I am not sure if the situation is as bad as it looks in the photos. Check the paper below, I hope it helps.

https://www.hindawi.com/journals/sv/2018/7645726

1- temporary supports

2- soil remove around piles

3- check and rehabilitation

It looks as if some piles deteriorated to the point of preserving some rebar but not much concrete or not concrete at all. I think you need some field data on pile cross sections to determine the following,

1. geometry and configuration (steel - concrete) of damaged sections

2. materials properties as they stand

3. extent of the damage

For the modelling, very likely you will need to,

a. specify plastic hinges on the top of piles for those sections with no other capacity but rotational. In some cases - depending on the state of deterioration, you might need to cancel out the pile therefore release all degrees of freedom at those points.

b. model those piles whose section is semi-preserved as if it had a change of section at the regions whith high deterioration.

Beyond this analysis, I think that the overall foundation needs replacing. Evidently, the soil is highly corrosive while concrete and rebar went into an irrevesible process of decay.

I hope this helps.