About the research
The Iowa Department of Transportation (DOT) replaced the existing railroad underpass on US 63 in Waterloo between Dane and Newell Streets with an overpass. The existing underpass was initially constructed below the water table; therefore, the Iowa DOT had to continuously use a groundwater dewatering system to prevent water from entering the underpass. With the overpass construction project, the Iowa DOT aimed to change the existing well system to avoid unknown impacts on the water table. However, groundwater level monitoring was required throughout construction to avoid endangering neighboring properties. The Iowa DOT used nine dewatering pumps to control the high water table in the area during overpass construction.
The work described in this report aimed to observe groundwater levels at and around the construction site and assess the effects of dewatering, which occurred between November 5, 2017 and March 25, 2019, and a groundwater suppression system (GSS), which began operation on October 15, 2019. Water levels were monitored in several observation wells and reported over 76 months that encompassed four periods: before dewatering, during dewatering, between dewatering and operation of the GSS, and during operation of the GSS.
Analysis of the time series of water levels and statistical analysis of the mean water levels and variances during the four periods provided a picture of the behavior of the water table and led to the following conclusions:
- Water levels in wells farther from the overpass were not greatly affected by the operations at the site. This observation confirms previous findings.
- Dewatering lowered the water table by a large amount around the construction area. The water levels rose quickly by the end of the dewatering period; mean water levels before and after dewatering (but before groundwater suppression) appeared similar, though the statistical analysis indicated that most mean levels were significantly different at the 5% level.
- The GSS has lowered the water table by about 1 to 2 ft. Statistical analysis supports the observation by showing that water levels during suppression were significantly different from the levels in periods without suppression or dewatering.
The variance of the water levels was smallest during the period with groundwater suppression. This observation suggests that the GSS has stabilized the water levels near the overpass.