Project Description

The Quad City Airport located in Moline, Illinois required the extension of one of its runways by 300 meters to meet future needs and provide a secondary runway for larger aircraft. Because an existing highway limited any extension at one end, the runway had to be extended toward a bluff at the other end. In order to maintain the proper flight path clearance, the elevation of the end of the runway would require as much as a 4.5-meter fill over existing soft alluvial soils. The width of the runway was 45 meters.

Surface Conditions
The subsoils consisted of a silty clay overlying fine sand of medium density. The upper 1.5 to 2.0 meters of the silty clay was medium stiff with "N" values ranging from 5 to 10 blows per 300 mm. This material was slightly desiccated and had moisture contents ranging from 22 percent to 30 percent. The lower portion of the silty clay layer was soft to very soft with "N" values from 1 to 4 blows per 300 mm and a moisture content ranging from 30 to 45 percent. The total depth of the silty clay layer ranged from 4.5 to 7.6 meters.

Design Concerns
The designers were very concerned with post construction settlement resulting from the added fill, in order to maintain the proper grade for the runway. Settlement projections varying from 120 mm near the end of the existing runway to in excess of 300 mm at the greatest fill height could not be tolerated. Various solutions such as removal and replacement, surcharging, and wick drains were investigated. The high cost of removal and replacement eliminated this option. The time rate of settlement eliminated the option of just building the embankment and waiting for settlement to occur. Surcharging would not only be costly because of the need for removal and disposal after completion of settlement, but it would also create a slope stability problem.

The design also called for a twin set of 1.80 meter diameter culverts to be extended under both the runway and taxiway. While these culverts could be designed for the total amount of settlement anticipated, it was necessary that they be at the final elevations prior to opening of the runway.

Therefore, the use of wick drains to accelerate the settlement, combined with a small amount of surcharge that could be used elsewhere on the site, was chosen as the solution.

Another concern to the designers was the availability and cost of a clean sand to be used for a drainage blanket. The problem was solved by designing a system of strip drains, utilizing a 300 by 25-mm strip drain, which was outletted into a gravel and pipe trench under the outer edge of the embankment. Each strip drain was connected to four rows of wick drains by extending the wick material over to the strip drain. This was the first use of horizontal strip drains in lieu of a drainage blanket on an airport runway or taxiway.

Design
Results from both field and laboratory testing indicated that a maximum total settlement of 365 mm would occur under approximately 1.7 m of surcharge and 4.5 m of fill. Based on consolidation coefficients, it was determined that a 1.5-meter triangular pattern spacing of the wick drains would result in a settlement time period of approximately 6 months, once full height was attained. The project was divided into two phases since the taxiway wasn't as critical as the runway. The runway project was designed first with a wick drain quantity of approximately 77,000 linear meters. The area was extended to cover part of the taxiway so that the future project of the taxiway could be completed without any settlement effect to the runway.

Project Results
The wick drain installation began in the middle of June and just under 10,000 wick drains averaging 7 meters in depth were installed in a 3-week time period. The horizontal drains were completed shortly thereafter. Actual settlements ranged from 100 to 400 mm in the worst area, and were completed in 3 to 4 months, within the design time of 6 months.

Since stability was not a concern, only settlement plates were necessary to determine the amount and time rate of consolidation.

Project Cost
This project was somewhat unique in that the wick drains were bid on a per drain basis instead of the more common unit of linear meter. The unique unit of measurement was selected because the wick drains were short, but had to be extended to the strip drains at the surface. Subcontractor prices for the wick drains were approximately $11.50 per installed drain or slightly in excess of $1.30 per linear meter with a mobilization charge of $10,000. Installed horizontal strip drain prices were $4.80 per linear meter.