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[Agnibesh]

DEAR SEFIANS,

I have a case of design of SBR basin in sewerage treatment plant ehre the HFL is above the base slab level of tank.
Hence uplift check to be considered.
My approach has been -
Design of walls and wall footing in limit state of collapse, limiting crack width to 0.1mm, and the base slab as on grade slab until the dead weight of the structure = 1.25 x the buoyant force.
Now, in this case of tank, in floatation check the FOS is greater than 1.5, hence i was going for the base slab for nominal thickness.
But the approver is suggesting to design the net base slab area independently considering only the dead weight of base slab against the buoyant force, which gives an abrupt section of 200mm thick top and bottom layer each with infilled sand layer 1150mm thickness.

The base slab is cast monolithically with the wall footing hence the action of independent base slab is justified or not ???

Anyone expert can confirm that which approach is justified ???


Please follow the link for sketches for referral ----
https://1drv.ms/u/s!AmlA3W51AzC-rlTGLD1EANvjyl0r?e=3VHmBD

[维克ram.jeet]

Upliftment control :

In open basins , not storing clear water , there is option to go for PRV's ( Pressure release valves ) which allows subsoil water to go in basin but not Vice versa , during Construction / maintenance phases. During operation , water ( sewage/ waste water ) upto TWL is available which counters upliftment.

Other option is to provide lean mix over RCC base slab providing 1.25 times uplift weight . A thin Topping with reinf to control temp is also needed.

Low cost under reamed piles ( if allowed ) can be provided to gain balancing force against uplift . Rcc base slab in this case do not require filling over it , but to be designed as flat slab .

Sandwiched sand filling between two slabs is generally not a much favored option , due to uneven compaction , chances of water ingress in sand . Top rcc base slab in large basins canot be designed for uplift due to large span between walls. However if two base slabs sandwiching sand are interconnected with vertical rcc members* , the integrity is assured.

* Vertical rcc member can be in form of 100mm dia with 1 no bar at center embedded Ld in top and bottom slabs sandwiching the sand , it may be at some spacing say 1.2 to 1.5 m crs grid as per design dictate.

[spsvasan]