Strain Accumulation And Non-coaxial Characteristics Of Clay Under Principal Stress Rotation Induced By Traffic Loading

Marine sedimentary clay formed in Quarternary period is widely distributed in the southeast of China.On the soft clay ground in Wenzhou,a great number of transportation facilities have been constructed,such as highways,roads,railways and airfield runways.Excess settlement occurs on these transportation facilities is mostly due to traffic loading,leading to a large deformation,instability and risk of failure,following with the great amount of maintenance costs and even life threatening.Considering the longevity and complexity of traffic loading,it is necessary to develop experiments investigation on soft clay under long-term multi-dimensional coupling stress paths of traffic loading.Meanwhile,advanced and delicate test apparatuses lead to the difficulties of applying these test results to actual projects because it is unlikely for practicing engineers to conduct many complex tests.Therefore,it is imperative to study the cyclic deformation behaviour of soft clay and also the substitutability of conventional test results to complex cyclic tests.In this study,a series of cyclic tests on natural soft clay and its reconstituted clay under long-term multi-dimensional coupling stress paths of traffic loading were conducted by GDS dynamic triaxial tests apparatus and dynamic hollow cylinder apparatus.The main contents and conclusions are as follows:1.Based on the triaxial tests results on natural soft clay,it is found that the axial strains developing in the monotonic and cyclic tests shared the same surface in the three-dimensional space p'q-?a(?apeak).Therefore,the strains of the clay after a large number of loading cycles can be evaluated by those under monotonic loading.This study established a relationship between cyclic and monotonic test results.A simple procedure was proposed.The prediction of this method was verified by the test data from the previous research.To further investigate the generalization of this method,the results of two-way cyclic triaxial tests at two different frequencies and monotonic triaxial tests were compared.Similarly,the cyclic response of tests at different stress levels and frequencies all share the same relationship line in plane?res-?a,and the relationship is the same for top points and bottom points.2.In undrained condition,a series of long-term cyclic torsional tests on reconstituted soft clay under pure principal stress rotation at maintained intermediate principal stress coefficient were conducted at different stress levels.It is found that the intermediate principal stress coefficient has certain effect on the deformation behaviour of soil samples.Generally,the relationship between the amount of non-coaxial and maximum principal stress is independent of number of cycles.The average amount of non-coaxial at one certain cycle linearly decreases with the increasing deviatoric stress.The intermediate principal stress coefficient has little effect on this relationship.When the stress level is extremely low,the amount of non-coaxial is approximately 45°.The loading rate has little effect on the average amount of non-coaxial,but has certain effect on its amplitude of fluctuation.The amount of non-coaxial linearly decreases with the principal strain increments.The intermediate principal stress coefficient has little effect on this relationship.3.Static and cyclic hollow cylinder tests have been conducted on undisturbed Wenzhou soft clay to study its anisotropy and cyclic deformation behaviour under traffic loading.It is found the shear stress level has significant effect on the development of strain and accelerates the failure of soft clay.And the difference between these three series of tests increases as the VCSR value increases.The phenomenon can be explained by the relationship between cyclic stress paths and the enveloping surface obtained by static tests.Taking shear stress level into consideration,the allowable CSR is measured to be about 0.22 for Wenzhou soft cay.An experimental model of ? and an experimental model of axial permanent strain ?pz have been established with CSR,? and number of cycles.The relationship between ?pz and? appears as a unique line in semi-log plot,regardless CSR,?,and number of cycles.4.A series of undrained long-term(10,000 cycles)cyclic torsional tests on the natural Wenzhou soft clay after KO consolidation had been conducted on GDS hollow cylinder apparatus.And it was proposed that the allowable cyclic stress ratio(0.114?0.143)could be the criterion of settlement control for road engineering.And compared with the results of triaxial tests,when considering the rotation of principle stress axes,the control of dynamic stress in road engineering should be stricter.5.Based on the tests results of undrained long-term(10,000 cycles)cyclic torsional tests on the natural Wenzhou soft clay after KO consolidation,an experimental model of axial permanent strain ?pz have been established considering the effect of principal stress rotation.Meanwhile,the amplitude of cyclic stress at different depth was calculated according to the tonnage of heavy truck and traffic flow.Finally,the experimental model were applied to the prediction of settlement of pavement in engineering practice.The results show that this prediction is better than the traditional prediction based on the static loading or even cyclic triaixal loading.Also,it is conventional to transfer the condition of loading with principal stress rotation or without principal stress rotation by changing the value of ? in this experimental model.It means that the model could be built on the cyclic triaxial tests.According to the above-mentioned statement,the results of cyclic triaxial tests could be predicted by traditional monotonic triaxial tests to some degree.It will greatly facilitate the translation of the researches in laboratory to the engineering practice.6.The two main effects on the settlement of pavement are the thickness of embankment and stress history.At the same condition,the settlement induced by traffic loading will greatly decrease with the increasing thickness of embankment and preloading level.It is because the increasing thickness of embankment and the high preloading level decrease the cyclic stress ratio,and the decreasing of cyclic stress ratio helps to reduce the detrimental effect of principal stress rotation.Besides,the heavier truck causes higher cyclic stress and deeper effect on the subgrade.The tonnage of truck has much more effect on the settlement of pavement.