Analysis of Vertical Column Support Systems for Stabilization of Roadway Subgrade Settlements

Every year, patching and resurfacing projects are undertaken to repair pavement distress and structural failure due to problematic subgrade soils. These conditions seriously impact roadway function and safety and create substantial costs to remediate the problems caused by subgrade settlement. Many of the subgrade settlement issues occur as a result of very soft to soft cohesive soils, very loose to loose granular soils, saturated soils, and/or organic soils, all of which yield low strength and subgrade support characteristics. The settlement problem can usually be addressed by using conventional remediation methods or by using vertical column support methods.Conventional methods to remediate subgrade settlements caused by problem soils include removal of weak soils and replacement with new suitable engineered fill, near-surface chemical stabilization such as lime or cement, or preloading/surcharging with or without wick drains. However, when problem soils are relatively deep, or long term settlement tolerances are low, these conventional methods can sometimes prove ineffective or too costly. New technologies and extended application of old technologies has led to some relatively limited use of vertical column support methods for the remediation of roadway subgrade settlements. A specific search was performed to identify other research studies performed on the use of vertical column support systems for the remediation of existing roadways exhibiting subgrade settlements, however, no other similar research studies were located. Some previous research studies on the use of vertical column support systems used to support embankments for new roadways, road widening, and bridge approaches were reviewed, although these research projects were not for the remediation of existing roadway settlements. The use of vertical support columns can be employed either as a deep foundation system or as a ground improvement technique.Vertical column support systems create relatively stiff column elements which allow for reduced loads on the surrounding weaker soils and improved support of the overlying roadway systems. Vertical column composition varies greatly and includes concrete, grout, stone/aggregate, sand, or soil-cement inclusions within the existing soil matrix. Some methods not only create strong vertical column elements at the locations they are installed but also improve the surrounding weak soils through a number of differing techniques. As a result, the current vertical column support methods can generally be grouped into two distinct subsets: ground improvement and deep foundations. Examining the available vertical column support methods and identifying the cost effective methods which provide increased subgrade support and decreased settlements will assist transportation agencies by improving roadway safety, reducing future pavement rehabilitation projects, lowering repair costs, as well as lowering the overall cost to the road users and society as a whole