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.     

饱和黏土不排水剪切的热破坏

试验资料表明：饱和黏土在不排水常载升温条件下会产生很大的变形并最终可发生热破坏现象,因此,升温也成为一种加载方式。基于姚仰平等提出的热UH临界状态模型,推导了受温度影响的热不排水抗剪强度表达式,对不同超固结度饱和黏土在升温下的不排水抗剪强度模拟与试验结果对比表明：所提公式能够合理反映试验资料中受温度影响的不排水抗剪强度变化规律。利用热UH模型对不排水常载-升温过程进行了模拟,模拟结果表明,热UH模型可合理地反映不排水常载-升温条件下饱和黏土的应力-应变关系以及强度变化规律。针对能影响热破坏过程的几种因素如升温初始时的偏应力比、升温幅度、超固结度以及先期固结压力等进行了讨论分析,得到了各因素对于饱和黏土不排水剪切下的应力-应变关系及强度特性的影响规律。     

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.     

Effectiveness of deep cement mixing walls with top-down construction for deep excavations in soft clay: case study and 3D simulation

This paper presents the observed and simulated effectiveness of deep cement mixing walls created using top-down (DCM-TD) construction techniques for a deep excavation in soft Bangkok clay. The wall system consisted of four rows of 0.7-m-diameter DCM columns, and the bracing system consisted of two 0.25-m-thick basement slabs and seven temporary struts. The effectiveness of the wall system compared to that of other wall systems was evaluated using the measured results of previous case studies. A 3D numerical analysis was performed to calculate forces in the basement slabs and bending moments in the DCM wall. Finally, series of parametric analyses of both DCM-TD and deep cement mixing walls created using bottom-up (DCM-BU) construction techniques were carried out, and their results were compared to highlight the effectiveness of DCM-TD and its applicability to excavations at greater depths. The field and numerical results show that DCM-TD is more effective than DCM-BU in terms of the limitations of lateral wall movement, the bending moment in a DCM wall and the thickness of a DCM wall for various depths because of a larger system stiffness. Therefore, DCM-TD is very effective and suitable for use in potential future deep excavations in urban areas.

     

Factors affecting the stability of deep excavations in clay with consideration of a full elastoplastic support system

Acta Geotechnica - This study investigates the stability of internally braced excavations in thick, saturated clay using a finite element method with reduced shear strength. The support system was...     

软土地区深基坑连续墙最大变形简易计算方法

采用有限单元法研究了影响软土地区地下连续墙最大侧向变形的主要参数。针对基坑开挖深度H、基坑开挖宽度B、单位宽度地下连续墙系统刚度S、支撑结构的轴向刚度 及黏土归一化的不排水抗剪强度 为不排水抗剪强度, 为有效垂直应力）5个参数进行分析研究,通过回归分析研究结果,给出地下连续墙最大侧向变形的简易计算方法。利用简易计算方法,计算实际工程中基坑案例的地下连续墙最大侧向变形,并与现场监测结果进行对比,验证了计算方法的准确性,可为以后预估地下连续墙最大侧向变形及检查设计提供参考。     

A numerical exercise for the definition under undrained conditions of the deep tunnel front characteristic curve

In spite of the increasing diffusion of tunnel boring machines, conventional tunnelling is still largely employed in the excavation of both deep and shallow tunnels characterized by a particularly irregular tracing. Under difficult ground conditions, in conventional tunnelling, the front is frequently reinforced by using fibreglass tubes partially removed during the excavation. This technique is expensive, time-consuming and its design is based on either empirical or very simplified theoretical formulas. Thus, the ultimate objective of the research developed by the authors is to introduce a more sophisticated design approach for this front reinforcement technique. A first step in this direction is this numerical study, in which the mechanical response of deep tunnel faces under undrained conditions is analysed by employing the front characteristic curve: a useful tool largely employed in the literature in analogy with what done for the cavity. The main result of this paper is the “Front Mother Characteristic” curve, obtained by introducing appropriate non-dimensional variables, allowing the designer, once both the system geometry and the soil mechanical properties are assigned, to assess the displacements of tunnel fronts without performing any numerical analysis.     

A novel strut-free retaining wall system for deep excavation in soft clay: numerical study

This paper presents a novel strut-free earth retaining wall system for excavation in soft clay, referred to as the rigid and fixed diaphragm (RFD) wall retaining system. The RFD system is comprised of four main structures—diaphragm walls, rib-walls, cross walls, and buttress walls—and a complementary structure—the cap-slab. The characteristics of the RFD system are: (1) the formation of a continuous earth retaining wall by constructing diaphragm walls along the circumference of the excavated zone; (2) the formation of a rigid and fixed retaining wall system by a series of rib-walls and cross walls; and (3) the formation of a rigid retaining wall by buttress walls and the cap-slab. Furthermore, the performance and mechanisms of the RFD system were investigated carefully through three-dimensional finite element analyses. The results demonstrated that the system stiffness of the RFD system was a major factor controlling deformations induced by excavation. Moreover, the excavation geometry determined the dimension of each component of the RFD system.

     

A simplified analysis approach for the effect of the installation of adjacent XCC pile on the existing single XCC pile in undrained clay

Acta Geotechnica - In this paper, a simplified two-stage analysis method is introduced to predict the lateral response of the existing X-section Cast-in-place Concrete (XCC) pile caused by adjacent...     

Deep tunnel fronts in cohesive soils under undrained conditions: a displacement-based approach for the design of fibreglass reinforcements

The fronts of tunnels excavated under particularly difficult ground conditions by employing conventional tunnelling methods are commonly supported: the stabilization is usually achieved either by improving the mechanical properties of the soil (injections, jet grouting, soil freezing, etc.) or by introducing linear inclusions. This last technique, consisting in the introduction of pipes (usually made of fibreglass reinforced polymers) in the front, is particularly popular since it is very simple to adapt the reinforcement geometry, length and number to the different conditions encountered during the excavation. The design of this reinforcement technique is nowadays based on very simplified approaches: on either empirical formula or the limit equilibrium method. In a previous paper, the authors numerically studied the mechanical response of unreinforced fronts in cohesive soils and defined a non-dimensional front characteristic curve. In this paper, the authors intend to take into consideration the role of reinforcements by following the same approach. A procedure allowing the definition of the reinforced non-dimensional front characteristic curve, once the reinforcement pattern is assigned, is introduced. The practical use of this curve is described.

     

敏感环境下基坑数值分析中土体本构模型的选择

数值分析已成为敏感环境下基坑工程分析的最重要手段,其关键是选择合适的土体本构模型和计算参数。在分析了岩土数值分析中常用土体本构模型特点的基础上,通过算例较系统地对比了各类模型在基坑开挖数值分析中的适用性。敏感环境下的基坑工程需重点关注墙后土体的变形,从满足工程需要和方便实用的角度出发,建议采用能考虑黏土的塑性和应变硬化特征、能区分加荷和卸荷且刚度依赖于应力水平的硬化类弹塑性模型,如MCC模型和HS模型进行分析。具体工程实例的分析,表明了硬化类弹塑性模型在敏感环境下基坑开挖数值分析中的适用性。     

Thermo-poro-mechanics under adsorption applied to the anomalous thermal pressurization of water in undrained clays

Acta Geotechnica - Pore fluid pressurization, one of the main causes of soil instability, is known to be anomalously high for interstitial water in clay submitted to undrained heating. This anomaly...     

A novel approach to estimate the variations in stresses and fault state due to depletion of reservoirs

Geomechanical changes may occur in reservoirs due to production from reservoirs. Study of these changes has an important role in upcoming operations. Frictional equilibrium is one of the items that should be determined during the depletion as it may vary with respect to time. Pre-existing faults and fractures will slide in regions where there is no frictional equilibrium. Hence, it may be concluded that reduction in pore pressure can initiate the sliding of faults. Whereas, it is also possible that faults will not exist after a certain time as production recovers the equilibrium. Casing shearing or lost circulation may be occurred due to faulting. In reservoirs which depletion leads to frictional equilibrium, decrease of fractures and faults leads to some variations in permeability. Hence, predicting the effect of depletion on frictional equilibrium is required in dealing with casing shearing or lost circulation in drilling of new wells. In addition, permeability modeling will be more precise during the life of reservoirs. Estimation of necessary time to create or vanish faults is vital to be successful in production from wells or drilling new wells. Achieving or loosing of equilibrium mainly depends on in situ stresses and rate of production. Estimation of the in situ stresses at the initiation state of reservoir is the key to study the state of faults. The next step is to predict the effects of depletion on in situ stresses. Different models are suggested in which decrease of horizontal stresses is predicted as function of pore pressure variation. In these models, different assumptions are made such as simplifying the poroelastic theory, ignoring the passing time, and considering the geometry of reservoir. In this article, a new model is proposed based on theory of inclusions and boundary element method. This state-of-the-art model considers the geometry of reservoir. In addition, changes of in situ are obtained as a function of time to reach to a more precise model capable of applying during the reservoir life. Finally, the model is imposed on real cases. The effect of depletion on faults is studied in reservoirs of normal and strike-slip stress regimes, and conventional and proposed models are compared. For this aim, in the first step, mechanical earth models of these two reservoirs are built using logging and coring data. Stress polygon method and poroelastic horizontal strain model are used for strike-slip and normal regimes, respectively. In reservoir 1 which is in a strike-slip stress regime, a fault is distinguished in formation microimaging (FMI) log. For this reservoir, the required time to achieve to frictional equilibrium is calculated. In the reservoir 2, the potential depth of fault sliding is analyzed and required time for faulting in that depth is predicted. The predicted time for satisfaction of frictional equilibrium using the proposed model has a significant difference with the predicted time using the previous methods. In addition, the proposed model predicts that different parts of reservoir 2 are willing for faulting during depletion. The previous model determines only one point that faulting may happen. It is necessary to use this new model to consider different important factors such as geometry and time to gain more accurate predictions.     

A lateral soil resistance model for XCC pile in soft clay considering the effect of the geometry of cross section

In this paper, a series of well-calibrated finite-element analyses are performed to quantify the influence of the geometry of cross section on the load transfer mechanism of X-section Cast-in-place Concrete (XCC) pile under lateral load, aiming to propose a lateral soil resistance model for XCC pile in soft clay. Based on the results of the numerical parametric analysis, the failure mechanism of soil flow and the ultimate lateral soil pressure are investigated to reveal the underlying mechanism that controls the cross-section geometry-dependency response. Finally, a general p-y formula for XCC pile, which can well capture the lateral behavior of XCC pile considering the various cross section geometries, is developed. In addition, compared with the traditional circular cross section pile with the same area, the XCC pile is more effective in terms of resistance to lateral load.

     

A review of the ANDRA''s research programmes on the thermo-hydromechanical behavior of clay in connection with the radioactive waste disposal project in deep geological formations

This paper presents the different studies realized or launched by ANDRA in collaboration with different contractors, including laboratory and in situ experiments, as well as physical and numerical modelizations, related to the thermo-hydromechanical behavior of clays and clayey materials. Clays are considered as both potential host rocks and sealing materials, among other geological formations and materials, respectively.

The study of a high-level or long-lived nuclear waste disposal concept is necessarily a step-by-step procedure, with two main objectives: on one hand, ensuring a good disposal design, including feasibility of the vaults and their stability during the operating phase with the eventual continuation of the phase of waste retrievability, i.e. on a long period of time not shorter than one hundred years. On the other hand, the objective is to preserve the long-term clay properties with regard to radionuclide behavior, in particular those properties which influence their containment capacity.

Swelling clays as an engineered barrier material have been considered by ANDRA for a long time. Buffer material as an interface between the nuclear waste packages and the geological barrier and tight core of dams in drifts and shafts are the two types of constructions for which the clay conditioning and performance have been investigated. In both of these applications, the understanding of the THM behavior of the swelling clay is required. In the first case the THM load parameters are much more severe and the geochemical imbalances are of greater consequences.

R & D programmes are necessarily integrated into the design process of the disposal concept. The results will allow selection between technical solutions estimated as valuable candidates for validation within the project duration, i.e. 13 years, and those solutions which cannot be. In return the design process indicates what are the more profitable axis for the future R & D efforts.     

A mass balance approach to estimate the dilution and removal of the pollutants in stream water polluted by acid mine drainage

 A few simple mass balance equations were developed to simultaneously estimate how much the pollutants from acid mine drainage (AMD) in stream water are diluted and removed during their migration. The application of the equations requires knowledge of the variations in the concentrations of the dissolved pollutants and the stoichiometry of the precipitation reaction of the pollutants when none of the pollutant shows a conservative behavior along the stream path. The calculation should be restricted to the pollutants showing much higher concentrations in the polluted main stream water than in the combining or diluting water of the same target area. The mass balance equations were applied to estimate the dilution factor and precipitation fractions of pollutants in Imgok Creek such as Fe, SO₄ and Al from the AMD of Yeongdong mine. The results show that the estimation, especially for SO₄ and Al, significantly depends on the kinds of the precipitates. When FeOHSO₄ and AlOHSO₄ are assumed to precipitate, the maximum removal fractions of SO₄ and Al by precipitation are respectively 34% and 46% of the original input, which is much higher than the values estimated when SO₄ is considered to be perfectly conservative. It indicates that the stoichiometry of precipitation reaction is very important in the interpretation of the pollutant dilution and migration and assessment of environmental impacts of AMD. The applicability of the mass balance equations may still need to be verified. However, examining the calculated dilution factor and precipitation fractions with the equations can provide invaluable information on not only the behavior but also unexpected input of the pollutants in the stream water polluted by AMD and other point sources. Received: 12 November 1997 · Accepted: 30 March 1998     

土钉墙+排桩在明挖隧道深基坑支护中的几个问题

就当前土钉墙+排桩在明挖隧道深基坑支护中的几个问题,开展了有针对性的研究。采用理论分析和数值计算方法,结合工程实例,深入探讨了土钉墙坡比i,排桩桩径-间距（D-S）,以及设有内支撑时,桩-撑相对刚度等参数的选取原则。结果表明：（1）i越小,土钉墙越稳定,但排桩弯矩、水平位移越大。在土钉墙自身滑动稳定前提下,i宜取大值。（2）D-S越大,排桩水平位移越小,排桩混凝土、钢筋量越少,造价越低,但桩间土的防护越困难。在工程地质好、地下水位低的情况下取大桩径、大间距,反之取小值。（3）弱桩强撑的支护效果好,但对施工干扰大;强桩（非悬臂）弱撑造价低。选择桩-撑的相对刚度需从工程地质、周边环境要求、工期、造价等因素综合考虑,方能达到安全可靠,经济合理的目的。     

Use of Regional Geochemical Survey Data and exploratory statistics to estimate natural condition concentrations for trace metals in stream sediments: a case study for the Cook Inlet watershed,Alaska

The United States and other nations use the term natural conditions or reference conditions in legislation providing for the protection of aquatic ecosystems. Natural or reference conditions are generally used to describe environmental conditions in an aquatic resource before any human introduced pollution. The natural or reference conditions are used in establishing water quality criteria for protection of aquatic resources, helping classify aquatic resources within a regulatory category, and for assessing effectiveness of aquatic resource restoration actions. Regional scale geochemical mapping of stream sediments is available for many countries that when combined with exploratory data analysis and geospatial data can be used to establish screening level natural conditions for trace metals in stream sediments. This approach is applied to the Cook Inlet watershed in Alaska using the US Geological Survey National Geochemical Survey stream sediment trace metals data. Upper concentration limits are developed for As, Cr, Cu, Pb, Hg, Ni, and Zn in the Cook Inlet watershed to aid in screening sediments for potential anthropogenic contamination.     

Geomechanical and geochemical effects on sandstones caused by the reaction with supercritical CO2: an experimental approach to in situ conditions in deep geological reservoirs

The geochemical and geomechanical behaviour of reservoir rocks from deep saline aquifers during the injection and geological storage of CO₂ is studied in laboratory experiments. A combination of geochemical and geomechanical studies was carried out on various sandstones from the North German Basin. After the mineralogical, geochemical and petrophysical characterization, a set of sandstone samples was exposed to supercritical (sc)CO₂ and brine for 2–4 weeks in an autoclave system. One sample was mineralogically and geochemically characterised and then loaded in a triaxial cell under in situ pressure and temperature conditions to study the changes of the geomechanical rock properties. After treatment in the autoclaves, geochemical alterations mainly in the carbonate, but also in the sheet silicate cements as well as in single minerals of the sandstones were observed, affecting the rocks granular structure. In addition to partial solution effects during the geochemical experiments, small grains of secondary carbonate and other mineral precipitations were observed within the pore space of the treated sandstones. Results of additional geomechanical experiments with untreated samples show that the rock strength is influenced by the saturation degree, the confining pressure, the pore fluid pressure and temperature. The exposure to pure scCO₂ in the autoclave system induces reduced strength parameters, modified elastic deformation behaviour and changes of the effective porosity in comparison to untreated sandstone samples. Experimental results show that the volume of pore fluid fluxing into the pore space of the sandstones clearly depends on the saturation level of the sample.     

Modelling the behaviour of a retaining wall in residual soils for a cut and cover construction of a deep station in Metro do Porto

This paper presents a re-appreciation of the ground characterisation and the criteria to select the most representative geomechanical parameters to consider in a numerical model to predict the behaviour of a retaining wall of a deep excavation in highly weathered granite rock masses and residual soils. This study was focused in the construction of a deep station of Metro do Porto, which involved a cut and cover solution, with unusual proportions (in plant and in depth), built in the typical Oporto's granite weathered profiles, being the excavation conducted with retaining walls consisting of multi-anchored concrete piles. Specific sections were carefully instrumented, due to the presence of historic buildings in the vicinity. The definition of representative model parameters was based on precise laboratory tests over high quality soil samples, including oedometer and high-precision triaxial tests. Geotechnical and geological characterisation of all the area for the original design, was initially based on in situ tests, such as SPT and rock masses classification, and on the local experience on this type of ground. After this construction, the assumptions of parameterisation, using a constitutive modelling based on new laboratory tests over high quality block samples, allowed a reanalysis of the assumptions on the design phase. A back-analysis of monitored displacements and forces during the construction was made, assuming the designed structural solutions, which were in fact implemented in construction, but considering the new approaches on the definition of the geomechanical parameters for the prevailing weathered rock masses, necessary for the numerical simulation based on the commercial software Plaxis®, using the Mohr-Coulomb and “Hardening-Soil” models. Some specific changes of the constructive sequence during the excavation and activation of supporting system were attained, by looking at the information found during the construction. The results of this parametrical re-approach and analysis of the singularities of highly weathered granite and corresponding residual soils masses for modelling of retaining walls of large excavations are discussed.