An Inside Look at Carving a Mountain

Crazy Horse Memorial seal. Click for the Home page

An inside look at carving a mountain

Drilling on Crazy Horse mountain below the carved face. Click for a larger view.

Spectacular view of a drill at work on Crazy Horse.

Carving Crazy Horse is a demanding challenge in explosives engineering. The members of the Crazy Horse mountain crew are experts at precision blasting and have an outstanding safety record.

When most people think of blasting rock, they imagine using dynamite. The primary explosive used on the Crazy Horse mountain carving, however, is detonating cord ("Det cord"), which is much more suitable to this kind of explosives engineering. Dynamite expends all of its energy in an area just a few inches in length - the length of the stick of dynamite. "Det cord" expends the same amount of energy, spread over as much as 70 feet.

This provides much better control and does much less damage to the rock that is not removed by the blast.

All blasts at Crazy Horse are designed, drilled and executed to protect the rock that is left after the blast. This is the opposite of most blasting operations, where the main concern is the final size and location of the material being blasted away. Crazy Horse carvers prepare their blasts using a system that explosives engineers call “pre-splitting.” It is similar to perforating a piece of paper to allow it to tear evenly. The rock to be removed is drilled on all sides with a series of parallel drill holes. "Det cord" then explodes throughout the entire length of each of the drill holes, cleanly removing the desired rock while leaving the remaining rock undamaged.

The distance to what will be the finished surface of the mountain carving (distance from "grade") dictates the distance between drill holes and the amount of explosive per foot of "det cord" used.

Each drill hole is loaded with a single strand of the appropriate size of "det cord." After the "det cord" is in place, all holes are filled with small crushed stone. The stone fills the open space in the hole and better focuses the explosive energy to split the rock.

For horizontal holes, silica sand is blown into the hole with special equipment. The thickness of the rock to be removed can be up to 7 or 8 ft with the water gel, or as little as 12 - 18 inches with 50 grain cord.

A closeup of detonating cord. Click for a larger view.

Closeup of Det cord connecting adjacent drill holes.

Blasts are detonated by a sophisticated electronic system that sends a digital pulse to a computer chip embedded in the explosive blasting cap. This makes it almost impossible for an accidental firing of the explosives. The electronic system is also able to trigger the explosives at different times in different drill holes with very high precision.

The equipment used by Crazy Horse Memorial can provide accuracy to within one millisecond. This is important because on most blasts, the explosives in adjoining drill holes, or groups of holes, are deliberately fired a fraction of a second apart, to reduce vibrations in the rock left behind. Delay patterns are calculated to keep vibration below three inches per second at grade, which is the rock that will be the finished surface of the mountain carving. This is extremely low compared to most types of blasting. It is accomplished by keeping the total pounds of explosive fired per delay very low (often under 0.5 lb/delay). Seventeen millisecond delays are typically used between each section of the blast, each of which might include four or five individual drill holes.

Most of our holes are drilled up to 35 feet deep with machine drills, using 1 7/8 inch steel bits. We do drill by hand 1 3/8 inch or 1 5/8 inch holes up to 21 feet deep when the track drills cannot be maneuvered to the drilling site.

Workers on the Crazy Horse mountain carving. Click for a larger view.

The mountain crew often works under dangerous conditions. Safety is a major consideration.

In 1998, after the face of Crazy Horse was dedicated, the mountain carvers switched gears from highly detailed finish work to blocking out the horse’s head, which requires moving much more rock at a time.

When the crew returns to finishing work, which will be on the horse’s head, it will be accomplished by drilling to isolate small blocks of rock (100 to 1,000 pound chunks). These blocks may be blasted off using very light explosives.

In very detailed areas of the carving, drill holes will be located so close together that they almost contact each other and mechanical wedges will be used as an alternative to explosives.

A jet finishing torch is used for the final finishing of the surface of the mountain carving. The jet removes drill marks and smoothes the final surface. The torch runs on diesel fuel and compressed air. The 3,300-degree (F) jet flame causes tiny fragments of rock to flake off as the result of heat expansion, leaving a polished surface. The variety of minerals in the pegmatite granite makes torching a challenge because each mineral reacts to the heat differently.

Crazy Horse Memorial has received many generous gifts of construction equipment and materials over the years. Such equipment has served the Memorial well, but the operation faces a constant challenge of replacing aging equipment. The mountain crew still uses some donated equipment that is 30 or more years old. In the past few years, for example, equipment gifts included the following:

Caterpillar Company gave Crazy Horse a D6 track-type tractor, commonly known as a bulldozer,
A much-needed excavator was donated by Case Credit and Case Construction Equipment, divisions of Case New Holland, Inc.,
A track drill was given by Atlas Copco CMT US, and,
The Bobcat Company gave a skid-loader to Crazy Horse Memorial.
Altas Copco Compressors, Inc., donated an XAS96 air compressor.

Picture of equipment recently contributed to Crazy Horse. Click for a larger view.

Recent gifts of equipment to Crazy Horse Memorial.

The primary source of funding for the work of carving Crazy Horse comes from visitor admission fees. Generous contributions and donations of equipment from corporations provide critical additional resources to make continued work on the mountain carving possible.