SOILS AND FOUNDATIONS Vol. 47, No. 5, 887–896, Oct. 2007 Japanese Geotechnical Society RELATIONSHIP BETWEEN THE ATTERBERG LIMITS AND CLAY CONTENT ENNIO POLIDORIi) ABSTRACT This study investigates the liquid limit (Casagrande's method) and plastic limit (rolling and thread method) of six in- organic soils and their respective mixtures with ˆne silica sand. It was observed that the liquid limit and plastic limit values of the mixtures tested, except those with a low clay percentage, are linked to the respective clay size contents by a linear relationship. The Atterberg limits were subsequently recalculated using the equations of the regression lines of the mixtures governed by linear law with the clay percentages. The plotting of the plastic limit as a function of the liq- uid limit of these data made it possible to determine the relationship among the liquid limit, the plastic limit and clay fraction valid for inorganic soils that contain platey clay minerals and for clay size contents that are not too low. Hence, on the basis of the interdependence among the parameters considered (WL, Wp, Ip, CF, A), for a given inorgan- ic soil, knowing only two of three parameters (WL, Wp, CF ) that are measurable using standard tests, the values of other three parameters can be obtained. Key words: Atterberg limits, clay, laboratory tests, plasticity, soil classiˆcation (IGC:D1/D3) kedly, while the liquid limit of the clay mineral kaolinite INTRODUCTION is not in‰uenced (Di Maio and Fenelli, 1994). Hence, for If a clayey soil is mixed with ever increasing amounts of a given soil, the values of the Atterberg limits, which are water it becomes softer and softer and a point will be the result of the combination of all the factors, provide reached at which the soil ceases to behave as a plastic insight into that soil's plasticity characteristics for every material and becomes essentially a viscous ‰uid. Atter- possible combination of the factors that in‰uence the berg (1911) suggested a method for deˆning this change, plasticity of a soil. and the water content of the soil at this point is its liquid There are few studies in literature on the Atterberg limit, WL. Likewise, Atterberg deˆned the change from a limits of soils as a function of their clay size contents and plastic to a semi-solid state, and the water content of the the results of these studies are not always in agreement. soil at this point is its plastic limit, Wp. The methods to These studies have focused primarily on the liquid limit determine the liquid and plastic limits, later developed by of clay minerals mixed with silica sand (Seed et al., 1964a, Casagrande (1932, 1958), are considered standard inter- 1964b; Sivapullaiah and Sridharan, 1985; Tan et al., national tests. These limits and the numerical diŠerence 1994; Nagaraj et al., 1995; Kumar and Muir Wood, between them, the plasticity index, Ip, are very useful to 1999). Seed et al. (1964b) in a study on the Atterberg characterize, classify and predict ˆne soils engineering be- limits of the clay minerals kaolinite, illite and montmoril- haviour. lonite and their respective mixtures with sand, concluded The Atterberg limits of a soil depend on its composi- that, for clay percentages which are not too low, the liq- tion (quantity and type of clay minerals) and so-called dy- uid and plastic limits are both linked by a linear relation- namic factors (Veniale, 1983) such as, pH, temperature, ship to their clay size contents. The respective regression cation exchange capacity, type and quantity of cations in lines pass through the origin of the axes. Nevertheless, the the solution, etc., which vary in space and time for natur- plastic limit values reported by Seed et al. (1964b) of the al soils. An example of dynamic variables can be found in montmorillonite-sand mixtures are less than those of the continuous alteration of the environment by human kaolinite-sand mixtures with the same clay percentages. activities, such as the impact of acid rain and chemical Conversely, White (1949) in his study of the Atterberg products used in agriculture. Dynamic factors can have a limits of the most common clay minerals concluded that strong eŠect on the liquid limit value, though such eŠects the plastic limit of montmorilloniteÀ(illite)Àkaolinite. may vary according to the type of clay minerals. For ex- The same conclusion was traced later by Mitchell (1993). ample, as the concentration of salts increases, the liquid Several attempts to link plasticity index with liquid limit of the clay mineral montmorillonite decreases mar- limit, mostly through the empirical correlations, ignoring i) Institute of Applied Geology, University of Urbino `Carlo Bo' `Sogesta' Scientiˆc Campus, Italy (ennio.polidori＠uniurb.it). The manuscript for this paper was received for review on December 11, 2006; approved on May 29, 2007. Written discussions on this paper should be submitted before May 1, 2008 to the Japanese Geotechnical Society, 4-38-2, Sengoku, Bunkyo-ku, Tokyo 112-0011, Japan. Upon request the closing date may be extended one month. 887 888 POLIDORI the content of non-clay particles (À2 mm) are reported in fraction, CF (º2 mm) in each soil was lowered in succes- literature (Casagrande, 1948; Seed et al., 1964b; Nagaraj sive steps by adding sand to obtain changes of 10z and Jayadeva, 1983; Sivapullaiah and Sridharan, 1985; weigth of CF up to a minimum of 10z (for bentonite Panadian and Nagaraj, 1990). ``c''). The silica sample used in the mixtures is composed This study, based on compositional factors (amount of 85z ˆne sand. and type of clay minerals), investigates how the liquid The geotechnical characterization was performed ac- limit and the plastic limit vary as a function of clay size cording to international ASTM standards (D 422 and D contents in inorganic soils with platey clay minerals. On 4318). First the grain size distribution was determined the basis of the average values (using equations of the and then the liquid limit (Casagrande's method) and the regression lines) of the experimental data collected a plastic limit (rolling thread method) of the soils and mix- relationship between the Atterberg limits and the clay fractions is then investigated. Since the non-platey clay minerals such as halloysite, allophane, attapulgite have characteristics very diŠerent from that of platey clay minerals (e.g., high plastic limit, low index plasticity) (Mitchell, 1993), they are excluded from the present research as organic soils are. GEOTECHNICAL AND MINERALOGICAL CHARACTERIZATION Experiments were carried out on six inorganic soils and on their respective mixtures with ˆne silica sand. Three of the soils were composed of bentonite, one was composed of kaolinite (commercially available), another was com- posed of 1:1 mixture of kaolinite- bentonite whereas the Fig. 1. Liquid limit, WL and plastic limit, Wp as function of clay frac- last soil was a natural soil belonging to the Formation of tion CF (º2 mm) of soils and their respective mixtures with ˆne sili- Varicoloured Clays (upper Cretaceous-lower Eocene) ca sand. 1＝kaolinite-sand mixtures; 2, 5 and 6＝bentonite ``a'', Central Italy. Their characteristics are summarized in ``b'' and ``c''-sand mixtures, respectively; 3＝(1:1) kaolinite-ben- tonite ``c''-sand mixtures; 4＝varicoloured clays-sand mixtures Tables 1 and 2. In the soil-sand mixtures (Fig. 1) the clay Table 1. Index properties of inorganic soils used in present study Soil Soil type Sand Silt Clay WL Wp Ip A r 1 kaolinite — 8 92 62 36 26 0.28 2.64 2 bentonite ``a'' 5 20 75 153 36 117 1.55 2.76 3 (1:1) soils 1–6 mixture — 19 81 170 38 132 1.63 2.69 4 varicoloured clays — 14 86 193 41 152 1.77 2.79 5 bentonite ``b'' 3 27 70 260 39 221 3.16 2.75 6 bentonite ``c'' — 30 70 343 42 301 4.30 2.75 3 Sand, silt, clay, WL, Wp and Ip are in z. r (density) is in gWcm . A＝activity. Table 2. Mineralogical composition of inorganic soils used in present study Soil Soil type Fraction size C D Mineralogy 1kaolinite zº2 mm — — Kaolinite (97z) with a good degree of crystallinity and illite (3z) zÀ2 mm — — Aggregations of kaolinite crystals. 2 bentonite ``a'' zº2 mm — — Montmorillonite (100z)withgooddegreeofcrystallinity. zÀ2 mm 3 — Quartz and calcium carbonate. 4 varicoloured clays zº2 mm — — In order of respective quantities: vermiculite with low degree of crystallinity, interbedded illite-vermiculite, kaolinite and traces of illite. Presence of quartz. zÀ2 mm 3 — Quartz and calcium carbonate. 5 bentonite ``b'' zº2 mm — 4 Montmorillonite (96z) with good degree of crystallinity and dolomite. zÀ2 mm 2 — Quartz and calcium carbonate. 6 bentonite ``c'' zº2 mm — 8 Montmorillonite with low degree of crystallinity, interbedded illite- montmorillonite. Presence of dolomite and amorphous ferrous hydroxides. zÀ2 mm 2 14 Dolomite, quartz and calcium carbonate. C (calcium carbonate) and D (dolomite) are in z ATTERBERG LIMITS AND CLAY CONTENT 889 tures with sands. Some standards, in addition to etc.). The combination of these factors provides a very Casagrande's method, have included the fall-cone wide range of slope values (from 0.67 to 4.86 in Fig. 1), method. Both methods have advantages and disavan- where the minimum and maximum values belong to tages. For example, (with reference to this study) the cone kaolinite and montmorillonite in monovalent ionic form, penetration method is not suitable for very expansive respectively. WL and CF are deˆned in percentages. soils (Wasti and Bezirci, 1986). In a saturated soil also composed of non-clay particles, Grain size distribution of the silica sample and the soils assuming that all of the water is associated with the clay was obtained using the sieve and hydrometer methods, phase (Seed et al., 1964b; Mitchell, 1993), the constant respectively.