DYNAMIC DEEP COMPACTION TREATMENT OF COLLAPSIBLE SOILS IN WYOMING

Collapsible alluvial soils represent a significant hazard along Interstate 25 in northern Wyoming. Because of the need to treat collapsible soil layers greater than 20 ft thick, the Wyoming Dept. of Transportation (WYDOT) selected deep dynamic compaction (DDC) as the principal mitigation method on three large sections of interstate highway between Kaycee and Buffalo between 1989 and 1990 and in three large sections north of Casper between 1999 and 2005. The total treated area was over 850,000 sq. ft and involved over 156,000 drops of the tamping weight using a large capacity crane. The DDC work typically involved a 20 ton weight, 4 ft in diameter, with a drop height of 100 ft. Generally, primary drop points were spaced at 10 ft and 12 ft on centers transverse and parallel to the direction of traffic, respectively. Secondary drops were spaced at the center of four primary drop points. Five drops were typically made at each primary drop point and 2 at each secondary point. The number of drops per point was typically limited to 5 or 6 for the primary points and 2 to 3 for the secondary points. The applied energy per volume increased from about 60% of the standard Proctor test energy in the 1989-1990 work to about 95% for the second set of tests. These relatively high energy levels are common for treating collapsible soils. Inspectors monitored the number of drops and the crater depth for each drop point. The average crater depths were typically between 5 and 7 feet deep after treatment and the crater diameter typically increased to about 10 feet at the ground surface. Dynamic cone penetration tests were performed at several locations along each roadway section before and after treatment. Substantial increases in cone penetration resistance occurred when soil types consisted of silty sands (SM) or low-plasticity silts (ML or CL-ML) and the natural water content was relatively low. In these cases, the average penetration resistance increased from an average of 5 to 7 blows/ft to an average of 25 to 30 blows/ft. However, in cases where the soil profile contained layers of plastic clay (CH) with higher natural water contents, little improvement in penetration resistance was observed. Under these conditions, the clay soil appeared to absorb a significant percentage of the impact energy rather than transmitting it to the deeper layers.