Fundamentals of Deep Excavations

Chang-Yu Ou

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000431254
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Excavation is an important segment of foundation engineering (e.g., in the construction of the foundations or basements of high-rise buildings, underground oil tanks, or subways). However, the excavation knowledge introduced in most books on foundation engineering is too simple to handle actual excavation analysis and design. Moreover, with economic development and urbanization, excavations go deeper and are larger in scale. These conditions require elaborate analysis, design methods and construction technologies.

This book is aimed at both theoretical explication and practical application. From basic to advanced, this book attempts to achieve theoretical rigor and consistency. Each chapter is followed by a problem set so that the book can be readily taught at senior undergraduate and graduate levels. The solution to the problems at the end of the chapters can be found on the website (http://www.ct.ntust.edu.tw/ou/). On the other hand, the analysis methods introduced in the book can be used in actual analysis and design as they contain the most up-to-date knowledge. Therefore, this book is suitable for teachers who teach foundation engineering and/or deep excavation courses and engineers who are engaged in excavation analysis and design.

Chapter 1 Introduction to the analysis and design of excavations

DOI: 10.1201/9780367853853-1

When Terzaghi (1943) first considered the stability of excavations, he defined those whose excavation depths were smaller than their widths as shallow excavations while those with depths larger than their widths were deep excavations. Years later, Terzaghi and Peck (1967) and others, including Peck et al. (1977), revised that excavations whose depths were less than 6 m could be defined as shallow excavations and those deeper than that as deep excavations, considering that the use of sheet piles or soldier piles grows uneconomical once the excavation depth goes beyond 6 m. Generally speaking, the analysis methods for shallow excavations are comparatively simple. In fact, more and more excavation projects are located in urban areas. To avoid damage to adjacent properties caused by excavation, diaphragm walls are commonly used as retaining walls. What’s more, computer programming has done most of the analysis and design, which applies to all depths, following the same theories. Therefore, it isn’t meaningful to distinguish between deep and shallow excavations any more.

Analysis of deep excavations is usually required before going into design. Deep excavation analysis is a typical soil–structure interaction problem. Soil is a nonlinear, inelastic, and anisotropic material. Its behavior is normally affected by water contents. Some types of soils have the characteristics of consolidation and creep. Theoretically, analysis of deep excavation involves simulations of elastoplastic behavior of soil, interface behavior between soil and retaining walls, and the excavation process. A reasonable excavation analysis in practice should make use of conventional soil mechanics and simple structural mechanics, along with appropriate modifications according to field observation. For a detailed discussion of excavation analyses, please see Chapters 5–8.

A complete deep excavation design includes a retaining system, a strutting system, a dewatering system, excavation procedure, a monitoring system, and building protection. Figure 1.1 illustrates the general course of deep excavation design, which will be paraphrased as follows:

Figure 1.1 Flowchart of analysis and design of an excavation.

1.1 Geological investigation and soil tests

Deep excavation projects include the construction of building basements and subway stations, whose depths may range from several meters to 30 or 40 m. The process of an excavation may encounter different kinds of soils underneath the same excavation site—from soft clay to hard rocks. The closer the construction site to a hillside, the more complicated the geological condition. The geological condition determines the type and construction of retaining walls and greatly influences the excavation behavior as well. In addition to the geological condition, the distribution of groundwater also contributes to the excavation behavior. For example, it may fall below the hydrostatic water pressure in an urban area because of the long-term overuse of groundwater. On hillsides, there might exist an artesian aquifer, which has a rather high pressure. In seaside areas, seawater may permeate into the soils and tides will make it fluctuate daily. To sum up, the geological investigation of an excavation project aims at the soil conditions underneath the construction site and the distribution of groundwater.

There are many soil tests for deep excavations. These include tests of basic soil behavior, such as unit weight, specific weight, water content, and Atterberg limit, and tests of mechanical behavior, such as consolidation and strength. According to the information from the soil tests, engineers can judge whether the soil is a drained or an undrained material. Since the strength differs significantly between drained and undrained materials, the choice of analysis methods and retaining walls varies accordingly. The more precise the results of soil tests, the more reasonable the analysis results and the more economical are the retaining and excavation systems.

1.2 Conditions of the adjacent properties

From the perspective of mechanics, deep excavation necessarily gives rise to movement of the soils near the excavation site. However, if the movement or settlement is too large, it will damage neighboring buildings or public facilities. Some buildings or facilities are especially sensitive to settlement, a little of which may bring about cracks in beams or columns, while others can stand more settlement. The allowable settlement of a building or a facility is highly correlated with its foundation type, construction material, structural type, and age. Therefore, investigation of the condition of adjacent properties and public facilities before designing an excavation project is required to determine the allowable settlement, which in turn determines the type of retaining and strutting systems and the selection of auxiliary methods.

1.3 Confirmation of the conditions of an excavation site

According to the shape, area, and elevation of the excavation site, along with geological conditions, the distribution of groundwater, and conditions of neighboring properties, we can decide on a provisional retaining method and an excavation method. Therefore, it is necessary to have a thorough understanding of the conditions of the excavation site.

1.4 Design criteria

Whether an excavation is successful is significant to the lives and properties of many people. Thus, an appropriate design criterion must be selected before design.

A deep excavation design criterion should include at least the method of stability analysis, the methods of simplified and advanced deformation and stress analyses, a dewatering scheme, the design of structural components, and property protection.

This book explores theories, as well as their application to deep excavation, from both the theoretical and practical perspectives. Getting familiar with the contents should not only help the reader to understand excavation behavior and design and analyses, but also help develop a suitable excavation design criterion.

1.5 Collecting case histories of the nearby excavations

The first job of excavation design is to decide the type of retaining wall and the excavation method. Though we can choose the most reasonable methods based on geotechnical theories, geological conditions, and neighboring property conditions, an excavation analysis doesn’t always predict the excavation behavior exactly because geological investigation may not cover all kinds of soils to be encountered during excavation and because the simulation of excavation process may not be complete. A case history of nearby excavation, equivalent to a full-scale excavation experiment, helps design the excavation project no matter if it was successful or not in the end.

1.6 Auxiliary methods

A deep excavation may have difficulty meeting design criteria, due to poor geological conditions or deteriorated adjacent buildings. Even if it reaches the criterion, it may be very expensive. Auxiliary methods can help solve the dilemma. These include soil improvement, buttress walls, cross walls, micropiles, and underpinning. Please see Chapter 11 for the design of auxiliary methods.

1.7 Excavation analyses

Excavation analyses consist of stability analyses, deformation analyses, and stress analyses. Stability analyses, including base shear failure analyses, sand boiling analyses, and upheaval analyses, aim at avoiding failure or collapse. Base shear failure analyses and sand boiling analyses can determine how deeply the retaining wall should penetrate into the soil. Upheaval analyses can decide on dewatering schemes at different stages. For stability analyses of excavation, please see Chapter 5. For dewatering analyses, see Chapter 9.

Deformation analyses are to find the lateral deformation of retaining walls, the heave of the excavation bottom, and the settlement of the soil outside the excavation zone. The lateral deformation and the settlement of the soils affect not only the safety of the retaining wall but also the adjacent properties. As to the heave of the excavation bottom, it is correlated with the capacity of the strutting system.

Stress analyses involve those of strut load and of bending moment and shear of retaining walls. The data on the strut load are necessary for the detailed design of struts or anchors, while those of the bending moment and the shear are relevant to the choice of the appropriate type and dimension of retaining walls, and sometimes to the design of reinforcements. For methods of stress and deformation analyses of excavations, please refer to Chapters 6–8.

1.8 Layout of the strutting system

A strutting system comprises either horizontal struts or anchors, which contribute to the resistance to the lateral earth pressure generated by excavation. Stability analyses determine the penetration depth of a retaining wall. After finishing the stability analyses, the tentative locations and vertical distances of the struts can be determined. The procedure of installation and of the later removal of the struts for the construction of floor slabs basically determines the locations and vertical distances of the struts. After analyzing deformation and stress, the type and size of the struts are accordingly decided. For the detailed design of strutting systems, please see Chapter 10.

1.9 Monitoring system

In spite of thorough geological investigations, soil tests, rigorous analyses, and design before excavation, excavation theories, based on many hypotheses, can hardly cope with many uncertainties of geological conditions. Therefore, excavation has to be carried out along with monitoring instruments, which tell, immediately, changes in stress and displacement generated by excavation. The engineers in charge can thereby check the safety of excavation at any time. For large-scale excavations, the geological uncertainty increases and a monitoring system is urgently required. For the items and design of monitoring systems for deep excavation, please see Chapter 12.

1.10 Protection of neighboring properties

Due to the unbalanced earth pressures on the two sides of a retaining wall, excavation produces displacement of retaining walls and settlement of the ground. The buildings and public facilities within the range of the settlement may have differential settlement. When settlement goes beyond the allowable amount, the nearby buildings or public facilities may be damaged. The damages may turn out to be structural or non-structural. To avoid such damages, prediction of the settlement is necessary to decide whether and how to take protecting measures. There are many ways to protect neighboring properties. Some of them can be deduced from theoretical analyses. Others rely on engineering experience and empirical data. Chapter 11 introduces various measures to protect adjacent properties and related analyses.

Chapter 2 Engineering properties of soils and geotechnical analysis

DOI: 10.1201/9780367853853-2

2.1 Introducti...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• About the author
• 1 Introduction to the analysis and design of excavations
• 2 Engineering properties of soils and geotechnical analysis
• 3 Excavation methods and lateral supporting systems
• 4 Lateral earth pressure
• 5 Stability analysis
• 6 Stress and deformation analysis: simplified method
• 7 Stress and deformation analysis: beam on elastic foundation method
• 8 Stress and deformation analysis: finite element method
• 9 Dewatering in excavations
• 10 Design of retaining structural components
• 11 Excavation and protection of adjacent buildings
• 12 Monitoring systems
• Appendix A: Conversion factors
• Appendix B: Soil properties at the TNEC excavation site
• Appendix C: Definition of plane strain
• References
• Index