1.1 Background and Motivation
To cope with the rapid growing demand of the underground space, it is necessary to construct multi-level basement for new buildings. The wall displacement induced by the excavation would increase with the excavation depth of multi-level basement. In urban area with soft clay deposit like Taipei city, it is often required that the diaphragm wall displacement of a deep excavation be limited to a low level to minimize the damage to the adjacent buildings.
As the auxiliary measures such as cross walls and buttress walls are installed in the excavation zone, not only the undrained shear strength of clay within the excavation zone would equivalently increase, but the three-dimensional effect would also be obvious. The so-called three dimensional effect accounts for the presence of buttress walls and cross walls in strengthening the stiffness of excavation support system, and the wall deflection can significantly be reduced if there is a pronounced three-dimensional effect in the excavation zone.
Clough et al. (1989) first presented a relationship among the wall displacement, system stiffness and factor of safety against basal heave for excavation in clay. By applying the relationship to the excavation in soft clay, the maximum lateral displacement of the diaphragm wall can be rapidly predicted. However, the requirement for the
excavation projects to control wall displacement are more rigorous in recent years, and it is desirable that the maximum wall displacement be controlled within 0.5% of the excavation depth, or below 0.3% of the excavation depth in some instances. Under the circumstances, Clough’s chart is no longer applicable to estimate the wall deformation at such low displacement level. It is obvious that there is a need to incorporate the effect of cross walls in Clough’s original chart of system stiffness and to extend Clough’s design curves toward the area of high system stiffness and high factor of safety against basal heave. Due to extensive use of the cross walls in modern excavations, the potential factor of cross walls that impacts the displacement of diaphragm wall should be further explored.
1.2 Research Objectives
Cross wall is an essential part for modern excavation to refrain wall deflection, to reduce the ground settlement induced by excavation and to increase the passive resistance.
As the strengthening effect of cross walls is particularly evident in soft clay, the cross walls are widely installed in recent years. In this research, two parts are discussed in detail.
First, the system stiffness and factor of safety against basal heave proposed in Clough’s original chart will be modified to incorporate the effect of cross walls, which lacks in the present form of Clough’s chart. Clough’s original design curves will also be extended to meet modern requirements and keep its original advantage about rapidly estimating the maximum wall displacement. Second, parametric studies through three-dimensional
numerical software will be conducted to identify the important factors that govern the effectiveness of cross walls.
By fulfilling these two parts, we can have a better understanding on the behavior of diaphragm wall for excavation with cross walls, and utilize the modified Clough’s chart to reasonably estimate the wall displacement for excavations with cross walls. The objectives of this study are itemized as follows:
1. To modify the Clough’s original curves to incorporate the effect of cross walls and extrapolate the original curves. In addition, the applicability of the regression equation is evaluated by other cases. Afterwards, an appropriate range of the regression equation is described.
2. To identify the important factors that control the diaphragm wall for the excavations with cross walls. The optimal spacing of cross walls is also examined.
1.3 Research Outline
The flow chart of this study is shown in Figure 1.1, and the outline is given as follows:
1. Chapter 2 reviews previous studies on system stiffness, Plane Strain Ratio as well as the effect of auxiliary measures on the retaining wall. In addition, various characteristics and effect of cross walls are presented.
2. Chapter 3 demonstrates the cross walls that play a significant role in reducing the deflection of diaphragm wall through three case histories. Comparisons between
numerical results by PLAXIS 3D, field observation and predictions by Clough’s chart are discussed.
3. Chapter 4 revises the original Clough’s chart by extending the curves into the uncharted area. The pronounced effect of cross walls would lead to an increase of both system stiffness and factor of safety against basal heave. In addition, this outcome would be applied to the previous three cases.
4. Chapter 5 conducts three-dimensional numerical parametric analyses to explore the key factors that governs the strengthening behavior of cross walls.
5. Chapter 6 validates the effectiveness and applicability of the above two parametric studies, the revised Clough’s chart and the key factor of cross walls, by reviewing four additional case histories.
6. Chapter 7 summarizes the studies, and provides recommendations for future work.
Figure 1.1 Flow chart
• Model validation of three case histories with cross walls
Revision of Clough’s chart (Chapter 4)
• Extrapolation of design curves
• Combined system stiffness
• Adjusted factor of safety against basal heave
Parametric Studies on the Effect of Cross Walls (Chapter 5)
• Numerical scheme
• Spacing of cross walls
Applications of Parametric Studies (Chapter 6)
• Application of the revised Clough’s scheme on 4 additional cases
• Application of effect of cross walls spacing on 7 cases with alternating sand/clay layers
Conclusions and Recommendations (Chapter 7)
• Summary, conclusions and recommendations