With the large-scale triaxial apparatus, triaxial tests of gravel soil for a 300m high embankment dam were carried out; the highest confining pressure is to 5 MPa. The experimental results show that the stress-strain curve of gravel soil is a hyperbolic one; and that shear strength envelope of gravel soil is nonlinear. According to physical meaning of Duncan-Chang model, initial modulus Ei should be obtained in the area of minimum value of axial strain ea; and ultimate principal stress difference (sigma(1)-sigma(3)) ult from a bigger range of ea; so propose that Ei is obtained in the area of epsilon(a)=0%-3% and (sigma(1)-sigma(3)) ult in the area of epsilon(a)=5%-15%. The proposed method is more reasonable than the traditional method because the actual strain of high earth dam is about 1%,. As the confining pressure of the triaxial test is in a large range, confining pressure is divided into three parts, such as 0.2-1.0 MPa, 1.5-3.0 MPa, 3.5-5.0 MPa; and the tangent modulus in each range of confining pressures are gained respectively. It is proved to be reliable by the comparison between test data and computing data.
