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Diaphragm wall with tiebacks, English units

A. Project description

In this example we will design a diaphragm wall supported by tiebacks, supporting a 35 ft excavation. Tables 1 and 2 present the soil properties and the stratigraphy respectively. Table 3 presents the external loads. Table 4 presents the wall properties. Table 5 presents the support properties. The general ground elevation is 0 and the general water elevation is -26.2 ft.

Table 1: Soil properties

Name

Soil Type

Φ'

(deg)

c'

(psf)

Su

(psf)

γt

(pcf)

γdry

(pcf)

Kx

(ft/sec)

Kz

(ft/sec)

Eload

ksf

Eur

ksf

qskin,u

psi

F

Sand

30

0

-

127

121

0.00033

0.00033

313

939

7.2

Clay

Stiff clay

28

0

3131

127

121

3.2808

3.2808

417.5

1252.6

18.1

Table 2: Stratigraphy (Boreholes)

Soil Layer Name

Top Elevation (m)

OCR

Ko

F

0

1

0.5

Clay

-10

1

0.5

Table 3: External loads

Name

Type

Position

(X0,Z0)

(X1,Z1)

Q0 (ksf)

Q1 (ksf)

0

Surcharge

On the ground

(-50,)

(-35,0)

0.2

0.2

1

Surcharge

On the ground

(-32,0)

(-3,0)

0.1

0.1

Table 4: Wall properties

Type

Diaphragm (slurry) wall

Concrete

Fc 3 ksi

Steel

Grade 75

Top Elevation (ft)

0

Height (ft)

70

Thickness (ft)

2

Hor. Spacing (ft)

3

N of top rebars

6

Top bars #

#8

N of bottom rebars

6

Bottom bars #

#8

Shear reinf. #

-

Shear reinf. hor. Spacing, Sh (in)

-

Shear reinf. ver. Spacing, Sv (in)

-

Table 5: Support properties

Name

Type

Elevation on the wall (ft)

Angle (deg)

Free length (ft)

Fixed length

(ft)

Hor. Spacing

(ft)

Prestress

(kips)

0

Tieback

-6

20

17

50

10

35

1

Tieback

-16

20

17

50

10

35

2

Tieback

-26

20

17

50

10

35

B. Modeling with DeepEX

B1. Use of DeepEX

  • Soil properties: The soil properties in DeepEX can be defined in the General tab of DeepEX software. By pressing the button, the soil properties form appears (Figure 1). Here we can add, delete and modify available soils by changing their type, the general properties like unit weights, strength parameters and permeability, modify the elastoplastic parameters and modify the bond resistance for tiebacks. A soil can be used in a boring more than one time. A number of estimation tools that help the user estimate values are also included.

Figure 1: Edit soil properties dialog.

  • Borings (Soil layers): The stratigraphy in DeepEX can be defined in the General tab of DeepEX software. By pressing the button , the soil layer dialog appears (Figure 2). In this dialog we can edit the borings available for use in the project. In each boring the user can add soil layers. To do this, we can type the new soil layer’s elevation, choose the soil type from the list of soil types and define the new layers OCR and Ko. In addition, by clicking on Edit button, we can modify the selected soil’s properties . The coordinates X and Y refer to the plan location of the boring and do not affect analysis results.

Figure 2: Edit soil layers dialog.

  • Wall sections: The wall sections in DeepEX can be defined in the General tab of DeepEX software. By pressing the button , the Edit wall properties dialog appears (Figure 3). Here we can choose the wall type and dimensions, choose the wall sections and edit the rebar options for concrete walls.

Figure 3: Edit wall properties dialog.

  • Walls: By double-clicking on the wall in the model area of DeepEX, the Edit wall data dialog appears (Figure 4). Here we can define the wall section, the elevation of the top of the wall, the wall height and the number of wall nodes for the limit equilibrium analysis.

Figure 4: Wall data dialog

  • Supports: We can add supports in the model from the General tab of DeepEX. By selecting an option from the drop down menu, a support or other related support entities can be drawn on the model. The following options are available:

Figure 5: Support options.

After drawing the support on the model, the Edit support data dialog appears (i.e. for tieback supports, Figure 6).

Figure 6: Edit support data dialog.

  • Loads: We can add supports in the model from the General tab of DeepEX. The drop down menu contains tool buttons for adding external loads (surcharges) and some related surcharge options. The following options are available:

Figure 7: Load options.

After drawing the load on the model, the Edit surcharge dialog appears (i.e. for sitrip surcharges, Figure 8).

Figure 8: Edit distributed load dialog.

  • Surface options: We can modify the surface options in order to create inclined soil surfaces in the General tab of DeepEX.By clicking on the button , we can edit the surface options. These options are presented in the table below. In addition, we can change the elevation next to the wall.

Figure 9: Surface options.

  • Water elevation: In this area we can define the water elevations next to the wall.

Figure10: Water elevation – settings.

With the draw custom water surface tool, we can draw a non-horizontal groundwater table. To do this, select this option and then start clicking the left mouse button from left to right (press enter to complete). To delete the custom water line, move on top in the model and press delete.

The draw a U line tool is used to draw a line of constant pore pressure in the model. This line is only used in slope stability analysis.

The define user water pressures tool launches the dialog for defining custom values of water pressures next to the walls. Please note that in the non-linear engine, two consecutive zero values of water pressure still count in increasing the total vertical stress by γw (see theory manual).

  • By clicking on the button , the Ground water table dialog shows up.

Figure 11: Ground water table.

B2. Example simulation

In DeepEX 2014 we can design our projects using construction stages. After the model is designed, the software calculates each construction stage, reassuring that the model is stable, since the last stage is not always the critical one. Next, we provide the steps in each construction stage, in order to simulate the project in DeepEX.

  • Stage 0 (Figure 12)

  1. Define the soil properties

  2. Define the soil layers (stratigraphy)

  3. Define the wall section and wall properties

Figure 12: Model, Stage 0.

  • Stage 1 (Figure 13)

  1. Excavate on the right side of the wall to Elevation -8

Figure 13: Model, Stage 1.

  • Stage 2 (Figure 14)

  1. Apply first tieback support row at elevation -6

Figure 14: Model, Stage 2.

  • Stage 3

  1. Excavate on the right side of the wall to Elevation -18

  • Stage 4

  1. Apply second row of tiebacks at Elevation -16

  • Stage 5

  1. Excavate on the right side of the wall to Elevation -28

  • Stage 6

  1. Apply third row of tiebacks at Elevation -26

  • Stage 7 (Figure 15)

  1. Excavate on the right side of the wall to Elevation -35 (final excavation level)

  2. Apply surcharges on the left side of the wall

Figure 15: Model, Stage 7.

In DeepEX we can design several design sections in the same model. We can add new design sections as new or as copies of the existing ones, doing several modifications in the model, or just defining different standards, calculation or analysis methods.

  1. Results in DeepEX

Since the model is ready, we can choose to calculate the design section, pressing on the button .

After the analysis is succeeded, the Summary table appears. Table 6 below includes some critical checks and values for each construction stage. Figures 16 to 19 present some graphical results from the results tab of DeepEX.

Table 6: DeepEX critical results/stage

Figure 16: Moment diagrams, Stages 5 and 7 respectively.

Figure 17: Shear diagrams, Stages 6 and 7 respectively.

Figure 18: Wall deflection diagrams, Stages 2 and 5 respectively.

Figure 19: Effective horizontal soil pressures, Stages 1 and 7 respectively.

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