[Image above] Some areas of southwest Japan flooded in July 2022 as Typhoon Aere approached the region, bringing heavy rain. Japan’s climate and geographical location put it at flood risk from numerous sources. Credit: Bloomberg Quicktake: Now, YouTube


While the 2022 hurricane season is just now roaring to life in the Atlantic with Hurricanes Fiona and Ian, over in the Pacific, Japan is recovering from Typhoon Nanmadol while Vietnam, Thailand, and the Philippines deal with Typhoon Noru. Meanwhile, Pakistan has faced severe flooding for months that left nearly a third of the country underwater.

The intensity and speed at which these storms developed is unprecedented, yet experts believe such unpredictability will become more common as climate change fuels more frequent and more extreme weather events.

While the effects of these storms on infrastructure and people are immediately obvious, less obvious is how such events can fundamentally change the landscape to be even more susceptible to damage in the future.

For example, in August, we discussed how heat waves and drought can cause clay soil to contract, leading cracks to form and grow in a house’s foundation and walls. On the flip side, too much water can decrease the aggregate stability of soils, making it more likely that future floods will lead to large-scale landslides and embankment breaches because of the deteriorated soil.

Cement treatment, or cement stabilization, methods have attracted significant attention as a way to fortify soils against disaggregation and mitigate future damages. It involves mixing soil with cement and water to generate reactions that create a matrix between the soil particles, which gives the soil strength.

The short-term (cement hydration) and long-term (pozzolanic reaction) strength development mechanisms of cement-treated soils are well known. The long-term stability of these soils, however, is not well investigated.

In a recent study, researchers from the Japan Cement Association, Hiroshima University, and the Paul Scherrer Institute in Switzerland investigated samples of cement-treated soil that came from a long-term (22-year) field test conducted by a technical committee of the Japan Cement Association.

Unlike most cement-treated soils, the samples in this study did not contain Portland cement. Though standard, this cement is not sufficiently effective in volcanic cohesive soils, which are present in most of flood-prone Japan. Instead, high-sulfate cement, which can host the formation of ettringite to increase initial strength, is widely used to treat soils in Japan.

The field test took place in Narashino, Chiba prefecture, Japan, an area covered with a volcanic cohesive clay called Kanto loam. This clay is commonly present throughout eastern Japan but is generally unsuitable as a construction material because of its high water and amorphous contents.

Nine cement-treated columns with a diameter of 0.45 meters and length of 2 meters were constructed and buried at the Narashino site. The first column was excavated after 28 days, while the next seven were excavated after 0.5, 1, 2, 3, 5, 7, and 10 years.

After the eighth excavation at 10 years, the ninth and final column was relocated to shallow ground in Sakura (15 km west from Narashino), which had soil properties similar to the original Narashino soil, except for a lower water content. After an additional 12 years (total age of 22 years), the last column was reexcavated for additional analysis.

The researchers used a variety of techniques to analyze the samples, including unconfined compressive tests, needle penetration, powder X-ray diffraction, pH determination, electron probe microanalysis, and geochemical thermodynamic calculations. These tests allowed them to provide a detailed description of how the pozzolanic reaction evolved in the cement-treated soil.

Ultimately, they drew two main conclusions.

  • Long-term stability. The treated soil maintained its strength for 22 years. Its early strength development was due mainly to formation of ettringite and C–S–H through cement hydration, while long-term strength development occurred because of strätlingite formation by a pozzolanic reaction between cement hydrates and the soil.
  • Strength reduction. Calcium leaching and carbonation at the column surface significantly reduced strength at the deteriorated region. Relocation of the test column enhanced the carbonation deterioration.

The researchers write that future studies could include the detection of whole hydrates, including amorphous phases, along with analysis of the detailed dissolution process of soil in the alkaline environment with various types of cement and soils.

Also, “For establishing a universal concept to predict strength development via geochemical modeling, the other factors such as pore size distribution, soil particle skeleton, and water surface tension should be considered in addition to phase changes,” they conclude.

The paper, published in Cement and Concrete Composites, is “Twenty-two-year investigation of strength development and surface deterioration of cement-treated clay in an in-situ field test” (DOI: 10.1016/j.cemconcomp.2022.104783).

Author

Lisa McDonald

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  • Environment