Good morning

Please I need good advise on how to construct soak away/septic tank in a land where we normally see water after 3 feet digging.

Ask builders who regularly works around the Lekki/Ajah axis. They cast the four walls of the septic tank from ground level to what ever height you want and begins to sink in to ground by excavating the sands inside. At the same time pumping out the water continuously.

When I had mine done, this was the sequence albeit I might miss some points but you get the drill after reading this.

Decide on where it will be and the size etc.
Carpenter uses 1 by 12 wood or marine to construct like a box or frame structure.
Iron bender bends iron rebars for the formwork.
Carpenter constructs the inner frame
Iron bender tidies everything up.
You buy cement, sharp sand and also original water proof cement/additive.
The cement is mixed up and soak away casted from the surface.
Leave it to dry for over a week or so.
Bury the soakway by digging the ground from inside.
Have a pumping machine running, pumping out the water continuously.
If you require the base to be sealed, then again you will need water proof cement, DPC, some wood to block a hole or something. Then the base gets sealed leaving a little hole. I dont know why they do this.
Have the pumping machine continue to pump out the water.
Then after this, the small hole is also sealed as well.
Then you get plaster sand for the internal to be plastered.
Ensure that you have a cover for the top.

This is what I remember.