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Example Application 10

Analysis of a Concrete

Diaphragm Wall

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Cross section through diaphragm wall

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Soil Properties

Excavation and Dewatering Depths

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Strut Properties

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Modeling Procedure

Step 1 Generate the model grid and assign material models and properties for the six soil layers. Include the wall

as beam elements attached to the grid via interfaces. Assign boundary and initial conditions representing a

water table at -2m depth.

Step 2 Solve for the initial equilibrium state. Assume a large-strain condition.

Step 3 Perform the excavation to -4m depth in four steps:

a. Dewater to -5m depth.

b. Excavate to -4m depth.

c. Add a pre-loaded strut at -3m depth.

d. Record the wall displacement and strut load.

Step 4 Repeat Step 3 for dewatering to -9m depth, excavation to -8m depth, and adding a pre-loaded strut at

-7m depth.

Step 5 Repeat Step 3 for dewatering to -13m depth, excavation to -12m depth, and adding a pre-loaded strut at

-11m depth.

Step 6 Repeat Step 3 for dewatering to -16m depth, excavation to -15m depth, and adding a pre-loaded strut at

-14m depth.

Step 7 Repeat Step 3 for dewatering to -19m depth, and excavation to -18m depth.

Step 8 Repeat Steps 3 through 7 for struts with no pre-loading.

Step 9 Compare wall displacement results and strut axial force results.

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In the [Model options] dialog, select [GwFlow] and [Adjust tot. stress] configuration options, and [Include structural elements] user interface option.

The system of units are [SI: meter-kilgram-second].

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A project title is assigned, and a project file diap.prj is created and stored in a working directory.

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Step 1-1 In the [Geometry Builder] import the boundary geometry file diap.dxf. (This geometry can be obtained from a dxf file or drawn in the

[Sketch] tool.) Note that half of the excavation is modeled, and a line of symmetry is positioned at x = 9.0. The excavation width is taken to be

18m. Also, the geometry is slightly modified so that the bottom of the wall coincides with the boundary between Soil 5 and Soil 6.

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Step 1-2 Create construction lines to divide the model into quad blocks.

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Step 1-3 Add a vertical line to extend the wall edge. Add another vertical line at x = -4.0 to assist adding finer zoning around the wall region.

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Step 1-4 Use the [Cleanup] stage to clean up any bad vertices and edges.

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Step 1-5 Check that the model is divided into 4-sided blocks in the [Blocks] stage. Extract 18 quad blocks for building the FLAC grid by pressing

[Build]. Press [OK] to exit the [Geometry builder].

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Step 1-6 Enter the [Edit] tool. Select [Boundary] edit stage and assign standard boundary conditions using the [Automatic boundary cond.]

mode.

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Step 1-7 Select the [Set attach] edit mode and click on the box associated with the bottom edge at x=0 to change this to an attached edge.

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Step 1-8 Select the [Set beam-interface] edit mode, and click on the remaining edges at x = 0 to change these edges to beam-interface edges.

These will represent the diaphragm wall.

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Step 1-9 Select the [Set properties] edit mode and click on a beam-interface edge. An Interface properties dialog opens, and interface stiffness

and friction properties are assigned.

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Step 1-10 Select the [Mesh] edit mode and turn off [Use automated zoning]. Manually set the number of zones in the excavation region as

shown.

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Step 1-11 Manually set the zone size as shown throughout the entire model.

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Step 1-12 Select the [Adjust ratio] tool and set a geometric ratio of 0.9 for the blocks, as shown, to produce a gradual transition from small zones in

the excavation region to larger zones away from the excavation.

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Step 1-13 Select the [Materials] edit stage, and then select [Define regions]. Click on the edges, as shown, to create separate regions in the model

to assign the different material types.

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Step 1-14 In the [Assign] mode, click on [Database] and load in the DIAP.GMT material database.

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Step 1-15 Assign the six soil materials to the regions in the model, as shown.

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Step 1-16 Click [OK] to exit the [Edit] tool. Click [Execute] to create the commands to send to FLAC. A warning message appears to indicate that

zones at the boundary farthest from the excavation region have an aspect ratio greater than 10:1. This is considered acceptable, so [Yes] is

selected to create the commands. A finer mesh could be created to remove the high aspect ratio zones if desired.

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Step 1-17 The FLAC commands are shown in the [Record] pane.

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Step 1-18 Enter the [Material]/[GWProp] tool and select [Set all]. Enter the porosity of 0.3 in the Model Groundwater properties dialog. Press

[Execute] to send the command to FLAC.

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Step 1-19 Enter the [Structure]/[SEProp] tool and assign the beam properties representing the diaphragm wall.

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Step 1-20 Assign gravity with the [Settings]/[Gravity] tool.

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Step 1-21 Turn the groundwater flow calculationoff and set the water density using the [Settings]/[GW] tool.

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Step 1-22 FISH function ININV.FIS assigns pore pressure, total stress and effective stress distribution for horizontally layered stratigraphy. Input

is water table location and effective horizontal to vertical stress ratios. Enter the [Fish editor] pane and open the FISH function. Press the execute

button to execute the function.

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Step 1-23 Save the state as dw_ini.sav.

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Step 2-1 The calculation is run in large-strain mode. Use the [Settings]/[Mech] tool and selct [Large-strain] in the Mechanical settings dialog.

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Step 2-2 Calcculate the equilibrium state using [Run]/[Solve] and [Solve initial equilibrium as elastic model].

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Step 2-3 Plot the maximum unbalanced force using the [Plot]/[Quick] tool to check that equilibrium is reached. Save the state as dw_equil.sav.

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Step 3-1 Reset the displacements and velocities to zero with the [Clear? Displmt & velocity] button in the [In Situ]/[Initial] tool.

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Step 3-2 In the [Insitu]/[Initial] tool, the dewatering is performed by lowering the pore pressure to vary from 0.0 at y=-5m to 317 kPa at y=-34m.

The mouse is dragged over this region inside the excavation and [pp] is selected. Press [assign] to open the dialog to assign the pore pressure

variation.

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Step 3-3 Zero pore pressure is assigned to gridpoints above y=-5.0.

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Step 3-4 Zero saturation is assigned to gridpoints above y=-5.0. Press [Execute] to send the commands to FLAC.

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Step 3-5 Enter the [Plot]/Scanline] tool and add a vertical scan line to use with the pore pressure distribution plot. Press [Create Scanline] to

create the line, and [Exit] to exit the tool.

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Step 3-6 Select [Run]/[Solve] to calculate the equilibrium state with dewatering to y=-5m.

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Step 3-7 Save the state as dw_w1.sav.

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Step 3-8 Enter the [Utility]/[History] tool and monitor x-displacements as 5 gridpoints along the wall.

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Step 3-9 Enter the [Material]/[Assign] tool and null zones corresponding to an excavation depth of y=-4m.

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Step 3-10 Use [Run]/[Solve] to calculate the equilibrium state.

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Step 3-11 Save the state as dw_e1.sav.

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Step 3-12 Enter the [Structure]/[Beam] tool and add a one-segment strut connected to the wall beam at y=-3m. Locate the other end of the

beam along the centerline at x=9.0, y=-3.0.

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Step 3-13 Pin the strut to the wall using the [Pin] mode.

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Step 3-14 Change the beam property ID to B2 using the [PropID] mode.

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Step 3-15 Assign strut properties for strut number 1 in the [Structure]/[SEProp] tool.

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Step 3-16 Pre-load the beam to -100000 Pa and fix y-velocity and rotation in the [Structure]/[Node] tool.

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Step 3-17 Use [Run]/[Solve] to calculate equilibrium.

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Step 3-18 Reset the load to 0.0 and initialize and fix the x-velocity of the node in the [Structure]/[Node] tool.

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Step 3-19 Use the [Run]/[Solve] tool to calculate the equilibrium state with strut 1. Save the state as dw_s1.sav.

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Step 3-20 Enter the [Fish editor] and open wall_disp.fis. Horizontal displacement along the wall will be stored in tables. In_wall =30 and

n_table = 1.

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Step 3-21 Initialize strut forces in tables by executing init_strut_table.fis in the [Fish editor].

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Step 3-22 Store strut loads in tables using strut_ax_load.fis. Execute in [Fish editor] with nstage = 1, nstrut = 11 and el_num = 30.

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Step 3-23 Save the state as dw_r1.sav.

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Step 4-1 Repeat Steps 3-2 to 3-6 and dewater to -9m.

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Step 4-2 Repeat Steps 3-9 to 3-10 and excavate to -8m.

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Step 4-3 Repeat Steps 3-12 to 3-19 and install pre-loaded strut 2.

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Step 4-4 Repeat Steps 3-20 and 3-22 to record the wall displacement and strut loads.

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Step 5-1 Repeat Steps 3-2 to 3-6 and dewater to -13m.

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Step 5-2 Repeat Steps 3-9 to 3-10 and excavate to -12m.

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Step 5-3 Repeat Steps 3-12 to 3-19 and install pre-loaded strut 3.

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Step 5-4 Repeat Steps 3-20 and 3-22 to record the wall displacement and strut load.

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Step 6-1 Repeat Steps 3-2 to 3-6 and dewater to -16m.

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Step 6-2 Repeat Steps 3-9 to 3-10 and excavate to -15m.

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Step 6-3 Repeat Steps 3-12 to 3-19 and install pre-loaded strut 4.

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Step 6-4 Repeat Steps 3-20 and 3-22 to record the wall displacement and strut load.

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Step 7-1 Repeat Steps 3-2 to 3-6 and dewater to -19m.

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Step 7-2 Repeat Steps 3-9 to 3-10 and excavate to -18m.

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Step 7-3 Repeat Steps 3-20 and 3-22 to record the wall displacement and strut load.

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Step 8-1 Clone a separate branch for adding strut support without pre-loading. Right-click on branch dw_s1.sav and create a separate branch.

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Step 8-2 Comment out the three commands shown and add STRUCT NODE 31 FIX X Y R in branch noload_dw_s1.sav. Press [Rebuild] to rebuild

the branch.

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Step 8-3 Rebuild branches noload_dw_r1.sav, noload_dw_w2.sav and noload_dw_e2.sav.

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Step 8-4 Comment out the three commands shown and add STRUCT NODE 33 FIX X Y R in branch noload_dw_s2.sav. Press [Rebuild] to rebuild

the branch.

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Step 8-5 Rebuild branches noload_dw_r2.sav, noload_dw_w3.sav and noload_dw_e3.sav.

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Step 8-6 Comment out the three commands shown and add STRUCT NODE 35 FIX X Y R in branch noload_dw_s3.sav. Press [Rebuild] to rebuild

the branch.

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Step 8-7 Rebuild branches noload_dw_r3.sav, noload_dw_w4.sav and noload_dw_e4.sav.

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Step 8-8 Comment out the three commands shown and add STRUCT NODE 37 FIX X Y R in branch noload_dw_s4.sav. Press [Rebuild] to rebuild

the branch.

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Step 8-9 Rebuild branches noload_dw_r4.sav, noload_dw_w5.sav, noload_dw_e5.sav and noload_dw_r5.sav.

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Step 9-1 x-displacement histories at five elevations along the wall for pre-loaded struts.

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Step 9-2 x-displacement histories at five elevations along the wall for no pre-loading in struts.

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Step 9-3 Shear forces in the diaphragm wall at the final excavation stage (with pre-loading).

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Step 9-4 Shear forces in the diaphragm wall at the final excavation stage (no pre-loading).

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Step 9-5 Moments in the diaphragm wall at the final excavation stage (with pre-loading)

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Step 9-6 Moments in the diaphragm wall at the final excavation stage (no pre-loading)

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Step 9-7 x-displacements of diaphragm wall at the end of each excavation stage for pre-loaded struts

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Step 9-8 x-displacements of diaphragm wall at the end of each excavation stage for no pre-loading in struts

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Step 9-9 Actual axial forces in struts at the end of each excavation stage for pre-loaded struts

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Step 9-10 Actual axial forces in struts at the end of each excavation stage for no pre-loading in struts