Tutorial: Braced Support of a Vertical Excavation

Problem Statement

Note

To view this project in FLAC3D, use the menu command Help ‣ Examples…. Choose “Structure/Beam/BracedSupport” and select “BracedSupport.prj” to load. The main data files used are shown at the end of this example. The remaining data files can be found in the project.

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. [CS: again, the following used to link back to “simple tutorial”], compared to that shown in :flag2:-defcrossrefFilename{ftd127}ref{127F1001}- in defcrossrefFilename{ftd127}ref{127S1001}volI.

Figure 1: Braced excavation supported with a beam.

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

Data File

BracedSupport.dat

model new
model large-strain off
model title "Braced support with a beam of a vertical excavation"
; Create simple brick
zone create brick size 6 8 8
zone face skin ; label boundaries
; Assign material model and properties
zone cmodel assign mohr-coulomb
zone property bulk 1e8 shear 0.3e8 friction 35
zone property cohesion 1e10 tension 1e10 density 1000
; Gravity and boundary conditions
zone face apply velocity-normal 0 range group 'East' or 'West'
zone face apply velocity-normal 0 range group 'North' or 'South'
zone face apply velocity-normal 0 range group 'Bottom'
; Initial conditions
model gravity 9.81
zone initialize-stresses ratio 0.3
model solve ratio-local 1e-5
model save 'initial'
; Reduce zone strength, excavate 
zone property cohesion 1.25e3 tension 1e3
zone cmodel assign null range position (2,2,5) (4,6,10)
model large-strain on
zone gridpoint initialize displacement (0,0,0)
; Create beam brace
struct beam create by-line ( 2, 4, 8) ( 4, 4, 8) segments=2
struct beam prop young=2.0e11 poisson=0.30
struct beam prop cross-sectional-area=6e-3 moi-z=200e-6 ...
                 moi-y=200e-6 moi-polar=0.0
model solve ratio-local 1e-5
model save 'final'