In the design of cement-soil composite foundations, how is the strength of the cement soil determined?

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Regarding the design of composite foundations using cement-soil mixing piles, codes require that the strength of the cement soil be determined through laboratory mix design tests. However, in most cases, the design is carried out without such laboratory data; it is therefore necessary to first establish an estimated value for the compressive strength of the reinforcement material before proceeding with further design calculations. Determining this value appropriately is a problem that plagues designers, and it is also a question frequently asked by experts on the Zhizhi platform. Here, a response is provided based on relevant information. According to field study data, when cement is used as the curing agent and other conditions remain constant, the strength of the cemented soil varies depending on the cement content in the same soil layer. Grade 32.5 cement can be used for the project, with a cement content of 7% to 12%. When the cement content is greater than 10%, the strength of the cemented soil can reach 0.3 MPa to over 2 MPa. Generally, the cement addition ratio αw is set between 12% and 20%. For reinforced soil mixing piles (walls) made of steel bars, due to their high water-cement ratio (1.5–2.0), in order to ensure the strength of the reinforced soil, cement of grade 42.5 or higher should be used, with an addition rate of not less than 20%. The compressive strength of cemented soil increases as the corresponding cement content increases, but due to differences in the soil properties of the site and construction conditions, the increase in cement content does not correspond exactly to the percentage increase in the strength of the cemented soil. The strength of cement directly affects the strength of cemented soil; for every 10 MPa increase in cement strength, the strength of the cemented soil, fcu, increases by approximately 20% to 30%. Based on general engineering experience, when the solidifying material is cohesive soil and there is no laboratory mix design data available, the unconfined compressive strength fcu of the pile shaft can be estimated using a value of 1.0–1.6 MPa as a reference (when the cement content is 12%–15%). The strength of cemented soil increases as age progresses, and it continues to increase significantly after 28 days. To reduce costs, a curing time of 90 days is adopted as the standard age for load-bearing mixing pile specimens both domestically and internationally. For the mixing piles that provide support and bear horizontal loads, taking into account the impact of the excavation period, the standard strength grade for cement-treated soil can be taken as that at 28 days age. Based on the compressive strength tests, it was found that, under identical conditions, there is a roughly linear relationship between the compressive strengths of cement soil at different ages. The empirical formulas are as follows: fcu7 = (0.47–0.63)fcu28; fcu14 = (0.62–0.80)fcu28; fcu60 = (1.15–1.46)fcu28; fcu90 = (1.43–1.80)fcu28; fcu90 = (2.37–3.73)fcu7; fcu90 = (1.73–2.82)fcu14. In these formulas, fcu7, fcu14, fcu28, fcu60, and fcu90 represent the compressive strengths of cement soil at 7 days, 14 days, 28 days, 60 days, and 90 days respectively. After three months of age, the strength of cemented soil increases slowly. The strength of the cemented soil at 180 days is 1.25 times that at 90 days, and the increase in its strength continues even after 180 days.