夯击能应用于高填土地基的一些探讨

大量工程实践表明 ,对填土及软土地基进行夯击加固时 ,夯击能的选取不仅与填土层的厚度有关 ,而且与填土层下覆土质的工程地质条件等因素密切相关。本文结合南方某港口填土地基加固工程实例 ,对在高填土地层中夯击能的使用问题进行了探讨 ,分析了当填土层的下卧层分别为坚硬土层和软弱土层、以及当填土层的填料分别为粗颗粒料和含水量较高的粘性土等细粒填料时 ,填土层加固的机理 ,制定了在不同条件下选取夯击能的标准 ,并根据现场检测资料 ,评价了该场地的地基加固效果 ,进一步验证了所确定夯击能标准的可靠性。     

强夯密实处理块碎石填料试验研究

在高填方工程中,压实质量是工程成败的关键因素。强夯法所具有的优点,使其在许多地基处理工程中都有广泛的应用。某山区高填方工程,根据工程的实际需要,采用强夯法对山体爆破块碎石填料进行密实处理。为此,开展现场试验研究。首先对爆破的块碎石填料进行了颗粒分析试验和压实试验,分析其可填性和压实特征。按照试验参数和工序,分别进行了2000kNm、3000kNm单点夯试验;同时夯后进行了密实度试验、颗粒分析试验。由此,确定了合理的强夯施工参数。最后根据面层载荷试验的检测,以此确定的施工参数在该工程取得了良好的处理效果。     

强夯法加固填土地基的现场试验与微观结构分析

对位于北京西北郊的某工程场地的填土地基在不同夯次、不同夯能条件下进行了现场试验,发现地面夯沉量的变化具有阶段性的特点。通过对采自不同夯击阶段的试样进行SEM图像定量分析,得到了夯击过程中地基土的微观结构参数与夯沉量变化的相关关系,从微观角度初步掌握了强夯过程的结构控制规律。     

土石比对碎石土强夯地基加固效果影响规律瑞利波检测分析

为了探讨土石比及大粒径块石对碎块石土强夯加固效果的影响规律,对3组不同土石比条件下碎块石土强夯地基的瑞利波检测数据进行了对比分析。夯后波速曲线具有明显的突变性,并随着土石比的减小其突变程度增强,突变段与大粒径块石有较好的对应性。波速增加比例曲线具有垂直分区特征,土石比对强夯加固区厚度影响较大。大块石对周边土体存在较强的扰动作用,夯后波速曲线往往表现为突然锐减,特别是当其位于夯后场地表层或较多大块石距离较近时表现更加明显。当大块石位于强加密区内时,加固深度随着大块石埋深增加而减小,且其埋深越靠近波速增加峰值点时加固深度越浅。研究成果对碎块石填土强夯地基的设计和施工具有参考价值。     

土石混合料剪切特性及块石破碎特征

通过室内大型直剪试验和基于PFC2D的颗粒离散元数值模型,探讨考虑块石破碎的土石混合料的剪切特性及块石破碎特征。以土石混合料室内大型直剪试验和筛分试验为基础,提出了一种能真实描述块石形态特征并准确反映块石破碎效应的土石混合料颗粒离散元数值建模方法,模拟并分析了6种含石量土石混合料在4种不同法向应力作用下的剪切特性及块石破碎特征。结果表明：土石混合料抗剪强度随含石量的增大而增大,且基本符合摩尔-库仑（M-C）强度准则,随着含石量增大,内摩擦角呈现“慢-快-慢”增长趋势,黏聚力则呈现先增后减再增大的变化趋势。剪切后土石混合料块石破碎形式可归纳为表面研磨、局部破碎、完全破裂、完全破碎4种方式。提出了一种新的颗粒破碎指标,该指标能够准确描述粒径大于5 mm的块石的破碎程度,其随含石量和法向应力的增大而增大。通过对土石混合料颗粒离散元数值模型剪切面的分析发现,剪切面出现“剪斜”现象,其起伏程度随含石量增大愈加明显,且随着含石量增大剪切面附近剪裂隙数量增多,土石混合料在剪切过程中的破坏为拉-剪混合破坏。     

土-石混合填土强夯动应力的简化与修正算法

强夯法是一种加固某些土体的施工操作较为简单的方法,合理分析其夯击作用在土体中产生的动应力是工程实践中的关键环节之一。针对成都平原地区的典型土-石混合填土,通过现场夯击试验,测得了在4 000 k N·m的夯击能作用下土体中不同深度处的竖向夯击动应力,鉴于既有算法与试验值的差异,从两个方面确定了适于土-石混合填土夯击动应力的简单计算方法。一方面针对较为复杂的既有理论公式问题,采用敏感性分析与回归分析法,建立了以夯锤质量、夯锤落距、夯锤半径、土体密度、土体动力剪切模量、土体泊松比、土体阻尼比、夯锤入土的速度损失率等8个参数表征夯击点冲击应力的线性回归方程;另一方面采用竖向动应力传递指数对理想弹性静力学理论公式进行修正,得到了基于拟静力法的土体中夯击动应力计算表达式,进而可分析确定强夯有效加固深度。试验结果显示,土-石混合填土的竖向动应力传递指数约为1.673 57,土-石混合填土地层的强夯有效加固深度约为8.0 m。     

重庆高填方填筑材料基本力学性质研究

西南地区高填方项目场地多为爆破后削山填谷抛填而成。以重庆高填方土石混合料为研究对象，开展了室内物理特性及直剪试验研究，分析了不同固结压力下含石率及含水率对土石混合料抗剪强度指标影响规律。结果表明，土石混合料黏聚力与含石率呈正相关，在含石率从20%增至40%时，黏聚力增量效果最明显；初期填料的内摩擦角随含石率增加呈现先增大后减小的趋势，后期随着含石率增加，内摩擦角减量效果显著。黏聚力与含水率整体呈负相关，且含水率从5%增至8%的情况下，黏聚力的减小幅度明显；填料内摩擦角随着含水率增大同样呈现先增大后减小的趋势。根据室内试验结果，给出了适用于项目场地抗剪强度指标取值范围，相应的关键参数可以为类似工程提供参考。     

泥砂互混吹填土自重沉积及颗粒分布规律

开展大尺寸沉降柱试验,获得不同初始含砂比、不同初始含水率的泥砂互混吹填土在自重作用下的泥面沉降量和沉降速率的基本特征,结合分层抽取法获得试样沿高度方向的密度剖面与颗粒分布变化规律。试验结果表明,泥砂互混吹填土沉积和颗粒分布规律与初始含砂比、初始含水率有较大关系,增加初始含砂比能显著增加沉降量,且存在一个合理含砂比,超过合理含砂比后沉降量反而降低,初始含水率越低该种现象越明显;沉降-时间曲线基本可以分为3个阶段,其中第Ⅱ阶段呈现出比较明显的非线性特征,随着初始含水率降低,3个阶段和非线性特征逐渐不明显;密度剖面呈阶梯变化特征,并随初始含水率和初始含砂比的降低而逐渐过渡到线性特征;初始含砂比和初始含水率均影响着泥砂互混吹填土的颗粒分布模式,初始含砂比和初始含水率越大越有利于粒径大于0.075 mm的颗粒沉积;初始含砂比越大,底部沉积的粒径小于0.005 mm颗粒相对越多,体现出群体沉降特征。     

高填方地基土工离心模型试验技术研究

以模型试验的相似理论为基础,结合九寨黄龙机场高填方地基工程实例,详细阐述了高填方地基离心模型试验技术,即对九寨黄龙机场104 m高填方地基的离心模型试验,采用等应力局部模型设计方案,以剔除法和等量代替法配制填料,以等效法模拟软弱地基强夯处理,并以增大离心加速度的方法模拟高填方地基的填筑加载过程;试验结果表明,高填方地基的总体沉降特征为"沉降大、压实快";土体的压实沉降主要发生在施工期;试验中高填方地基施工结束时土体的沉降固结度已完成90%以上,施工结束后土体产生的沉降量还不到土体总沉降量的10%.     

山坡地貌填土场地雨季强夯施工工艺

通过分析强夯法处理某山坡地貌填土场地工程,针对该工程填土厚度差异大、施工参数变异大、雨季施工等难点,提出信息化施工、分锤分区夯击等措施;具体分析工后差异沉降的控制及降雨对于夯击施工及质量的影响。对于山坡地貌填土场地工后差异沉降的控制及雨季强夯施工具有一定的借鉴作用。     

含石量对土石混合体剪切特性影响的颗粒离散元数值研究

采用基于PFC2D接触黏结模型的离散元数值模型,探讨含石量变化对土石混合体剪切特性的影响。利用室内大型直剪试验对表征土石混合体细观力学性质的模型参数进行标定,模拟分析了4种不同含石量土石混合体在4种不同围压作用下的剪切特性。试验结果表明:相同法向应力作用下,含石量越高,峰值剪应力越大,达到峰值剪应力时的剪应变越大,软化后强度越高,剪胀特性越强,剪切"跳跃"现象越明显。土石混合体摩擦角随含石量增高而增大;黏聚力随含石量增高而减小,含石量超过50%后基本不变。相同含石量的土石混合体,法向应力越高,峰值剪应力越大,应变软化特性越强。土石混合体剪切过程中以克服摩擦能和应变能为主,动能变化几乎为零。     

Modeling of granular bed‐stone column‐improved soft soil

The present study pertains to the development of a mechanical model for predicting the behavior of granular bed‐stone column‐reinforced soft ground. The granular layer that has been placed over the stone column‐reinforced soft soil has been idealized by the Pasternak shear layer. The saturated soft soil has been idealized by the Kelvin–Voigt model to represent its time‐dependent behavior and the stone columns are idealized by stiffer Winkler springs. The nonlinear behavior of the granular fill has been incorporated in this study by assuming a hyperbolic variation of shear stress with shear strain as in one reported literature. Similarly, for soft soil it has also been assumed that load‐settlement variation is hyperbolic in nature. The effect of consolidation of the soft soil due to inclusion of the stone columns has also been included in the model. Plane‐strain conditions are considered for the loading and foundation soil system. The numerical solutions are obtained by a finite difference scheme and the results are presented in a non‐dimensional form. Parametric studies for a uniformly loaded strip footing have been carried out to show the effects of various parameters on the total as well as differential settlement and stress concentration ratio. It has been observed that the presence of granular bed on the top of the stone columns helps to transfer stress from soil to stone columns and reduces maximum as well as differential settlement. Copyright © 2007 John Wiley & Sons, Ltd.     

碎石土地基在不同含石量下桩-土水平作用特性及m值的研究

四川输电线路经过的山区场地中,碎石土地基分布普遍,而碎石土是一种介于岩石和土体之间特殊的岩土体,水平受荷碎石土桩基础在不同含石量下水平承载特性具有较大的差异,现行规范给出的地基水平抗力系数的比例系数m值取值范围较为宽泛。研究碎石土地基在不同含石量下桩-土水平作用特性与m值取值是输电线路塔桩基设计中有待解决的问题。通过室内单桩水平静载试验,得到了不同含石量的碎石土地基对桩顶位移、桩身内力、地基水平抗力系数的比例系数m值的影响,以及不同含石量下m值的变化趋势。对比分析得到试验特征规律,研究桩身弯矩、剪力曲线与桩侧土压力曲线,不同含石量条件的m值变化趋势。结果表明:随着碎石土地基含石量提高,桩身最大弯矩值呈非线性增大,且最大弯矩值约在埋深0.3 m截面位置处;碎石土含石量的提高,地基土水平抗力会有所增大,桩侧土压力零点位置也会有所提高;m值随着含石量的提高而增大。含石量每提高10%,m值约增大1.15~1.40倍,该项研究可作为地基水平抗力系数的比例系数m值取值的一个参考。     

Numerical modelling of soil nails in loose fill slope under surcharge loading

A plane–strain numerical model has been developed to mimic a nailed loose fill slope under surcharge loading. The model has been used to back-analyse a field test that was conducted to examine the behaviour of soil nails in loose fill slopes under surcharge loading. Incremental elasto-plastic analyses coupled with pore water diffusion have been performed to study the internal deformation, water content redistribution in the soil, and the performance of the soil nails during and after the application of surcharge loading. The model parameters describing the mechanical and hydraulic properties of the nailed slope were obtained from field or laboratory tests. Different modelling techniques and boundary conditions for mimicking soil–nail interaction in loose fill material have been examined. Comparisons between numerical predictions and field measurements demonstrate that a new interfacial model, denoted as the embedded bond–slip interface model, is more suitable for mimicking the interfacial behaviour. Despite the simplicity of the numerical model, the predicted responses are in close agreement with the field test results, in particular the mobilisation and distribution of nail forces in response to surcharge loading. Both the numerical and the field test results suggest that soil nails are capable of increasing the overall stability of a loose fill slope for the loading conditions considered in this study. The increase in confining stress along the soil nails near the surcharge area is central to the overall stabilising mechanism. On the contrary, the nail forces mobilised near the nail heads are much smaller, indicating that the beneficial effect of having a structural grillage system at the slope face is limited for the range of surcharge pressures considered in this study.     

Axi-symmetric Analysis of Geosynthetic-reinforced Granular Fill-soft Soil System with Group of Stone Columns

The paper presents a mechanical model to predict the behavior of geosynthetic-reinforced granular fill resting over soft soil improved with group of stone columns subjected to circular or axi-symmetric loading. The saturated soft soil has been idealized by spring-dashpot system. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement layer, respectively. The stone columns are idealized by stiffer springs. The nonlinear behavior of granular fill and soft soil is considered. Consolidation of the soft soil due to inclusion of stone columns has also been included in the model. The results obtained by using the present model when compared with the reported results obtained from laboratory model tests shows very good agreement. The effectiveness of geosynthetic reinforcement to reduce the maximum and differential settlement and transfer the stress from soft soil to stone columns is highlighted. It is observed that the reduction of settlement and stress transfer process are greatly influenced by stiffness and spacing of the stone columns. It has been further observed that for both geosynthetic-reinforced and unreinforced cases, the maximum settlement does not change if the ratio between spacing and diameter of stone columns is greater than 4.     

土石混合料剪切特性影响因素的离散元数值研究

土石混合料剪切特性控制着高填方边坡的稳定性。土石混合料特殊的结构、物质及粒径组成等极其复杂,为探究土石混合料剪切特性影响因素及其规律,在室内试验基础上,基于PFC2D构建了颗粒离散元数值模型,分析了颗粒级配、初始孔隙率、块石尺寸及块石形状等因素对土石混合料剪切特性的影响。结果表明:土石混合料的剪应力-剪切位移曲线主要包括4个阶段:弹性变形、局部剪切、剪切破坏以及残余变形;块石形状、颗粒级配及初始孔隙率对土石混合料的破坏模式无明显影响,破坏模式均为应变软化型;当土石混合料的颗粒级配较差时,剪应力随剪切位移的增加出现较大波动,曲线出现明显"跳跃"现象;当含石量一定时,相同法向应力条件下,块石尺寸越大,土石混合料的抗剪强度越大,且随着法向应力的增大,不同块石尺寸的试验组之间抗剪强度差值也越大,块石尺寸对土石混合料抗剪强度的影响越明显。     

滑带土原位直剪试验条件的不确定性及基于BP网络的抗剪强度参数估计

滑带土原位直剪试点的物质构成、含水率、应力历史均具有随机性,预剪面边界约束、恢复初始固结状态的程度、剪切过程中的排水条件等因素则具有模糊性,这些不确定性因素导致试验成果离散、代表性受到局限。以原位直剪强度参数以及破坏面现场定性地质描述、含水率测试、颗粒分析结果训练BP网络,利用BP网络的高度容错性和强大的非线性映射能力,估计滑带土的抗剪强度参数,可在一定程度上可克服试验成果离散的影响。     

Shear deformation and strength of the interphase between the soil–rock mixture and the benched bedrock slope surface

A series of benched excavations were typically carried out on the bedrock slope surface to improve the stability of the soil–rock mixture (S–RM) fill slope. It is difficult to devise an in situ, large-scale direct shear test for the interphase between the S–RM fill and the benched bedrock slope surface. This study introduced a comprehensive approach to investigate the shear deformation and strength of the interphase. First the soil–rock distribution characteristics were analyzed by test pitting, image analysis, and sieve test. Then the PFC2D random structure models with different rock block size distributions were built, and large-scale numerical shear tests for the interphase were performed after calibrating model parameters through laboratory tests. The stress evolution, damage evolution and failure, deformation localization (based on a principle proposed in this paper), rotation of rock blocks, and shear strength were systematically investigated. It was found that as the rock block proportion and rock block size (rock block proportion of 50 %) increase, the fluctuations of the post-peak shear stress–displacement curves of the interphase become more obvious, and the shear band/localized failure path network becomes wider. Generally, smaller rock blocks are of greater rotation angles in the shear band. The peak shear stress and internal friction angle of the interphase increase, while the cohesion decreases with growth of the rock block proportion. However, all these three parameters increase as the rock block size (rock block proportion of 50 %) increases.     

Response of multilayer geosynthetic-reinforced bed resting on soft soil with stone columns

In the present study, a mechanical model has been developed to study the behavior of multilayer geosynthetic-reinforced granular fill over stone column-reinforced soft soil. The granular fill and geosynthetic reinforcement layers have been idealized by Pasternak shear layer and rough elastic membranes, respectively. The Kelvin–Voight model has been used to represent the time-dependent behavior of saturated soft soil. The stone columns are idealized by stiffer springs and assumed to be linearly elastic. The nonlinear behavior of the soft soil and granular fill is considered. The effect of consolidation of soft soil due to inclusion of the stone columns on settlement response has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution and results are presented in nondimensional form. It has been observed that if the soft soil is improved with stone columns, the multilayer reinforcement system is less effective as compared to single layer reinforcement to reduce the total settlement as there is considerable reduction in the total settlement due to stone column itself. Multilayer reinforcement system is effective for reducing the total settlement when stone columns are not used. However, multilayer reinforcement system is effective to transfer the stress from soil to stone column. The differential settlement is also slightly reduced due to application of multiple geosynthetic layers as compared to the single layer reinforcement system.     

Nonlinear Behaviour of an Embankment on Floating Stone Columns

This paper describes a benchmark problem for a floating stone columns supported embankment. In the 3D model, a thick soft subsoil deposit represented by Singapore marine clay was treated by floating stone columns to support a 2.0 m high embankment. The 3D numerical results were compared among different constitutive models such as the hardening soil model, soft soil model, modified Cam-clay model and Mohr-Coulomb model, highlighting the effect of non-linearity in the overall performance of the ground structure. The advanced constitutive soil models allow more realistic soil response to be investigated. From the numerical results, the Mohr-Coulomb model with elastic-perfectly plastic stress-strain response was able to give comparable total settlements but failed to demonstrate some intrinsic behaviours obtained with advanced soil models. These intrinsic behaviours include the mode of failure, displacements profile, stress transfer mechanism and rate of consolidation. This benchmark example has given us an improved understanding of the performance of floating stone columns when a more realistic soil behaviour is modelled.