Slake-Durability Index (SDI) Tests

An abundant group of rock materials, especially those with a high clay content, are often prone to weakening either by slaking or disintegration when exposed to short term weathering processes of a wetting and drying nature. Special types of tests are therefore required to predict the exact extent of their mechanical performance and these tests are often referred to as “index tests”. It is worth mentioning that index tests are best used in classifying and comparing one rock sample with another, which is why for this particular test an untreated shale sample was tested against a DECL treated shale sample, in order to determine the effectiveness of the DECL product on the stability and durability of the Kimberley shales. The following slake-durability tests were conducted at Rocklab in Pretoria according to the ISRM’s specifications and was used to simulate a short term natural wetting and drying process for the Kimberley shales.

Two samples were sent in for analyses, one treated with the Sasbind DECL product (chosen as the best result of the above mentioned AWT test) and the other left untreated. The durability of

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both samples were classified according to their slake-durability index after the second (2nd) wetting and drying cycle using the classification table as recommended by ISRM and proposed by Gamble (1971) (see Table 10 below).

Since the fabric of a rock has a very important effect on the properties being measured during the given SDI test, both samples were ensured to represent two undisturbed rock specimen as far as possible, having being treated or handeld as little as possible prior to commencement of these tests. The apparatus used to test the durability of these rocks essentially consisted of:

 A test drum comprising a 2 mm standard mesh cylinder of unobstructed length (100 mm) and diameter (140 mm).

 A horizontal trough axis to contain the test drum and allow free rotation.

 A motor drive capable of rotating the drum at a speed of 20 rotations per minute (rpm).  An oven capable of maintaining a temperature of 105° to within 3°C for a period of at least

12 hours.

 A scale capable of weighing the drum plus sample to an accuracy of 0.5 grams.

These components of the SDI test equipment are shown below in Figure 44:

Figure 44 - Critical dimensions of the slake-durability test equipment (Franklin & Chandra, 1972). Table 10 - Slake-durability index (SDI) classification system according to Gamble (1971).

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The exact procedure as followed by the laboratory personnel at Rocklab, to run the SDI test for the purpose of this project, is summarized below:

1. Two representative shale samples were selected and cut into ten equidimensional rock lumps, each with a mass of 40 – 60 grams, to give a total sample mass of 450 – 550 grams each.

2. The first sample was left untreated whilst the second sample was treated in the exact same manner as discussed in Section 5.2.3 – Table 7 with the DECL product, Sasbind.

3. After coating the treated rock sample with two protective layers of the Sasbind DECL product, both samples (treated and untreated) were dried to a constant mass at a temperature of 105°C.

4. The mass (A) of the drum plus each individual sample was then recorded.

5. The trough was filled with slaking fluid (in this case, tap water) and the drum was rotated for 200 revolutions during a period of 10 minutes.

6. Thereafter the drum plus the retained portion of the sample was dried again to a constant mass of 105°C.

7. The mass (B) of the drum plus the retained portion of the sample was recorded for a second time.

8. Steps (5) – (6) were repeated again and the mass (C) of the drum plus the retained portion of the sample was recorded for a third time.

9. Finally the drum was brushed clean and its empty mass (D) was also recorded.

To accurately calculate the slake-durability index for both the treated and untreated shale samples, the second cycle of the SDI test was calculated as a percentage ratio of the final to initial dry sample mass as shown below:

[ SDI (Idz) = −

𝐴− x 100% ]

In order to fully understand the results and accurately interpret the state of the samples after completion of the test, which is directly related to a samples resistance towards weathering and a good visual indication of its durability, the following symbols were used to denote the end state and structure of both rock specimen (treated and untreated) after completion of the fourth cycle of the SDI test (see Tabel 11):

1 - Specimens were still intact after 4 cycles of slake durability tests.

2 - Some specimens were broken into a few big pieces after 4 cycles of tests. 3 - Some specimens were broken into small pieces after 4 cycles of tests. 4 - Specimens were broken into a lot of small pieces after 4 cycles of tests.

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The results will be presented and discussed in Chapter 6. The reason for conducting a short term SDI test on both a untreated and DECL - treated shale sample was to:

 Predict the exact extent of the mechanical performance of the Kimberley shales, both treated and untreated, under exposure to a short term weathering process of a wetting and drying nature.

 Classify and compare one rock sample with another (treated vs. untreated) as to determine the effectiveness of the DECL product on the stability and durability of the Kimberley shales and to document the differences in behviour during commencement of the test.

 Determine the slake-durability index of the Kimberley shales in their natural state (untreated) by using international ISRM standards. This allowed the Kimberley shales to be classified and categorized against other shale samples from around the world that underwent the same test under the same standards.

 Evaluate whether the DECL product (Sasbind) would have a strengthening effect on the disintegration and slaking rate of the Kimberley shales and if so, to what extent.

5.3.7 Summary

The methodological procedure as followed and described above produced very valuable and accurate results in terms of proposing a viable solution towards the defined slope stability problem at the Kimberley “Big Hole” Mine. The desktop study (i.e. aerial photography, drone modelling and pixel tracking) produced very informative maps and illustrations of sidewall boundary migration at the Big Hole Mine over the past few years and a lot of valuable conclusions could be drawn from this work, including the entire extent of slope stability problems and the rate at which they ensue. Furthermore, the petrographic analysis in combination with a full geochemical analysis proved extremely useful in identifying the different types of clay minerals present within the internal structure of the rock and provided enough mineral phase diagrams for identifying the different mineral phases in each rock sample. From this, the swelling and shrinkage potential of the Kimberley shales could be deduced, which gave a lot of insight into the suggested weathering rate of the rocks as a function of time and water content.

Table 11 - _{Calculated results of the slake-durability index (SDI) tests as carried out by Rocklab in Pretoria on both an} untreated shale sample as well as a DECL treated (Sasbind) sample.

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In terms of the laboratory tests as conducted and described above, all four tests provided valuable results in terms of assessing the effectiveness of the different DECL products on the durability and weathering resistance of the Kimberley shales. A strong correlation between the results of the cyclic wetting and drying test, the comparative accelerated weathering test and the slake-durability index test could be drawn. As for the conducted absorption test, the results showed a slight deviation when compared to the rest of the test results, which could possibly be attributed to human and mechanical errors during measurement of each sample.

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