Choosing the testing material type was dependent on the stone availability in the area in the Western Cape. The density was deemed to be the most critical determiner among other factors. It was incumbent that the most available riprap rock material be used for the incipient failure tests. Most importantly the chosen riprap had to adhere to a density close to the range 2600- 2700 kg/m3. Rocks with density in that range are generally used since they provide high stability and resilience in riprap design projects. Different stone suppliers in the Cape Town region were visited, and only two were chosen to source the required riprap. It was found that the most available angular riprap rock was the grey hornfels in Figure 21.
Figure 21: Pile of hornfels riprap rock
Hornfels riprap rocks are durable and dense, thus they were chosen for this specific study. The hornfels riprap rocks were sourced from the Tygerberg group of quarries. Two quarries were used to source the required riprap size for the research. The quarries that were visited were the AfriSAM Pty Ltd and Ciolli Bros Pty Ltd quarries. The quarries were located in the Durbanville area in the Western Cape.
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3.2.4.2 Riprap Size
The riprap median diameter stone sizes were chosen based on scale of 1:15 model to prototype scale ratio. The tested median stone sizes were D50 = 0.038 m and D50 = 0.075 m. Initially, three stone sizes were to be tested. The middle stone size, the D50 = 50 mm, was no longer considered for testing due to time constraints and labour limitations, as well as the preliminary findings during the testing of Test series one. During the testing of the D50 = 0.038 m median stone size, the riprap on the bed area failed before the bank. Thus, for the next set of testing series, it was decided that the D50 = 0.05 m would not be tested, and the larger D50= 0.075 m median stone size would be tested.
One of the main challenges for this research was obtaining the required stone size from a quarry, as the angular riprap stone was not readily available at the laboratory. Even when the riprap rocks were obtained from the quarries, they still required to be sorted and graded. These are typical disadvantages that would be directly encountered by engineers when opting to implement a design for riprap river revetment projects.
The following procedure was followed to obtain the required riprap size and grading:
• A large pile (more than 4.6 Tonnes) of mixed size riprap stones from the quarries were transported to the laboratory.
• The required stone sizes were determined as per the specifications to pass the specified grading (see section 3.2.4.6 for grading specification).
• Standard sieve diameters were used to sort and sieve the required stone sizes from the pile.
• The riprap stones were placed separately according to each size determined by sieving. • The stones were mixed as per the grading requirements specified in section 3.2.4.6.
Figure 22 displays the large diameter sieves that were used to perform the sieving. The stone
size was defined according to the sieve in which it was retained as opposed to the sieve that it passed through. Therefore, a stone defined as being a 26 mm stone was retained on the 26 mm square aperture sieve.
3-65 Figure 22: Standard metal sieves used for sorting the required stone sizes.
An important observation made during the sieving of the stones was that the physical stone sizes retained in each sieve were larger than the specified aperture of the sieve. This was due to the irregular shape of the stones. Thus, it was critical to reduce the use of stones that were needle-like shaped as they could go through some sieves even though they were larger than the sieve in which they were retained. This was noted, and consistency in terms of defining the stone size was used throughout the tests. This is also important for any designer who might use the findings of the thesis, to know that the stone sizes were defined according to the retaining sieve diameter aperture.
It was also important that the stone sizes were stored safely and should not mix. This was important during the grading and mixing because the correct stone size proportions were critical to obtaining the required grading. Therefore, separation of the stones by size was achieved by using blocks and wooden boards at the laboratory as shown in Figure 23.
19 mm sieve 38 mm sieve 53 mm sieve 26 mm sieve Square aperture sieves
3-66 Figure 23: Stone size storage and separation.
Lastly, during Test series one, the D50 = 0.038 m stone was tested first. On the second testing series, the D50 = 0.075 m stone size was tested. In the results of the first two testing series, the riprap dumped on the bed area failed before any failure on the side bank. As a result, the challenge for Test series three was the unavailability of stones larger than D50 = 0.075 m. Moreover, to go to a quarry, sort and mix riprap with a larger median stone size than D50= 0.075 m would be labour intensive. As a result, it was decided that the D50 = 0.075 m riprap on the bed area must be glued on the bed area, so that the riprap on the bed area does not fail before the riprap on the side bank. The investigator managed to test the failure conditions on the side bank of the physical hydraulic model on the downslope testing area. Thus, only the stability of the D50 = 0.038 m and D50 = 0.075 m median stone sizes were investigated.