MSP International Airport
Hydrology
To provide access to the infield area, the MAC has built three tunnels (see Tunnels section of the Web site) and is currently constructing the last one. The tunnel under the new runway (the 17/35 Tunnel) and a second tunnel under a taxiway (Y-3 Tunnel) on this same road are complete and open to the public. A third tunnel (W-Y) also provides infield access but is located inside airport security. The fourth tunnel (the 4/22 Tunnel) will go beneath Runway 4/22 and connect the south and north sides of the airfield. See Figure 1B (pdf) of Monitoring Information.
Due to the depth of the excavations and the makeup of surrounding soils, initial engineering strategies would have required considerable pumping of groundwater (dewatering) to keep the tunnels dry during construction. In response to concerns about the impact of dewatering, the MAC opted to add groundwater barriers around the construction of the three tunnels that have been completed. The barrier for each tunnel differs depending on soil characteristics and depth of the excavation.
The groundwater barrier for the 17/35 Tunnel was installed using jet grouting. Jet grouting creates a cement wall below the ground surface surrounding the entire excavation. The process dramatically reduced the amount of groundwater getting into the construction site and therefore reduced the dewatering that was needed. As a result, there was no impact to groundwater levels beyond the airport boundary.
To be certain that dewatering did not impact surrounding resources, the MAC implemented an aggressive monitoring program to measure the amount of ground water extracted, monitor the ground water levels in the area, and monitor lake levels in the area. The monitoring program was conducted in cooperation with the Lower Minnesota River Watershed District (LMRWD), the Minnehaha Creek Watershed District (MCWD) and the Minnesota Department of Natural Resources (DNR). Dewatering for construction of the three tunnels was completed in the summer of 2002, and groundwater levels have since returned to pre-dewatering levels.
The shorter tunnel on Cargo Road (Y-3 Tunnel) used sheet piling driven down into the ground to secure all the sides of the construction site prior to commencing dewatering. The soil was removed inside the sheet piling and a concrete floor constructed. This provided a complete barrier around the tunnel excavation, and the groundwater dewatering system was shut down. The tunnel was then constructed under "dry" conditions inside the barrier.
The W-Y Tunnel located inside airport security (W-Y Tunnel) was constructed at the same time the Y-3 Tunnel was built. Due to the close proximity of the tunnels, it was prudent to slightly stagger construction to avoid overlapping of dewatering activities. The W-Y Tunnel is the shallowest of the three tunnels and used an impermeable liner installed after the site was initially dewatered and excavated. Sand was placed on top of the liner to secure it. Then the dewatering system was shut down. This allowed for construction of the tunnel under "dry" conditions with no extended dewatering required.
The final vehicle tunnel (4/22 Tunnel) is currently under construction. Dewatering for this tunnel began on October 13, 2004. Based on 3 years of dewatering experience on the previous tunnel dewatering efforts, it was determined, in consultation with LMRWD, MCWD and the DNR, that a ground water dewatering barrier would be unnecessary for this project.
MAC will continue to monitor groundwater in the area to ensure that there is no adverse impact to the surrounding resources. The 4/22 Tunnel is scheduled to be complete by the end of 2006.
The following reports are the most recent results of our monitoring activities:
| Figures |
This illustraion presents the locations where surface water and groundwater levels are being recorded.
Figure 1A |
This illustration presents the locations where surface water and groudwater dewatering is a component of construction projects at the airport.
Figure 1B |
This illustration contains five figures illustrating the ground and surface water elevations on and off airport property.
Figure 2 |
| Tables |
The surface water and groundwater elevation information that has been compiled for the monitoring stations presented in Figure 1 are included in this table.
Table 1A |
This table presents groundwater elevations in piezometers located at the Runway 17-35 Vehicular Tunnel project site.
Table 1B |
This table summarizes the average weekly groundwater extraction rates for dewatering activities at several airport construction projects, including the 17-35 vehicular tunnel project.
Table 2 |
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