This article discusses the modeling of piles in Slide. Topics discussed include:
- Intro to modeling piles in Slide
- Two methods for modeling a sheetpile wall
- Pile material property specification
One can model piles in Slide as long as you correctly average the pile properties in the third dimension (“into” the screen). The pile force is added to the strength of the soil which is equal to C/S, where C is the shear strength and S is the spacing. The direction of the force is tangential to the slip surface or perpendicular to the pile (defined by the user) and its location is at the support/slip surface intersection. If the pile is not failing in shear, it may be possible to use some other value indicative of the failure mechanism you think will happen. A good reference for support in a limit equilibrium analysis is Chapter 8 of Duncan and Wright’s Soil Strength and Slope Stability text:
Duncan, J.M. and Wright, S.G. (2005). Soil Strength and Slope Stability, John Wiley and Sons Inc.
There are a few different ways to model a sheetpile wall in Slide. If you want to model the global failure of a surface below the sheetpile wall, a thin material layer with infinite strength is suggested. This will force the failure surfaces to go below the wall.
If you are modeling failure through the wall, then you can model the wall as a micropile support element or a material layer. See the Micro Pile help topic for more information.
Attached to this article are two models of the same slope. In one case the pile is modeled with a support element, and in the other case a material layer is used. In the models piles with 0.2 m2 area, spaced at 1m, are assumed. This gives a pile material thickness of 0.2m if the pile wall is modeled as a material. If the spacing is different, the thickness of the pile material also needs to be adjusted. For example, if the spacing was 2m, then the pile wall thickness would be 0.1m. The results for the two models are quite close.
Support examples: pile (left) and material layer (right)
You can also refer to Example #49 in the Slide Slope Stability Verification manual (Part 2). All example files are installed with the Slide program.
Model Geometry for Slide Slope Stability Verification #49
If you are modeling failure through the wall, the issue then becomes what you use for the shear strength (material) or shear force capacity (micropile). If the mode of failure is assumed to cause rupture of the wall itself, then you can use the shear strength of the wall material. This is usually something like the tensile capacity of the steel or joints but often there are codes for these types of numbers. If failure is assumed to create a passive type bearing failure of the toe material, then this strength should be used. Often people will use the passive resistance (F=Kp*gamma*H*H/2), RSPile or another program to determine the strength of piles assuming the failure surface intersects the pile at a certain location.
The following reference describes the aforementioned analysis for soil nails:
Forest Service Engineering Staff (1994). Application Guide For Launched Soil Nails, United States Department of Agriculture. EM 7170-12A.
What you need to determine is the shear force that the support will contribute.
If you use a material layer, then you need to determine the appropriate thickness based on the material strength. In the example attached to this article, the pile material cohesive strength is 300kPa. With a material thickness of 0.2 intersected by the slip surface the support is contributing 60kN of shear, per unit out of plane. This is the same as defining a micropile with a shear strength of 60kN and a spacing = 1 meter, as you can see in the "pile passive" file. In the case of the micropile, Slide does not need any thickness, only the input shear stregth is considered.
For modeling a simple sheet pile wall using micropiles, the "spacing" of the micropiles is a fictitious number. We use a spacing of 1 for simplicity, and then enter a value of micropile shear strength which gives the equivalent shear strength per unit out of plane, as the sheet pile wall of a given thickness.
Remember that the Slide analysis is 2-dimensional, and support forces are averaged over the unit out of plane distance, regardless of how you model the support.