Measures to Control Consolidation Settlement of Existing High-Pressure Gas Mains through Ground Improvement-Based Protection

Abstract

An embankment approximately 2.5 m in height was planned in an area where an existing 0.6m-diameter, high-pressure gas main was buried (with a preload portion to be dismantled in 20 months). An approximately 8 m-thick, soft, cohesive soil layer (N value = 0) was found under the high-pressure main, and deformation in the vertical direction was restricted at some points at the planned site. Uniform settlement of the high-pressure gas main was thus impeded, generating excessive stress. Though this stress did not exceed the allowable stress, given the importance of the gas main, a stress-reduction measure was designed. An improved structure was designed to accommodate the high-pressure gas main between piles constructed by deep mixing and a gate constructed by shallow improvement. The embankment load was borne by this improved structure, allowing control over the extent of consolidation settlement. To adapt the extent of consolidation settlement to that in an area where this measure was not implemented, three-dimensional FEM analysis was used to design a longitudinal improvement depth. Results from 20 months of field observation following construction of the embankment showed that measured consolidation settlement corresponded well with the value estimated in the design phase, corroborating the effect of the improved structure as designed.