ENGINEERING GEOLOGY/Rock Properties and Their Assessment 577 Table Ethylene glycol soak test index values Time class Degree of disintegration class day days 4 10 days 11 15 days 16 20 days 21 30 days Over 30 days 10 11 10 Class : 4–10 days Class : 11–15 days Class : 16–20 days Class : 21–30 days Class : Over 30 days An index value can be derived from the integration of the degree of disintegration and time taken to reach the worst condition In other words, a scale of relative durability values can be developed, ranging from in which specimens are more or less unaffected to 11 when they are totally disintegrated (Table 5) The ethylene glycol soak test has been used to distinguish certain unsound basalts and dolerites, commonly described as slaking types, from sound types, particularly in relation to their use as road aggregate Slaking refers to the breakdown of rocks, especially mudrocks, by alternate wetting and drying If a fragment of mudrock is allowed to dry out, air is drawn into the outer pores and high suction pressures develop When the mudrock is next saturated the entrapped air is pressurised as water is drawn into the rock by capillary action This slaking process causes the internal arrangement of grains to be stressed Given enough cycles of wetting and drying, breakdown can occur as a result of air breakage, the process ultimately reducing the mudrock to gravel-sized fragments The slake-durability test estimates the resistance to wetting and drying of a rock sample, and is particularly suitable for mudrocks and shales The sample, which consists of 10 pieces of rock, each weighing about 40 g, is placed in a test drum, oven dried, and weighed After this, the drum, with sample, is half immersed in a tank of water and attached to a rotor arm, which rotates the drum for a period of 10 at 20 revolutions per minute (Figure 13) The cylindrical periphery of the drum is formed using a mm sieve mesh, so that broken down material can be lost whilst the test is in progress After slaking, the drum and the material retained are dried and weighed The slake-durability index is then obtained by dividing the weight of the sample retained by its original weight, and expressing the answer as a Figure 13 Slake durability test apparatus percentage The following scale of slake-durability is used: under 25%, very low 25–50%, low 50–75%, medium 75–90%, high 90–95%, very high over 95%, extremely high However, it has been suggested that durable mudrocks may be better distinguished from non-durable types on the basis of compressive strength and three-cycle slake-durability index (i.e those mudrocks with a compressive strength of over 3.6 MPa and a three cycle slake-durability index in excess of 60% are regarded as durable) In fact, the value of the slake durability test as a means of assessing mudrock durability has been questioned as the results obtained frequently not compare well with those of other durability tests This has led to a number of adaptations being made to the test, for example, ethylene glycol has been used instead of water, the time taken to carry out the test has been extended and the number of cycles has been increased Failure of weak rocks occurs during saturation when the swelling pressure (or internal saturation