Hello, everyone.
This is Enta.
I don't know if it's because of the typhoon, but lately it's been raining on and off, and it's so muggy, so muggy, so muggy...
I'm breaking out in a weird sweat lol
Is it because I'm an old guy!?
But that's beside the point.
I’d like to write about some common questions regarding the spacing of rebar insertion work.
This is about pitch.
A common question regarding this pitch is how to determine the pitch, especially on mountains with steep terrain.
What exactly is the pitch in rebar insertion work?
Reinforcement installation (rock bolting) is a common method of slope reinforcement that involves inserting reinforcing bars into drilled holes and bonding them to the rock mass using mortar or grout.
The basic process is that the construction drawings specify the placement intervals—such as “2.0 m pitch”—and the construction crew lays the concrete according to those specifications.
The design specifications say something like a 2-meter pitch, right?
Therefore, please pour the concrete at 2-meter intervals.
That’s how it goes, but it’s easy to think of it as just a mountain that’s been cleaned up perfectly, lol.
If the slope is perfectly straight, it's easy to determine the pitch.
That's exactly the kind of steep mountain I'm talking about!
You don't have to think about this at all.
With a 2-meter spacing, it's easy to install.
For a smoothly cut slope, all you have to do is place a tape measure against it and divide the length by 2.0 meters to get the design specifications, so almost no site supervisor has any trouble with this.
The problem comes after this.
The basic premise is to determine the placement location based on the design cross-section.
The key point here is that the rebar insertion work has been calculated based on the cross-sectional area.
If four were installed in the design cross-section (planned cross-section, main survey line), then four are absolutely required there!
First, where is the design cross-section?
Please refer to the drawings and design calculations and verify No. ○○ on-site against the drawings.
We always pour a specific number of columns—such as four or five—into that cross-section.
Looking at it the other way around, as long as the specified number of items—such as 4 or 5—is included as designed, that’s fine.
We'll distribute them to the left and right based on that standard.
The number of reinforcing bars in the design cross-section is the result of a calculation performed during the stability analysis to determine, “How many reinforcing bars with this resistance are required for the assumed slip surface at this depth?”
"Pitch" refers to the process of determining the placement locations—specifically, where to position this fixed number of piles on the slope.
If you remember to follow this order—first establishing the premise that the positions (pitch) and number of cross-sections must never be misaligned, and then using projections to determine their positions—you won’t get confused on the job site.
The same approach applies even on steep slopes | Determine the placement location using projections
So, what happens when it turns into a pile like this?
Even in rugged, hilly terrain, the basics remain the same.
Here's what I have in mind.
Nothing has changed.
This applies to all slopes—please think of it as a projection.
If you were to measure by holding the tape measure as it follows the undulating contours of the ground, and the result looked like the illustration above, the pitch would vary, right?
This would make installation really difficult.
This is strictly the projection pitch!
The reason we determine the placement position using a projection rather than the actual terrain is that stability analyses are based on the assumption that the slope is a plane (cross-section).
If you simply trace the unevenness with a tape measure to determine the positions, the concrete placement will be too dense in the recessed areas and, conversely, too sparse in the protruding areas, resulting in a distribution of resistance that deviates from what was intended in the design.
The idea is that if you align the positions using a projection, the design-calculated placement density will be maintained no matter how much the natural ground undulates.
The Manual on Soil Reinforcement Methods for Natural Slopes states that, regarding the spacing of reinforcement materials, if the spacing is too narrow, the individual piles will be unable to provide their full reinforcing effect due to a phenomenon similar to the group effect of piles; conversely, if the spacing is too wide, the surface layer of the slope becomes prone to weathering, and three-dimensional partial collapses are more likely to occur due to rainfall or earthquakes.
Based on practical experience with road and railway standards (empirical methods), a general guideline is to set the minimum installation spacing at approximately 1.0 m and the maximum installation spacing at approximately 1.5 m (up to 2.0 m when integrating with a rigid surface material, such as bedrock, where sufficient lateral friction resistance can be expected). (This approach can also be considered in the absence of specific design specifications.)
Furthermore, this spacing refers solely to the density of the layout as projected, not to the actual measured distance as it exists in the natural ground.
If you confuse these points, the placement of concrete will end up being either excessively dense or, conversely, too sparse on undulating slopes.
Unlike ground anchors, the soil reinforcement method is characterized by the placement of numerous reinforcement elements throughout the entire ground mass; therefore, it states that there is no need to be extremely precise regarding the installation angle, spacing, or angle of installation for each individual anchor do not need to be extremely precise; it is also stated that if obstacles such as gravel prevent installation at the planned location, the angle or position may be slightly adjusted to accommodate this. [p. 137]
Basically, it just means you don't have to worry too much about the pitch—it's fine as long as it's somewhat in tune. lol
How much of a difference is there between projection and natural terrain?
Since it’s hard to convey this just through words, let’s look at a simple example.
Suppose there is a raised area (protrusion) about 0.5 m high exactly in the middle of a section where the projection pitch has been set to 2.0 m.
If you trace this protrusion directly with a tape measure and mark the “2.0 m” point, you’ll find that—because you’ve actually taken a detour by climbing up and down the protrusion—that mark hasn’t yet reached 2.0 m in projected distance.
It's the same principle as mountain climbing.
Even a mountain hut that’s 2 km away on the map (as the crow flies) can end up being nearly 3 km away when you actually walk along a mountain trail with ups and downs, right?
It’s the same principle as when an uneven surface causes you to take a longer route, so the actual walking distance ends up being greater than the horizontal distance.
In other words, if you use “2.0 m, measured with a tape measure along the natural contour of the ground” as the placement position, the actual spacing—as seen in the projection—will be closer together than specified in the design.
On the other hand, if you don’t follow the sequence of first determining the 2.0-meter position using a projection and then marking that position on the natural ground (and, if there’s a protrusion, extending the tape measure slightly further to find the actual position on the natural ground), the measurements won’t line up.
Well, in that regard, it looks better if you balance it with the crossbeams and position them just right.
If you just go by pitch alone, it ends up looking bad, so in the end, it’s all about looks lol
When to Use a Square Pattern vs. a Staggered Pattern
Here's another effective layout pattern to use when assigning elements via projection.
According to the book, while a square bolt arrangement is the standard, a rectangular arrangement is more economical when stability can be maintained even with increased spacing in the depth direction; however, since this increases the risk of three-dimensional partial collapse, a staggered arrangement is considered effective under such conditions.
In areas prone to instability—such as near riverbanks or sections with abundant spring water—it’s best to simply use a staggered arrangement to increase density, which will also help prevent gaps from forming.
The trick here is also to decide on the pattern on the projection drawing first, and then transfer it to the natural ground.
The Same Principle Applies to Horizontal Planes | Determined by the Center-to-Center Projection Distance
The same principle applies here, even when viewed horizontally. (This is a cross-section viewed from directly above.)
Suppose we have a slope like this.
We will install a rebar inserter here.
Let's assume a pitch of 2.0 meters.
All right! Since it’s 2.0 meters, let’s measure it by placing the tape against the ground!
When you do that, it will look like this:
That's what I wrote earlier ↑↑↑, right?
This is what it looks like when measured in a mountain-shaped pattern using tape.
However, this is not a 2.0-meter pitch.
It's all about projection!
The evaluation criterion is whether the horizontal distance between the heads (centers) of the rebar inserts, as seen on the plan view, is 2.0 meters. In other words, this distance is managed strictly as the center-to-center distance on the drawing—not as the actual length measured along the surface of the natural ground.
The same approach can be applied to legal frameworks as well.
This is a 2.0-meter pitch.
No matter how uneven the natural terrain is, it's just a projection!
The legal framework is up in the air.
Please look at the shadow.
That's when the shadow is created by placing the frame along the ground.
I hope this gives you an idea of what I mean.
So, even if the legal clearance measured along Jiyama Street is 2.5 meters, as long as the distance from centerline to centerline is 2.0 meters, it’s acceptable.
As long as you get the general idea, that's fine!
Whether it’s rebar placement or formwork installation, this concept of projection can generally be applied to any type of work that involves arranging reinforcing materials or structural members in a grid or frame pattern.
Conversely, when you’re having trouble with the intersection spacing within the legal framework, it’s a good idea to get into the habit of first checking whether what you’re measuring is the natural ground surface or a projection.
Only as-built measurements should be taken as-is.
However, please measure the finished shape based on the undulations of the hill!!!
The settlement is an "m" settlement, right?
It's important to measure carefully according to the terrain!
Allocations and layouts are based on projections, while as-built measurements and final settlements are based on actual field measurements of the natural ground surface.
I’d like you to keep in mind that the purposes of layout and as-built measurements are different.
There was a time when I, too, was convinced that “pitch” meant “the distance measured with a tape measure,” lol.
It makes perfect sense once you think of it in terms of determining the position through projection, but it’s probably something many people don’t realize until it’s explained to them.
By the way, on site, you determine the vertical alignment by eye while lowering and adjusting the plumb line, right!?
That's a projection, lol.
See you later.
Approaches to Placing Ground Anchors on Undulating Slopes (Part 1)
