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Traditional restoration strategies are designed to provide 100% restoration in the event of any single failure. The impact of simultaneous failures in the network is not frequently studied. Simultaneous failure scenarios become an important consideration when the network is subject to stringent reliability requirements. We examine and compare the robustness of two familiar restoration strategies, 1+1 and shared mesh, and two new proposals, enhanced shared mesh and enhanced shared mesh "plus", with respect to the resilience to simultaneous failures. We propose and utilize efficient algorithms to evaluate demand-pair availabilities for each restoration strategy. Our algorithms for the shared mesh and enhanced shared mesh designs represent new work in this area. We check the accuracy of our availability algorithms with a Monte Carlo simulation. We use the algorithms in conjunction with two real networks to compare the restoration strategies.