A Solution to the Braced Excavation Collapse in Singapore By Javier Artola B., Civil Engineering Stevens Institute of Technology, 2003 SUBMITTED TO THE DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING IN CIVIL AND ENVIRONMENTAL ENGINEERING AT THE MASACHUSE'TS-I INSTfI'TE OF TECHNOL OGY MASSACHUSETTS INSTITUTE OF TECHNOLOGY MAY 3 1 21 JUNE 2005 (.Tvier Artnla_ All riihts reserved_ LIBRARI ES The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part. Signature ofAuthor_ ',-A -_ _ -Departin'nt of Civil and Environmental Engineering May 24, 2005 Certified by Andrew ttle Professor of Civil and Environmental Engineering Thesis Supervisor n. Accepted by Andrew J. Whittle Chairman, Departmental Committee for Graduate Students iR .IVES A Solution to the Braced Excavation Collapse in Singapore By Javier Artola Submitted to the Department of Civil and Environmental Engineering On May 24, 2005 in Partial Fulfillment of the Requirements for the Degree of Master of Engineering in Civil and Environmental Engineering ABSTRACT At about 3:30pm on April 20, 2004, a 30m deep excavation adjacent to Nicoll Highway in Singapore collapsed, resulting in four casualties and a delay of part of a US$4.14 billion subway project.
This thesis examines the flaws in the original design of the bracing system, which have been cited as causes of the failure. The Author then proposes a revised design for the braced excavation system. The Plaxis finite element program was used to simulate the excavation process and compute forces on the major structural elements in the original design. Some pertinent background information on this program is provided throughout the thesis in order to better understand the significance of certain errors in the input data of the original model that ultimately led to the incorrect assumptions and calculations of the original design.
A new model using this same program was regenerated with a corrected set of input assumptions, thereby leading to reasonable estimates of structural forces. These results were then used to propose a revised design of the excavation support system and compare this design to the original used in the excavation project. There are several lessons that could be learned from this structural failure, one being the need to acknowledge the limitations built in advanced analysis software systems, and another being the importance of ascertaining that the user understands every feature of the product. A cost estimation of the proposed design is given and compared to the original design in order to evaluate the viability of the proposed design in the construction bid.
Finally, some important conclusions are drawn from this study that should be applied to future large-scale construction projects where public safety and welfare is at stake. Thesis Supervisor: Andrew J. Whittle Title: Professor of Civil and Environmental Engineering Acknowledgements I would foremost like to thank my parents for their unwavering support of my interests and goals, in academia and elsewhere. For this thesis, I owe a great deal to Professor Andrew Whittle, without him I would have never been exposed to this interesting research.
His guidance and efforts encouraged me to find a solution to this problem and led me to the culmination of my thesis project. Professor Jerome Connor has been a wonderful mentor and inspiration to me, and I would like to acknowledge his wisdom and support in every aspect of my life at MIT. I would like to acknowledge Pat Dixon and Cynthia Stewart, for their support and patience in the submission of my thesis. I would also like to acknowledge my dearest girlfriend, Wendy, for all her help and support and for being that joyful thought in the most stressful times.
Finally, I would like to acknowledge the families of the victims of this tragedy, may God be with you and your loved ones in the afterlife, and may their deaths serve as a remembrance of the commitment and responsibility that we - engineers - have pro bono publico (for the good of the public). -3- Table of Contents 1. Review of Slurry (Diaphragm) Wall Excavation Systems.1 General Methods of Slurry Wall Construction .2 Cross-Lot Braced Slurry Wall Excavations. The Original Design .1 Overview of the Project .2 Design of M3 Support System .4 Design of Structural Elements .1 Design of Diaphragm Wall in Type M3 Area .2 Design of the Strutting System for Diaphragm Wall in Type M3 Area .3 Design of Strut-Waler Connection .1 The Under-design of the Diaphragm Wall Using Method A .1 Background and Errors in the Input Data of the Plaxis Finite Element Program .2 The Impact of Method A and Method B on the Diaphragm Wall Design .2 The Impact of Method A and Method B on the Strutting System Design .3 Under-design of Strut-Waler Connection .1 Incorporation of C-channel Stiffeners in Waler Beam Connections.2 Omission of Splays in Strut-Waler Connections.
A Revised Design for the Type M3 Excavation Area .1 Revised Plaxis Model .2 Design of the Diaphragm Wall .3 Design of the Strutting System .4 Design of Waler Connection. 51 -4- Table of Figures Figure 1: Trenching Equipment. 9 Figure 2: Typical Construction Sequence of Slurry Walls. 10 Figure 3: Typical Excavation Sequence in Cross-lot Excavations.
12 Figure 4: Preloading Arrangement and Measured Brace Stiffness. 12 Figure 5: Overview of Circle Line Construction Stages 1 to 5. 13 Figure 6: Overview of Cut and Cover Tunnel Adjacent to Nicoll Highway. 14 Figure 7: Overview of M3 Area.
15 Figure 8: Soil Profile and Design Support System for M3 Section. 16 Figure 9: Cross-section for wall Type M3. 21 Figure 10: Strut-Waler Connection. 22 Figure 11: Strut-Waler Connection Channel Stiffeners.
26 Figure 12: Site Before and After the Collapse. 27 Figure 13: Mohr-Coulomb Failure Model. 30 Figure 14: Diaphragm Wall Deflections under Methods A and B. 34 Figure 15: Diaphragm Wall Bending Moments under Methods A and B.
35 Figure 16: Inclinometer Readings I-104 & 1-65. 36 Figure 17: Stiffener Plate and Waler Beam Web Buckling. 39 Figure 18: Load-Displacement Curves of the C-channel and the Plate Stiffener Connections.40 Figure 19: Types of Strut-Waler Connections. 41 Figure 20: Sketch of Proposed Reinforcement for Diaphragm Wall.
45 Figure 21: Bending Moment Envelope Diagram for a 1.2m Thick Diaphragm Wall. 45 Figure 22: Maximum Deflection Diagram for a 1.2m Thick Diaphragm Wall. 46 Figure 23: Diaphragm Wall and Waler Connection Detail for the 9th Level of Struts. 49 -5- List of Tables Table 1: Soil Profile Description.
17 Table 2: Summary of Plaxis Input Parameters in Original Design. 19 Table 3: Plaxis Parameters under Different Design Methods. 31 Table 4: Strut Loads at Type M3 Area under Design Methods A and B .37 Table 5: Summary of Soil Parameters used in Revised Plaxis Model. 43 Table 6: Summary of the Strutting System Design.
47 Table 7: Summary of the Original and Revised Designs for the Strutting System. Introduction Braced excavation systems are widely used in a variety of construction projects, such as cut-and-cover tunnels and building basements. Common malpractice or negligence in the design and construction of such systems can result in large-scale losses of capital and human lives. There are several examples of excavation collapses and corresponding studies that investigate their origins.
This thesis examines one in particular: the 30m deep excavation collapse adjacent to Nicoll Highway in Singapore, which occurred on April 20, 2004. There have been various reports that explain the causes of this collapse. The final report of the Singaporean Ministry of Manpower (MOM) Committee of Inquiry has just been released, and is cited frequently throughout this thesis. However, it is not the author's intent to further analyze these studies, but instead to use the information already available to propose an alternate and effective design for the excavation system.
A finite element model using the soil-structure analysis program Plaxis v.0 was generated for this excavation using the proper parameters to obtain data on the required design capacities for the temporary diaphragm wall, strutting system, waler connection, and other elements of the project. All the design procedures are explained in detail throughout this thesis. The original design was performed as per the British code BS8002 for soil-strut interaction and BS5950 for structural steel design. However, the proposed design was done using the American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Highway Bridges (1 4 th Edition) and the American Institute of Steel Construction (AISC) Allowable Stress Design (ASD) Manual of Steel Construction (9 th Edition).
The final design of the excavation system was obtained through an iteration process of the model and design criteria. Review of Slurry (Diaphragm) Wall Excavation Systems 2.1 General Methods of Slurry Wall Construction Slurry wall design and construction demands attention to a variety of factors such as slurry materials (i. processing), excavating equipment, and panel size. For example, the depth of the slurry wall may be determined by the soil conditions present at the site, or the site layout may limit panel sizes.
One often encounters existing utilities or nearby buildings in urban excavations and they may need to be protected or relocated. In addition, water-stopping details should be given special consideration because slurry walls are frequently part of the permanent structure. Working schedules can also be impacted by the requirements for traffic maintenance. Construction procedures should therefore address these and other relevant issues in order to optimize the construction project as a whole.
A slurry wall is constructed by linking a series of slurry wall panels in a predetermined sequence. The panels are excavated to specified dimensions while at the same time slurry or another stabilizing fluid is circulated in the trench. Excavation equipment may range from simple clamshell buckets to hydraulic clamshells to hydrofraises (Xanthakos, 1994, Parkison & Gilbert, 1991, Ressi, 1999, Bauer, 2000). In addition, individual contractors have developed their own f(typically) patented trenching equipment.
Figure 1 displays a variety of trenching equipment employed in slurry wall construction. -8- a (a) (b) (C) Id) {e) (f) (a) (h) 1) Figure 1: Trenching Equipment (Xanthakos, 1991) (a) Clamshell bucket attached to a kelly. (b) Vertical percussive bit with reverse circulation, (c) Percussive benching bit. (d) Rotary benching bit.
(e) Rotary bit with vertical cutter. (f) Rotary drilling machine with reverse circulation. (h) Bell-mouth suction rotary cutter with direct circulation. (i) Horizontal auger machine.
Figure 2 presents the basic steps in typical slurry wall construction. The first step is to clear the site of any possible obstructions. Guide walls are then built to stabilize the upper few feet of soil and to guide the trenching equipment (controlling the vertical orientation of the panels). End-stops are inserted into the panel after trenching is completed in order to help form water- tight joints connecting adjacent panels.
The end-stops are withdrawn after the adjacent panel is trenched. After a panel is excavated to the specified dimensions, then a reinforcement cage is placed into the slurry filled trench. Reinforcement cages may be spliced if the required cages are too heavy for the lifting equipment. The bottom of each panel is cleaned prior to concreting because sands and other soils may form intrusions that undermine the integrity of the wall (i.
its water-tightness, stiffness, and strength). Concrete is then carefully tremied into the trench and continuously displaces the slurry therein. The top few inches of the panel are always chipped in order to bring the fresh concrete to the surface because the slurry is trapped in the top inches of the panel. -9- An important issue in the concreting process is the segregation of concrete aggregates during fast concreting.
Slurry can become trapped within the tremied concrete, thereby creating soft zones within the slurry walls.