Lateral Earth Pressure on Cylindrical Concrete Shafts

Cylindrical concrete shafts are one of the main components of any concrete storm water or sewerage system. The state of stresses within the soil surrounding these shafts are not fully understood, since it is different than the well known stresses behind retaining walls due to the radial stiffness of the shaft. This paper concentrates on studying the lateral pressure around such shafts. The presented study consists of two main parts: an experimental program that comprises full-scale testing of cylindrical shafts under in situ conditions in a controlled laboratory environment, and a numerical modeling investigation using the finite element method. The research results showed that the lateral earth pressure ranged between above the at-rest to slightly below the active earth pressure based on the soil type. The shafts in clayey soils experienced larger lateral pressure values than those in sandy soils. The effect of compaction was found to be significant at the top two meters of backfill and negligible below that. The live load was found to have a marginal effect on the pressure close to the top of cylindrical shaft and this effect decreases with depth (i.e. distance from top of cylindrical shaft). Comparisons were made between the lateral earth pressure observed in this study and some proposed procedures by the Canadian Highway Bridge design code for calculating pressure behind retaining walls.