I dont have the answer to question #2, but referring to what type of water...both are acceptable for a causeway. Thanks, Candice

As already said it doesn't matter if you use either salt or clear water for a causeway. I assume that your consultant is probably saying 1/3rd of the thickness mainly because it will make backfilling and compacting far easier. Other than that I can't think at the moment of any other reason.

The value and yield of soils with high contents of salts are significantly reduced, causing severe socioeconomic and environmental problems in the long term. The accumulation of salts from improper soil and water management is a serious problem worldwide. The global cost of irrigation-induced salinity is equivalent to an estimated US$11 billion per year. To stop the loss of arable land due to salt accumulation it is necessary to use appropriate soil and water management practices. Recognizing the symptoms of salt-affected soils in time may save costly reclamation efforts and further land losses. i hope to know answers of your questions .....