Yanhu Mu

Laboratory testing on heat transfer of frozen soil blocks used as backfills of pile foundation in permafrost along Qinghai-Tibet electrical transmission line

Generally, construction for pile foundation in permafrost has to be carried out in winter to minimize the thermal distribution to the underlying or surrounding permafrost. Thus, there exists a problem that it is hard to meet the stipulated requirement to the compaction degree of the backfilled frozen soil blocks around the pile foundation excavated quickly. In order to study the effect of froze soil blocks on the heat transfer process between pile and permafrost during the construction of the Qinghai-Tibet electrical transmission line in winter, some laboratory tests were carried out for the highly porous frozen soil blocks and the naturally compacted thawed soil body, respectively. In addition, the thermal conductivities were calculated under different temperature gradient according to the measured thermal data. Results show that the convective heat transfer occurs in the highly porous frozen soil blocks at negative temperature corresponding to winter time, which is favorable for refreezing the pile foundation and lowering permafrost temperature. However, backfilling the highly porous frozen soil blocks hardly meet the requirement of compaction degree. It has dual effect on the stability of tower foundation depending on the specific site conditions such as permafrost temperature, ice content, soil type, permeability, hydraulic condition, and embedded depth of pile. Results also show that the equivalent thermal conductivity of the frozen soil blocks is over five times more than that of the thawed soil body on average. This is because the convective heat transfer occurs in frozen soil blocks in winter, which has stronger heat exchange effectiveness and can diminish refreezing time. Tests have revealed the process of heat transfer of frozen soil blocks used as fills around the pile foundation in permafrost, verified its thermal semiconductor effect, and accumulated and expanded data of the thermal conductivity.

Effect of Freeze-Thaw Cycles on Mechanical Behavior of Compacted Fine-Grained Soil

A large amount of work has been conducted to study the effect of freeze-thaw cycles on the geotechnical properties of various soils. But less laboratory work has been focused on the effect of number of freeze-thaw cycles and on quantitative relationship between mechanical behavior and freeze-thaw cycles. This study undertook a series of tests including freeze-thaw (FT) (after 0, 2, 5, 11, 21, and 31 freeze-thaw cycles respectively), unconfined compression (UC) and unconsolidated-undrained triaxial compression (UUTC) tests. These tests aim to assess the influence of freeze-thaw cycles on the mechanical behavior of compacted fine- grained soil, to establish correlation between the mechanical behavior and freeze-thaw cycles, and to facilitate the prediction of changes in geotechnical properties of soil. Freeze-thaw cycles notably influence the stress-strain curve reducing the UC strength by 11%, elastic modulus by 32%, and cohesion by 84% after 31 freeze-thaw cycles in comparison with those soils unexposed to freeze-thaw. The angle of internal friction is slightly increased by 1 to 2°. The weakening and deteriorating effects of freeze-thaw on the compacted fine-grained soil are confirmed. They can provide a scientific basis for design consideration of cold-region infrastructures, and for countermeasures against frost heave and thaw settlement.

Effects of freeze-thaw cycle on engineering properties of loess used as road fills in seasonally frozen ground regions, North China

Compacted loess is widely used as fills of road embankments in loess regions of northern China. Generally, densely-compacted loess can satisfy the requirements of embankment strength and post-construction deformation. I lowever, uneven subsidence, pavement cracks and other related damages can affect the integrity of loess subgrade after several years of operation, and even cause some hazards, especially in North China, where the strong freeze-thaw erosion occurs. In this study, cyclic freeze-thaw tests for both densely and loosely compacted loess samples were performed to determine the variation in engineering properties such as volume, void ratio, collapsible settlement, microstructure, and the related mechanisms were addressed. The experimental results showed that an obvious water migration and redistribution occurred within the samples during freeze-thaw cycles. Ice lenses and fissures could be identified in the upper frozen layers of the samples. After freeze-thaw cycles, the dry densities of the nipper layers of samples changed significantly due to strong freeze-thaw erosion. The dry densities decreased for the dense sample and increased for the loose sample. It can be found that dense samples become loose, while loose samples became dense with the increasing number of freeze-thaw cycles. Their related void ratios changed reversely. Both void ratios tended to fall into a certain range, which verified the concept of a residual void ratio proposed by Viklander. The loosening process of densely compacted samples involves the formation of large pores, volume increase and density reduction as well as the related changes in mechanical properties because freeze-thaw cycles may be important contribution to problems of loess road embankments. Adverse effects of freeze-thaw cycles, therefore, should be taken into account in selecting loess parameters for the stability evaluation of road embankment in seasonally frozen ground regions.

Experimental Analysis and Discussion on the Damage Variable of Frozen Loess

The damage variable is very important to study damage evolution of material. Taking frozen loess as an example, a series of triaxial compression and triaxial loading-unloading tests are performed under five strain rates of 5.0 × 10−6–1.3 × 10−2/s at a temperature of −6°C. A damage criterion of frozen loess is defined and a damage factor is introduced to satisfy the requirements of the engineering application. The damage variable of frozen loess is investigated using the following four methods: the stiffness degradation method, the deformation increase method, the dissipated energy increase method, and the constitutive model deducing method during deformation process. In addition, the advantages and disadvantages of the four methods are discussed when they are used for frozen loess material. According to the discussion, the plastic strain may be the most appropriate variable to characterize the damage evolution of frozen loess during the deformation process based on the material properties and the nature of the material service.

Influence of Wetting-Drying Cycle in Road Cut Slope in Loess in Northwest China

Wetting-drying cycle is a strong weathering process which considerably changes the engineering properties of metastable loess. It has been considered in slope stability evaluation and slope surface protection. In this study, rainfall intensity, volumetric water contents and ground temperatures in loess road cut slopes were monitored at two study sites, Dingxi and Pingliang, northwest China, to determine infiltration depth of rainfall and frost depth. The monitored data indicated that water contents from the ground surface to −20 cm deep changed significantly, less changed below the depth of −40 cm. The surficial loess of road cut slope had experienced strong wetting-drying cycles in the rainy season. Based on the monitored data, the wetting-drying and direct shear tests were carried out in laboratory to estimate the influence of wetting-drying cycle on dry density and shear strength of undisturbed loess samples. The mean dry density and shear strength of Dingxi loess samples decreased from 1.60 g/cm3 and 30.3 kPa to 1.30 g/cm3 and 17.8 kPa after 30 cycles, respectively. Concerning the Pingliang soil samples, they decreased from 1.66 g/cm3 and 31.7 kPa to 1.40 g/cm3 and 15.7 kPa, respectively. Wetting–drying cycle significantly reduced the dry density and strength of loess samples and became a key factor leading to the failure of loess road cut slope.

A New Strength Criterion for Frozen Clay Considering Temperature Effect

To study the strength property of frozen soil under complex stress states, a series of directional shear tests on remoulded frozen clay were conducted under four mean principal stresses (p = 1, 3, 4.5 and 10 MPa) and four coefficients of intermediate principal stress (b = 0, 0.25, 0.5, and 0.75) at three temperatures (−6, −10, and −15 °C) using the hollow cylinder apparatus (HCA). The experimental results indicated that stress-strain curves of frozen clay all performs as strain hardening under directional shearing. In the p-q plane, the strength of frozen clay increases with increasing mean principal stress at first, and then decreases with a further increase of p. An elliptic strength criterion is proposed to describe this variation law. The test results also showed that the strength of frozen clay increases with decrease of temperature significantly. Then, temperature parameters are introduced into the elliptic function to consider the temperature effect.

Influence of Warm Oil Pipeline on Underlying Permafrost and Cooling Effect of Thermosyphon Based on Field Observations

Ground temperatures under a buried warm China-Russia oil pipeline were monitored to evaluate permafrost thawing and cooling performance of thermosyphons installed near the pipe in sporadic permafrost regions. Field observations demonstrated that warm oil thawed the underlying permafrost and increased the active layer thickness. Thermosyphons can cool the soils surrounding the pipe and effectively mitigate the permafrost thawing depending on their number and working duration. But now the heat dissipated from the warm pipeline and construction disturbance were not completely removed by two pairs of thermosyphons after two winters of operation. There was still a thawed layer beneath the thermosyphons even in winter. Further long-term monitoring of the cooling performance of thermosyphons is needed. This study provided basic data and analytic references for other similar cold region pipelines.

Experimental Study on the Strength Characteristics of Frozen Clay on π Plane

A set of directional shearing tests using hollow cylinder apparatus (HCA) was carried out on remolded frozen clay at −6 °C to study the strength properties of frozen soils in π plane. During the shearing, the stress Lode angles was fixed at five different values (θσ = −30, −16.1, 0, 16.1 and 30°) under four mean principal stresses (p = 1, 3, 4.5 and 10 MPa). Through the tests, the strength locus of frozen soil in π plane with θσ ranging from −30 to 30° were gained experimentally for the first time. The test results show that the strength envelope of frozen clay changes from a curve-sided triangle to a circle with increasing p. And a combination of the Spatially Mobilized Plane (SMP) criterion and Mises criterion, the Generalized Non-linear strength theory, can better describe this strength envelope evolution law in π plane.

Characteristics of thawed interlayer and its effect on embankment settlement along the Qinghai-Tibet Railway in permafrost regions

The formation of thawed interlayer beneath embankment can result in embankment settlement in permafrost regions. Based on the data on ground temperatures and deformations beneath the embankment, observed in-situ along the Qinghai-Tibet Railway in permafrost regions from 2006 to 2013, characteristics of the thawed interlayer beneath the embankment and its influence on the embankment settlement are studied. The results indicate that the thawed interlayer hardly forms beneath the natural field, and beneath the embankments from the Qinghai-Tibet Railway the thawed interlayer develops widely, and it can be refrozen totally in the regions with lower mean annual ground temperature, and developed further in the regions with higher mean annual ground temperature. The thawed interlayer is closely related to the embankment settlement. The ice content of permafrost underlying the thawed interlayer influences the settlement of embankment. The higher the ice content is, the larger the settlement is, and vice versa. The increase in thickness of thawed interlayer mainly results from the decline of artificial permafrost table in high-temperature permafrost regions.

Damage evolution and recrystallization enhancement of frozen loess

A series of multistage triaxial compression, creep, and stress relaxation tests were conducted on frozen loess at the temperature of −6℃ in order to study the damage evolution and recrystallization enhancement of mechanical properties during deformation process. The effect of strain rate, confining pressure, and hydrostatic stress history in the degradation laws of mechanical properties is investigated further. The strain rate has a significant influence on the stress–strain curve which dominates the evolution trend of mechanical properties. The mechanical behaviors (strength, stiffness, and viscosity) of frozen loess all exhibit evident response for the consolidation and pressure melting phenomenon caused by the confining pressure. The multistage loading tests under different hydrostatic stresses are capable of differentiating the development characteristics of mechanical properties during axial loading and hydrostatic compression process, respectively. The testing results indicated that the recrystallization of the ice particle in the frozen soils is an important microscopic factor for enhancement behaviors of mechanical parameters during the deformation process. This strengthening degree of mechanical properties is determined by temperature, duration time, deformation degree, and stress state during the recrystallization process. The phase transformation led by pressure melting and ice recrystallization is a nonnegligible changing pattern of frozen soils microstructure, which has apparent role in the damage evolution of mechanical properties.