Here’s a refresher on the rebar inserter role—which I’m sure you’re all familiar with—or an introduction to the role for newcomers!
Reinforcement insertion work (rock bolting) involves inserting reinforcing bars into holes drilled in the rock, injecting cement slurry to ensure full anchorage, and
This is a construction method that ensures the stability of slopes and embankments by integrating them with the natural ground.
From my perspective as a slope engineer with over 30 years of on-site experience in the design, installation, and construction management of rock bolts,
Everything from drilling, grouting, and placement to equipment and construction management46 Practical Q&AsI've compiled them here. (There may be more to come.)
We answer questions from public works officials, general contractors, consultants, and young engineers using real-world insights from the field.
What Is a Reinforcement Installer (Rock Bolt Installer)? | The Basics
Reinforcement Installer (Rock Bolt Installer)In a nutshell, that means
「A construction method in which holes are drilled into the natural ground, rebar is inserted, and the rebar is fully anchored with cement slurry to reinforce the natural ground."."
The official name is"Soil Reinforcement Method for Natural Ground"。
It is one of the most versatile slope stabilization methods designed to prevent slope failures and ensure slope stability, and is widely used in public works projects by the Ministry of Land, Infrastructure, Transport and Tourism, NEXCO, and prefectural governments.
The Three Major Characteristics of Reinforcing Bar Installation Workers
- ✅ Full-Surface Fixation Method — Unlike ground anchor construction, the rebar becomes integrated with the natural ground along its entire length
- ✅ No need to be nervous — Do not apply tension (just tighten it with the screw just enough so it doesn't come loose)
- ✅ It has a wide range of applications — We offer a wide range of services, including the reinforcement of cut slopes, fill slopes, slope faces, and existing retaining walls
- ✅Increased Circumferential Friction Resistance and Shear Stress Reinforcement
Differences from Ground Anchor Construction
Although they are often confused, rebar insertion work and ground anchor work areThe mechanisms are completely differentThis is the construction method.
| Item | Reinforcement Installer (Rock Bolt Installer) | Ground Anchor Construction |
|---|---|---|
| Fixing Method | Full-Length Anchoring (Full Length of Rebar) | Adhesion area only (tip only) |
| Nerves | None (Tighten just enough so it doesn't come loose) | Yes (tensioned by the design tension) |
| Pull-out Test | Just to confirm that it won't come out | Acceptance testing at approximately 1.2 times the normal level |
| Pressure-receiving plate | A square washer measuring approximately 150 × 150 is sufficient. | A large pressure plate is required |
| Main Uses | Preventive Reinforcement of Cut Slopes and Existing Slopes | Measures Against Large-Scale Landslides and Seismic Reinforcement |
Construction Sites Where Rebar Installers Are Hired
- Preventing Slope Collapses on Cut Slopes — Preventive Reinforcement for Roads and Residential Land Development
- Reinforcement of Existing Retaining Walls — Unbeatable when used in combination with the Moldam method
- Measures for the Red Zone (Special Landslide Hazard Area)
- Reinforcement of the Rear Sides of Abutments and Piers
- Solutions for Sloped Sites on Small Lots and in Urban Areas — Self-drilling type that can be used even in areas where machinery cannot be brought in
▶ For information on combining the Moldam Method with the rebar insertion method, see “What Is the Moldam Method? | Complete Edition”Please also take a look at this.
▶ List of All Articles in the Rock Bolt Installation Category
Drilling, Grouting, and Construction Management | 46 Practical Q&As
Below, I’ve compiled a list of questions I’m often asked on the job, along with their answers, in no particular order.
From quality control in public works projects to the detailed know-how of skilled craftsmen,The reality of the field that isn't found in other literatureI'm writing this from the heart.
*This page is updated regularly. Please feel free to send us any questions or comments.
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What is the height of the scaffolding from the drilling location?
Although it depends on the drilling angle, this is generally a good guideline.
Skid-type 1.5-metric-ton-class core drill = 30–50 cm
Skid-type 2.5-metric-ton-class core drilling rig = 30–60 cm
Lightweight drilling machines (such as spring drills) = 20–30 cm
Crawler-type core drill = -20 to -30 cm (soil foundation)
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Is caulking necessary for the head section?
It is necessary.
There's a reason for this, and...
These days, hard-type rust-preventive oil is used, so it’s generally fine, but,
The head, exposed to the scorching summer sun.
The internal rust-preventive oil may dissolve and seep out from areas such as the back of the pressure plate.
As a countermeasure, caulking is required around the pressure-bearing plate and the cap.
Please use weather-resistant caulk.
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"Where do I look for the load?"
The table listing the resistance values for each reinforcement material in the design calculation report'sTdPlease take a look.
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"Are grout and cement slurry the same thing?"
In the case of grout, the suspensions and solutions used for grout injection are also considered grout.
Water glass and CB injection are also types of grout.
In other words, it refers to a fluid that is injected to fill cavities, voids, gaps, and the like. (wiki)
That would be what’s known as “cement milk.”
By the way, cement slurry is a mixture of cement and water.
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"Since they're completely different, shouldn't we apply different loads!?"
Do whatever you like. You can apply different loads to each section, or you can apply a single combined load—the management method remains the same either way.
And the results remain the same.
I think it just creates more paperwork and adds to the hassle.
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"So, what exactly do you learn from making a stress management chart!?"
It's just a matter of whether the rebar came out or not.
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"Why Won't the Reinforcing Bar (Rock Bolt) Come Out in the First Place!?"
We're digging a hole, inserting rebar, and pouring cement slurry into it.
As the cement slurry hardens, it bonds with the surrounding rock,
It is designed so that it cannot be pulled out due to the friction between the soil and the rebar.
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"Do we even need measuring instruments like dial gauges in the first place!?"
From a conceptual standpoint, measuring in millimeters may make it easier to see where adhesion failure is occurring.
However, since rebar installation (rock bolting) involves full-depth anchoring, if adhesion failure occurs,
It slides right out, so I don't really feel the need for a measuring tool.
Personally, I don't think it's necessary.
(I know this goes against the control chart, but...)
I think, “Just apply some force and see if it comes out or not—that’s all there is to it!!!” But
Please manage construction in accordance with either the Design and Construction Manual for Soil Reinforcement Methods for Natural Ground or the NEXCO guidelines.
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"Why does the strength of the cement slurry vary?"
Rest assured when conducting compressive strength tests on cement slurry test specimens!There's definitely going to be some variation!!!!!
In addition, because it has a small cross-sectional diameter (φ5–100 mm) and high strength (24 N or more),
The results will vary depending on how it breaks.
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"Can't we reduce the variation!?"
It's not that it can't be done.
PreviouslyThe Ambiance of Fiber-Reinforced Cement Milk
As you can see, the fiber really adds some chewiness!
I've had experiences where the strength became relatively stable.
(Since there is only five days' worth of data, its reliability is questionable.)
I'd like to have a little more experimental data.
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"Resources on Self-Drilling Rock Bolts!"
The cost estimation guidelines for the National Special Construction Certification Program were recently revised.
Currently, the only materials used for cost estimation and similar tasks are the following:
"National Association for the Protection of Specific Slopes (Incorporated Association), Rock Bolt Cost Estimation Data (Reference), Fiscal Year H17""National Association for the Protection of Specific Slopes: Rock Bolt Cost Estimation Examples (for Reference), Fiscal Year H17""Okabe Co., Ltd., Civil Engineering Division: OSDP Rock Bolt Method—Cost Estimation Reference Materials, Fiscal Year H17"
"National Association for the Protection of Specific Slopes: Rock Bolt Cost Estimation Data (Reference) FY H27"
"National Association for the Protection of Specific Slopes: Rock Bolt Cost Estimation Examples (Reference) Fiscal Year H27"
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"I've never tried self-piercing, but does it really exist!?"
Back in the day (about 15 years ago), it was quite common, and for sections of the borehole wall that couldn’t stand on their own, we relied almost exclusively on self-drilling.
However, in practice, when the borehole wall collapses, there are many areas where grouting is insufficient even with reinforcing bars in place, so this method has come to be used only for temporary structures.
Since double-pipe drilling is now the standard practice, the use of self-drilling has decreased.
The Ministry of Land, Infrastructure, Transport and Tourism has recently started using it again.
Although the reason is unclear,
I suppose it’s because it’s advantageous for drilling in places where machinery can’t reach.
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"How many meters can you drill with a manual core drill!?"
For cost estimation purposes, 2.0 m using rope scaffolding with a borehole diameter of 45–50 mm
Single-tube scaffolding installed at 3.0 m
Based on experience, the maximum height for self-drilling construction using rope scaffolding is 3 meters!!
We can't lift it any higher than this, and we can't bring up the cuttings (the debris from drilling) either.
My hand is tingling, too!
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"How do you do a self-needle injection!?"
There's a hole right in the middle of the rebar (self-drilling rebar)!!
If you attach the injection adapter there and inject,
Milk comes out of the air hole in the tip.
As long as it goes all the way to the top, that's fine!!
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"Do I Need a Tip Cap!?"
That's not necessary.
It wasn't there before, but before I knew it, it was there.
(I know the real reason, but I can't say it because it would cause trouble for some people lol)
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"There's a material called 'Sheath'—is that necessary?"
To get straight to the point,No need!
Nowadays, since we fill in the areas where the cement slurry has leveled out with mortar,
Water never gets in.
However, there was a time in the past when the back surface was not treated after reinforcing steel installation and grout injection.
That's when the idea of a sheath was conceived.
The purpose of this protection is to keep water from coming into direct contact with it.
However, there is now a possibility that it is not adhering properly.
Make sure the adhesion length isn't too short.
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"Do I really need a torque wrench to tighten the head!?"
I don't need it!
In the past, we used to convert the design values to torque, but,
Right now, I'm giving it a good squeeze with the Pilen.Tighten it so it doesn't come looseThat's all.
Applying tension improperly can cause the cable to come loose or affect the formwork or sprayed surface.
However, if the amounts were aggregated for accounting purposes, they may be included in the total.
Please be careful.
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"Do we really need to multiply the design load by 1.2 during acceptance testing!?"
No need! As long as it doesn't pull out under the design load, it's fine.
Even though it's merely a check to see if anything is missing,
I don't think it's worth applying more load than necessary and risking the coating peeling off.
The design load is sufficient.
It’s important that there are no gaps and that it’s firmly adhered across the entire surface.
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"If the rock is too poor and the borehole wall won't stay stable on its own, is a double-pipe system the only option!?"
Please use a double-pipe installation.
Lately, some consultants have been saying that since their N-value is 10 or higher, they’re self-sufficient.
I understand how you feel, but,
The stability of the borehole wall cannot be measured by the N-value.
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"The design angle is specified for the rebar insertion work, and it's not at a right angle to the slope—can we change it!?"
It's possible.
From a construction standpoint, it is better to install rebar (rock bolts) at a right angle to the slope.
Since you can install the frame without having to think about anything, even when cutting out the box, this reduces the number of installation errors.
What’s more, since slopes often have uneven terrain, if the angle is set in stone, then...
This results in waste during the installation of formwork and the insertion of rebar.
If the angle is too steep, the taper plate (angle adjustment base) and other components may
It will become necessary, and costs will rise.
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"The slope area is larger than what was specified in the design. We can install rock bolts, but what should we do!?"
For a rebar installer (rock bolts), if there are 10 bolts in the design cross-section,
The design cross-section is divided into 10 sections. These are then unfolded horizontally.
So, if the area increases, we can conclude that it’s okay to hit that much more, but,
Please consult with the local government office.
The same applies when there are two or three cross-sections to consider; please refer to the closest cross-section.
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"If I drill a hole and inject material as shown in the book's illustrations, does the diameter of the hole increase?"
It won't work at all.
When the drilling diameter is φ65, the hole diameter is 65 mm.
Even if you inject fluid and apply pressure, the diameter will not change.
If pressure high enough to cause the diameter to expand were applied, the peak would likely collapse.
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"Some local governments actually allow self-piercing!"
Yes, there is! It's also approved by the Ministry of Land, Infrastructure, Transport and Tourism.
At NEXCO, a pioneer in the field of rebar installation (rock bolting), this work is classified as temporary.
I hardly ever use it. That's because the injection is unstable and unreliable.
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"Is it okay to place rock bolts at the edge of the legal framework?"
No problem.
For rebar installation (rock bolt installation) work, it is considered acceptable not to have any overhang.
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"Is it possible to install self-drilling rock bolts in all mountains that cannot stand on their own?"
We can definitely handle the drilling. However, there are significant concerns regarding the grouting process.
We cannot guarantee the injection in any way.
In addition, there are cases where the rock is too hard to drill with an iron bit, and
You can drill holes with a bit that has a cutting edge.
Since the ground was hard, we used a chip bit, and
When drilling holes, self-drilling rods often break.
The reason the rod breaks is that the threaded part has an extremely coarse thread.
It wobbles a lot. It'll break because of that wobble.
There are ways to make it less likely to break, but they're hard to explain.
To put it simply, if you eliminate the wobble, it will be less likely to break.
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"Why is grouting unstable in self-drilling rock bolts?"
If the drill wall cannot stand on its own, it means the drill slime is unlikely to appear.
Even if you inject grout there, it won't reach that damaged area.
Furthermore, and most importantly, if the implant does not settle on its own, there is a very high likelihood that the injection site will become blocked.
If the injection port is blocked, injection will not be possible.
In the past, as far as I know, about half of the self-perforations I’ve seen weren’t injected properly.
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"Is grouting guaranteed with double-pipe drilling?"
That's for sure.
Actually, if you drill while rotating the outer casing,
Once drilling is complete, the natural ground is stable on its own.
The fragile borehole wall is eroded only around the perimeter due to the rotation and impact vibrations of the outer casing.
As it compacts, it will eventually stand on its own.
Since we’ll be injecting the mixture to replace the soil and cement slurry, it’s expected to be less prone to collapse.
(Of course, there are times when it falls apart.) -
"What happens if you drive rock bolts into a fracture zone?"
Since the cement slurry isn't coming up, please use the bag packer.
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"How much does a suspended core drill weigh?"
It's generally between 700 kg and 1,100 kg.
Since smaller tools naturally have less impact force, their drilling capacity is relatively low.
Depending on the model, we generally use a 190-horsepower compressor.
The higher the air flow, the smoother the drilling process will be.
Do not use a high-pressure compressor.
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"Is a greased cap necessary?"
In the past, since the material was untreated, head protection was necessary, but now that full plating is standard,
I don't think it's necessary, but we don't have enough data to say for sure.
NEXCO actually has the data, but it has not been made public.
NEXCO hasn't been capping recently.
Those are the results based on that data.
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"How long does the plating last?"
I remember reading somewhere a long time ago that, generally speaking, the crown area lasts about 25 to 50 years, and the hairline lasts 50 years or more.
(I can't say this based on another company's materials.)
Recently,Service Life = Zinc Coating Thickness (g/m²) ÷ Corrosion Rate (g/m²·year) × 0.9
The following formula is shown.
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"Is BoltScale® sufficient for positioning errors?"
It's okay.
As long as you can explain it clearly, there won't be any problems.
It's worse not to manage it.
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"Is caulking necessary for square washers?"
Actually, it's not necessary.
This is not necessary unless you have a grease-filled cap.
If you use a grease-filled cap, it will melt in the summer and leak out from under the square washer, so
Caulking is needed.
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"Which is better for sealing square washers—caulk or mortar paste?"
Since mortar paste can't be applied thickly, it will eventually crack and wear away.
If you use weather-resistant clear or gray caulk, it will last quite a while.
(The place I visited 10 years ago was still there.)
However, I have seen birds eating black caulk.
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"So, what's the Clip Spacer® really like?"
It’s very convenient in areas where wooden framing or sprayed mortar has been applied, but,
If you install it on a section of soil with no fill, it will sink.
It cannot be installed if the sheath is already in place. (It can be installed if the sheath is added later.)
According to the craftsman, once you try it, you'll be hooked.
I recently received a notice from the local government.
"Is the installation we've done so far okay? Isn't it attached to the bottom?"
I told them that up until now, I’d been doing it by propping things like rocks underneath, lol.
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"What is the maximum length for a single piece of rebar that I can order?"
Suspended single-hole drilling is OK up to 6 meters (any longer than that is too heavy).
With a double-pipe skid-type drilling rig, you can drill up to 5 meters (any deeper than that is too heavy).
If possible, I'd prefer the real thing.
The reason is that,
This is because it makes it easier to investigate later whether the cable has been cut or not.
In an ultrasound examination, if a coupler is present, the distance that can be measured is limited to that point.
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"Is it okay to use a connector?"
You're free to use it, but it's better not to.
If the design is 5 meters long and you split it into 3-meter and 2-meter sections, connecting them with couplers is a hassle.
Also, even if you tighten it securely with a pipe wrench, it comes loose easily because the screw threads aren't fine enough.
Also, most importantly, if there were an investigation and there were suspicions that the line had been cut,
1. If it's genuine, we can investigate it.
It's also part of the job to ensure that a third party can review the work at any time without any issues.
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"How do you go about conducting the survey?"
Recently, we've been using non-destructive testing equipment.
Ultrasound examinations are the main focus.
Therefore, you should use single-piece sections whenever possible.
If there's a coupler, you can only measure up to that point.
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"Is there a way to ensure that the cement slurry adheres firmly to the rebar?"
Immediately after injection, be sure to move the rebar up and down firmly. (Shake)
This ensures that the natural soil, rebar, and cement slurry bond firmly together.
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"What do you plan to do if you absolutely can't drill at the design angle (within the specified range)?"
In most cases, simply making the rebar longer will solve the problem.
Actually, making it about 50 cm longer would solve the problem.
I can't calculate that part alone, and making it longer will increase the length of the root zone (undisturbed soil mass).
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"We're supposed to install five piles across the cross-section, but because of the slope's shape, we can only install four. What should we do?"
Please drive five piles. The design does not take the slope direction into account.
Since you can hit five balls on a flat surface, the calculation is based on five balls.
The actual slope faces a certain direction, and it appears that the slope frame is oriented either to the right or to the left.
Since the calculation is designed to accommodate five items, please arrange them so that five fit, unless you need to recalculate.
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Which machines are easy to operate?
Here is a list of rock bolt drilling machines, ranked in order of ease of operation.
1. Inverter Chisel (Down-the-Hole Hammer)
This machine is lightweight and electric.
In any case, it's very easy to use, so even a beginner can drill holes with it.
No other machine makes drilling this easy.
Since both the rotational and propulsive forces are electric, it’s very powerful.
We mainly perform single-hole drilling.
2. Pneumatic Drilling Machines
For skid-type (fixed) lightweight machines, the air system is
The movements are quite tight and fast.
It takes a little getting used to.
It spins fast, but it doesn't have much torque, so it's hard to handle when it gets stuck.
He has tremendous power at the plate.
You can assume the same applies to suspended drilling rigs.
We mainly perform single-hole drilling.
3. Rotary Percussion
When it comes to modern double-pipe construction methods, they generally involve rotary percussion.
In any case, it's extremely dangerous if you're not used to it.
It's easy to break your arm.
As far as double-pipe systems are concerned, it's hard to imagine anything else at this point.
Drilling can be done regardless of soil conditions, but it all comes down to the operator's skill.
I have other drilling machines, but I'll skip the ones that might get me in trouble lol.
In any case, when drilling, you must constantly monitor changes in soil conditions, rotational pressure, feed pressure, and so on,
Drilling is performed by adjusting the air flow, water flow, and pressure.
Just keep in mind that drilling holes isn't as easy as it seems.
It’s all thanks to the artisans’ skills.
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"The tunnel face is vertical—how are we supposed to drive rock bolts at right angles to the face?"
I can't hit a zero.
In a design where the slope is at a right angle, if part of the slope is at a right angle, then
Please perform the installation at -10 degrees.
The minimum angle for rock bolts is minus 10 degrees.
You cannot add any more milk.
There's nothing you can't do with a packer, but,
The default is minus 10 degrees.
If possible, temperatures of minus 15 degrees or higher make it easier to process the head, so,
Please explain the situation to the local government office and ask them to modify part of the design.
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Should the injection be from the top? Or from the bottom?
Grout injection is
Always start from the bottom. Always.
Even if there is no water or anything else inside the casing, start from the bottom.
The rule is to start from the bottom. There are no exceptions.
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Is it possible to change the anchor material?
Yes, that's possible.
These days, it doesn't matter which anchor material manufacturer you use.
Call the manufacturer you want to use directly, and
If I just say, “I want to use this, so please change the design,”
Most manufacturers will accommodate this request.
However, when it comes to price, we can't match the prices offered by association members.
Also, the requirements for membership are surprisingly strict, so it's hard to get in.
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How often should a drilling machine be greased?
We do this every morning.
You shouldn't put too much in, but,
Just 2 to 3 pumps per grease nipple each day is sufficient.
Around the gasket of the swivel cleaning adapter is
Please fill it up until it's full.
It's best to perform maintenance on the rotating and striking parts every day to prevent problems.
If the machines don't break down, it will lead to cost savings.
Thank you for your continued support.
Sorry for the silly question...
To begin with, the pressure plate used in rock bolting isn't actually under tension, and
What kind of technical significance is actually expected from this?
Thank you as always.
To get right to the point,
The rock bolt's pressure plate doesn't actually serve any purpose, lol.
Basically, we expect this to increase shear strength.
A plate that’s just big enough to prevent us from being swept away when the mountain shifts is sufficient.
The square washers currently in use—approximately 150 × 150 × t9—are sufficient.
NEXCO certainly does a thorough job with that aspect.
Thank you for your guidance.
...Oh, I see. lol
I was working on some documents, and once I started getting into the nitty-gritty of the fundamentals, I got all confused lol.
It really helped me understand the difference between what people say and what actually happens, and I feel much clearer now.
That really helped. Thank you so much!
Thank you for your continued support. The information you post is extremely helpful and serves as a great reference. Thank you very much.
Could you tell me one thing?
In the case of temporary construction work intended to reinforce the excavated surface, is it really not necessary to apply galvanization or install head caps for rust prevention?
Thank you as always.
Generally speaking, items classified as "temporary" are not galvanized.
The government office will probably say it's because it's a temporary facility.
If you ask me whether it’s necessary, I’d have to say it’s better to have it, but it’s not a big deal if you don’t.
Actually, from the contractor’s perspective, plating is the better option, but...
The same principle applies to the head cap.
If you're considering life-cycle costs, galvanizing is definitely the better option; however, for temporary structures unrelated to the main construction work, it may not be necessary.
These days, even galvanized rebar often does not have head caps.
Thank you for your continued support.
Thank you very much for your prompt reply.
I'll use this as a reference.
Also, since this is a very informative blog, I’ll be sure to keep following it.