اندازه‌گيري مكش بافتي اوليه‌ي خاك‌هاي متراكم‌شده بر روي بازه‌ي ترشدگي منحني مشخصه‌ي آب - خاك با استفاده از روش جابجايي محوري

APPLICATION OF AXIS TRANSLATION TECHNIQUE FOR MEASURING INITIAL MATRIX SUCTION OF COMPACTED SOIL ON WETTING PORTION OF SWCC

در اين نوشتار به بررسي كارايي استفاده از روش جابجايي محوري براي اندازه‌گيري مكش بافتي اوليه‌ي گونه‌هاي مختلفي از نمونه‌هاي خاك متراكم‌شده‌ي ماسه، لاي، و رس پرداخته شده است. تجهيزات آزمايشگاهي مورد استفاده به گونه‌يي بود تا از رخداد بسياري از عوامل خطاي متداول در اين روش جلوگيري شود. نمونه‌هاي خاك با استفاده از روش تراكم استاتيكي در مقادير مختلف نسبت تخلخل متراكم‌ و مكش بافتي اوليه‌ي آن‌ها با استفاده از روش جابجايي محوري و همچنين بازه‌ي ترشدگي منحني مشخصه‌ي آب - خاك نمونه‌هاي خاك نيز اندازه‌گيري شده‌ است. نتايج به‌دست‌آمده حاكي از سازگاري مقادير مكش بافتي اندازه‌گيري‌شده با منحني مشخصه‌ي آب - خاك نمونه‌ها بوده است.

The axis translation technique is the most commonly used technique of controlling suction. Nevertheless, the axis translation is not diversely utilized for measuring soil suction, due to many difficulties concerning the application of this technique, such as lengthy test procedure, evaporation and condensation, air diffusion, soil volume changes, change of soil pore water distributions, and the interface of soil and measuring equipment. In this paper, the applicability of the axis translation technique is determined in measuring the initial matrix suction of a variety of compacted soils, including sand, silt, and clay. Elaborate experimental laboratory equipment is set up to minimize the occurrence of common errors associated with the application of axis translation techniques for suction measurement. The cell of the apparatus was equipped with porous stones and five bar ceramic disks in its bottom pedestal and top cap to minimize the lengthy procedure time and possible change in soil pore water distribution associated with controlling and measuring soil matrix suction using the axis translation technique. Additionally, the pressurized air entering the cell was passed through a closed container, half filled with water, before going to the cell, to minimize the loss of soil water content during the test. Soil samples were statically compressed inside a load frame in different compaction states with standard proctor optimum water content. The degree of saturation of samples was increased during the compression stage. Therefore, the compacted soil samples would be expected to locate on the wetting portion of the soil-water characteristic curve. After the compaction stage, soil samples were installed inside the apparatus and confined with a lateral pressure of 20 kPa to provide good contact between the end sides of specimens and the surfaces of the matrix suction measuring devices. Consequently, pore air and net confining pressure were identically and gradually increased to 300 and 320 kPa to shift the sample negative pore water pressure to a positive measurable value. Analogous positive pressures were stabilized in both ends of the samples within four to eight hours. Additionally, the wetting portion of the soil-water characteristic curve of the soil under study was measured with the testing equipment to validate the values of initial matrix suction measurements. The comparison of results shows that the sample soil suction measurements are located on their wetting soil-water characteristic curve.