mike,
i might be able to help some i hope. i have installed a lot of manholes in floodplain areas and will share some simple formulas off a old set of plans i have. step 1) weight of pipe which 48" 4" thick is 683 lbs per linear ft. step 2) weight of water displaced which is 1069 lbs per linear ft. so for example at 10' deep pipe weight is 6,380 lbs and displaced water weight is 10,690 lbs leaving you with a upward force of 3,860 lbs or almost 40 lbs per ft. these figures were used with backfill all around the pipe. concrete does not weigh as much under water as it does in air. dont ask me why but more than one engineer has told me that so not using backfill around the pipe increases your upward force drastically. i didnt figure the weight of your concrete base because i dont know it but i can tell you the exact setup your using with dirt all around it will float if the groundwater is high enough because they make us mechanically anchor them. i think you need lots of weight to make this work and it needs to somehow become one piece not sections. the turnbuckle from top section to anchor point like u mentioned may work. they make us pour concrete collars.
