Adaptive sampling strategy for characterizing spatial distribution of soil liquefaction potential using cone penetration test

Characterizing spatial distribution of soil liquefaction potential is critical for assessing liquefaction-related hazards (e.g. building damages caused by liquefaction-induced differential settlement). However, in engineering practice, soil liquefaction potential is usually measured at limited locations in a specific site using in situ tests, e.g. cone penetration tests (CPTs), due to the restrictions of time, cost and access to subsurface space. In these cases, liquefaction potential of soil at untested locations requires to be interpreted from limited measured data points using proper interpolation method, leading to remarkable statistical uncertainty in liquefaction assessment. This underlines an important question of how to optimize the locations of CPT soundings and determine the minimum number of CPTs for achieving a target reliability level of liquefaction assessment. To tackle this issue, this study proposes a smart sampling strategy for determining the minimum number of CPTs and their optimal locations in a self-adaptive and data-driven manner. The proposed sampling strategy leverages on information entropy and Bayesian compressive sampling (BCS). Both simulated and real CPT data are used to demonstrate the proposed method. Illustrative examples indicate that the proposed method can adaptively and sequentially select the required number and optimal locations of CPTs.     

A step forward towards a comprehensive framework for assessing liquefaction land damage vulnerability: Exploration from historical data

The unprecedented liquefaction-related land damage during earthquakes has highlighted the need to develop a model that better interprets the liquefaction land damage vulnerability (LLDV) when determining whether liquefaction is likely to cause damage at the ground’s surface. This paper presents the development of a novel comprehensive framework based on select case history records of cone penetration tests using a Bayesian belief network (BBN) methodology to assess seismic soil liquefaction and liquefaction land damage potentials in one model. The BBN-based LLDV model is developed by integrating multi-related factors of seismic soil liquefaction and its induced hazards using a machine learning (ML) algorithm-K2 and domain knowledge (DK) data fusion methodology. Compared with the C4.5 decision tree-J48 model, naive Bayesian (NB) classifier, and BBN-K2 ML prediction methods in terms of overall accuracy and the Cohen’s kappa coefficient, the proposed BBN K2 and DK model has a better performance and provides a substitutive novel LLDV framework for characterizing the vulnerability of land to liquefaction-induced damage. The proposed model not only predicts quantitatively the seismic soil liquefaction potential and its ground damage potential probability but can also identify the main reasons and fault-finding state combinations, and the results are likely to assist in decisions on seismic risk mitigation measures for sustainable development. The proposed model is simple to perform in practice and provides a step toward a more sophisticated liquefaction risk assessment modeling. This study also interprets the BBN model sensitivity analysis and most probable explanation of seismic soil liquefied sites based on an engineering point of view.     

饱和南京细砂初始液化后特大流动变形特性试验研究

针对地震中倾斜场地砂土液化流动大变形特征与机理,以具有明显片状颗粒结构特征的南京细砂为研究对象,采用了英国GDS公司的动态空心圆柱扭剪仪,开展了饱和南京细砂液化后特大流动变形特性的循环扭剪试验研究,主要分析了橡皮膜效应、有效围压、循环加载幅值和初始静剪应力等因素对南京细砂液化流动大变形特性的影响规律及其机理。试验结果表明:在其它条件不变时,随着有效围压增加,饱和南京细砂抗液化强度和抗单向液化流动累积变形强度都有所增加。当保持围压不变时,饱和南京细砂抗单向液化流动累积变形的强度有增加的趋势。随着初始静剪应力比的变大,南京细砂的抗液化和抗单向累积变形强度特征可以分为三个不同阶段。在无初始静剪应力条件下饱和南京细砂液化后仍以循环液化流动变形为主和单向流动累积变形为辅。但是,随着初始静剪应力比的增大,饱和南京细砂抗液化强度和抗单向液化流动累积变形强度明显降低,即主要发生单向液化流动累积变形破坏。当初始静剪应力比接近或超过循环动剪应力比时,饱和南京细砂的抗液化和抗单向流动累积变形的强度又明显增大,但仍以液化单向流动累积破坏为主。当初始静剪应力比继续增大到一定程度时,饱和南京细砂已经很难液化,其抵抗单向累积变形的强度也明显降低,主要原因应为此时试样发生的是塑性累积大变形破坏。同时,试验结果表明饱和南京细砂的液化后流动大变形特性也明显区别于含圆形颗粒为主的日本丰浦砂。     

汶川地震中道路边坡工程震害分析

 发生在山区的地震对边坡工程所造成的危害,在致灾机制和破坏形式方面具有鲜明的特征。结合汶川地震灾区道路边坡工程震害实例,分析路堑、路堤以及与桥隧相连的各类边坡及相应支挡结构的震害机制和破坏形式。锚索(杆)地梁或预应力锚索抗滑桩加固的边坡具有较好的抗震性能,其原因是这些结构已与坡体联接在一起而形成一个整体,在地震波作用下结构与坡体的位移和变形能够很好地协调一致。铺设土工格栅或施加加筋材料的路堤边坡工程具有较好的抗震性能,一般填筑路堤特别是高路堤,其抗震性能较差。根据沙土液化和软弱黏性土层震陷造成的震害实例,提出含水沙质地层路堤边坡应注意坡脚沙土液化造成的震害,应采取措施防止软弱黏性土层地基震陷造成路面破坏以及坡脚震陷造成的边坡失稳。山区隧道洞口边、仰坡的抗震设计应重视支挡结构的耐震性。建于坡体上的桥台、桥基和桥路过渡段的安全性与坡体稳定性直接相关,应切实加强这些结构所在边坡的抗震设计。对于依山傍水而建的顺河桥,相关边坡的失稳危害桥梁时,应对其采取抗震措施。目前公路、铁路工程抗震规范涉及边坡工程及支挡结构的内容极少,研究成果可为规范的修改和补充提供有益的参考。     

Shaking table tests on wall-type gravel and rubber drains as a liquefaction countermeasure in silty sand

In this paper, the seismic performance of wall-type gravel and rubber drains as a liquefaction countermeasure in silty sand has been addressed using a series of 1 g shaking table tests. In these tests, the liquefaction resistance of silty sand was studied by changing soil relative density, silt content, number of walls, and materials of drainage walls. In order to evaluate shear wave propagation, the generation and dissipation of pore water pressure (PWP), and the ground surface settlement, various accelerometers, displacement and PWP transducers were placed. The obtained results indicated that an increase in the relative density and the number of wall-type gravel and rubber drains in liquefiable silty sand reduces the settlement, liquefaction-induced deformations, as well as the excess PWP and eventually improves the liquefaction resistance. Totally, it can be noted that the reinforced silty sand with wall-type gravel drains revealed less excess PWP and settlement than that with wall-type rubber drains.     

潛盾隧道液化評估及抗液化對策之探討

液化對潛盾隧道之影響及設計檢核内容,一般包括液化時上浮之穩定性、震後產生之不均勻沈陷及環片因液化引致之額外應力等。整理目前工程寅務上常用之分析方法,並探討各分析方法之限制及待研究突破之問題點;整理台北、高雄捷迎潛盾隧道段抗液化處理對策及各項處理對策之優缺點,建議可行之抗液化處理對策,以供爾後類似工程之參考。     

砂土液化后再固结体变规律表征与离心模型试验验证

针对一定相对密度的饱和砂土,首先开展单元体K₀固结试验和振动液化试验研究,发现饱和砂土液化后体变规律受再沉积和再固结两种机制制约：其中再沉积部分与所受振动历史密切相关,尤其是液化触发后的应变历史,土骨架累积剪应变比越大、再沉积体变越大;而再固结部分受先期固结历史和循环振动历史影响显著,再固结曲线会沿原有正常固结曲线趋势发展,其稳定段再压缩指数比相同条件下的正常固结曲线的压缩指数稍大。据此提出了考虑先期固结和振动历史的砂土液化后体变模型和简化算法,将再沉积和再固结两者统一表达成再固结体变,并建议了再固结压缩指数和假设起始应力的确定方法。进一步开展了水平场地地震液化离心机模型试验,监测模型固结和振动液化过程的沉降,从模型尺度进一步揭示砂土液化后体变规律,并初步验证了本文模型与简化算法的有效性。     

德阳松柏村典型液化震害剖析

2008 年 5 月 12 日中国发生的 8.0 级汶川大地震中, 位于 Ⅶ 、 Ⅷ 度交接处的德阳松柏村液化破坏十分明显, 喷砂类型丰富,液化伴随大量地裂缝,液化加震现象十分显著 , 具有典型研究价值。对该村进行了详细的现场测试并 与以往液化震害进行了对比分析,结果表明： 尽管松柏村地表主要喷出物是中砂和粗砂,但实际液化土与喷出物差别显著,主要为砂砾土液化;以往还没有提出过适于工程应用的砂砾土液化判别方法,而超重型动力触探（ DPT ）具有设备简单、经济实用、测试数据连续的优点,可作为砂砾土液化预测的核心指标; 松柏村大量地裂缝现象不是由次断层造成而是由土层液化引起,而液化产生地裂缝的基本条件为地表较平坦（ <3% ）以及液化土层水平分布不均匀; 液化减震的基本条件为上覆非液化层足够厚且较为密实,但定量评价方法还有待进一步研究。     

The uplifting behavior of shallow tunnels within the liquefiable soils under cyclic loadings

The liquefaction of soils under earthquake loadings has always been a main concern for geotechnical engineering practices. As an earthquake causes the ground to liquefy, the effective stress and hence the shear strength of the soil decreases sharply, and large deformations happen in the area. This phenomenon occurs only rarely when the liquefaction occurs at a large depth. However, deformations increase extensively when this layer is located in shallow depths near the ground level. In this case super structures and also underground structures may be severely damaged. The tunnels which are constructed in this layer may be affected by the liquefaction as well. In this condition the liquefaction may cause settlement in the ground, deformation in the tunnel shield, buoyancy in the underground buildings, reduction in bearing capacity and increases in lateral spread and pore pressures. In this paper the FLAC software has been used to model the pore pressure changes during earthquakes that lead to soil liquefaction. The studies thus far have been focused on the impact of the soil liquefaction on the tunnels constructed in this area. According to the studies, the buoyancy and uplifting forces due to liquefaction have major effects on the behavior of underground structures. Increasing the soil parameters such as friction angle and damping ratio causes the liquefaction effects on the tunnel to decrease, and increasing the geometric parameters such as tunnel diameter and location depth causes these effects to increase.     

基于地震变形控制的隧道地基注浆抗液化加固效果评价

 目前理论上主要有两种液化防治思路：一种是限制超孔隙水压力的产生与发展;另一种是减少液化产生的过大变形。目前,基于前一种设计思想的液化处理方法和研究较多,而对基于变形控制的处理方法实施效果的研究则相对有限。从后一种思路出发,基于Biot固结理论,利用循环弹塑性本构模型,通过数值模拟方法,对上海某电厂取排水隧道的地震液化问题进行模拟,建立一种平面应变条件下隧道地震液化变形的数值模拟方法,以评估其注浆预防液化的加固效果。其中重点分析了地震作用下加固前后该隧道地基的位移、加速度、超孔隙水压力等动力反应。结果表明,采用适当的抗液化处理措施可以较好地抑制地基液化变形,这为工程场地抗震设计及液化防治提供了科学依据,对类似工程的可液化场地处理具有良好的借鉴作用。     

Liquefaction-induced damage evaluation of earth embankment and corresponding countermeasure

Liquefaction of sandy soils is a big threat to the stability and the safety of an earth embankment laid on saturated soils. A large number of liquefaction-induced damages on embankment due to different types of earthquakes have been reported worldwide. In this research, the dynamic behaviors of earth embankment and the reinforcement effects of grouting as remediation method, subjected to moderate earthquake EQ₁ and strong earthquake EQ₂, were numerically investigated. The seismic behaviors of ground composed of cohesionless sandy soil and cohesive clayey soil were uniformly described by the cyclic mobility (CM) model, which is capable of describing accurately the mechanical property of the soil due to monotonic and cyclic loadings by accounting for stress-induced anisotropy, over-consolidation, and soil structure. It is known from the numerical investigation that the embankment would experience destructive deformation, and that the collapse mode was closely related to the properties of input seismic motion because high intensities and long durations of an earthquake motion could lead to significant plastic deformation and prolonged soil liquefaction. Under the strong seismic loading of EQ₂, a circular collapse surface, combined with huge settlement and lateral spread, occurred inside the liquefication zone and extended towards the embankment crest. In contrast, in moderate earthquake EQ₁, upheaval was observed at each toe of the embankment, and instability occurred only in the liquefied ground. An anti-liquefaction remediation via grouting was determined to significantly reduce liquefaction-induced deformation (settlement, lateral spreading, and local uplift) and restrain the deep-seated circular sliding failure, even though the top sandy soil liquefied in both earthquakes. When the structure was subjected to EQ₂ motion, local failure occurred on the embankment slope reinforced with grouting, and thus, an additional appropriate countermeasure should be implemented to further strengthen the slope. For both input motions, the surface deformation of the considered embankment decreased gradually as the thickness of reinforcement was increased, although the reinforcement effect was no longer significant once the thickness exceeded 6 m.     

Liquefaction in the Kanto region during the 2011 off the pacific coast of Tohoku earthquake

The present text is a summary of liquefaction events that occurred in the Kanto Region during the 2011 off the Pacific coast of Tohoku Earthquake, excluding those events in the Tokyo Bay area that are discussed elsewhere. Most liquefaction occurred in the abandoned river channels and former lakes that had been recently filled with liquefiable soils. Noteworthy was the damage to private housing lands and river dikes that had not been well treated against liquefaction risk. Many sites experienced repeated liquefaction after former earthquakes in 1923 or 1987, if they existed in those times. In contrast to the recent liquefaction-prone soils, more-aged sands scarcely liquefied because of what is called the “ageing” effect. The present study analyzed cases in Tokyo area to quantitatively assess the temporal development of ageing so that liquefaction risk in more aged soils might be reasonably assessed. In the appendix, remarks are made of the reliability of air-photo survey in quick detection of liquefaction sites.     

Centrifuge modeling of hybrid foundation to mitigate liquefaction-induced effects on shallow foundation resting on liquefiable ground

A hybrid foundation is developed in this study to mitigate the liquefaction-induced effects on shallow foundations. The proposed hybrid foundation is a combination of a gravel drainage system and friction steel piles with spiral blades, framed under the footing. The motivation behind having a gravel drainage system, as an integral part of the hybrid foundation, is its ability to improve the liquefaction resistance of the ground in the most economical way. However, case histories and the development of recent research have highlighted that gravel drainage systems have exhibited poor performances and could not prevent ground liquefaction during strong ground motion. To counteract these shortcomings, friction steel piles are provided which are supposed to yield frictional resistance during earthquakes and are presumed to minimize the rocking/tilting behavior of the foundation-structure system even if the ground undergoes liquefaction. The evolution of excess pore water pressure, specifically in the vicinity of the foundation-structure system, dominatingly influences the settlement mechanism of shallow foundations. Centrifuge test results show that the presence of gravel drainage can minimize the post-liquefaction settlement of shallow foundations through the rapid dissipation of excess pore water pressure. Moreover, friction piles are able to minimize the tilting/differential settlement of shallow foundations. It is found that the proposed hybrid foundation provides the desired function of reducing the overall liquefaction-induced effects on shallow foundations resting on liquefiable grounds.     

Liquefaction assessments by field-based methodologies: foundation soils at a dam site in Northeast Turkey

Recent examples show that strong earthquakes can cause damage to dams, notably tailings or hydraulic fill dams or relatively small earth fill embankments. Liquefaction is known to be one of the most dangerous consequences of the dynamic loading of an embankment dam and hence must be considered during the dam site selection and construction stages. This paper presents the liquefaction analyses undertaken for the Demirözü dam site in Northeast Turkey. The liquefaction assessments are mainly based on field-performance data using SPT- and CPT-based methods in conjunction with the results of the available cyclic triaxial tests. The presence of a very shallow groundwater table, loose sandy layers and the seismic features of the region result in a high susceptibility to liquefaction. The results obtained from the SPT- and CPT-based criteria for evaluating the liquefaction potential are generally consistent and show that the thickness of the liquefiable layers increases in the area between the left bank and the central part of the proposed dam. Further studies are required to evaluate the suitability and efficiency of measures which could be taken to avoid liquefaction-induced problems at the dam site.     

Optimized back-analysis for tunneling-induced ground movement using equivalent ground loss model

Application of an optimization technique for the back-analysis of tunneling-induced ground movement is presented in this paper. The conjugate gradient method was adopted for the optimization procedure and the gap parameter model was utilized to evaluate the ground movement. According to the results of back-analysis using the monitoring data of the Taipei Rapid Transit System, the angle of the influence zone of ground settlement is approximately 45° for tunneling in clayey soils, and that for sandy layers ranges from 30 to 50°. Backfill grouting will reduce the physical gap of shield machine in the order of 60–80% for tunneling in clayey soils and in the range of 70–85% for sandy layers. Semi-empirical equations were established to predict the tunneling-induced ground movement in the silty clay and silty sand of Taipei basin. Case studies were presented to demonstrate that the adoption of the method can provide a reasonable prediction of the ground settlement due to shield tunneling.     

砂砾土液化判别的基本方法及计算公式

2008年汶川8.0级大地震中液化现象显著且砂砾土液化占很大比重,而我国一些地区砂砾土分布广泛,发展相应液化预测和判别方法十分必要。我国规范液化判别方法来源于砂层（细粒土）液化资料,且按规范规定标准贯入试验不适于砂砾场地,故现有规范中基于标贯的液化判别方法对砂砾土不可行。以汶川大地震液化震害调查和现场测试为基础,提出了基于超重型动力触探试验（动探试验）的砂砾土液化判别方法并建立了计算模型和公式。结果表明：砂砾土液化判别由初判和复判两部分组成,初判以排除不可能液化及可不考虑液化影响情况为目标,复判则可采用动探击数N₁₂₀为基本指标的计算模型。初判包括地质年代、埋藏条件和含砾量3个条件,复判模型则由动探击数基准值、含砾量、砂砾土埋深、地下水深度和地震烈度等5个参数组成。根据此次地震液化砂砾土埋深及地下水位变化范围较大的特点,采用归一化方法导出动探击数基准值,利用优化方法推导出砂砾土深度及地下水位的影响系数。提出的砂砾土液化判别方法,较全面地考虑了砂砾土液化的影响因素,复判模型和公式表达简单明了,回判成功率较高,且与现有规范具有连续性,便于工程应用。     

严重液化场地单层建筑地基基础的选型及分析

对一栋拟建于严重液化场地的单层建筑,提出两种地基及基础的方案选型,通过理论计算和经济技术指标的详细对比,最终确定采用粉土液化平均震陷量的经验方法作为调整地基抗液化措施的设计依据。     

滨海吹填造陆地区岩土工程勘察问题分析与评价

围海造陆是当今沿海国家及城市开发土地资源的重要措施之一。以天津滨海某工程勘察为例,根据建设项目的构筑物特点、地基基础方案及场地土工程性质,采用了钻探取样、标准贯入试验、剪切波测试、十字板剪切测试、浅层平板载荷试验、室内试验等多种勘察手段获得了准确详实的地质资料,并对勘察过程中涉及到的冲填土、液化土、软土震限、负摩阻力、大面积群桩施工影响等关键技术问题进行了分析与评价,为该工程的设计及施工提供了可靠的岩土参数及建议。     

广州地铁砂土层液化判别

在广州地铁工程砂土地震液化判别过程中,考虑了地铁结构与液化土层的相互作用。通过大量的现场实验、室内动三轴实验,总结了水平场地、区间、车站土层液化分布情况和液化特点;为了提高液化判别精度,进一步详细地对比和检验了现场和室内的判别结果,分析了液化土层与结构的空间相对位置以及结构对液化势的影响,所采用的多参数和多手段的液化判别技术为合理的抗液化设计提供帮助。     

双圆盾构隧道土体地表沉降特性

介绍了双圆盾构隧道这种新型隧道形式,与圆形盾构隧道相比,双圆盾构隧道具有占用地下空间小、施工效率高、掘削土量少等优点,但双圆盾构隧道引起的土体位移相对较大,影响范围也比较广。基于双圆盾构隧道的施工特点,通过计算圆形盾构的土体地表沉降,运用土体位移叠加法,研究了双圆盾构隧道引起的土体地表沉降的特性,建立了双圆盾构隧道直径、埋深和地层损失等因素与土体地表沉降的关系。结果表明:双圆盾构隧道的地表沉降槽的形态与圆形盾构隧道相似;双圆盾构隧道的地表沉降量大,影响范围广;双圆盾构隧道的地表沉降与埋深和直径之比有关。