Jacking Force Prediction: An Interface Friction Approach based on Pipe Surface Roughness
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This study identifies mechanisms controlling interface shearing between pipes and granular materials and develops a predictive jacking force calculation model. The surface roughness of six pipe materials, including Hobas (Centrifugally Cast Fiber Reinforced Polymer Mortar), Polycrete (Polymer Concrete), Permalok Steel (Rolled Steel with a Painted Surface), Wet Cast Concrete, Packerhead Concrete, and Vitrified Clay pipe, have been characterized to determine the role of surface roughness on the soil-pipe interface shearing mechanism. Interface shear tests were performed between pipe materials and two characteristically different granular soils: Ottawa 20/30 sand and Atlanta Blasting sand. Shearing behavior between the sands and the pipe materials was evaluated to determine the mechanisms of shearing on materials with varied roughness values. Interface friction values were established for the pipe materials and soils. A model was developed to jacking forces based on modifications to Terzaghi's Arching Theory for predicting normal stresses and interface friction coefficients developed in the laboratory. Field research on fourteen case histories of microtunneling and pipe jacking projects was presented. Pertinent project details were provided including pipe materials, site geometry, geotechnical information, construction sequencing, lubrication injection, and jacking force records. Jacking force records for each project were separated into isolated segments along the alignment to analyze jacking stresses. Unlubricated segments of the microtunneling drive records were analyzed to compare actual and predicted jacking forces using the proposed model. The predictive model was compared to other models currently available for predicting the frictional component of jacking forces. Lubrication effects on jacking forces were analyzed to determine how the interface friction coefficient changed once lubrication was applied to the pipeline. Two types of lubrication strategies were identified and predicted lubricated jacking forces were shown. A step-by-step guide for using the jacking force predictive model was presented for design applications and estimating lubricated interface friction values.