بررسی وابستگی سرعت موج برشی به اندازه دانه‌های خاک ماسه‌ای

در محدوده کرنش‌های کوچک (ε≤〖10〗^(3)%)، سرعت موج برشی (vs) و متعاقب آن مدول برشی حداکثر (gmax) یکی از مولفه‌های اساسی برای انجام محاسبات ژئوتکنیکی و تحلیل‌ دینامیکی خاک‌ها می باشد. تاثیر اندازه دانه ها در هنگام انتشار امواج بر رفتار دینامیکی خاک، یکی از مسائل مهم و مورد بحث محققین است. در گذشته تاثیر اندازه دانه های خاک بر سرعت موج برشی، معمولاً در دامنه محدودی از اندازه دانه‌های خاک مورد برسی قرارگرفته است. اگرچه نتایج این تحقیقات، تاثیرهای متفاوت اندازه دانه های خاک بر سرعت موج برشی را نشان می دهد، اما نتیجه ای قطعی از تاثیر اندازه دانه های خاک بر سرعت موج برشی ارائه نگردیده است. در این تحقیق به روش آزمایشگاهی و با استفاده از دستگاه المان خمشی، تاثیر اندازه دانه های خاک بر سرعت موج برشی در دامنه وسیعی از اندازه دانه های خاک ماسه ای خشک، تحت فشارهای همه جانبه از 50 تا 500 کیلو پاسکال در دستگاه سه‌محوری بررسی شد. به کمک الک‌های استاندارد astm خاک ماسه ای در 10 گروه تفکیک شد. از هر گروه، نمونه های سه‌محوری به روش تراکم کاهش یافته و با رعایت نسبت تخلخل یکسان تهیه و مورد آزمایش قرار گرفت. نتایج بررسی ها نشان میدهد که سرعت موج برشی به‌اندازه‌ی دانه ها وابسته است، به‌طوری‌که در قطر متوسط دانه ها از 225/0 تا 29/1 میلی‌متر با افزایش قطر، سرعت موج برشی افزایش و برای قطر متوسط دانه ها از 29/1 تا 14/7 میلی‌متر با افزایش قطر سرعت موج برشی کاهش می یابد.

Investigating the Interrelationships Between Shear-Wave Velocity and Particle Size of a Sandy Soil

Under small strains (ε≤10−3%), the shearwave velocity (Vs) and its resultant maximum shear modulus (Gmax) are important parameters in geotechnical engineering calculations and soil dynamics analyses. At present, the shear wave velocity of sand is typically determined using measurement and theoretical analysis methods. The measurement methods include insitu and laboratory tests. Insitu tests are commonly conducted using a borehole method or a surface wave dispersion analysis method. Laboratory tests include bender element tests, resonant column tests, ultrasonic tests, and dynamic triaxial tests. In this regard, the evaluation of the influences of soil particle size on the dynamic behaviour of soils during wave propagation has been an important issue in geotechnical engineering. Heretofore, the effects of particle size on shearwave velocities in soils have been examined using various experimental techniques. Most of this research was carried out over a limited range of particle sizes, and the results indicated various effects of particle size on shearwave velocity: there has been no comprehensive and unambiguous outcome describing the influences of particle size on shearwave velocity in soils. This research focused on the influences of particle size on shearwave parameters in a particular type of sandy soil. A digitally controlled triaxial testing machine equipped with bender elements was used. A significant advantage of bender element test is that it can be incorporated in standard soil mechanics apparatuses such as triaxial and oedometer devices, and the approaches for data interpretation are relatively simple. This research aims to experimentally examine the effects of a wider range of particle sizes on shearwave velocity and other shearwave parameters, transmitted in dry sandy soils, using a bender element apparatus embedded in a triaxial testing machine under confining pressures of 50500 kpa. In this research, the sandy soil was initially categorized into 10 different groups using ASTM standard sieves, and all triaxial samples were prepared with an identical void ratio. The void ratio plays a vital role in the determination of the maximum shear modulus of soil. For all ranges of particle size, the maximum and minimum void ratios were determined, in order to provide an acceptable level of comparison among the results, all samples were prepared with a single void ratio of 0.80. In this study, homogeneously identical samples were assumed as a prerequisite for all experiments. Therefore, it was necessary to take practical measures to ensure this crucial prerequisite in all specimens. In this regard, various experimental methods may be used to achieve a desirable void ratio, including the wet and dry tamping method, dry pouring technique, and water precipitation methods. In this study, the dry tamping method was carried out to prepare similar specimens with an identical void ratio. To measure the shearwave travel time, the frequencies between 5 and 12 kHz were used. The significant results obtained in this study were as follows. 1) With reference to different methods of determining the shearwave travel time, the results of this research showed that the crosscorrelation and peaktopeak methods gave the most reasonable values of the shearwave velocity. 2) The outcomes revealed that, in a particular soil sample, as the excitation frequency increases, the received signals possess significant amounts of higher frequency components, and surprisingly, these signals are similar in shape. 3) Particle size influences the shape of the received signals, such that the frequency content of received signals in both fine and coarse grained soils are quite similar, but mediumsized soils increased with increasing confining pressure. 5) The results showed that the increasing size of soil grains leads to increased shearwave velocity in a particular range of particle sizes, and decreased shearwave velocity in the other range. 6) Although the effects of particle size on shearwave velocity were the main subject of this study, it seems that this factor alone cannot dominate, and other factors must also be considered, such as the type and shape of particles and the surface roughness.