Hello, everyone.
This is Enta.
The rain has really been coming down in extreme bursts lately, hasn't it?
It pours down for a short while, then clears up right away.
It feels like a dam releasing water—it hits you with a thud just when you've forgotten about it.
In an era when short-term forecasts of linear precipitation zones are crucial...
But that's beside the point.
"There was supposed to be grout there, but after it crumbled, the rebar was exposed."
I think this is something that people who’ve been working on-site for a long time will recognize as a common occurrence, but it seems to come as quite a shock to government officials or general contractors seeing it for the first time, lol.
"Did they actually do the injection properly?" lol
In this post, I’ll explain the reasons behind this by comparing the actual site conditions with the contents of the design and construction manual.
A "hollow" rock bolt seen on a riverbank slope
First, let’s take a look back at the situation on site.
The project focused on slopes adjacent to rivers, and slope reinforcement frames and self-drilling rock bolts were selected as measures to prevent surface slippage.
The reason self-drilling rock bolts were chosen is likely because the walls of the drilled holes were not stable enough to stand on their own.
In fact, this mountain had undergone significant sandification, and the entrance to the borehole was crumbling to pieces.
In geological conditions where the borehole wall cannot be stabilized, self-drilling rock bolts—which allow for simultaneous drilling and grouting while using the rod itself as casing—should have been a reasonable choice.
This is the site where the slope, which had been reinforced to prevent surface slippage, collapsed due to a deep-seated landslide on a scale that exceeded expectations.
When I looked at the rock bolts that were lying around after the collapse, I noticed that there was almost no rust on the rebar. I believe this is evidence that the grouting was effective.
However, people who see this photo tend to have the same reaction.
"There's no grout anywhere. It's bare, isn't it?"
I completely understand how you feel.
When I first saw this, I thought the exact same thing, lol.
Why Does Only the Grout Disappear When It Collapses? | The Adhesion Mechanism Between Core Material and Anchoring Material
Why is it that rock bolts, which should have been securely anchored, end up exposed after a collapse?
According to the Manual for Earth Retaining Structures: Design and Construction, the resistance force when a reinforcement member (rock bolt) is pulled out is determined by the smallest of the following three values.
- Ultimate circumferential friction resistance between the grout and the installation subgrade
- Ultimate Bond Strength Between the Core Material (Reinforcing Bar) and the Anchoring Material (Grout)
- The breaking strength of the core material (rebar) itself
In other words, this concept holds that, among the three points of contact—rebar, grout, and the ground—the weakest one will fail first.
As long as the load remains within the normal design load range, the system operates stably within these three safety margins.
However, in cases where the slip surface moves suddenly on a scale that exceeds expectations—such as in a deep-seated collapse—significant shear and tensile forces not accounted for in the design will be instantly applied to the grout surrounding the reinforcing bars.
Although grout has high compressive strength, the interfaces between the grout and the reinforcing bars, as well as between the grout and the soil, are brittle when subjected to unexpected stress concentrations and tend to fail suddenly.
When a sudden displacement—such as a collapse—occurs, one of two things happens first: either the bond between the rebar and the grout breaks, or the friction surface between the grout and the borehole wall fails.
The moment it breaks, the grout surrounding the rebar cracks into tiny pieces and peels away from the rebar.
It’s like an image of a structure collapsing explosively, leaving only the rebar exposed.
So, I actually think it’s more typical for grout not to adhere to rock bolts after a collapse.
Even if someone from the government office asks, “Isn’t there any grout in there?”, it’s probably best to be prepared to calmly explain, “It just came loose and fell off when the structure collapsed,” lol.
Risks Unique to Riverbank Slopes | Flooding and Scour at the Foot of the Slope
Another point I’d like to mention is that this site was located along a river.
Under the Ordinance on the Structural Standards for River Management Facilities, etc., revetments are installed when constructing bottom stops and similar structures to prevent scouring of riverbanks and levees caused by changes in water flow.
Conversely, this means that riverbanks where revetments or toe protection are insufficient are prone to scouring at the toe every time flooding occurs.
Technical documents on levees published by the National Institute for Land and Infrastructure Management also report cases of recent large-scale flooding in which erosion at the toe of the levee was particularly pronounced.
Even if retaining walls and rebar reinforcement have been installed as measures against shallow-seam sliding, if the toe of the slope is scoured away and loses its support, there is a very real possibility that the entire slope above it will be swept up in a deep-seam landslide.
At a job site I worked on, there was a riverbank slope constructed using a combination of retaining wall panels and rebar inserts. During a record-breaking downpour, the entire back side of the retaining wall was washed away into the river, leaving only the rebar inserts sticking out like Pocky sticks.
Back then, it was still the era of film cameras, and I still regret not having kept those photos. That photo was really great!
You almost never see them arranged in such a neat “Pocky” pattern!
If the Mie River and National Highway Office still had those records... (though there’s no way they do... it was about 25 years ago)
Be sure to take plenty of photos so no one suspects you've had injections!
So far, we’ve been saying that “it’s inevitable that the grout will peel off if the structure collapses,” but that doesn’t mean the grouting process can be done carelessly.
If the grout is left insufficiently filled, bare rebar will rust immediately, and even galvanized rebar will develop white rust early on—something that an expert can spot at a glance.
The injection really needs to be done perfectly!
Otherwise, the mountain won't hold up, lol.
To that end, it’s important to make sure you get the full experience and take plenty of photos.
Knowledge is the key to staying calm when you see a disaster site!
See you later.
