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, whereit 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/m3c = 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

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/m3)

Worked example 2 :

1 2 3

65

4

8 m

1.5 m3.5 m 1.5 m1.5 m

0.5 m

3 m1 m

1 = 301 = 16kN/m3

c1 = 0

2 = 202 = 20kN/m3

c2 = 15 kN/m2