بررسي مطلوبيت روش بارش ماسه براي نمونه‌سازي خاك‌هاي ماسه‌اي در مدل‌هاي آزمايشگاهي

Investigation of Desirability of Sand Pluviation Technique in order to Sample Making of Sandy Soils for Laboratory Models

روش باران ماسه يا به عبارت ديگر «بارش ماسه در هوا»، روشي براي آماده‌سازي مدل‌هاي آزمايشگاهي است كه به منظور انجام برخي آزمايش‌هاي خاص، مانند كاليبراسيون نفوذسنج‌هاي استاتيكي و ديناميكي به كار مي‌رود. هنگام استفاده از مدل‌هاي آزمايشگاهي، كاليبره شدن دستگاه نمونه‌ساز به منظور توليد دقيق نمونه‌هايي با شرايط دلخواه و دانسيته‌ي نسبي، اهميت دارد. اين مقاله به بررسي نتايج به دست آمده از كاربرد دستگاه نمونه‌ساز به روش بارش ماسه، براي ايجاد نمونه‌هاي ماسه‌اي در دانسيته‌هاي مورد نظر پرداخته است. در اين راستا با تركيب آرايش‌هاي مناسب براي قطر منافذ صفحات بارش، فاصله‌ي منافذ و ارتفاع بارش، مي‌توان به دانسيته‌ي نسبي ماسه‌ي مورد مطالعه دست يافت. نتايج نشان مي‌دهد كه اين روش، توانايي توليد نمونه‌هاي ماسه براي مدل‌هاي آزمايشگاهي در دانسيته‌هاي نسبي 20 تا 100 درصد را دارا است و مي‌توان ماسه‌اي با شرايط يكنواخت ايجاد نمود. به طور كلي در حالتي كه سرعت بارش پايين است، مي‌توان به دانسيته‌هاي نسبي بالاتر و متعاقب آن وزن واحد حجم بالاتري دست يافت. همچنين اين موضوع كه در حالت ارتفاع بارش بيش از 500 ميلي متر، تغييري در ميزان دانسيته به دست آمده ايجاد نمي شود، نيز تاييد گرديد.

Dry pluviation, or the “raining of sand through air”, is a commonly employed technique used to prepare sand samples in the laboratory to some specified initial state. It is important, however, that the techniques are calibrated so that the density of the samples produced is known accurately and precisely. Several methods have been used to prepare sand samples for use in experimental studies. This paper presents the results of a study in which the characteristics of pluviated samples of Stockton Beach silica sand are determined as a function of deposition rate and free-fall height. The experimental arrangement used to generate the data for this study is a simple set with 500 mm of diameters mold. Sand was rained from the hopper, through holes ranging in size from 6mm to 15mm diameter, at spacing’s (S) ranging from 20 to 100mm, with holes arranged in a triangular pattern. In all cases, a single diffuser screen, with an aperture size of 2.36mm was employed (except where the pluviation rate was so great that sand accumulated on the screen, at which point, the diffuser was removed). The fall heights (free fall distance traveled by the sand from the diffuser to the top of mould) considered were 61mm, 236mm, 481mm, 641mm. A 1 liter sampler mold was used to catch the sand. The dry pluviation technique is an effective method of preparing sand samples to a wide range of target densities. A comprehensive set of data has been obtained and reported to assist in the design of pluviation systems. Data is also provided on the consistency of the achieved densities, and how this varies as a function of the target density. In general, it is found that the higher densities achieved under slower rates of deposition are more repeatable. There is no unique relationship between shutter porosity and the deposition rate, but rather there is a trend for shutters with fewer, larger holes to achieve significantly greater deposition rate than shutters with a greater number of smaller holes, with the same porosity. Clearly, retardation of flow at the margins of the shutter holes is reduced in situations of fewer, larger holes, as the ratio of circumference to area is smaller in these cases. The results show that samples with relative densities from around 20% to 100% are readily achieved by simple combinations of hole size and hole spacing. In general, it is apparent that the dry pluviation techniques is more suited to the preparation of samples of greater density, with many combinations of hole size and hole spacing able to achieve relative densities between about 70 and 90%. Densities between 20 and 60% are achieved less readily, while densities below 20% are the least readily achieved. Previous observations that sand fall heights greater than 0.5m have no affect on density are confirmed by the results of this study.