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70 deg Slope - Circular Slope Surface Method

In this example we will design a soil nail wall (soil nails with shotctete facing), supporting a 70deg slope surface. The model is created and analyzed with SnailPlus - Slope Stability and Soil Nailing Software.

A. Project Description

In this example we will design a soil nail wall (70 deg from horizontal), with 5 rows of soil nails. Tables 1 and 2 present the soil properties and the stratigraphy respectively. Tables 3 and 4 present the soil nail and the Shotcrete facing properties respectively.

Figure: Project model.

Table 1: Soil properties.

Soil Layer

Soil Type

Design parameter

φ’ (deg)

C’/Su (psf)

γ (pcf)

γdry (pcf)

q,skin (psi)

ks

(kcf)

F

Sand

30

0

120

120

15

20

S1

Sand

34

0

13 0

100

20

30

S2

Till

36

200

135

135

28

70

 

Table 2: Stratigraphy.

Soil Layer

Top Elevation (ft)

OCR

Ko

F

-0

1

0.5

S1

-6

1

0.441

S2

-18

1

0.412

 

Table 3: Soil Nails.

Load

Section

Installation Angle

Length

Horizontal Spacing

Row 1

#9 Bar

15 deg

30 ft

6 ft

Row 2

#9 Bar

15 deg

30 ft

6 ft

Row 3

#9 Bar

15 deg

30 ft

6 ft

Row 4

#9 Bar

15 deg

25 ft

6 ft

Row 5

#9 Bar

15 deg

19 ft

6 ft

 

Table 4: Shotcrete Facing and Plates.

Shotcrete Type

Permanent Only

Facing Thickness

8 in

Front Reinforcement Size

W 5

Front Reinf. Spacings (HxV)

8’’x8’’

Base Reinforcement Size

W 5

Base Reinf. Spacings (HxV)

8’’x8’’

Head Plate Section

PL8x8x1.25

 

B. Modeling with SnailPlus

The original model will be created using the SnailPlus Model Wizard:

Define Model Parameters:

Figure: Model Parameters in the SnailPlus Wizard.

In this dialog, we define the ground surface elevations, wall inclination and the soil nails layout (original spacing, installation angle and length). We can select the option “Include all intermediate stages”. This action will generate all project stages, allowing us to examine the slope stability of each stage independently.

Define Shotcrete Facing and Head Plate Parameters

Figure: Shotcrete and Head Plate Parameters in the SnailPlus Wizard.

In this dialog we define the shotcrete facing parameters (thickness, rebar sizes, rebar spacings and materials), and we can also select a plate section for the soil nail head plates.

Define Analysis Options

Figure: Analysis Options in the SnailPlus Wizard.

In the last Wizard tab we can define the analysis options. We will use the automatic slope search method for the slope surfaces and we define to use ACI and FHWA Allowable Stress Design methods for the structural design.

Define soil types and stratigraphy

From the General tab of SnailPlus we can access the Soil Types dialog, where we can create a list of soils and define the soil properties of all soil types we wish to use in the project.

Figure: Edit Soil Properties in SnailPlus - General Soil Properties.

Figure: Edit Soil Properties in SnailPlus - Bond Ressistance and Subgrade Modulus.

Also, in the General tab, we can select to edit the project Stratigraphy. In the Edit Borings dialog, we can create a list of borings. For each boring we can define the top of the soil layer elevation and the soil type for each boring soil layer.

Figure: Define Stratigraphy in SnailPlus.

The SnailPlus Model Wizard generates all construction stages automatically. The following Figures present the models and the defined stratigraphy in Stages 2 and 5 respectively:

Figure: Generated Model – Stage 2.

Figure: Generated Model – Stage 5 (Final Stage).

C. Slope Stability Analysis Settings

In this example we will use circular slope surfaces. In each construction stage, we can access and modify the created rectangle of centers limits. We can also access and modify the position of the radius search points below the wall.

In the model area we can access (double-click) on the created rectangle of centers and edit the properties (dimensions, position, number of increments etc.).

Figure: Center Search Limits in SnailPlus.

Finally, in the General tab of SnailPlus we can define the Slope Stability Analysis method. In this example we will use the General Limit Equilibrium Method (GLE – Morgenstern-Price).

Figure: Define Slope Stability Analysis method in SnailPlus.

D. Soil Nail Wall Analysis and Results

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

After the analysis is succeeded, the Summary table appears. The table below includes some critical checks and values for each construction stage.

Table: SnailPlus critical results/stage

From the table above we can see that at the soil nails are under-stressed (the provided structural and geotechnical capacities are quite larger than the applied loads). We can review the particular results of each soil nail row (graphically or in tables as described below) and decide how to optimize the model, in order to achieve a more efficient solution (decrease the soil nail rebar size, increase the spacing, decrease the soil nail length etc.).

The following figures present some graphical results from the results tab of SnailPlus.

Figure: Graphical Results: Slope Surface, FS, Soil Nail Reactions – Stage 2.

Figure: Graphical Results: Slope Surface, FS, Structural and Geotechnical Ratios – Stage 5.

Figure: Graphical Results: Soil Stresses and Slope Slices – Stage 5.

By double-clicking on the shotcrete facing or any soil nail row in the model area, we can access the detailed table results for the selected item in the selected construction stage.

Figure: Shotcrete Facing Detailed Results.

Figure: Second Soil Nail Row Detailed Results.

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