高温冻土固化机理与效果研究

Warm frozen soil is known as plastic frozen soil which has the characteristics of more unfrozen water content, higher compressibility and lower shearing strength. At present, the warm frozen soils are widespread in the subgrades of both the Qinghai-Tibetan Highway and the Qinghai-Tibetan Railway in permafrost regions, which has caused a lot of hazards to the embankments such as settlement and cracks. In the constructin of the above two engineerings, the measures of "permafrost protection" and "cooling embankment" were taken in order to preserve the stability of the embankments. However, these measures are certainly insufficient considering the problem of globle warming. In this project, we plan to adopt the ground treatment methods for normal soils and to apply soil stablizers to reinforce the warm frozen soils. By laboratory tests at negative temperatures, we intend to investigate the effect of types and ratios of the stablizers on unfrozen water content and pore structure in the solidified warm frozen soils. Meanwhile, we will test the compressibility, the shearing strength and the thawing temperature of the solidified warm frozen soils, and compare their variations before to after they are solidified. Afterwards, we plan to conduct some grouting experiments in the subgrade along the Qinghai-Tibetan Highway at the warm permafrost sections, and carry out some in-situ pressuremeter tests to check the effectiveness of the reinforcement of grouting. In this way, we hope to reveal the mechanism of solidification of the warm frozen soils, and to show improvement of the physical and mechanical properties of the warm frozen soils affected by the stablizers. After all, we expect to propose some new ideas to treat the thaw settlement hazards to the embakments of the Qinghai-Tibetan Highway and the Qinghai-Tibetan Railway in permafrost regions.

高温冻土又称塑性冻土，具有未冻水含量高、压缩性强、抗剪强度低等特点，目前已在青藏公路、铁路冻土路基中广泛发育，造成路基沉陷、开裂等病害。为了保持冻土路基的稳定性，既有工程中普遍采用了"保护冻土"与"冷却路基"的方法。然而，在全球升温的背景下，这种措施必然存在一定的局限性。本项目拟借鉴常规土地基处理方法，采用土壤固化剂对高温冻土进行加固。通过负温环境下的室内试验，重点研究不同种类固化剂及其配比对高温冻土中未冻水含量与孔隙结构的影响，并对固化处理前后高温冻土的压缩性、抗剪强度、融化温度等指标进行对比测试。然后，选择青藏公路高温冻土路段开展现场注浆试验，并采用旁压仪对其加固效果进行原位测试。由此揭示高温冻土的固化机理，阐明土壤固化剂对高温冻土物理力学性质的改良效果，为冻土路基融沉病害整治提供一条新的思路。

高温冻土又称塑性冻土，具有未冻水含量高、压缩性强、抗剪强度低等特点，目前已在青藏公路、铁路冻土路基中广泛发育，造成路基沉陷、路面开裂等病害。本项目突破冻土地基加固中传统的降温思路，借鉴软土地基处理方法，采用土壤固化剂对高温冻土进行改良。主要开展了负温环境下3类常见的土壤固化剂（离子类、高分子类、无机化合物类）改良高温冻土物理力学性质的机理与效果研究。根据高温冻土固化前后物理性质（水分相态、微观结构）的变化分析固化机理，通过固化前后力学性质（抗压强度、压缩系数）的对比评价改良效果，得出以下主要结论：（1）离子类土壤固化剂对高温冻土具有明显的固化效果，其作用机理是有效地降低了冻土中的未冻水含量。（2）高分子类土壤固化剂同样对高温冻土具有明显的固化效果，并且其作用机理与离子类土壤固化剂相似。但是，本项目选用的2种高分子类土壤固化剂在掺量较大时会导致固化土的冰点降低，未冻水含量增加，力学性质发生劣化。（3）硅酸盐水泥配合适当的添加剂使用，可以显著改善高温冻土的力学性能。但是，其力学强度主要来自固化土中水化产物形成的联结与骨架支撑作用，水分条件已居于次要地位。（4）将硅酸盐水泥与硫铝酸盐水泥混合在一起使用，可以进一步增强高温冻土的固化效果。其作用机理是硫铝酸盐水泥掺入硅酸盐水泥后，一方面加快了钙矾石的生成速率，另一方面减轻了钙矾石对凝胶类水化产物的破坏，由此达成了二者在水化反应中的动力学协调关系。（5）青藏高原多年冻土地基处理的现场试验结果表明，固化剂与冻土的拌合以及固化土的回填、压密、养护等施工条件与工艺流程对固化效果具有非常重要的影响，在今后的工程实践中必须对此类问题高度重视并开展专题研究。