Fence posts in runny sand

Hello,

I am looking to install a fence but I tried to dig the holes with a post digger I realised the sand is very runny. Very, very runny. It just slides off the sides of the post digger.

I tried wetting the sand but even then it keeps collapsing in.

So I was thinking of another idea:

• Dig the hole, 700mm deep, let the sand collapse (resulting with quite a wide hole)
• Take a PVC pipe 225mm wide, stick it all the way in the ground
• Push the sand back around the PVC pipe, compact and tighten up
• Stand the fence post inside the PVC pipe and fill with cement

It's just 2-3 posts that will need to do that way.

Does anyone see any risks with this approach?

Thanks.