1. GEOTECHNICAL ENGINEERING ECG 503 LECTURE NOTE 10 TOPIC : 3
GEOTECHNICAL ENGINEERING ECG 503 LECTURE NOTE 10 TOPIC : 3.0 ANALYSIS AND DESIGN OF RETAINING STRUCTURES 18 SEPTEMBER 2008

2. 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.

3. Braced Excavation – Determination of Forces in Struts
TOPIC TO BE COVERED
Braced Excavation – Determination of Forces in Struts
Cofferdam

4. 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

7. 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 :

8. Class A – Firm clay and flexible wall
Pressure Envelope :
Class A – Firm clay and flexible wall
0.2H
= unit weight
H = height of cut
0.2H H
0.3H

9. Class B – Stiff to very stiff clay and flexible wall
Pressure Envelope :
Class B – Stiff to very stiff clay and flexible wall
= unit weight
H = height of cut H
0.3H

10. Class C – Coarse soil dry
Pressure Envelope :
Class C – Coarse soil dry
= unit weight
H = height of cut H
0.2H

11. 1 P1 2 P2 P3 3 = Apparent pressure S = Spacing strut c/cd1 d 1 P1
d2 / 2 d2
d2 / 2 2 P2
d3 / 2 d3
d3 / 2 P3
d4 / 2 3 d4
d4 / 2
= Apparent pressure
S = Spacing strut c/c
1 = P1 / S (d1 + d2 /2)

12. Lateral earth pressure varies with depth
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.

13. b. Clay, soft to medium stiffness where H  4  = H [ 1 – (4c) ] or
By Peck,
a. Sand,  = 0.65HKa
b. Clay, soft to medium stiffness where
H  4
 = H [ 1 – (4c) ] or
 = 0.3 H
which ever is the bigger C H

14. Pressure Envelope For SandH
 = 0.65HKa
Pressure Envelope For Sand

15. Pressure Envelope For Cuts in Soft to Medium Clay
0.25H
0.75H
Pressure Envelope For Cuts in Soft to Medium Clay

16. Pressure Envelope For Cuts in Stiff Clay
0.25H
H  4 C
0.5H
 = 0.2H to 0.4 H
Purposely for design, take average
0.25H
Pressure Envelope For Cuts in Stiff Clay

17. 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)

19. Design Procedure
ii. 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)

20. Design Procedure
iv. Knowing the strut load at each level and the intermediate bracing, then select the proper section from steel construction manuals.

21. EXAMPLE 1
Draw the earth pressure envelope and determine the strut loads. Strut are placed at 3m c/c 6m 1m
= 18kN/m3
c = 35 kN/m2
 = 10 2m 2m 1m 1m 3m 3m 3m 3m

22. 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 A
= 18.2 kN/m3
 = 20 3m B 3m C
1.5m
3.5m
3.5m
3.5m
3.5m

23. 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.

24. Contains three basic types which is :
a. Circular Cofferdam
b. Diaphragm Cofferdam
c. Cloverleaf Cofferdam

26. Design consideration:
a. Cell geometry
b. Cell fill materials
c. Sheet piles

27. LATERAL EARTH PRESSURE
Stability Analysis
Worked example 2 :
A concrete gravity wall is shown in Figure below. Determine :
FOS against Overturning
FOS against Sliding
The pressure on the soil at the toe and heel
(Note : Unit weight of concrete is 24kN/m3)

28. 1 2 3 6 5 4
8 m
1.5 m
3.5 m
0.5 m
3 m
1 m
1 = 30
1 = 16kN/m3
c1 = 0
2 = 20
2 = 20kN/m3
c2 = 15 kN/m2