August 29, 2013

Moving water to get fans closer to the field

By DAVID WINTER Hart Crowser

As is often the case, good engineering helps make exciting design a reality.

At Husky Stadium the redesigned field level and lower bowl bring the fans closer to the action on the field -- and bring the field closer to the groundwater below. It was up to Hart Crowser's engineers and hydrogeologists to figure out a way to control the water to create a dry surface for construction and a stable subgrade for the playing field.

Conditions below

From about the eastern 30-yard line all the way to Lake Washington, lakebed deposits and recently placed fill underlie the stadium area. The top of the hard glacial soil dips towards the lake, with the fill and sediments layered above. This intermingled geology means that the water level in the lake affects the water level below the area just east of the stadium.

Lake Washington water levels are about 16 to 18 feet above sea level. The water level on the eastern goal line is about 30 feet, meaning that groundwater flows toward the lake from the eastern side of the stadium.

On the western side the conditions are different. The ground from the eastern 30-yard line toward campus has very dense glacial soils just below the surface, but it is mingled with thick layers of sand that collect rain from all over campus and the University District. All this groundwater flows towards the stadium. So while the groundwater levels below the field might be 30 to 32 feet, the water can be under pressure from the infiltration occurring on campus.

Photos by Jesse Overton/Hart Crowser [enlarge]

Temporary dewatering wells kept the construction ground surface passable, even during wet winter months. A large lined settling pond was used to collect runoff. Ponded water is from rainfall.

The water levels vary a little seasonally. They are higher in the winter and spring by a foot or so, and lower in the summer and fall. The level on the Lake Washington side of the field is fairly consistent year-round.

Field level drops

The extended lower bowl seating became possible by moving the running track to another location and lowering the field level 4 to 5 feet. The unintended consequence of improved sight lines and greater intimacy with the field is a playing surface only about 1 foot above the seasonal high groundwater level.

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A permanent French drain lowered the groundwater so the field subgrade could be built. It cuts off the groundwater flow before it gets to the field.

Underneath the artificial turf are about 2 feet of prepared subgrade. To provide this base it meant the field excavation had to be 2 to 3 feet lower than the playing surface -- about 2 feet below the groundwater level. So not only did the final design need a drainage system to keep the water well below the playing surface, it also needed a way to control the water to allow the field section to be built in the dry.

Hart Crowser's hydrogeologists constructed a groundwater flow computer model to predict the changes in water level using various controlling systems. During construction they estimated that a network of five pumping wells installed just west of the construction zone and slightly north of the center of the field would cut off the groundwater flows from the main campus and lower the water table below the field by several feet.

These five wells were installed at the beginning of construction and pumped an average of about 50 to 60 gallons per minute. (As a comparison, a garden hose will flow at about 10 gallons per minute.) Not a lot of water for such a big area -- but enough to lower the water level at the field level by 3 feet and allow the excavation and construction work for the field to occur under workable conditions.

3 sources of water

The temporary dewatering wells had to lower the water level enough to allow the permanent groundwater cutoff system to be installed. The permanent groundwater cutoff system had to control the water levels to allow the field subgrade and playing surface to be constructed.

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Three independent drain systems work below the surface to keep the field dry.

Water could inundate the field from three different sources:

1. Rain falling directly onto the field.

2. Rain running off the seating onto the field.

3. Groundwater flowing into the area from campus.

Hart Crowser designed three separate water control systems to keep this from happening:

1. A network of cross drains parallel to the yard lines and about a foot below the field catches water and prevents buildup of water pressures below the turf. The turf itself is highly permeable, as is the gravel subgrade layer below. Water falling onto the field as rain or snow will easily pass through the turf and gravel, be collected by the cross drains, and be conveyed to an outfall to Lake Washington. Even in the pouring rain the field remains playable.

2. A trench drain surrounds the field at the base of the lower bowl and collects runoff from the bowl seating. The trench drain is 2 feet deep and filled with gravel, with a perforated pipe at the bottom. Water collects in the pipe and flows to the outfall instead of running onto the field.

3. The groundwater capture and conveyance system is deeper but very simple. A French drain 5 feet deep surrounds the field on the three upgradient sides (north, south and west). The French drain is filled with gravel and has a perforated pipe at the bottom. Groundwater intersects the drain and pipe, which carries it to the outfall. This keeps the groundwater level at least 3 feet below the field.

All three of these systems act independently, and all are necessary to keep the field in top playable condition

David Winter, PE, LEED AP, is vice president of Hart Crowser. He was principal-in-charge of the geotechnical, environmental and seismic design and construction of the Husky Stadium redevelopment and ICA track relocation projects.

 

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