Extrimenal Assessmant of Three Types of Envelope Used in Subsurface Drainages

Due to the large area of saline and swamp lands throughout the country and the clear need for drainage and also great desire to subsurface drainage systems. It is a top priority to supply materials and specifically select proper drainage envelope. One of the selecting criteria is discharge of pipes. In this study a physical model with dimentions of 4*1*1.3 is used in order to simulate natural conditions and hydraulic conductivity of envelopes. The model as a part of a simulated drainage trench has an adjustable water table. Three kinds of envelopes such as mineral (sand and gravel), organic (rice husk) and synthetic (PP450) ones which are used in the country prepared and the tests performed after installing each envelope around drainage pipes in simulated trench. In all experiments, the discharge through mineral envelope was far more than organic and synthetic ones at different water levels (60, 70 and 80 centimeters from pipe axis). This discharge in mineral envelope was 4-5 times higher compared to synthetic one and the proportion for organic envelope was 3-4 times higher that is due to the higher porosity of the two types of envelopes. While the rice husk cause reduction in discharge compared to sand but can be used as a good choice for envelope in subsurface drainage.