Page - 509 - in Book of Full Papers - Symposium Hydro Engineering

Fortunately, with close coordination with the Bengawan Solo Water Council representing the various user groups in the basin, the start of a construction window can be controlled by monitoring releases from the reservoir. For the 2017 Construction season the drawdown of the reservoir was very successfully timed to the lower the water levels below Elevation 131.0 to within the exact day (15- Aug-2017) that had been negotiated. However, the timing of the end of the construction season is not so easily solved as it is dependent on the onset of early rains which induce flash flooding and rapid rising reservoir levels. No amount of “coordination” can control the onset of early rains; therefore, a physical solution is necessary. The two options were considered; using cofferdam against rising waters or raising the base platform for the construction. Second Challenge – Very Soft Foundation The second challenge of construction is the soft sediments, which cannot directly support placement of closure dike. The improvement of the soft soils in order to support the weight of the embankment is to be solved with the use of Deep Mixing Soil Improvement (DMSI). 2. DMSI (DEEP MIXING SOIL IMPROVEMENT) – GENERAL Deep Mixing Soil Improvement (DMSI) is a physical method to form soil- cement columns in soils in order to increase the bearing capacity and / or reduce settlement of soft ground. Unlike a bored pile where a boring machine excavates a hole, removing the soil which is replaced with reinforcing bar and concrete to form a solid concrete pile, Deep Mixing involves a machine which augurs or drills a mixing blade down a hole and then mixes cement with the soil in-situ, (in place) to form a soil cement columns. The figure below shows the basic sequence in three steps. In Step 1 - Twin augurs drilling down through the soil until reaching the hard bottom. Then in Step 2- the augurs are rotated back up and at the same time cement slurry is injected from the blades and mixed into the soil. When in Step 3 the augurs are extracted, the remaining column is a mixture of soil and cement. This equipment then moved to the next location and the process repeats. 509