New design method for frozen earth structures with reinforcement

Difficult soil conditions and earlier failures using more conventional construction methods on deep shafts and approach structures at a construction site showed the need for a frozen ground support system. Ground freezing was particularly suitable for the project because it did not require lowering of the water table, thus avoiding potential problems for existing nearby structures. Preliminary design methods showed the potential for large tensile stresses around openings in the shaft wall, hence the need for reinforced frozen wall sections.; Irregular wall geometry of the proposed frozen earth structure and poor soil conditions required specialized computational methods for design and evaluation of structural stability. Conventional design methods were found to be too conservative and would result in frozen earth structures that were excessively large and cost prohibitive. Also, conventional analysis techniques were unable to account for the complex frozen wall geometry required for construction.; A laboratory testing program was conducted to determine elastic and time-dependent material properties of the weaker soils in both uniaxial compression and tension. The finite element method was used to evaluate stresses and displacements at critical locations which would lead to unstable and unsafe conditions during construction.; Composite steel-concrete reinforcement members were designed and placed so as to transfer loads to less critical wall areas. These reinforcement members were installed prior to ground freezing. Stress and deformation changes within the reinforced frozen wall were evaluated for the excavation and construction phase using frozen soil creep parameters and a nonlinear finite element analysis. Field measurements during construction showed agreement with predicted wall movements.; The use of measured elastic and time-dependent soil properties with the finite element method for evaluation of the frozen wall structural behavior was considered a significant success. This approach can deal with complex frozen wall geometry and should be suitable for a variety of design requirements.