Clays and silts are tested using the technique. Flow of water through the specimen is observed by monitoring the rate of fall of water in the tube. It is essential that soils of very low permeability are sealed inside the cylinder to prevent seepage along the sides of the specimen. Before testing, the specimen must be completely saturated with water as the presence of air will restrict the ow of water. Three glass stand pipes a stand with nine glass tubes of 6mm, 10mm, 20mm, 25mm, 40mm, 50mm, 60mm, 70mm, 75mm, bore tube is provided with over ow arrangement for constant head tests. The remaining tubes are used for falling head test.