THe following sentences are about using scrap tires for a number of purposes and some of the sentences which I makred red are very confusing. I need your help.
1. Dr. Hoening is on to the next wave in building materials: used tires.
2. They have been studying engineering applications for these scrap tires.
3. There are some 500 million used tires stockpiled in the U.S., with a growth rate equal to about 250 million per year, creating hazards.
4. When they( the tires) are placed in landfills, they tend to rise the surface, which has caused 33 states to forbid landfill placement altogether.