Water Storage Reservoir Slurry Wall Construction

The purpose of the Delta Water Storage Reservoirs Project (Project) was to design and construct a soil-bentonite cutoff wall to control groundwater during sand and gravel mining and eventually to be used to create a sealed water storage reservoir meeting the Colorado’s State Engineers Office (SEO) requirements for lining of gravel pits referenced in the August 1999 State Engineer Guidelines for Lining Criteria for Gravel Pits (1999 SEO Guidelines).
 
The client selected Forgen as the design-builder for this project. Forgen worked with their selected design partner to assist in design with constructability reviews, cost evaluations and scheduling scenarios to optimize construction. Preconstruction services began in early 2020, while construction started in July 2020. Mining of the sand and gravel resource is expected to take up to ten years but requires the cutoff wall to facilitate much of the mining in the dry.

Forgen installed a soil-bentonite (SB) cutoff wall providing a sealed water storage facility near the confluence of the South Platte and Cache la Poudre Rivers in Weld County, Colorado. The cutoff wall was constructed in Cell-1 where the ultimate buildout of the project will include three storage cells. Forgen addressed only Cell-1 which was located on the northeastern portion of the site. The conceptual cell geometry provided gave a generalized concept to be used for final design. Forgen and the designer performed an evaluation of different cell geometries as part of the design phase. Forgen performed a constructability review of the final design proved by Deere and Ault Consultants.

Approximately 79% of the total perimeter length of the cutoff wall required excavations deeper than 85 feet below existing ground surface to a maximum depth of 115 feet due to an existing Palo Channel. The final design required a storage amount of 3,395 acre-feet and was achieved through construction of 650,000 square feet of slurry wall with a total length of 7,880 linear feet. The cutoff wall was installed using two headings with PC1250 long reach excavators and a crane equipped with clamshell. The cutoff wall is currently the deepest soil bentonite cutoff wall constructed in the South Platte River Basin utilizing conventional construction methods.

Our team was responsible for installing the work platform which was prepared by grading the existing perimeter of the site with on-site soils per the final design and specifications. The platform was designed and installed to provide a dry and level surface capable of supporting the equipment necessary to construct the cutoff wall. The final platform was approximately 95 feet wide with an 80-foot mixing area and an additional 15 feet support zone along the centerline of the trench.

The soil-bentonite cutoff wall was built by excavating a narrow trench, (min 3 feet wide), while pumping slurry into the trench and maintaining its level at or near the top of the trench during the excavation process to prevent collapse of the trench walls. The trench was keyed into underlying suitable bedrock formations as determined by the engineer. This trench was then backfilled with low permeability material that displaced the slurry and formed a permanent hydraulic barrier.

Forgen used two separate headings for installation of the cutoff wall to reduce the overall duration of the project. Excavation of the first heading began with a Komatsu PC-1250 Excavator equipped with a boom and stick capable of reaching depths to 80 ft. The second heading also used a Komatsu PC-1250 and a Liebherr HS 855 Crane with a Stein K610 clam attachment. The use of the clamshell allowed for deeper 115 feet+ excavations. The equipment had the necessary breakout strength to excavate the key into the competent bedrock formation or upon encountering refusal to meet the design depths in the profile. Deeper key excavations were anticipated due to the paleo channel to key into the underlying aquitard advancing through weathered and/or higher permeability bedrock.

The project met or exceeded all quality control standards set forth in the design including a hydraulic conductivity of less than 1 x10^-7 cm/sec. More importantly the project was completed safely and exceeded the SEO Design Standard for the project by having a leakage rate of 70% of the calculated allowance in the provisional leak test.

The lasting impact of this work will be significant for the area. The project will enhance water management and storage capacity, providing a reliable water supply for agricultural use, especially during dry periods. The cutoff wall, forming a critical component of the storage reservoir, ensures long-term stability and efficiency by preventing water leakage, thus maximizing water storage capacity. The final design required a storage amount of 3,395 acre-feet and was achieved through the construction of 650,000 square feet of slurry wall with a total length of 7,880-lf. The cutoff wall was installed using two headings with PC1250 long-reach excavators and a crane equipped with clamshell.

This improvement in water storage infrastructure will benefit local agriculture, supporting crop irrigation and contributing to the stability and growth of the local farming community. The reservoir will also help manage water resources more effectively, reducing the risk of water shortages and enhancing the overall sustainability of water usage in the region.