Spent last week tearing out an old creek bridge with a 20 ton excavator - a few things I didn't expect

When we did some work on an older truss bridge last year, the biggest headache wasn't the crushing force itself, but managing the vibrations once we moved past the main structural supports. I had a Cat 320 in the bank and we were running a realy tight sequence. The initial impact is one thing, but once you start swinging the bucket to isolate chunks, the low frequency resonance travels through the abutment almost instantly. It’s more than just a shake; it’s a hum you feel deep in the seat.

We spent a lot of time just monitoring the surrounding enviornment beacuse that resonance could crack the approach ramp supports if we hit the wrong frequency. We had to transition from massive drops to a meticulous cutting process using hydraulic shears to keep the vibration spikes down. That controlled cut technique was critical for us. If you try to drop a major section and the structural integrity is allready compromised, you are just risking a massive secondary failure.

I found that using a specific dampening mat under the outriggers when we were staging near the abutment helped stabilize the whole operation. It’s a small thing, but it takes the vibration right out of the tracks when you’re running heavy swings. It made the whole setup feel much more stable under the weight of the machine. It made the whole cleanup process a little less of a nightmare too.

When I did the County Road bridge demolition last year, the bigger concern wasn't the concrete falling, but what happened beneath the supports. You think you are on solid ground, but the soil around the piers on those older structures is often soft silt or old fill. The original post talked about the deck flexing, which is true, but the real danger is the foundation shifting. We used a smaller Volvo excavator, and instead of just starting the cuts, we had to do a full geotechnical check on the approaches before we began swinging the bucket. That dirt is unforgiving when a massive point load is applied.

We didn't use the power shear to break the main load points because we worried about kicking up enough subsurface dust to destabilize the banks. We instead opted for surgical cutting with a diamond-bladed hydraulic saw to sever the connections at the abutment. That took twice as long as a smash, but it kept the pressure on the surrounding earth very low. It felt much safer to control the break method rather than relying on brute force for the initial detachment.

It is also smart to consider the runoff, even for a small project. When you cut those large sections, you create a lot of fine aggregate dust that mixes with the creek runoff. We had to put up temporary silt fencing right around the work zone just to manage the sediment. It sounds like a boring logistical hassle, but dealing with the county inspectors over creek pollution is a nightmare that outweighs any time saved by crashing through the deck.

The cleanup being longer than the tear down tracks with my expirience too, though ours was a small culvert replacement, not a real bridge. What I remember most is how much of the concrete ended up in the water anyway no matter how careful the operator was, and then the county made us pull every sizeable chunk back out before they'd sign off, which nobody had budgeted time for. Curious wether your crew had to sort the steel out on site or hauled the slabs off whole, because around here the recycling yard pays differently depending on how much rebar is still in the pile.

That resonance thing is real, second guy is spot on about it coming through the seat before you hear anything. On ours I started parking one track on a stack of old timber mats where the bank met the approach and it made a noticible difference in how much the machine rocked on full swings. Only thing I'd add is check those mats after every break, we had one sink into the fill a couple inches after a big drop and didn't catch it for an hour.

Funny reading this, on jobs back in the 80s we never put the machine on the span at all. We'd either lift deck sections off with a crane or knock it loose from underneath with a long boom sitting on the bank. Slower but you slept fine at night. One thing I'd tell any crew now, half those old county bridges have the original drawings filed away somewhere, courthouses or the state DOT archives. If you can find them you know exactly where the rebar runs and how big the deck pieces really are. We pulled plans for a 1953 job once and it saved us a world of guessing.