eRocks

This sample is a fine-grained limestone that consists predominantly of calcite, with a high volume of allochems in the form of various types of bioclasts. Minor coarser-grained calcite and narrow quartz veinlets also traverse this sample.

4.1.1 Macroscopic Features

This sample is a fine-grained, pale grey sedimentary rock that is competent (flexed during formation) in nature, and exhibits an abundance of fossil fragments (Figure 4.11.1 part D). Narrow calcite veinlets can be seen traversing the sample.

4.1.2 Mineralogy and Petrography

The bulk of this sample consists of a fine-grained calcite matrix together with abundant medium to coarse-grained allochems. The fine-grained matrix that hosts the variety of allochems (Figure 4.11.1 part A+B+C) consists of calcite grains, that are typically <5µm in size and may be referred to as a micrite or carbonate mud.

The allochemical components of this sample consist largely of different types of bioclasts, including brachiopods and various foraminifers, namely endothyracids (Figure 4.11.1 part A+B). Several narrow cross cutting calcite veins were also observed (Figure 4.11.1 part C). Examination of this rock using qualitative energy dispersive analysis revealed the presence of narrow quartz veinlets (Figure 4.11.1 part D) that possibly formed during compaction.

(a and b) Colour, transmitted light photomicrographs that illustrate the nature of the fine-grained calcite matrix (greenish grey) with abundant fossil fragments, mainly different types of bioclasts. (c) Narrow cross cutting calcite (grey-white) veinlets are present. (d) A false coloured backscattered electron image illustrating the presence of a narrow quartz veinlet (mauve) in the fine-grained calcite (orange) matrix.

Samples were examined macroscopically and then a diamond saw was used to section each sample, after which representative samples were used in the preparation of a thin and a polished section. Thin sections were studied using transmitted light microscopy and individual phases within the sample were identified by their optical properties.

Polished sections were examined using reflected light microscopy with individual phases being identified by their optical properties. Backscattered electron imaging was also used to determine the composition of individual phases by the use of energy dispersive microbeam techniques.

All figures referred to in the text are presented at the end of this chapter in section 4.11. in addition to a caption below each image explaining important features.

A lump sample (14-10mm) is mounted in resin and pushed against a rotating lap for a period of 500 revolutions with a force of 2kg. The percentage of the original sample weight is the AAV. This test gives an insight into mineral cleavage behaviour and intergranular bond strengths.

This test is a variation of the ACV where a load specific to the force required to produce 10% fines is investigated where fines are -2360ï?­m. The standard range into which the undersize should fall is 7.5% to 12.5%. The force to produce 10% fines is given by:-

Where x if the maximum force in KN and y is the mean percentage passing -2360ï?­m of two tests.

A sample of 2 kg is subjected to a continuous compressive force with a total load of 400kN. The rate of loading is 40kN per minute. As with the AIV the percentage passing 2360�m is calculated. With both the AIV and ACV a lower value indicates a “stronger� rock.

The AIV was incorporated into the British Standards in 1955 as a measure of crushed rock strength. A sample of lumps (14-10mm) is impacted with a piston fifteen times and the product is sieved using a 2360ï?­m aperture. The percentage material passing denotes the AIV. The test is then repeated and should be within a numerical range of one.

The British aggregate industry has a set of standard tests that are used to characterise the physical properties of aggregate products.

This technique was also adapted from the ceramics and hard metals industry by Middlemiss and King [1994]. A range of parameters can be obtained from this procedure including fracture toughness, crack length, crack density and extent of cracking. The procedure involves making an indentation into the test piece with a micro-hardness tester. The image of the indentation area is then captured digitally and then the cracks can be traced onto the screen of an image analyser. This allows the total crack length to be calculated. There are limitations with this approach to finding hardness, for example the energy associated with the indent has to gauged correctly to avoid total breakage, and some materials are not suitable such as those where grain size is similar to that of the indentation point.

This method is limited to very soft or very hard rocks [ISRM Brown (Ed) 1981]. The principle of the test is the determination of the amount of energy absorbed into a rock when impacted upon by a set amount of energy (0.74Nm). The orientation of the hammer is critical because values can be affected if the hammer is incorrectly used.

Carter and Sneddon [1977] found that the method was quick to perform, more than twice as many tests could be performed when compared to the standard UCS tests. They concluded that the results correlated well with UCS and could therefore be used as a quick alternative to a laboratory test. It was also suggested that the method was not as direct as the point load test because it does not fracture the sample. Aggistalis et al [1996] found a high positive correlation between the Schmidt hammer results (r=0.86) and the degree of weathering.

This device has been developed at the Utah Comminution Centre [Bourgeois 1992] and has proved to be very successful in recording impact fractures of particulate material. It allows the generation of force time histories when a particle is subjected to an impact. Dropping a steel ball of known mass and diameter from a known height onto the test particle, which is resting on a rod, generates the impact. At the base of the rod there are strain gauges which record the compression wave resulting from the impact. From the generated output the minimum force to fracture can be determined. From this using the relationship below the particle strength can be calculated:-

Sigmap = 4Fi / pi2d

Where Fi is the fracture initiation force and sigmap is the measured particle strength.

There were some doubts concerned with the reliability of the results achieved, due to the uncertainty from the fact that the initial fracture may not always be clearly defined and that particles below 2mm gave the greatest concern regarding accuracy.

The shape and orientation of the particle are factors that are not addressed in the testing method. For example a smaller particle gives the ball further to drop, therefore removing the standardisation of the method. The presentation of the particle is not considered with respect to the fact that a particle may break more easily in one plane of orientation than another.