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GEOTECHNICAL ENGINEERING
ECG 503
LECTURE NOTE 08
3.0 ANALYSIS AND DESIGN OF
RETAINING STRUCTURES
LEARNING OUTCOMES
Learning outcomes:
At the end of this lecture/week the students would
be able to:
Able to design the sheet pile wall – cantilever
and anchored using free earth support method
Braced Excavation – Determine the Forces in
Struts.
TOPIC TO BE COVERED
Sheet Pile Wall – Cantilever and
Anchored Sheet Pile
Analysis by Free-Earth Support
Method
Braced Excavation – Determine
the Forces in Struts.
CANTILEVER SHEET PILE
• Cantilever sheet pile basically depend
mainly on the passive resistance on front of
the wall to retain the stability of the sheet
pile
• If the cantilever sheet pile is too deep,
therefore the are need to tie back the sheet
pile with dead man anchor
• For clay soil, there are possibilities that the
sheet pile will rotate in the point of fixity
and thus will result on increasing the
passive resistance
• FOS will be used due to consideration of
passive resistance in the front of the wall.
• Effective depth, d can be calculated by
equilibrium moment.
• Another 20% will be added to the effective
depth due to consideration of the existing of
void ration in the soil skeleton.
Analysis by Free-Earth Support Method For
Sheet Pile (Cantilever and Anchored)
The depth of penetration below the
effective depth will not enough to
produce the fixity at the end of the pile
(i.e. at Point C)
Therefore, FOS will be used in the
design. The total of passive resistance
mobilized, Ppm = Pp / FOS
Types of
s/pile wall
Limit states-
Sheet piles
Excessive
deflection
Rupture/bending
Cantilever type
Limit states-
Sheet piles
Global instability
Steel sheet pile - general installation & usage
Concrete
sheetpile-
Sg. Klang
Trenching
& = Fatality
K.L.
Braced sheetpile cofferdam with king piles – Kpg. Bahru
Flat sheet pile
As cellular
cofferdam - Arbed
Normal s/pile as
cellular cofferdam-
Sg. Paka
Failure of steel sheet pile cofferdam – Sg. Saribas
Failure of temp sheetpile wall - Kelana Jaya
Faiure of temp basement cofferdam –K.L.
Anchored s/pile
wall – Sg MudaBraced/propped
wall – J.B.
1. FOS on Ka: such as Ka/F where F varies from 1.5 to 2.0
Several definitions:
Factor of safety introduced to compensate for
uncertainty in loads, water cond., soil para, limit deformation.
2. FOS on c’ and Ø’ on the passive side before multiplying by Kp
where F=1.5 to 2.0 for sands and
F= 1.2 to 1.5 for clays
3. FOS on net passive total pressure (BSC piling Handbook) -
not recommended due to high value of FOS compared to
other methods. FOS on effective strength is very low.
4. FOS on all effective strength parameters (eff. & pasv. pressures)
where F=1.5
5. Increase depth of penetration by 20%
D’ achieved from moment equilibrium at FOS=1 is then increased
by 1.2 to 2.0 i.e. embedment depth D = Fd x D’
6. Revised FOS by Burland, Pott & Walsh’s
Taking moment about the tie, mobilised mmt of soil above
dredged level equivl to resisting mmt by soil below dredged level
Stability Analysis
LATERAL EARTH PRESSURE
Figure below shows the cantilever sheet
pile being design using free earth
support method. If the ka = 0.33, kp = 3
and FOS = 1.5, determine the effective
depth, d for the sheet pile given.
Worked example 1 :
Stability Analysis
LATERAL EARTH PRESSURE
THE PROBLEM
Pa
Ppm
4.5 m
d
H
C
Ka = 0.33
Kp = 3.0
FOS = 1.5
C = 0
= 18 kN/m3
Stability Analysis
LATERAL EARTH PRESSURE
Figure below shows the cantilever sheet
pile being design using free earth
support method. Determine the
effective depth, d required to stabilized
the sheet pile.
Worked example 2 :
Stability Analysis
LATERAL EARTH PRESSURE
THE PROBLEM
4 m
d m
C
= 25 kN/m3
= 100
C = 0 kN/m2
= 19 kN/m3
= 200
C = 0 kN/m2
FOS = 2.0
Stability Analysis
LATERAL EARTH PRESSURE
Figure below shows the anchored sheet
pile design using free earth support
method. Given the unit weight is
18.5kN/m3. The soil parameter is C = 0
and =330. Using FOS = 2.0 for passive
resistance, determine the effective
depth, d required and calculate the
force magnitude in the tie rod of the
anchored sheet pile.
Worked example 3 :
Stability Analysis
LATERAL EARTH PRESSURE
THE PROBLEM
1.5 m
d m
C
= 16 kN/m3
= 200
C = 0 kN/m2
sat = 23 kN/m3
= 250
C = 0 kN/m2
FOS = 1.5
sat = 20 kN/m3
T
1.5 m
2 m
Synopsis
Needs for further trenching, where
it carried out, design consideration +
analysis of components (design of the
elements)
Objective
Able to design the bracing and other
components to support trench excavation.
Able to analyzed the design. Trenching
normally temporary structure
Design Components :
Select appropriate size of wale, struts,sheet pile or soldier beam
Basis of selection : Based on the estimated lateral earth pressure
Theoretically aspects of lateral pressure :
Pressure Envelope :
Class A – Firm clay and flexible wall
0.2H
H
0.3H
0.2H
= unit weight
H = height of cut
Pressure Envelope :
Class B – Stiff to very stiff clay and flexible wall
H
0.3H
= unit weight
H = height of cut
Pressure Envelope :
Class C – Coarse soil dry
H
0.2H
= unit weight
H = height of cut
d1
d2
d3
d4
P1
P3
P2
d
d2 / 2
d2 / 2
d3 / 2
d4 / 2
d3 / 2
d4 / 2
1
2
3
= Apparent pressure
S = Spacing strut c/c
1 = P1 / S (d1 + d2 /2)
• Lateral earth pressure varies with depth.
Each strut being designed for maximum
load to which it is subjected.
• Thus, braced cut being designed using
apparent pressure diagram determined
from measured struts load in the field.
• By Peck,
a. Sand, = 0.65HKa
b. Clay, soft to medium stiffness where
H 4
= H [ 1 – (4c) ] or
= 0.3 H
which ever is the bigger
C
H
H
Pressure Envelope For Sand
= 0.65HKa
0.75H
Pressure Envelope For Cuts in Soft to Medium Clay
0.25H
0.5H
Pressure Envelope For Cuts in Stiff Clay
0.25H
0.25H
H 4
C
= 0.2H to 0.4 H
Purposely for design, take
average
Design Procedure
• Design procedure to determine strut load :
i. Draw the pressure envelope of the propose
strut levels (soldiers beam are assumed to
be hinged at the strut level, except for the
top and bottom ones)
Design Procedureii. Determine the reaction for the two simple
cantilever beam (top and bottom) and all
others are simple beam (A, B1, B2, C1, C2
and D)
iii. Used the formulae to calculate strut loads
PA = (A) (s)
PB = (B1 + B2) (s)
PC = (C1 + C2) (s)
PD = (D) (s)
Design Procedure
iv. Knowing the strut load at each level and
the intermediate bracing, then select the
proper section from steel construction
manuals.
EXAMPLE 1
• Draw the earth pressure envelope and determine
the strut loads. Strut are placed at 3m c/c
6m
1m
2m
2m
1m
1m 3m 3m 3m 3m
= 18kN/m3
c = 35 kN/m2
= 10
EXAMPLE 2
• A braced cut shown in Figure below were constructed in a cohesionless soil having a unit weight, = 18.2 kN/m3 and an angle of internal friction, = 35. The trust located at 3.5m centre-to-centre in a plan. Determine the trust load at levels A, B and C
5m
2m
3m
3m
1.5m
3.5m 3.5m3.5m 3.5m
= 18.2 kN/m3
= 20
A
C
B
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