Red Cabin Sinkhole

sinkhole
house
Published

July 8, 2024

Modified

July 11, 2024

Red Cabin Entryway

Red Cabin Entryway

Introduction

Yesterday morning I noticed that our PurpleAir sensor wasn’t reporting. It’s normally plugged into an outdoor GFCI outlet on the red cabin, but with all the rain we’ve gotten over the past few days, that outlet wasn’t working reliably. I’d assumed it was all the rain, but when I walked out there to look, a sinkhole was starting to form under the back corner of the arctic entryway to the cabin. The ground was dropping and had pulled the electrical cable between the red cabin and shed tight, tripping the breaker.

When I got out there, the cribbing supporting the corner was no longer supporting the weight, so I jacked it up as close to the edge as I could and moved the cribbing away from the hole. Throughout the day, the hole got deeper and more fully developed, but hasn’t gotten much larger than the original extent shown in the video and photo below.

In this photo you can see the unsupported corner above the hole, and the cribbing I moved to around the center of the back wall of the back cabin. As long as the hole doesn’t get much larger, this temporary fix seems to be holding, but it’s not going to work once we get three feet of snow on the roof.

Sinkhole

Sinkhole

Here’s a video for context. I shot this on my iPhone, and converted it to a more web-suitable format (smaller, better supported by all browsers) using ffmpeg’s vp9 encoder

Code
ffmpeg -i IMG_1495.mov -c:v libvpx-vp9 -b:v 2M -pass 1 \ 
  -an -f null /dev/null && \
ffmpeg -i IMG_1495.mov -c:v libvpx-vp9 -b:v 2M -pass 2 \
  -c:a libopus red_cabin_sinkhole.webm 

My first thought was to pick up and move the red cabin away from the hole. The main building is a 12 feet by 24 feet box, and the entryway is an 8x8 foot box stuck onto the side of the cabin. I figured a large forklift with long forks and a wide base could easily pick up the cabin and I could just move it slightly and set it back down onto shifted cribbing. I’d need to disconnect the electrical connections from the house; drain, disconnect and move the heating oil tank; and separate and move the entryway portion before or after moving the cabin.

But looking more closely at the cabin itself, it’s more than ten feet from the creek, and even the corner closest to the sinkhole is not in danger. The real issue is that the entryway part of the building is shifted back from the front face of the cabin. This yields a small deck in front, and preserves the window on that side of the cabin but also places the entryway closer to the creek. You can see the deck and window in the top photo.

Rather than moving the whole thing, I’ll bridge the sinkhole with a beam to support the far side of the entryway.

The following is a diagram as viewed from the front of the entryway. The big box on the left is the entryway, the long horizontal rectangle is a beam, and the small squares are the cribbing on the ground.

Plan

Plan

Risks with this plan:

  • The unsupported section of the beam could be as long as 20 feet so there may be a dip in the beam at the very corner of the entryway. That could result in a gap where the entryway meets the red cabin, something that was already a problem before this happened.
  • The sinkhole could expand. This would be especially bad if it expands toward the front of the cabin because the front edge of the entryway is currently resting on what is currently solid ground. I probably don’t want the entire entryway hanging out in space, resting on multiple beams across the hole.
  • Actually getting the beam in place will require temporarily supporting the back half of the entryway on new cribbing or jacks so I can put the new beam at the back edge where the cribbing currently sits.

Positives:

  • Avoiding all the issues related to moving electrical connections and the heating oil tank, cutting trees, leveling the ground, etc.
  • Reasonably easy to accomplish without heavy equipment or lots of labor.
  • Cheap: a 20 foot 6x8 timber is only $150. That’s quite a bargain compared with all the costs of moving the cabin.

There is one other option, and that’s to remove everything from the entryway, and demolish it. Put a door on the side of the cabin (where there probably was one originally), and forget about an entryway. I think this is the fallback option should the hole get too large for the beam to work, or the beam proves to be too weak to support the weight of the entryway in winter.

Execution

I got a 20 foot 6x8 timber from Northland Wood, gingerly driving it home on the trailer, eight feet hanging off the back. I moved it behind the red cabin by rolling it on short sections of plastic pipe, then jacked up the back of the entryway using a pair of bottle jacks. Because I didn’t know where the joists were, the jacks were pressing on a piece of 2x8 to spread out the load. Once the back of the entryway was supported by the jacks and the cribbing was free I moved the beam in place of the old cribbing.

Entryway resting on bottle jacks

Entryway resting on bottle jacks

The back of the entryway was light enough I could lift each corner with a handyman jack. With the far end resting on the ground, I raised the back corner closest to the red cabin until the jack was free, raised it a bit higher, then set cribbing under the beam and let the jack down. Raising the other end was even easier due to the leverage of being at the far end of the beam from the entryway.

Here’s what it looks like now.

Beam in place

Beam in place

And a video for more context. Even though the hole looks larger since yesterday, I think the overall size at the surface is the same, it’s just fully hollowed out now.

I expect I will need to adjust the height of the beam as the cribbing sinks into the ground. And the way the water is moving in the hole, I should probably fill it with gravel. Even if this section of land eventually flows all the way down to the creek, filling it should slow the process and maybe even allow water to pass more easily into the creek without removing the ground along with it.

2024-07-09 Update

I ran a string line across the top edge of the beam and measured the deflection of the beam at the corner of the entryway that’s hanging out over the sinkhole. Right now the beam is bending about ¾ of an inch, which is fantastic. Not counting my chickens, since the beam may continue to bend under constant load, and the winter snow load will add considerably to the weight, but this is a good outcome right now.

Beam deflection

Beam deflection

2024-07-11 Update

The good news is that the beam isn’t sagging much more than ¾ of an inch since I installed it. The bad news is we got almost an inch of rain last night and all that water moving downhill to the creek has doubled the size of the hole. Much of the expansion has been toward the front of the entryway, which has put the front corner in danger, but it also expanded away from the cabin.

In the photo below you can see some of the cribbing for the front corner of the entryway (on the left) now hanging over the hole and the back side of the hole is much closer to the end of the beam. There’s a ditch forming behind the beam where you can see the V formed by trees falling as the land subsides.

Expansion

Expansion

I got two cubic yards of gravel delivered and shoveled it into the hole. I started pumping water out of the driveway and over the hole, and set up a couple pipes to redirect as much of the water away from the hole as possible. I think if I can get the current pulse of water from last night’s rains into the creek and we don’t get a bunch more rain, I might be able to stop the expansion before it takes the entryway. But it’s very much touch and go at this point.

2024-07-12

Despite pumping continuously all night and piping the water away from the hole, it was still expanding today. I got another 6x8 and supported the front side of the arctic entryway over the hole with the new beam. This required moving the deck out of the way, but so far so good.

Red cabin entryway up on beams

Hole