With the ever-increasing demand for coal due to the construction of new Eskom coal fired power stations, Total Coal South Africa have expanded their Dorsfontein operation to mine other seams of coal in the area. This expansion included the construction of a new 30m high coal stockpile which comprises a Reinforced Earth basin to house the stockpile with a tunnel running underneath it. The tunnel is used to transport the coal from the stockpile via a conveyor belt system. Due to the type of equipment being installed in the tunnel, it was imperative that the total and differential settlements were minimised.
The field investigation comprised Dynamic Probe Super Heavy and Continuous Surface Wave tests and test pitting. Laboratory testing of disturbed and undisturbed samples was carried out to identify the properties of the in situ material. The results obtained revealed this material to be of very poor quality. The situation was exacerbated by the presence of a shallow ground water table. Measures were thus required to minimise the settlement.
A brainstorming session was held with the consulting engineer to Dorsfontein (Alan Robinson) and ARQ’s Mark Laughton and Alan Parrock to identify the most cost-effective solution to the problem. Following much deliberation it was established that the installation of stone columns in conjunction with a high strength geosynthetic would act as a “piled raft” below the tunnel minimising settlement.

The stone columns are installed by placing 1m³ of very hard rock (UCS>100MPa) and ramming it into the ground to a depth of approximately 1m using a Rapid Impact Compaction rig. This rig drops a 12 ton weight through a height of 1.5m at a rate of 45 blows per minute. Once the rig has reached a depth of 1m, the depression is filled with rock and the process repeated until the column reaches a depth of between 5 and 6m. Following the installation of the stone columns in a grid-like pattern, a high strength geosynthetic is placed over the area and spans between the columns. The stone columns form the “piles” and the geosynthetic with a 300mm layer of G6 material placed above it forms the “raft”. The figures below show the position of the stone columns.

Finite element analyses which were carried out using Phase2 showed the use of this technique greatly reduced the expected settlement from 190mm down to just 45mm with the expected differential settlements brought well within acceptable limits.
A slope stability analysis was also carried out on the external slopes using Prokon’s SlopBG. Earthquake loading was simulated by applying a horizontal acceleration of 0.12g in the analysis. Initial outputs revealed the un-reinforced slopes to have a factor of safety of 0.58 and a probability limit (the probability that the factor of safety is below 1) of 100%. A secondary analysis was carried out in which the slope was reinforced with 7 layers of Kaytech’s RockGrid PC200/200 at a vertical spacing of 3m. This analysis showed the factor of safety had increased to 1.18 as can be seen in the figure below. A probabilistic analysis was carried out on the model and the probability limit as above had decreased to 1:550 which is well within acceptable design limits.

Amazing how some geosynthetic, a bit of stone and a “giant hammer” can make such a big difference. 

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