What are these circular plugs on heavy timber steps for?

Those circular marks are actually a series of overlapping wooden plugs used to repair defects or knots in high-quality timber, specifically what we call **Schalungsholz** or shuttering boards in Switzerland. While they might look decorative or structural, thier primary purpose in a construction context is to create a perfectly flat, uniform surface for pouring concrete. If a piece of wood has a large resin pocket, a loose knot, or a nasty split, it leaves an imprint in the finished concrete. By drilling out the defect and plugging it, they ensure the final wall or pillar comes out smooth. The 'circle within a circle' look happens becuase of the overlapping technique. You can't just drill five holes at once or the wood between them woud splinter or the drill bit would wander. A carpenter or a specialized machine drills the first hole, glues in a tight-fitting plug, and lets it set. Then, they drill the second hole so it partially bites into the first plug. By repeating this process across the length of a split or a large knot, you get that overlapping chain. It's basicly a round version of a **dutchman repair**. In Zermatt and throughout the Alps, you see these boards reused for all sorts of temporary structures once they've served their time on a concrete site. Those bridge steps were likely salvaged from a formwork job. Since the plugs are glued in under pressure with waterproof adhesive, they actually end up being more stable than the original knot or crack would have been, wich is why they hold up fine under foot traffic on a temporary bridge.

You won't typically see this in heavy structural framing like a roof truss because it’s more of a surface-level fix for finish quality. But for alpine construction where timber is used and reused til it's literally falling apart, these plugged boards are everywhere. It's a standard way to extend the life of a expensive piece of lumber rather than tossing it just because of one localized grain issues.

That makes so much sense about them being reused! I honestly thought it was some fancy way to stop the wood from snapping, but I'm such a newbie at this. Does this process have a specific name for the drill bit they use? I can't imagine a regular drill bit staying centered while hitting a plug that's already there. Also, if they use these for concrete, do the plugs ever pop out when they get wet or does the glue actually make them permanent?

I'm not totally sold on the shuttering board theory here. Have you ruled out that these might just be simple boat-style plugs used specifically for structural stabilization in high-moisture zones? Even if they were salvaged, a 'chain' of plugs across a grain split sounds more like a mechanical tie-in than a surface fix for concrete. If they were just for aesthetics, wouldn't they use a single larger patch? I've seen similar patterns used to stop checking in bridge timbers where the goal is to bridge the grain, not just smooth the face.

Spot on about the shuttering boards. You'll see those 'chains' all over Swiss construction sites becasue the wood is high-grade spruce or fir. One tiny thing to add: those plugs are almost always cut with a Forstner bit or a specific plug cutter that has a center point, but the real trick is they use cross-grain plugs. If they used end-grain plugs, they'd shrink and swell differently than the board, but these are cut so the grain runs parallel to the plank. That’s why they don't just pop out when the bridge gets rained on; they move in sync with the rest of the wood.

Oh wow, I'm totally blown away by the 'chain' technique! I'm just starting out with a basic tool kit and this sounds like such a cool way to save a board. Since everyone is talking about how these plugs handle moisture and concrete, it made me wonder about the glue. If they're using these in the rainy Alps, what kind of adhesive acutally holds up for years without failing? I'm also super curious—could you do this same overlapping trick with diffrent colored woods like walnut and maple just to make a cool pattern on a tabletop, or would the different expansion rates make the whole thing crack apart?

Update: Thanks for the info! I went back and looked closer after reading about the shuttering board theory. You guys were right—the grain in the plugs runs perpendicular to the board, so they're definitly filling knots. I'm still curious about that 'chain' though. If they're using a standard plug cutter, how do they keep the bit from skipping when it hits the edge of the previous plug?