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International Conference on Case Histories in (1988) - Second International Conference on Geotechnical Engineering Case Histories in Geotechnical Engineering

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A Proposed Procedure for the Identification of Dispersive Soils

H. J. Von M. Harmse University of Potchefstroom for Christian Higher Education, Potchefstroom,

F. A. Gerber Department of Water Affairs, Pretoria, South Africa

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Recommended Citation Von M. Harmse, H. J. and Gerber, F. A., "A Proposed Procedure for the Identification of Dispersive Soils" (1988). International Conference on Case Histories in Geotechnical Engineering. 3. https://scholarsmine.mst.edu/icchge/2icchge/2icchge-session3/3

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This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. Proceedings: Second International Conference on Case Histories In Geotechnical Engineering, June 1-5,1988, St. Louis, Mo., Paper No. 3.14 A Proposed Procedure for the Identification of Dispersive Soils H.J. Von M. Harmse F.A. Gerber Professor of Pedology, University of Potchefstroom for Christian Hydrologist, Department of Water Affairs, Pretoria, South Africa Higher Education, Potchefstroom, South Africa

ABSTRACT: The piping failure of the dam and many other small dams in South Africa, despite the use of apparently sound material and good control during construction, emphasizes the need for a method to unambiguously identify dispersive soils. Physical and chemical tests of one hundred and seventy soil samples were evaluated against the double hydrometer method, after removal of free salts. The chemical methods based on characterization of the exchange complex (CEC and ESP) gave consistently more reliable results than the physical tests, such as the pinhole, crumb and sticky point tests and even the double hydrometer test when free salts are not removed.

INTRODUCTION cl uded that the cause of fai 1 ure was dis­ persive clays. Subsequent and earlier in­ vestigations confirmed that piping failure in The Senekal dam, an off-channel storage re­ small dams and the soil erosion associated servoir, is situated in the ap­ with dispersion is of widespread occurrence proximately 3 km upstream from the town of in South Africa (Donaldson, 1973; Harmse, Senekal in the Orange , South Af­ 1973). The piping failure of the Senekal dam rica. and many other sma 11 dams, as we 11 as the influence of dispersion on the erodibility of The country rock underlying the alluvium, soils, emphasized the need for a method to which is also the parent material of the identify soils which may be susceptable to soils used for construction, compromises dispersion unambiguously. mustones and arkosic sandstones of the Ade- 1 aide Subgroup (Beaufort Group l, of the Karoo Sequence. The foundations are situ­ ated on alluvial soils and the wall was con­ structed using the B-horizon and subsurface DISPERSION horizons of a solonetzic soil excavated from a borrow pit 500 m away. The materia 1 was blended and the moisture content and den­ Although the behaviour and characteristics of sity were checked at regular intervals du­ dispersive soils are reasonably well under­ ring construction (Wagner, et !l• 1981). stood and adequately explained by the double 1 ayer theory (Bolt and Bruggenwert, 1976 l, a The earth wall which has a maximum height of satisfactory and analytical method for the 8 m, is 1100 m long and contains a reservoir identification of dispersive soils remains a of approximately 19 ha with a capacity of problem. This is probably mainly due to the fact that dispersion is a_ physical manifesta­ 1.4 x 10 6 m3 (Wagner, et !l· 1981 l. tion of chemical and mineral-dependent phy­ sico-chemical properties of soils (Green­ land and Hayes, 1978). The relative influ­ Fi 11 i ng of the reservoir commenced in No­ ences of these factors has not been con­ vember 1974 by pumping water into the dam, sidered in the past, especially the influence but was stopped a week later when leaks were of the clay mineralogy. detected on the downstream toe of the em­ bankment. The water level was about 3 m a­ The clay mineralogy of soils is the result of bove the floor. The flow increased rapidly several factors acting upon the parent mate­ and failure through piping occurred approxi­ rial, such as climate and associated intensi­ mately four days later (Wagner, et al, ty of weathering. Accumulation and loss of 1981). A detailed geotechnical investiga­ the products of weathering within soil pro­ tion to determine the cause of failure was files are often related to their position in initiated by Jones and Wagner during 1975. the landscape, vegetation and the duration of The results of the geotechnical investiga­ these weathering influences. tion are described in detail by Wagner, et al, (1981). After an initial physical, chf= Only for a limited number of combinations of 'iiilcal and physical-chemical analysis of 9 conditions do soi 1 forming factors and pro­ samples from the core (Table ll it was con- cesses have a uniquely determining effect on

411 Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu the clay mineralogy. The clay mineralogy of from broken edges and protoni zati on of soils therefore depends only rarely upon the exposed oxygen and OH-groups. direct influence of the parent material from which it developed. Most of the physical 3. Oxides. hydrous oxides and hydro xi des of properties of mineral soils, including dis­ iron and aluminium of tropical areas with persion, hydraulic conductivity, swelling low net negative charge or cation and erodibility, can be related to the interaction between the composition of the exchange (1-5 melOOg-l of clay). Dis­ soil solution and the clay mineralogy (Low, persion is expected to be virtually non 1968; Bolt and 8ruggenwert, 1976) . existent in natural soils containing

TABLE 1. Average physical and chemical properties of 19 Samples from the core of the Senekal dam

Chemical Properties Index Properties Grading pH EC ESP LL PI S11U ClayS * **

Ava rage 8,3 14,6 24,8 42,5 26,5 31 53 Std. De vi at ion 0,4 50,4 12,5 10.1 7,7 5.1 9,8 Coeff. Deviation 0,06 0,35 0,50 0,24 0,29 0,16 0,19

* EC • Electric conductivity in mSm- 1 ** ESP Exchangeable sodium X 100 C£C

In general, soil clays are mixtures of mine­ these minerals as dominant components of ralogically related clay components. the clay fraction. Some of these clay components, however, have This case history reports on an investigation an irregular structure that makes unique of the phenomena of dispersion to determine identification in terms of the mineralogical an unambiguous method for the positive iden­ components difficult. Futhermore, identifi­ tification of dispersive soils. cation of the minerals in the clay fraction of the soil requires complex procedures and The approach adopted was to apply and expensive equipment. It can11ot be app 11 ed evaluate the results of various dispersion for routine analysis of large numbers of tests in order to investigate the interaction samples, especially in semi-arid countries of soil properties. This approach resulted where mixed layer minerals and dispersive in a reduction of the number of tests and led soils are widespread. The cation exchange to the selection· of a limited number of re­ liable methods which give consistent results capacity CEC, expressed · in melOOg-l of for the identification of dispersive soils. clay), or net negative charge on the col­ The results of these tests are also in­ loidal fraction of mineral soils may, how­ dispensable for the determination of the ever, be used to define broad categories amount of gypsym required for the reclamation within which certain minerals in the soil of dispersive soils. could be expected to have related properties (Dixon ·and Weed, 1977). This can be applied during routine analysis for the positive identification of dispersive soils. These categories are: METHODS AND MATERIALS 1. The 2:1 types of phy11 osi 1i cates which include hydromica, vermiculite, chlo­ One hundred and seventy soil samples ranging rites and smectites. The CEC (40-150 me in CEC lOOg-l clay from 5 to 140, and in ESP 100g-l clay) of these clay minerals is values from 0 to 20 per cent, were selected mainly derived from ionic substitution and prepared and subjected to both physical in either the octahedral or tetrahedral and chemical methods of evaluation. Cation layer, or both. exchange capacity, exchangeable cations, par­ ticle-size distribution and electric con­ 2. The 1: 1 types of phy11 osi 11 cates. such ductivity were determined by standard as kaolinite and halloysite, with inter- procedures (Hess, 1971). mediate to low CEC-values (5-40 meioog- 1 The results of nine physical and five of clay). The negative charges of these chemical dispersion tests were evaluated. clays are pH dependent and arise mainly

Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu From these fourteen methods, four were From figure 1 it is evident that variation selected on the basis of consistency of the results (Gerber and Harmse, 1986). in clay mineralogy (CEC lOOg-l of clay) influences the ESP values at which total The four methods chosen were: dispersion will occur. All samples with ESP values above 15 per cent were dispersive 1. The sodium concentration in a saturated (Figure 1). paste (Sherard, Dunningan and Decker, 1972). Soils with low cation exchange values of 15 2. The percentage exchangeable sodium (ESP method) (Sherard, Decker and Ryker, me 'IOOg-l of clay were completely non-dis­ 1972). persive at ESP-values of 6 (Figure 1). Soils 3. The ESP + PEMg method (PEMg = percentage with high CEC-values and plasticity indices exchangeable magnesium) (Harmse, 1980). greater than 35 (PI), swell to such an extent 4. The percentage dispersion or double that the influence of dispersion on the hydrometer method (Anderson, 1951; van stabi 1 ity of structures are expected to be Zyl, 1973) was used as a criterium for insignificant. This explains why these soils dispersion after removal of free salts. can be used for hydrosealing if judiciously This method was selected because of the placed. From the research it became evident fact that the amount of clay in sus­ that the results of the physical methods, pension in distilled water is a measure namely the pinhole, double hydrometer, sticky of dispersion (Anderson, 1951 l and be­ point and crumb tests, will be invalid if the cause of the consistency of the results. water quality is not considered, or if the soil solution contains soluble free salts. This is due to the influence of free salts in the soil and water on dispersion (Bolt and Bruggenwert, 1976) (Figure 2). The inability RESULTS AND DISCUSSION of these methods to identify dispersive soils unambiguously was proven in all instances where free salts were present, ~hich is more During the re-evaluation of 67 of the sam­ often than not the case with sodium saturated ples, 46 of which were dispersive (Table 2) natural soi 1 s (Gerber and Harmse., 1986 l. and 21 non-dispersive (Table 3) by the dou­ ble hydrometer method, attention was also given to the influence of clay content, CEC value, phosphate, pH, electric conductivi­ ties and type of cation adsorbed on the ex­ change complex. The curves shown in Figure 1 were estab 1 i shed from these resu 1ts. The " influence of free salts in the soil solution on the flocculation of dispersive soils is shown in figure 2. The threshold values of free salts at which flocculation occurs are also clearly dependent upon the clay mine- ralogy as manifested in the CEC lOOg-l clay.

----cec-- 1oo1-1 Clcyl5-201me

TO 12 14 16 11 20 22 VERY DISP'I!RSIYE .. SODIUM ADSORPTION RATIO ISARI • !SELF HEALING IP (Na•) c~r"' '

FIGURE 2. The influence of free salts with a high sodium content on floccula­ tion and dispersion as influenced by the composition of the colloidal fraction of mineral soils.

40 10 fO 11)0 110 CATION !XCHAMGE CAP!

Determination of CEC by the summation of extractable cations (Sherard, Dunningan and FIGURE 1. Diagram illustrating the influence Decker, 1972) may lead to conflicting re­ of composition of the clay fraction sults. This method can be invalid if soluble salts occur in the soil solution or if the as manifested in CEC lOOg-l of clay base saturation of the soil is less than 100 and ESP on dispersion (according to per cent. Application of the results re­ all four methods evaluated). presented in figures 1, 2 and 3 enables easy

Second International Conference on Case Histories in Geotechnical Engineering 413 Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu TABLE 2. Analytical data of soils with a percentage dispersion of more than 25% according to the double hydrometer method (free salts removed)

Sample pH EC CEC ESP Clay CEC Dispersion No H20 % (me lOOg-l) % (me) Clay *

A3573 8.1 5,3 21,4 9,6 59 36 26,8 A4767 7,2 6,5 16.7 14,6 60 28 27.1 A4813 8,3 22,8 26,6 11.9 63 42 28,5 A4773 7,8 22,3 10,9 14,2 65 29 29.1 A2514 7,8 10,0 11.4 12.7 68 17 29,6 A3770 8,6 11 • 5 27,6 8. 1 44 63 32,7 A3731 7. 1 7,7 17.9 17.2 46 39 33,0 A2472 6,8 2,7 9,4 8,2 44 21 33,5 A3204 8,0 8,4 25,3 11 • 6 62 41 36,7 A2744 7' 1 6,3 29,0 1 3. 1 36 81 38,5 A4753 8,4 24,0 24,8 12,0 65 38 39,0 A4594 8,3 28,3 23,3 13 '6 65 36 39,4 Al565 7,2 5,4 9,0 7,6 53 17 41 • 9 A3946 8,2 15,5 25,6 6,9 43 60 43,0 A4729 7,4 11 • 9 18,7 7,4 64 29 43,8 A3289 6,8 3,2 21 ,4 7,6 54 40 43,9 A3633 s;8 8,6 18,7 7,6 36 52 44,0 A4688 7,5 19,0 22,4 10' 9 61 37 49,5 Al654 9. 1 10,5 28,0 8. 1 44 64 49,5 A3005 7,7 4,4 27' 1 4,8 45 60 50,0 Al535 8,2 13,9 25,6 11 • 1 60 43 52,8 A3908 8,2 10,6 37,3 7,9 63 59 54,8 A2884 7,3 4,3 17,2 19,0 65 26 56,3 A2754 7,8 8,5 30,9 5,0 45 69 57,9 A3184 8,31 6.1 24, 1 5,7 46 52 59,3 A2804 6,8 5,3 20,0 16,5 50 40 59,8 A3755 9,0 10,8 26,1 15,5 55 48 60. 1 A4793 7,7 17,5 22,0 17,6 55 40 60,8 A3514 8,4 10,5 35,4 14,9 49 72 63,6 Al592 9,2 40,6 41 ,'6 11 '6 65 64 64,9 A3603 8' 1 18,9 18,3 16,5 64 29 66,7 A2674 6,9 8,7 20,0 18,7 52 38 69,2 Al655 9,1 13,0 31 '8 6,7 42 76 69,7 A3425 8,7 20,4 28,6 15,0 58 49 70,9 A4744 7,8 10,5 13,4 11,0 53 25 74,6 A2524 8,8 8,8 24,7 11.7 41 60 80,0 A2764 7,4 8,5 14,2 22,4 57 25 86,2 A4693 8,1 17,8 18,7 20,0 61 31 86,2 Al595 9,3 64,5 21 '2 36,6 65 33 93,9

EC Electrical conductivity in mSm- 1_ CEC Cation e~change capacity (melOOg 1 sot 1) ESP _ - Exchangeable sodium percentage _ CEC lOOg 1 clay- Cation exchange capacity per lOOg of clay (me lOOg 1) * percentage dispersion (double hydrometer method, free salts removed)

and unambiguous determination of the dis­ fl'uence as sodium. The effect of magnesium persive potential of a soil, even in water alone and of magnesium with sodium on dis­ containing free salts. The following pro­ persion has not been quantified succesfully cedure should be followed: nor has the influence of organic material been researched but should receive further 1. With the aid of figure 3 determine if attention. the soil is dispersive. 2. Determine the degree of dispersivity with the aid of figure 1. The presence of lithium on the adsorption complex can be expected to have the same in-

Second International Conference on Case Histories in Geotechnical Engineering 414 Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu TABLE 3. Analytical data of soils with a percentage of dispersion according to the double hydrometer method (free salts removed) of less than 25%

Sample pH EC CEC ESP % Clay CEC lOOg-l % Dispersion No H2o * * * * clay * *

A5740 4,9 3,0 6 ' 1 4,9 35 l 7 12' 3 A5750 5 '8 7,9 20, l 2, 2 47 43 12,8 Al670 7. 2 4,5 1 7. 5 3, 2 47 37 14. 2 A3268 5 '6 5,4 17,4 2' 5 60 29 14,4 A3428 6,4 7,5 23,0 l • 7 45 51 14,8 A3368 5,9 8,7 15, 2 3, 1 46 33 15, l A3313 5. 7 2,7 l 7 '2 2,4 62 28 16,7 A239l 7. 3 4,0 22,7 2,5 30 76 20,0 A2715 6,8 14,5 28,4 2,8 62 43 18,4 A3549 6. 5 6. 3 19' 8 2, 2 60 33 18.9 A2854 6. 5 9,6 23,0 2,3 68 34 19,0 A239l 7 • 9 l 0. 0 23,4 2 '9 32 73 17,5 A3134 7,4 6,5 22,2 2,4 31 72 20,0 A2300 6,6 5,2 2 3' l 2,4 40 58 20,9 A3613 6,0 9,5 25,6 2,0 51 50 21 , 8 A3444 6,4 8,4 18,8 2,3 69 27 22,4 A5749 6' 6 7,9 10,0 3,7 41 24 24,2 A2726 6, 2 8,0 l 7, 6 2,4 58 30 25,0 A3074 7, 2 8,0 19,2 3,4 60 32 25,6 A2814 6. 3 5,5 21 , l l • 8 54 39 26,2

* For explanation of abbreviations see Table 2.

of soils. It is therefore logical that these factors be considered in evaluating the potential of soils to disperse to the degree where it would render earth structures un­ safe. The results of the analytical proce­ dures proposed in this study, lead to the conclusion that the tendency for the poten­ tial of soils to disperse, especially those containing free salts, may be evaluated more consistently with the aid of physical-che­ mical methods. The results should enable in­ vestigators to identify potentially dis­ persive soils with greater reliability and to determine the amount of gypsum required for reclamation of dispersive soils readily.

ACKNOWLEDGEMENTS

The faci1 ities of the Hydrological Research FIGURE 3. Proposed procedure for identi­ Institute, Department of Water Affairs were fication of dispersive soil used for this research. This paper is "pu­ from which free salts were not blished with the permission of the Depart­ removed (Harmse, 1980). ~ent of Water Affairs.

CONCLUSION REFERENCES

Dispersion is a phenomenon which is mainly Anderson, H.W. 1951. Physical charac- controlled by the chemical and mineralogical teristics of soils related to ero~ion Soil composition as well as the free salt content cons. J. 13. pp 129-133. Bolt, G.H. and Bruggenwert, M.G.G. 1976. Soil chemistry.

415 Second International Conference on Case Histories in Geotechnical Engineering Missouri University of Science and Technology http://ICCHGE1984-2013.mst.edu Basic elements. Elsevier Sci. Amsterdam. Wagner, F. von M., Harmse, H.J. von M., Stone, P. and Ellis, 1981. Chemical Dixon, J.B. and Weed, S.B. 1977. Minerals w. in the soil environment. Soil Sc1. Soc. treatment of a dispersive clay reservoir. Of ifriler'i'"'C"a,'" Made son. Proc. of the lOth Conf. on Soil Mechanic­ Donaldson, G.W. 1973. Piping failures in sand Found. Eng., Stockholm, pp 285-791. small earth dams in South Africa. Symposium. Transportation and the en­ vtronment, Durban. Ann. S.A. Inst. of Civil Eng. Gerber, F.A. 1983. Evaluering van die fi­ sies-chemiese eienskappe van dispersiewe grond en die metodes vir identifisering van dispersiewe grond. M.Sc. Thesis, 1983, PU for CHE (Unpublished), pp 127.

Gerber, .F.A. and Harmse, H.J. von M. 1986. 'n Evaluering van die fisies-chemiese ei enskappe van di spersi ewe grond en die metodes vir identifisering van disper- siewe grond. Technical report no. TR · 123. Dept. of Water Affairs, pp 136.

Greenland, D.J. and Hayes, M.H. 1978. The chemistry of sci 1 constituents. WiTeY !ntersc1ence, New York. Harmse, H.J. Von M. 1973. Die invloed van die tipe geadsorbeerde katione en die grondoplossing op die erodeerbaarheid van gronde. Symposium Transportation and the environment, Durban. Ann. S.A. Inst. of Ci vi 1 Eng. Harmse, H.J. von M. 1980.Dispersiewe grond, hul ontstaan, identifikasie en stabili­ sasie. Ground Profile No. 22, pp 10-31. Hesse, P.R. 1971. A textbook df sci 1 che- mical analysis. John Murray Publ s. London. Jones and Wagner. 1975. Geotechnical in- vestigation on dam fai 1 ure at Senekal. Consulting Engineers report to Cahi & Gertenbach, J. & W. report No. G2/2/75. Low, P.F. 1968. Mineralogical requirements in soil physical investigations. Special Pub. No. 4, SSSA. Sherard, J.L., Decker, R.S. and Ryker, N.L. 1972. Piping in earthdams of dispersive clay. Proceedings of the speciality con­ ference on the performance of earth and earth supported structures. Purdue Uni­ versity. Published by ASCE. Sherard, J.L., Dunningan, L.P. and Decker, R.S. 1972. Identification and nature of dispersive soils. Geot. Eng. Div. J. 162 pp 278-301.

van Zyl, D.J.A. 1973. Gronderosie deur water: 'n Fisiese proses. Annale Vpn die vyfjaarl i kse konvensi e van die Sui d- Afrikaanse Instituut vir Siviele In- genieurs. · Ver.voerwese en Omgewing, pp 16-25.

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