I Always Get Slaughtered, Part 2 — Dirt Problems
“Dirt gives me nightmares,” Manny Bucks groaned as we continued guzzling lattes and crunching biscotti.
“Yeah, finicky stuff, dirt,” I agreed.
“I have more horror stories about dirt than even about Level 3 Idiots. Like, for instance, the bad CAD job.”
“Right — the one where your engineer designed the road in the wrong location. Then you moved it to avoid a lot of expensive fill. You mean there’s more to that story?”
“Ohhhh yeah. I wound up building that road three times. It’s true: when we moved the road, we reduced the amount of fill considerably. But, still, there were several thousand cubic yards of fill required. My crew was instructed to use native cut material from elsewhere on the site for the fill sections, which they did. However, they bladed the fill into place in three-foot thick lifts, compacting it with a dozer.”
“Ouch! Double jeopardy. No way that fill would ever pass a compaction test.”
“Tell me about it. The dump truck we used to proof-roll it nearly capsized in the swells. Made ruts like log flumes. It was a disaster.” (Proof-rolling is a quick method to check for adequate compaction in which a full dump truck is slowly driven over the finished subgrade. If the wheels don’t sink in, there’s good compaction — subgrade passes. If the wheels cause ruts or if the soil bucks and heaves, subgrade isn’t properly compacted — failure.
“Didn’t your guys read the specs?” I asked. “Don’t they know that they’re supposed to fill in thin lifts, no more than about eight inches, then compact with a heavy vibratory drum roller?”
“I thought so. But I didn’t have my project manager on site that day and the crew figured they’d save a little time. Claimed the paving crew was breathing down their necks and they had to hurry.”
“Great, so now, not only do you have to pay to have the subgrade ripped out and redone, the entire project schedule is shot to heck.”
“I wish the blood-letting could have been limited to that. Yes, we had to rip out the entire fill section, about 5,000 cubic yards. Then we replaced it in thin lifts, compacting correctly as we went. But midway through the repair it started to rain. The fill material was borderline too wet to begin with and the added moisture put it beyond any hope of achieving 95% compaction. But my crew either didn’t care or didn’t know better and plowed ahead anyway. Again, there was no project manager on site to objectively assess the situation. The crew thought they were being real troopers for getting the job done under such adverse conditions.”
“Regular dirtwork heroes. This story is almost too painful to endure.”
“But wait — there’s more. So the crew finishes and we proof-roll again. This time about half the fill section fails. Now there’s steam coming out of my ears. I send my project manager out to stand over the job while the crew, again, hogs out the bad material and this time imports 2,500 cubic yards of pit-run gravel (also known as “bank-run” or “ballast” — a sand and gravel mix specifically for structural fill.) This was placed in 6-inch lifts and our guys hammered the holy heck out of it with the biggest vibratory roller available.”
“Whew! And it finally passed, right?”
“Wrong! It was no better than before! Turns out the pit run itself was too wet. We’d just had several days of hard rain and the gravel pit shipped pit run that was too wet. Once again we were hosed. Some smart aleck operator came along afterward and said that we over-compacted. That we should have static-rolled the pit run instead of using a vibrating drum.”
“Well, he may have had a good point. But maybe not. When compacting too-wet pit run, especially if it contains a lot of fines (silt or clay-silt), you’re sometimes better off running the compactor with the vibrator off (static rolling). However, this only works if the pit run is just marginally too wet. If it’s a lot too wet, there’s nothing you can do except wait until it dries or get different pit run.
“In your case,” I continued, “even though you failed your proof-roll, if you had the luxury of waiting a few weeks, that too-wet pit run would have set up nice and hard and passed, no problem. Tell me that’s what happened.”
“No, that didn’t happen. Because the project is fast-track, I had no choice but to tear out the failing sections yet again, get new, more granular (less silty) pit run from a different source 10 miles further away and redo it a third time. We static-rolled it in thin lifts and finally it passed. What should have been a $50,000 dirt job wound up costing three times that amount.
Good grief. To summarize the lessons learned:
- Always place structural fill in thin lifts.
- Pay close attention to moisture content. If it’s too dry or too wet, it will not compact properly.
- Dirtwork is highly weather-dependant. It’s a lot easier to add water to too-dry material than to take water away.
- Compact each lift with a heavy vibratory roller. If the fill is slightly too wet, static compaction may work better.
- Most important, have a well-qualified expert on site testing and evaluating as the work progresses.
- Fail at this and the consequences are severe. Dirt can easily turn into the stuff of ghoulish nightmares.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author, and software producer for the building industry. Check out his new book, "Cracks, Sags, and Dimwits –Lessons To Build On," available at www.lulu.com, Amazon and Barnes and Noble.
Send e-mail to firstname.lastname@example.org. Tim reads every one.
This column cannot be reprinted without permission from the author.
The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.