LESSONS LEARNED FROM TWO INVESTIGATION CASES OF GROUND DISTRESSES DUE TO DEEP EXCAVATION IN FILLED GROUND
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Ground distresses such as ground settlement/subsidence, lateral movement, cracking, etc, are usually the main concern for any excavation project. The contributory factors of ground distresses could be from various aspects and sometimes are similar in nature. This paper presents the processes of geotechnical investigation, remedial design, construction monitoring for two case histories of ground distresses occurring on a retained platform due to excavation in filled ground. Desk study, site inspection and subsurface exploration have been deployed to reveal the evidences and identify probable causes of the distresses. Back analyses utilizing finite element computer program “Plaxis” proved useful to reveal the inherent mechanisms of ground distresses. Lessons learned from the investigations are documented as useful mementos for future projects of similar nature.Keywords:
Ground subsidence
Ground movement
Settlement (finance)
Common ground
Desk
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Inclinometer
Extensometer
Instrumentation
Settlement (finance)
Piezometer
Geotechnical investigation
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The survey of damaged engineering buildings is in many cases very demanding in terms of the selection of a right exploration method in relation to the results obtained for subsequent engineering works, time for survey implementation, and violations arising from survey activities. Heterogeneity of materials of a natural and anthropogenic origin is a fundamental axiom which can subsequently lead to either a distortion or a failure threatening statically the existence of a building structure. On the test object of a pavement, after some time of its use, severe deformations became evident whose causes and future evolution were not known. Within the design of survey techniques being able to quickly and efficiently uncover the causes of failures, the GPR (Ground Penetrating Radar) investigation was included which as an indirect, non-destructive survey method very quickly helped to clarify he causes of failures of the building structure.
Ground-Penetrating Radar
Massif
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Based on an engineering project, this paper initially establishes an observation point for foundation pit and then determines monitor warning value. During project construction, we carried out an experiment on the horizontal movement and settlement and inclination of adjacent buildings and promptly monitored the foundation pit., Scientific analysis of the data is presented. This work is designed to provide for effective measures to implement security alerts for foundation construction. Detailed analysis examines the causes of deformation of foundation pit and offers a reasonable treatment measure. Results offer some scientific basis and technical measures to guarantee deep foundation project construction security and more knowledgeable engineering construction. With the rapid development of urbanization in China, the deep excavation works requirehave been put forward strict demand regulations concerningdue to the requirements of the spatial location, structural stability and using function. Deep excavation engineering is mostly carried out in areas of heavy traffic and dense construction. The complexity associated with deep excavation depth and difficult construction creates environments where serious accidents can occur. The deep excavation work is a wide-ranging and integrated engineering process. Previous research on accidents in national deep foundation pit engineering found the general accident ratio was about 20% of that of the deep excavations work (Tang, 1997),. Most accidents in urban areas were caused by foundation pit support. In deep excavation engineering, both the strength and deformation of the supporting structure and the surrounding environment affected by pit deformation should be considered (Sun, 2006). The pit support systems are always temporary facilities with fewer safety considerations and more hazards. Working status and conditions are more complicated and uncertain. Thus, during the construction process, dynamic monitoring and control is very important. The content of deep excavation-site monitoring generally includes the horizontal displacement of supporting structure, tilt displacement of neighboring buildings, sedimentation of adjacent roads and so on. A monitoring crew should provide timely feedback information (Liu, 2006) to detect any problems and provide early warnings for reducing disasters. A monitoring program that provides critical information and manages deep excavation construction scientifically and effectively is the key to successful deep excavation construction (Liu et al., 2007).
Foundation (evidence)
Construction site safety
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Various ground improvements have been used to support embankments on soft ground. The success of the ground improvement works depends on many factors from planning, investigation, analysis, design, specification of works, construction and closed supervision by design consultants. Flaws in any of the above stages would compromise the effectiveness of a ground improvement. The failure of ground improvements can either be short-term ultimate limit state failure (e.g. slip failure and tension cracks) or long-term serviceability limit state problems (e.g excessive differential settlement). This paper presents three case histories of failures related to ground improvement works in soft ground, namely vacuum preloading with vertical drains, stone columns and piled supported embrankments. The causes of failures, remedial works proposed and lessons learned are discussed.
Serviceability (structure)
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Foundation (evidence)
Soil liquefaction
Dynamic compaction
Geotechnical investigation
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At present, there are few well-documented case studies of circular shaft construction, making it difficult for designers to estimate reliable ground movements arising from such construction. This paper describes field observations of ground surface settlement assembled during the construction of 27 circular shafts built for three major tunnelling projects in London: Crossrail, National Grid's London Power Tunnels project and Transport for London's Northern line extension. Two categories of shaft construction were identified: support before excavation (SBE) and excavation before support (EBS). For the SBE category, the shaft was first supported by pre-installed walls followed by excavation of the soil between the pre-installed walls. For the EBS category, the ground was progressively excavated in sections followed by construction of the shaft lining. Interpretation of the field observations showed the importance of the shaft construction method on ground movements. Settlements were much more significant for EBS shaft construction than for SBE shaft excavation, although settlement arising from the installation of pre-installed walls or dewatering operations should not be overlooked. Normalised charts are presented to help the industry make estimates of settlements due to circular shaft construction in London, with due consideration for different shaft geometries and construction methods.
Settlement (finance)
Dewatering
Human settlement
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The ground in which utility service pipelines are buried inevitably controls, to a large degree, the structural performance and progressive deterioration of the pipelines themselves. In a parallel programme of research to the UK Mapping the Underworld (MTU) project, a study of the fundamental properties of the ground, and how they change with the seasons and local physical and chemical contexts, is being conducted at the University of Birmingham, UK. While the results of this study feed into both the operational protocols for the MTU multi-sensor location device and the associated knowledge based system (KBS) that is being created to aid its deployment (both topics being the subjects of separate papers to this conference), the suite of complementary research projects on the ground and its properties provide valuable insights to the pipeline engineer. Geophysics is being used by the research team to explore the state of the ground with the aim of highlighting areas of concern for the structural health of pipelines buried in the ground. Studies of cast iron pipeline corrosion mechanisms have focussed on the changes that the reaction products cause to the surrounding soils, with a particular emphasis on clay soils, and one interesting finding is that these clay-iron reaction products can make the pipelines `invisible' to standard geophysical location devices. Moreover there are other features in the ground that are being targeted (voids, ground wetting and softening due to leakage, ground weakening due to progressive erosion), and these features effectively make the ground more or less `visible' to geophysical technologies. Alongside this work, bespoke tests have been developed for use on site to `calibrate' the geophysics, thereby enhancing the signatures of the features. This paper introduces these parallel research projects and draws out the important findings for pipeline engineers charged with establishing the condition of existing buried assets.
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Extreme ground behaviour in high-stress rock masses such as rockburst prone and squeezing ground conditions are encountered in a range of underground projects both in civil and mining applications. The occurrence of such ground behaviour types are difficult to predict and special design and construction measures must be taken to control them. Determining the most appropriate support system in such grounds is one of the major challenges for ground control engineers because there are many contributing factors to be considered, such as the rock mass parameters, the stress condition, the type and performance of the support systems, the condition of major geological structures and the size and geometry of the underground excavation. The main characteristics and support requirements of rockburst-prone and squeezing ground conditions are herein critically reviewed and characteristics of support functions are discussed. Different types of energy-absorbing rockbolts and other support elements applicable for ground support in burst-prone and squeezing grounds are introduced. Important differences in the choice and economics of ground support strategies in high-stress ground conditions between civil tunnels and mining excavations are discussed. Ground support benchmarking data and mitigation measures for mines and civil tunnels in burst-prone, squeezing and heavily swelling grounds conditions are briefly presented by some examples in practice.
Benchmarking
Rock burst
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Ground distresses such as ground settlement/subsidence, lateral movement, cracking, etc, are usually the main concern for any excavation project. The contributory factors of ground distresses could be from various aspects and sometimes are similar in nature. This paper presents the processes of geotechnical investigation, remedial design, construction monitoring for two case histories of ground distresses occurring on a retained platform due to excavation in filled ground. Desk study, site inspection and subsurface exploration have been deployed to reveal the evidences and identify probable causes of the distresses. Back analyses utilizing finite element computer program “Plaxis” proved useful to reveal the inherent mechanisms of ground distresses. Lessons learned from the investigations are documented as useful mementos for future projects of similar nature.
Ground subsidence
Ground movement
Common ground
Settlement (finance)
Desk
Cite
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During underground construction, the behavior of the ground is influenced by characteristics of the rock mass with situ stresses and ground water, cross section of the excavation area, excavation method, and the rate of excavation. These fundamental features are considered to ensure the support and stability of underground excavations and achieve long-term successful operation. However, the ground composition of the Himalayas hinders tunnel excavation, especially in case of mechanized tunneling; this causes time and cost overruns. This study has reviewed the recently completed Neelum–Jhelum Hydroelectric Project; the project complexities, geological environments involving significant overburden and tectonic stresses, and effects of the excavation method on tunnel stability were analyzed. The major challenges that were encountered during construction are discussed herein along with their countermeasures. An analysis of project-related data reveals that latest techniques and approaches considering rock mechanics were used to complete the project; the existing approaches and methods were accordingly verified and extended. Apart from ground composition, the excavation methods used play an important role in the occurrence of severe rock bursts. Thus, the findings of this study are expected to be helpful for future tunneling projects in the Himalayas.
Overburden
Hydroelectricity
Tunnel Construction
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