Main Dam

Construction of the Tallest Asphalt Core Dam in the U.S.

Dam Height as of April 8, 2024: 183.5 feet

The Chimney Hollow main dam on the north end of the reservoir will be a rockfill structure with a hydraulic asphaltic concrete (HAC) core approximately 350-feet tall. The main dam will contain approximately 10 million cubic yards of rockfill and 75,000 cubic yards of HAC. Rockfill and aggregates will be sourced from within the reservoir footprint and the HAC will be batched on site.   


The plinth is considered a key piece of the main dam, essentially tying the above-ground features of the dam to the grout curtain and rock foundation below. The plinth acts as a stable surface to conduct grouting as it is held tight against the rock by thousands of steel anchors to resist the upward pressure generated in the process. The plinth also allows builders to create a water-tight bond between the foundation and the asphalt core. 

Two core samples taken from the grouting operations.
Onsite geologists log the subsurface geologic conditions of every other grout hole for the entire length (up to 230 feet), which allows our engineer, Stantec, to confirm and/or update their understanding of the engineering characteristics of the subsurface.

Foundation Grouting  

A two-row grout curtain and blanket grouting are required for foundation preparation. The grouting consists of drilling holes as deep as 230-feet below the foundation of the dam and pumping Portland cement grout into the holes to seal any voids, such as cracks and fissures, in the bedrock to reduce seepage. This also helps to prevent consolidation of the foundation under the weight of the main dam.  

It includes nearly 2,000 holes (28 miles of drilling) filled with 8,400 cubic yards of grouting material made of 7,000 tons of Portland cement.  

Asphalt Core Rockfill Embankment  

An asphalt core dam was selected for Chimney Hollow because the valley does not provide enough fine-grained clay material in the reservoir footprint to construct a clay core dam, such as those in Carter Lake and Horsetooth Reservoir. Asphalt core dams use a special hydraulic asphalt mix, unlike brittle paving asphalt, to create a flexible, very low permeability core. This technology is proven effective, with more than 200 asphalt core dams constructed worldwide having decades of safe operations.  

The main dam at Chimney Hollow Reservoir will be the second asphalt core dam in the United States and by far the tallest. The asphalt core dam design recommendation was provided by Stantec. It was reviewed and approved by the Colorado State Engineer’s Office Division of Dam Safety and an independent Project Review Board.  

The first section of the asphalt core was placed on Oct. 15, 2022. 

At the base of the core where it connects to the plinth, it is 6.6 feet wide for about 18 inches, then it moves to 5 feet wide for another 18 inches. This ensures that there is a good bond on the plinth. From there the width of the asphalt core is 3.3 feet wide and transitions to 2 feet wide at the top.