Natural and natural-artificial flood ice laminates up to 85 inches thick were tested for collapse under a variety of loading conditions. The data was empirically correlated using Westergaard's extreme fiber stress formula for elastic plates on an elastic foundation to establish a critical load modulus, which when exceeded resulted in immediate or delayed collapse of the ice sheet. For natural sea ice this critical load modulus is given by Sc = 215 - 2.95 T psi, where T (°C) is the ice center temperature. For laminates with ice center temperatures near −14°C the critical load modulus was found to be Sc = 100 + 1.42 P psi, where P is the percentage thickness of natural ice in the laminate. Seismic data showed only weak evidence of radial crack formation, possibly indicating plastic flow failure at the ice water interface. Circumferential cracks were observed both visually and using seismic methods at locations near those predicted by Nevel's theory for elastic wedges on an elastic foundation. Using this theory, the surface ice tensile strength for both natural and flood ice was found to be 75 psi. Brazil tensile strength tests yielded similar values, with a decrease in the strength for deeper sample locations in the ice.

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