tube Preparation a. The sampling trays are arranged in the turntable both as shown in figure 4. The closed-cell foam pad is placed in one of the trays and the tube lowered onto the pad to seal the tube during filling. b. The tube is filled with water in the temperature range 15 to 25 C and the water temperature is allowed to temperature stabilize (as discussed in section 2). c. The trays in the turntable bath should be submerged by at least 10 mm of water to seal on the bottom of the tube during the conduct of the trial. A2.2 Sample Preparation As discussed in section 4, pre-treatment of aggregates generally falls into one of the following categories: shock wetting, slow wetting and no pre-treatment required. The sample preparation follows the sequence: split sample to the required size, wet (if at all) at the required rate and place in the appropriate sample introduction device. A2.2.1 Sub-Sampling The field sample is gently split to obtain a representative sample of approximately 10 grams of oven dry mass. For a sample in a dry granular condition, use a riffle box. Where the sample is in a more cohesive state, it may be appropriate to cone and quarter the sample or use a sheet roll and split technique as described in sampling a dry mass between 9.5 and 10.5 grams (an estimate of the soil moisture content may first be required but with a little practice this can be estimated to the required accuracy). A suspended sample could be split using a riffle box; however, further investigations as to the most appropriate technique are required. All types of sediment require a duplicate sample so that the moisture content of the sediment can be measured (and recorded on Table 1). A2.2.2 Sample Wetting Shock Wetting The empty Sample Introduction Device is placed upon the balance and a small quantity of distilled water is poured into the introduction chamber (to a depth of approximately 2 mm). This ensures that the sample does not adhere to the bottom of the chamber and provides even wetting of the sub-sample.

15 The balance is tared and the sub-sample introduced. The mass is recorded in the table as Total Moist Mass (as shown in Table 1). The remainder of the chamber is filled with distilled water and the plunger is set such that the barrel is brimming with water. The aluminum flap, attached permanently as a hinge point is then placed over the chamber and temporarily sealed in place with vacuum grease. The injection barrel is then gently placed in the top of the tube. For the self-inverting bucket, the sample is simply placed in its cradle upright with the locking bar in place. This may now be gently lowered into the top of the tube ready to commence the trial. Controlled Wetting A range of controlled wetting pre-treatments were reviewed by Grieve (1979). Slow wetting on a tension plate and in a humidifier are two popular methods. Transferring the wetted sample to the introduction device must be made with a minimum of disturbance, therefore the use of a filter paper in the wetting procedure should be avoided. Preparation of a sample in suspension Samples collected in the field frequently are saturated (e.g. runoff samples). To permit the testing of such samples in the Griffith Tube the sediment must be gently separated from the suspension. This can be simply performed by pouring the sample through a 63µm sieve, using distilled water to wash the sample vessel. Some material finer than 63µm will pass through the of the Griffith Tube. If pipette or hydrometer analysis are planned, retain the < 63µm fraction. If the prime use for the tube is to be the testing of suspended samples then a device which permits whole sample introduction must be considered. It is envisaged that such a device would consist of a two-chambered apparatus similar to the barrel of the Visual Accumulation Tube (Inter-agency Committee on Water Resources, 1957). A2.3 Trial Conduct Samples may be taken at times to suit either the results required and/or the settling characteristics of the sample. For instance, the sample times presented in Table 1 are suitable for a sample containing rapidly-falling water-stable aggregates. The times are arranged in an approximately logarithmic schedule to obtain a total of twelve samples. A finer division of the samples may be required for some purposes. Step A1. For the sample injection system trial time t=0 corresponds to the lifting of the tube from the foam pad. The mass of the column of water draws the sample into the tube and settling commences. Step A2. for the self-inverting cup, the trial is commenced by moving the turntable such that the tube lies above the first empty sampling tray. The circuit is closed on the electro magnet so that the locking bar lifts and the cup overturns. The time of overturning is t=0. Step B. At the sample times (see Table 1), the turntable is rotated so that the next tray is beneath the exit of the tube. Rotation of the sampling trays by hand proved to be a straightforward task.

16 Step C. Step B is repeated at the specified intervals until the trial is completed. Step D. The joining seals between the trays are removed in the waterbath and the samples gently lifted from the bath in order. Some water can be decanted from each of the trays before drying. Where only one set of trays is available, it may be necessary to transfer the contents of the trays to other drying containers. In either case, the oven dry mass of the oven-bound container must first be obtained and entered in Table 1. Any washing necessary in transferring the sample must be with distilled water. Step E. After drying, the jar and soil mass is recorded. It is suggested that the containers be weighed in the same order each time with approximately the same oven to balance time to avoid moisture absorption errors. Step F. Calculations are made as per table 1 and the settling velocity curve may be produced as per Figure 5.