Milliken Reservoir Cell 2 Slurry Wall

Odin Construction Solutions recently completed the construction of a soil-bentonite cutoff wall to a maximum depth of 48’. The project, located in Milliken, Colorado, was the second slurry wall installation for the Milliken Reservoir, providing cutoff of alluvial groundwater to allow the mining of sand and gravel and future below grade water storage. The project included installation of an approximately 7,700’ long soil-bentonite cutoff wall; future outlet pipe installation and backfill at depth; grading, including excavation, transport, and backfill; value engineering and installation of a dewatering cutoff wall for utility installation; and miscellaneous infrastructure and appurtenances improvements. The project entailed multiple active utility crossings in which extra precautions and care were required during slurry wall installation. The project team members worked closely with the various utility owners to locate all active/non-active lines to allow for inspection prior to slurry wall installation. Each utility crossing was protected during slurry wall installation in a manner acceptable by the respective owner.  

Prior to constructing the soil-bentonite cutoff wall, the cutoff wall alignment was established, and area cleared and graded to provide a stable working platform for the specialty Long Reach Excavators. Before mobilization to the project site, supplemental fines for mixing the trench backfill were staged along the alignment by a separate owner-managed contract. Two Long Reach Excavators, one PC-1250 and one PC-800 were utilized on two separate headings for slurry wall installation. The excavators were supported by a centralized slurry mixing pond and QC lab, and individual backfill mixing and placement crews for each heading. 

Throughout the latter half of the project, Odin utilized UAS technology to assist in the planning of yet to be performed scopes of work as well as for inspections of the site for confirmation of punchlist and closeout items. The nadir photos taken by the system were stitched together to help locate buried utilities and identify the need, or lack of need, for road crossings over the completed wall. The oblique photos captured during the closeout of the project helped ensure that all areas disturbed during the construction of the wall were left in an acceptable condition for the Client.

In addition to the base scoped cutoff wall, an approximately 1,100’-long dewatering cutoff wall was value engineered and installed to allow for the mass excavation and grading of a working platform 35’ below grade. This working platform was used to install twin 36” ductile iron pipes which will be used for water management of the reservoir post mining. The dewatering wall was installed with a large enough footprint to allow the future contractors to excavate and construct a wet well down to the pipe elevations without significant dewatering effort, which would be needed in the sandy soils immediately adjacent to the South Platte river. In order to facilitate a watertight seal between the piping, the slurry wall, and the bedrock, a clay plug and concrete encasement were chosen by the Engineer. The clay plug was excavated to a sufficient width to allow for tie-in with the cutoff wall, approximately 15’ wide, and of sufficient depth to create a solid watertight key between the clay and the bedrock, approximately 6’ deep. The concrete encasement was cast around the pipes through the clay plug, providing additional protection for the pipes during the tie-in and to provide an impermeable transition between the relatively irregular pipe trench and the clay plug. During the tie in with the permanent cutoff wall, staking was performed of the asbuilt clay plug after the excavation was backfilled, allowing for an accurate tie in along the center of the plug. 

After the excavation and backfill of the entirety of the slurry wall was performed, several crossings were constructed over the wall. These crossings were coordinated with the utility owners which would be using the roads to ensure they met their exact needs. The crossings were constructed utilizing uniaxial geogrid and geotextile to distribute the load of a fully loaded oil truck and were covered with a crushed rock and road base materials. 

The Milliken Reservoir slurry wall was successfully completed within the specified construction window, to the satisfaction of the United Water and Sanitation District, and most importantly without injury.

Project Details

Client
United Water and Sanitation District
Location
Platteville, Colorado
Start Date
August 2019
End Date
December 2019
Service
Cutoff Walls, Dam, and Levee Rehabilitation, Groundwater Seepage Barrier Walls and Passive Treatment
Performed As
Prime

Project Highlights

  • Construction of a 7,690 LF (approximate 313,000 SF) soil-bentonite cutoff wall by the slurry trenching technique
  • 7,690 LF consolidation cap
  • 35,000 CY pipe excavation
  • Dewatering Cutoff Wall Installation
  • Mass Excavation, grading, transport, and backfill 
  • 36” DIP Installation for future pump station installation
  • 13,940 man-hours
  • Crew size of 13 craft personnel and 4 managers
  • Personal protection level D