关键词: 冻融循环, 植被混凝土, 团聚结构, 淋溶流失率

Abstract:

Vegetation concrete （VC） ecological protection technology is an effective solution for the vegetation recovery of bare steep slopes， which has been increasingly applied in cold regions in recent years. When the technology is implemented， the nutrient retention ability of VC substrate is essentially concerned. Under the actions of freeze-thaw cycles， fertility of the VC substrate as well as natural soil is thought to degrade gradually. It has been recognized that the nutrient retention ability of soil is significantly correlated with its physical structure. Similarly， the nutrient retention ability of VC substrate could be supposed to be physical structure-dependent. To enhance the comprehensive performance of VC substrate in cold regions， the investigation of nutrient retention ability is required， which nevertheless is still little identified. In this study， a series of freeze-thaw cycle experiments for VC substrate were conducted， and the effects of initial water contents and freeze-thaw cycles on characteristic parameters of water-stable aggregates and leaching loss rates of major nutrient substances were studied. A freeze-thaw cycle for specimen treatment， performed by a fast air freeze-thaw test machine， was defined as the freezing process of 12 hours at -20 ℃ in addition to the thawing process of 12 hours at +20 ℃. Except for the non-treatment， namely without freeze-thaw cycle， 7 treatments were considered to prepare the specimens， including 1 cycle， 2 cycles， 4 cycles， 8 cycles， 16 cycles， 32 cycles and 64 cycles. According to the field experience in practice， the lower and upper initial water contents of specimens were designated to be 18% and 24%， respectively. The results showed that the water-stable aggregates of the VC substrate were mainly composed of the particles with size ranging from 0.05 mm to 0.25 mm， which contained the proportions over 50% of total mass for all specimens. With increasing initial water content， the water-stable micro-aggregates transformed into the macro-aggregates， among which the particles of ≥1~2 mm were found to hold the maximum increase rate in proportion. Other parameters， which could quantitatively represent the characteristics of aggregate structure， also showed that the aggregate stability increased with initial water content. In addition， the contents of particles smaller than 0.25 mm were positively related to freeze-thaw cycles， while that of the particles larger than 0.25 mm showed the inverse trend. This indicated that the average value of aggregate particle sizes decreased with freeze-thaw cycles. It was noticed that the dispersion rates of aggregate increased with initial water content， which showed that destructive action to aggregate caused by freeze-thaw cycles was greater than the reinforcement provided by the increasing cement hydration products. Furthermore， the freeze-thaw cycles required for the aggregate characteristic parameters of VC to reach the stable state were more than that for natural soil. It may be due to that natural soil would go through the repetitive process of decomposition and aggregation， while destruction process of cement hydration products was irreversible. For the fertility， a high initial water content was associated with the increasing contents of major nutrient substances. Contents of organic matter， ammonium nitrogen （NH₄⁺-N）， available phosphorus （PO₄^3--P） and potassium （K⁺） still increased with freeze-thaw cycles， while content of nitrate nitrogen （NO₃^--N） decreased. Moreover， the leaching losses of these nutrient substances increased with freeze-thaw cycles obviously. From the Pearson correlation analysis， the leaching loss rates of major nutrient substances were found to correlate closely with the aggregate characteristic parameters. In consideration of significance levels and absolute values of correlation coefficients， geometric mean diameter （GMD） could be suggested as the reasonable index to describe the nutrient retention ability of VC substrate. The results may contribute to illustrate the underlying reason for VC substrate fertility degradation under freeze-thaw cycles and provide theory basis for countermeasure.

Key words: freeze-thaw cycles, vegetation concrete, aggregate structure, leaching loss rate