Soft Soil Foundation Pit Research Articles

Protective effects of measures for railway subgrade adjacent to excavation of a parallel pit

The Hongqiao Airport–Pudong Airport urban railway construction project in Shanghai, includes a 156.1-m-long deep strip foundation pit that runs closely parallel to the subgrade of the Shanghai–Hangzhou High-speed Railway, operating at 300 km/h. With a maximum excavation depth of 25.5 m and a minimum distance of only 10.6 m between the outer edge of the pit and the subgrade slope foot, controlling the deformation of the foundation pit in soft soil and ensuring the safe operation of the adjacent high-speed railway present significant challenges. Various protective measures were implemented to mitigate subgrade deformation caused by pit excavation, including partition excavations, 1.5-m-thick diaphragm walls, a servo steel strut system, isolation piles between the subgrade and diaphragm walls, and water-resistant barriers. The foundation pit, soil, and railway subgrade deformations during excavation were measured and analysed. The results demonstrate that the protective measures limited lateral diaphragm wall deformation to within 0.14% of the excavation depth. The maximum horizontal and vertical displacements of the subgrade caused by excavation were 3.2 mm and 1.5 mm, respectively. As a result, trains safely passed through this section at a limited speed of 120 km/h. The findings presented provide useful reference information for similar projects.

Effect Analysis of Steel Sheet Pile Supporting for Utility Tunnel Foundation Pit in Water-Rich Soft Soil

Given the structural stability of steel sheet pile support in a water-rich soft soil area, the interaction mechanical mechanism between steel sheet pile support and soft soil was analyzed in the paper. Based on the limit equilibrium theory of soil mass and the coordinated deformation effect of retaining structure and soil, the force equation and action coefficient considering the cohesion of soil and the friction angle of steel sheet pile to soil were obtained. According to the characteristics of the soil around the foundation pit, reasonable supporting spacing (d = 4 m) could be obtained considering the friction angle of the steel sheet pile to soil properly without the cohesion of soil. The monitoring results show that the axial force during the support process is more reasonable considering the cohesion of soil and the friction angle of interaction with the wall and soil. The axial force of the measuring point was affected by the construction site and the layout of the utility tunnel foundation pit. The location with large deformation of the steel sheet pile and the most unfavorable range from the top of the foundation pit was 0.5–0.7 H. For the mechanical analysis of the interaction between steel sheet pile and soil, the apparent cohesion and friction angle can be taken as 0.05 rH and φ/3–φ/2, respectively. The foundation pit in the water-rich soft soil area could operate stably with a better waterproof effect by the steel sheet pile under different working conditions.

Simplified Calculation of Lateral Displacement of a Cement‐Soil Gravity Retaining Wall

A common retaining structure in Shanghai’s soft soil foundation pit is the cement‐soil gravity retaining wall. In this research, a streamlined calculation method is suggested to examine the lateral displacement of the cement‐soil gravity retaining wall’s deformation features. First, the lateral pressure of the retaining wall in a nonlimit passive state is calculated by separately estimating water and soil pressure. Then, the work performed by the lateral pressure of the retaining wall and the internal energy of deformation is calculated and analyzed according to the law of energy conservation to obtain the maximum lateral displacement at the top of the retaining wall. At last, the influences of wall insertion ratio, wall width, cement mixing ratio, and soil internal friction angle on the lateral displacement of retaining wall are analyzed with finite element numerical simulation. The results of the streamlined calculations are contrasted with the measured findings when paired with engineering scenarios, which demonstrates that the predicted lateral displacement is reasonably accurate with an average error of less than 15%.

A Review of Key Technologies Development of Super High‐Rise Building Construction in China

Super high‐rise building is a typical architectural form of urban modernization, and it is also the trend of urban architectural development. This paper reviews the development history of super high‐rise buildings in China combined with the engineering practice of representative super high‐rise buildings since the 1980s, such as the Shanghai Center, Shanghai Oriental Pearl Tower, Jinmao Tower, Shanghai World Financial Center, Canton Tower, and Shanghai Tower. The development process of the key technologies of super high‐rise projects from the aspects of several technologies, such as soft soil pile foundation and foundation pit technology, concrete structure construction technology, integral formwork equipment technology, and steel structure comprehensive installation technology, is also elaborated. At the same time, the Shanghai Tower is selected as a typical case to share the innovative technology of China’s tallest building. Finally, prospects for development of super high‐rise construction technology are presented.

The Influences of Bladder Properties on the Working Performance of the Multibladder-Type Inflatable Anchor

Due to the high bearing capacity and better controllability of bladder parameters, the multibladder-type inflatable anchor shows a great application prospect in the engineering construction in soft soil areas. In this study, based on a soft soil foundation pit project in Guangzhou Province, China, the multibladder-type inflatable anchor was applied to the engineering practice for the first time, and the working characteristics of the multibladder-type inflatable anchor were analyzed and discussed through field experiments. Primarily, the optimization design and experimental facility of the multibladder-type inflatable anchor were introduced in detail to realize the refinement and perfection of the anchorage technological process. Besides, the influence degrees of the bladder number, diameter, thickness, space, and length on the bearing characteristics and deformation control of the anchor were studied by using the control variable method, as well as the influencing mechanisms. Subsequently, the advantages of the multibladder-type inflatable anchor were presented by comparing its anchoring efficacy with that of the traditional grouting anchor. Ultimately, the analysis results were used to verify the reliability of the multibladder-type inflatable anchor and provide the experimental basis for its popularization and application.

Analysis of Lateral Displacement Law of Deep Foundation Pit Support in Soft Soil Based on Improved MSD Method

Foundation pit envelope and foundation pit excavation solution design is a multidisciplinary problem that could be linked to a series of safety issues in the geotechnical engineering of an actual construction project. Moreover, the construction of large deep foundation pit in soft soil often faces greater risks and challenges as the support structure deforms more easily and unpredictably. In order to improve the deformation prediction of deep foundation pit engineering precision and efficiency and to ensure that the construction of deep foundation pit engineering is safe and efficient, in this article the traditional MSD (mobilizable strength design) theory research and analysis, combined with the Jinan formation characteristics of a tunnel in JInan, and new parameters were introduced to the original MSD method theory: the wall itself within the bending strain energy U and support compression elastic potential energy of V. A new law of conservation of energy is constructed, and finally, an optimized MSD method has been proposed, this method is shown in the article. Finally, the results of foundation pit deformation calculation were compared among the optimized MSD method, finite element calculation method, and field monitoring data analysis method, so as to demonstrate the reliability of the prediction system of optimized MSD method and finite element analysis method. The results show that, by optimizing the MSD calculation method, the horizontal displacement of the retaining wall varies depending on the excavation depth of the foundation pit and the form of internal support with the overall peak value of displacement between 0∼0.2% H (H being the excavation depth); the deformation of retaining wall increases gradually with the increase of excavation depth of the foundation pit, and the peak position of deformation gradually moves down with the excavation of foundation pit. The trend of these changes is consistent with the results of the finite element method and field data analysis method, proving that the optimized MSD method is reliable in predicting the deformation of the foundation pit under specific stratum conditions.

Application of new pumping technology in dewatering in deep soft soil pit

The soft soil stratum has low carrying capacity, poor permeability and is hard to be drained, which affects the excavation speed of foundation pit. This paper introduces the application of a new type of dewatering technology in deep soft soil foundation pit dewatering. This technology adopts air compressor and vacuum pump to improve the water collection efficiency and pumping efficiency effectively and gains good economic and environmental benefits.

Applied research on slope reinforcement with cement mixing pile in deep soft soil foundation pit

Deep Quaternary soft soil layer with poor mechanical properties are widely distributed in the Pearl River Delta. In this paper, based on engineering examples, a numerical simulation method was used to study the action mechanism of new reinforcing techniques of grading slope combined with cement mixing piles. The effect of construction conditions, the thickness of the cement mixing piles, and reinforcement of the passive zone on the deformation and overall stability of a foundation pit were analyzed. Finally, the simulation results and the actual monitoring data were compared and analyzed to further verify the rationality of the model. The research reveals that the step-slope combined with cement mixing pile reinforcement has significant effects on limiting the deformation of deep foundation pit and ensuring the stability of the pit. The findings of this study can serve as a reference for similar researches[1].

Deformation of deep and large foundation pit in soft soil of Fuzhou Subway

In order to study the deformation law of deep and large foundation pits in the soft soil layer of Fuzhou, 22 lines of 2#, 5#, and 6# lines of the rail transit system in Fuzhou area are covered by 1 m underground continuous wall or 850SMW method piles + multi-layer. The measured data of the inner-supported long-shaped subway foundation pits were statistically analyzed and compared with the research results of similar projects at home and abroad. The results show that (1) the maximum horizontal displacement δhm of the continuous wall of the subway pit is 0.078–0.15% Hm, the average value is 0.118% Hm; (2) the maximum horizontal displacement point of the retaining structure is located at a distance of about 8 m; (3)the maximum settlement value δvm is in the range of 0.057–0.124% Hm, and the influence range is about 4.5 Hm; (4) the range of δvm/δhm of the foundation pit of the subway station is 0.13–1.07, and the average value is 0.96; (5) the range of the column bulge value δcu of the station foundation pit is 0.05–0.11% Hm. The research results of this paper can provide reference for the design and construction of other subway (3#, 4#, 7#, 8#, and 9#) stations and similar foundation pit supporting structures in Fuzhou.

Study on Supporting Structure Performance of Deep Soft Soil Foundation Pit near Sea under Waves, Tides, Vibration, and Unbalanced Loads

In addition to the vibration loads caused by offshore structure construction, the offshore foundation pit can also be affected by additional hydraulic loads such as waves, tides, and typhoons, hindering our understanding of the mechanical performance of the foundation pit supporting structure. In order to illuminate this mechanical performance under waves, tides, hammering, and unbalanced loads, this paper analyzed the supporting structure with theoretical and numerical simulations. The results were verified with field monitoring data. Moreover, the influences of loads, including construction vibration loads and unbalanced loads, on structure and soil were discussed. By applying these results to the engineering design, this research could provide a reference for studying mechanical properties of relevant supporting structures and the construction of offshore foundation pits.

Deformation analysis of retaining structure of deep foundation pit in soft soil under over-excavation

In the construction of deep foundation pits in soft soil areas, over-excavation of foundation pits and failure to set up support in a timely manner often occur. Due to the creep properties of soft soil, the envelope structure will be sharply deformed and cause engineering accidents. With the secondary development program of the elastic-viscose-plastic constructive model, simulate the conditions of normal construction and over-excavation of foundation pit in Soft clay area, analyze the time change regulation of retaining structures and safety state of foundation pit, in order to achieve effective control of the deformation and protect the buildings adjacent to foundation pit.

Unloading Mechanical Effect Analysis of Retaining Structure of Deep Foundation Pit in Soft Soil

The large-scale foundation pit construction site is small in area and complicated in conditions, which is easy to affect the environment. At the same time, it is prone to cracking or even collapse of nearby buildings due to uneven surface. The low-ranking official network around the foundation pit is easily damaged, and the surrounding roads are cracked. Many problems. At the same time, soft soil has the characteristics of high compressibility, low strength, large water content and small bearing capacity. It shows remarkable structural and rheological characteristics, which makes it more and more challenging to ensure the safety and stability of soft soil foundation pit engineering. In this paper, the deep foundation pit deformation of super soft soil is predicted and controlled, and the deep foundation pit engineering of soft soil is taken as the research object. The mechanical characteristics of soil unloading process are discussed, and the strength and deformation law of unloading soil are studied. The actual constitutive model and soil parameters are numerically simulated by the finite element method, which objectively reflects the mechanical state of the foundation pit excavation unloading soil, which is of great significance to ensure the safety and long-term stability of large underground engineering construction.

Investigation of the Axial Force Compensation and Deformation Control Effect of Servo Steel Struts in a Deep Foundation Pit Excavation in Soft Clay

This study presents a comparative analysis of the deformation control effect of the hydraulic servo steel struts and ordinary steel struts of a foundation pit based on the measured axial force of the steel struts, lateral wall deflection, and ground surface settlement due to pit excavation. The results indicate that ordinary steel struts installed via axial preloading exhibit a disadvantageous axial force loss with a maximum value equal to 86.7% of the axial preloading force. When compared with ordinary steel struts, the hydraulic servo steel strut exhibits a superior supporting effect. The hydraulic servo steel strut adjusts the axial force in real time based on the deformation of the retaining structure and the axial force of the struts. Thus, the ratio of maximum lateral deflection to the excavation depth of a deep foundation pit in soft soil is less than 0.3%. Concrete struts undergo unsupported exposure during the excavation process, leading to sharply increasing deformation of the retaining structure. Therefore, regarding a foundation pit with strict requirements for deformation control, the use of hydraulic servo steel struts rather than concrete struts is recommended.

A Research on the Stability and Support Technology of Pit in Half Water in the Soft Soil Foundation

Semi-shallow water soft soil foundation pit usually have unbalanced earth pressure and soil strength is low. Therefore its support is more difficult in this situation. Anchor-grouting tube technology in geotechnical engineering as a new type of grouting technique has been widely applied in the engineering, this paper applies the technology of soft soil foundation pit supporting, and expounds the concrete application form, whether through site for the application of anchor tube process of soft soil foundation pit respectively for steel sheet pile lateral displacement monitoring, and based on the finite difference method for foundation pit deformation and sliding field is studied. The calculation results show that the technology can effectively control the deformation of foundation pit in soft soil, especially for deformation control.

Study on the Rheological Properties and Constitutive Model of Shenzhen Mucky Soft Soil

In order to obtain the basic parameters of numerical analysis about the time-space effect of the deformation occurring in Shenzhen deep soft-soil foundation pit, a series of triaxial consolidated-undrained shear rheology tests on the peripheral mucky soft soil of a deep foundation pit support were performed under different confining pressures. The relations between the axial strain of the soil and time, as well as between the pore-water pressure of the soil and time, were achieved, meanwhile on the basis of analyzing the rheological properties of the soil, the relevant rheological models were built. Analysis results were proved that the rheology of Shenzhen mucky soft soil was generally viscous, elastic, and plastic, and had a low yield stress between 90 and 150 kPa. The increase in pore-water pressure made the rheological time effect of the mucky soft soil more remarkable. Thus, the drainage performance in practical engineering should be improved to its maximum possibility extent to decrease the soft-soil rheological deformation. Lastly, a six-component extended Burgers model was employed to fit the test results and the parameters of the model were determined. Findings showed that the extended Burgers model could satisfactorily simulate the various rheological stages of the mucky soft soil. The constitutive model and the determination of its parameters can be served as a foundation for the time-space effect analysis on the deformation of deep soft-soil foundation pits.

Simplified Method for Estimating the Inner Support and Internal Force through Stand Column Uplift of Foundation Pit in Soft Soil

In the excavation process of Foundation Pit in Soft Soil area, it will cause the large uplift of stand column, and may cause the instability and failure of foundation pit in case of being serious; but the additional internal force of support system caused by the uplift and sinking of stand column currently has no practical calculation model in the foundation pit design. This article discusses the reason and mechanism for uplift of stand column in the foundation pit, proposes the simplified calculation method of estimating the additional bending moment of inner support through uplift of stand column in allusion to two main influence factors of bottom heave and vertical load caused by the excavation of foundation pit, inversely calculates the permissible additional bending moment of inner support based on this, verifies by combining the monitoring data for uplift of stand column in the actual project, and finally, further proposes the project measures corresponding to the uplift of stand column, hoping to provide beneficial reference for the similar project.

Basal Heave Stability Analysis of Deep Foundation Pit in Soft Soil

In order to avoid the limitations in traditional methods of basal heave stability analysis for deep foundation pit, numerical analysis was adopted by establishing a two-dimensional elasto-plastic finite element model. Resistant heave stability safety factor was calculated using the fitting formula acquired from the simulated results, and then the upheaval value and plastic range under critical conditions were studied. The results show that the soil stress will be redistributed with the increasing of excavation depth until upheaval damage taken place. The upheaval value will be larger, the plastic range will be greater, and the basal heave stability will be worse with the decreasing of plastic parameters. Meanwhile, the basal upheaval peak position will transform gradually from excavation center to diaphragm neighborhood with the decreasing of plastic parameters. There is a critical value associated with supporting parameters for soil plastic parameters and the possibility of upheaval destruction increases significantly if the plastic parameters are less than the critical value.

A Comprehensive Analysis of Soft Soil Foundation Pit Accident

Factors affecting the supporting structure in a soft soil foundation pit are analyzed in this paper. Based on the information obtained from accident investigation, design factors and construction factors, such as insufficient embedded depth of pile and insufficient anchor pullout resistance, are discussed in detail. The analysis and conclusions can serve as a good reference for similar projects design and construction.

Analysis on Excavation Procedure in Large Soft Soil Foundation Pit of Shanghai Expo Axis

On the basis of the large soft soil foundation pit of Shanghai Expo axis and the Section 1 of underground complex project, this paper proposes the appropriate excavation procedure according to the results obtained by using centrifuge model tests and time hardening creep model, which uses ANSYS to compute foundation pit deformation caused by different excavation timeline, sequence and longitudinal excavation width of remained berm. The results of numerical calculation were approximately close to the horizontal displacement of the underground diaphragm wall measured both on site and from centrifuge model tests, so numerical calculation could well reflect the deformation behavior of excavation. The study also showed that more than 80% of underground diaphragm wall deformation caused by soil creep occurred within 60 days after the remained berm being excavated. In order to decrease the deformation, it would be useful to conduct plate structures as soon as the remained berm started being excavated. Remained berm and middle plate had good control over underground diaphragm wall deformation. When the remained berm was excavated by jumpily digging method, it would be advisable to firstly excavate the vicinity of underground diaphragm wall where there were no required protected objects nearby. The longitudinal excavation width of remained berm was proposed to be 20 meters in the north part and 30 meters in the south part.

Simulation Experimental Research for the Influence of Embedded Ratio on SMW’s Working Traits

Through model test, the displacement of the top of the wall and the strain of the core pile with excavating were measured and the deformation and damage characteristics of the soft soil foundation pit supported bySMWwas revealed. By parameter sensitivity analysis and back analysis,ABAQUSwas applied to build the numerical simulation model ofSMWin soft soil and the model parameters were obtained and the model was optimized. The impact of the embedded depth on the displacement of the top of the wall ,strain of the core pile, the bending moment of the core pile and the surface subside were systematic studied. The curve fitting equations of the above mentioned action effect variables with embedded ratio were obtained. Studies have shown that embedded depth was significantly influenced the working properties ofSMW. When the embedded ratio reaches a critical value, the nonlinear variation characteristics of the above mentioned action effect variables will occur and the signs of foundation pit's failure will occur.