ارزیابی سرعت موج برشی در خاک‌های دانه‌ای با استفاده از المان‌های خمشی و ستون تشدید

سرعت موج برشی در خاک ها یکی از پارامترهای مهم در مطالعات دینامیکی خاک ها محسوب می شود. این پارامتر در نمونه های المانی خاک از طریق آزمایش های المانی یا در محل از طریق آزمایش های ژئوفیزیک قابل ارزیابی است. از جمله روش های پرکاربرد در آزمایشگاه جهت تخمین سرعت موج برشی در خاک ها استفاده از المان های خمشی و آزمایش ستون تشدید است. در این مقاله با مقایسه نتایج به‌دست‌آمده از المان های خمشی و ستون تشدید، مقادیر فرکانس مناسب در آزمایش المان خمشی در خاک های دانه ای به دست آمده است. از طرف دیگر تاثیر نسبت منافذ، تنش همه‌جانبه و درصد شن بر روی سرعت موج برشی مورد بحث قرار گرفته است. نتایج نشان می دهد که با کاهش نسبت منافذ یا افزایش تنش همه‌جانبه، سرعت موج برشی به صورت غیرخطی افزایش می یابد. از طرف دیگر با افزایش مقدار شن  تا 50 درصد به ماسه میزبان، سرعت موج برشی افزایش یافته و بعد از آن با افزایش شن، سرعت موج برشی کاهش می یابد.

Evaluation of Shear-Wave Velocity in Granular Soil Using Bender Element and Resonant Column

Shear wave velocity, Vs, is a useful soil mechanical property for determining the low shear strain (γ ≈ 0.0001%) elastic shear modulus, which is required for both static and dynamic response analyses of earth structures. It is an important geotechnical soil property for the design and analysis of geotechnical structures. It can be employed to determine the maximum or smallstrain (≤103%) dynamic shear modulus (Gmax or G0) of the soil mass. Vs can be easily obtained using laboratory or in situ testing techniques. Laboratory tests can be carried out on undisturbed soil samples by simulating the field stress conditions or reconstituted soil samples from a site. However, obtaining good quality undisturbed samples of granular soil deposits is very difficult and moderately expensive. In most geotechnical investigations, the seismic properties (P or Swave velocity and the dynamic elastic properties) of granular soil layers from in situ tests (such as the seismic cone penetration test) cannot be determined because of the high cost of undisturbed specimens and the need for highly specialized personnel and equipment. There is increasing interest in the use of Vs to study soil particle properties (e.g. shape, elastic properties, gradation) and the soil state and fabric (e.g. void ratio, boundary stress). In the current study, the bender element and resonant column tests were conducted on sandgravel specimens. A bender element is an electromechanical transducer composed of twolayers of piezoceramic plates that are crosssectionally polarized. This allows for straightforward wave velocity measurement of soil specimens. The BE converts electrical energy to mechanical energy (movement). The resonant column device is commonly used in the laboratory to measure the lowstrain properties of soil. This includes the dynamic properties (shear modulus and damping ratio) of soil specimens at strain levels of 106 to 5×103 conducted according to ASTM standards. In the RC test, Vs can be calculated using the solution of the equation for the linear vibration of a columnmass system. The RC device is the most reliable laboratory method for measurement of Vs. The aim of this research was to explain the effects on Vs of the relative density, mean effective stress, grading characteristics, consolidation stress ratio and initial fabric anisotropy produced during specimen preparation. Five gradations of gravelly and sandy material were used to study the influence of grading characteristics, consolidation stress ratio, depositional method, relative density and mean effective stress on the dynamic properties of granular soil. The pure sand was clean, uniformlygraded fine sand with a mean grain size of 0.6 and a silt content of less than 1% that was classified as SP according to the unified soil classification system (USCS). The pure gravel was uniformlygraded soil with a maximum particle size of less than 16 mm that was classified as GP according to the USCS. The measured values from the resonant column and bender element tests also were compared. Comparison of VsBE and VsRC shows that the results obtained by both techniques were in acceptable agreement for all specimens; however, there was a slight difference between the two techniques at low values of Vs in which VsBE was consistently lower than the corresponding values of VsRC. The results of these tests were employed to develop a generalized relationship for predicting the Vs of granular soil. The Vs model was validated using experimental data from the current study and from previous studies. The results indicate that the proposed model is capable of predicting the Vs of granular soil.