Umpteen different possible reasons, but two theories for that particular spot based on eyeballing the picture. . .I don't understand why they build these berms in order to install the retaining wall and sound barrier pilings. Can't they drill the holes and place the pilings from ground level?
This is on the Lowell Line (the previous set of photos). They build this temporary embankment up almost the level of the top of the retaining wall, then drill shafts and install the pilings through it, then clear it away. They did this behind the DPW and along the high school site as well. Only think I can think of is that it serves to brace the pilings in place while the concrete foundation cures.
Also they need a level platform to sit on. Most of the drilled shafts on the southwest side of the corridor are getting installed through what is an existing slope. Imagine a right triangle - and trying to drill straight down while you're perched on its hypotenuse at a 45 degree angle.The drill rigs are designed to drill below the operator cab, and they need a considerable area accommodate their swing.