Ground settlement in PLANT areas are major concern as it might affect integrity of structures and safe operations. In a recent incident at GASCO Sulphur Handling Terminal (Ruwais), substantial ground settlements were observed at sulphur granulator unit area which affected nearby foundations/structures. Various reasons can contribute to settlement issue and this paper presents the methods employed to determine the root causes, assessing the impact on existing structures/foundations and identifying corrective actions required to ensure safe operation.
A detailed study was carried out to ascertain the extent of settlement and to identify the reasons. In order to assess the severity of settlement and its pattern, topographical survey was performed. Geotechnical investigations including boreholes, trail pits with associated field & laboratory tests were executed to study the ground conditions, soil chemistry, collapse potential and strength characteristics of ground. Equipment vendor were also consulted during various stages of study to discuss allowable operational limits. Study results revealed the root cause of ground settlement and appropriate remedial methods were finalized considering techno-economic factors.
Topography results determined the affected foundations/structures and settlement extent. Geotechnical investigation findings indicated that the settled area is a reclaimed land consists of dredged hydraulic fill. The fill material was poorly compacted calcareous soil, contains high proportion of carbonates & gypsum. Carbonate sand is susceptible to dissolve in presence of acidic water & has collapse potential properties. Leaky chemical drain channels were noticed at affected locations.
Many possible causes of settlement were analyzed and evaluated based on investigation results. It is concluded that principal root cause of the settlement is leakage of acidic/non-acidic water from drain channels into ground which had weakened and consolidated the calcareous soil.
Static and dynamic analysis were performed for settled foundations/structures to ensure its stability under various potential loading conditions. Necessary modifications and repair work details are finalized based on analysis results. To avoid further settlement, various options for ground improvement are studied including compaction grouting, underpinning of foundations, jacking/relevelling of super structure, compensation grouting & replacement of foundations.
Study outcome showed that ‘compaction grouting’ is most suitable & economic long term solution. This method involves drilling and injection of vertical and inclined holes in the ground at affected areas with high pressure grouting and thus displace & compact the soil to prevent settlement.