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Automatic Failure Surface Method - SnailPlus Example

80deg Slope - Automatic Slope Search Method


In this example we will design a soil nail wall (soil nails with a shoctrete facing) supporting an 80deg 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 (80 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.

Automatic Failure Surface Method - SnailPlus Example

Figure: Project model


Automatic Failure Surface Method - SnailPlus Example

Automatic Failure Surface Method - SnailPlus Example

Automatic Failure Surface Method - SnailPlus Example

Automatic Failure Surface Method - SnailPlus Example

B. Modeling with SnailPlus

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

Define Model Parameters:


Automatic Failure Surface Method - SnailPlus Example

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

Automatic Failure Surface Method - SnailPlus Example

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


Automatic Failure Surface Method - SnailPlus Example

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.


Automatic Failure Surface Method - SnailPlus Example

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


Automatic Failure Surface Method - SnailPlus Example

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.


Automatic Failure Surface Method - SnailPlus Example

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:



Automatic Failure Surface Method - SnailPlus Example

Figure: Generated Model – Stage 2.


Automatic Failure Surface Method - SnailPlus Example

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


C. Slope Stability Analysis Settings

In this example we will use the automatic slope method. Using this method, we actually define the left and right slope search limits. SnailPlus examines all possible slope surfaces that pass within the defined limits and returns the most critical surface along with the calculated slope stability safety factor and the calculated soil nails and head plate results for each examined construction stage.


In the model area we can access (double-click) on the created left and right limits and define the start and end position of each limit.


Automatic Failure Surface Method - SnailPlus Example

Figure: Automatic 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).


Automatic Failure Surface Method - SnailPlus Example

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

Automatic Failure Surface Method - SnailPlus Example

From the table above we can see that at least one of the soil nails fails structurally or geotechnically. 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 (increase the soil nail rebar size, reduce the spacing, increase the soil nail length, increase the soil nail installation angle etc.).


In the same summary table, we can see that the calculated slope stability safety factor is small (less than one) in some construction stages. This means that the design is unsafe, so we have to reconsider some model parameters (soil nails layout, soil nail wall inclination etc.).


Finally, we can see that the shotcrete facing is quite under-stressed. We can definitely use a thinner facing or try less reinforcement (reduce mesh size or increase vertical/horizontal spacing).


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

Automatic Failure Surface Method - SnailPlus Example

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


Automatic Failure Surface Method - SnailPlus Example

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


Automatic Failure Surface Method - SnailPlus Example

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


From the images above, we see that the top two soil nail rows fail geotechnically (GEO >1). This is logical, since these rows are fully embedded in the top two very soft soil layers, with relatively small skin friction.


In this case, we should consider to decrease the horizontal distance between the nails and increase the embedment length of the top two soil nail rows.


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.

Automatic Failure Surface Method - SnailPlus Example

Figure: Shotcrete Facing Detailed Results.


Automatic Failure Surface Method - SnailPlus Example

Figure: Second Soil Nail Row Detailed Results.


 



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