To determine the slake durability index of given sample.

Scope:-

This test is intended to assess the resistance offered by a rock sample to weakening and disintegration when subjected to two standard cycles of drying and wetting.

Apparatus:-

The apparatus consists essentially of the following;

(a). A test drum comprising a 2.00 mm standard mesh cylinder of unobstructed length 100 mm and diameter 140 mm with solid fixed base. The drum must withstand a temperature of 105^oC. The drum has solid removable lid. The drum must be sufficiently strong to retain its shape during use, but neither the exterior of the mesh nor the interior of the drum should be obstructed, for example by reinforcing members.

(b). A trough to contain the test drum supported with axis horizontal in a manner allowing free rotation, capable to being filled with a slaking fluid such as water to a level 20 mm below the drum axis. The drum is mounted to allow 40 mm, unobstructed clearance between the trough and base of the mesh.

(c). A motor drive capable of rotating the drum at a speed of 20 rpm. The speed should be held constant to within 5 % for a period of 10 minutes.

(d). An oven capable of maintaining a temperature of 105^oC to within 3^oC for a period of at least 12 hours.

(e). A balance capable of weighing the drum plus sample to an accuracy of 0.5 g.

SLAKE DURABILITY TEST

This test method is used to estimate qualitatively the durability of weak rocks in the service environment.

Background: Rock properties change with time due to processes such as exfoliation, hydration, slaking, solution, oxidation, abrasion, etc.

Exfoliation: Rock being disintegrated sheet by sheet.

Hydration : Reaction with water.

Slaking: Weakening of rock due to repetition of wetting and drying.

Oxidation: Reaction with oxygen.

Abrasion: Smoothening of surface when scraped by other material.

Since these processes cannot be reproduced in the laboratory, resistance or rock (durability of rock) against them is evaluated in terms of some appropriate indices.

Slake Durability Test:-

A drum of 100mm in length and 140mm in diameter is rotated, half immersed in water, at 20 rounds per minute. The drum is made of a 2mm sieve. About 500g of rock is broken into 10 pieces and put in the drum. After rotation for 10 minutes, the percent of rock retained inside the drum, on a dry weight basic, is reported as the slake durability index, I_d (Goodman, 1980). A smaller I_d means that a greater amount of rock was broken into small pieces and lost through the sieve. 

Gambles' Slake Durability Classification (Goodman, 1980):-

Group Name	%Retained after one 10 min cycle (dry weight basis)	%Retained after two 10 min cycle (dry weight basis)
Very High Durability	> 99	> 98
High Durability	98 - 99	95 – 98
Medium High Durability	95 – 98	85 – 95
Medium Durability	85 – 95	60 – 85
Low Durability	60 – 85	30 – 60
Very Low Durability	< 60	< 30

                       (After Dr. I. Towhata, University of Tokyo)

Procedure:-

• A representative sample is selected comprising ten rock lumps each with a mass of 40-60 g. to give a total sample mass of 450-550 g . The maximum grain size of the rock should not be more than 3mm. Lumps should be roughly spherical in shape and corners should be rounded during preparation.
• The sample is placed in a clean drum and is dried to constant mass at a temperature of 105⁰C usually requiring from 2 to 6 hr in the oven. The mass A of the drum plus sample is recorded. The sample is then tested after cooling.
• Te lid is replaced, the drum mounted in the trough and coupled to the motor.
• The trough is filled with slaking fluid, usually tap water at 20⁰C to a level 20 mm below the drum axis and the drum rotated for 200 revolutions during a period of 10 minutes to an accuracy of 0.5 minutes.
• The drum is removed from the trough, the lid removed from the drum and the drum plus retained portion of the sample dried to constant mass at 105⁰C. The mass B of the drum plus retained portion of the sample is recorded after cooling.
• The drum is brushed clean and is mass D is recorded.