Deflection Of Diaphragm Wall Research Articles

Numerical analysis of effect of friction between diaphragm wall and soil on braced excavation

A plane strain finite element model was established to investigate the effect of friction between diaphragm wall and soil on braced excavation. The behavior of interface between diaphragm wall and soil was simulated with the interface model of ABAQUS. Parametric studies were conducted with different diaphragm wall external friction angles δ. The results show that deflection of diaphragm wall and ground surface settlement decrease with the decrease of δ. However, the reduction effect on diaphragm wall deflection is the most significant at the depth where the maximum wall deflection occurs and can be neglected at the wall base. The ratio between wall deep inward component and wall cantilever component reaches its peak value 2.7 when δ=5°. The ratio of the maximum ground surface settlement to the maximum wall lateral deflection decreases at a reduced rate with the increase of δ. For excavation with braced diaphragm wall, the effect of friction between diaphragm and soil on the deflection of diaphragm wall and ground settlement, especially the distribution of ground surface settlement behind diaphragm, should be taken into account.

Estimating Wood Structural Panel Diaphragm and Shear Wall Deflection

Estimating shear wall and diaphragm deflection is important for several reasons, including (1) classifying diaphragms as rigid or flexible; (2) estimating intrastory drifts to compare to code specified limits; and (3) estimating the potential for excessive deflection, which may cause costly damage. Historically, shear wall and diaphragm deflection has not been a critical design consideration. Much more attention has been paid to this issue after the 1994 Northridge, Calif. earthquake. The Northridge earthquake caused designers and code officials to place more emphasis on deflection, specifically for classifying diaphragms as either rigid or flexible. Building code changes that limited the use of narrow shear walls were also a result of the Northridge earthquake. Although building codes have provided methods for estimating diaphragm and shear wall deflection, this information is scattered and incomplete. The intention of this paper is to provide a comprehensive summary of the currently recognized methodolo...

Reducing swelling pressure on the freezing method of ground treatment by drilling and coring measures for tunnelling works

The Outer Metropolitan Discharge Channel Project was implemented to relieve flooding problem at the outskirts of Tokyo. Vertical Shaft No. 2 is one of the vertical shafts constructed in diluvial sand and clay formations. The ground treatment work by Freezing Method was used to create a frozen earth wall to prevent water seepage and collapse at the launching portal during dismantling of diaphragm wall and initial launching stage of the shield machine. After considering the design strength of frozen earth, lateral pressure and safety factor, the thickness of the frozen earth during dismantling of the diaphragm wall was designed as 4.8 m while for initial launching stage of shield machine, the thickness of the frozen earth from the entrance packing was designed as 7.95 m. Totally, about 3500 m3 of frozen earth were generated. Due to the existence of clay in the frozen earth, a higher swelling pressure to the clay layer resulting from freezing was anticipated to occur during construction. As a counter measure to protect the vertical shaft and diaphragm wall, drilling and coring was carried out into the ground formation to the full depth of clay layer and an additional two metres into the underlying sand layer. The drill holes absorbed the displacement due to swelling stemming from freezing thereby reducing the swelling pressure. In order to determine the degree of impact of swelling pressure on the vertical shaft and diaphragm wall as well as the efficacy of drilling and coring into the ground formation in relieving swelling pressure, the following factors of steel reinforcement stress and deflection of the diaphragm wall as well as displacement of the internal space of vertical shaft were measured. The measurement results revealed that drilling and coring had managed to keep the stress, deflection and displacement below the respective established control values. In addition comparison was made between: (1) average swelling pressure estimated from maximum deflection of diaphragm wall; (2) average swelling pressure estimated from displacement of internal space of the vertical shaft; and (3) average swelling pressure deduced from a theoretical equation proposed by Takashi without subject to drilling and coring. The results show that the average swelling pressure estimated from maximum deflection of diaphragm wall on the dismantling date of diaphragm wall (i.e. 85th day after the freezing operation commenced) was 12 tf/m2. This value is about 70% of 17 tf/m2 i.e. the average swelling pressure without drilling and coring as deduced from Takashi's equation. Furthermore, the average swelling pressure estimated from displacement of internal space of the vertical shaft on the 153rd day after the freezing operation started (i.e. when the frozen earth for initial launching of shield machine was ready) was 18 tf/m2, which was about 80% of 23 tf/m2 i.e. the average swelling pressure without drilling and coring as deduced from Takashi's equation. In conclusion, it was found that drilling and coring in ground formation had reduced the swelling pressure resulting from freezing to 70% to 80% of the case without drilling and coring. Thus, this has proven that drilling and coring in ground formation is an effective way to reduce swelling pressure stemming from freezing method of ground treatment. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Design and construction of MRT project Contract 825 of CCL1 in Singapore

Contract 825 of CCL1 consists of 4 underground MRT stations and tunnels going under and adjacent to existing MRT lines and old buildings within difficult and changeable ground conditions. A summary of the scope and challenges that exist within the project is presented together with some of the key aspects of the design and construction of stations, which were used to overcome these challenges. Methods such as using top-down construction method with diaphragm walls as main soil retaining structure, extensive geotechnical instruments to monitor the deflection and settlement of diaphragm wall, adjacent ground and existing building and MRT structures. The instruments provide very useful information to control the site activity and reducing disturbance to existing structures. Underpinning work carried out at DBG station is also described in this paper. All the four stations are built using the top-down construction method. A key consideration for choosing this construction method was to reduce soil movements and ground water lowering induced by the 28 to 35 m deep excavation, within close proximity to existing buildings and MRT structures. This together with requirements to reduce the disruption on the heavy traffic flow and disturbance to the environment led to the choice of top-down construction with diaphragm walls acting both as temporary and permanent soil retaining structure. The selected method required a close integration between design and construction teams as the diaphragm walls were designed both for temporary and permanent conditions. The planning of the different traffic diversions to enable necessary access and openings for construction activities had also to be considered in the D-wall design. The design philosophy and principles using Plaxis analysis is described in Section 3. The varying soil conditions called for different types of equipment for D-wall excavation. The experiences during D-wall installation are reflected in Section 4. The extension of the Dhoby Ghaut station is carried out very close to the existing station and MRT tunnels imposing restrictions on the construction activities. Of special interest is the underpinning works for the B2-link between two existing stations on each side of the new one. This is described in Section 5. As the project involves deep excavations and tunnels close to and under existing sensitive structures and MRT lines, instrumentation systems to provide feedback and assurance on design assumptions and predictions are important. The full range of geotechnical instruments has been employed on the contract with specific instrumentation schemes designed for specific structures. For the in-tunnel instrumentation, real time prism monitoring and real time track electrolevels have been utilized following the approach taken on earlier MRT projects, (Shirlaw et al 2000). For the historical surface structures individual schemes have been designed taking into account the predicted movement, the nature of the foundations, and settlement history, as far as feasible. These schemes involve limited but targeted real time electrolevels backed up by precise leveling and structure monitoring, together with recording of ground and diaphragm wall movements and piezometric changes within the various different strata. The monitoring is linked to sophisticated data collection allowing rapid processing into graphical format for ease of interpretation and dissemination to involved parties. This allows maximum benefit to be gained from the data in the form of controlling the construction works and feedback into the design process. (A). Reprinted with permission from Elsevier. For the covering abstract see ITRD E124500.

Groundwater and ground movement around deep excavation

The Tsuen Wan West Station of Kowloon Canton Railway Corporation was constructed on both previously and recently reclaimed land. The project comprised approximately 600 m of cut-and-cover approach tunnels and a 400 m long station structure constructed in a 20 m deep excavation. Extensive instrumentation monitoring was carried out to monitor groundwater levels, ground movements and displacement of adjacent structures during the course of construction. This paper presents background information on the construction site and the monitoring results. It was found that the groundwater regime on the seaward side was strongly influenced by tidal movements throughout the monitoring period, whereas the landward groundwater regime showed a diminished tidal influence soon after construction of the diaphragm wall. Rock grouting carried out beneath the diaphragm wall was effective in mitigating drawdown of piezometric level outside the wall. The magnitude of the drawdown was found to be a function of diaphragm wall depth. Data on ground settlement, diaphragm wall deflection and groundwater were interactively discussed as the construction progressed. Finally subsurface settlement data were reviewed to highlight the adverse effect of pre-trenching for diaphragm wall construction and excessive drawdown in the weak rock layer.

Neural Network Forecast Model in Deep Excavation

Diaphragm wall deflection is an important field measurement in deep excavation. The monitoring data are applied to evaluate the construction performance to avoid a supporting system failure or damages incurred to adjacent structures. Despite the numerous case histories of construction projects and several forecasting methods, no method accurately forecasts the performance of construction due to the complicated geotechnical and construction factors affecting the behavior of the diaphragm wall. This work predicts the diaphragm wall deflection by using the adaptive limited memory–Broyden-Fletcher-Goldfarb-Shanno supervised neural network. Eighteen case histories of deep excavations with four to seven excavation stages are selected for training and verification. In addition, the knowledge representation adopts measured wall deflections of previous excavation stages as inputs to the network. Doing so substantially reduces the importance of soil parameters, which are often extremely fluctuating and difficult to assess. Simulation results indicate that the artificial neural network can reasonably predict the magnitude, as well as the location, of maximum deflection of the diaphragm wall.

PREDICTION OF LATERAL DEFLECTION OF DIAPHRAGM WALL IN DEEP EXCAVATIONS

既設構造物等に近接した深い掘削において, 土留め壁の水平変位および周辺地盤における地表面沈下等がどの程度発生するかを影響解析するために土の構成則を考慮した数値解析などが行われるが, より簡便な手法による変位の概略値推定も望まれている。本論文では, 既往計測結果等をもとに地盤および施工条件を勘案した係数の相関から, 深い掘削における地下連続壁の最大水平変位を簡便に推定する手法を提案する.

Lateral Wall Deflections of Braced Excavations in Clay

The lateral deflections of sheetpile or diaphragm walls are often used to evaluate the performance of braced excavations. This paper examines the influence of a number of factors affecting the lateral deflections of strutted or anchored sheet‐pile walls in clay using the finite element method, such as the undrained shear strength, the width and depth of excavation, the wall stiffness, the penetration depth, and the depth to hard stratum beneath the bottom of the excavation. A simple procedure to estimate the lateral deflection is proposed. It has been assumed that the walls are flexible and the lateral deflections are governed by the plastic yielding of the soil below the bottom of excavation. The yielding is inversely proportional to the factor of safety with respect to basal heave. A nonlinear hyperbolic stress‐strain relationship has been used in the analysis. Deflections computed by this method compare favorably with field records and published results.