Performance of Relief Well Systems along Mississippi River Levees

In the late 1940s, the principal levees along both sides of the Mississippi River from Alton to Gale, Ill., were raised to a 1-in-50-year flood level—a 500-year frequency in urban areas. In 1950, the St. Louis District initiated an investigation of seepage beneath these levees. As a result of this investigation, 2,480 relief wells were installed along 292 mi of mainline and tributary levees during the 1950s. During the flood of 1993, the stage of the Mississippi River along the mainline levees equaled or exceeded the design stage, the highest river level to which the middle Mississippi River levees had ever been subjected. The writers were responsible for collecting and analyzing the data and information relating to the condition, efficiency, and performance of the relief wells installed in the 1950s along the above stretch of the Mississippi River. The study showed that the relief well systems that were designed and constructed during the 1950s with limited design data performed successfully and prevented any significant sand boils or piping for a design river stage. The parameters involved in design and performance of the underseepage control systems were largely confirmed within the normal variations of the parameters involved and field conditions. From analysis of the data collected during the 1993 high water, a program for maintenance and repair of the relief well system was developed. This paper presents a brief description of the phenomena of underseepage, the parameters involved in the design and performance of underseepage control measures, and a comparison of design values with field performance for a 50-year high water against 300 mi of Mississippi River levees in 1993, of which 45 miles are now protected from underseepage by relief wells. This paper also describes the importance of not only building levees high enough to hold back the design high water of a river but also the design, construction, and maintenance of relief well systems to prevent failure of levees as a result of sand boils and/or piping for the various subsurface conditions that exist along levees in the alluvial valleys of major rivers.