| Soft clay in China’s coastal areas are very widespread with the characteristics of high water content, high compressibility and low shear strength. Cement-soil pile composite ground has been widely used in highways, railways, airport runways and other major infrastructure construction as a method to reinforce the soft clay foundation and improve its earthquake resistant behavior in recent years. At present, many studies have been done on the mechanical properties of the cement-soil under static loads, but the researches on strength and deformation characteristics of cement-soil subjected to dynamic loads were extremely less.Based on the failure mechanism of cement-soil pile composite ground under seismic loading, the cyclic shear tests of cement-soil were carried out by using GDS dynamic simple shear apparatus to study the effect of cement ratio, seismic loading amplitude, frequency,phase and the seismic loading multidirection on dynamic deformation, dynamic strength and degradation characteristics. The results were as follows:(1)The dynamic shear strain developed more slowly and the dynamic strength increased gradually with the cement ratio increasing. However, the results of the increase of cyclic stress ratio were opposite. It was shown that the critical cyclic stress ratios were 0.267, 0.286 and 0.327 for the cement-soil with cement ratio of 5%, 10% and 15%, respectively. Using the failure criteria of turning strain and the method of normalization, the model between cyclic stress ratio and failure number was established. The degradation index of cement-soil reduced with the increase of cyclic stress ratio and cyclic number under cyclic loading, and the Semi-logarithmic model could more accurately describe the degradation characteristics of cement-soil. The effect of frequency on the dynamic strength of cement-soil with different cement ratio exhibited different properties.(2) With the increasing of strain amplitude, dynamic shear modulus decreased and damping ratio increased. It reflected that the elastic deformation of cement-soil reduced gradually and the damage and plastic deformation in the structure were increasing. When cement ratio was greater than 10%, the porosity and compressibility of cement-soil significantly reduced. But the structure of cement-soil became stronger, and the effect of consolidation stress on maximum dynamic shear modulus became weaken. The relationshipbetween maximum dynamic shear modulus and cement ratio was linear, and the relationship between consolidation stress was also approximately linear. The normalized dynamic shear modulus and damping ratio concentrated in a narrow area, and the proposed model could accurately determine the trend of dynamic shear modulus and damping ratio.(3) The turning strain occurred in the development of dynamic shear strain and the relationship between turning strain and the logarithmic failure number was linear. Using the failure criteria of turning strain, the model between dynamic strength and failure number was established. The dynamic strength of cement-soil decreased with the increase of failure number and three criteria dynamic strength were obtained corresponding to three phases.There was a critical strength value of cement-soil under bidirectional cyclic loading with different phases. When the loading below this critical value, no matter how big the cyclic number was, the cement-soil would not produce a large strain and damage.(4) The phase of horizontal bidirectional dynamic loading had a significant impact on the dynamic strength of cement-soil. When the phase was 0 degree, the dynamic strength of cement-soil was the highest. However, the dynamic strength was the lowest when the phase was 90 degree. Compared with unidirectional cyclic loading, the dynamic strength under bidirectional cyclic loading of cement-soil decreased about 30% under the usual seismic loading equivalent numbers(10 to 30) when phase was 0 degree. In actual engineering design process, phase and multidirection could not be ignored. |
