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Geotechnical News • September 2013
43
perfectly because there are many
factors that may influence its stabil-
ity from time to time. Therefore,
the stability of the slope should be
analysed with various approach so
that the most critical situation can be
determined. There are various types
of slope repairing methods that have
been established by the specialist and
researchers dealing with different
types of slope failure. In practice,
the method to be used is normally
confronted with the local practical
constraints mostly its effectiveness,
construction time and cost. This
paper will looked into the effective-
ness of anchored steel sheet pile wall
as a slope repairing technique in term
of FOS of the repaired slope and its
construction cost.
Methodology
Three sets of analysis were carried
out using PLAXIS in which each
indicated the original slope profile,
repaired slope profile with CBP
wall and repaired slope profile with
anchored steel sheet pile wall respec-
tively to obtain FOS of the slope and
comparison is made. Besides, the cost
analysis of proposed anchored steel
sheet pile wall was developed with
the information of a quantity surveyor
from Puchong, Selangor. Cost com-
parison is made with the CBP wall to
define which is more economical.
Case Study
The low-lying ground generally con-
sists of alluvial deposits and at rela-
tively higher ground. This typically
represents an unstable mass, relatively
weak material and found burying very
weak alluvium soils. The soil gener-
ally consists of sandy silt, clayed silt
and silty sand. It was predicted that
the collapsed was likely triggered by
the extremely heavy rainfall. Ten
boreholes, six observation wells,
three inclinometers and one stand-
pipe piezometer were planned and
implemented to investigate the cause
of failure. The overburden material
was found to be generally weak with
SPT-N value ranging from 0 to 15.
Inclinometers
are suggested
at the slope to
determine slip
surface and
detect the slope
creep movement.
Two slip surfaces
have been identi-
fied as shown in
Figure 1.
Results
Atterberg Limit Test and Particle
Size Distribution
This classification was based on Brit-
ish Soil Classification System. Most of
the soil samples collected near the slip
surface is clayey silt of intermediate to
high plasticity.
Subsoil and Groundwater Level
Based on the data of exploratory
borehole obtained, the subsoil layers
and its properties can be determined.
The top layer generally composes of
clayed silt with SPT N value rang-
ing from 3 to 10. Following layers
compose of sand with some gravel.
Six numbers of observation wells and
a standpipe piezometer were used to
monitor the groundwater level. The
critical groundwater table within the
failed slope was found ranging from
2.89m to 2.77m (from OW-3) below
ground level which can consider high.
Design and analysis
Two sets of design will be carried out,
one for the CBP wall and other for
the proposed anchored steel sheet pile
wall. Both designs were using the
same FS of 1.5. All design parameter
used are based on the subsoil data
and some considerable assumptions.
Wall friction, δ = 18.7° and φ’design
= 23.9° according to BS 8002:1994
clause 3.2.6. soil cohesion, c’ = 0 kN/
m2 as recommended by ASTM Steel
Manual Design, for permanent steel
structure. While the design retained
height, H = 3.0m and critical ground-
water level was assumed as 3.0m
below ground surface. The lateral
earth pressure coefficient is calculate
based on Coulomb’s theory in which
the friction between soil and the wall
is take into account. Ka (horizontal
component) = 0.461 and Kp (horizon-
tal component) = 2.100. Introduce a
Figure 1. Slip surface plane.
(a) Depth of CBP wall Vs Shear force (b) Depth of CBP wall Vs bending
moment.
Figure 2. Shear force and bending moment acting on CBP wall.