Độ lớn của các tham số x là sự thống nhất cho một đất bão hòa và không cho một đất khô. Mối quan hệ giữa x và mức độ bão hòa, S, đã thu được bằng thực nghiệm. Các thí nghiệm được thực hiện trên phù sa cohesionless (Donald, 1961) và đất đầm (Blight l l%), như thể hiện trong hình. (a) và @), tương ứng. | 40 3 STRESS STATE VARIABLES where ua - pore-air pressure X a parameter related to the degree of saturation of the soil. The magnitude of the X parameter is unity for a saturated soil and zero for a diy soil. The relationship between X and the degree of saturation s was obtained experimentally. Experiments were performed on cohesionless silt Donald 1961 and compacted soils Blight 1961 as shown in Fig. a and b respectively. Figure demonstrates the influence of the soil type on the X parameter Bishop and Henkel 1962 . Bishop et al. 1960 presented the results of triaxial tests performed on saturated and unsaturated soils in an attempt to substantiate the use of Bishop s equation . Eq. . Bishop and Donald 1961 published the results of triax-ial tests on an unsaturated silt in which the total pore-air and pore-water pressures were controlled independently. During the tests the confining pore-air and pore-water Degree of saturation s b Figure The relationship between the X parameter and the degree of saturation s. a X values for a cohesionless silt after Donald 1961 b X values for compacted soils after Blight 1961 . pressures . ơ3 ua and uw were varied in such a way that the ơ3 - ua and ua - uw variables remained constant. The results showed that the stress-strain curve remained monotonic during these changes. This lent credibility to the use of Eq. however the test results equally justify the use of independent stress state variables. Aitchison 1961 proposed the following effective stress equation at the Conference on Pore Pressure and Suction in Soils London in I960 a a ỳp where p pore-water pressure deficiency Ý a parameter with values ranging from zero to one. Jennings 1961 also proposed an effective stress equation at the same conference a Ơ fip where p negative pore-water pressure taken as a positive value 3 a statistical factor of the same type as the contact area. This factor should be measured experimentally. Equations .
