The block
As a basic structural element I chose a cylinder. The cylinder consists of used plastic bottles which are put together in centrifugal direction in multiple layers and a plastic plate to which the bottles are fixed by their necks. I tried to create block with solid wall so there are almost no holes between the bottle around the perimeter. I used some basic math to attain it.
The mass
I wanted to design a tower which will be really easy to construct The blocks are just stored one on another with no connective elements between them. One storey consists of 19 blocks placed in hexagonal grid. Another stories are rotated to improve firmness of whole construction.
Conclusion
Because I had to use exact number of plastic bottles I designed the blocks first. One block has 500 bottles within it and weight of 25 kg. In one storey is 19 blocks and whole tower is made of 15 stories. All values are adjustable and the final number of bottles is shown immediately.
Download file: PETower tutorial