For a street bench on grass, my go-to concrete base is a reinforced poured concrete pad rather than setting the bench legs directly into soil. Grass areas hold moisture, and soil shifts with rain, drought, and freeze-thaw cycles. Without a proper concrete base, a street bench will start wobbling, sinking, or rusting at the feet within a year or two.
A practical concrete base for a street bench on grass can be one of two forms: two separate concrete footings under each bench end, or one continuous shallow concrete pad. For most 6-foot street benches, I recommend a continuous pad about 6 ft 6 in long by 2 ft wide and at least 4 inches thick. If you prefer separate footings, make each one at least 2 ft by 2 ft by 4 inches. Use a 3,000 psi or higher concrete mix and add wire mesh or rebar to control cracking.
Before pouring, remove the grass and topsoil, then compact a 4-to-6-inch gravel base. This improves drainage and reduces movement. The top of the concrete should sit about 1 to 2 inches above the surrounding grass to shed water. If you mow around the bench, keep the pad nearly flush with the lawn and use a broom finish for slip resistance. Slope the surface about 1/4 inch per foot away from the bench.
For anchoring, use galvanized wedge anchors, sleeve anchors, or cast-in-place anchor bolts. Stainless steel or hot-dip galvanized hardware is best for outdoor use. If the bench has pre-drilled feet, make a template and set anchor bolts in wet concrete, or drill after the concrete has cured.
If the bench is lightweight and the grass sits on firm, well-drained soil, precast concrete deck blocks can work as a quick base. But for public areas, commercial benches, or anywhere with soft soil, a poured reinforced concrete pad is the more durable choice.