Hydraulic fracturing pressure of concentric double-layered cylinder in cohesive soil

This study aims to investigate hydrofracturing in double-layered soil through theoretical and experimental analysis, as multilayered soils where the difference in mechanical properties exists are generally encountered in practical engineering. First, an analytical solution for fracturing pressure in two different concentric regions of soil was presented based on the cavity expansion theory. Then, several triaxial hydraulic fracturing tests were carried out to validate the analytical solution. The comparison between the experimental and analytical results indicates the remarkable accuracy of the derived formula, and the following conclusions were also obtained. First, there is a linear relationship between the fracturing pressure and confining pressure in concentric double-layered cohesive soil. Second, when the internal-layer soil is softer than the external-layer soil, the presence of internal soil on the fracturing pressure approximately brings the weakening effect, and the greater strength distinction between the two layers, the greater the weakening effect. Third, when the internal-layer soil is harder than the external-layer soil, the existence of the internal-layer soil has a strengthening effect on the fracturing pressure regardless of the proportion of internal-layer soil. Moreover, the influence of strength distinction between the two layers on the fracturing pressure is significant when the proportion of internal-layer soil is less than half, while it’s limited when the proportion is more than half. The proposed solution is potentially useful for geotechnical problems involving aspects of cohesive soil layering in a composite formation.     

Hydraulic fracturing stress measurements in Hong Kong

Since 1990, a total of 149 hydrofrac stress measurements to about 200 m depth were conducted in 18 boreholes as part of several geotechnical site investigation programs in the Hong Kong area. The in situ tests were carried out by using the wireline hydrofrac technique to move the straddle packer tool within the 76 mm or 101 mm diameter boreholes. Although the tests were performed both in fractured and unfractured crystalline rocks and the boreholes are located in areas of pronounced topographic relief, the results yield a consistent orientation of the maximum horizontal stress of N 108°±28°. Above 150 m depth, the vertical stress S_v due to the weight of the overburden with given rock density is the minimum principal stress, while the few deeper data available suggest that the minimum horizontal principal stress is the least principal stress. The derived stress magnitudes can be reasonably summarized by the following normalized stress-ratios:

where z is the depth in meters and S_h and S_H are the minimum and maximum horizontal principal stresses. Due to the considerable scatter of the stress data at shallow depth above 100 m, it is suggested that further detailed in situ stress measurements be undertaken in areas where large-scale underground excavations are planned.     

Hydraulic fracturing in a sedimentary geothermal reservoir: Results and implications

Field experiments in a geothermal research well were conducted to enhance the inflow performance of a clastic sedimentary reservoir section. Due to depths exceeding 4050 m, bottom hole temperatures exceeding 140 °C, and open hole section (dual zone), technically demanding and somewhat unprecedented conditions had to be managed. The fracturing operations were successful. Fractures were created in two isolated borehole intervals and the inflow behaviour of the reservoir was decisively enhanced. The effective pressures applied for fracture initiation and propagation were only slightly above in situ pore pressures. Nevertheless, the stimulation ratio predicted by fracture performance modelling could not be achieved. Multiple reasons could be identified that account for the mismatch. An insufficient fracture tie-back, as well as chemical and mechanical processes during closure, led to reduced fracture conductivities and therefore diminished productivity. The insights gained are the basis for further fracture design concepts at the given and geologic comparable sites.     

爆振加固法中的水力劈裂及作用范围研究

爆振加固法是一种高效、经济的地基加固方式,其炸药量的选择和药包布置间距的确定对爆振加固的作用效果起决定作用。由于现有的爆振加固计算方法较为繁琐,根据经典爆炸理论、Mohr-Coulomb准则和土体强度参数,从水力劈裂角度提出更便于工程应用的爆振作用范围确定方法,研究了炸药量和土体参数对爆振作用范围的影响,并与试验结果进行了对比,两者对应较好。研究结果表明,在炸药量一定的情况下,药包埋置深度h越大,则土体围压作用越大,爆振作用范围越小;土体强度参数c、φ越大,则土体强度越高,爆振作用范围越小。最后,对爆振加固方法在使用过程中的药量选择及药包布置间距的确定给出了一定的工程建议。     

基于XFEM的土体水力劈裂模拟

水力劈裂会对心墙堆石坝的安全造成严重的负面影响。宏观上,土体水力劈裂可看作是土体在水力楔劈作用下使局部裂缝(薄弱面)进一步发展的破坏过程。XFEM是一种可以有效描述裂缝的数值模拟方法。采用XFEM结合Biot固结理论,并对裂缝单元进行处理,从而以弥散式的裂缝状态和嵌入式的裂缝形态共同描述了土体的水力劈裂过程,并以一个模型算例和挪威Hyttejuvet坝的实际工程算例对方法进行了验证。该工作的成果有助于理解土体水力劈裂的成因和过程,可用于对土工结构物的流固耦合破坏分析。     

复杂环境下大体积土石方开采技术

以贺州车站3号取石场为例,介绍了在复杂环境下综合利用浅孔爆破、深孔爆破、药壶爆破等技术进行石方开采,通过对爆破参数的不断完善与调整,经过现场实践,总结得出了一套针对复杂环境下的石方开采技术,为今后类似工程的施工提供理论依据与实践经验。     

Disk shaped piezo-ceramic transducer for P and S wave measurement in a laboratory soil specimen

Piezo-ceramic elements are customized as transducers for the measurement of mechanical properties of materials in the engineering field. This was made possible with the industrial production of piezo-ceramic elements in various shapes and sizes. This paper describes the development of a single flat disk shaped piezo-ceramic transducer for measuring both compression and shear wave on an identical soil specimen. Procedures for interpreting results, initial verifications of its performance and applications are also presented. Two types of piezo-ceramic elements, one for measuring P waves and the other for S waves, were placed together in a metal housing, which worked as a wave measuring transducer installed in a triaxial apparatus. Three kinds of granular geo-materials, fine, medium-coarse and coarse sands, were tested. Small strain shear stiffness, G_max, of the tested sands was evaluated by various techniques, including proposed disk transducer method, trigger and accelerometer method and traxial small strain cyclic loading. Shear moduli obtained from all the techniques fell in a similar range within allowable scatters and it was confirmed that the disk transducer was one of applicable wave measurement technique for laboratory soil specimens.     

盐渍土地区混凝土耐久性试验用试件尺寸的评价

采用2种不同尺寸的混凝土试件进行室内混凝土耐久性快速破坏试验,寻求适用于室内混凝土耐久性快速破坏试验的最合理的混凝土试件尺寸。结果表明:采用尺寸为100 mm×100 mm×400 mm的混凝土试件比采用尺寸为40 mm×40 mm×160 mm的混凝土试件进行耐久性快速破坏试验的试验结果具有较好的准确性。     

A novel specimen preparation method for TJ-1 lunar soil simulant in hollow cylinder apparatus

Conventional methods for hollow cylinder apparatus (HCA) specimen preparation are not applicable for TJ-1 lunar soil simulant due to its wide particle size distribution. A novel method to prepare uniform TJ-1 specimen for HCA tests is put forward. The method is a combination of the multi-layering dry-rodding method and a new under-compaction criterion in the multi-layer with under-compaction method (UCM). In the novel method, the specimen is prepared with 5 layers by dry-rodding and the UCM is used to determine the height after each layer is compacted. The density uniformity of specimen is evaluated by the freezing method to find out the best under-compaction criterion. Two HCA specimens with the same target density are prepared by the novel method and examined in the tests of pure rotation of the principal stresses. Their conformable mechanical behaviors ascertain the effectiveness of the method to produce uniform and reproducible HCA specimens. Four groups of HCA tests are carried out to investigate the anisotropic and non-coaxial behaviors of TJ-1 lunar soil simulant. The results indicate that the principal stress direction, the deviator stress ratio, the stress level and the coefficient of the intermediate principal stress significantly influence the strength and deformation properties of TJ-1 lunar soil simulant.     

土工三轴试验试样全表面变形测量的实现

应用 基于亚像素角点识别的计算机图像处理技术,提出并实现了一种土工三轴试验试样全表面变形观测的图像测量方法。通过一台 CMOS 摄像机,借助放置于改制压力室中的平面反光镜,实现了对试样全表面变形的实时测量。在此基础上,对原始数据进行了畸变修正及像素当量归一化,并通过引入相邻时间间隔每个计算邻域内试样等向膨胀的计算假定,求得了试样全表面变形场。最后,以饱和砂土三轴排水剪切试验为例,探讨了全表面变形场在分析试样局部化变形中的应用。     

水泥土强度的试件形状和尺寸效应试验研究

对不同形状、尺寸、端部边界约束条件以及龄期的水泥土试件进行了无侧限抗压强度试验 ,探讨了水泥土强度的试件形状和尺寸效应。     

Hydraulic conductivity behaviour of soil blended with geofiber inclusions

Efficiency of fiber reinforcements to ensure the sealing efficiency of the landfill cap soil barriers so as to isolate the waste from the environment was demonstrated in the present study. Evaluation of hydraulic conductivity of soil barrier materials with different types of fibers, fiber dosage and fiber lengths are very important to ensure the sealing efficiency of the fiber reinforced soil barriers. An attempt was made to evaluate the hydraulic conductivity of the soil barrier material at a known effective stress using a flexible wall permeameter. Soil samples of 100 mm diameter and 100 mm height were prepared and tested in the present study. In all the cases, the hydraulic conductivity test phase was started after the completion of initialisation, saturation and isotropic consolidation phases of the soil samples. In the present study, seventeen (17) hydraulic conductivity tests were conducted on two different soil types for studying the influence of fiber content, fiber length and fiber type on the hydraulic conductivity of fiber reinforced soil. The fiber content, f used were 0.25%, 0.50% and 0.75% and the fiber lengths, l were 30 mm, 60 mm and 90 mm. Two types of fibers namely polyester (PET) fibers and polypropylene tape (PP-T) fibers were used for hydraulic conductivity tests. The repeatability of test results was also demonstrated. As the fiber content and fiber length were increased, initially there was a marginal decrease in hydraulic conductivity of the soil and thereafter marginally increased. Short fibers and low fiber contents were found to have greater influence in reducing the hydraulic conductivity of the soil and the variation was found to depend on the soil type also. Even with long fibers, the hydraulic conductivity of selected barrier material remained within the permissible limit required for a barrier material. The hydraulic conductivity of PP-T fiber reinforced soil is more, compared to hydraulic conductivity of PET fiber reinforced soil at all the fiber contents varied in the present study. The use of Scanning Electron Microscopy (SEM) is also attempted for the interpretation of the results.     

三轴土样局部变形的数字图像测量方法

在已完成的土样径向变形数字图像测量基础上 ,实现了土样轴向变形和径向变形的同步测量 ,不仅可测量土样整体的平均变形 ,而且可测量土样的局部变形 ,还给出了图像测量系统的理论精度和实际测量精度。用砂土、粉煤灰两种饱和试样进行固结排水剪切试验 ,对采用图像测量方法得到的三轴土样不同部位的局部测量结果和采用传统测量方法得到的整体平均测量结果进行了比较     

改性土体材料圆柱体试件轴心抗压试验研究

为改善传统土坯砌体民居的土体材料的基本力学性能,通过在土体材料中分别掺入不同掺量的麦秸秆、生石灰或粗砂,形成改性土体材料,制作了直径100 mm×高110 mm圆柱体试件,通过试件抗压强度试验的试验现象和结果的分析,研究不同掺和量对试件基本力学性能的影响,分别提出掺和麦秸秆、生石灰或粗砂试件的最佳掺和比例范围.试验结果表明,当麦秸秆掺和量达到一定范围时,极限荷载和抗变形能力提高明显,当生石灰或粗砂掺和量达到一定范围时,极限荷载提高显著但抗变形能力下降.     

麦秸秆加筋盐渍土的尺寸效应与制样问题处置

制备均匀且整体性较好的试样是保证室内实验结果可靠的前提条件。提出了两端压实的静力制样法,围绕制样中出现的几个问题,制定了适宜的应对措施。同时完成了不同直径的盐渍土和麦秸秆加筋土试样的抗压强度实验,分析了试样抗压强度的尺寸效应,以及试样直径与加筋条件间的比例关系。结果显示:采用两端压实的静力制样方法制备加筋土试样是可行的;两种试样均存在明显的尺寸效应;由于麦秸秆的加筋作用,使得麦秸秆加筋土不同尺寸试样的抗压强度尺寸效应大于盐渍土的,且其抗压强度的尺寸效应随试样尺寸的增大而越来越明显;适宜加筋长度为0.316倍试样直径,适宜加筋率0.25%。研究结果可为制备合格的加筋土试样提供参考,也有益于在未来的研究中选择更合理的麦秸秆加筋条件。     

Flow around a cubic body immersed in a turbulent boundary layer

This paper reports the results of an experiment concerning flow in the vicinity of a cubic body immersed in a turbulent boundary layer which is fully developed along a tunnel floor. By changing the flow direction and body height, measurements were made of the pressure and drag acting on the body, and of the flow pattern and pressure distribution on the floor. The results indicate the changes taking place in the pressure distribution over the surface of a cubic body, the pressure drag acting on it, and the flow pattern and pressure distribution over the floor surface as the angle of incidence α is varied.     

Hydraulic and purification behaviors and their interactions during wastewater treatment in soil infiltration systems

Four three-dimensional lysimeters were established in a pilot laboratory with the same medium sand and either an aggregate-laden (AL) or aggregate-free (AF) infiltration surface and a 60- or 90-cm soil vadose zone depth to ground water. During 48 weeks of operation, each lysimeter was dosed 4 times daily with septic tank effluent (STE) at 5 cm/d (AL) or 8.4 cm/d (AF). Weekly monitoring was done to characterize the STE, percolate flow and composition, and water content distributions within the lysimeters. Bromide tracer tests were completed at weeks 0, 8, and 45 and during the latter two times, ice nucleating active (INA) bacteria and MS-2 and PRD-1 bacteriophages were used as bacterial and viral surrogates. After 48 weeks, soil cores were collected and analyzed for chemical and microbial properties. The observations made during this study revealed a dynamic, interactive behavior for hydraulic and purification processes that were similar for all four lysimeters. Media utilization and bromide retention times increased during the first two months of operation with the median bromide breakthrough exceeding one day at start-up and increasing to two days or more. Purification processes were gradually established over four months or longer, after which there were high removal efficiencies (>90%) for organic constituents, microorganisms, and virus, but only limited removal of nutrients. Soil core analyses revealed high biogeochemical activity within the infiltrative zone from 0 to 15 cm depth. All four lysimeters exhibited comparable behavior and there were no significant differences in performance attributable to infiltrative surface character or soil depth. It is speculated that the comparable performance is due to a similar and sufficient degree of soil clogging genesis coupled with bioprocesses that effectively purified the wastewater effluent given the adequate retention times and high volumetric utilization's of the sand media.     

关于“水泥土强度的试件形状和尺寸效应试验研究”的讨论

笔者认真学习了陈更生等同志的“水泥土强度的试件形状和尺寸效应试验研究”(《岩土工程学报》2002年第5期,以下简称“原文”)一文,文中所讨论的内容是笔者一直比较感兴趣的问题,受益颇深。但有以下几点疑问与原文作者商榷。(1)原文在前言中说由于圆柱体与立方体“两者在形状和尺寸上均存在较大的差异,这样就造成用一种条件下的实际强度去检验、评判另一种条件下的设计强度,其可比性就差,严格地讲不存在可比性”。笔者认为此结论有失偏颇。众所周知,在深层搅拌法的设计中,将室内水泥土试块的强度试验结果乘以强度折减系数(与施工及拟建工程的性质等诸多因素有关)来指导实际工程设计的方法已是非常普遍的方法,为众多的规范和规