ENGINEERING GEOLOGY/Rock Properties and Their Assessment 579 with water just above the level of the specimen Strain is measured in all three perpendicular directions No strain is allowed to develop in the test specimen during the swelling pressure test This is accomplished by tightly mounting a cylindrical specimen inside a rigid ring that provides radial constraint The specimen absorbs water through porous end plates Any axial strain that develops is monitored and compensated for by increasing the axial load The stress required to prevent expansion when equilibrium conditions are established is equal to the swelling pressure index particular rock is defined by its coefficient of permeability or hydraulic conductivity Grades of permeability are given in Table Determination of the permeability of many rock types in the laboratory is made by using a fallinghead permeameter (Figure 15A) The sample is placed in the permeameter, which is then filled with water to a certain height in the standpipe The stopcock is then opened and the water allowed to infiltrate the Table Grades of permeability (IAEG, 1979)Ã Permeability Permeability considers the ability of a rock to allow the passage of fluids into or through it without impairing its fabric In ordinary hydraulic usage, a substance is called ‘permeable’ when it permits the passage of a measurable quantity of fluid in a finite period of time and ‘impermeable’ when the rate at which it transmits that fluid is slow enough to be negligible under existing temperature-pressure conditions The permeability of a Permeability Class ms Greater than 10 10 10 10 10 10 10 10 10 Less than 10 Ã Description Very highly Highly Moderately Slightly Very slightly Practically impermeable International Association of Engineering Geology Figure 15 (A) Falling head permeameter (B) Radial percolation test apparatus