Research on the Influence of Tunnel Invert Excavation on the Rheological Deformation of Different Levels of Surrounding Rock
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The closed section of the inverted arch, formed by the surrounding rock, acts as a bearing ring. Combined with the upper initial support, it ensures stable initial support. However, excavating the inverted arch can disturb the original balance, significantly affecting the tunnel’s stability. To determine the optimal exposure length and excavation length of the elevation arches at different rock levels, numerical analyses were conducted. These analyses used the classical Burgers creep intrinsic structure model for the three-step excavation mode. Various closure distances and exposure distances of the elevation arch were considered. The study aimed to investigate the influence of these factors on the stability of the primary lining, comparing it with the maximum displacement of the vault. The results indicate that the strength of the surrounding rock primarily affects the displacement of the arch crown. Lower rock strength corresponds to greater arch crown displacement. Additionally, increasing the closure distance of the inverted arch leads to increased arch displacement. On the other hand, the exposure distance of the inverted arch has minimal impact on arch displacement. Longer exposure distances result in greater arch displacement. These findings can serve as a basis for improving current standards and adapting them to meet the spatial requirements of large-scale mechanized operations.Keywords:
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There are different degrees of degradation in the excavation unloading,rebound of rock and rock quality around the underground chambers.Taking underground powerhouse chambers in Jinshajiang river for an project background,mechanical characteristic analysis and quality evaluation of rock mass during excavation and unloading are discussed based on the theory of unloading rock mass mechanics and finite difference method.The rock mass evaluation is finished by RMR method according to the mechanical parameters after rock excavation and unloading.The results show that with the excavation of chambers,the unloading area in surrounding rock is evolves constantly and the depth and range become larger and larger;after the excavation finished,there are different breakthrough areas in three main powerhouse chambers;the rock quality of unloading areas in surrounding rock is changed from class Ⅱ into class Ⅲ after excavation.
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The creep and creep recovery are important characteristics of Build-up system (BU-system). So, this paper is focused on the creep and creep recovery of several BU-systems. The creep and creep recovery of two different types of force transducers, including RTN and column type, are measured when applied 20%, 40%, 60%, 80%, and 100% of the rated loads and after unloading, respectively. Afterward built-up by these transducers, BU-systems with different structures and capacities are measured in order to investigate the creep and creep recovery behaviors of them and the different between the BU-system and single transducer.
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In this paper, creep characteristics of mild steel which are to be determined by long term creep tests are interrelated with that of short term creep tests. It leads to the prediction of long term creep limit from the data of short term creep tests together with that of single long term creep test datum. The prediction is demonstrated by examples. Temperature dependence of creep characteristics of the material is also discussed, and a proposition is made on the estimation of creep limit at arbitrary temperatures from the data of creep tests conducted at a reference temperature.
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Creep life prediction is very important to maintain safe operation of thermal power plants for a long-term period. In this study, an alternative creep life prediction method was proposed based on a relationship between creep exponent n and creep coefficient k. A high temperature indentation creep test was conducted to identify the creep exponent and creep coefficient for each specimen with different creep damage ratio. Subsequently, the relationship between the creep coefficient and creep exponent for each creep damage ratio was focused on. As a result, it was confirmed that a new parameter B introduced based on both creep coefficient and creep exponent linearly increase as the creep damage ratio increases. Furthermore, the B is uniquely determined by the creep damage ratio regardless of the temperature and stress conditions. These results indicate that the creep life can be predicted with a high accuracy using the B.
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An incremental format of creep model was presented to take account of the development of concrete creep due to loading at different ages. The formulation was attained by introducing a horizontal parallel assumption of creep curves and combining it with the vertical parallel creep curve of the rate of creep method to remedy the disadvantage of the rate of creep method that significantly underestimates the amount of creep strain, regardless of its simple format. Two creep curves were combined by introducing an ageing parameter whose value was obtained from two sets of time-dependent laboratory experiments on cylindrical specimens. The presented creep description takes the advantage that a single creep curve due to the initial loading describes the entire development of creep under the persistent change of creep-causing stress. Further, the creep formulation takes advantage of being consistent with the incremental format of age-dependent constitutive formulation. The performance of the presented creep equation was investigated with time-dependent laboratory experiments on cylindrical specimens and compared with the performances of four existing creep models.
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A new tensile creep model that integrates the tensile strength at creep temperature is investigated for its generic applicability in predicting the long-term creep strengths from short-term creep test data for high Cr creep-resistant steels using creep and tensile strength data measured for a grade of 11Cr steel. The results show that, when the long-term creep strengths are specified by stresses producing the required minimum creep rate, they can be accurately predicted using short-term creep test data. However, when they are specified by stresses giving the required creep rupture time, using only short-term creep test data will lead to over-predictions. The microstructure evolution origin of such over-predictions is traced to the Z-phase precipitation during creep in creep-resistant steels with more than 9 wt.% Cr. The conventional concept on the relationship between creep test stress and creep mechanisms is also re-evaluated in light of the new results.
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This paper studies the creep and creep rupture properties of 63 Sn-37 Pb solder. Creep and creep rupture tests were carried out using 63 Sn-37 Pb solder specimens at 313 K, 353 K and 398 K to develop a creep constitutive equation and to obtain rupture time. Short primary and long tertiary creep stages were found but there was no clear steady creep stage at the three temperatures. A creep constitutive equation was proposed as a function of stress and temperature. The proposed equation predicted the experimental creep strain within a factor of 1.25. A new equation for predicting creep rupture time was also proposed. The equation predicted the experimental rupture time within a factor of two.
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본 연구에서는 지하공간 건설을 위한 주방식 공법에 대해 기존 전통적인 지하공간 건설공법에 대비한 공사비 측면의 경제성 검토를 가정된 전형적인 지반조건에 대해 수행하였다. 경제성 비교검토를 위해 지하철 역사 등 대규모 지하공간 조성에 적용되고 있는 전통적인 공법 중 하나인, 2아치 터널 굴착 공법을 비교대상 공법으로 선정하고 동일한 공용 단면적을 확보하도록 굴착하였을 때 소요되는 공사비를 산정하여 비교하고 타당성을 검토하였다. 주방식 공법에 적용할 보강 공법 등은 기존 공법에서 사용하는 방식을 차용하는 것으로 가정하였으며 특히, 굴착 공사의 경우, 주방식 공법은 대체적으로 직사각형 단면을 사용하나 기존 공법과의 비교를 위해, 그리고 공간 천정부 및 암주부의 지반 자체 지지력을 최대한 증가시키기 위해 아치형의 터널 형태로 굴착하는 것으로 가정한 검토를 수행하였다. 검토 결과, 건설 요구 공간이 넓어질수록, 암반의 상태가 양호할수록 기존 전통 터널공법 대비 주방식 공법의 경제성이 더욱 향상될 수 있는 것으로 파악되었다. This study aims to investigate an economical efficiency of two excavation methods with respect to the room-and-pillar method for the underground space and conventional excavation method, i.e. 2-arch tunnelling method. For feasibility study, an excavation cost for both room-and-pillar method and 2-arch tunnelling method was estimated when the same space in operation was required. It was assumed that properties of reinforcements and rock were adopted from literatures. However, an excavation shape of the room-and-pillar method was assumed not to be the rectangular shape which is a general type in the room-and-pillar method but to be an arch shape in order to compare with the conventional excavation method (2-arch tunnelling) and to achieve the maximum bearing capacity of the structure during excavation. Consequently, the wider space in use or required and the better condition of rock we assumed, the more economical advantage we have in the room-and-pillar method than the 2-arch tunnelling method.
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