地下连续墙施工影响应力重分布的数值模拟

对武汉市某超大型超深基坑10幅邻近地连墙跳跃式施工过程进行了三维有限差分数值模拟。数值模拟步骤依次为泥浆护壁成槽开挖、混凝土浇筑及混凝土硬化3个过程。泥浆护壁成槽开挖及混凝土浇筑分别采用常静液压力和变静液压力的方式加载,混凝土的硬化过程采用变弹性模量和泊松比的线弹性实体单元完成。数值计算结果与实测数据吻合较好。对单个跳跃式开挖过程墙上土压力的监测揭示了地下连续墙施工影响应力重分布的变化规律。模拟施工完成后10幅地下连续墙上的土压力值沿墙长度方向随静止土压力值上下波动,波谷出现在槽段连接处附近,波峰接近槽段中心轴,波动幅度大小与土体深度有关。分析表明,泥浆压力、混凝土灌注压力及土压力差值是影响墙后应力重分布波动幅度的主要原因,适当的泥浆重度及合理的注浆方式能避免土体扰动。     

超深地连墙槽壁侧压力演变模式及其施工扰动分析

地连墙施工扰动的精细分析对预测基坑开挖的环境影响十分重要,尤其是超深基坑。因此,收集分析了某102 m超深地连墙施工过程中的泥浆压力和混凝土压力现场实测数据,总结了槽壁侧压力的演变规律和竖向分布模式,提出了混凝土浇筑过程中槽壁侧压力的三折线模型并验证。三折线模型可以再现槽壁侧压力先增大后减小的趋势,并可退化为双折线模型。最后采用三折线模型建立了精细化数值模型,模拟了百米地连墙的成槽开挖及混凝土浇筑过程,分析了连续墙施工对槽段周围土体应力与变形的影响,并将计算结果与双折线模型结果进行了对比。结果表明：超深地下连续墙施工会引起周围土体的应力重分布,其影响范围在沿槽段方向为1.6倍槽段长度,在垂直槽段方向为4.3倍槽段长度;土体应力重分布有沿竖向和水平向传递两种机制,且以水平向传递为主。在上海软土地区,地下连续墙混凝土的浇筑会对槽壁产生挤压作用,引起槽段体积增大,进而导致混凝土浇筑量的增大。     

框格式地下连续墙非线性分析及稳定计算

根据地质及结构特点,建立了能反映框格式地下连续墙力学特性的典型子模型,即槽段有限元模型。基于ABAQUS有限元软件,采用经典的摩尔-库仑（Mohr-Coulomb）屈服准则、非关联流动法则,对框格式混凝土地连墙槽段进行了施工全过程模拟,详细分析了结构在最不利工况下混凝土连续墙的应力与变形,还用强度折减有限元方法分析了该最不利工况下结构的整体稳定性,论证了框格式连续墙这种地基处理方式的合理性与可行性,地下连续墙在应力变形及稳定方面能满足工程需要。     

西安地铁隧道穿越饱和软黄土地段的地表沉降监测

以西安地铁一号线朝阳门站一康复路站区段饱和软黄土地铁隧道为研究对象,通过施工期现场地表沉降变形监测,分析了在饱和软黄土特殊地层条件下隧道浅埋暗挖法施工引起的该区段地表沉降变形规律以及地表沉降槽分布特征。结果表明：在饱和软黄土隧道开挖时,随着掌子面的推进,隧道顶地表沉降可分为沉降微小阶段、沉降显著发展阶段、沉降缓慢阶段和沉降稳定阶段;单线隧道开挖后的最大地表沉降量为18．89mm,双线隧道开挖后的最大地表沉降量为36．4mm;已开挖隧道对围岩土体的扰动作用使得后开挖隧道的地表沉降发展较大;双线隧道的地表沉降槽宽度接近单线隧道沉降槽宽度的2倍,因此可以将其近似为单线隧道地表沉降槽宽度与双线隧道轴线中点距离之和;单线隧道开挖后地表沉降槽宽度为8．4～9．3m,双线隧道开挖后地表沉降槽宽度为16．2～17．5m;隧道开挖施工的沉降槽宽度参数为0．435～0．467,单线隧道开挖后的地层损失率为0．765％～1．324％,双线隧道开挖后的地层损失率为1．231％～2．200％。     

河湖相软土区某深基坑开挖变形特征分析

为研究在昆明市河湖相软土地区的复杂地质条件下，基坑开挖施工过程中的变形特征，以昆明市某地铁站深基坑的工程实例为背景，结合基坑开挖施工过程中支护结构及周围土体变形监测数据，运用MIDAS/GTS NX有限元软件，建立三维模型进行全过程整体的数值模拟分析，对比数值模拟结果和监测数据。结果表明：数值模拟结果与监测数据相比，两者结果差值较小，变化趋势基本一致，验证了有限元数值模拟软件在软土地区深基坑工程中运用具有可行性；地连墙顶竖向位移和墙顶附近土体沉降受基坑坑底软土隆起的因素影响较大；各监测点变形均小于控制值；基坑周边土体沉降和地连墙体变形符合基坑开挖变形规律，验证了基坑设计支护的合理性。研究结果可为昆明河湖相软土地区基坑工程提供经验借鉴。     

PLAXIS数值模拟在深基坑地连墙支护中的应用

应用PLAXIS建立了深基坑地连墙支护模拟模型,该模型可对地连墙支护结构,进行开挖支护施工过程的动态模拟分析。通过现场对基坑水平位移、竖向位移、锚杆内力等进行监测并得到相应数据。将模拟模型与现场观测数据进行对比分析,说明建立的数值模拟分析模型,能够反映地连墙支护的基坑,在开挖过程中的受力和变形特征。这也为深基坑地连墙支护技术的设计和施工提供指导,并对深入了解地连墙支护作用的机理有较大意义。     

机械预切槽法开挖软土隧道地层变形研究

软土中的机械预切槽法是超前支护的一种，在预筑拱的保护下可以安全高效地开挖隧道。在隧道力学和弹塑性理论的基础上，利用自行开发的三维程序，分析了机械预切槽法与常规全断面法开挖隧道时引起的地层变形。计算结果显示：机械预切槽法形成的预支护作用在控制地层变形方面较常规全断面法具有明显的优势，预切槽法可以减少隧道施工对地层变形的影响范围和影响程度，可以用于对地表沉降控制较严格地区的隧道施工。     

软土地层基坑开挖扰动及土体再压缩变形研究

软土地层基坑开挖会对坑底土体产生严重扰动,受开挖扰动影响,坑底土体应力状态和力学性质将发生变化。因此,正确评价开挖扰动程度及扰动对土体工程性质的影响十分重要。通过对现有施工扰动评价方法的总结,以太湖隧道基坑工程为例,采用有限元模拟方法研究了不同开挖深度下坑底中心土体扰动度分布规律及强扰动区深度。进一步,以不排水抗剪强度为评价指标,建立了基于孔压静力触探(CPTU)锥尖阻力的黏性土开挖扰动评价方法,并与有限元扰动评价结果进行了对比,取得了较为一致的结果。最后,采用考虑土体扰动的沉降计算方法,结合有限元扰动度计算结果,对不同基底附加应力下坑底扰动土体的沉降变形进行了计算。计算结果表明,扰动会显著增加地基沉降量,当基底附加应力从100 kPa增加至150 kPa时,考虑土体扰动的地基沉降量与不考虑土体扰动的地基沉降量比值将从1.43增加至2.24。     

顶管工程中的地层损失参数和土体变形计算

目前的地层损失参数和土体变形计算方法不能反映顶管施工的动态过程,重新定义顶管施工过程中的地层损失参数,使之可以反映超挖、欠挖等不同工况。基于Mindlin解,改进间隙参数g,从而可以考虑管壁与土的摩擦力。基于开挖面周围土体的扰动分区,修正Loganathan土体变形公式,并将改进的间隙参数g代入修正公式,计算顶进距离不同时的土体变形,获得顶管推进过程中地表测点的动态变化。算例分析表明,计算结果与监测数据吻合较好。     

软土地层矩形顶管掘进引起地表隆沉变形分析

准确预测并及时控制软土地层矩形顶管掘进过程中引起的地表隆沉,可有效降低掘进施工对紧邻结构设施的影响。结合弹性力学Mindlin解和随机介质理论,进一步考虑顶管开挖面附加推力、非均匀分布且具有软化特性的机体-土体侧摩阻力、受触变泥浆特性影响的管节与土体间的侧摩阻力,管节附加注浆压力及基于开挖面收敛模式的土体损失共同作用,推导得到矩形顶管掘进期间地表隆沉位移解析解。经与3个工程算例的实测结果进行对比分析,发现所提方法可预测矩形顶管在软土地层掘进引起的地表隆沉变形规律。分析结果表明：顶管开挖面前方地表表现为隆起;随着顶管开挖面的远离,摩阻力、注浆压力对地表的影响逐渐减小,开挖面后方地表主要受土体损失作用发生沉降;土体损失引起的地表沉降量受开挖面收敛模式影响。     

Displacements induced by the installation of diaphragm panels

The walls of a deep excavation in cohesionless soils below the water table have been supported by a reinforced concrete diaphragm with T-shaped panels. To improve the safety against the risk of local collapse during the panel excavation, the soil surrounding the panels has been treated by deep mixing to a depth of 6?C10?m. The horizontal displacements, induced in the surrounding soil by the installation (deep mixing, slurry supported excavation, placing of the reinforcement cage, concrete casting and curing) of the diaphragm, have been measured by means of inclinometers. It is claimed that they can be a significant fraction of the total displacements induced by the excavation. A back analysis of the observed displacements shows that the deformation process is essentially elastic and can be satisfactorily modelled provided the values of the soil stiffness are properly selected.     

Modelling of earth and water pressure development during diaphragm wall construction in soft clay

The influence of a diaphragm wall construction on the stress field in a soft clayey soil is investigated by the use of a three‐dimensional FE‐model of seven adjacent wall panels. The installation procedure comprises the excavation and the subsequent pouring of each panel taking into account the increasing stiffness of the placed fresh concrete. The soft clay deposit is described by a visco‐hypoplastic constitutive model considering the rheological properties and the small‐strain stiffness of the soil. The construction process considerably affects the effective earth and pore water pressures adjacent to the wall. Due to concreting, a high excess pore water pressure arises, which dissipates during the following construction steps. The earth pressure finally shows an oscillating, distinct three‐dimensional distribution along the retaining wall which depends on the installation sequence of the panels and the difference between the fresh concrete pressure and the total horizontal earth pressure at rest. In comparison to FE‐calculations adopting the earth pressure at rest as initial condition, greater wall deflections and surface ground settlements during the subsequent pit excavation can be expected, as the average stress level especially in the upper half of the wall is increased by the construction procedure of the retaining structure. Copyright © 2004 John Wiley & Sons, Ltd.     

考虑土体硬化的基坑开挖性状及隆起稳定性分析

基坑开挖过程中,土体应力路径、卸载回弹再压缩特性与简单加载或卸载不同,采用常规的理想弹塑性模型模拟基坑开挖,得到的围护墙位移、坑内土体回弹以及坑外沉降较大。分析了基坑开挖不同区域土体的性状,采用土体硬化模型模拟基坑开挖的卸载与土体硬化行为,结合工程算例,对比土体硬化模型和理想弹塑性模拟以及实测的围护结构土压力、围护墙水平位移和坑外土体沉降,并利用强度折减法分析基坑的稳定性。计算结果表明,考虑土体硬化的HS模型有限元方法能体现土体卸载再加载与开挖的特性,所得土压力、围护结构水平位移以及基坑抗隆起稳定性符合软土地区基坑工程的实践。     

Influence of diaphragm wall installation on the numerical analysis of deep excavation

This paper presents a numerical analysis of the influence of initial stress state on the response of deep excavation supported by retaining wall. Indeed, the influence of diaphragm wall installation prior to excavation works may affect the soil response and lateral wall deflection induced by excavation process. The first part of this paper gives a short review of the numerical methods aimed to reproduce the retaining wall installation. Numerical analysis of a deep excavation in two‐dimensional and three‐dimensional conditions is then performed according to the methods previously presented. In three‐dimensional conditions, diaphragm wall installation is performed considering a sequence of panels, described by their number and length. Results of three‐dimensional calculations confirm that stress state is disturbed by wall installation, which has a sensitive effect on the ground response induced by soil excavation. It is also noted that these results are not easily reproduced in two‐dimensional conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Efficiency of excavations with buttress walls in reducing the deflection of the diaphragm wall

Installation of buttress walls against diaphragm walls has been used as an alternative measure for the protection of adjacent buildings during excavation, but their mechanism in reducing movements has not yet been fully understood. This study performs three-dimensional finite element analyses of two excavation case histories, one in clay with T-shape buttress walls and another in dominant sand with rectangular buttress walls, to establish analysis model. Then, a series of parametric study were performed by varying soil types, types and length of buttress walls based on the above-mentioned excavations. Results show that the mechanism of buttress walls in reducing wall deflections mainly came from the frictional resistance between the side surface of buttress wall and adjacent soil rather than from the combined bending stiffness from diaphragm and buttress walls. The buttress wall with a length <2.0 m had a poor effect in reducing the wall deflection because the soil adjacent to the buttress wall had almost the same amount of movement as the buttress wall, causing the frictional resistance little mobilized. Since the frictional resistance of buttress walls in a deep excavation has fully been mobilized prior to the final excavation depth, the efficiency of buttress walls in reducing the wall deflection in a deep excavation was much less than that in a shallow excavation. Rectangular shape of buttress walls was of a better effect than T-shape in the shallow excavation because frictional resistance between buttress walls and adjacent soil played a major role in reducing the wall deflection rather than bearing resistance of the flange. When the excavation went deeper, the difference in reducing the wall deflection between the R-shape and T-shape became small.     

港口大型圆形地下连续墙施工技术

日照港石臼港区东煤南移项目翻车机房围护结构采用圆形地下连续墙。文章阐述了在吹填地层港口环境下,针对圆形地连墙厚砂层坍塌、内径小且弧度大、圆弧形钢筋笼、偏槽等难点问题,采取了场地加固、设置导墙外扩角、改进开挖方法、特制加工平台、多方法纠偏等处理措施,取得了较好的效果,为类似工程提供一定的参考。     

Three-dimensional numerical analysis of deep excavations with cross walls

Previous plane strain analysis of a case history has shown that cross walls in an excavation can effectively reduce movements induced by deep excavation. This study performed three-dimensional numerical analyses for 4 deep excavation cases with different installations of cross walls, including different excavation depths, cross wall intervals and cross wall depths. Both the observed and computed wall deflections for the 4 cases were compared with those of the same excavations that were assumed with no cross walls installed to demonstrate the effectiveness of cross walls in reducing lateral wall deflections. The results show that the cross wall also had a corner effect similar to that of the diaphragm wall. The deflection of the diaphragm wall was smallest at the location of the cross wall installed and then increased with the increasing distance from the cross wall, up to the midpoint between two cross walls. Many factors such as in situ soil properties, diaphragm wall properties, construction procedure, cross wall depth and so on may affect the amount of reduction in lateral wall deflections due to the installation of cross walls. Under the same condition, the amount of reduction was highly dependent on the depth of cross walls, distance to the cross walls and the cross wall interval.     

粉质砂土土钉墙水平位移与土钉轴力的FLAC3D研究

以物理模型土钉墙的破坏性试验获得的土体的基本参数和模型的尺寸为基础,应用FLAC3D软件建立土钉墙数值模拟模型。通过数值模型模拟基坑开挖与支护过程,并监测该过程中的土钉墙墙体沿深度方向水平位移情况和各层土钉的轴力变化情况以及它们之间的关系。支护过程结束后,在墙顶分级施加竖向荷载直至墙体产生较大变形,研究了土钉墙在超载状况下的工作状况以及破坏过程,并与物理模型土钉墙的破坏性试验结果进行对比。研究发现,开挖过程中墙体水平位移底部大于顶部,呈“勺形”分布;墙体水平位移最大处附近的土钉轴力也最大;粉质砂土土钉墙变形超过基坑开挖深度的4‰后,墙体的稳定性会极大降低;粉质砂土土钉墙没有下卧软弱层时,在地面超载作用下其破坏形式为体内破坏,表现为部分土体沿滑裂面向下滑动。     

双矩形钢模管互导干作业地下连续墙

地下连续墙常采用泥浆护壁、水下灌注混凝土进行施工,槽壁稳定和槽段接头防渗一直是施工控制的难点。泥浆本身易于造成环境污染,而泥浆制备的水耗和废料处置也增加了工程造价。为解决这些问题,开发了一种新技术——双模管互导干作业地下连续墙技术。该技术采用钢模管护壁和双矩形模管导向沉管挤土或沉管干取土的成墙工艺（分别适用于墙厚300～500mm和600～1 000mm）,不需要泥浆和水下灌注墙混凝土,可以在同一平面内成墙、实现墙单元无缝连接。实践表明,该技术墙体防渗性能好,成墙效率高,连接质量可靠,不仅适用于深厚软粘土地层的基坑围护工程,也可应用于类似地质条件下水库工程的防渗加固。