1816
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Table 3. Comparison of forces in wall and deflection (BP30#)
Embed-
ment
(m)
Bending
Moment
(kN-m/m)
Shear
Force
(kN/m)
Deflection
(mm)
Traditional
Approach
(Geoguide 1)
25.26
10,320
1,269
184.8
C580
21.24
19,610
2,403
193.9
% Change
- 16%
+ 90%
+ 89%
+ 5%
# BP30 is a pile with the highest retaining height of 19.4m
Based on the table above, there is an average decrease of 3m
embedment by using C580 approach which is about 14%
reduction to the original proposed embedment. However, the
advantage of the reduction of embedment by using C580 results
in a substantial increase in structural forces. Thus, it would
require an increase of structural capacity by increasing the steel
ratio in these cases.
5 COST SAVING
Although, the bending moment and shear force of the cantilever
bored pile wall increased 90% in average as shown in Tables 2
and 3 above, the increase of the steel reinforcement percentage
is only from 1.32% to 1.78% mainly due to the use of 3 m
diameter bored pile with high bending capacity. The cost
comparison using the two appraches is presented in Table 4.
Table 4. Summary of cost difference
Average
pile length
(m)
Steel
percentage
(%)
Total cost per pile
(million HK$)
Traditional
Approach
(Geoguide 1)
43.0
1.32
3.03
C580
39.5
1.78
2.74
% Change
- 3.5
+ 0.46
- 0.29
(Saving of 10%)
The increase of the construction cost of the bored piles is not
directly proportional to the increase of pile depth. Normally, it
is an exponential increase with depth. Thus, the reduction of a
pile embedment results in a saving in the overall construction
cost. From the table above, there is about 10 % of overall cost
saving on the cantilever bored pile retaining wall system when
using C580 approach. This represents an amount of HK$ 9.6
million saving on this project.
Besides the cost saving, there is also a non-quantified saving
by using C580 approach such as the reduction of construction
risk for deeper pile, (Sze et al 2005).
6 PEFORMANCE AND MONITORING
A 2.0 m deep capping beam was installed onto the pile heads
before excavation and deformation markers were provided for
the measurement of the wall top movement by land surveying
methods. The excavation commenced in December 2011 and
was completed by stages in June 2012, The condition of the
cantilever wall during the excavation stage in January 2012 is
shown in Figure 7.
SERVICE
RESERVOIR
ROAD
BP30
BP11
Figure 7: Excavation of the cantilever bored pile wall (January 2012)
The monitoring results of the wall top movement by land
survey is compared with the design predictions are shown in the
Table below. It is concluded that the use of C580 is a safe
approach for the design of a permanent cantilever bored pile
wall.
Table 5. Comparison of predicted and actual wall deflection
Traditional
Approach
(Predicted)
C580
(Predicted)
Actual lateral
movement
BP11
146.7 mm
146.7 mm
21 mm
BP30
184.9 mm
193.9 mm
20 mm
The actual movement from monitoring results maybe less
than predicted due to:
1. the beneficial effect from the capping beam is not
accounted for in the design
2. the slopping ground on the retaining side was
significantly reduced in height due to the formation of a
temporary piling platform and hence loading was reduced
3. local variations in the bedrock weathering profile was
not accounted for in the design
A comprehensive and continuous monitoring of the wall
movement was conducted throughout the construction period
comprising the followings will be discussed in future:
1. Piezometers provided behind the wall to monitor the
groundwater conditions.
2. Inclinometers and B-OTDR (Brillouin Optical Time
Domain Reflectometer) optical fibres were installed into ducts
reserved in the piles to monitor the behaviour of the cantilever
bored pile wall.
7 CONCLUSIONS
The use of C580 design approach together with partial factors as
suggested in “Notes on Design of Excavation and Lateral
Support Works Using the Limit State Partial Factor Method”
(BD 2005) is a safe and economic design for a cantilever bored
pile wall. In general, there is a cost saving by using C580
design in comparison with the traditional design approach as
described in Geoguide 1 (Second Edition)
However, the pile deflection profile and the long term effect
(i.e creep effect) of a cantilever bored pile wall system using
C580 design approach should be further reviewed.
8 ACKNOWLEDGEMENTS
Authors would like to thank Dr. P.L.R. Pang of Geotechnical
Engineering Office, HKSAR for the review and guidance of the
first C580 permanent cantilever wall design report.
