Tank Pad foundations (Fig. 1) are-\r\n\r\n \tUsed for Vertical storage tanks, Circular in shape having larger diameter\r\n \tUsed where there are no anchorage requirements for the tank (as specified by the tank vendor)\r\n\r\nComponents of Tank Pad foundation:\r\n\r\n \tTank Pad body\r\n \tTank Pad Shoulder\r\n \tFinishes\r\n\r\nTank foundation functions and requirements :\r\n\r\nThe functions of a tank foundation are:\r\n\r\n \tTo spread and transfer the load from the tank and its contents via tank foundation body and shoulder to the sub-grade so that the settlement remain within the allowable limits.\r\n \tTo provide a smooth surface with sufficient bearing capacity and stability for tank construction and operation\r\n \tTo channel rain water away from tank shell and tank bottom quickly\r\n\r\nThe requirements for the shoulder of the tank foundations are:\r\n\r\n \tTo provide sufficient lateral support to the tank foundation during construction, operation and maintenance activities\r\n \tTo resist edge settlement beneath the tank shell\r\n \tTo resist erosion by wind and\/or water\r\n\r\nDesign Parameters:\r\n\r\nThe tank foundation shall be designed to ensure that it is capable of:\r\n\r\n \tabsorbing subsoil deformations to ensure deformations in the tank base remain within limits\r\n \tthe possible unequal foundation pressures\r\n\r\nUnder certain circumstances, soil improvement under the tank foundation shall be considered in order to:\r\n\r\n \tprovide a foundation with sufficient strength\r\n \treduce large settlements\r\n\r\nThe minimum elevation of the tank foundation at the base of the tank shall be 0.60 m above the adjacent terrain.\r\n\r\nFig. 1: Example of Typical Tank Foundation\r\n\r\nDrain pipes shall be placed around the circumference of the foundation in order to detect leakages (if any) and to prevent the build up of pore pressure due to liquid accumulating in the tank pad.\r\n\r\nGeo-textile membrane\u00a0 shall be installed under the surface finishing to the shoulder if there is a possibility of washout of fine materials\r\n\r\nA stability analysis of the shoulder and tank foundation shall take into account the following aspects :\r\n\r\n \twind loads\r\n \tearthquake loads\r\n \tinitial height above adjacent ground level\r\n \thighest possible ground water level\r\n \tangle of slope of the shoulder\r\n \tgeotechnical properties of the tank foundation materials and subsoil\r\n \tload provided by the tank and its contents during hydrostatic testing, operation and maintenance\r\n\r\nTank Foundation Shoulders:\r\n\r\nThe shoulder width shall be selected such that the stability of the foundation, shoulder and subsoil is ensured. The minimum width of the tank foundation shoulder (S) depends on several aspects:\r\n\r\n \theight of the tank (H)\r\n \tdensity of the product\r\n \tslope of the tank foundation edge\r\n \theight of the tank foundation (T)\r\n \tThe shoulder shall have a gradient of 1:10. The slope to the shoulder shall not exceed a gradient of 1:1.5.\r\n\r\nMaterials:\r\n\r\nTank pad foundations are constructed with durable, granular materials, such as crushed rock, coarse sand, etc.\r\n\r\nThe tank foundation body shall be constructed from clean granular materials which meet the following requirements:\r\n\r\n \tnot crushable\r\n \tlow compressibility\r\n \thigh friction properties\r\n \tlow silt content\r\n \tfree draining\r\n \tinsensitive to weathering, chemical changes\r\n \teasy to compact\r\n \tnot sensitive to liquefaction (especially when constructed in earthquake zones).\r\n\r\nWell-compacted sand, especially coarse sand, meeting the above requirements providing the chemical and mechanical stability of the minerals is guaranteed.\r\n\r\nIn order to prevent capillary rise of the groundwater the upper 200 mm of the tank foundation shall comprise coarse sand.\r\n\r\nTank foundation finishes:\r\n\r\nThe purpose of the surface finishing layer under the tank\u00a0 is:\r\n\r\n \tTo act as a barrier to corrosion promoted by water or water vapour, together with chemicals which may be present in the tank foundation or subsoil\r\n \tTo promote a uniform distribution of stress from the tank bottom to the tank foundation\r\n \tTo allow thermal expansion of the tank bottom\r\n\r\nThe function of the surface finishes of the tank foundation shoulder is to protect the foundation from damage caused by weathering, erosion and construction, operation and maintenance activities.\r\n\r\nThe surface finish layer under the tank shall be a mixture of sand and bitumen.\r\n\r\nThe final levels of the placed and compacted surface finishes under the tank edge between any two points 10 m apart around the periphery of the tank shall not be greater than +\/-\u00a06\u00a0mm.\r\n\r\nThe tank foundation shoulder may, like bund walls, be covered by a mixture of sand, bitumen and cement or lime (i.e. a wet sand mix).\r\n\r\nThe surface finish to the shoulder shall have a gradient slope of 1 vertical in 10 horizontal from the underside of the annular plate of the tank, to avoid ingress of water under the tank.\r\n\r\nDuring construction of the tank foundation, necessary tests are performed on each 0.3m thick layer of placed and compacted.