I inspect over 400 well and septic systems a year in Mid-Michigan. Most of our septic systems are the more conventional Trench type fields with gravel and plastic 3 inch pipe. In many new builds they are going to elevated sand mounds that would be called seepage beds. I have been to trade shows with companies selling these new domed chambers but have not yet seen one installed. I have the same thoughts that you had about damage. It would seem anything larger than a garden tractor would fall through. As part of a thorough inspection you would prob the field looking for signs of excessive fluid. The plastic dome would not allow you to do this. Also, we dig up every septic tank and look inside. We are looking to see if septic fluid is rising up in the tank above the discharge baffle which is a red flag to field failure. These new large chambers in the field would make this impossible because they would mask many problems. It is going to be very hard to inspect these type of system in the future. Several county's around me have standards for septic system inspections and standards for indentifing failures but they have not address these new systems.
