The world's largest underground storage facility for Liquefied Petroleum 0Gas (LPG) was constructed in Cretaceous granitic rock in the Inland Sea near Kurashiki in western Japan. This facility is being operated using a water curtain system in order to control the flow of groundwater and pore pressure, thereby ensuring long-term safe LPG storage. Grouting was also conducted during the excavation of rock caverns so as to control the hydraulic conductivity of the rock mass appropriately.
Pre-excavation grouting was conducted under high grouting pressure (about 4.2 MPa) in order to control the hydraulic conductivity below 0.35 Lugeon. Also post-excavation grouting was carried out if the inflow exceeded the requirement. After the excavation of the grouted areas, the effects of grouting were checked by measuring the inflow rate and the pore pressure around the storage caverns. If needed, geological survey, analysis of measurement data, and analysis of grouting data were carried out and grouting designs were rationally revised step by step in terms of grouting equipment, grouting materials, grouting patterns, and criteria of additional holes.
The main grouting material was super-fine cement for the whole of the cavern. However, a micro-fractured zone (mf-zone) was encountered, where improvement effect with cement grout was limited despite many changes in grouting design made. In this zone, colloidal silica grout (CSG) was used as a supplement, after confirming its suitability for long-term sealing at the Kurashiki Base. Because of little precedent of using CSG for rock grouting in Japan, specifications of CSG grouting were reviewed and adjusted based on information-based technique. The results of pre-grouting with CSG in the cavern indicates that the mf-zone where improvement of sealing effect was not sufficient when using cement grouting, has been improved using CSG.
As a result, the inflow to the LPG caverns and the groundwater pressure around the caverns after the completion of construction were within the scope of the prediction.
