ecg503 week 10 lecture note chp3

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GEOTECHNICAL ENGINEERING


ECG 503


LECTURE NOTE 10


TOPIC : 3.0 ANALYSIS AND
DESIGN OF RETAINING
STRUCTURES


18 SEPTEMBER 2008

LEARNING OUTCOMES

Learning outcomes:

At the end of this lecture/week the students would
be able to:


Understand natural slope and made engineered
soil slope assessment which include rainfall
induced failure and role of suction.

TOPIC TO BE COVERED


Braced Excavation


Determination of Forces in
Struts


Cofferdam


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

0.3

H

〮0

H



= 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

0.2

H



= unit weight

H = height of cut

d
1

d
2

d
3

d
4

P
1

P
3

P
2

d

d
2
/ 2

d
2
/ 2

d
3
/ 2

d
4
/ 2

d
3
/ 2

d
4
/ 2


1


2


3



= Apparent pressure

S = Spacing strut c/c



1

= P
1

/ S (d
1

+ d
2

/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,


㴠=⸶5


a



b. Clay, soft to medium stiffness where






H



4












䠠嬠ㄠ


(4c) ]

or











㴠=⸳

H

睨楣栠敶w爠楳⁴桥⁢楧来r


C


H

H

Pressure Envelope For Sand



= 〮㘵

䡋H

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



㴠〮0

䠠⁴漠‰ 㐠



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 Procedure

ii. Determine the reaction for the two simple
cantilever beam (top and bottom) and all
others are simple beam (A, B
1
, B
2
, C
1
, C
2

and D)

iii. Used the formulae to calculate strut loads



P
A
= (A) (s)



P
B

= (B
1

+ B
2
) (s)



P
C

= (C
1

+ C
2
) (s)



P
D

= (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

3.5m

3.5m

3.5m


= 18.2 kN/m3



㴠㈰


A

C

B

CELLULAR COFFERDAMS


Used to enable construction works in
water bound areas eg. rivers, lake
and sea



Stability depend mainly on
interaction of the soil to fill the cell
and the steel sheetpiling.


Contains three basic types which is :



a. Circular Cofferdam


b. Diaphragm Cofferdam


c. Cloverleaf Cofferdam


Design consideration:



a. Cell geometry


b. Cell fill materials


c. Sheet piles


Stability Analysis

LATERAL EARTH PRESSURE

A concrete gravity wall is shown in Figure below. Determine :


a.
FOS against Overturning


b.
FOS against Sliding


c.
The pressure on the soil at the toe and heel



(Note : Unit weight of concrete is 24kN/m
3
)

Worked example 2 :

1

2

3

6

5

4

8 m

1.5 m

3.5 m

1.5 m

1.5 m

0.5 m

3 m

1 m


1

= 30



1

= 16kN/m
3

c
1

= 0


2

= 20



2

= 20kN/m
3

c
2

= 15 kN/m
2