Power stump removal action

Removing a 4' diameter eucalyptus stump is tough. It's tougher when it's on a hillside and, despite exposing 6 feet of the roots, you can't chainsaw it apart because it's full of dirt and rocks. Here's what it looks like after chomping off all you can with a 28" chainsaw above the dirt.



Driving a stump grinder up to it won't work, because the grinder weighs 300lb and has a reach of only 2 feet.

So, build a ramp to the top of the stump with the handy Bobcat!



Then, after chewing off 2 yards of stump chips with much noise and great effort, remove the ramp and excavate the remaining root bits. And you now have a big hole in the ground.



And a big pile of dirt.

It's looking like a wet stamp job

One of the bits of jargon I've picked up from talking with landscape contractors is "wet stamp design." This is a design, plan, or schematic that is known to be vetted and pre-approved by the local building & planning department. Using a wet stamp means you don't have to hire a structural engineer to create a custom design for the job, which saves money and time.

There is a seriously overengineered wet stamp CMU retaining wall design for the County of Marin that Novato approves of that looks to me like it could hold back a hill of mud. I didn't like it: it specs that a 4.5' wall needs a 4.5' wide base, with lots of rebar and a keyway (I'll try to post a drawing later). But after going through the custom design process for a poured concrete retaining wall with a geotech and a structural engineer, it turns out I practically do have a hill of mud to deal with.

Dry clay is rock hard, with a load bearing capacity of 8000 pounds per square foot (psf) or more. But wet clay is only 2000 psf and likes to slide. A design that will safely withstand the pressure of that wet clay is nearly as hefty as the wet stamp design. Since I don't have to pay for ~$1500 of engineering to use the wet stamp design, it's starting to look attractive.

I can put off a final decision for another week, since this weekend I'll be removing a massive eucalyptus stump for our hillside with chainsaw, 25hp self-propelled stump grinder, Bobcat (to build a ramp to the stump for the grinder) and excavator (to pull out the roots and ground up bits.) If I have time left over I'll start the excavation for the wall footing.

Here's David last weekend, having at the stump with the chainsaw to cut vertical grooves. It was my plan then to pull the stump apart at the grooves with the excavator. This was a complete failure, since the stump is massive, strong, and full of dirt that dulls the saw blade in seconds. The pros cut off the wood and grind up the stump.

Designing for the ice age

I spoke with the geotech who stopped by my place around lunchtime as planned. He says it's just as he thought, my yard is part of the landslide material that started at the top of the mountain and stopped a couple miles away in the valley. I'm about in the middle, on the foot of the mountain.

The soil in my yard is "landslide deposit junk" of clay, boulders, and everything in between. The slide happened about 20,000 years ago at the end of the last ice age.

The geotech says that the soil is rock hard now but will turn into mud a few weeks into the rainy season and slide into the yard. He also said he'd give me a "great cat rate" because our cat Hirame stalked him around the yard so much that he was delayed getting back to the truck where his wife took Hirame's picture. He's a cat person himself, among his cats is a 16 year old Siamese that they take camping and horse riding.

His quick spreadsheet calculation says a 5' wall needs 18" piers on 6' centers, 6.5' deep. That's doable, especially if I don't hit any boulders...

Geotechnical

After a lot of dialing I found a structural engineer with a bit of free time to design a pier wall for me, but he needs a "geotech" (soils engineer) to "write a memorandum" (technically describe the soil stability parameters) first. Fortunately there's a geotech who lives close by my house and rides his horse along the fire road right behind our lot a couple times a month! He told me yesterday he watched us build the playhouse (he liked the framing).

He'll be stopping by today to glance at the site to form an opinion. I think he might even do this gratis!

I've told both engineers that my goal is to drill the pier holes this weekend, but given that it's already Friday morning, I don't think it's gonna happen. I'll be installing baseboard moulding inside instead.

Update: I dropped by the equipment rental yard this morning to check out the 18" drill bits for a Bobcat mounted drill rig. They tell me I can swap from bucket to drill and back myself with no tools. Awesome.

Timid concrete DIYer?

One of the structural engineers I spoke with advised me to reconsider a CMU wall built on top of a pier foundation, for two reasons:

1. I'll have to bring a concrete truck to the site at last twice: once for the piers, and again to fill in the CMU cells. The truck trip is as expensive as the concrete, he implied.
   
2. It'll probably be a lot easier overall to build forms for the wall and do one big pour than build a big CMU wall.
   

The subtext of #2 was that I was being a big weenie, should get over my fear of five foot high concrete forms (think of the hydrostatic pressure at the bottom!), and just build the thing properly. He pointed out there were several good ways to screw up a CMU wall, too, like dropping mortar into the cells and closing them off, or simply building it crooked.

Engineering

Before I can rent more heavy equipment and get on with this wall, I need to know exactly what kind of wall I'm building. The City says any wall over 2' in height must have a building permit, and they have only two pre-approved designs for walls between 2' and 5' 8". One of those is a cantilevered CMU design that for my 4' 8" wall would be 4' 6" wide at the foundation. That's a freakin' sidewalk, it would stick 2' into the usable yard space and require massive amounts of excavation and backfill.

The other is a wooden wall that uses piers sunk 4' into the ground, which I like a lot better because being only 18" wide it saves space and materials. But the wood will only last 10 - 15 years around here and can't be covered with a nice stone finish, so I'm not building one of those.

I went back to the City building permit troupe* today to plead my case for a hybrid wall that use 18" rebar & concrete piers sunk 5' into the ground, tied to the wall with top to bottom rebar. Robert, the building permits guy, looked at my drawing and agreed it's great idea! Narrower, more efficient with materials than the cantilever, easy to build.

"So can you approve this design?"
"No."
"You had me going there."
I wasn't really surprised, it was a fool's hope anyway.

The City must have a professional structural, civil, or soils engineer's signature on the design before they'll approve it. "That way when the mud flows into your neighbor's yard, we've all got someone to point the finger at." Ah, liability avoidance, the second great purpose of planning departments (after revenue generation).

I called around to a dozen or so local structural and civil engineering guys / firms. Three answered their phone, and one actually agreed to take the job for about $800. I was expecting to pay $2000 for this engineering, so that's a bright spot in this diversion.

(* They have hired a new code enforcement guy, about 55 years old, who thinks he's the comic relief for the planning department. The place felt like a continuous amateur improv show today.)