Editor's Note: Recently, Japan developed a ground improvement method — the CXP grouting method — suitable for reinforcing alkaline liquefiable ground with pH of 10 or above. Let's take a look~~~

Development Background

In underground construction in Japan, chemical grouting is often used to achieve ground improvement. To better promote consolidation of the grouted mass, sodium silicate is often added to the grout; however, once the pH of the ground exceeds 9, the addition of sodium silicate cannot ensure the strength and long-term durability of the improved soil, creating an urgent need for a new method to solve this problem.

The CXP Grouting Method

The CXP grouting method (Complex Polymer), jointly developed by Konoike Construction and Toagosei Co., Ltd., is a chemical grouting method in which CXP grout is permeated into sandy ground to reinforce liquefiable ground. The CXP grout refers to a grouting liquid with magnesium acrylate as the main agent, mixed with polyaluminum chloride and additives.

After the CXP grout is injected, the gel (complex polymer) formed by polymerization does not undergo hydrolysis or other decomposition reactions, thus ensuring the long-term durability of the improved soil.

When applied to alkaline ground with pH of 10 or above, CXP grout ensures the strength and long-term durability of the improved ground, and the improved soil does not affect aquatic organisms, meeting the specified standards of Japan's Soil Contamination Countermeasures Act.

Trial Application

The method was trial-applied in a ground improvement project at a chemical plant in Nagoya, where ground improvement was required beneath hazardous materials storage tanks. The soil layer beneath the tanks (GL -1.45 m to -4.65 m) is a fill layer with N-values of 5 to 9, composed of sandy soil with silt at pH 10.3; the layer is alkaline with a fines content of 10%–24%, and the stratum is presumed to have liquefied most likely due to an earthquake.

Inclined drilling and grouting

Since the tanks above the soil layer could not be moved, it was decided to adopt the CXP grouting method, injecting grout from the outer perimeter of the oil containment dike to directly beneath the tanks to complete the ground improvement.

Based on the results of mix proportion tests, the CXP grout concentration was set at 6% and the gel time at 50–60 min, forming a total of 8 spherical improved soil columns with a diameter of 2.5 m and a center-to-center spacing of 2.0 m.

Construction was carried out using the double-packer dynamic injection method on the premise that no deformation occurred in the tank foundations and the oil containment dike. After the full planned volume was injected, samples were taken from the improved columns beneath the tanks, confirming that the uniaxial compressive strength of the improved columns sufficiently met the design target strength (in-situ strength 46.7 kN/m2).

Conclusion

Through this trial application, the feasibility of the CXP grouting method was verified, and it was also demonstrated that the method is suitable both for ground improvement directly beneath structures and for alkaline liquefiable ground with pH exceeding 10.

Going forward, Konoike Construction and Toagosei Co., Ltd. will actively promote the application of this method in other projects.