SHEAR STRENGTH OF MONTMORILLONITE AND KAOLINITE RELATED TO INTERPARTICLE FORCES by

Shear strength of saturated, remolded clay mineral samples was measured in an attempt to relate strength to forces acting between clay particles. The fine fractions of the clays, purified when necessary, were consolidated to different void ratios and subjected to quick, translatory shear in a direct shear device. Relative values for shear strength were obtained in this way. Interparticle forces in the clays were altered and the effect on shear strength was observed. For a high-swelling, Na-saturated montmorillonite, strength at constant void ratio decreased with increasing salt concentration. To explain the decrease in strength with decrease in force of repulsion between particles a model is suggested where strength resuls from the force of repulsion resisting displacement of particles in the shear plane. Addition of small amounts of dispersing and of soil-conditioning chemicals had little effect on shear strength. Calcium montmorillonite had a lower strength than sodium montmorillonite when compared at the same void ratios. This is related to the lower surface area of interaction and lower repulsion found with divalent ions. Water content at the liquid limit showed the same dependence upon interparticle forces in the montmorillonite clays as did shear strength. In kaolinite, interparticle forces of attraction result in a structure or particle arrangement which has the major influence on shear strength. A clay at low pH which was flocculated in a network, edge-to-face structure owing to presence of positive edge-charge had higher strength than a dispersed clay, but a clay flocculated by electrolyte which produced a lamellar, face4o-face structure had a lower strength. I N T R O D U C T I O N S h e a r s t r e n g t h of c lays u s u a l l y h a s b e e n a n a l y z e d o n t h e bas i s of t h e p r i n c i p l e s of m e c h a n i c s t h a t were f o u n d u s e f u l fo r cohes ion l e s s g r a n u l a r soils. R e c e n t l y i n t e r e s t h a s g r o w n i n t h e p h y s i c o c h e m i c a l p a r t i c l e i n t e r a c t i o n t h a t g ives r i se to t h e s e o b s e r v e d m e c h a n i c a l p r o p e r t i e s ( L a m b e , 1958; 1 Department of Agricultural Physics, Macdonald College and Department of Civil Engineering. 210 StIEA.R STRENGTtI OF MONTMOI~ILLONITE AND KAOLINITE 211 l~osenqvist, 1959). Shear strength is considered to result from the difference between the interparticle forces of attraction and repulsion, with interparticle attraction due mainly to van der Waals forces, and repulsion to the charge on the clay mineral surface. This study is an at tempt to test this concept experimentally and to relate measured shear strength of two well-defined clays to the interparticlc forces of attraction and repulsion and the resulting particle arrangement. Relative values of shear strength have been measured for samples of remolded clays treated in various ways to alter the interparticle forces. Differences in measured strength have then been related to differences in these forces. The natural structure of clays, in which other forces would contribute to shear strength, is not present in these samples. A C K N O W L E D G M E N T S The authors wish to express their appreciation to the National Research Council, Canada, for the Grant in Aid of Research under which this work was done. E X P E R I M E N T A L P R O C E D U R E S