冻融循环作用是导致寒区岩石性质劣化的重要原因。岩石冻融损伤程度取决于温度条件和其含水状态。虽然关于岩石的冻融损伤规律和损伤机制已有大量研究，但大多考虑岩石在饱和状态的损伤演化，而忽略了天然岩石的吸水过程及真实孔隙水赋存状态的影响。地表天然岩石的吸水过程大多为毛细管力驱动，在毛细渗流方向上水分呈梯度分布状态。毛细吸水岩石的冻融损伤规律和损伤机制应与饱和岩石差异显著，需要进行深入研究。

本文首先通过对砂岩进行毛细吸水实验及浸泡吸水实验，测量吸水过程中样品的吸水质量、吸水锋面高度、核磁共振T₂谱和核磁共振分层含水率。研究了毛细吸水砂岩水分的迁移过程及赋存状态。在此基础上，对毛细吸水、浸泡吸水及抽气饱和后的样品进行冻融循环，测量冻融循环后，样品的吸水过程，以及含水率、电阻率、波速、核磁共振T₂谱、核磁共振分层含水率以及样品表观形貌。并对不同冻融循环次数下的样品进行单轴和巴西劈裂实验。研究冻融循环对吸水过程的影响，以及冻融循环条件下毛细吸水砂岩物理参数演化规律及力学性质劣化规律。研究结果表明：

(1)毛细吸水迁移过程中，水会以气体状态进行扩散并冷凝，吸附在吸水锋面前的孔隙表面。通过核磁共振T₂谱分析得到的吸附水信号值随吸水时间的变化。将其与样品吸水锋面高度随吸水时间变化的曲线进行对比，可以发现在吸水锋面到达样品顶部之前，吸附水信号值达到稳定。这印证了吸水峰面上方存在水汽的扩散和凝结。毛细水迁移过程及赋存状态受到孔隙率及层理方向的影响。平行层理砂岩吸水速度比垂直层理砂岩的吸水速度更快。砂岩内部水分的最终赋存含量则主要取决于样品本身的孔隙率大小。毛细吸水及浸泡吸水过程都会导致砂岩内存在无法排出的气体。毛细吸水与浸泡吸水的水分迁移过程及赋存状态存在差异。

(2)冻融循环对毛细吸水过程及浸泡吸水过程均有明显影响。冻融循环导致了样品孔隙结构的变化。吸水条件不同，冻融循环导致的孔隙结构变化也存在差异。冻融循环对抽气饱和砂岩造成的损伤最明显。冻融循环对毛细吸水砂岩孔隙结构的改变沿高度方向有所差异。吸水底端的孔隙结构受冻融循环影响更大。

(3)冻融循环导致了样品力学性质的劣化。吸水条件不同，冻融循环导致的样品力学性质的劣化存在差异。对于抽气饱和砂岩，其单轴抗压强度及抗拉强度随冻融循环次数的降低较为明显。毛细吸水样品沿高度方向的抗拉强度存在明显差异。

(4)结合砂岩在孔隙尺度和层理尺度上的结构观测结果，提出了代表性孔隙结构单元。通过对代表性孔隙结构单元中水分迁移的分析，认为层理结构对砂岩毛细吸水过程的影响体现在平行层理砂岩和垂直层理水砂岩中不同的水分迁移模式。含水量及水分分布状态的差异会导致样品在冻融循环作用下，产生损伤的差异。抽气饱和砂岩内部由于水分含量高，导致冻融循环作用造成其结构损伤相对于毛细吸水及浸泡吸水砂岩更为明显。而对于毛细吸水砂岩，由于其内部水分赋存状态的差异，导致了损伤的差异。

The freeze-thaw cycle is an important reason for the deterioration of rock properties in cold area. Although there are a lot of researches on the mechanism of rock freeze-thaw damage, most of them consider the damage evolution of rocks in saturated state, but ignore the influence of natural rock water absorption process and real pore water occurrence state. The water absorption process of natural rocks on the surface is mostly driven by capillary force, and the water distribution is gradient in the direction of capillary seepage. The freeze-thaw damage law and damage mechanism of capillary water-absorbing rock should be significantly different from that of saturated rock, which needs further study.

In this paper, firstly, the capillary water absorption experiment and immersion water absorption experiment were carried out on the sandstone. The water absorption mass, height of the water absorption front, NMR T₂ spectra and stratified moisture distribution were measured during the water absorption process. The water migration process and water distribution in sandstone are studied. Then the freeze-thaw cycle experiment was carried out on sandstone under different water absorption conditions. After the freeze-thaw cycle, the water absorption process, moisture content, resistivity, wave velocity, NMR T₂ spectrum, NMR layered moisture content, and sample apparent topography are measured. Uniaxial and Brazilian splitting experiments were performed on samples with different freeze-thaw cycles. The influence of freeze-thaw cycle on water absorption process and the evolution law of physical parameters and deterioration law of mechanical properties of capillary water absorption sandstone under freeze-thaw cycle were studied. The results demonstrate that: 

(1)Vapor diffusion and condensation occurred ahead of the water absorption front. The variation of adsorbed water signal value with water absorption time obtained by nuclear magnetic resonance T₂ spectrum. Comparing this with the curve of the height of the sample absorbent front with the water absorption time, it can be seen that the adsorption water signal amplitude stabilizes before the water absorption front reaches the top of the sample. This confirms the diffusion and condensation of water vapor above the water absorption peak. The migration process and water redistribution mode are affected by porosity and layer structure. Parallel layered sandstone absorbs water faster than perpendicular layered sandstone. The final water content in the sandstone depends on the porosity of the sample. Both capillary water absorption and immersion absorption processes can lead to the presence of gases in the sandstone that cannot be vented. There are differences in the migration process and water redistribution mode of the capillary water absorption and immersion absorption processes.

(2)The freeze-thaw cycle has obvious influence on capillary water absorption process and soaking water absorption process. The freeze-thaw cycle results in the change of pore structure. The pore structure changes caused by freeze-thaw cycle are also different under different water absorption conditions. The damage caused by freeze-thaw cycle is most obvious to saturated sandstone. The change of pore structure of capillary water absorbing sandstone by freeze-thaw cycle is different along the height direction. The pore structure at the bottom of water absorption is more affected by freeze-thaw cycle. 

(3) The freeze-thaw cycle leads to deterioration of mechanical properties of sandstone. The deterioration of mechanical properties caused by freeze-thaw cycle are also different under different water absorption conditions. For pumped saturated sandstone, the uniaxial compressive strength and tensile strength decrease obviously with the freeze-thaw cycles. The tensile strength of capillary absorbent samples along the height direction is obviously different. 

(4) Combining observations of the sandstone structure at pore-scale and layer-scale, the concept of representative pore-structure element (RPE) is proposed. Based on analysis on water migration in RPEs, we suggest that effects of the layer structure on capillary imbibition in sandstone are embedded in the different water migration modes in P_ar (samples with bedding planes parallel to the height) and P_er samples (samples with bedding planes perpendicular to the height). Due to the difference of water content and water distribution inside sandstone, the difference of freeze-thaw damage is further caused. Freeze-thaw damage from pumped saturated sandstone is the most obvious. And the damage from capillary absorbent sandstone is different along the height direction.