Greetings to all the hydraulic engineering gurus on the forum. My question refers to a situation where two adjacent septic tanks were installed underground in a high water-table area. The bottoms and sides of the tanks are now bulging inwards, somewhat dislodging the interconnecting pipes thus allowing groundwater to stream into the tanks. (The cause of the tanks bulging is my assumption based on a visual inspection) Each HDPE tank has a capacity of 12500 litre and weighs 450kg, has a diameter 2.8m, its height is 2.2m with a manhole of 450mm. See They were connected together with a 110mm PVC pipe at its side connector, some 200mm from its bottom. It also only has a 110mm breather pipe connected onto the inlet sewer line.
It was installed in compliance to a technical drawing by my employer. This drawing asks for a concrete footing onto which the tanks strapped with stainless steel cable, attached to eyebolts, embedded in the footing with chemical anchors. Backfilling around the sides of the tanks is a 5% cement sand mix compacted at 300mm wide to the top of the tanks. However I later discovered this process conflicts with the manufacturers design and method by far.
The manufacturer specifies that the tanks should be encapsulated (bottom and side) in a shell of soil-crete mix of 1:6 ratio (about 16%) but adds "for excavated holes deeper than 2150mm or where a high water table is anticipated, backfill material must consist of a concrete mix of 5% cement and 95% selected granular material" So the question is why would they specify "16% cement" for regular ground type and a mere 5% cement mix for deep/waterlogged ground type? (Again I assume this 5% may be a misprint that should have read 15%). These tanks are used for conservancy to be emptied regularly by means of honey suckers, implying that they must be able to “move or breath” since cold and hot as well as full and empty does make a difference in measurement or movement of the tanks. Hence all pipes joined to tank must have a degree of flexibility. Not the case with the drawing, which specified rigid interconnection pipe 300mm long.
The drawing allows water to seep in-between the concrete footing and tanks thus creating an up-thrust under tanks which caused them to deform as it pushes up against the strength of the anchors and the weight of the backfilling holding them down. As such I deemed the design/ drawing to be flawed. Since the tanks were emptied a few times, sucking water from one tank only while the other tank doesn't have a breather. Can the suction cause a lower atmospheric pressure in the tank causeing it to bulge inwards? Aided by the out higher outside atmospheric pressure and the groundwater pressure. Would you say the tanks deformed due to the 5% cement which I gauge aught to have been 15% or the due to buoyant force getting in under the tanks. Please disregard my assumptions give a detailed explanation as what would cause these tanks to deform/collapse. Thank you.
It was installed in compliance to a technical drawing by my employer. This drawing asks for a concrete footing onto which the tanks strapped with stainless steel cable, attached to eyebolts, embedded in the footing with chemical anchors. Backfilling around the sides of the tanks is a 5% cement sand mix compacted at 300mm wide to the top of the tanks. However I later discovered this process conflicts with the manufacturers design and method by far.
The manufacturer specifies that the tanks should be encapsulated (bottom and side) in a shell of soil-crete mix of 1:6 ratio (about 16%) but adds "for excavated holes deeper than 2150mm or where a high water table is anticipated, backfill material must consist of a concrete mix of 5% cement and 95% selected granular material" So the question is why would they specify "16% cement" for regular ground type and a mere 5% cement mix for deep/waterlogged ground type? (Again I assume this 5% may be a misprint that should have read 15%). These tanks are used for conservancy to be emptied regularly by means of honey suckers, implying that they must be able to “move or breath” since cold and hot as well as full and empty does make a difference in measurement or movement of the tanks. Hence all pipes joined to tank must have a degree of flexibility. Not the case with the drawing, which specified rigid interconnection pipe 300mm long.
The drawing allows water to seep in-between the concrete footing and tanks thus creating an up-thrust under tanks which caused them to deform as it pushes up against the strength of the anchors and the weight of the backfilling holding them down. As such I deemed the design/ drawing to be flawed. Since the tanks were emptied a few times, sucking water from one tank only while the other tank doesn't have a breather. Can the suction cause a lower atmospheric pressure in the tank causeing it to bulge inwards? Aided by the out higher outside atmospheric pressure and the groundwater pressure. Would you say the tanks deformed due to the 5% cement which I gauge aught to have been 15% or the due to buoyant force getting in under the tanks. Please disregard my assumptions give a detailed explanation as what would cause these tanks to deform/collapse. Thank you.
