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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