Foundation Pit Excavation Research Articles

Fluid-Solid Coupling Effect on Numerical Simulation of Deep Foundation Pit Deformation in Soft Soil Area

Groundwater is abundant in soft soil areas, which has a significant impact on the excavation deformation of foundation pits. In this paper, based on the monitoring of deep foundation pits with waterproof curtains in Shanghai deep foundation pits, COMSOL Multiphysics is used to model the layers of the foundation pits and establish a two-dimensional seepage-consolidation coupled model for hierarchical dewatering excavation. The feasibility of numerical simulation of regional foundation pits, the modeling method of the foundation pit seepage model is explored, and the presence or absence of waterproof curtains, and the influence of aquitards on the horizontal displacement of foundation pits and surface settlement outside the pits is analyzed. The research shows that the simulated foundation pit deformation values are in good agreement with the actual monitoring values and that the effect of dewatering and seepage has a great influence on the foundation pit deformation. The waterproof curtain has a significant effect on reducing the drop in the water level outside the pit and controlling the surface settlement. After installing a waterproof curtain, the amount of ground settlement is reduced, but the disadvantage is that the deformation of the enclosure structure increases. Finally, the influence of aquitard on the deformation of foundation pit excavation is simulated, and the distribution characteristics of the flow network diagram under different permeability coefficients are analyzed. According to the analysis of the foundation pit deformation law and flow network diagram, it is considered that the waterproof curtain can effectively reduce the influence of aquitard on foundation pit deformation.

Site Measurement Study on Mechanical Properties of SMW Piles of Building Structures in Sandy Soil Areas

SMW (soil mixing wall) piles have been widely used in soft soil areas such as Jiangsu, Shanghai, Tianjin and so on, and they have many advantages, such as retaining the structures of foundation pits. In order to promote the application of SMW piles in sandy soil areas such as Henan province, SMW piles were used in a deep foundation pit project of a high-rise building in Zhengzhou. Three SMW piles in the middle area of the foundation pit were selected for site measurement to determine the mechanical properties of SMW piles in sandy soil areas. Several typical test sections were determined along the height of the pile. The vibrating string type of the reinforcement dynamometers were set on the H-shaped steel of each test section, and the stress distribution of the H-shaped steel along the depth of the pit was obtained via testing. The axial force, bending moment and shearing force of the H-shaped steel were further calculated, and the affecting factors and development laws of the internal force distribution of the H-shaped steel were analyzed in detail. The research shows that, at the stage of foundation pit excavation, the overall stress of H-shaped steel increases gradually. The axial force of H-shaped steel in an SMW pile is mainly affected by such factors as the weight of the H-shaped steel, the weight of the crown beam and the first support system, the weight of the breast beam and the second support system, and the frictional resistance of the cemented soil. The bending moment and shearing force of H-shaped steel are mainly affected by such factors as the lateral soil pressure and the concentrated forces of the two support systems. When the foundation pit was excavated to the base, the development of and changes in the law of internal force with regard to the H-shaped steel was analyzed. When the overall internal force of the H-shaped steel is at its maximum, the maximum absolute values in terms of the axial force, bending moment and shearing force are −481 KN, 371 KN·m and 123 KN. In the process of foundation pit excavation and backfilling, the point of contraflexure of the H-shaped steel moves down gradually, and the fixed end of corresponding SMW pile also moves down and stabilizes below the base. These results may provide a reference for the design and construction of SMW piles of building structures in sandy soil areas.

Safety Evaluation and Energy Consumption Analysis of Deep Foundation Pit Excavation through Numerical Simulation and In-Site Monitoring

Foundation pit excavation is common in urban construction, while safety evaluation is always significant in every specified project. The soil material properties, groundwater level, excavation method, supporting structure, monitoring points’ arrangement, and so on distinguish from one site from another. Thus, many studies have looked into the safety and reliability of designated projects. This paper was based on the co-construction underground tunnel project of a deep foundation pit excavation in Suzhou, China. This paper aimed to perform a safety evaluation on this foundation pit by means of numerical simulation for parameter influence analysis, as well as scientific comparison with in-site monitoring data. To minimize the energy consumption and contribute to the carbon neutrality, a brief energy consumption analysis was also conducted. The results indicated that the maximum deformation of the foundation pit bottom is 4.5 cm and the deformation of the foundation pit is within the allowable range. The maximum horizontal displacement of each excavation is approximately at 10 m to 12 m of the diaphragm wall and the largest deformation is 28 mm. The maximum ground settlement is less than 16 mm, which confirmed the safety during excavation. It is ideal that the above deformation law will provide a reference for similar projects. Furthermore, this research simulated and monitored the whole cycle of foundation pit excavation, and contributes to savings in energy consumption and limiting of carbon emissions.

Study on Design and Deformation Law of Pile-Anchor Support System in Deep Foundation Pit

In this study, a deep foundation pit project of Nanlishi Road in the Xicheng District of Beijing was taken as the engineering background. Based on the monitoring method of that project and referring to its design scheme principle, this study applied advanced monitoring technology methods such as anchor axial force and deep horizontal displacement monitoring. The mechanism of pile–soil interaction, the stress change and deformation law of the three-pile and two-anchor support systems of deep foundation pits, and the stability of deep foundation pit support in an anhydrous sandy pebble stratum, were studied in depth. Results show: The axial force of the anchor rod had great loss in the early stage of prestressed tension locking; with the deepening of foundation pit excavation, the lateral pressure of stratum increased gradually, and the prestress of the anchor increased until the end of excavation, where it tended to be stable; the maximum horizontal displacement of the pile was smaller than the design value, and the maximum horizontal displacement was not at the top of the pile; the axial force of the prestressed anchor varied with the formation pressure and surrounding load; the tension of the lower anchor had a certain influence on the axial force of the upper anchor. Except for the east side of the foundation pit, the anchors of the first layer were all stabilized at about 140 kN, and the anchors of the second layer were stabilized at about 150 kN. The third row of anchors on the north side was stable at around 170 kN. By analyzing the variation law of stress and deformation of the supporting structure of the foundation pit, the timeliness of the data during the construction process was improved, and a reference is provided for the informatization construction of related working conditions.

Numerical Simulation and Field Test of the Interaction between Existing Station and Enclosure in Open Excavation and Adding Stories Construction

When constructing open excavation and adding stories on the upper part of an existing station structure, the new foundation pit contacts with the existing station at zero distance; subsequently, the construction has a greater impact than that of other close constructions. To explore the cross-impact of new and existing structures on open excavation and story-adding above the existing station, in this study, through numerical simulation and field measurement, we analyze the deformation law of the existing station and foundation pit retaining structure in the excavation and story-adding construction above the existing station. The following conclusions are obtained. The excavation of the upper foundation pit leads to the overall uplift of the existing station, and the sidewall exhibits an internal extrusion trend, which is caused by an increase in the horizontal load on the station caused by the unloading of the vault and expansion of the formation plastic zone. Affected by the existing station structure, some foundation pit retaining structures are end-suspended piles; the bottoms of the end-suspended pile slips are evident. This study offers control measures for the primary support connection between the pile bottom and initial retention and protection of the existing station, deep hole grouting reinforcement of the stratum behind the pile, and reserving earth berm at the pit bottom to reduce the deformation of the end-suspended pile. The research results can provide a reference for constructing the added story of a subway transfer station.

Effect of principal stress direction during consolidation on the shear strength properties of natural granite residual soil

In numerous real-life civil engineering practices, including multi-stage embankment construction and foundation pit excavation, the direction of the major principal stress σ1 becomes rotated. In these cases, the granite residual soil may be subjected to inclined consolidation (IC) with σ1 being inclined, because of the relatively high permeability as a result of the fissures formed during weathering. While the effect of the σ1 direction during the shear on the strength of granite residual soil (inherent strength anisotropy) has been primarily established, little is known about how the soil strength is affected by the direction of σ1 during consolidation. This paper presents the effects of IC on the shear strength properties of natural granite residual soil through undrained hollow cylinder torsional shear tests. The effect of the soil structure is also considered by testing remolded soil specimens. The results reveal that while IC changes neither the shape of stress–strain curve nor the specimen features at failure, it leads to an increased ultimate shear strength in terms of both the undrained strength and stress ratio, with the remolded soil being more affected. The presented data provide new insights into the understanding of residual soil strength behaviors.

Experience Analysis of Construction Foundation Pit Excavation and Monitoring Measures

Experience Analysis of Construction Foundation Pit Excavation and Monitoring Measures

Study for Longitudinal Deformation of Shield Tunnel in Side of Foundation Pit Based on Virtual Image Technique

A prediction model of enclosure structure deformation caused by the excavation of foundation pit is established. Based on the virtual image technique, the soil deformation caused by sidewall deformation is calculated. Meanwhile, the deformation model considering rotating and staggering of shield tunnel is used to establish a calculation method of longitudinal deformation of adjacent shield tunnel. The result suggests that: (a) The horizontal displacement of adjacent shield tunnel is normally distributed. (b) With increasing retaining structure deformation, it is obvious that deformation value and range of adjacent shield tunnel also increase. (c) As the distance between the adjacent shield tunnel and the enclosure structure increases, the horizontal displacement of the side shield tunnel decreases nonlinearly. (d) The shield tunnel with shallow buried depth is greatly affect by the excavation of adjacent foundation pit. As the depth of the tunnel increases, the impact gradually decreases.

Deformation Characteristics of the Foundation Pit of the Underpass Viaduct and Grey Relational Analysis of the Maximum Horizontal Deflection of the Retaining Wall

With the rapid development of rail transit, the excavation of new foundation pits will inevitably approach or pass through existing structures. Therefore, choosing a reasonable support scheme can effectively control the deformation of the foundation pit and ensure the safety of existing structures. This paper investigates the excavation of a typical foundation pit in Guangdong that underpasses a viaduct as the research object. This study uses field monitoring and numerical simulations to analyze the deformation characteristics of the supporting structure and introduces grey relational analysis (GRA) to investigate the key factors that influence the maximum horizontal deflection of the retaining wall. The results indicate that all monitored values are substantially less than the warning value specified in the code. Moreover, the horizontal deflection of the retaining wall and the horizontal displacement of the top of the wall at the measuring points near the existing bridge piles are smaller than at other measuring points. This support system has a high safety factor and a large space for optimization. The main factors affecting the maximum horizontal deflection of the retaining wall are as follows: second strut horizontal spacing > first strut horizontal spacing > layered excavation thickness > retaining wall thickness > retaining wall stiffness. The results of this study will provide technical guidance for the design of similar projects and optimization of their construction.

Analysis of Deep Foundation Pit Pile-Anchor Supporting System Based on FLAC3D

Relying on a deep foundation pit project in Beijing, using FLAC3D three-dimensional finite-difference software simulation combined with displacement monitoring data verification method, the deep foundation pit excavation and three-pile and two-anchor rod support system in anhydrous sand pebble stratum are systematically analyzed, and summed up the variation law of formation stress, internal force of soil nail, axial force of bolt, stress and displacement of pile in the process of excavation and support of deep foundation pit. The results show that during the excavation of the foundation pit, the maximum horizontal displacement of the sidewall is not at the top of the pile and the top of the slope but at a certain distance below the top of the pile. The axial force of the anchor rod is unevenly distributed along the length direction, the axial force of the free section is equal, the axial force of the anchoring section decreases in turn, and the prestress of the anchor rod spreads to the anchoring section after tensioning and locking. The soil-nailed wall is a passive force-bearing system, the force is small during the excavation and support process, and the axial force distribution along the length direction is large in the middle and small at the two ends. By analyzing the variation law of stress and deformation, the timeliness of the data in the construction process is improved, and an effective and reasonable case reference is provided for the informatization construction of simulated relevant working conditions.

Innovations of the UH model associated with a clue of stress–strain chain

One of the most fundamental issues in geotechnical engineering is deformation calculation. Within the elastoplastic constitutive model framework, a new concept of stress–strain chain is proposed for the analysis of the computational process of plastic strain. The connotation of the chain is then demonstrated through seven stages, starting with six-component stress and ending with six-component plastic strain. The inadequacies of previous research on plastic strain computation induced by several factors such as anisotropy, dilatancy, hardening law, and hardening parameter are evaluated utilizing the stress–strain chain. Meanwhile, based on the stress–strain chain, innovations of the UH model such as the transformed stress method, the unified normalized compression line, and the unified hardening parameter are introduced and validated. It can be seen with the stress–strain chain that plastic volumetric strain and its assignment criterion are the keys to deformation calculation. Representative innovations of the UH model facilitate the efficiency of plastic strain calculation by accurately determining plastic volumetric strain and appropriate ratio relations. Through a foundation pit excavation case, the values and ratios of strain components predicted by the UH model are more reasonable and reliable. This demonstrates that the UH model enhances the accuracy of deformation calculations greatly.

Centrifuge Model Investigation of Interaction between Successively Constructed Foundation Pits

A series of centrifuge model tests were conducted to study the interaction between successively constructed adjacent foundation pits. The stress, deformation, and earth pressure on retaining structures and the settlement of the soil between the two adjacent foundation pits during successive construction were investigated by a comprehensive instrumentation program. To reveal the effect of the construction sequence, both the stress and deformation of successively constructed foundation pits were compared. The results showed that the stress and deformation of the retaining structure in the foundation pit constructed first were larger than those in the foundation pit constructed later. Due to the inward displacement of the soil around the foundation pits excavated first, the first strut of the foundation pit constructed later underwent high tension during the construction of the first foundation pit. The lateral deformation of the retaining structure of the foundation pit excavated first increased with the increase of the excavation depth. However, the excavation of the second foundation pit reduced the earth pressure on the retaining wall between the two excavations, thus leading to the recovery of the inward deformation in the first excavation. However, the top of the retaining wall deformed into the first foundation pit during the whole construction. The settlement of the soil between the two foundation pits showed a superposition effect. During the construction of the two foundation pits, the settlement of the soil between them kept increasing. The active earth pressure on the middle wall of the foundation pit constructed later was lower than that on the middle wall of the first foundation pit. The excavation of the foundation pit constructed later had no significant effect on the passive earth pressure of the first foundation pit.

Study on the Influence of Excavation Unloading on Pile Depth in Sandy Soil

With the development of urban construction, uplift pile has been widely used in underground structure bearing buoyancy. The pile is buried deep underground, and the foundation pit is excavated on the pile foundation, the stress redistribution of the soil after the foundation pit excavation will reduce the lateral stress of the pile at the bottom of the pit, resulting in the reduction of friction resistance, but this effect is limited to a certain depth at the bottom of the foundation pit. Referring to the unloading influence depth corresponding to the vertical unloading coefficient in the spring back calculation of foundation pit and introducing the unloading influence depth corresponding to the horizontal unloading coefficient, it is deduced that the relationship between the horizontal unloading coefficient and the vertical unloading coefficient is based on the friction angle. Based on the bearing capacity control and numerical simulation, it is determined that the corresponding depth when the horizontal unloading coefficient is 0.95 is the unloading influence depth. The depth to width ratio of excavation has a great influence on unloading influence depth, and the internal friction angle of soil has little influence on unloading influence depth, while the influences of pile diameter and pile-soil friction coefficient can be ignored. The formula of unloading influence depth is derived in this paper. Comparing the unloading influence depth of sand and soft soil, the unloading influence depth of sand is greater than that of soft soil.

Analysis of the influence of super large area and super deep foundation pit excavation construction on surrounding subways

The shortest distance from the deep foundation pit envelope of the project to the subway station building is only 7.7 meters, and the deepest excavation depth of the foundation pit is about 24 meters. The surrounding environment of the foundation pit is complex, involving many buildings, the construction is difficult, and there is a great risk of construction safety. The effective control of the structural deformation of the existing subway station is the key to selecting the underground enclosure structure and construction process of the surrounding post-construction projects. Combined with the actual situation of the project, the three-dimensional numerical analysis method is used to analyze the impact of the construction of the foundation pit of the building on its adjacent structures in detail, and the relevant treatment measures are proposed. The research shows that the unloading effect caused by the large-scale excavation causes the soil outside the pit to move towards the inside of the pit. Under the influence of the soil deformation transfer effect, the Beiguan Station of the R1 line subway produces certain rebound, settlement and horizontal displacement, but the values of each deformation index are within the deformation control standard, the numerical analysis considering the influence of small soil strain can better reflect the influence of foundation pit excavation on the displacement, deformation and internal force of the existing structure. It can better match the actual engineering experience.

Research on stability of steep bank slope and reserved thin-walled rock cofferdam during excavation of intake foundation pit

The intake of water diversion project is generally located on the bank slope above the large reservoir or deep water area of a river. Due to the influence of water level change and hydraulic action, the rock mass of bank slope is seriously weathered and broken, leading that the rock mass has low strength and is easy to collapse or landslide. In this paper, the water intake engineering of the long-distance diversion project in central Shanxi Province is selected as the research object. The multi-point displacement meter is used to monitor the slope deformation after anchorage and concrete retaining wall support. Based on the FLAC3D V6.0 software, the numerical simulation of the excavation process of the steep reinforced river bank slope is carried out, and the long-term deformation of the slope is predicted. The excavation scheme of retaining outer thin-walled rock wall as water cofferdam is proposed. The excavation process of foundation pit, which is supported with concrete-filled steel tubular pile reinforcing cofferdam and concrete supporting beam, is simulated. It is found that the upper part of the steep slope supported by anchor cable and concrete retaining wall is stable, and the large deformation and plastic failure of the water-retaining thin rock wall are mainly affected by the disturbance of foundation pit excavation. The concrete-filled steel tubular pile support avoids the collapse of the reserved thin-walled cofferdam, and the horizontal beam in the foundation pit improves the stress state of the external wall of the foundation pit. The research results of this paper reveal the evolution mechanism of rock mass deformation and failure during excavation of foundation pit on steep bank slope, and provide guidance for excavation and support of foundation pit for similar bank slope.

Foundation Settlement Response of Existing High-Speed Railway Bridge Induced by Construction of Undercrossing Roads

The construction of undercrossing roads often has an adverse effect on the upper existing railway line, and can cause an uneven subsidence of the crossed section. In response to the influence of new road construction on the existing high-speed railway bridge, a highway passing under an existing high-speed railway bridge was taken as the research object. Based on the FLAC3D finite difference software, a computational model of new roads undercrossing a high-speed railway bridge was established to analyze the structural deformation response of a high-speed railway bridge. The results show that the excavation of foundation pits during new road construction causes an overall upward heaving of the soil layers between the pits, with a greater amount of heaving near the high-speed railway piers than that at other locations. Meanwhile, a degree of upward displacement of the high-speed railway bridge piers occurred. The filling of a road base structure generates an additional stress greater than the original consolidation pressure, inducing an overall subsidence of the soil layers between the pits. The final deformation during construction of new roads undercrossing the high-speed railway bridge shows that the displacement of adjacent piles is manifested as an overall settlement and as a horizontal displacement to the side of soil excavation.

Effect of damping ditch on blasting vibration reduction by field experiment and numerical simulation

In order to analyze the influence of a damping ditch on the blasting vibration, a large number of on-site monitoring tests with or without damping ditch were carried out based on the foundation pit excavation project. Based on the Sadovsky’s empirical formula, the optimized vibration velocity attenuation model was established by using the waveform subsection correspondence method and least square method. The calculation results are in good agreement with the measurement results. The average errors in radial direction and vertical direction are 17.04 % and 10.76 % respectively. A series of finite element models were established to study the influence of the damping ditch geometry on the damping effect. The experimental and numerical results show that the damping ditch can obviously reduce the blasting seismic effect, with the highest damping ratio in the area near blasting source. For different damping ditches, the damping ratio first increases and then decreases proportionally to the increase of distance between the measuring point and the blasting zone. The damping ditch depth has the best damping effect on blasting vibration, followed by the distance between the damping ditch and the blasting source, and finally the damping ditch width, which is consistent with the damping mechanism of theoretical analysis. The attenuation of the surface wave is only related to the damping ditch depth. The research results can provide a reference for the design of blasting vibration isolation.

Interactions of Foundation Pit on the Underlying Adjacent Existing Underground Structures

The influence of foundation pit excavation on deformation and stability of the supporting structure of underlying adjacent existing tunnel is a comprehensive geotechnical engineering problem, including unloading effect of foundation pit excavation and the coupling effect of deformation of the tunnel surrounding rock and supporting structure. Using numerical calculation methods, this paper changes the different excavation depths and widths of foundation pit and discusses the evolution law about force and deformation of tunnel supporting structure, and the changing relationship of the axial force of tunnel anchor with the distance between foundation pit and tunnel is obtained and compares the influence of different excavation way of foundation pit on the axial force of anchor rod near the existing tunnel. It is found that with the increase of distance between foundation pit and tunnel, the axial force of anchor rod at the top of tunnel under the foundation pit entirely symbolizes a trend of increases first and then decreases, the axial force of the leftmost anchor rod changes from pressure to tension, then decreases gradually. When the foundation pit depth increases, the axial force of the anchor rod at the top of the tunnel increases, the axial force of the leftmost anchor rod decreases, and the width of foundation pit absolutely has little effect on the axial force of the leftmost and the top tunnel.

Deformation Stability Response of Adjacent Subway Tunnels considering Excavation and Support of Foundation Pit

Abstract Since the safety and stability of the original tunnel structure are easily affected by the adjacent foundation pit excavation, it is strongly necessary to study the deformation evolution of tunnels during the adjacent foundation pit excavation. With regard to the two cases that tunnel is adjacently located at the right and bottom of foundation pit, the influence of different supporting methods, including pile support, bolt support, pile-bolt support, and shotcrete-bolt support, on the tunnel stability was investigated on the basis of the whole excavation process numerical simulation of deep foundation pit for determining the best foundation pit supporting beneficial to the stability of adjacent tunnel. The results indicate that both one-step excavation and multistep excavation have great influences on the displacement of adjacent tunnels, wherein the influences on the tunnel located at the right of foundation pit are greater than those at the bottom of foundation pit. Multistep excavation is recommended for the foundation pit adjacent to shallow tunnel. In the case of the tunnel located on the bottom of the foundation pit, the maximum stress generated around the tunnel is small, the maximum stress area is limited, and the displacement of tunnel monitoring points is also small. For the tunnel located at the right of the foundation pit, the pile-bolt supporting can effectively limit the displacement of soil between the tunnel and the foundation pit, reduce the maximum stress and the maximum stress distribution area, and effectively control the tunnel displacement.

Case Study on the Deformation Coupling Effect of a Deep Foundation Pit Group in a Coastal Soft Soil Area

Simultaneous construction of adjacent projects may lead to emergencies in a foundation pit group, which significantly affects the deformation and safety of foundation pits. In this study, the deformation characteristics of a deep foundation pit group and the mutual interactions among the adjacent foundation pits were observed by a monitoring system during excavation. Field data of the foundation pit group, including the lateral deflections of the enclosure pile, the ground subsidence, as well as the vertical column movements, were analyzed and compared with individual excavations in Shanghai. The field data showed that the excavation of the adjacent foundation pit reduced the lateral deformation of the enclosure structure, caused by the reduction of active earth pressure acting on the retaining pile. Furthermore, the foundation pit excavated later caused upward movements of the soil between them. However, the foundation pit excavated earlier had a negligible influence on the vertical column movements of the foundation pit excavated later. Due to the optimized excavation sequence of the deep foundation pit group, the deformation of this special excavation was well controlled.