Hazard Reduction in Deep Excavations Execution

In this article, the authors consider a completely new approach in design, which is related to the use of previously un-adapted technologies known to bridge engineering in geotechnical issues for prestressing of diaphragm wall during deep excavations execution. The bridge technology described here is the prestressing of concrete structures. Hazards related to deep excavations and methods of digging them, such as the ceiling method and top&down method, are presented. Current problems in supporting deep excavation slopes are related to the use of extensive quantities of materials (such as steel struts, ground anchors, or concrete and reinforcement steel). The authors’ method helps to achieve a higher level of sustainability, which is important in a modern approach to geotechnical engineering. The non-linear arrangements of the cables according to the occurrence of the prestressing moments for a given phase are presented. Results related to numerical analysis—showing the correctness of the method and cost optimization results, showing possible savings are presented. The article is a part of the set. In the second (already published) article titled “Modern Methods of Diaphragm Walls Design”, the authors present the concept of the calculation methodology for diaphragm wall design.

Modern Methods of Diaphragm Walls Design

This article addresses hazard reduction in deep excavations. The authors present a possible combination of prestressing of concrete structures (from bridge engineering) and prestressed structures of diaphragm walls from geotechnical engineering science. This innovative concept has not yet been shown in scientific articles. The “Sofistik” software (with TENDON module–SYSP/AXES/TOPP/TGEO) and its use is shown, with graphical presentations of the suggested solution. The authors compare the provided solution through usage of Sofistik and Plaxis software. The results show possible strengthening of sustainable construction by limitation of hazards and decreasing costs (via limitation of use of expensive steel reinforcement).

Research the Integration of Geodetic and Geotechnical Methods in Monitoring the Horizontal Displacement of Diaphragm Walls

This article investigates the integration of geodetic and geotechnical methods for monitoringthe horizontal displacement of diaphragm walls. The results show that when the horizontal displacementis measured by the geotechnical method using an inclinometer sensor, the center point at the bottom ofthe guide pipe is usually chosen to be the origin to calculate displacements of the upper points. However,it is challenging to survey the bottom point for checking its stability directly. If this bottom point moves,the observation results will be incorrect. Thus, the guide pipe must be installed in the stable rock layer.But in the soft ground, this rock layer locates more deeply than the diaphragm walls, so the guide pipecannot be laid out at the required location. Geodetic methods can directly observe the displacement of thecenter point on the top of the guide pipe with absolute displacement values at high accuracy. Because thedisplacements of observation points are determined at stable benchmarks, these values are considered thepipe's displacement. Thus, an integrated solution allows the center point on the top of the pipe to be theorigin to calculate the displacements of different points located inside the diaphragm wall. Then, thecalculated values are calibrated back to the inclinometer observed values to achieve highly reliabledisplacement, which reflects the moving of diaphragm walls. An experiment integrating the geodetic andgeotechnical methods is conducted with an observation point at a depth of 20 meters at a construction sitein Ho Chi Minh city. The deviations of the top point that are observed by the two methods are -4.37millimeters and -3.69 millimeters on the X-axis and the Y-axis, respectively. The corrected observedresults prove that the integrated solution has a good efficiency in monitoring the horizontal displacementof diaphragm walls. The bottom point observed by an inclinometer is unconfident enough to choose to bea reference point.

Evaluation of Deformation of Adjacent Buildings With Different Foundation Types Caused by Foundation Pit Excavation Under Combined Support of Isolation Pile and Diaphragm Wall

The process of excavation and unloading of a deep subway foundation pit will cause deformation of the surrounding buildings. There are significant differences in building deformation due to different methods of supporting the foundation pit and building foundation forms. This study takes the deep foundation pit project of the station as an example to investigate this difference. A three-dimensional numerical finite element model of a deep foundation pit has been established that considers different types of building foundations (independent foundation, box foundation, and pile foundation). The sensitivity of the two supporting methods of the diaphragm wall and the combined support of isolation pile and diaphragm wall regarding the maximum settlement value of the building, the horizontal inclination value, the slope angle, and the foundation angular distortions were analyzed. Finally, the sensitivity of the length of the isolated pile to the maximum settlement value and the horizontal displacement value of different types of building foundations are discussed. The results show that the combined support method of isolation piles and diaphragm walls has the highest supporting efficiency (93.5% of independent foundations and 42.3% of box foundations) for angular distortions of shallow foundation buildings. The efficiency of pile foundation support is the lowest (31.4%). For the combined support method of isolation piles and diaphragm walls, the maximum settlement value, and the value of horizontal displacement of the building will decrease with increasing the length of isolation pile. When the depth of isolation pile is greater than 24 m, the settlement deceleration rate of the independent foundation and the pile foundation slows down; when the depth of isolation pile is greater than 27 m, the settlement deceleration rate of the box foundation will slow down, and the deceleration rate of the horizontal displacement of the independent foundation and box foundation will slow down.

Local Failure and Reliability Analysis of Horizontal Struts in Deep Excavation Based on Redundancy Theory

The instability failure of many deep excavations supported by diaphragm walls (retaining piles) and horizontal struts is caused by the local failure of struts and the following large area chain effect. The lack of redundancy of struts is an important reason for the overall failure of supporting structures. In this paper, based on an actual excavation project, the numerical calculation model is established by Flac3D5.0, and the reliability of the supporting structure is analyzed based on the redundancy theory. The main conclusions are as follows: the redundancy of single support is large, and strut (6) (close to the middle of the excavation) is the most important. The redundancy is reduced due to continuous failure, and the redundancy is only 3.50 when strut (1)–(7) are all failed (half of the struts). The second row of the struts has the smallest redundancy, while the third row has the biggest redundancy.

Experimental and Numerical Investigation on the Effects of Foundation Pit Excavation on Adjacent Tunnels in Soft Soil

Excavations near an existing tunnel are often encountered in underground construction. The influence of the excavation on the adjacent tunnels is not yet fully understood. This study presented a centrifugal model test about excavation next to existing tunnels in soft soil foundation. The bending moment of diaphragm wall, surface settlement, tunnel deformation, and earth pressure around the tunnel were mainly studied. The influence of tunnel location is further studied by numerical simulation. During the stabilization stage of foundation pit, the diaphragm walls present convex deformation towards foundation pit, and the surface settlement outside the diaphragm wall appears to be the concave groove type. During the overexcavation stage, the diaphragm walls are almost damaged, and the shear bands are nearly tangent to the tunnels. The displacement of the tunnels and the surface settlement rapidly increase. The deformation of the diaphragm wall and the surface settlement are limited by the existing tunnel. The numerical results indicate that the change of tunnel location has little effect on the retaining wall but an obvious effect on the tunnel itself.

Geothermal Geotechnics development program as a commonly used solution in D-wall.

<p>The project relates to an idea consisting in the use of diaphragm walls constituting a substructure system most often used during the foundation of a large volume building structure in tight urban fabric. Additionally, it offers the possibility of using this substructure as near-surface geothermal geotechnics and in conjunction with adjacent soil as an interseason heat storage in the form of enclosed box. The effect of the following development program is expected to provide a product in the form of concrete elements, that are already required for structural reasons, as diaphragm walls and barrettes with an integrated geothermal installation that allows obtaining part of the heat energy necessary for the operation of a renewable energy building. The accumulated energy, in the form of a lower energy source will be used to heat the building in winter. In summer,  the reduced temperature of diaphragm walls in relation to weather conditions will allow the building to cool down, and thus will power air conditioning systems. This will feature not only concerns about environment aspects but also provides a long-term cost-saving solution that will limit building maintenance.</p><p>Presented, currently running, two years program is an effect of cooperation between experienced deep foundation contractor and The Institute of Heat Engineering, scientific unit. The development program, presented below, is based on the industrial research phase in which the lower heat source systems are modelled in Ansys Fluent and then the calculation results are reproduced under laboratory conditions on small physical 3x2x0.7m models. The results from measurements with temperature sensors and IR cameras are used to calibrate the FEM models and to determine the most optimal distribution of the pipes with the fluid carrier.  Stage 2 will allow the analysis of the impact of thermal stress generated by the geothermal installation on the construction of the diaphragm walls and the entire building using deformation sensors.  Development works in stage 3 will allow verification of the above assumptions using real commercial construction in the interseasonal cycle.</p><p>The most significant effect of the development programme, stage 4,  will be the creation of a simple tool, on the basis of empirical data collected during model works and prototype tests, to commonly determine the thermal balance for building structures under given ground conditions for commercial buildings. The aim of the tool, being acquired by a deep foundation contractor, is a popularization of the thermo-active ground structures <span>solutions </span><span>and promotion of geothermal energy utilization.</span></p>