When specifying anchoring for urban benches in a public square, security and durability are paramount. The most secure system I recommend is a combination of vandal-resistant, expansion-type concrete anchor bolts installed into a substantial concrete foundation.
For permanent installations, stainless steel wedge or sleeve anchors are superior. These are set into pre-drilled holes in cured concrete. When the bolt is torqued, it expands mechanically, creating an immense holding force that resists both upward pull and lateral shear. To deter theft, pair these with specialized anti-tamper bolt heads that require unique driver bits.
The foundation itself is critical. Benches should be anchored into a structural concrete footing, at least 300mm deep, not just a thin paving slab. For surfaces where core drilling isn't possible, such as historic pavements, a high-strength, surface-mounted steel baseplate system anchored at multiple points is a robust alternative. This plate is then chemically anchored or through-bolted to the substrate.
Ultimately, the "most secure" system is one designed for the specific site conditions and installed with professional precision. A certified structural engineer should always review the plan, ensuring the anchors are rated for the required load and the concrete's compressive strength is adequate. This proactive approach prevents loosening, vandalism, and liability, ensuring the benches remain safe, fixed public assets for years.