Braced Support of a Vertical Excavation

Problem Statement

Note

The project file for this example may be viewed/run in 3DEC.[1] The data files used are shown at the end of this example.

This example examines a trench that is supported with one strut bracing the excavation walls at the top. The following command sequence is for the simple case in which the brace is placed immediately upon excavation. The strut is represented as a single beam composed of two elements, and is created using the structure beam create by-line command. Use of this command ensures that the two ends of the beam are attached to the grid such that the translational degrees-of-freedom are rigidly connected and the rotational degrees-of-freedom are free.

Figure 1 shows the position of the brace, and Figure 2 illustrates the reduced displacements in the excavation walls.

../../../../../_images/bracedsupport-geom.png

Figure 1: Braced excavation supported with a beam.

../../../../../_images/bracedsupport-disp.png

Figure 2: Displacement contours and axial displacement in strut for braced excavation.

Data File

BracedSupport.dat

model new
model random 10000
model large-strain off
model title "Braced support with a beam of a vertical excavation"

; Create simple brick
block create brick  0 6 0 8 0 8

; cut trench
block cut joint-set origin 0 0 5
block cut joint-set dip 90 dip-direction 90 origin 2 0 5
block cut joint-set dip 90 dip-direction 90 origin 4 0 5
block cut joint-set dip 90 origin 2 2 5
block cut joint-set dip 90 origin 2 6 5
block join

block zone generate edgelength 0.5

; Assign material model and properties
block zone cmodel assign mohr-coulomb
block zone property bulk 1e8 shear 0.3e8 friction 35
block zone property cohesion 1e10 tension 1e10 density 1000

; Gravity and boundary conditions
block gridpoint apply velocity-x 0 range position-x 0
block gridpoint apply velocity-x 0 range position-x 6
block gridpoint apply velocity-y 0 range position-y 0
block gridpoint apply velocity-y 0 range position-y 8
block gridpoint apply velocity-z 0 range position-z 0

; Initial conditions
model gravity 9.81
block insitu topography ratio-x 0.3 ratio-y 0.3

model solve convergence 1
model save 'initial'

; Reduce zone strength, excavate 
block zone property cohesion 1.25e3 tension 1e3
block excavate range position (2,2,5) (4,6,10)

block gridpoint initialize displacement (0,0,0)

; Create beam brace
structure beam create by-line ( 2, 4, 8) ( 4, 4, 8) segments=2
;structure beam create by-line ( 2, 4.25, 7.9) ( 4, 4.25, 7.9) segments=1
structure beam property young=2.0e11 poisson=0.30
;structure beam property young=2.0e8 poisson=0.30
structure beam property cross-sectional-area=6e-3 ...
                 moi-z=200e-6 moi-y=200e-6 torsion-constant=0.0

model solve convergence 1

model save 'final'

Endnote