Absorption Tests

The rate at which mudrocks (and with no exception to the Kimberley shales) absorb water, is a critical parameter with regards to its permeability and a very important engineering property when considering the stability characteristics thereof. It is often said that there is a direct relationship between the permeability of a rock and its susceptibility to weather, meaning that the more permeable the sample, the faster it will deteriorate and disintegrate (Venter, 1980). Because the rate of weathering of a rock is more often than not a direct function of its permeability and absorption characteristics, a unique absorption test was conducted at the geotechnical rock

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laboratory at the University of Stellenbosch following similar tests by Deo (1972) and Venter (1980), only with a few minor changes tailored to the purpose of this project.

As part of trying to find a proper solution towards the defined slope stability problem at the Kimberley “Big Hole” Mine (which includes the vast susceptibility of the Kimberley shales to weather and deteriorate under natural conditions, especially during times of heavy rainfall), five different “dust and erosion control liquids (DECL)” were identified with the aim of: (1) decreasing rock permeability; (2) increasing rock slake index / resistance; and (3) strenghtening internal rock parameters, with the ultimate goal to slow down the weathering rate of the Kimberley shales and in turn enchance the stability thereof. All five liquids represent water repellent bases, which resists water infiltration through the surface. It is applied to and binds the soil and dust size particles on the surface of the rock together. In short, it forms a water resistant protective coat around the surface of the rock, ultimately acting as a soil modifier that not only enhances the physical properties of the rock, but also increases its resistance to deformation. In theory, all five DECL products should therefore decrease a rock’s permeability and the rate at which it absorps water.To test this theory, all five DECL products were integrated into the following absorption tests. The exact description as well as the unique chemical composition and characterisitcs of each individual liquid is attached in Appendix A and the following will only serve as a brief description of each:

 NanoSil is a water-soluble, UV and heat stable, reactive soil modifier with the ability to retain strength of soil (or rock) particles and resist deformation. It is most commonly used in the industry as a breathable soil waterproofing product for road bases and slopes.

 NanoBond is an acrylic co-polymer emulsion with the ability to bond to soil particles and resist soil erosion. It is most commonly used in the industry as a dust suppressant especially in side shoulders and slopes.

 NANO is an even mixture bewteen NanoSil and NanoBond. It is usually mixed together in the industry for one step waterproofing and bonding of compacted soils. It combines the effects of both NanoSil and NanoBond products to get the most effective results.

 Sasbind is a uniquely formulated water-based emulsion of modified acrylic polymers. It is suitable for the binding and stabilisation of various soil layers and types in the construction of roads and is also commonly used for application to the surfaces of already constructed roads that require dust palliation.

 Sasbind (+Bit) constitues the exact same product as “Sasbind”, only with an added component of bitument to the mixture.

Before absorption tests commenced, the samples were prepared and included the following steps: 1. Five equidimensional rock lumps, weighing between 50 and 60g each, were collected for

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test batch. Six individual absorption tests were undetaken, that is, one for each DECL product and one for an untreated reference batch as illustrated in Table 7.

2. Treated samples were all sprayed with two layers (or coats) of the respective DECL product on a 1:2 water to product ratio (i.e. 200ml of water mixed with 400ml of product to give a 600ml mixture) (see Table 7).

*Note: The DECL product “NANO” is a combination between DECL products NanoSil and NanoBond, which is why the dosage of the applied liquid to the rock surface in Test 4, differs from the rest of the indicated dosages (in other words 200ml of NanoSil was mixed with 200ml of NanoBond to give 400ml of product, but because the water to product ratio remains 1:2, 800ml of water was needed to complete the mixture).

3. Between treatments (or coatings), samples were left to dry for 24 hours in order to make sure the surface was completely dry before submurging in water as seen in Figure 39.

Figure 38 - Average sample size and shape for a single absorption tests.

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After the necessary sample preparation had taken place, absorption tests commenced and included the following steps (tests were conducted in the Geotechnical Laboratory at the University of Stellenbosch):

1. Samples were dried in the sun for 12 hours to get rid of any moisture still left in the rock. 2. Subsequently, rock lumps were weighed together as a batch for each individual absorption

test and the combined weight was documented in Table 8.

3. Each batch was then completely immersed in water for 15 minutes after which it was taken out, surface-dried and weighed again (see Figure 40).

4. After the first weighing-session, rocks were placed back in the water for another 15 minutes (submerged for a total of 30 minutes), taken out, surface dried and weighed.

5. The submergence period was then increased systematically with periods 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours and 1 day with the process of surface drying and weighing consecutively being repeated.

6. For the latter mass determinations the exact weighing times were not adhered to as strictly, as the readings were by then out of the high rate of absorption part of the curve.

Figure 39 - Sample preparation (or treatment) with DECL liquids prior to commencement of the individual absorption tests.

Figure 40 - Absorption tests 1 - 6, completely immersed in water for 7 days and weighed at regularly timed intervals. Each batch represents a different DECL product, whilst one contains an untreated reference sample.

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These tests were run for a total of 168 hours (or 7 days), to ensure that the maximium moisture content was recorded for each individual batch and that each sample reached its maximum saturation weight in order to represent the most accurate results. The following table represents the weighed mass of each sample at regularly timed intervals in grams and gives a good idea of the raw data that was used to calculate the overall absorption rate / moisture intake for each individual absorption test as a function of time (see Table 8).

The moisture content for each batch was further calculated for each time interval as a percentage of the total dry mass during regularly timed intervals. This was done by means of the following equation:

% 𝑜𝑖 𝑜 𝑖 = 𝑎 𝑎 𝑖_{igi a a g}− 𝑖 𝑖 𝑎 𝑎 ) x 100%

As a result, the calculated percentages of the total moisture intake (or absorption rate) for each individual sample as a function of time was tabulated and will be displayed graphically during the discussion of the results in Chapter 6.

The reason for conducting these absoprtion tests were to:

 Determine the general absoprtion characteristics of the Kimberley shales by means of assessing the absorption rate of untreated samples when immersed in water over a long period of time.

 Determine whether any of the DECL products would have an effect on the absorption rate of the Kimberley shales and if so, to what extent.

 To determine which one of the DECL products proved most effective in decreasing a rock’s permeability characteristics and showed the slowest absorption rate when immersed in water for a long period of time.

Table 8 - Raw data representing the weighed mass of each sample (or absorption test) at regularly timed intervals for a total of 7 days.

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