If you're installing urban benches in sandy soil, the short answer is that you need footings that resist both settlement and lateral shifting. Sandy soil drains well, but it can be loose, abrasive, and unstable around a shallow foundation.
For in-ground bench mounts, I would not rely on short post spikes or shallow concrete pads. The most dependable options are helical piles or screw anchors driven deep enough to reach denser, more compacted sand. These can be installed with minimal excavation and they anchor the bench legs against uplift and side loads. For heavier benches or cantilevered designs, use reinforced concrete piers. Dig the hole at least 36 to 48 inches deep or below your local frost line, place a sonotube, and pour a 12 to 18 inch diameter pier with galvanized anchor bolts cast into the top. In sandy soil, a bell-shaped or widened bottom helps spread the load, and a few inches of compacted crushed stone at the base improves stability before you pour.
If the bench is surface-mounted on a concrete walkway or plaza, the footing requirement is mostly about anchoring into the slab with proper bolts. But if the bench sits directly on sandy ground, use a wide precast concrete pad or reinforced ballast blocks. The pad should extend beyond the bench footprint and be at least 6 to 10 inches thick for lighter benches, and thicker for heavy or cantilevered units.
For lightweight benches, heavy-duty ground screws or auger anchors can work if they are rated for sandy soil and local wind loads. Choose anchors with a broad helix or flange rather than narrow straight rods, and torque them to the manufacturer's specification instead of stopping at a fixed depth. Sand capacity can vary a lot.
Corrosion is another factor. Sandy soil, especially near the coast, can be salty and abrasive. Use hot-dip galvanized or stainless steel anchors, bolts, and brackets. Add a compacted crushed-stone collar around the base to reduce erosion and keep sand from washing out around the foundation.
Before choosing a footing, check whether the sand is loose or compacted and whether the bench will be exposed to wind, vibration, or heavy public use. If you are unsure, have a geotechnical engineer or local installer confirm the bearing capacity. In loose sand, the safest approach is to over-excavate, compact the base, and use deeper or larger footings than the minimum specified.
In short, for sandy soil installations, helical piles and deep concrete piers are usually the most reliable footings for urban benches. Wide precast pads or ballast blocks can work for surface-mounted benches, and ground screws may be acceptable for lighter units if they are sized and torqued correctly.