Sept. 14, 2023

Six Zones Create Main Dam Structure

Chimney Hollow Reservoir’s main dam is a zoned asphalt core rockfill embankment dam (ACRD). Most embankment dams consist of many varying types of materials which are carefully placed in select locations within the dam to meet a specific function. The main dam includes six zones that are produced on site: 

Zone 1 – Hydraulic Asphaltic Concrete Core 
The primary water stop in the main dam is the core, located along the centerline of the main dam. An asphalt core has a lower permeability than a typical clay core by up to five orders of magnitude (10^-11 cm/s). To put it in perspective, it would take a molecule of water roughly 30 years to cross zone 1. Zone 1 at its widest is 3.3 feet and then narrows with embankment height to 2 feet due to decreasing pressures.  

Zone 2 – Transition Zone  
This zone acts as a confining layer to the asphalt and is placed at the same time as zone 1 with an asphalt core paver. Zone 2 is crushed, screened and washed material that is 3/8-inch to 2.5-inch and is on both sides of the asphalt core. 

Graphic of rockfill zones on the main dam

Zone 3 – Secondary Transition Zone 
This is another confining layer that prevents zones 1 and 2 from migrating into the coarse zone 4 material. Zone 3 is crushed and screened material that is less than 6 inches and is on both sides of the asphalt core. 

Zone 4 – Rockfill 
This zone provides the resistive mass of the dam and makes up most of the embankment material. Of the roughly 13 million cubic yards of material placed on the main dam, 11 million cubic yards are in zone 4. It is material that is less than 24 inches, on both sides of the asphalt core and is produced by drilling and blasting in the quarry and is hauled directly to the dam.  

Zone 5a – Fine Filter Blanket Sand 
This zone is placed over the entire downstream footprint of the main dam and is a washed sand that sits right on top of the foundation. It allows seepage water from the foundation to pass through but stops migration of foundation material.  

Zone 5b - Coarse Filter Banket
On top of zone 5a sits zone 5b, which is the same as zone 2 – a 3/8-inch to 2.5-inch washed material. It also allows water to move through but captures larger material from moving through.  

Zone 6 – Riprap 
On the upper 90 feet of the upstream portion of the main dam is a layer that protects the zone 4 rockfill from ice that may form on the surface as well as large waves that crash against the dam.   

Of the roughly 13 million cubic yards of material to be placed on the main dam, approximately 3.4 million cubic yards were placed as of as of Aug. 30.