Test Section Overview

The test section was selected because AKDOT&PF identified it as the section of road on the Dalton Highway with the softest spots (frost boils) observed in the spring of 2010. The road section was originally built directly on the tundra with the degraded granite. Sieve analyses indicated that the soil was classified as gravel with sand, according to USCS classification, and contained about 6% or more fines (material passing the #200 sieve). At the west edge of the road about 0.91 meter (3.0 feet) below the ground surface, the original tundra was found at about 1.36 meters (4.5 feet) below the centerline of the road section. The degraded vegetation layer with dark yellow color was about 0.05-0.1 meter (1-2 inches) thick. In situ crushed rocks and sand were encountered below the degraded vegetation. Groundwater was 0.15 meter (6 inches) below the tundra surface. Additionally, during the construction process, excess water was observed in the drainage ditch, which was located on the west side of the road.

In total, 22 pairs of sensors were installed in the pavement structure. Each pair of sensors consisted of a Campbell Scientific 107-L temperature sensor and a CS616-L moisture content reflectometer. An HMP45C air temperature/relative humidity sensor was also installed at the site to monitor the air temperature and relative humidity. Four layers of sensors were installed at depths of 0.45, 0.76, 1.06 and 1.97 meter(s) (1.5, 2.5, 3.5 and 6.5 feet) below the road surface. Two layers of wicking fabric were installed at depths of 0.76 meter and 1.06 meters (2.5 feet and 3.5 feet) below the road surface. In order to ensure that the geotextile could transport water in the direction parallel to the water flow direction, care was taken to make sure that the geotextile (wicking fabric) was installed along the transverse direction of the road section. On the east side of the roadway, the two wicking fabric layers were left exposed to the air at 1.21 meters (4 feet) off the shoulder. Sensor 22 was installed at the location closest to the drainage ditch, and could be used as a representation of the saturated moisture content in the pavement structure in summer time. The sensor wires were protected with aluminum conduit. A Campbell Scientific CR1000 data logger was adopted to record the data. All of the data acquisition devices were organized into an ENC14/16-NC-NM weather-resistant enclosure which was installed on the tundra about 6.1 meters (20 feet) from the west edge of the road.

Excavation
A test section of 18.1 meters (60 feet) was constructed at Beaver Slide from August 3, 2010 through August 5, 2010. The construction was performed according to the following procedure. The east (northbound) lane of the road was excavated to a depth of 1.06 meters (3.5 feet) below the original road surface, while the west (southbound) lane of the road was not excavated, to maintain the traffic flow. Before installing the first layer of wicking fabric, three pits were excavated below the centerline, the edge of the original road, and the edge of the excavation limit. These pits were about 1.97 meters (6.5 feet) below the original road surface, or about 0.91 meters below the mass excavation level. At the bottom of the excavation pits, three pairs of sensors were installed and are numbered 13, 6, and 2, respectively.

1st layer of Geotextile
The excavation pits were then backfilled with soils originally taken out, and were compacted according to AKDOT&PF general construction procedures. Sensor pairs 1, 5, 9, and 12 were then carefully installed and the first layer of wicking fabric was installed. Since water transportation in the wicking fabric is directional, care was taken to make sure the direction of the wicking fabric was along the transverse direction of the road section, so that water in the road structure was transported horizontally to the road shoulder.

Sensor Installation
Sensor 22 was installed at the location closest to the drainage ditch, and could be used as a representation of the saturated moisture content in the pavement structure in summer time. The sensor wires were protected with aluminum conduit. A Campbell Scientific CR1000 data logger was adopted to record the data. All of the data acquisition devices were organized into an ENC14/16-NC-NM weather-resistant enclosure which was installed on the tundra about 6.1 meters (20 feet) from the west edge of the road.

2nd layer of Geotextile

After the bottom of the excavation was graded, a pit was then excavated at about 5.44 meters (18 feet) from the center of the road to install sensor 20. The excavation pit was backfilled, and sensors 16, 19 and 22 installed, followed by a layer of the wicking fabric. The same procedures as those described above were followed. The excavated soils were used to backfill the excavation area with a thickness of approximately 0.30 meters (one foot). The backfill was then graded and compacted. After the soils were compacted, sensors 11, 15, and 18 were installed. A second layer of wicking fabric was installed and the excavated soils were brought back to backfill the excavation area with a thickness of approximately another one foot.

The backfill was then graded and compacted and then sensors 10, 14, and 17 were installed. After that, the rest of the soil was brought back to backfill the excavation to its original elevation. Approximately four feet of fabric was left exposed to the air on the east shoulder, the down-slope side of the embankment, where water will be allowed to drain out of the fabric and flow down the hill.

Data Acquisition
All of the wires for the sensors were protected using aluminum conduit to prevent damage from the traffic loads. The aluminum conduits were grouped together, buried in a small ditch in the transverse direction, and connected to a Campbell Scientific CR1000 datalogger. In addition to the temperature and moisture sensors inside the test section, an HMP45C Air Temperature/Relative Humidity weather station was installed to monitor the air temperature and relative humidity at the test site. The panel temperature of the datalogger was monitored by the CR1000. All of the data acquisition devices were organized into an ENC14/16 -NC-NM weather-resistant enclosure, which was installed on the tundra on the west side of the road.