Thermal Characteristics of the Embankment with Crushed Rock Side Slope to Mitigate Thaw Settlement Hazards of the Qinghai-Tibet Railway

Permafrost （perennially frozen ground） appears widely in the Golmud-Lhasa section of the Qinghai-Tibet railway and is characterized by high ground temperature （≥1℃） and massive ground ice. Under the scenarios of global warming and human activity, the permafrost under the railway will gradually thaw and the massive ground ice will slowly melt, resulting in some thaw settlement hazards, which mainly include longitudinal and lateral cracks, and slope failure. The crushed rock layer has a thermal semiconductor effect under the periodic fluctuation of natural air. It can be used to lower the temperature of the underlying permafrost along the Qinghai-Tibet railway, and mitigate the thaw settlement hazards of the subgrade. In the present paper, the daily and annual changes in the thermal characteristics of the embankment with crushed rock side slope （ECRSS） were quantitatively simulated using the numerical method to study the cooling effect of the crushed rock layer and its mitigative ability. The results showed that the ECRSS absorbed some heat in the daytime in summer, but part of it was released at night, which accounted for approximately 20% of that absorbed. Within a year, it removed more heat from the railway subgrade in winter than that absorbed in summer. It can store approximately 20% of the ＂cold＂ energy in subgrade. Therefore, ECRSS is a better measure to mitigate thaw settlement hazards to the railway.

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 QinghaiTibet Railway in permafrost regions from 2006 to2013, 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.

Study on temperature field and settlement of thawing soil under static and dynamic loading

A series of tests were conducted to analyze temperature field distribution and thawing settlement of a thawing soil under static and dynamic loading at various cooling and thawing temperatures. The results demonstrate： （1） the temperature field distribution of the thawing soil was not significantly influenced by the loading form under the tested loading conditions; similar results were obtained for samples at different dynamic loading frequencies and different dynamic loading ampli- tudes, which verified the independence of loading form and temperature field; （2） changed temperature field distributions were found in thawing soil with different cooling and thawing temperatures, and the cooling and thawing temperature of the samples were the main factors affecting their temperature distributions; （3） under the tested conditions, thawing set- tlements were little influenced by the thawing temperature and the dynamic loading frequency; and （4） a linear relation- ship existed between the thawing settlement and the cooling temperature, and a logarithmic function could be used to describe the relationship between the thawing settlement and the loading amplitude.

Characteristics of thawed interlayer and its effect on settlement beneath embankment in permafrost regions--A case study for the Qinghai-Tibet Highway

Based on ground temperatures and deformations monitored at the Xieshuihe site along the Qinghai-Tibet Highway(QTH)in permafrost regions from 2004 to 2015,variation of artificial permafrost table(APT),maximum frozen depth(MFD),thawed interlayer thickness(TIT)and ground temperature beneath embankment is analyzed,respectively.The results indicate that under the embankment,the change of APT occurred from October to December of that year and presented a deepening trend.The change of MFD occurred from April to June of that year with no obvious change trend,and TIT had an increasing trend year by year,which mainly resulted from the deepening artificial permafrost table.Mean annual ground temperature at 0.5 m depth was 3.91°C higher beneath the embankment center than that under the natural field.The rising ground temperature at shallow layer of embankment resulted in the development of thawed interlayer beneath the embankment and warming of underlying permafrost.Embankment settlement is closely associated with TIT.Greater settlement easily occurs when permafrost with higher ice content exists under the thawed interlayer,and in turn the settlement is smaller when permafrost with lower ice content exists under the thawed interlayer.

北方寒冷地区温度场作用下公路路表形变规律分析

为了更加具体、深入地理解寒区道路,研究其病害发生、发展规律,提高对道路结构在时间和空间上的认知,开展了北方寒冷地区路表形变规律的研究。通过选取典型公路路段,建立10个监测段落,对路基的冻胀、融沉变形持续进行现场高程监测。分析温度场作用下路表形变特征和响应机制,分析公路在自然条件下的时域形变特点、动态形变特征及差异性。分析周期监测数据,构建竖向相对形变、竖向相对形变速率、横断面间形变速率最大倍数等参数,比较不同段落、不同横断面和监测点间的形变及其差异。研究北方寒冷地区路表形变与温度场协调规律,结果表明:北方寒冷地区基于温度场的路表形变具有典型的周期特性。对比分析填、挖方路段纵横方向形变的差异特征,形变极值分布规律,以及结合热胀、冻胀、融沉阶段的相对形变规律及形变速率差异特点,结果发现:挖方路段形变速率是填方路段的10余倍;8月份至10月份的热胀速率大于10月份至11月份的热胀速率;融沉主要发生在3月份至5月份;而冻胀速率较为稳定,对于填方路段,11月份至次年1月份的冻胀大于1月份至2月份的冻胀,而挖方路段则相反。给出了寒冷地区路表形变的温度场时域响应规律、特点,为开展寒区路基、路面结构和材料研究,以及道路病害等研究提供了依据。

新型框架通风锚杆支护多年冻土边坡降温效果及力学机理研究

本文提出了一种具有通风降温、支挡锚固和减胀减震功能的新型框架通风锚杆冻土边坡支护结构,并阐述其工作机理。基于传热学和自然对流理论,建立新型框架通风锚杆与土体换热的计算模型,对其降温效果进行了分析;基于Winkler理论,建立冻胀和融沉阶段新型框架通风锚杆与土体相互作用的力学简化计算模型,分析冻胀和融化阶段新型框架通风锚杆受力性能。结合算例,采用提出的简化计算方法分析了新型框架通风锚杆支护多年冻土边坡的热学和力学效应,并通过模型试验与理论计算对比,对所提出理论计算方法的合理性进行验证。结果表明:①新型框架通风锚杆能够充分发挥冷季吸收冷量冻结土体、暖季屏蔽热量保护冻土的作用,并逐年抬升冻融交界面;②新型框架通风锚杆支护结构支护效果良好,能够减弱土体冻胀作用,提高多年冻土边坡的稳定性;③所提出简化计算方法以期为新型框架通风锚杆支护多年冻土边坡提供理论依据。

季冻区水平和立体加筋土体防冻胀融沉效果研究

冻胀融沉是季节性冻土区建筑物破坏的主要原因,实际应用表明,土工合成材料加筋路基具有良好的抗冻融循环损伤能力,然而不同加筋形式的有益影响尚未被量化。针对四川阿坝地区典型季节性冻土,开展了水平及立体加筋土冻融循环试验及平板载荷试验,研究了冻融循环对加筋土和未加筋土力学性能的影响,对比了水平加筋及立体加筋土的防冻胀融沉效果。研究发现:水平加筋和立体加筋均可一定程度上抑制土体的冻胀融沉,提高冻融循环后土体的承载力,抑制冻融循环的弱化作用。立体加筋加固效果优于水平加筋。经过5次冻融循环后,水平加筋使冻胀隆起和融化沉降分别减少5%,6%,立体加筋使冻胀隆起和融化沉降分别减少4%,17%;水平加筋对冻融循环土体的承载力和刚度分别提高75%,29%,立体加筋对冻融循环土体的承载力和刚度分别提高388%,40%。

注浆-冻结联合工法加固地铁隧道地表融沉特性研究

在地铁隧道修建过程中若遇到复杂不良地质,单一工法有时无法将冻胀融沉位移控制在预期范围内,而采用注浆-冻结联合工法施工,能够有效控制土体融沉位移的施工参数。本文以广州地铁三号线某折返线隧道冻结工程为背景,隧道所处地层为砂质黏土地层,采用有限元数值分析软件,建立水热力耦合模型分析地铁隧道冻胀融沉位移场的发展规律,并与实测数据进行对比验证了模型可靠性,建立不同注浆半径的地铁隧道模型。通过对比其注浆控制效果,得出该法能够控制地表融沉位移的最佳注浆半径,并在此基础上研究了不同水泥掺量对土体融沉位移的影响,进一步得出能够有效控制土体融沉位移的施工参数。结果表明,采用水泥预加固-人工冻结联合工法施工,注浆半径选取4~5 m对融沉的控制效果最佳,注浆半径在4 m以上时土体融沉位移的效率开始有了明显的减弱。水泥掺量大于9%时,改善土体融沉位移的效率开始提高;而水泥掺量达到12%以后,改善土体融沉竖向位移的效率变缓。注浆半径设定在4 m,水泥掺量设定为12%,此时与原施工方案对比,自然解冻末期地表融沉量减小了73%。本研究的设计参数可为其他类似工程施工提供理论依据。

人工冻结水泥改良土冻胀融沉特性研究

针对人工冻结法缺陷引起冻胀融沉的施工问题,采用室内试验的方式,研究不同水泥掺量和养护龄期作用下冻结水泥改良土冻胀融沉性能,并通过数值模拟与试验数据相互验证。研究表明,随水泥掺量的增加,样本冻胀融沉率逐渐降低;水泥掺量达到8%时,样本的冻胀融沉抑制率最优;养护龄期在14 d后,冻胀融沉率基本不变;在12%水泥掺量下,模拟值与实测值的冻结锋面发展规律较为接近;冻结初期,冻结锋面向下传递速率较快,越靠近暖段冻结锋面传递速率越慢,两者到达冻结锋面80 mm处,模拟值较快,差值为1.5 h。研究结果表明数值模拟能有效反映水泥改良土的冻胀机理。

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 postconstruction deformation. However, 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 upper 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.

宁波软土地区地铁联络通道冻结法工后长期运营沉降预测研究

宁波地区主要以海相淤泥质黏土或淤泥等软土为主,此类土具有含水量大、压缩性高、承载力低的特性,地铁围岩开挖施工面临垮塌、变形问题,冻结法施工往往是处理上述问题的有效方法。然而,地层解冻后注浆作业不及时、注浆效果差极易导致地层沉降,所以需要对地基沉降进行长期监测。为更好考虑软土地基下的隧道安全,以宁波句章路站-鄞州客运总站站-南部商务区站区间盾构隧道为例,采用冻结法对宁波句章路站-鄞州客运总站站区间1#联络通道进行施工,开展冻融工后联络通道长期运营沉降预测研究,并通过三维有限软件MIDAS/GTS建模,对其冻融沉降进行计算分析。根据布点及实时观测,对隧道及句鄞区间盾构隧道及1#联络通道开挖施工期间的结构沉降变形及长期运营沉降变形进行预测,预测结果与监测数据基本一致,对于软土地基冻融处理具有重要工程意义,同时也可为相关工程提供借鉴。

人工冻结水泥改良软黏土冻融及强度特性研究

为探究人工冻结水泥改良软黏土冻胀融沉特性及冻融前后强度变化规律,通过室内冻胀融沉试验和单轴抗压强度试验,揭示不同冻融状态、不同影响因素等条件下水泥改良软黏土冻胀融沉及强度特性。研究结果表明,软黏土冻胀率和融沉系数随冻结冷端温度降低而减小,且融沉系数大于冻胀率;掺入水泥后融沉系数均小于冻胀率,且随水泥掺量增加而降低,当水泥掺量超过5%时,融沉系数小于2%。水泥土起始冻结温度随水泥掺量增加而降低,当水泥掺量为9%时,起始冻结温度最低达到-1.53℃;冻融过程沿试样不同高度温度场变化分为积极冻结、维护冻结和融化三个阶段;冻融后水泥改良软黏土强度降低,25组试样平均降低56%,冻融后强度折减最大达76%。研究指出,改良土强度分别随水泥掺量增加而增大,随含水率增大而减小,随龄期增长而增大。

Compressibility of fine-grained soils subjected to closed-system freezing and thaw consolidation

In order to investigate the effects of freezing and thawing on the compressibility of fine-grained soils, freezing and thawing tests and subsequent compression tests with fine-grained soils in an oedometer were carried out. During the freezing, a part of the soils is loosened and another part is over-consolidated under the freezing pressure σE. The compression curves after the freezing and thaw consolidation are neither different from the normal consolidation curve nor from the rebound curve of an unfrozen soil, until the consolidation pressure σz = σE is arrived. Based on the experimental results, a theoretical model has been devel- oped to predict the frost heaves, the thaw-settlements and the compressive deformations of fine-grained soils after the thaw consolidation. The theoretical results are very close to the experimental results.

富水砂层冻结暗挖法C形联络通道设计与工程实践

为解决复杂环境和富水砂层中地铁盾构叠落区间C形联络通道安全实施的问题,采用以“先下、后上、再中间”为施工工序的分部暗挖工法和与之匹配的分时间、分区域冻结辅助工法。首先,根据埋深确定各部位的冻结壁厚度及其他主要冻结参数,并对冻土进行试验,取得各层位冻土强度指标;对冻结壁进行强度验算,验算结果满足安全系数要求。其次,合理布置地面垂直及隧道内水平冻结管并采取技术措施减小交叉冻结、冻胀的影响,保证冻结效果。再次,在联络通道各部位预埋注浆管,根据解冻进程,从下至上以“多点、少量、多次、均匀”的方式进行融沉注浆,控制结构和地层沉降。最后,施工期间对冻结壁、结构、地面及周边环境状态进行监测,并根据监测数据对施工进行修正指导。结果表明:采用该冻结暗挖法施工C形联络通道安全可靠,冻结壁强度高、止水性好、冻胀融沉小,结构受力变形及对周边环境的影响均较小。

下穿地铁车站冻结加固冻胀与融沉控制技术

基于某沿海城市新建地铁车站密贴下穿既有运营地铁车站暗挖通道水泥改良土地层二次冻结加固工程,通过既有地铁车站轨行区的自动化监测系统,分析既有运营车站下方土体冻结后,冻结过程中的冻胀及解冻后冻土消融对既有地铁车站的影响。在冻结前,提前对冻结区域土体进行注浆加固,改善冻土参数,降低冻胀对既有运营车站的影响。在冻结过程中,采取冻结区域止水、对冻土进行取土泄压、调整冻结顺序、调节冻结温度、开启泄压孔进行压力泄压、控制冻结盐水流量和优化开挖方式等措施,有效减缓冻土对现有地铁车站的的影响。在暗挖通道结构施工完毕后,通过填充注浆和融沉注浆2种方式,解决土体冻结消融后既有地铁车站沉降的问题。

黄土的冻胀融沉作用对水利设施的病害分析

季节性冻土区内,黄土的冻胀融沉受多种因素影响,冻结温度引起冻结深度和水盐迁移变化,影响冻胀融沉发生的程度,反复的冻胀融沉作用会对高标准农田灌区水利设施产生不同程度的危害,影响水利设施的正常使用。以陕西省宝鸡市千阳县高崖镇高标准农田灌溉区的黄土为研究对象,利用COMSOL数值仿真模拟软件研究了不同冻结温度状态下黄土冻结深度的变化规律,通过室内冻结试验,分析了冻结温度对水盐迁移及冻胀融沉的影响,探究了多因素条件下黄土的冻胀融沉作用对高标准农田水利设施产生的危害,并提出了相应的防治措施。结果表明:冻结温度降低时,冻结温度梯度增大,水盐迁移量增大;土层冻结深度随冻结温度的降低而加深,即一定温度范围内,冻结温度越低,土层冻结深度越大,冻胀融沉的影响范围越大,土体扰动范围越大,对高标准农田水利设施产生的危害就越大;采取“水盐热隔离法”,可隔离水盐迁移,降低水盐迁移对黄土冻胀融沉的影响,从根本上解决冻胀融沉对水利设施产生的病害。

黏土与固化土冻胀融沉试验研究

为降低渠道土体的冻胀融沉变形,减少渠道衬砌混凝土开裂,文中采用自制试验装置研究了宁夏黏土和水泥固化土冻融过程的温度、变形规律,并测试了多循环冻融变形曲线。结果表明黏土冻融变形经历了6个阶段:快速冻胀、降温收缩、冻胀稳定、升温膨胀、缓慢融沉和融沉稳定,验证了冰融化过程存在温度平台。经历多次冻融,黏土累积膨胀量随着冻融次数增加呈线性增加的趋势。固化土表现为冻缩融胀,变形幅度较小,多次冻融后累积变形基本为零。因此,若采用水泥固化土部分替换黏土,将有效减小渠道的冻胀融沉变形,有利于混凝土衬砌的结构防裂。

高地震烈度寒区钢-混凝土组合梁桥抗震韧性评价

综合考虑桥梁系统灾变全过程,提出了一种适用于高地震烈度寒区的钢-混凝土组合梁桥抗震韧性评价方法。基于现有统计数据,综合考虑桩基沉降损伤、疲劳损伤以及冻融损伤,建立了高地震烈度寒区钢-混凝土组合梁桥震前功能损失函数;将地震易损性分析与桥梁抗震韧性评价相结合,建立了桥梁系统震后恢复函数;提出了震前临界维修加固年限和临时保通状态。结果表明:在高地震烈度冻土区的钢-混凝土组合梁桥系统的临界维修加固年限随地震动峰值加速度的增大而减小,当地震动峰值加速度分别为0.10g、0.15g、0.20g、0.30g时,桥梁系统的临界维修加固年限预计分别为27年、24年、19年、8年;当桥位在Ⅸ度及以上设防区时,必须在桥梁方案设计中考虑增设减隔震装置;桥梁系统的恢复时间随地震动峰值加速度的增大而增大,韧性指标随地震动峰值加速度的增大而减小;桥梁在运营20年后发生地震作用时,地震动峰值加速度为0.10g和0.15g的地震动预计不会影响桥梁通行,地震动峰值加速度为0.20g~0.40g的地震动需要进行维修加固实现临时保通,预计恢复时间为1~42天,韧性指标为0.647~0.578,恢复至临时保通状态的韧性指标最大值为0.650。

Deformation monitoring and analysis at two frost mounds during freeze–thaw cycles along the Qinghai–Tibet Engineering Corridor

This paper presents various deformation-monitoring technologies employed to monitor the frost heave and thaw settlement of two mounds along the Qinghai–Tibet Engineering Corridor(QTEC), China. The QTEC is known as a critical infrastructure and passage connecting inland China and the Qinghai–Tibet Plateau(QTP). Three technologies—global navigation satellite system(GNSS), terrestrial laser scanning(TLS), and unmanned aerial vehicle(UAV)—were used to estimate the freeze/thaw–induced 3D surface deformation of two frost mounds. Our results showed that (1) the two frost mounds exhibited mainly thaw settlement in thawing periods and frost heave in the freezing period, but frost heave dominated after repeated freeze–thaw cycles;(2) different zones of the mounds showed different deformation characteristics;(3) active-layer thickness(ALT) and elevation changes were highly correlated during thaw periods;(4) integrated 3D-measurement technologies can achieve a better understanding and assessment of hazards in the permafrost area.

砂卵石地层冻结法施工融沉效应研究

本文针对富水砂卵石地层联络通道应用冻结法进行施工时的融沉特性进行研究,在数值模拟软件ANSYS中采用间接法进行热力耦合计算,模拟地铁联络通道冻结法施工的全过程,探讨富水砂卵石地层融沉阶段地表变形、双线隧道管片及联络通道结构的变形与受力特性,为实际工程施工提供有效的参考与建议。结果表明:随着砂卵石地层的逐渐解冻,地表沉降呈逐渐增大的趋势,但砂卵石地层的融沉效应小,融沉变形仅有1.85 mm;管片和联络通道结构的沉降变形值随砂卵石地层的解冻逐渐增大,在此过程中管片和联络通道结构的拉应力减小,压应力增大;实际施工中,应该重点监测管片结构在与联络通道结构连接处顶部的沉降变形、底部的受压情况以及联络通道结构底板处的沉降变形和受压情况。