Stability Of Foundation Pit Research Articles

Pipe piles and split-cables: reinforcing weak mine pits

To mitigate the instability problem of heavy equipment foundations in deep coal mine chambers under high geo-stress, this study proposes a combined reinforcement strategy for foundation pits, which are mainly composed of pipe piles and split-level vertical cables. The weak rock mass around the pit is reinforced using combined cables and grouting. The split-level-arranged cables with different lengths improve the tensional strength of the rock mass, whereas the pipe pile at the pit corner can enhance the shear strength. The overall performance of the structure in terms of bearing capacity can be significantly enhanced by pouring pipe piles to integrate them and the pump foundation to form a cohesive structure. Both the foundation integrity and foundation pit strength can be improved by stepwise support technology. Finally, an improved reinforcement strategy is implemented in a deep-pump chamber in the Huainan mining area. The monitoring results indicate that the foundation pit stability was successfully controlled and floor cracking and tilting of the pump foundation were avoided by using the improved support method.

Geo-environmental Suitability Assessment for Shallow Foundition Pit in Urban Zhengzhou City Based on GIS and AHP Methods

With the objective of the security and sustainability of underground space, the geo-environment suitability assessment for shallow foundation pit was conducted by GIS and AHP methods combining with the characteristics of the study area on the basis of the investigation of the current exploitation of underground space and the geological environment conditions in Zhengzhou City and the advanced experiences at home and abroad. Computer technology provides powerful tools and methods to the geo-environmental suitability assessment of underground space, and supports decision-making and planning. In the research, the suitability of geo-environment was assessed using the multi-objective linear weighted function model. The results indicate that the elements influencing the geo-environment suitability of shallow foundation pit in the study area mainly include stability of foundation pit, active earth pressure of slope, and shallow groundwater depth and water enrichment of aquifer according to the impact of evaluation factors on the foundation pit engineering. The results showed that the suitable area with an area of about 96.2 km2 is located to the west of Beijing–Guangzhou Railway, the medium-suitable area with an area of about 174.4 km2 is located to the east of Beijing–Guangzhou Railway and to the west of the Beijing-Guangzhou high-speed railway, the poor-suitable area with an area of about 111.3 km2 is located to the east of Dongfeng canal-Jingguang high-speed Railway and we should particularly focus on the impact of surface water, groundwater, soft soil and land subsidence during construction. The application of GIS and AHP method provided key technical support for the assessment. The geo-environment suitability evaluation of the foundation pit engineering by GIS and AHP methods has a strong pertinence and can offer technical assistance for the exploitation of underground space in Zhengzhou City.

Stability of a Deep Foundation Pit with Hard Surrounding Rocks under Different in-Time Transverse Supporting Conditions

This paper focuses on investigating the stability of a deep foundation pit with hard surrounding rocks at different excavation stages with different supporting schemes by means of numerical calculations. The supporting schemes in question were combinations of one fixed vertical support and four varied transverse supports. Drilled grouting piles were used as vertical supports, and the commonly used steel bracings and prestressed anchorages served as transverse supports. The parameters used to evaluate the stability of the foundation pit at different excavation stages included the lateral displacements of the surrounding rocks, the settlement of the surrounding ground, the axial forces of steel bracings, and displacements at the tops of the drilled grouting piles. Simulation results showed that when a transverse supporting scheme consisting of one-layer steel bracings and prestressed anchorages set at 9 m and 22.5 m underground, respectively, was adopted, the lateral displacements of the surrounding rocks and settlement of the surrounding ground at different excavation stages were the largest compared to those under the other three transverse supporting schemes, while the corresponding values were lower compared to those allowed in Chinese standard GB50007-2011, demonstrating that this kind of supporting scheme is effective in terms of ensuring the safety of the foundation pit at different excavation stages. Moreover, the setting techniques for this kind of supporting scheme were relatively simple, and the corresponding influences of supporting element arrangements on excavation techniques were the lowest. Therefore, one-layer steel bracings and one-layer prestressed anchorages constituted the most suitable transverse supporting scheme for excavating a deep foundation pit with hard surrounding rocks.

Monitoring and Prediction of Horizontal Displacement of Underground Enclosure Piles in Subway Foundation Pits.

Urban rail transportation and underground space development and utilization are initiatives to solve urban traffic congestion. The monitoring and prediction of the stability of underground enclosure piles in foundation pits is a key index for the dynamic evaluation of the stability of underground space engineering. In this paper, we focused on the problem that the dynamic prediction accuracy and stability of foundation pit retaining piles in the Qingdao area are not high. Based on the analysis of various time function curves and the physical meaning of the parameters, we proposed the Adjusted-Logistic time function model, which introduces three physical parameters to adjust the deformation velocity and deformation acceleration in different stages for better accuracy. It could predict the deformation process of underground enclosure piles under different geological engineering conditions. It was verified in the field that the root-mean-square error (RMSE) of the Adjusted-Logistic function was 0.5316, MAE was 0.3752, and R2 was 0.9937, which were better than those of Gompertz, Weibull, and Knothe time function models. Meanwhile, it showed that with the increase of excavation depth, the maximum horizontal displacement of the underground enclosure piles gradually decreased and finally stabilized at 0.62-0.71 H. We established a catastrophe model of the horizontal displacement cusp at the observation point of the underground enclosure piles by using the time series of the measured data. It could determine the weak location of the underground enclosure pile stability and realize the multipoint warning of the foundation pit stability, which would ensure safe construction.

Ultra-weak FBG sensing for identification and analysis of plastic zone of soil caused by supported excavation

Deformation caused by supported excavation makes significant impact on foundation pit stability. Evaluation of potential plastic zone is crucial for early warning of foundation pit instability. To address the shortcomings of existing evaluation methods, a new identification method based on the ultra-weak fiber Bragg grating (UWFBG) technology was proposed. To validate this approach, a UWFBG monitoring program was implemented in the boreholes as part of the Shanghai Metro Line 18 project. The monitoring results were utilized to determine the range of plastic zone for each borehole. It was noted that there are two plastic deformation zones formed with different mechanisms, one located near the base of the diaphragm wall (DW) and the other is distributed within the range of 6–19 m below the excavation face. Lastly, a criterion of early warning of foundation pit instability was presented. By analyzing the monitoring data, the highest risk level of this project was determined as “Attention”, implying the necessity of taking measures to prevent the local plastic zone from evolving into a full-scale sliding surface.

Numerical Analysis of Influences of Engineering Piles for Anti-Inrush of Deep Excavation under Confined Water

Aiming at the problem of anti-inrush of confined water in foundation pits considering the influence of engineering piles, a three-dimensional numerical analysis model of anti-inrush stability of deep foundation pits is established by using the finite element software ABAQUS, and the inrush failure mechanism and deformation behavior of the foundation pit bottom under the action of different confined water levels are analyzed. The effects of different pile lengths, pile spacing, waterproof layer thickness, and soil strength are studied, respectively, and the laws of pile foundation stress and soil deformation under the action of confined water are revealed. The results show that the existence of engineering piles has an obvious inhibitory effect on the stability of foundation pit confined water against inrush. By introducing the comprehensive correction coefficient of pile side friction, an analysis method for the anti-inrush stability of foundation pits considering the influence of engineering piles is proposed.

Field Experimental Study on Mechanical Properties of Frame Support Structure Composed of Subway Foundation Pit Retaining Pile and Steel Strut Rigid Joint

In order to study the advantages of frame support structure composed of rigid joint between steel strut and retaining pile in restraining the lateral displacement of retaining structure and maintaining the stability of foundation pit, foundation pit failure test was carried out. Compared with the traditional support structure, which can only bear pressure composed of steel strut and retaining pile lap joint, the variation characteristics of pile lateral displacement and support axial force of frame support structure under the influencing factors, such as weak stratum, pit side loading, groundwater level rise, and support drop, are studied. The test results show that, compared with the traditional steel strut, the pile‐strut frame support structure can not only bear the compressive stress, but also the tensile stress caused by the lateral displacement of the top of the retaining pile outside the foundation pit. The overall stability is better, and the axial force distribution of each layer of support is more uniform so as to reduce the overall lateral displacement of the retaining pile. In the process of foundation pit collapse, the frame support structure will not have the problem of strut falling, which can maintain part of the support capacity, prolong the collapse time of retaining piles, strive for more escape time for front‐line construction workers, and reduce the number of casualties.

Study on the Characteristics of Self-Stabilizing Height Distribution for Deep Foundation Pit Vertical Sidewall in Binary Strata of Upper Soil and Lower Rock

The self-stability height of the foundation pit sidewall is an important criterion for evaluating the safety degree and designing the supporting structure. The strength reduction elastic-plastic finite element numerical calculation method has been adopted in this paper. Based on comparative analysis of the stability characteristics for deep foundation pit in binary strata of upper soil and lower rock under multiple working conditions, the potential fracture surface of deep foundation pit and the evolution law of corresponding safety factor have been revealed under different Hs and H. A new idea that the vertical soil sidewall height (Hs) and the vertical rock sidewall height (Hr) are used as two independent evaluation indexes, respectively, for deep foundation pit stability in binary strata of upper soil and lower rock has been put forward. The distribution characteristics and variation law of Hs0 and Hr0 under different Hs and different H have been revealed, respectively. The spatial distribution map of the self-stabilizing height for deep foundation pit vertical sidewall in upper soil and lower rock binary stratum has been constructed, and the mathematical fitting equation between Hr0 and Hs has been obtained. Finally, combined with the implementation effect of the deep foundation pit project of Ningxia Road Station for Qingdao Metro Line 3, the rationality of the conclusions is verified. The research results provide theoretical basis for quickly determining the self-stability characteristics of foundation pit vertical sidewall.

Laboratory investigation of cemented silt improved with bentonite and phosphogypsum in DCM walls

Engineering properties of cemented soil in deep cement mixing (DCM) walls significantly affect security and stability of foundation pit, especially in regions where silty and sandy layers are widely distributed. This article investigates properties of cemented silt improved with calcium bentonite and phosphogypsum, in order to realise economic and environmental benefits. A series of laboratory tests, including unconfined compression strength (UCS) tests, hydraulic conductivity tests and scanning electron microscope (SEM) tests are carried out to examine strength, deformation, hydraulic behaviour and microstructure of solidified silt. The results show that the growth of unconfined compressive strength is as high as 86 ∼ 166 per cent with the addition of optimal content of calcium bentonite and phosphogypsum. The hydraulic conductivity of cemented silt decreases by one to two orders of magnitude with increasing additives and curing time. Moreover, the SEM pictures account for the existence of an optimal content of phosphogypsum explicitly. The microstructure of specimens reflected by SEM pictures shows a high consistency with UCS tests and hydraulic conductivity tests. Laboratory tests and cost analysis indicate that calcium bentonite and phosphogypsum have potential applications as binders in construction of DCM walls.

System Stability of Bored Piles with Steel Supports for Deep Foundation Pits in Sandy Pebble Stratum

Background: At present, there are a few research studies on the stability of the supporting structure of bored piles with steel supports for deep foundation pits, and it is even less common to use this support method for deep foundation pits in sandy pebble stratum. This paper studied the stability of the deep foundation pit support of the Southwest Jiaotong University Station in Chengdu, China, using the stability analysis theory of bored piles with steel supports for deep foundation pits. Objective: The study aims to show the stability of steel supports for deep foundation pits. Methods: The overall stability, anti-overturning stability and uplift stability of the foundation pit are important components of the stability analysis of bored piles with steel supports for deep foundation pits. Aiming at the designed method and parameters of the bored piles with steel support for the deep foundation pit, in-depth theoretical analysis, and calculation of the overall stability, antioverturning stability, and uplift stability of the deep foundation pit are conducted, respectively. Results: The results show that the stability of the deep foundation pit using bored piles with steel support meets the requirements, indicating that the design of this support form is safe and reasonable, thus ensuring the smooth progress of the project. Conclusion: The stability analysis of deep foundation pits in sandy pebble stratum provides ideas for the optimization of deep foundation pits and a reference for the stability analysis of deep foundation pits in sandy pebble stratum and other types of the stratum.

Mechanical properties of a new string capsule drainage anchor and its supporting effect in deep foundation pit

With the increase of the scale and depth of urban underground space construction in coastal soft soil area, the traditional anchoring technology has been difficult to meet the requirements of deep foundation pit excavation and support. This paper puts forward a new type of string capsule drainage anchor supporting structure, introduces its structure composition, analyzes its working mechanism and mechanical characteristics, the calculation formula of ultimate bearing capacity of small cylindrical shear failure between anchor and surrounding soil is given, finally, the supporting effect of the new structure in deep foundation pit engineering is simulated and analyzed by finite element software. It is concluded that the stress of the new type of anchor can be divided into three stages: static earth pressure, plastic zone development and plastic zone breaks through; the new anchor can quickly dissipate the pore water pressure in the surrounding soil, which is more conducive to the stability of foundation pit; the axial force distribution presents a “sawtooth” shape, and the axial force increases sharply in the expanding section; the supporting effect of the new anchor is more obvious than that of the traditional anchor, which can more effectively inhibit the horizontal displacement of the slope.

Optimization Design Research of Deformation Control in Qingyuan Xinhe Garden Foundation Pit

As there are many factors affecting foundation pit excavation and enclosure structure design, it is strongly necessary to carry out dynamic design and information-based construction, and to perform anti-analysis through monitoring information in deep foundation pit engineering. Taking Qingyuan Xinhe Garden foundation pit project as the engineering background and based on the deformation control optimization design theory, this paper analyzed the deformation of the foundation pit and the settlement volume of the surrounding buildings by combining on-site monitoring measurement and numerical simulation, which can provide an important reference for the safety and stability of foundation pit.

The Application of Principal Component Analysis in Stability Evaluation of Foundation Pit

The foundation pit stability is the key and difficult point in foundation pit engineering research. The stability and safety of the foundation pit should be ensured not only by its excavation, but also by the surrounding buildings.Because the stability of foundation pit is decided by many complicated factors, it has a certain randomness.This paper adopts the principal component analysis method combined with engineering examples to calculate and analyze the six influencing factors that affect the stability of foundation pit: land subsidence, groundwater level, pile top settlement, horizontal displacement of pile top, reinforced concrete brace and inclination.The results show that inclination and pile top displacement have the most important influence on the stability of foundation pit. According to this study, the principal component analysis method has higher accuracy in the stability evaluation of foundation pit and has certain reference value, which can provide theoretical basis for the stability evaluation of foundation pit in related projects.

Comparative Study of Pile Pillared Support and Pile-anchor Retaining in Deep Pit

Support pattern is the most important factor affecting the stability of foundation pit. In order to study the stable state of deep foundation pit, this paper selects pile pillared support and pile-anchor retaining which are high accident rate for comparison, and optimizes the construction scheme combined with the actual deep foundation pit project. The deformation of the supporting structure and the settlement of the foundation pit of Huiquan Square are used to analyze by FLAC3D. The variation range and trend of the internal force of the steel support and the axial force of the anchor cable are analyzed under the different values of the soil layer parameters, such as elastic modulus, cohesion and internal friction angle. The results show that the internal force of pile pillared support is greatly affected by the change of cohesion and the anchor axial force is greatly affected by the change of elastic modulus and internal friction angle. Meanwhile, the influence degree of each soil layer parameter on the internal force of support structure is different, which provides reference suggestions for the selection of support pattern of deep foundation pit.

Study on the Influence of Soil Parameters Change on the Stability of Foundation Pit

Combining with engineering examples, the influence of grouting on the stability of foundation pit is analyzed by comparing the parameters before and after grouting. The numerical analysis software FLAC is used to simulate the original foundation pit without grouting. The relationship between the maximum horizontal displacement and cohesion, friction angle and elastic modulus is explored. The fitting formulas of the influence of elastic modulus, friction angle and cohesion on displacement are put forward. Enclosure design provides basic data.

The Informational Dynamic Design and Construction Technology Application in the Soft Rock Tunnel

Informational dynamic design and construction technology are emerging new technologies at home and abroad. There are many applications in deep foundation pit stability, side slope stability, tunnel engineering and bridge engineering. This paper takes the West Qinling soft rock tunnel as an example to discuss engineering pre-design, construction and information collection, and pre-design modifying. Finally, this paper lists some problems in the construction of the soft rock tunnel project.

珠三角深厚软土区公路隧道深基坑施工安全控制研究

珠三角地区的软土具有含水量大、压缩性高、承载力低等特性，对基坑工程的设计和施工都提出了更高的要求，深基坑支护设计和施工面临诸多挑战。因此，本文以珠三角公路隧道深基坑工程为背景，分析了基坑施工状况，重点介绍了施工中存在安全问题。釆用岩土有限元分析软件对该工程的开挖过程进行模拟,分析基坑边缘的堆载和支护时机对基坑稳定性影响。结果表明：基坑边缘荷载控制在20 kpa以内，荷载离基坑边缘距离超过5 m，钢支撑及时架设，可大大减小桩身位移值。研究结果对类似软土公路隧道深基坑的开挖具有一定的借鉴意义。 The soft soil in the Pearl River Delta area has the features of high water content, high compressi-bility and low carrying capacity, etc., which have put forward stricter requirements for the design and construction of foundation pit project. There are many challenges in the design and construc-tion of deep foundation pit engineering. Therefore, this paper, taking foundation pit excavation in the section of highway tunnel, analyzed the present situation during excavation of foundation pit. The geotechnical finite element analysis software simulated the excavation of foundation pit, which analyzed the influence of pile load and support timing on foundation pit stability. The results show that: the load of the foundation pit edge is within 20 kpa; the distance of the load from the foundation pit edge is over 5 meters; the steel support was erected at the first half of the excavation. The results can be used for reference to other similar venues construction.

Analysis of influence of seepage on stability of foundation pit

Numerical analysis is executed to deep foundation pit by using the finite element software. In combination with the site data, the consequences with the consideration of groundwater seepage are contrasted to the consequences without the consideration of groundwater seepage including the stability of the foundation pit. The analysis indicates that the result with the consideration of seepage is relatively close to the site data. Compared with the result without consideration of groundwater seepage, the maximum horizontal offset of the pile increases approximate 5mm and the location of the maximum horizontal displacement moves 2m down. So seepage is harmful to the retaining structure of foundation pit and should be considered in the design of foundation pit.

Stability Analysis of Foundation Pit Under Bottom Over-Excavation and Top Overloading Conditions

The soil nailing wall is a common support method used in foundation pit supporting system. The overloading and over-excavation maybe be met during the processing of excavating and those all effect the stability of foundation pit. The effect of over-excavation and overloading on foundation pit stability are analyzed in this paper using FEM combined with the project in Luoyang city. The results show that local over-excavation may form a continuous slip surface in slope and lead to instability of foundation pit. The external load on the slope top may lead to greater lateral displacement and settlement. The distribution area of settlement under external load shows as triangular and the maximum depth is almost equal to load width. The overloading on the top of foundation pit may accelerate collapse processing when the toe of slope is over excavated, but over excavation still may lead to instability of foundation pit although no overloading on the top. This should be paid more attention in project.

Impact Analysis of Support Structure of Unsymmetrical Loading Deep Foundation Pit Adjoining Highway

The air shaft deep foundation pit 6 is influenced by subgrade unbalance loading of highway 312, specifically its deep horizontal displacement of supporting piles and subgrade settlement of expressway 312 under unsymmetrical load effect. This paper carries out construction monitoring and numerical simulation analysis and gives a detailed study on the influenced factors of support structure deformation. Calculation results show that subgrade unsymmetrical load has a great influence on deep horizontal deformation of supporting piles. The maximum horizontal displacement at the bias side is about three times more than non-bias side’s; when the distance of subgrade to foundation pit is the same as the depth of excavation, the influence of subgrade on pit can be ignored; as the cohesion and internal friction angle increases, the horizontal displacement of fender piles decreases. However, enlarging the embedded depth of supporting piles has no significant influence on the stability of foundation pit.