An improved specimen preparation method for marine shallow gas-bearing sand sediments and its validations

Instabilities of shallow gas-charged seabed are potential geological hazards in ocean engineering.In practice,the conventional field sampling techniques failed to obtain undisturbed gas-bearing sediments from the seabed for laboratory mechanical testing because of sensitive gas exsolution and escape from sediments.However,preparation of representative remoulded gas-charged specimens is a challenging issue,because it is rather difficult to quantitatively control the gas content and obtain uniform distribution of gas bubbles within the specimen.Given the above problems,this work proposes a reliable approach to reconstitute the high-saturation specimen of gas-charged sediments in the laboratory by an improved multifunction integrated triaxial apparatus(MITA).This apparatus is developed based on an advanced stress path triaxial system by introducing a temperature-controlled system and a wavemonitoring system.The temperature-controlled system is used to accurately mimic the in situ environments of sediments in the seabed.The wave-monitoring system is utilized to identify exsolution point of free gas and examine the disturbance of gas to specimens during gas exsolution.The detailed procedure of gassy specimen preparation is introduced.Then,the quality of prepared specimens using our improved apparatus is validated by the high-resolution micro-X-ray computed tomography(μCT)scanning test,from which bubble occurrence and size distribution within the gassy sand specimen can be obtained;and preliminary mechanical tests on gassy sand specimens with various initial saturation degrees are performed.The proposed specimen preparation procedure succeeds in proving the postulated occurrence state of gas bubbles in coarse-grained sediments and accurately controlling the gas content.     

A Study on the Engineering Properties of Sand Improved by the Sand Compaction Pile Method

In order to directly evaluate the effects of soil improvement by the Sand Compaction Pile (SCP) method on the density, deformation, and static and liquefaction strength characteristics of sandy soils, a series of field and laboratory tests were performed. Laboratory tests were performed on high-quality undisturbed samples obtained from sandy soils both before and after soil improvement by the SCP method. The high-quality undisturbed samples were recovered by the in-situ freezing sampling method. The drained shear strength (internal friction angle, φ_d), liquefaction strength (R₁₅: cyclic stress ratio needed to cause 5% double amplitude axial strain in 15 cycles), and cyclic deformation characteristics (G~γ and h~γ relations) were determined by performing a series of laboratory tests on the undisturbed samples. Both the in-situ density and the relative density were measured on the undisturbed samples used in the laboratory tests. A standard penetration test (SPT) and a suspension-type P-S wave logging test were performed to investigate the soil profile of the test site before and after the sand compaction. Both the static and the liquefaction strengths of the sandy soils obtained in the laboratory tests were also compared with those estimated by empirical correlations used in practice based on the SPT N-value and soil gradations.     

白鹤滩水电站层间错动带的剪切特性

基于白鹤滩水电站层间错动带的室内和现场剪切试验结果,对层间材料、层间材料/母岩接触面(土/岩接触面)的剪切力学特性进行研究。试验结果表明,层间材料和土/岩接触面都表现出无剪胀行为的理想弹塑性力学特性;层间材料的蠕变特性不明显;土/岩接触面的抗剪强度与接触面形貌密切相关,接触面越粗糙,其抗剪强度就越高。比较层间材料的室内和现场直剪试验的抗剪强度可知,试样具有强烈的不均匀性。试样扰动对其抗剪强度的影响要比材料变异性的影响小。尺寸效应使得室内试验获得的内摩擦角要比现场试验获得的大。黏粒含量是影响层间材料抗剪强度的重要因素:随着黏粒含量的增加,内摩擦角减少,黏聚力增大。层间材料的抗剪强度对初始饱和度的变化不敏感。室内试验和现场试验结果在黏粒含量和初始饱和度影响上表现出良好的一致性,说明试样的尺寸效应和变异性只是影响其抗剪强度的次要因素。     

Estimation of In-Situ Undrained Strength of Soft Soil Deposits by use of Unconfined Compression Test with Suction Measurement

Unconfined compression tests have been widely used in Japan for the purpose of determining undrained strengths of clay samples, but the unconfined compressive strengths are usually scattered even if specimens tested seem to have been subjected to the same stress history. This is due to the characteristic of the test itself, which is performed under unstable confining stress given by capillary pressure of the sample itself. In order to make clear the cause of scattering of measured strengths, three series of tests simulating the process from sampling to unconfined and triaxial compression tests on saturated soil samples are performed with three remolded and three undisturbed clays together with an undisturbed peat, and the influence of stress release and mechanical disturbance of the test specimen on the undrained strength is examined. Based on the test results, a practical method for estimating the in-situ undrained strength of soft soils from the results of a routine unconfined compression (UC) test together with suction measurement is proposed. From comparisons of the undrained strength profiles estimated by the proposed method with the strength profiles obtained by in-situ sounding test, it is found that the strength obtained by UC test can be reasonably corrected to estimate the in-situ undrained strength of soft soil deposits.     

Characterization of a Cohesive Soil Bed using a Cone Pressuremeter

A new cone pressuremeter has been developed at the University of Engineering and Technology, Lahore-Pakistan. The new device is called the Akbar Pressuremeter (APMT). This paper is based on the APMT testing of a cohesive soil deposit comprised of low plastic lean clay (CL) to sandy silty clay (CL-ML). The APMT testing, using a full-displacement technique, standard penetration tests and undisturbed samples, was carried out at two locations. The soil strength and type were determined using the undisturbed samples. The applied pressure-cavity strain curves of the APMT tests performed at different levels were analysed to determine soil strength and stiffness. The undrained shear strength of the undisturbed samples was determined in the laboratory by unconfined compression tests. This paper provides a comparison of the parameters interpreted from the pressuremeter and those determined from other field and laboratory methods.     

海床砂土应力分析及动态强度特性

探讨了海床土壤与陆上土壤应力状态的差异,在三轴试验中利用反水压力作为静水压力以模拟原海床水深,回复海床土壤在原海域环境中的应力状态,并探讨水深对海床砂土动态强度的影响。另外,探讨了海床砂土受到长、短波浪周期作用时,其动态强度特性。研究结果显示,水深越大,海床砂土的液化阻抗强度越低;海床土壤在长周期的波浪作用下,有较高的液化阻抗强度,同时引起海床砂土液化有一最低的门坎值。     

由于应力释放引起的深水土样扰动性研究(英文)

从深水海底取得的土样要经受很大的应力释放。模拟孔隙水中溶解有不同含量气体的深水黏土取样过程 ,对Lierstranda黏土进行了一系列三轴和高压固结试验。试验结果表明 ,自重应力下的孔隙比变化与孔隙水中的气体饱和度 η 成线性增长关系。其它测得的土性参数 ,如不排水强度、峰值应力下的应变、剪胀系数和先期固结压力等均随着 η 有规律地变化 ,显示土样的扰动性也随着 η 的增加而增加。据此 ,本文提出了对可能含有气体的深水土样的最佳贮藏与处置方案。     

国际土力学及基础工程协会简介

国际土力学及基础工程协会(ISSMFE)于1936年成立,同年在美国哈佛大学召开了第一届会议,在这之后,除特殊情况外,都是每四年召开一次国际会议。我国于1957年参加国际土力学及基础工程协会,并派茅以升、陈宗基为代表出席在英国召开的第四届国际会议。此后,二十余年中,主要由于台湾问题,一直没有再出席国际土力学及基础工程协会的会议。历届会议的概要见表1。     

小应变下土体刚度非线性分析的试验研究

精确的试验测试显示,土体在小应变下呈非线性作用,用常规的室内试验和传统的线弹性分析方法不能真实描述土的刚度变化,只能采用原位试验和非线性分析方法来确定。基于Bolton等人提出的平面应变下不排水膨胀试验的非线性分析理论,研究了剑桥自钻式旁压试验过程中土体刚度随应变的变化规律,并与线性分析结果进行了对比。试验和分析结果表明:小应变下土体刚度具有高度的非线性,随应变的增加发生衰减,但在不同应变区间衰减程度不同;土体割线模量值大于切线模量值,其差值随应变的增加逐渐减小;传统的线性分析低估了土体弹性作用范围和强度,而采用非线性分析能确定不同应变下土的刚度参数,真实反映土的非线性作用特点。     

边坡岩土体抗剪强度原位试验研究

针对碟子沟风井场地边坡特征,在边坡典型位置的岩石与土层界面和软弱泥岩夹层处分别进行了原位剪切试验,获得了相应的抗剪强度指标。通过与滑带土相同工况下的室内试验强度指标对比,发现室内试验指标较原位剪切试验获得的内聚力和内摩擦角低,并分析了室内外试验结果差异的原因。试验研究表明,边坡岩土体的大型原位剪切试验对边坡稳定性分析和工程治理优化具有重要价值。     

Static liquefaction-triggering analysis considering soil dilatancy

The failure of a sloping ground due to static liquefaction occurs when the shear stress applied by a monotonic triggering load exceeds the undrained yield (peak) shear strength of the saturated liquefiable cohesionless soil. Current practices for determining the in-situ undrained yield strength for grounds subjected to static shear stress rely on either a suite of costly laboratory tests on undisturbed field samples or empirical correlations based on in-situ penetration tests, which fail to account for the effect of soil dilatancy in decreasing the degree of strain-softening and the brittleness of cohesionless soils with an increasing penetration resistance. In this study, the effect of soil dilatancy on the static liquefaction failure of cohesionless soils is characterized by an empirical relationship between the soil brittleness index and the undrained yield strength from a database of 813 laboratory shear tests collected from the past literature. The application of this relationship for estimating the static liquefaction-triggering strength of cohesionless soils under sloping ground conditions is validated by comparing several cases of liquefaction flow failures. Finally, a procedure is briefly demonstrated for evaluating the triggering of static liquefaction in a dyke to the north of Wachusett Dam and Duncan Dam which incorporates the dilatancy behavior of cohesionless soils in a semi-empirical procedure based on in-situ penetration tests.     

Behavior of geogrid–reinforced sand and effect of reinforcement anchorage in large-scale plane strain compression

This paper presents experimental investigations on the behavior of geogrid–reinforced sand featuring reinforcement anchorage which simulates the reinforcement connected to the wall facings in numerous in-situ situations. A series of large plane strain compression tests (the specimen 56 cm high × 56 cm wide × 45 cm long) was conducted. Standard Ottawa sand and 4 types of PET geogrids exhibiting 5% stiffness in the range of 750–1700 kN/m were used in this study. The specimens were tested by varying the relative density of sand, confining pressures, geogrid types, and reinforcement-anchorage conditions. Experimental results indicate that relative to unreinforced specimens, both anchored and non-anchored geogrid reinforcements can enhance the peak shear strength and suppress the volumetric dilation of reinforced soil. The studies on anchorage revealed that anchoring the reinforcement can restrain the lateral expansion of reinforced specimens, resulting in a substantial increase in shear strength and a reduction in volumetric dilation. The strength ratios of non-anchored specimens appeared to be insensitive to the reinforcement stiffness, whereas the strength ratios of the anchored specimens increased markedly with increases in soil density, reinforcement stiffness, and system deformation (i.e., axial stain). Geogrid anchorage contributed a large percentage of the total shear-strength improvement, nearly 3-times more than the contribution of the soil–geogrid interaction in non-anchored specimens. Lastly, an analytical model was developed based on the concept that additional confinement is induced by reinforcement anchorage, and the predicted shear strength of the anchored soil was verified based on the experimental data.     

豫东地区压实粉土地基岩土工程分析与评价

针对豫东地区在新农村建设过程中多采用压实填土作为多层民用建筑地基持力层的工程实践,通过压实粉土的室内试验和原位测试,研究了压实粉土的工程力学特性,分析了粉土作为填土填料的基本物理力学性质,对比研究了不同含水率下压实粉土与原状粉土的强度与变形性状。试验结果表明:施工质量控制好的压实粉土,在天然湿度状态下其物理力学特性较好,有着较高的强度和刚度,而不同的含水率对压实粉土的强度与变形特征较原状粉土影响大,在施工过程中应严格控制压实粉土的含水量。     

Stiffness decay characteristics and disturbance effect evaluation of structured clay based on in-situ tests

The structure of the sedimentary clay influences its mechanical behavior in non-negligible ways. This paper proposes an effective approach for investigating the in-situ stiffness characteristics from shear modulus and strain decay curves (G–γ curves) based on self-boring pressuremeter and seismic dilatometer tests. To evaluate the excavation disturbance effects on the structured clay, the stiffness parameters from pre-bored pressuremeter tests are compared with the results of self-boring pressuremeter tests. The result indicates that the complete in-situ G–γ curves can be acquired by integrating the strain-dependent tangent shear modulus G_t from self-boring pressuremeter tests and the small-strain modulus G₀ from seismic dilatometer tests. Simultaneous observations of the G–γ curves with hyperbolic shapes in semi-logarithmic coordinates at the same strain scale show the similarity of the stiffness decay mode of the soil at different depths. The increase in the measured values of G_t and G₀ with depth can be attributed to the improved consolidation pressures and cemented strength in the structured clay. Additionally, the G/G₀–γ curves measured by the in-situ tests generally agree well with the results predicted by the Stokoe model. The excavation disturbance weakens the stiffness of the structured clay, as evidenced by G_t from the pre-bored pressuremeter data being significantly smaller than that from self-boring pressuremeter tests at the same depth. Based on quantitative analysis, the disturbance degree computed from the measured results has a low sensitivity to the soil depth and a strong negative correlation with the strain level of the soil. This study provides an effective method for predicting the stiffness parameters of structured soil based on in-situ tests.     

基坑中基于CPTU软土不排水强度确定及应用

不排水抗剪强度是土的重要力学特性参数。在岩土工程施工期稳定性分析中,需用不排水抗剪强度值。目前,不排水抗剪强度的确定主要有室内试验和十字板剪切试验。但前者受土样扰动影响大,后者十字板强度是土体各滑动面上抗剪强度的较小值,在基坑工程验算坑底隆起时,结果不够准确。本文首先总结了国内外基于CPTU的Su确定方法,在典型软土基坑工程中进行了孔压静力触探试验,根据不排水试验值与锥尖阻力的关系,反演出圆锥系数的值。基于反演的圆锥系数,把CPTU预测的不排水抗剪强度应用到基坑的隆起分析中,与基于室内试验数据和基于十字板强度的计算结果进行了比较,结果表明:基于CPTU的软粘土不排水抗剪强度指标计算结果合理,符合工程实际,值得在工程设计中推广。     

热传导CPT探头的研发与应用

土体热导率是能源地下工程、高压电线埋设、冻土路基等工程设计中的重要参数,是评估土体换热性能及地下温度场计算的关键,但目前缺乏有效的原位测试手段。基于瞬时线热源温度消散理论,研发了可测试原位土体热导率的静力触探(CPT)探头。根据理论假设与现有CPT系统尺寸,确定了探头具体长度、直径、内部构造、温度采集点位,并提出相应测试步骤与热导率计算方法。利用COMSOL有限元分析软件对测试过程进行模拟验证,结果表明探头实际传热符合线热源假定,且计算方法适用于一般热导率土体。对于热导率较小(小于0.6 W/(m?K))土体,需适当延长测试时间。现场应用表明,土体原位热导率略高于取样土室内测试结果,表明取样扰动可能降低土体导热性能,最后对取样测试及工程设计提出改进建议。     

油罐软粘土地基性状

首先,通过K₀固结三轴试验等室内试验探讨了金山粘土的应力应变关系以及强度和刚度各向异性。采用有限单元法研究了圆形贮罐上部结构、垫层与地基共同作用以及土体各向异性对沉降的效应。其次,在试验研究的基础上,提出一组考虑初始应力状态和土体固有各向异性的非线性弹性系数实用方程式。通过强度发挥度,可以把土体刚度和强度联系起来。最后,应用比奥（Biot）固结理论有限单元法分析了二只大型油罐（容量分别为10000m³和30000m³）地基试水期间的固结过程,分析中考虑了上部结构、环基和垫层的刚度。计算沉降过程线和孔隙压力过程线与现场实测值接近。     

Effects of Sample Disturbance on Small Strain Characteristics and Liquefaction Properties of Holocene and Pleistocene Sandy Soils

To investigate liquefaction properties of sandy soils, undrained cyclic loading tests are usually performed. However, it would be difficult to simulate fully the actual soil behaviour through the laboratory tests because the tested sample can be disturbed even though it is taken by in-situ freezing technique. In this study, by using three kinds of in-situ frozen sandy soils which were taken from Holocene and Pleistocene deposits and their reconstituted samples, their volume change properties were measured during freeze and thaw processes at different confining pressures of 30 kPa and 98 kPa. In order to investigate the effects of the possible sample disturbance on the liquefaction resistance, small strain characteristics were measured as well, which would reflect the soil structure. Decreases in the small strain characteristics and the liquefaction resistance were observed in case of the Holocene specimens that were thawed at the confining pressure that was lower than the in-situ overburden stress. On the other hand, in case of the Pleistocene specimens, the effects of the confining pressure during the thaw process on the small strain characteristics and the liquefaction resistance were small. Such contrastive feature between the Holocene and the Pleistocene samples could be linked with the difference in the types of their natural aging effects.     

武广客运专线灰岩残积层红粘土变形参数研究

地基土的变形模量是地基变形计算中的重要参数,一般要通过代价昂贵的平板载荷试验来获取。而室内试验虽可方便地获取土的压缩模量,但因取样过程中的扰动,致使压缩模量与变形模量之间并不符合理论关系公式。我们基于室内试验与现场平板载荷试验的对比分析,结合已有经验公式,得出适用于武广客运专线红粘土路段的变形模量经验取值公式,为由室内试验参数估算现场土体的变形模量提供理论依据。