Hello, everyone.

This is Enta.

Have you ever heard that, when grouting ground anchors, “applying pressure makes them stronger” or “the cement slurry penetrates the soil”?

I, too, used to think that injecting the solution all the way to the free end—rather than applying pressure only at the fixed end—might reduce the risks.

You're not entirely off the mark. However, upon further investigation,"Pressure grouting of the anchor body" and "grouting of the free-length section" serve different purposes.That's right. If we lump these two together, the discussion about construction management will go off track.

Strength cannot be determined based solely on the shape of the pressurized test specimen.

You can see a mark here that seems to lead toward the center, right?

These are test specimens in which the cement slurry was cured under pressure using a packer. Looking at their shape, it is clear that the pressure had an effect.

However, based on this photo alone, we cannot say that “the compressive strength has increased.” To compare strength, compressive strength tests must be conducted using the same mix design, curing conditions, age of the concrete, and test specimen dimensions. After all, appearance and test results are two different things.

Pressure injection is not a one-size-fits-all solution; its effectiveness varies depending on soil conditions.

This is a test specimen that was cured without pressure. It has a layered appearance, similar to what is typically seen when test specimens are taken in the usual manner.

According to the "Ground Anchor Design and Construction Standards" of the Japanese Geotechnical Society, pressure grouting isAppropriate methods based on the ground conditions around the anchor bodyIt is said to be performed in this way. In other words, it doesn't mean that "applying pressure will fix everything."

(Naoki Katayama) According to the research paper, it appears that no grout infiltration due to pressure was observed within the scope of tests conducted on homogeneous model soils. Grout dehydration was observed in sandy and sandy-gravel soils, and the results also showed that, depending on the conditions, both the frictional stress and the grout strength increased.

The key point here is not to apply these results to all sites. The study itself was conducted on homogeneous model soil. Since natural soil varies in terms of cracks, groundwater, grain size, and density, the effectiveness of the applied pressure will also vary.

Another paper published by the Japan Society of Civil Engineers reports that, in sandy soils, the dehydration of grout makes it difficult for pressure to effectively act on the surrounding soil, while in cohesive soils, fracturing may occur depending on the conditions. This indicates that pressure injection cannot be explained by simple “permeation” alone.

Although the free length section is not a resistive element, the filling and injection process is quite important.

The anchor body is the part that transfers the tensile force received from the tendon to the ground. The free length of the tendon is the part that transfers the tensile force acting on the anchor head to the anchor body.

Even if the free length is fully backfilled, this does not mean it should be included in the calculation of frictional resistance along the circumference in place of the anchor body. (In principle, only the anchored section is considered.) If the free length is strongly restrained by the soil along its length, this will also affect the tendon’s original function of extending freely.

However, that doesn’t mean it’s better not to have the free-length section filled and injected.

The standards manual explains that the purpose of grouting is to fill the voids between the outer surface of the free-length section of the sheath and the surrounding soil with grout to enhance corrosion protection, and to prevent loosening and weathering of the soil around the borehole wall. According to the 2012 edition of the standards, grouting is mandatory for anchors classified as Corrosion Protection Class II or higher.

So, to be precise,"The anchor section must be constructed exactly as designed, and the subsequent free-length section must also be backfilled reliably using a method appropriate for the purpose."That's right. And this means that the friction in the free length also accounts for the friction at the anchorage point. If an anchor is driven all the way to the free length and then pops out, that means it was installed incorrectly in the anchorage soil to begin with, lol.

 

See you later.

[Reference Materials]

• Naoki Katayama, “The Effect of Grout Injection under Pressure on Friction Stress in Anchor Bodies,” Journal of the Japanese Society for Landslides, Vol. 45, No. 5, 2009, pp. 376–382
• Kazuo Kurihara, Kenji Saito, Yasuyuki Kobayashi, and Masahito Tamura, “A Study on Grout Pressurization in Ground Anchors,” Transactions of the Japan Society of Civil Engineers, No. 453, 1992, pp. 145–154

How to Make a Packer for Ground Anchor Construction (Part 1)