Chapter 5 - Causes and Findings of the Collapse by FYP - The Collapse of Nicoll Highway

Reason 1: Under Design of Diaphraagm wall using Method A

FYP — 2015-02-10 10:32:54 UTC

Method A over-predicted the undrained shear strength; underestimated the wall bending moment and deflection, and resulted in the under-design of the diaphragm wall.

Method B should have been used in this circumstance. The use of Method A instead of Method B resulted in an underestimation of the predicted bending moments and deflections by about 50% or a factor of 2 for the original design in M3.

Reason 2 : Under Design of the Water connection in the strutting system

FYP — 2015-02-10 10:45:47 UTC

The estimation of the load on the water connection for double struts assumed that splays would absorb one third of the load in the struts, thus splays were omitted in this event which result in only 70% of load in the struts instead of 100%. Also, the axial design capacity was about 70% of the assumed design load for the connection due to the inappropriate use of C-channels. This two factors cause the design capacity of the connections to be only one-half the required design strength.

These design errors resulted in the failure of the 9th level strut-waler connections together with the inability of the overall temporary retaining wall system to resist the redistributed loads as the 9th level strutting failed. The catastrophic collapse then ensued.

Reason 3: Abuse of Back Analysis in Type M3

FYP — 2015-02-10 11:21:04 UTC

The two critical back analysis at Type M3 were geotechnically flawed. There were repeated breaches of the instrumentation review levels at Type M3. All the experts agrees that on the basis of the second back analysis for Type M3, work should not have been allowed to proceed in that area.

Reason 4; Failure to insitute regular, dose, effective monitoring system

FYP — 2015-02-10 11:25:46 UTC

Close monitoring was necessary throughout but no readings were taken for I-104 as Paul Broome explained that both I-65 and I-104 were tracking very similar movements. Site Supervisor also explained that inclinometer was covered with ‘a lot of soil’, preventing workers from collecting data.

Reason 5:Inclinometers at key locations of the diaphragm wall were not monitored daily during critical periods

FYP — 2015-02-10 11:27:09 UTC

There were over 2000 monitoring instruments in C824, including inclinometers, vibrating wire piezometers, strain gauges and load cells. They find the instrumentation monitoring system in C824 has fallen short. There were serious shortcomings in both the management and the application of instrumentation data for safe construction control.

Reaason 6: Unsafe acts

FYP — 2015-02-10 11:31:25 UTC

No stop work order was issued in the face of unsafe acts, unsafe conditions and unsafe attitudes.

The many site problems from March 2003 to April 2004 showed a lack of an informed safety culture.

Some of the administrative errors

FYP — 2015-03-01 13:40:46 UTC

a) Not learning the right lesson from numerous warning signs and incidents in the C824 site going back to the middle of 2003. Many of these incidents were critical failures with severe potential consequences. The Launch Shaft 2 incident was one such failure.

b) Lack of proper design expertise and experience of the design personnel;

c) Failure to institute a regular and close monitoring regime;

d) The systematic pattern of the builder’s scant respect and callous approach to the use of the results of each back analysis.

e) Inadequate implementation of the hazard and consequences analysis;

f) Inadequate risk assessment for new or unfamiliar technologies and impact on public safety;

g) Lack of safety management system;

h) Unresponsive organization and human factors;

i) Inadequate safety culture;

j) Unsafe acts and practices;

k) Production pressures to complete schedule regardless of safety concerns;

l) Poor teamwork and communication;

m) Poor engineering judgement in critical areas;

n) Untimely remedial work;

Some of technical factors

FYP — 2015-03-01 13:54:38 UTC

The technical factors included the following which resulted in the inability of the system to take the re-distributed loads.

a) In sufficient toe-penetration of the diaphragm walls;

b) The occurrence of a plastic hinge in the diaphragm wall approximately 3 weeks before the collapse;

c) Peculiar conditions at the M3 area which are:

i. The presence of deep buried channel

ii. The presence of 66 kV cable crossing

iii. Curves diaphragm wall (on plan)

d) The instrumentation and monitoring system was ineffective;

e) The hacking of Tunnel Eyes at the TSA Shaft caused an increase in the time required for the installation of the 10^th level struts at the M3 area;

f) Large spans of excavation remained unsupported for extended period;

g) Lack of quality during construction;