Plastic marine piling can be manufactured into several stock sizes from 6" diameter to 15" diameter up to 65' in length and can be reinforced with fiberglass  rebar or Glass fibers

Composite Structural Piling Reinforced with fiber glass pricing

8" diameter composite piling $27.00 a foot  standard lengths 8' 10' 12' 20 and 25'. Piling can be spiced together. Quantity discounts available 

The plastic matrix can be modified to include different sizes of fiberglass rebar