Study On Suctions And Shear Strength Of Unsaturated Soils And Experimental Verification

As an essential portion of basic framework of soil mechanics, strength theory of saturated soils has been well established and widely used in geotechnical engineering practice. However, unsaturated soils are widely distributed and display more complex behaviour compared with saturated soils in engineering practice. Therefore the strength characteristics and strength theory of unsaturated soils have not been well recognized due to the complexity of their multi-phase constitution and interaction of particles, pore water and pore air occupied in the void. In fact, the theories of stress and strength of unsaturated soils are essential and fundamental for analysis of deformation and evaluation of stability of earth structures and foundations comprising of unsaturated soils. Therefore, it is necessary to establish the system of stresses and theory of strength for different degrees of unsaturated soils. Considering the fact that the three-phase composition and individual characteristics of different components as well as their difference from the saturated soils, the difference between the pore-air pressure and pore-water pressure constitutes a matric suction in addition to the net stress which is imposed on soil skeleton. The matric suction plays an important role in deformation and strength properties. The saturation or water content will define the distribution state and transport or seepage properties of pore air and pore water in the voids and then will directly or indirectly influence strength and permeability of unsaturated soils. In order to fully understand the engineering behaviour of unsaturated soils, it is of primary importance to establish the classification system of unsaturated soils according to the saturation and water content and then to define the net stress and matric suction for different states of unsaturated soils which is the key to establish the system of stresses of unsaturated soils. In this thesis, based on theoretical analyses for an ideal assembly of regular particles, the theories of suction and strength are established and then is attempted to be used for interpretation of strength characteristics of actual unsaturated soils. Then some especially-designed experimental tests are performed to verify the fundamental concepts and theory presented in theoretical analyses. These studies form the basis of unsaturated soil mechanics. The following portions are included in this dissertation.1. Based on thermodynamics and geometry, nonlinear formulae are established and numerically programmed with an iterative approach to determine three dimensional shape of contractive skin between two mono-sized ball particles and matric suction and saturation degree can be determined accordingly for an ideal assembly of regular ball-typed particles with isolated water. Considering both effects of magnitude and effective area of matric suction on granular media, equivalent matric suction is presented as average of total matric suctions imposed on all sections of a particle surface over the whole surface of the particle and generalized soil-water characteristic curve is given for relating the equivalent matricsuction to water content. It is shown that conventional matric suction is higher than its equivalent partner and the variation of equivalent matric suction with saturation degree is not as prominent as that of matric suction. Both matric suction and equivalent matric suction decrease rapidly with the increase of the spacing between particles. Additionally, both soil-water characteristic curve and generalized soil-water characteristic curve are dependent on the contact angle of pore water to the particle surface.2. In the most-popularly accepted classification for unsaturated soils, the unsaturated soils are classified as three types with different characteristics such as continuous water with discontinuous air (CWDA) and continuous water with continuous air (CWCA) and discontinuous water with continuous air (DWCA). In fact, surface tension exists on the surface of pore water. Overlapping of pore water along the surface of particle makes the surface tension impose a tension on the particle. Such a tension plays a similar role to that of matric suction and is different essentially from matric suction. The equivalent action of such a tension averaging over the action area of matric suction is defined as tension suction. Its existence is associated with overlapping condition of pore water along the surface of particle. Hence, it is necessary that CWCA unsaturated soils should be further divided into overlapping and non-overlapping CWCA unsaturated soils according to whether contractive skin is overlapping on the surface of soil particles or not. For the unsaturated soils in both DWCA and overlapping CWCA states, both matric suction and tension suction exist due to the overlapping of contractive skin on the surface of soil particles. Therefore, DWCA unsaturated soils and overlapping CWCA unsaturated soils can be combined into overlapping unsaturated soils in which direct effective on strength by surface tension exist in essence. Similarly, CWDA unsaturated soils and non-overlapping CWCA unsaturated soils can be also grouped into non-overlapping unsaturated soils. In this type of unsaturated soils, no direct effect of surface tension exists on strength while matric suction may exist or not exist depending on the conditions of pore water. In this way, the unsaturated soils can be grouped into two types such as overlapping and non-overlapping unsaturated soils according to the existence or non-existence of tension suction. By using such a classification system of unsaturated soils, the state of voids in unsaturated soils can be authentically related to the interaction mechanism between skeleton and voids. Therefore such a classification can interpret the essential behaviour of unsaturated soils and is more rational scientifically.3. In order to examine the characteristics of suction, theoretical analyses are made for an ideal assembly of mono-sized ball-typed particles under the state of discontinuous water with continuous air (DWCA). Based on thermodynamics and geometry, the equations respectively for computing the matric suction and tension suction are theoretically derived. The ratio of tension suction with respect to matric suction is defined as suction ratio in order to observe the correlation between tension suction and matric suction. For the assembly system of particles with zero spacing, the dependency of the suction ratio on saturation radius is observed through computations. It is shown that for unsaturated medium withdiscontinuous water with continuous air (DWCA), the suction ratio is much larger than zero and generally lager than unity in most cases. Therefore the tension suction plays independent role in addition to matric suction and cannot be overlooked. Both an exponential-type empirical relation and a nonlinear formula are established respectively for the relationship between suction ratio and saturation radius and for the relationship between suction ratio and saturation by using curve-fitting technique. Furthermore, the sum of both tension and matric suctions is defined as resultant suction. At the same time, the equivalent tension suction and equivalent resultant suction are introduced in order to take the effect of action area into account. It is found that the equivalent tension suction increases while the equivalent matric suction decreases with the increase of saturation and the equivalent resultant suction is almost unvaried.4. For the ideal model of granularly assembled particles, the critical conditions among the DWCA and overlapping CWCA and non-overlapping CWCA states are developed through theoretical analyses. For the overlapping CWCA unsaturated medium, the equations respectively for computing the matric suction and tension suction are theoretically derived on the basis of thermodynamics and geometry. Through computation for different contact angles of contractive skin on the surface of ball-shaped particles, the dependency of both matric suction and tension suction and their equivalent partners on saturation are achieved. It is found that for unsaturated medium under the state of overlapping CWCA, the tension suction is higher than matric suction and both suctions are reduced rapidly with saturation. The matric suction will disappear at a certain lower saturation, around 30% while tension suction will vanish at a certain saturation about 35-45%. All types of suctions rapidly increase with contact angle of contractive skin on the surface of ideal particles. Therefore, both the concepts and determination procedure of all suctions proposed above can reproduce the overall effect of mineral components of soils on both various suctions and soil-water characteristic curves of unsaturated soils in a certain degree. When the saturation is lower than the critical saturation that is transition from DWCA state to overlapping CWCA state, the overlapping length of contractive skin on the surface of particles will increase with saturation, leading to increase of the tension suction with saturation. On the contrary, when the saturation is higher than the critical saturation, the overlapping length of contractive skin on the surface of particles will decrease with saturation, resulting in decrease of tension suction with saturation. At the same time, equivalent matric suction monotonically decreases basically with the increase of water content from zero to 100%.5. In the study of unsaturated soils, it is common to directly employ the equivalence hypothesis suggested by Bishop which can be stated as that the effect of interaction among particles on the deformation and strength behaviour of soils can be equivalently replaced by externally-applied average isotropic confining pressure. For an ideal system regularly assembled by mono-sized particles, the effects resulting from externally-applied hydrostatic confining pressure and attraction among particles are respectively analyzed foi two cases inwhich there is no absence of particle and there exists absence of particle. It is shown that Bishop's hypothesis is rational in the case of no absence of particle while it is invalid in the case of existence of absence of particle. Under such a circumstance, the effect of confining pressure on friction along the interface among particles, interlocking and volumetric deformation are much more considerable than that of interaction or attraction among particles. However, it is rather difficult to define the effects of both confining pressure and internal attraction among particles on the deformation and strength of loosely-packed media as well as the interrelationship of both effects because of the limitation of current experimental and measurement techniques. Therefore, Bishop's hypothesis is still employed here to study contribution of various types of suction to strength in order to reduce the complexity. When linear Mohr-Coulomb's criterion is utilized, the strength of unsaturated soils induced by equivalent suctions increases linearly. When effective area of suctions on particle is taken into account, contribution of both matric suction and tension suction to strength can be superposed together and can be represented by the equivalent resultant suction. In order to define equivalent resultant suction, relationship between equivalent resultant suction and matric suction should be established since no proper procedure is currently available for directly defining or measuring the tension suction and the acting area of both tension and matric suctions. Empirical relation between matric suction and equivalent resultant suction is defined with use of curve-fitting technique based on the computed results. In the DWCA state, equivalent resultant suction is linearly related to matric suction and accordingly strength is linearly dependent on matric suction. In overlapping CWCA state, when matric suction disappears for a certain degree of saturation, tension suction may still exist independently and therefore non-zero equivalent resultant suction exists. The tension suction which may exist even under the condition that the matric suction vanishes yields non-zero resultant suction and therefore results in an intercept of ultimate strength. For the non-overlapping state, the unsaturated property of soils has no any effect on strength as no any suctions exist.6. Based on orthographic test principle, the experimental data of collective tests are re-evaluated to justify Fredlund's two stress-state variables for unsaturated soils respectively. The collected tests include unsaturated triaxial shear tests of both remolded loess soils and Nanyang expansive soils, the unsaturated direct shear tests of the Madrid grey clay, and unsaturated compression tests of reconstituted loess soils. It is demonstrated through the orthographic analyses that the dependencies of shear strength of unsaturated soil on net normal stress and matric suction are independent and there exists no cross interaction of the effects of net normal stress and matric suction on strength. Therefore the effects of net normal stress and matric suction on strength can be separately taken into consideration and then directly summed together to achieve the overall strength due to both net normal stress and matric suction. As a consequence, the overall strength induced by unsaturation of soils can be expressed by net normal stress and matric suction where the effect of the tension suction is implied. Based on the correlation relationship of matric suction and equivalent resultantsuction obtained for the ideal assemble of regularly-packed particles, in the range of saturation from 0 to 100%, the unsaturated state of unsaturated soils can be generally divided into two states which consists of generalized overlapping unsaturated state including both DWCA and overlapping CWCA and generalized non-overlapping unsaturated state including non-overlapping CWCA and CWDA. As a result, according to the existence and correlation of both matric suction and tension suction and the dependency of strength on suction or different unsaturated states, the strength behaviour of unsaturated soils can be described by four staged straight lines. Considering non-uniform of particle size and irregularity of particle shapes and multiply of grain size distribution as well as complexity of different mineral components in practice, an empirical formula of shear strength of unsaturated soils is proposed in terms of matric suction which is continuous and differentiable. The fundamental variation characteristics of strength with matric suction can be reproduced by the proposed formula, e.g., differential of strength with matric suction may approach to infinite due to the presence of tension suction when the matric suction vanishes while strength should approach to an asymptote at a higher value of matric suction.7. In order to verify the rationality and reasonability of the theories of suctions and strength of unsaturated soils proposed above, a number of experimental tests are well designed and performed. The conventional triaxial shear apparatus, pressure plate and GDS unsaturated triaxial shear apparatus are respectively employed to experimentally study fully saturated samples, unsaturated samples with higher saturation and naturally-dried unsaturated samples with iower saturation which are all initially saturated. The strength characteristics and failure features of unsaturated soils are examined at two extreme states including higher saturation and lower saturation or DWCA. It is displayed that the friction angle related to suction is larger than that associated with net normal stress for unsaturated soils under higher saturation while the strength linearly increases with increase of matric suction for unsaturated soils under lower saturation. It is implied that an internal force in addition to matric suction which exists in unsaturated soils will impose its contribution to the strength. Such an additional internal force may be resulted from the tension suction. It can be observed from the failure state of samples that the failure mechanism of unsaturated soils is different from that of saturated soils.8. By applying strength theory for unsaturated soils and Fredlund's theory of two stress-state variables, an one-dimensional constitutive model for vertical deformation of unsaturated soil under ^-consolidation state while the critical stable height of vertical cut slopes is evaluated with consideration of dry cracks at ground. (1) Unsaturated soils will undergo deformation as ground is soaked or underground water table rises. The increase in water content can result in, on one hand, compression due to rise in specific weight of soil, and on the other hand, rebound due to decrease in matric suction. The comprehensive effect is controlled by these two types of factors. Based on generalized Hook's law of elasticity and the empirical relationship between matric suction and saturation given by Brooks and Corey,an incremental-type of constitutive model is established for unsaturated soils loaded under the condition of A^-consoilidation. Combined with the layer-wise summation method, the proposed model can be used to assess deformation of unsaturated soils induced by the soaking of ground, rise or lowering of water table as well as over-evaporation. Assessment on an example displays that deformation characteristics of the unsaturated soil is not only dependent on soil properties, but also associated with the variation of profile matric suction distribution and thickness of soil layer as well as stress states. The procedure can also be used to evaluate the settlement of footing on unsaturated soil foundation. (2) Based on the representation procedure of two stress state variables, the formulas for estimating the critical stable height of vertical cut slopes of unsaturated soils are established respectively for two cases that matric suction is constant or linearly decreased along depth. It is shown that the critical height is dependent on both matric suction and strength parameters. On one hand, matric suction can enhance stability of vertical slope since shear strength will increase with the matric suction. On the other hand, matric suction will initiate surface cracks on ground, leading to reduce the stability of slopes. Therefore it is not always true that the matric suction plays a positive rule in the slope stability. The feature of variation of the critical height with matric suction is associated with comprehensive effects of related parameters of soil. Compared with the condition that shrinkage cracks on ground are overlooked, the critical heights of slope considering dry shrinkage cracks get smaller. The study is instructive and useful in excavation of deep foundation pits without supports.