ساخت دستگاه بارش ماسه نيمه خودكار جديد براي ساختن مدل هاي فيزيكي ژئوتكنيكي

Developing a Portable Curtain Sand Pluviator for Reconstitution of Soil Models

ساخت مدل هاي آزمايشگاهي يكي از پر كاربردترين روشهاي پژوهشي در مهندسي ژئوتكنيك است. مدلهاي آزمايشگاهي پديده هاي ژئوتكنيكي را در مقياسي كوچكتر شبيه سازي مي كنند تا تاثير پارامترهاي مختلف بر روي آن بررسي شود. يكي از اصلي ترين پارامترهاي خاك در مدلسازي تراكم نسبي خاك است. روش بارش ماسه يكي از روشهاي مورد قبول براي ساخت نمونه هاي آزمايشگاهي و مدل هاي فيزيكي براي دستيابي به تراكم هاي نسبي معين در پژوهش هاي ژئوتكنيكي است. ساخت مدلهاي خاكي با لايه هاي يكنواخت، تراكم نسبي مشخص و تكرارپذير، يكي از اهداف روش بارش ماسه است. اين پژوهش به معرفي يك دستگاه بارش پرده اي ماسه نيمه خودكار مي پردازد كه جهت ساخت مدلهاي فيزيكي در آزمايشگاه مكانيك خاك دانشگاه فردوسي مشهد ساخته شده است. در اين پژوهش نتايج حاصل از آزمايش هاي صورت گرفته بر روي دستگاه بارش توسط ماسه 161 فيروزكوه ، تحليل و بررسي شده است. تاثير عوامل مختلف بر تراكم نسبي لايه هاي خاك مانند ارتفاع بارش، شدت بارش، ضخامت پرده بارش و سرعت حركت پرده بارش در اين دستگاه بارش مورد مطالعه قرار گرفته است. نتايج آزمايش ها نشان مي دهند كه با افزايش ارتفاع، كاهش ضخامت پرده بارش و افزايش سرعت حركت پرده، تراكم نسبي ماسه افزايش مي يابد. همچنين با بررسي آزمايش هاي مختلف، تكرارپذيري و يكنواختي بسيار مطلوبي جهت ساخت مدلهاي فيزيكي در دامنه وسيعي از تراكم هاي نسبي ( بين 3 تا 93 درصد ) مشاهده شده است و پيشنهادهايي براي ساخت مدل هاي فيزيكي به اين روش ارائه شده است.

Physical modeling is one of the most applicable researching methods in Geotechnical Engineering. Physical models simulate Geotechnical Engineering phenomenon in small scale to evaluate the effect of different parameters on them. In geotechnical physical modeling, reaching to a targeted specific weight (γ) and Density Ratio (DR) for sand beds are important. Air pluviation is one of the most adoptable methods for preparation of uniform and repeatable sand beds of required density in physical modeling. In this method sand in a container falls from an opening bellow it through the air. With different openings, there are different types of air pluviation. It may consist of a single or multiple nozzles, single or multiple sieves or a narrow aperture that pours a sand curtain. A pluviator can be stationary or portable. Stationery pluviation is a traditional method that commonly used for preparing small samples. In this method the hopper is station and nozzle outlet is small and sand pours in a limit surface of sample, therefore uniformity of sand beds decreased. Also in the horizontal direction, stationary pluviation results in a great segregation in soils which contain fines. In a Travelling pluviator the hopper usually moves above the area of interest, in a certain pattern and the sand pours uniformly from nozzle or aperture in the sample or model box. This paper presents the details of a test series with a portable curtain rainer pluviator that has been developed for modeling the sand beds in model box in geotechnical laboratory in Ferdowsi University of Mashhad. The new portable sand pluviator has been designed and developed for preparation of sand beds in a box with large dimensions (1.8 m length, 0.4 m width, 0.8 heights). The main function of the pluviator was reproducing sand beds behind the wall in the model box. The apparatus consists of a hopper with a capacity about 20 kg which is placed on a rigid modular frame. The hopper frame is connected to a modular wheeled frame that could move back and forth longitudinally by a belt on a pair of rails. The modular frames could justify the height of sand fall during pluviation and keeps the sand fall height constant. The belt is connected to a series of gearwheels that moved by a stepping motor. The direction of motion is reversed automatically when certain steps of moving finished. The velocity of the hopper could be controlled in the range of 0.4 to 4 cm/s. The sand in the hopper exits from an aperture which is connected below the hopper and could be replaced. Different aperture widths used to change the deposition intensity. In these tests the Firoozkooh sand NO.161 used for calibration. The influence of different parameters such as Height of fall, sand flow Curtain velocity, sand curtain width and Sand deposition intensity (DI) evaluated on Relative Density (RD) of sand beds with obtaining several samples during calibration. As the results clearly shows, by increasing the velocity of container, decreasing the curtain thickness and increasing the height of fall, relative density will increase. The test results show the very good repeatability and uniformity in sand for physical models in a large domain of Relative densities (3% to 93%).