Can I reduce the LEM shear in cantilever excavations with DeepEX?
- May 19
- 3 min read
DeepEX provides access to three different approaches for designing and verifying cantilever excavations: Limit Equilibrium Method (LEM), Beam-on-Elastic-Foundation (Non-Linear), and Finite Element Analysis (FEM – 2D). Each method interprets earth pressures and wall response differently, and while results are often comparable, the shear force profile in cantilever excavations can vary significantly between methods, particularly when LEM is used.
In LEM, users can choose between the Free Earth Method and the Fixed Earth Method, each of which generates a different distribution of moment and shear below the excavation level (Figure 1). DeepEX allows engineers to switch between methods and apply modifications to the shear diagram when necessary.
Why LEM Produces High Shear at the Base in Cantilever Excavations
When using the Free Earth Method, the wall is assumed to rotate about the point of zero moment, resulting in an unbalanced shear distribution. Because shear must close to zero at the wall tip, the method tends to produce a large negative shear spike near the bottom of the excavation. This is a well-known characteristic of the method and does not necessarily represent physical behavior; instead, it reflects the simplified LEM equilibrium assumptions.
The Fixed Earth Method, on the other hand, balances both moment and shear at the base of the wall. This eliminates the large shear spike and produces a more stable and smoother shear diagram. However, the method generally produces larger wall embedment requirements, because the model forces the system to satisfy full equilibrium.
Figure 2 presents a result of the two analyses in the same model using the Free Earth Method (subfigure 2a) and the Fixed Earth Method (subfigure 2b), where it is possible to observe that the first method leads to higher shear when compared to the second method.
How DeepEX Allows You to Adjust or Reduce Cantilever Shear
DeepEX includes a built-in option to limit or reduce the negative shear in LEM for cantilever excavations (Figure 3). Under the Analysis tab, users can activate a setting that applies a shear adjustment factor, reducing the maximum negative shear value produced by the Free Earth Method and smoothing the diagram.
This allows users to:
Maintain the advantages of the Free Earth Method
Avoid unrealistically high negative shear values
Produce diagrams that better reflect engineering judgement
Compare adjusted values with Fixed Earth and FEM results
This adjustment does not affect the computed required embedment depth or the moment diagram; it only adjusts the plotted shear for interpretation purposes.
Figure 4 illustrates the result of the application of the limitation to the negative cantilever shear, using the Free Earth Method, resulting in a reduction of the shear of roughly 9.5% of the initial value.
Comparison With FEM
Engineers using the 2D FEM optional module will notice that FEM typically produces:
A more realistic shear distribution
A smoother transition below the excavation level
No requirement for artificial shear spikes
A natural representation of soil–structure interaction
For this reason, FEM is often preferable when evaluating structures where shear is a controlling design parameter.
Conclusion
Yes—DeepEX allows you to reduce the LEM shear spike in cantilever excavations. By selecting between the Free Earth and Fixed Earth Methods, and by using the shear-limiting option in the Analysis tab, you can adjust the shear diagram to better reflect practical engineering behaviour. For the most realistic representation, the FEM module can also be used, providing a fully soil-structure-interaction-based prediction of shear and bending along the wall.
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