Control and Prevent Land Subsidence Caused by Foundation Pit Dewatering in a Coastal Lowland Mega City: Indicator Definition, Numerical Simulation and Regression Analysis

Coastal mega cities are often commercial centers because of convenient traffic. Safe elevation above sea level is vital for their sustainable development. Global climate change and sea level rising increase flood risk especially in the lowland subsidence area. Shanghai of China was selected as research background. Although groundwater exploitation had been strictly restrained to control land subsidence and reserve safe elevation, lowering groundwater level during underground excavation cannot be avoided. Foundation pit dewatering (FPD) was intensively performed in underground exploitation during urbanization and city renewal. The FPD settlement accelerated land subsidence. Controlling FPD subsidence was urgent. Normally, the maximum horizontal influence radius of foundation pit excavation was less than three times excavation depth (H), and the 3H settlement was only caused by the FPD. The 3H maximum settlement was defined as the evaluating indicator of FPD land subsidence, and the corresponding 3H drawdown was defined as the control indicator of land subsidence. The FPD conceptual models were established on the basis of estimation and investigation of foundation pit information, including pit area, pit shape, pit depth, and curtain depth. Numerical models were established and a total of 5650 FPD numerical simulations were performed to investigate the land subsidence and FPD drawdown. Multi-factor regression analysis was conducted to obtain relations between land subsidence and FPD drawdown. Regression models were established between the 3H drawdown and the shape, area, depth, and curtain depth of foundation pit on the basis of the numerical simulations. A typical example introduced to verify the regression models. The regression models were used to manage the FPD land subsidence by controlling the 3H FPD drawdown. The results can provide reference for the land subsidence control in a coastal lowland city.

Discussion on the Improvement of Technical Safety Scheme Compilation for Hazardous and Ultrahazardous Projects

This paper reviews the summary and analysis of special technical safety schemes for hazardous and ultrahazardous activities, supported by housing scaffolding, installation, and dismantling of outer wall attached tower crane, deep foundation pit with supporting structure, municipal bridge box girder formwork support, as well as grooved Larsen steel sheet pile, which are commonly seen in recent years, so as to enhance the pertinence, rationality, and economy of the special program to strengthen safety.

Numerical study on stress paths in grounds reinforced with long and short CFG piles during adjacent rigid retaining wall movement

Ensuring the safety of existing structures is an important issue when planning and executing adjacent new foundation pit excavations. Hence, understanding the stress state conditions experienced by the soil element behind a retaining wall at a given location during different excavation stages has been a key observational modelling aspect of the performance of excavations. By establishing and carrying out sophisticated soil–structure interaction analyses, stress paths render clarity on soil deformation mechanism. On the other hand, column-type soft ground treatment has recently got exceeding attention and practical implementation. So, the soil stress–strain response to excavation-induced disturbances needs to be known as well. To this end, this paper discusses the stress change and redistribution phenomena in a treated ground based on 3D numerical analyses. The simulation was verified against results from a 1 g indoor experimental test conducted on composite foundation reinforced with long and short cement–fly ash–gravel (CFG) pile adjacent to a moving rigid retaining wall. It was observed that the stress path for each monitoring point in the shallow depth undergoes a process of stress unloading at various dropping amounts of principal stress components in a complex manner. The closer the soil element is to the wall, the more it experiences a change in principal stress components as the wall movement progresses; also, the induced stress disturbance weakens significantly as the observation point becomes farther away from the wall. Accordingly, the overall vertical load-sharing percentage of the upper soil reduces proportionally.

Analysis of Situ Test Results of Earth Pressure and Pile Internal Force under Pile-Anchor Support in a Deep Foundation Pit in Xi’an

At present, in deep foundation pit engineering, on the one hand, practice is ahead of theory, and on the other hand, theory can not correctly reflect the actual construction process and environmental effects. In order to further study the distribution and change law of earth pressure and internal force of pile body in deep foundation pit pile-anchor supporting system, field monitoring test of earth pressure and pile body reinforcement stress was carried out. The monitoring results show that before excavation, the distribution of earth pressure has a great relationship with the layering of the soil, and it is distributed in sections along the depth. Compared with the theoretical static earth pressure, the measured data of the upper depth is relatively small; after excavation, the overall earth pressure is distributed along the depth in a “z” shape under the non-limiting state. As the excavation progresses, the magnitude of the reduction of the earth pressure varies from place to place, and the magnitude of the decrease of the soil with better properties is not large; after the excavation, the stress and earth pressure of the pile reinforcement correspond to each other, and the distribution is also nonlinear. The existence of anchor tension has an obvious effect on improving the internal force of the pile. The selected earth pressure calculation methods have some discrepancies in the calculation of the earth pressure value of the project, and they need to be further improved. The research in this paper can provide reference and reference for the calculation of earth pressure and support design of pile-anchor supported foundation pit.

Soil Pressure and Deformation Analysis of Small Size Deep Foundation Pit

In this paper, the influence of space effect on soil pressure and deformation of deep foundation pit was considered, and the finite soil pressure calculation model was established. The soil pressure of deep foundation pit was calculated by assuming the slip surface and using the finite soil limit equilibrium theory. Then, PLAXIS 3D finite element software was used to establish finite element models of different plane sizes and depths. The distribution regulation of side wall soil pressure and deformation of deep foundation pit was calculated. Finally, the results of finite soil pressure calculation was compared with finite element method. The results shown that: The soil pressure of small deep foundation pit was affected by space effect, and the soil pressure and deformation decrease significantly along the foundation pit depth. Shear fracture Angle was related to the ratio of width to depth of foundation pit, and it was no longer a constant value of 45°+φ/2. Therefore, the spatial effect should be considered in the calculation of soil pressure of small deep foundation pit. The research results can provide some guidance for the design and calculation of similar small size deep foundation pit.

Full-Scale Field Test On Retaining Structure Enhanced With Soil Nails And Prestressed Anchors

Retaining structure enhanced with soil nails and prestressed anchors is found good at constraining the horizontal displacement and therefore ensuring the stability of the foundation pit during excavation. Based on these advantages, such retaining structure is widely used in foundation excavation practice. This paper presents results of a series of in-situ tests conducted to investigate the mechanical behaviors of retaining structure enhanced with soil nails and prestressed anchors. Behaviors of three different retaining structures enhanced with i) soil-nails; ii) soil-nails and prestressed anchors without unbonded part; iii) soil-nails and prestressed anchors with a 2.5m unbonded length, were monitored during staged excavation to investigate the influences of i) the prestressing force and ii) unbonded length of the prestressed anchors on the performance of the entire retaining system. It was found that the affecting the stress and deformation of composite retaining system, which is in agreement with the other published results in the literature. The variation of the magnitude and distribution of soil nail force responding to the anchor prestressing force however showed no systematic trend. The unbonded length of anchors, which is suggested to be the main factor affecting the structural stability in dense materials in the literature, is found to have little influence in loose fill materials used in this study. Studies presented in this paper are useful for the rational design and serviceability analysis of the composite soil-nailed retaining structure enhanced with prestressed anchors.