SOIL FORMATION AND CHARACTERIZATION
2.8 CLA Y PARTICLE-WATE R RELATIONS
Montmorillonite Minera l
Montmorillonite i s th e mos t commo n minera l o f th e montmorillonit e group . Th e structura l arrangement o f thi s minera l i s compose d o f tw o silic a tetrahedra l sheet s wit h a centra l alumin a octahedral sheet . All the tips of the tetrahedra point in the same direction and toward the center of the unit. The silica and gibbsite sheets are combined in such a way that the tips of the tetrahedrons of each silica sheet and one of the hydroxyl layers of the octahedral shee t form a common layer . The atom s common t o both the silica and gibbsite layer become oxyge n instead of hydroxyls. The thickness of the silica-gibbsite-silica unit is about 10 A (Fig. 2.5). In stacking these combined units one above the other, oxyge n layer s o f eac h uni t ar e adjacen t t o oxyge n o f th e neighborin g unit s wit h a consequence tha t ther e i s a very wea k bon d an d a n excellent cleavag e betwee n them . Wate r ca n enter between th e sheets, causing them to expand significantly and thus the structure can break into 10 A thic k structural units. Soils containin g a considerable amoun t o f montmorillonit e mineral s will exhibit high swelling and shrinkage characteristics. The lateral dimensions of montmorillonite particles rang e fro m 100 0 t o 500 0 A wit h thicknes s varying fro m 1 0 to 5 0 A. Bentonit e cla y belongs t o th e montmorillonit e group. I n montmorillonite , there i s isomorphou s substitutio n of magnesium and iron for aluminum.
Illite
The basi c structura l uni t o f illit e is simila r t o tha t o f montmorillonit e excep t tha t som e o f th e silicons are always replaced b y aluminum atoms and the resultant charge deficienc y is balanced by potassium ions . Th e potassiu m ion s occu r betwee n uni t layers . Th e bond s wit h th e nonexchangeable K+ ions are weaker than the hydrogen bonds, but stronger tha n the water bond of montmorillonite. Illite , therefore , doe s no t swel l a s muc h i n th e presenc e o f wate r a s doe s montmorillonite. Th e latera l dimension s o f illit e cla y particle s ar e abou t th e sam e a s thos e o f montmorillonite, 100 0 t o 500 0 A , bu t th e thicknes s o f illit e particle s i s greate r tha n tha t o f montmorillonite particles, 50 to 500 A. The arrangement of silica and gibbsite sheets ar e as shown in Fig. 2.6 .
Soil Formatio n an d Characterization 15
10A
T I I I I
T\ loA
I I I I I I I I I I I I I I I I H T M — Potassium moleculesII I M il I I I I I I III II I
\
1 1 1 1 1 1 1 1 1 1 1 1 I I 1 1 1 1 1 V«— Fairly strong bond Silica sheet Gibbsite sheet
Figure 2.6 Structur e o f illit e layer
behavior o f th e soi l mas s i s profoundl y influence d by th e inter-particle-wate r relationships , th e ability of the soil particles to adsorb exchangeable cations and the amount of water present.
Adsorbed Wate r j
The cla y particle s carr y a ne t negativ e charg e ^ n thei r surface . Thi s i s th e resul t o f bot h isomorphous substitutio n and of a break in the continuity of the structure at its edges. The intensity of the charge depend s t o a considerable exten t on tljie mineralogical characte r o f the particle. Th e physical an d chemica l manifestation s o f the surfac e charge constitut e th e surfac e activit y of th e mineral. Minerals ar e sai d t o have high or low surface activity , depending o n the intensity of the surface charge. As pointed out earlier, th e surface activity depends no t only on the specific surfac e but also on the chemical and mineralogical composition of the solid particle. The surface activity of sand, therefore, will not acquire all the properties of ^ true clay, even if it is ground to a fine powder.
The presenc e o f wate r doe s no t alte r it s properti e
changing it s unit weight. However, th e behavio r o l of coarse r fraction s considerabl y exceptin g ' a saturated soi l mas s consistin g o f fine san d might chang e unde r dynami c loadings . Thi s aspec t o f th e proble m i s not considere d here . Thi s article deals only with clay particle-water relations.
In nature every soil particle is surrounded by \^ater. Since the centers of positive and negative charges of water molecules d o not coincide, th e molecules behave like dipoles. The negative charge on th e surfac e o f th e soi l particle , therefore , attract s th e positiv e (hydrogen ) en d o f th e wate r molecules. The wate r molecules ar e arranged i n a definite patter n in the immediate vicinit y of the boundary between solid and water. More than one layer of water molecules sticks on the surface with considerable forc e and this attractive force decreases wit h the increase i n the distance o f the water molecule from the surface. The electrically attracted water that surrounds the clay particle is known as the diffused double-layer of water. The water located withi n the zon e of influence is known as the adsorbed layer as shown in Fig. 2.7. Within the zone of influence the physical properties o f the water are very different fro m those of free or normal water at the same temperature. Near the surface of the particle the water has the property of a solid. At the middle of the layer it resembles a very viscous liquid and beyond th e zone of influence, the propenles of the water become normal. The adsorbe d water affects the behavior of clay particles when subjected to external stresses, since it comes between the particle surfaces. To drive off the adsorbed water , the clay particle mus t be heated to more than 200 °C, whic h woul d indicat e tha t th e bon d betwee n th e wate r molecule s an d th e surfac e i s considerably greater than that between normal water molecules.
Particle surfac e
Adsorbed
water Distance
Figure 2. 7 Adsorbe d wate r laye r surrounding a soil particl e
The adsorbed film of water on coarse particles is thin in comparison wit h the diameter o f the particles. I n fine grained soils, however, this layer of adsorbed water is relatively much thicker and might even exceed th e size of the grain. The forces associate d wit h the adsorbed layer s therefor e play an important part in determining the physical properties of the very fine-grained soils, but have little effect o n the coarser soils .
Soils in which the adsorbed film is thick compared to the grain size have properties quite different from othe r soil s havin g th e sam e grai n size s bu t smalle r adsorbe d films . Th e mos t pronounce d characteristic o f the forme r i s their ability to deform plasticall y without cracking whe n mixed wit h varying amounts of water. This is due to the grains moving across one another supported by the viscous interlayers of the films. Such soils are called cohesive soils, for they do not disintegrate with pressure but can be rolled into threads with ease. Here the cohesion is not due to direct molecular interaction between soil particles at the points of contact but to the shearing strength of the adsorbed layers that separate the grains at these points.
Base Exchang e
Electrolytes dissociat e whe n dissolve d i n wate r int o positivel y charge d cation s an d negativel y charged anions . Acids break u p into cations of hydrogen and anions such as Cl or SO4. Salts and bases split into metallic cations such as Na, K or Mg, and nonmetallic anions. Even water itself is an electrolyte, because a very small fraction of its molecules always dissociates into hydrogen ions H+
and hydroxyl ions OH". Thes e positively charged H+ ions migrate to the surface of the negatively charged particle s and form what is known as the adsorbed layer . These H+ ions can be replaced by other cations such as Na, K or Mg. These cation s enter the adsorbed layer s an d constitute what is termed a s an adsorption complex. Th e process of replacing cations of one kind by those of another in a n adsorption comple x is known as base exchange. B y base exchang e is meant the capacity of
Soil Formatio n and Characterization 1 7
Table 2. 4 Exchang e capacit y o f som e cla y mineral s Mineral group Exchang e capacity (meq per 10 0 g)
Kaolinites 3. 8
Illites 4 0
Montmorillonites 8 0
Table 2.5 Cation s arranged in the orde r of decreasing shear strength of cla y
NH/ > H+ > K+ > Fe+++ >A1+++ > Mg+ > Ba++ > Ca++ > Na+ > Li+
colloidal particle s to change the cations adsorbed o n their surface. Thus a hydrogen clay (colloi d with adsorbed H cations) can be changed t o sodium clay (colloid wit h adsorbed N a cations) by a constant percolation of water containing dissolved Na salts. Such changes can be used to decrease the permeability of a soil. Not all adsorbed cations are exchangeable. The quantity of exchangeable cations in a soil is termed exchange capacity.
The base exchange capacity is generally defined in terms of the mass of a cation which may be held on the surface of 100 gm dry mass of mineral. It is generally more convenient to employ a definition o f base exchang e capacity in milli-equivalents (meq) per 10 0 gm dry soil. On e meq is one milligra m o f hydroge n o r th e portio n o f an y io n whic h wil l combin e wit h o r displac e
1 milligram of hydrogen.
The relative exchange capacity of some of the clay minerals is given in Table 2.4.
If one element, suc h as H, Ca, or Na prevails over the other in the adsorption comple x o f a clay, the cla y i s sometimes give n the name o f this element, fo r example H-cla y or Ca-clay. The thickness and the physical properties of the adsorbed film surrounding a given particle, depend to a large extent on the character of the adsorption complex. These films are relatively thick in the case of strongly water-adsorbent cations such as Li+ and Na+ cations but very thin for H+. The films of other cations have intermediate values. Soils with adsorbed Li+ and Na+ cations are relatively more plastic at low water contents and possess smaller shear strength because the particles are separated by a thicker viscous film. Th e cation s i n Table 2. 5 are arranged i n the order o f decreasing shea r strength of clay.
Sodium clays in nature are a product either of the deposition o f clays in sea water or of their saturation by saltwater flooding or capillary action. Calcium clays are formed essentiall y by fres h water sediments. Hydroge n clay s ar e a result of prolonged leachin g o f a clay by pur e or acidi c water, with the resulting removal of all other exchangeable bases .