Correction of the Ground Subsidence Risk Ratings during Open Cut Excavation

Currently, design engineers and owners of utility systems are trying to utilize innovative methods for installing critical grade pipelines more economically, with minimal disruption to surface activities, and in compliance with environmental concerns. A new technology, the AXIS Vacuum Microtunneling (VMT) system, was developed as a trenchless solution for installation of on-grade pipelines. This paper demonstrates the advantages of VMT through the results of a field evaluation of a sewer project in a pristine residential neighborhood in Glenview, Illinois. Three separate installations of (250 mm) diameter PVC pipes at grades of 0.55% in sections of 176 ft (55 m), 278 ft (85 m), and 331 ft (100 m) in soft clay soil conditions were analyzed. The field analysis studied productivity and costs comparing VMT with traditional open-cut in an attempt to quantify the advantages of adopting this trenchless technology. A cost comparison determined a depth of 10 ft (3 m) as being the point at which VMT begins to be more cost-effective compared to traditional open-cut. Productivity averaged 25.89 ft/hr (7.92 m/hr) for the three VMT installations compared to approximately 18 ft/hr (5.5 m/hr) estimated for a similar open-cut option.

ABSTRACT: A stress-path testing program was proposed and performed to investigate shear strength and deformation behaviors due to an open cut excavation in overconsolidated clay. Stress changes due to the staged excavation for different locations of the slope were estimated and applied on undisturbed specimens using a triaxial testing system. Axial load, chamber pressure, and back pressure were actively controlled to implement the various stress paths. Vertical and horizontal deformations were monitored during the stress paths, and then an undrained compression or extension shearing stage followed. Test results for three cases from the testing program: the crest and the toe of the slope, and the base of the excavation are presented. The clay deposit was found to be non-expansive, based on the deformation behaviors. Effective stresses appeared to govern the vertical and horizontal strains. Failure envelopes for compression and extension were established from the stress path testing. The mobilized undrained shear strength from the stress path approach is substantially lower than one from the conventional method.

The design of open cut excavations in newly reclaimed land overlying soft soils is often highly sensitive to the undrained strength parameter values. Under such circumstances, the use of the Observational Method may introduce significant programme and quantity savings by allowing the designer to adopt best estimate, rather than moderately conservative parameters. The Hong Kong Boundary Crossing Facilities (HKBCF) is located on newly reclaimed land located to the East of Chek Lap Kok Island. The Passenger Clearance Building (PCB) is located in the middle of the HKBCF and provides the customs and immigration facilities for passengers entering Hong Kong from the Hong Kong-Zhuhai-Macau Bridge and the Tuen Mun-Chek Lap Kok Link. The construction of the PCB basement required an excavation of approximately 10 m depth over an area of approximately 200 by 200 m. The typical soil profile consists of 15 m of sand fill overlying 20 m of very soft to soft Marine Clay. Ground improvement with prefabricated drains and surcharge was adopted by the reclamation Contractor to improve the Marine Clay strength.

Installation of combined sewer overflow (CSO) consolidation conduits in difficult waterfront conditions would have been no easy task as originally designed and bid, using cut-and-cover methods, yet an aggressive Value Engineering (VE) proposal was accepted and the installation accomplished using pipe jacking methods. Five-hundred-and-ten (510) feet of conventional pipe jacking was successfully accomplished to install 54-inch and 30-inch consolidation conduits as part of the Narragansett Bay Commission's (NBC) CSO Control Facilities Program. This paper will present site conditions, a comparison of the original cut-and-cover design and the VE pipe jacking proposal, provide construction details and challenges. The conduits were installed in a narrow strip of land between historic waterfront structures and the Providence River with a portion of the alignment beneath high-tension towers and transmission lines. The ground conditions consisted of urban fill, a soft organic silt deposit, and abandoned power tunnels, slips and wharfs. Bid documents directed an open cut with an Owner-designed secant pile wall support of excavation and installation of reinforced concrete pipe (RCP) for the conduits, and the Contractor's proposed VE largely eliminated open cut and included pipe jacking with sheeted pits. The VE proposal was negotiated and the Contractor successfully completed the pipe jacking using an open-face shield, ground improvement using chemical grouting, Permalok steel casing and high density polyethylene (HDPE) pipe. Several large obstructions, including concrete-filled vaults, abandoned tunnels, timber piles, and steel sheet piling were encountered and excavated from within the pipe jacking shield. Challenges and risks associated with open-cut excavation in a difficult waterfront environment were significantly reduced by implementing the pipe jacking VE change, but were not entirely eliminated.

The resettlement of residents to new urban areas in the Three Gorges Reservoir region, China, has generally adopted the local resettlement method, but engineering disturbances in conjunction with heavy rainfall have contributed to landslide instability. This paper discusses the influential factors and formation mechanism of the Jinjiling landslide in Jiangdong District, Wushan County, Chongqing, China. This study concludes that the cutting and filling of slopes have led to loading of the rear edge and open cut excavation at the downhill side of landslide. More importantly, the filling of channels with surface soil has blocked the surface water discharge channel along the landslide body, thereby transforming surface water into groundwater. Therefore, under heavy rainfall, the groundwater level in the Jinjiling landslide rose substantially, resulting in large deformation in approximately August 1, 2018. After emergency treatment, including groundwater pumping, the landslide deformation slowed considerably. The rising groundwater caused by filling of the slope was the key factor influencing the landslide. Based on the deformation characteristics and formation mechanism of the landslide, comprehensive control measures referred to as the “drainage+unloading+support” scheme are proposed, and drainage should be the focus of this type of landslide protection project. The results of this study may provide a reference for similar slope engineering projects.

Geotechnical Risk Assessment and Back Analysis of Ground Movements Induced by Tunnel and Open-Cut Excavations

Open cut has traditionally been the preferred method of pipeline installation traversing watercourses. It is well understood and accepted that open cut excavation of the channel bed and banks during construction causes temporary disturbance to watercourse and aquatic habitat. Horizontal directional drilling, direct push and other subsurface installation methods can potentially avoid channel bed and bank disturbance but may have unique environmental effects such as frac-outs of drilling fluid. Although highly dependent on site conditions, open cut crossings are generally less costly than comparable subsurface installation methods. When a pipeline is installed in an open cut, the pipe is typically installed on a gravel or sand bed, laid in place, surrounded by a sand pack and surrounding soils placed back in the cut in a manner that attempts to replicate the soil lithology, horizons and native compaction of the cut. It has long been thought that this sediment and soil disturbance and backfilling has the potential create a zone of geomorphological weakness at the cut where soil and sediment become dissimilar to the surrounding channel bed and banks and can result in the acceleration of bed scour, bank erosion, widening and slope instability. In this paper we examine the longer-term effects of open cut pipeline installations on the geomorphic characteristics of watercourses. Over the course of four years of field investigation, nearly 750 pipeline watercrossings throughout Ontario were visited and assessed for geomorphic stability and depth of cover. The fluvial geomorphology of Ontario is diverse and ranging from alluvial, sinuous, unconfined, low gradient watercourses in the southwest to karst-influenced morphologies in eastern Ontario and often greater slope, confined and bedrock dominated watercourses in the north. By examining the field-based geomorphological characteristics of pipeline watercourse crossings in Ontario installed by open cut and crossing a wide range of fluvial geomorphological types we will explore and draw empirically-based conclusions on whether open cuts do in fact affect the long term geomorphological conditions of the watercourse.

Collapse of a 7.08-m-deep open cut excavation for the bank of a navigation lock in sensitive soft clay in Zhejiang, China, is presented in this paper. Cone penetration tests, field vane shear tests (VSTs), and traditional laboratory tests have been performed to investigate the soil properties after the collapse. According to the test results, the soft soil layers of mucky clay and muck have been found to be disturbed because of the effects of the boring holes. For bored concrete piles, the stresses release due to the cut slope, thus its shear strength is much lower than that measured from laboratory tests. The shear strength reduction technique based on a finite-element method program has been used to study the excavation behavior and overall factor of safety (FOS) of the excavated slope considering geometries of original design, later adopted schemes and soil properties associated with various test types. The computed FOS values obtained from VST results are lower than the corresponding recommended values and just a little higher than the limited value. Thus the excavated slope may easily collapse triggered by additional vehicles and surface surcharge loads. The reasons of excavation collapse can be explained as the misuse of soil properties for designers, and inadequate construction process management. The investigation of the failure provides experiences and lessons for the design and construction of similar projects in sensitive soft clay.

As a representative city located in the Loess Plateau region of China, Lanzhou is affected by various environmental and engineering factors, such as precipitation, earthquake subsidence, and building construction, which all lead to frequent geological disasters. Obtaining information on land subsidence over a long time series helps us grasp the patterns of change in various types of ground hazard. In this paper, we present the results of using Interferometric Synthetic Aperture Radar (InSAR) to monitor land subsidence in the main urban area of Lanzhou from 26 October 2014 to 12 December 2021. The main influential factors leading to subsidence were analyzed and combined via machine learning simulation to assess the land subsidence risk grade distribution of a building unit. The results show that the annual average deformation rate in Lanzhou ranged from −18.74 to 12.78 mm/yr. Linear subsidence dominated most subsidence areas in Lanzhou during the monitoring period. The subsidence areas were mainly distributed along the Yellow River, the railway, and villages and towns on the edges of urban areas. The main areas where subsidence occurred were the eastern part of Chengguan District, the railway line in Anning District, and the southern parts of Xigu District and Qilihe urban area, accounting for 38.8, 43.5, 32.5, and 51.8% of the area of their respective administrative districts, respectively. The random forest model analysis results show that the factors influencing surface subsidence in Lanzhou were, in order of importance, precipitation, the distribution of faults, the lithology of strata, high-rise buildings, and the distance to the river and railway. Lanzhou experienced excessive groundwater drainage in some areas from 2015 to 2017, with a 1 m drop in groundwater and 14.61 mm surface subsidence in the most critical areas. At the same time, extensive subsidence occurred in areas with highly compressible loess ground and most railway sections, reaching a maximum of −11.68 mm/yr. More than half of the super-tall building areas also showed settlement funnels. The area at a very high risk of future subsidence in Lanzhou covers 22.02 km2, while the high-subsidence-risk area covers 54.47 km2. The areas at greatest risk of future subsidence are Chengguan District and Qilihe District. The city contains a total of 51,163 buildings in the very high-risk area, including about 44.57% of brick-and-timber houses, 51.36% of old housing, and 52.78% of super-tall buildings, which are at especially high risk of subsidence, threatening the lives and properties of the population. The deformation results reveal poor building safety in Lanzhou, providing an essential basis for future urban development and construction.

Ground subsidence is a typical geological hazard in urban areas. It endangers the safety of infrastructures, such as subways. In this study, the ground subsidence risk of Shanghai metro lines was mapped and assessed. Firstly, PS-InSAR was used for the ground subsidence survey, and subsidence intensity was divided into five classes according to subsidence velocity. 10 subsidence causal factors were collected and the frequency ratio method was applied to analyze the correlation between subsidence and its causal factors. Then LightGBM model was used to generate a ground subsidence susceptibility map. And receiver operating characteristic curve and area under the curve (AUC) were adopted to assess the model. And AUC is 0.904, which suggests the model's performance is excellent. Finally, a risk matrix was introduced to consider the intensity and susceptibility of ground subsidence. The risk of ground subsidence was mapped and classified into five levels: R1 (very low), R2 (low), R3 (medium), R4 (high), and R5 (very high). The results showed that the risk of subway ground subsidence exhibited a regional-related characteristic. Metro lines located in areas with higher ground subsidence risk levels also had higher ground subsidence risk levels. Meanwhile, the statistical results of subway ground subsidence risk levels showed that subway stations were safer than sections.

Coal mines play an important role in the global energy supply. Monitoring the displacement of open-pit mines is crucial to preventing geological disasters, such as landslides and surface displacement, caused by high-intensity mining activities. In recent years, multi-temporal Synthetic Aperture Radar Interferometry (InSAR) technology has advanced and become widely used for monitoring the displacement of open-pit mines. However, the scattering characteristics of surfaces in open-pit mining areas are unstable, resulting in few coherence points with uneven distribution. Small BAseline Subset InSAR (SABS-InSAR) technology struggles to extract high-density points and fails to capture the overall displacement trend of the monitoring area. To address these challenges, this study focused on the Shengli West No. 2 open-pit coal mine in eastern Inner Mongolia, China, using 201 Sentinel-1 images collected from 20 May 2017 to 13 April 2024. We applied both SBAS-InSAR and distributed scatterer InSAR (DS-InSAR) methods to investigate the surface displacement and long-term behavior of the open-pit coal mine over the past seven years. The relationship between this displacement and mining activities was analyzed. The results indicate significant land subsidence was observed in reclaimed areas, with rates exceeding 281.2 mm/y. The compaction process of waste materials was the main contributor to land subsidence. Land uplift or horizontal displacement was observed over the areas near the active working parts of the mines. Compared to SBAS-InSAR, DS-InSAR was shown to more effectively capture the spatiotemporal distribution of surface displacement in open-pit coal mines, offering more intuitive, comprehensive, and high-precision monitoring of open-pit coal mines.

Ground subsidence is a significant safety concern in mining regions, making large-scale subsidence forecasting vital for mine site environmental management. This study proposes a deep learning-based prediction approach to address the challenges posed by the existing prediction methods, such as complicated model parameters or large data requirements. Small baseline subset interferometric synthetic aperture radar (SBAS-InSAR) technology was utilized to collect spatiotemporal ground subsidence data at the Pingshuo mining area from 2019 to 2022, which was then analyzed using the long-short term memory (LSTM) neural network algorithm. Additionally, an attention mechanism was introduced to incorporate temporal dependencies and improve prediction accuracy, leading to the development of the AT-LSTM model. The results demonstrate that the Pingshuo mine area had subsidence rates ranging from −205.89 to −59.70 mm/yr from 2019 to 2022, with subsidence areas mainly located around Jinggong-1 (JG-1) and the three open-pit mines, strongly linked to mining activities, and the subsidence range continuously expanding. The spatial distribution of the AT-LSTM prediction results is basically consistent with the real situation, and the correlation coefficient is more than 0.97. Compared with the LSTM, the AT-LSTM method better captured the fluctuation changes of the time series for fitting, while the model was more sensitive to the mining method of the mine, and had different expressiveness in open-pit and shaft mines. Furthermore, in comparison to existing time-series forecasting methods, the AT-LSTM is effective and practical.

PDF HTML阅读 XML下载 导出引用 引用提醒 厦门市地面沉降影响分析与风险评价 DOI: 10.5846/stxb202007031727 作者: 作者单位: 作者简介: 通讯作者: 中图分类号: 基金项目: 国家自然科学基金（41571148）；中国科学院战略性先导科技专项（XDA23030103） Impact analysis and risk assessment of urban land subsidence in Xiamen City Author: Affiliation: Fund Project: the National Natural Science Foundation of China(41571148); the Strategic Priority Research Program (A) of the Chinese Academy of Sciences(XDA23030103) 摘要 | 图/表 | 访问统计 | 参考文献 | 相似文献 | 引证文献 | 资源附件 | 文章评论 摘要:城市的地面沉降一直以来都是沿海城市密切关注的问题。在沿海城市对高层建筑群诱发的地面沉降进行深入研究具有重要意义。获取地面沉降监测点的高程信息，分析了厦门市地面沉降情况与空间差异性。重点探讨建设用地、建筑容积率与土地利用转化信息这3种因素对厦门市地面沉降的影响，并对厦门岛地面沉降风险进行评估，可为厦门岛的高层建筑三维空间布局与优化研究提供先验知识。结果表明：（1）在2001-2015年厦门市整体的高程存在下降的趋势且较为缓慢，同时厦门市的城市建设规模不断变大且建设面积增速不断增大，与厦门市的城市地面的沉降发展趋势相同。（2）厦门市的高层建筑密度需达到一定程度才对地面沉降有影响，厦门岛的建筑容积率与地面沉降没有存在显著的相关性。（3）在不同土地利用转化建设用地对地面沉降的影响中：原水域用地沉降最为明显，建成时间越长，总沉降量越大；原耕地用地转化成高层建筑之后沉降有明显发生，也随着建成时间的延长有逐渐增加沉降量的态势；原园地、林地用地沉降只在少部分地区发生且与建成时间无明显差别。（4）厦门岛可用于城市建设区域中的53.34%是地面沉降中风险等级以上的。此外，本研究给出防御防治城市地面沉降风险的有效建议与措施。本研究结果可为厦门市城市化进程中的地面沉降监测、规划设计与决策等提供重要的数据支撑，为城市可持续发展提供科学的依据；也可为国内外其他城市的地面沉降监测分析和可持续发展研究提供理论依据和借鉴意义。 Abstract:Urban land subsidence has always been a problem that coastal cities pay close attention to. It is of great significance to conduct in-depth research on land subsidence which was induced by high-rise buildings in the coastal cities. In this study, the elevation information of land subsidence monitoring sites were firstly obtained to analyze the spatial difference of land subsidence in Xiamen City. Secondly, the influence of construction land, floor area ratio and land use conversion information on land subsidence in Xiamen City was discussed emphatically. The land subsidence risks of Xiamen Island were evaluated, which can provide a priori knowledge for the research on three-dimensional layout and optimization of high-rise buildings in Xiamen Island. The results show that: (1) from 2001 to 2015, the overall elevation of Xiamen City presented a downward trend and was slow. Meanwhile, the urban construction scale continued to increase and the construction area increased in Xiamen City, which were the same as the development trend of urban land subsidence in Xiamen City. (2) The density of high-rise buildings needs to reach a certain degree to have an impact on the land subsidence in Xiamen City. There is no significant correlation between floor plot ratio and land subsidence in Xiamen Island. (3) Among the impacts of land subsidence due to different land transformed construction land, the original water area total land subsidence is the most obvious. The longer the completion time, the greater the amount of subsidence. After the original cultivated land conversion into high-rise buildings, the settlement obviously occurred. With the extension of the construction time, the settlement increases gradually. The settlement of the original garden and forest land only occurs in a small number of areas and there is no significant difference from the completion time. (4) Xiamen Island can be used in urban construction areas where 53.34% of it is above the risk level of land subsidence. In addition, the effective suggestions and measures are also given to prevent the risk of urban land subsidence in this study. The results of this study can provide important data support for land subsidence monitoring, planning, design and decision-making in the urbanization process of Xiamen City, and provide a scientific basis for the sustainable development of the cities. The results can also provide theoretical basis and reference significance for the land subsidence monitoring analysis and sustainable development research for other cities at home and abroad. 参考文献 相似文献 引证文献

Land subsidence is the lowering of land-surface elevation mainly due to human activities including groundwater withdrawal, oil or gas pumping. It is a critical threat to the sustainable development of urban constructions and social economy. Long-term and large-scale over-exploitation of groundwater, which supplies about two-thirds of the total regional water requirement, is a main cause for land subsidence in Beijing Municipality, China. It is essential to assess land subsidence risks for decision-makers to prevent the disaster. This study takes the up-middle part of alluvial-proluvial plain fan of the Chaobai River in Beijing as an example area, where Huairou Emergency Groundwater Resource Region is located. It evaluates the risk of land subsidence between 2006 and 2008 by adopting the analytical hierarchy process with sensitivity analysis (AHP-SA) method. Six criteria used for the evaluation are thicknesses of compressible sediment and the quaternary strata, changes in groundwater level of the unconfined and confined aquifer system, building density and recharge from precipitation infiltration. Criteria weights were determined on the basis of the interrelation coefficients between land subsidence and six factors. The highest weight was assigned to thickness of compressible sediment, followed by the thickness of quaternary strata and change in confined aquifer groundwater level. The weight of recharge from precipitation infiltration is the lowest. Distribution of land subsidence during the same period, which was derived by using Persistent Scatter Synthetic Aperture Radar Interferometry technique, was used to verify the accuracy of the risk assessment map. The land subsidence risk map shows that the high risk region covers about 150 km 2 and is mainly distributed in the southwest of the study area. Area of moderate risk accounts for nearly 1/3 of the whole area which is in the middle-southern area. No-hazard zone is located in the northern area, which is the upper area of the alluvial-pluvial plain fan. The evaluation results are least sensitive to the recharge from precipitation infiltration. Risk map is more sensitive to the change of confined aquifer groundwater level in that when the weight changes for more than ± 2%, an obvious shift between high and moderate subsidence risk regions occurs. The risk map is also sensitive to the thicknesses of the quaternary and compressible sediment. However, since these two factors cannot be controlled by human beings, it is necessary for decision-makers to pay more attention to limit the drawdown of groundwater level. The output of this study provides a better insight to land subsidence hazard management.

As ground subsidence accidents in urban areas that occur due to damage to underground utilities can cause great damage, it is necessary to predict and prepare for such accidents in order to minimize such damage. It has been reported that the main cause of ground subsidence in urban areas is cavities in the ground formed by damage to underground utilities. Thus, in this study, attribute information and historical ground subsidence information of six types of underground utility lines (water supply, sewage, power, gas, heating, and communication) were collected to develop a ground subsidence risk prediction model based on machine learning. To predict the risk of ground subsidence in the target area, it was divided into a grid with a square size of 500 m × 500 m, and attribute information of underground utility lines and historical information of ground subsidence included in the grid were extracted. Six types of underground utility lines were merged into single-type attribute information, and the risk of ground subsidence was categorized into three levels using the number of ground subsidence occurrences to develop a dataset. In addition, 12 datasets, which were developed based on the conditions of certain divided ranges of attribute information and risk levels, and 12 additional datasets, which were developed using the Synthetic Minority Oversampling Technique to resolve the imbalance of data, were built. Then, factors that represented significant correlations between input and output data were singled out and were then applied to the RandomForest, XGBoost, and LightGBM algorithms to select a model that produced the best performance. By classifying the ground subsidence risk levels through the selected model, it was found that density was the most important influencing factor used in the model. A risk map of ground subsidence in the target area was made through the model; the map showed the trend of well-predicted risk levels in the area where ground subsidence was concentrated.