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Effect of Microscopic Observation On Unsaturated Soil Behaviour
Abstract
The soil structure is very important in understanding the behaviour of soil particularly in unsaturated case. The behaviour of unsaturated soil is highly influenced by microscopic structure of soil. Many electronic and optical microscopic observation have been made in this study. This paper shows the water is never been uniformly distributed with the soil matrix. This affect the water flow, capillarity and shear strength in unsaturated soil. The smaller the particle radius has the greater effect on the behaviour of unsaturated soil.
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SEM and mercury intrusion porosimetry are used in parallel to identify the structure of a medium sensitivity Champlain clay. The clay structure is observed firstly on intact, remolded, and over-dried soils and secondly on soils consolidated at various levels in one-dimensional compression. Both methods of investigation reveal for the intact soil the existence of an aggregated structure characterized by an interaggregate and an intra-aggregate porosity. Remolding affects interaggregate links but does not destroy aggregates. The observation of clay structure at various levels of one-dimensional compression shows that the collapse of the structure is progressive, the largest interaggregate pores being the first affected.-from Author
Role of Climate Types of Problems Typical Profiles of Unsaturated Soils Need for Unsaturated Soil Mechanics Scope of the Book Phases of an Unsaturated Soil Terminology and Definitions Historical Developments
Pore size distributions, compaction characteristics, and undrained strength and deformation data were related in tests to molding water content and type of laboratory compaction. The soil used was the commercially available illite clay. The pore size distribution measurements were obtained, after freeze drying, for a diameter range of about 600 /fgm/fgt to approximately 0. 016 /fgm/fgt. Samples compacted at moisture contents less than the Proctor optimum moisture contents showed brittle compressive failures at low strains.
The sizes and connections of the pores in a network of tetrahedral pores have been derived using the center coordinates of 3367 randomly close-packed equal spheres. The center coordinates of each sphere were measured by J. L. Finney, who took apart an actual packing of spheres. A. C. Wright mathematically divided the packing into 14,870 irregular tetrahedra by joining the center coordinates of spheres which were geometric neighbors. Their aim was to model the structure of liquids and glass. The present work uses their data in a different way and analyzes the capillary properties of the individual tetrahedral pores together with the network effects of the combination of pores during simulated drainage and imbibition. The meniscus drainage curvatures of the faces (bonds on the network) of the pores were calculated using the Mayer and Stowe–Princen method using a value of zero for the contact angle. Imbibition curvatures of the cavities (sites on the network) were initially estimated from the Haines insphere approximation. However, the constraint that a tetrahedral pore can, in isolation, at best show no hysteresis enables a new approximation to be proposed for the imbibition curvature of cavities formed by spheres. Drainage was simulated by using the meniscus curvature associated with each bond in conjunction with the bond network to determine whether a pore actually drained. Imbibition used the site curvatures. The results indicated that the tetrahedral (diamond) lattice is a good approximation to the pore network structure in a random packing of spheres. However, neither the bonds nor the sites are randomly distributed on the network. In drainage the bonds cooperate so as to make drainage harder. Conversely, in imbibition the sites cooperate so as to make imbibition easier. Thus the degree of nonrandomness in the location of the pore sizes in the packing produces an effect in drainage opposite to that produced in imbibition.
The curvature of an interface in a pore depends upon the shape of the pore and the operative contact angle that the interface makes with the solid surface. Even relatively simple pores formed by the surfaces of equal spheres have a complex shape including nonaxisymmetric cross-section and converging-diverging geometry. For such pores, a theory for meniscus behavior has been devised that uses a combination of a theory for meniscus curvature in rods together with the toroidal approximation of Purcell. The results of the theory show that converging-diverging geometry tends to compensate for the effect of contact angle. This is because the position at which the nonzero contact angle meniscus has maximum curvature in a converging-diverging pore is not the narrowest part of the pore throat. Due to this compensation, the effect of contact angle on maximum meniscus curvatures for drainage is approximately proportional to cos 2/3 [theta] (rather than the cos [theta] appropriate for cylindrical tubes). Experiments on pores formed by PTFE spheres using partially wetting liquids confirmed the theoretical prediction. Contact angle measurements on the PTFE spheres also demonstrated that, because of microscopic surface roughness, receding contact angles (these being operative with respect to drainage) on ground surfaces are significantly lower than values for smooth surfaces.
The purpose of this study was to examine the relation of frost heave to pore-size distribution of compacted silty soils, and to propose that frost-susceptibility criteria based upon the distribution of porosity are more logical and versatile than those based upon texture and grain size.Rapid freezing laboratory tests were conducted to evaluate the heaving rate. The soils were compacted at different energy levels and water contents, and consisted of three different combinations of silt and kaolin.Mercury intrusion tests were performed to obtain the pore-size distributions of the compacted soils. Since this procedure requires the soil to be free of moisture, the soil samples were freeze-dried. This type of drying minimized the volume change and consequent structural modification expected from air or oven drying.The relation of frost heave to pore size was obtained using the method of linear regression. For a given soil, it is the quantity of interaggregate pores that controls the frost heave. These pores are, in turn, controlled by the compaction variables of water content and effort.
The new considerations resolve the differences of opinion raised by R. A. Fisher. It appears that the author has throughout supported a case which belongs correctly to the falling moisture series. Fisher considered a very limited series for the rising moisture case and a hypothetical reversible series, and has been in error in so far as his treatment was regarded as exclusive.(Received September 16 1929)
Mercury intrusion porosimetry is one of the techniques used to measure pore size distributions of dry soil samples. It is known that incorrect pore size distributions can be obtained for some porous materials due to sample damage occurring during mercury intrusion, but little evidence relating to soil samples has been published. The pore size distributions of aggregates from three soils, differing in texture, strength and porosity, were measured after heating to 300 °C by mercury intrusion porosimetry. After the first intrusion, all of the intruded mercury was removed from each sample by heating under vacuum. Samples were then re-intruded. The pore size distribution curves for both intrusions were very similar, indicating an absence of sample damage during the first intrusion run.
The curvature of the meniscus between two equal rods and a plate, all in mutual contact, was calculated using a method proposed by Princen. Experiments were performed using capillary rise and bubble movement to test these theoretical values for both perfectly wetted systems and for systems which gave reproducible advancing and receding contact angles. The theory agreed with the experimental values to within the experimental accuracy thus verifying a remarkably successful way of calculating pore capillary properties, and indicating that the method may have much wider application.
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