You can have the most expensive gasoline tamping rammer in the world, but if you don't understand the chemistry and mechanics of the dirt you are standing on, you are just making noise. Compaction is not just about beating the ground; it’s about reducing the void ratio between soil particles so they lock together. The critical variable in this equation is water, specifically what geotechnical engineers call the "Optimum Moisture Content" (OMC).
If the subgrade is bone dry, running a rammer over it is useless. The dry clay or silt particles have too much friction between them; they will just shatter into dust and displace, blowing away in the wind rather than packing down. Conversely, if the dirt is completely saturated from a rainstorm, it becomes what we call "pumping" subgrade. Water cannot be compressed. If you hit a saturated trench with a jumping jack, the ground will feel like a waterbed. The rammer will sink, create a suction vacuum, and turn the trench into a soupy mess of mud.
In the field, I rely on the classic "squeeze test." You grab a handful of the soil you are about to compact and squeeze it tight in your fist. If it crumbles into dust when you open your hand, you need the water truck to spray the lift. If water drips through your fingers, you have to let the trench bake in the sun or mix in dry lime to soak up the moisture. If it holds its shape like a solid ball and breaks cleanly in half without crumbling, you have hit the OMC. That is the exact moment you fire up the impact rammer. At optimum moisture, the water acts as a lubricant, allowing the soil particles to slide past each other under the impact of the shoe, locking into a dense, impenetrable matrix.
