Chapter 2 - Project Review: Ground Condition and Design Concepts by FYP - The Collapse of Nicoll Highway

Pre-tender Site Investigations

FYP — 2015-02-10 14:32:32 UTC

A general site investigation was conducted including 14 boreholes piezocones and associated in-situ and laboratory test.

The borehole information at the time of tender showed the stratigraphy to comprise Fill, Estuarine deposits, Upper Marine Clay, Fluvial deposits, Lower Marine Clay, Estuarine or Fluvial deposits, Old Alluvium.

Post-tender Site Investigations

FYP — 2015-02-10 14:32:56 UTC

Additional site investigation was conducted in two phases. Between June and July 2001, a total of 60 boreholes and eight piezocones were sunk along the entire route. In February and March 2002, further soil investigation works were carried out in the vicinity of Crawford underpass.

Investigation Result

FYP — 2015-02-10 14:33:25 UTC

North of Kallang Basin is underlain by Marine Clay extended from depths of about 15m to 42m below ground level.

Groundwater table was generally at about 2m below ground level.

The topsoil layer at the collapse site comprised a fill of varying thickness, resulting from reclamation of the site in two stages. (North part was reclaimed about 50 years ago, South part was reclaimed about 20 years ago.)

The north and south sections, some 20m apart, show significant differences in stratigraphy below the base of the lower marine clay in the area of the collapse. For example, the Lower Estuarine Clay (E) is present below the Lower Marine Clay on north wall at the collapse area. It does not occur below the south wall.

Piezocones

FYP — 2015-02-10 14:37:12 UTC

Design Principles For the Retaining Wall

FYP — 2015-02-10 14:39:43 UTC

In accordance with the requirements stated in the Specifications and LTA Design Criteria of the contract,

The most salient points of adoptions:

Design of JGP Slabs

FYP — 2015-02-10 14:43:19 UTC

The report on the various wall type studies included a check on the capacity of the JGP slab. The initial design of the JGP slab included a 1.3m wide gap of untreated zones in the JGP slab spaced at 10m centres. The design also considered a 1.8m wide JGP “wailing” beam spanning the untreated zone and included a check on the capacity of the JGP wailing beam.

The study reports included two further checks on the capacity of the JGP slab. The first check considered the compressive stress and the second check considered the potential for buckling of the JGP slab. Both calculations used the total horizontal stress in the JGP slab obtained directly from the Plaxis analysis. The Plaxis computed horizontal stress was factored by 1.2 and compared with the design strength of the JGP to ensure that the factored stress was less than the design strength of the JGP. A similar procedure was used when the factored JGP stress was checked to ensure that it was smaller than the theoretical buckling stress of the JGP slab.

Instrumentation and Construction Monitoring

FYP — 2015-02-10 14:44:09 UTC

There were over 2000 monitoring instruments in C824, including inclinometers, vibrating wire piezometers, strain gauges and load cells. We 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. There were failings in the operation of the instrumentation system, particularly with respect to the real-time strut load monitoring system.