اثر دست خوردگي در رفتار ديناميكي نمونه هاي ماسه يي در آزمايش هاي ستون تشديد

REMOLDING EFFECT ON DYNAMIC BEHAVIOR OF SANDY SOIL SAMPLES USING RESONANT COLUMN TESTS

سختي و ميرايي از مهم ترين پارامترهاي ديناميكي هستند. يكي از دلايل تفاوت بين مقادير پارامترهاي ديناميكي حاصل از آزمايش هاي آزمايشگاهي با مقادير صحرايي،دست خوردگي نمونه هاست. در پژوهش حاضر، با انجام آزمايش ستون تشديد روي مصالح خاكي دست نخورده و دست خورده، رفتار ديناميكي خاك بررسي شده است. اثر تنش محدودكننده، رطوبت و ناهمساني تنش در روند تغييرات پارامترهاي ديناميكي بررسي شده است. نتايج بررسي ها نشان دادند كه دست خوردگي سبب كاهش سختي خواهد شد، اما تاثيري در ميرايي نخواهد داشت. با افزايش تنش محدودكننده، مقادير سختي و ميرايي به ترتيب افزايش و كاهش خواهد يافت. رطوبت، كاهش سختي را به دنبال خواهد داشت، در حالي كه در مقدار ميرايي تاثيري نمي گذارد. ناهمساني تنش نيز افزايش سختي را به دنبال خواهد داشت. روش نصف عرض توان نسبت به روش زوال دامنه ي ارتعاش، مقادير نسبت ميرايي بيشتري را ارايه مي كند. نتايج پژوهش حاضر با مدل هاي پيشنهادي ديگر پژوهش گران نيز مقايسه شده است.

Shear modulus and damping ratio are two most important dynamic parameters for seismic design of structures. Several attempts have been made to determine and study these two dynamic parameters using field and laboratory experiments. Sample disturbance is one of the reasons for the discrepancy between laboratory and in situ dynamic parameter test results. Failure to heed the effects of reconstituting on the dynamic parameters of soil under dynamic laboratory tests and analysis may cause serious damage. Hence, it is necessary to recognize the dynamic behavior of materials by conducting dynamic tests on cored materials and comparing the results with those of reconstituted specimen.In this research, the dynamic behavior of soil in small strains was studied by performing a resonant column test on cored and reconstituted samples. The core sample used in this study is clayey sand with high plasticity and the reconstitution of samples was carried out using the moist tamping method. Then, core sample in the dry condition and reconstituted samples in dry and moist conditions were subjected to resonant column test according to the ASTMD4015 standard under isotropic and anisotropic consolidated stresses of 150, 300, 500 and 700 KPa. The effects of consolidation stress, reconstitution, moisture, and anisotropic consolidated stress condition were studied by using the shear modulus and damping ratio versus shear strain diagrams. Furthermore, the damping ratio of the samples was determined by both free vibration decay and half power bandwidth method and the effect of calculation method was studied as well. The results of the study indicated that by increasing the consolidation stress, the shear modulus and damping ratio increased and decreased, respectively. Also, reconstituting reduced the shear modulus, but the variation of damping ratio versus shear strain for cored and reconstituted samples was negligible. Comparison of diagrams in dry and moist reconstituted samples showed that moisture could significantly reduce the shear modulus, but did not affect the damping ratio. Anisotropic condition might also cause increase in shear modulus. The damping ratio calculated from the half power bandwidth method was more than that obtained from free vibration decay in all samples.