BRACED EXCAVATION DESIGN

MUHAMMAD AZRIL HEZMI

PROF. KHAIRUL ANUAR KASSIM

PROF. Ir. DR. RAMLI NAZIR

INSPIRING CREATIVE AND INNOVATIVE MINDS

CONCEPT

• Consist of sheet piles driven into the soil to form sides of an excavation

such as construction of bridge piers and abutments.

• As excavation proceeds within the area enclosed by the sheet piles, struts

are added to keep the sheet pile in place.

• Displacement gets larger at depth.

• Critical design element are the loads on the struts which are usually

different because of different lateral loads at different depths, the time

between excavations and the installation procedures.

Contd…

• The analysis for the forces and deflection in braced excavation should ideally

consider the construction sequence, and numerical methods such as the finite

element.

• Semi empirical methods are often used in the design of shallow braced

excavation and also as a basic design of deep braced excavation.

• In the soft and Normally Consolidate soil, heaving may occurs at the base of

the excavation.

• Bjerrum and Eide (1956) suggested that the excavation could be view as

afooting and embedment depth Ho.

Contd….

so

ucheave

qH

cNFS

+=

γ)(

• The factor of safety against the bottom heave is given by:-

• For practical purposes the value of Nc is given by:-

L

BI

B

HifIN

and

B

Hfor

B

HN

co

cc

ooc

16.084.0,5.2...........9

5.2..........2.016

+=>=

≤

+=

Contd….

• If FOS < 1.5, the length of embedment should be extended below the base of

excavation for stability.

• The value of coefficient m = 1 except that when the soil below excavation is

deep, soft and normally consolidated clay, m value is approximately 0.4(FOS

heave).

• The strut load at each depth is assumed to be a hinge connection between strut

and sheet pile. Thus solution is using static equilibrium.

• Displacement of the wall is important in design consideration as adjacent

structire may affected.

• However it is better off to solve using numerical method.

PROCEDURE FOR ANALYSIS

• Check stability against bottom heave. If FOS < 1.5 the wall should be

extened below base.

• Determine the lateral stresses on the wall according to soil type.

• Treat the connections of the wall to the struts as hinges.

• Draw a free body diagram at each level of excavation.

• Solves the forces in the stuts using the static equilibrium equations on

each free body diagram.

Examples