Volume 2, Issue 1 March 2017

Case History-3

Control Measures for Installation and Removal of Temporary
Earth Retaining Walls
Dr. POH Teoh Yaw
Geotechnical Engineering Department,
Building and Construction Authority, Singapore
The process of wall installation and removal may cause excessive ground movements if not properly controlled. Proper
control measures should be implemented to minimise the effect of wall installation and removal, hence avoiding
unnecessary damage to the adjacent structures. This paper highlights some control measures for wall installation andremoval to minimise the impact of wall installation and removal. Case histories of wall installation and removal are alsopresented to demonstrate the important of implementing control measures during wall installation or removal.

■ Close proximity to existing buildings

Designer to allow
adequate clearance forsafe wall installation

As the city developed, buildings get closer. Due to highland prices in the city, developer may need to optimise
the land use. This often resulted in basement constructed
next to adjacent buildings. Such situation poses greatchallenges to the construction team. Installation ofretaining wall in close proximity to existing buildings
requires proper selection of machinery and tools andproper control and supervision of the piling works. In
addition, designer should also need to allow adequate
set-back distance for the safe installation of the retaining
walls. Damage to the neighbouring building may cause
unnecessary repair cost and also delay in construction
program. Figure 1 shows an example of installation ofretaining wall in close proximity to existing building.
Figure 2 shows a typical basement construction in close
proximity to existing building where various control
measures can be adopted to minimise potential damageto adjacent buildings.

■ Case study of installation of soldier pile wall byvibro-hammer
Figure 3 shows a plan layout for a housing developmentwith a basement. In the original design, 3 site
investigation boreholes were drilled. Results of these 3site investigation boreholes showed that the ground consisted of residual soils and weathered rocks of the Bukit TimahGranite. In view of the favorable ground conditions shown in all 3 boreholes and no concern of lowering of ground waterlevel, the designer has adopted a soldier pile wall with structural lagging in between the soldier pile as the retaining wallsystem. During installation of soldier piles using vibro-hammer, a few neighbouring houses was reported damage. Workwas stopped for 3 months for further investigation. As part of the investigation, 2 additional site investigation boreholes
along an existing drain was conducted. Results of these two new site investigation boreholes showed the presence ofvery soft clay and very loose sand with SPT N value of 0. The designer has concluded that the damage to adjacent houses
was caused by the installation effect of soldier piles using vibro-hammer in very soft soils.
Picture 1

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In view of presence of very soft soils, the designer has enhanced the retaining wall system with additional sheet pile wall
to provide an effective water cut-off wall during basement excavation. Standard Pressed-in method was adopted to install
the additional sheet pile wall. No damage was reported during the sheet pile wall installation. Such example suggested
that standard pressed-in method may be adopted as a control measure to minimise the impact of installation of sheet
piles wall for site located in difficult ground conditions and in close proximity to existing buildings.

Figure 2

Figure 3

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■ Case study of wall removal by vibro-hammer
Figure 4 shows a plan layout for a high-rise private residential development with 2-level of basement. Figure 5 shows thesection of the retaining wall system. The ground conditions consisted of Fill layer overlying a thick layer of soft marine
clay. Sheet pile wall and compressive ring slabs (where part of the permanent basement slab was used as compressive
ring to support the basement excavation) was adopted as retaining system. Such retaining wall system is getting popularamong designer in Singapore as it is a robust retaining system that able to provide effective support to restrict groundmovement, and hence minimizing potential damage to the adjacent buildings. Ground movements at the end of
basement construction was within its design limit as shown in Figure 6. The builder decided to remove two sections of
the temporary sheet pile wall next to open car park. Vibro-hammer was used for the sheet pile extraction. Excessive
ground movement was observed after the sheet pile extraction and the open car park and the adjacent road wasdamaged. This case study show that it is necessary to implement control measure such as proper equipment and method
statement during sheet pile removal to avoid potential damage to adjacent properties.

Figure 4

Figure 5

Figure 613Volume 2, Issue 1 March 2017

■ Control measures for wall removal
Figure 7 shows a summary of control measures may be adopted by the project parties to minimise the impact of removalof the temporary earth retaining wall. Proper implementation of such measures may help to minimise the risk of damage
to the adjacent structures.

Figure 7*This case history is based on the presentation at “IPA Seminar on Pressed-in Technology in Singapore”, which was heldon 2 March 2017.

■ A Brief CV of Dr. Ng Tiong Guan
Dr Ng Tiong Guan is the immediate past president of the Geotechnical Society of Singapore. He
graduated from the University of Technology Malaysia with Bachelor in Civil Engineering 1st Class
Hons. Degree in 1992 and obtained his PhD degree from the National University of Singapore in1999. He co-founded a specialist geotechnical consulting firm GeoEng Consultants in Feb 2002,
which became part of Golder Associates in Nov 2011. Dr Ng is currently the Principal of Golder
Associates and Executive Director of Golder in Singapore. Dr Ng specialises in analysis, design andsupervision of earth retaining system, deep foundation and ground improvement works.

■ A Brief CV of Dr. Poh Teoh Yaw
Dr. Poh is a Deputy Director with Building and Construction Authority which oversees andadministers the regulatory framework on building structure safety in Singapore. He is a geotechnicalspecialist with over 18 years of practical experience. He has authored over 18 publications ingeotechnical design and construction including those published in international peer-review journals
and conferences.

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