Module 6 : Design of Retaining Structures
Lecture 29 : Braced cuts [ Section 29.5 : Stability checks for designing a braced excavation: ]

•  Design of the structural members

•  Design of struts:

The struts are the structural members whose function is to transfer the earth pressure coming on the diaphragm walls due to the earth pressure from the surrounding soil. For calculation of the struts loads, Peck (1969) proposed apparent earth pressure diagrams to be used for the designing of the bracing systems. The diagrams are given in figure.

Fig 6.40. Apparent earth pressure diagrams (Peck, 1969)

For sands, p = 0.65 , where, = ( 1- sin )/( 1+sin )

H = height of the vertical cut.

For clays, p = ( 1 - ) ,

where , m = coefficient depending on the stability of the wall

For S<4, m = 0.6 0.8.

For S>4, m = 1.0. { S = }

Fig. 6.41 Apparent earth pressure acting on diaphragm wall

The apparent earth pressure acting on diaphragm wall is chosen as per the type of soil existing in the field. For each strut we get an effective zone over which the earth pressure acts. Usually the earth pressure zones extend from centerline of one strut to that of the other, which implies that each strut takes the earth pressure on either halves upto half the vertical spacing ( ) . As shown zones 1,2,3 apply pressure on the struts 1,2,3. For zone 4, it is assumed that the soil in that portion does not apply pressure and it is taken up by the underlying soil.

Each strut load is calculated by multiplying the effective area of action of earth pressure with the apparent earth pressure (p). Usually the vertical spacing of the struts are taken between 3-4 m. The highest strut load is taken up for choosing the section of the struts and same section is provided throughout.

•  Design of diaphragm walls:

For design of the diaphragm walls the wall is assumed to lie as a beam and the pressure distribution acting on it as shown in the fig.6.7.8.

Fig 6.42 Load on Diaphragm wall

From the pressure distribution the exact moment and forces acting on the struts and the wall can be calculated. However, for all practical purposes, the maximum bending moment acting on the wall ( ) = / 10, where l = . Accordingly the section of the diaphragm wall is chosen based on the moment acting on it.

•  Design of wales:

The wales are structural members which transfers the load from the diaphragm walls to the struts thereby acting as beams. The design of struts is done as simply supported beams as shown in fig. 6.7.9. Maximum moment on wales = (p. ). /8.

Fig. 6.43 Plan of struts and wales along with loading arrangement

•  Excessive ground movement:

The various structural members are constructed to minimize ground movements in the vicinity. However, wall cannot be infinitely rigid. Irrespective of placing of struts, diaphragm wall movement cannot be prevented. After some excavation is done, before a strut is placed there is a certain movement of the wall. Also, between subsequent placing of struts certain movement of wall occurs. As a result, the ground movement occurs locally.

If the joints are subjected to such movements excessive forces may generate leading to the distress of the structure. Therefore whatever ground movement occurs, it has to be limited to a minimum value. Total ground movement is the sum total of the ground movement and the bottom heaving. The idea of proving structural members is to minimize ground movements. More rigid the structure, lesser is the ground movement.

Peck (1969) proposed a graph which indicated the ground movements and their extent for a excavation site and site conditions as shown in fig.6.7.10.

Fig 6.44. Amount and extent of ground settlement (Peck, 1969)

Before conducting any excavation, depending on site and soil conditions, we can estimate the maximum settlement and extent of settlement that is going to occur when a excavation is carried out at that site. It is to be noted that Peck's analysis was based on experiments done over sheet pile walls. Therefore, if the rigidity of the structural members can be increased the settlement values can be minimized and whatever settlement we could have got for a sheet pile wall in zone III can be found to fall in zone II due to a more rigid structure. Hence after finding out the extent of settlement it has to be judged whether any surrounding structure falls within that range.