Andrew, they were tapered (pie shaped) -- apparently intended for curved wall landscape use. Front face was 48" wide, rear face was 31" wide. Also they were 18" tall, instead of the plan 24" tall. They weighed 2600 lbs instead of the rectangular block's 4800 lbs.
1.) If laid straight, the front faces abutted, but the embankment faces were spaced apart, the mfr's intention being that 3/4 inch stone would fill the gaps. Fine for static landscape use, but a bridge abutment has moving truck loads over, and tire impact loads, not to mention trees banging into them during a flood. I thought they'd shift too easily.
2.) They were only 42" deep - original design called for 48" deep blocks (4'x4'x2). This difference is particularly critical at the top course, where the steel bridge beams overlap only 1 foot onto the block. Loads could tip the top blocks up a little (or a lot) at each impact in my opinion. Dirt and gravel would enter at the back and further wedge the blocks up
Also affecting tipping resistance: they were less deep, didn't abut each other, thinner (18" vs 24") and weighed about half what he design called for. The centers of gravity of the blocks (and therefore resisting moment) was also further forward toward the wider front face. To me, if you multiply three or four reduction factors by each other, you're left with a very small percentage of the resistance to tipping originally intended.
I was talked into these blocks (when the excavator couldn't find 4x4x2 blocks locally) because they do have a couple of small tenons at the back that fit into mortise grooves, but no one mentioned that they were tapered wedge shaped overall with intended gravel infill. When I saw them going together, and asked where the straight blocks were (I thought the tapered ones were only for the end blocks, laid tight) I was told abut the gravel infill, and a two day "discussion" ensued.
Eventually, reason won.
