Limit States Evaluation of a Modular Polymer Stormwater Collection Structure with Full-Scale Physical Modelling

Abstract

Full-scale physical modelling was conducted to identify and evaluate limit states of a modular, polymer stormwater collection structure for shallow and deep burial cases. At minimum soil cover and when subjected to short-term design-truck loads, columns located directly beneath the load pad experienced the largest increments in force and the four central columns carried on average 31% of the applied nominal design truck load. Column demand at the factored service design truck load of 107 kN was equal to 0.45 and 0.64 times the single-column buckling resistance of the 0.6-m and 0.9-m-long columns tested. The columns as tested were found to satisfy load and resistance factor design requirements against short-term column buckling with a resistance factor of 0.7. An ultimate limit state to surface load with shallow cover was reached at surface loads between 218 and 238 kN. Failure was governed by top platen rupture and central column buckling for the longer columns and by platen rupture for the shorter. No structural limiting state was reached when a parked vehicle load of 89 kN was sustained for one week and column compression increased by less than 20%. The short-term lateral response along the edge of the structure during backfilling and under maximum burial were quantified using different soil materials, compaction methods, and column lengths. Tensile strains in edge columns were induced by lateral bending from soil placement and lateral earth pressures. Of the conditions tested, rammer-compacted poorly-graded gravel resulted in the largest short-term edge column tensions; however, even these values were 4.5 times smaller than the measured short-term strain limit. Physical modelling with elevated pressure and elevated temperature was then used to examine the buried system response subjected to vertical and lateral deep burial loading to infer longer-term behaviour. Similar modes and magnitudes of deformation were found between elevated pressure and elevated temperature including large lateral bending of the edge columns and column bending strains that developed in the columns away from the edge. Predictions from the elevated temperature results tend to indicate a stable long-term response – provided material resistance remains the same as tested, based on extrapolations in time at lower temperatures in what may be the first application of the stepped isothermal method to a full-scale soil-structure system. The prediction method appears to be promising and was proved to not be invalid, as requirements of having smooth and continuous master curves and time shift factors that followed an Arrhenius relationship were satisfied.