Abstract:

When the immersion area is small or the self-weight is less than its collapsible pressure, unsaturated loess does not collapse and its large pore structure can be retained. However, long-term hydraulic action significantly reduces the strength of saturated loess, potentially influencing the long-term stability of underground engineering projects. A field immersion test lasting 415 days was conducted to collect undisturbed saturated loess samples under various immersion duration, maintaining original in-situ stress and actual water infiltration conditions. Indoor and in-situ tests were conducted to examine changes in in-situ lateral stress during long-term immersion. The relationship between the softening mechanical behavior of loess and immersion duration was studied in terms of shear characteristics, compressive properties, and in-situ strength and deformation indicators. Results show that: (1) During prolonged immersion, loess transitions from a plastic to a soft or even flow plastic state, forming under-compacted saturated loess with a high void ratio, high water content, medium or high compressibility, and low strength. (2) During prolonged immersion, the in-situ horizontal stress of Q^eol₃ loess and Q^el₃ paleosol significantly increase, while the deep Q^eol₂ loess’s in-situ horizontal stress remains stable due to the strata’s arch effect. (3) Loess softening is time-dependent, and short-term indoor humidification samples are insufficient to show the gradual weakening of soil structure. After 120 days of continuous immersion, various strength and deformation indicators stabilize. (4) As immersion duration increases, the elastic segment of the stress-strain curve gradually shortens. Strength indicators, including unconfined compressive strength, peak shear strength, and in-situ horizontal bearing value, decrease significantly. The compression coefficient shows an exponential trend with increasing load, resulting in a significant increase in value and advancement of the peak interval. Additionally, both in-situ lateral pressure modulus and subgrade coefficient undergo substantial reduction. (5) In the stable stage, formation heterogeneity always exists. The unconfined compressive strength of saturated Q^eol₃ loess and Q^eol₂ loess is approximately 21%−32% of Q^el₃  paleosol, and the bearing capacity is 47% of Q^el₃ paleosol. The horizontal bedding coefficient K_x of saturated  Q^eol₂ loess is 67% of Q^el₃ paleosol, while that for saturated Q^eol₃  loess is 28%.

Key words: saturated loess, field immersion test, soil softening, immersion duration, mechanical indicators