Hello, everyone.
This is Enta.
I received a question today, so I’d like to try answering it.
Q:
On a difficult mountain site, after excavation work is complete, we install the formwork, but
In the past, construction was carried out using ingenuity and resourcefulness.
Thank you for your question.
When the natural ground conditions are poor, it is common for the slope to remain uneven even after it has been cut away with heavy machinery.
If you cut it out using BH exactly as described in the design document, it should look like this.
Before cutting into the mountain, this was the vision of the government agency and the general contractor; the design was intended to allow for the legal framework to be installed neatly and easily.
Naturally, according to the design plans, the legal framework is also expected to take this form.
So, the plan is to go straight down from top to bottom in one swift motion.
But in reality, it looks like this.
So, parts are falling off all over the place, and it's in a real state of disrepair.
If the difference in elevation between the highest and lowest points on this uneven surface were as much as 0.3 to 1 meter, it would be a total disaster.
If we were to install the formwork using the high point as a reference, there would be an enormous amount of mortar waste on the side of the formwork closest to the natural ground, beyond the red line.
Furthermore, work efficiency within the legal framework drops sharply, leading to an overall increase in costs.
If the frame is installed so that it follows the natural contour of the ground as closely as possible, the result will be as shown above; however, this will significantly increase assembly time and raise material costs.
I think building along the natural ground level will require the most labor and materials.
So, the person asking the question has chosen to smooth out the bumps and make it fit within the legal framework, and I think that’s the better option as well (taking safety into account).
So the red part serves as the base.
We'll be building this area using sprayed mortar.
It is common practice to apply what is known as "low-mix mortar" to this area. (At least according to textbooks and other sources.)
That "low-mix mortar" is,C:S = 1:6–8
It is stated that... (From the Q&A on Slope Protection Works)
Furthermore, in the process of lining the back of a zabuton frame,C:S = 1:8–10
It is said that...
The Slope Protection Association does not specify any particular strength requirements.
As a backing material for zabuton framesThe strength is approximately 10 N/mm².It is said that...
In practice, the interpretation is that as long as the material has strength equal to or greater than that of the natural ground, it will settle well when the retaining wall is installed, and there will be no structural issues.
Furthermore, since the substrate onto which the spray coating was applied is considered to be the natural ground, issues regarding strength are largely irrelevant.
However—and this is the important part—thisIt's hard to come up with the money for a low-budget productionThat's right!
It’s hard enough for government agencies to provide justification, and they’re likely to be even more reluctant if it involves a site subject to an audit.
Therefore, we will use market-standard sprayed mortar with a thickness of 5 cm, for example.
It's just faster that way.
Assuming a cumulative total,Sprayed Mortar with Low-Content BinderLet's say we created one.
I think it will definitely cost more than the market rate for sprayed mortar.
In that case, I think it would be clearer to use market-standard sprayed mortar and set the strength to 18 kN or higher.
Use the standard formula as well.
That is my opinion.
I’m sure there are various other ways of thinking and approaches, but I also think they differ from prefecture to prefecture.
Please do your best in that area.
Negotiations like this might just be the best part of being a site supervisor and a general contractor.
I'm rooting for you!
See you later.
