Static vs. Dynamic Analysis of a Cantilever Sheet Pile and an MSE wall
Sometimes, permanent wall systems have to withstand earthquake loadings.
Cantilever wall systems are known to accumulated deformations during sustained earthquake loading since they don’t restrain accumulated displacements.
On the other hand, MSE wall systems are known to behave relatively well during earthquakes in part due to their ability to transfer and share load among multiple geo-reinforcement layers.
This article examines and contrasts the seismic analysis results of a cantilever wall with a 0.1g acceleration for typically assumed analysis scenarios:
- Limit-Equilibrium with Mononobe-Okabe seismic loads
- Non-linear analysis with Mononobe-Okabe seismic loads
- Finite-element analysis with pseudostatic acceleration
- Finite-element analysis with a harmonic dynamic acceleration, 4 hertz, and 1 second duration
Figures 1 through 3 present critical seismic analysis results for deflections and bending moments for all cases.
From these figures we can see that the LEM, NL, and FEM with pseudostatic loading produced relatively similar maximum bending moments.
The dynamic analysis exhibited different bending moments at each duration, with a maximum bending moment about 33% greater than the traditional approaches.
Figures 4.a, 4.b, and 4.c compare results from an MSE wall analysis.
Figure 4.a presents pseudostatic FEM results while 4.b presents the results of the dynamic analysis. Figure 4.c presents the results of a traditional analysis with the coherent earth method and Mononobe-Okabe seismic pressures, while Figure 4.d assumes the simplified stiffness method as outlined by AASHTO.
The pseudostatic and dynamic analyses produced relatively similar deformations with the dynamic analysis producing slightly greater reinforcement loads. Both FEM analyses produced slightly smaller reactions versus the simplified stiffness method by AASHTO.
The coherent pressure MSE analysis method produced significantly greater loads vs. all other methods.
Concluding, incorporating seismic dynamic effects in excavation support systems can produce more stringent requirements for cantilever wall systems.
For MSE wall systems, it appears that the stiffness method is in greater agreement with both pseudostatic and dynamic finite element methods versus the coherent earth method.
Figure 1: LEM analysis with seismic loading results
Figure 2: NL analysis with seismic loading results
Figure 3: FEM pseudostatic analysis with seismic loading results
Figure 3a: FEM dynamic analysis results at 0.81 seconds
Figure 3b: FEM dynamic analysis results at 0.99 seconds
Figure 4.a: MSE wall with pseudostatic seismic load
Figure 4.b: MSE wall with dynamic seismic load
Figure 4.c: MSE wall with MO seismic load in conventional analysis and coherent analysis method
Figure 4.d: MSE wall with MO seismic load in conventional and stiffness analysis method
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