At 14,000 Feet, No Easy Answers

Remaking the Pikes Peak Summit House required a Strong Foundation, Creative Thinking

By William Hoffmann

Standing at 14,115 feet above sea level, Pikes Peak is one of the most visited mountains in North America. It is home to majestic views, a National Historic Landmark and more than 500,000 visitors per year. It is also home to some of the harshest building conditions I’ve experienced in my 40-plus years as an engineer.

The weather on Pikes Peak is cold, icy, windy and generally unforgiving. By October, temperatures can plummet to zero, with wind chills falling even lower. Snowstorms sweep in quickly, with wind gusts reaching up to 100 mph, and even up to nearly 195 mph in rare cases. During the winter, temperatures can drop to -40 degrees. Roof snow loads can approach 125 pounds per square foot vs. an average 30 pounds per square foot in nearby Colorado Springs, elevation 6,000 feet. Ground conditions on Zebulon’s namesake are bedrock and alpine permafrost, soil and rock that remains at or below freezing temperatures all year to depths of up to 200-plus feet, only warming above freezing in direct sunlight or due to external sources.

Despite these conditions, the peak will soon house a brand-new $56 million, 38,000-square-foot Pikes Peak Summit Complex. The three-building campus consists of a visitor’s center with dining and rooftop terraces, observation decks and interactive displays, and a high-altitude research and communications center. Because the building will rest on federal land, it must carry a LEED Silver certification and is being prepared to qualify for the Living Building Challenge, one of the world’s most rigorous sustainability programs.

Our task as geotechnical consultants to the owners – a group of six governmental agencies and private companies – was to build a foundation and structure that could withstand the unforgiving conditions and could be built within the mountain’s remarkably short building season – six months at best. Considerations also needed to be made for the altitude – tough on materials and people – and the location at the top of a mountainous road rife with hairpin turns too tight for large trucks to navigate.

Needless to say, every move had to be calculated and recalculated. Lead structural engineer Steve Horner, HCDA Engineering, Inc., summarized it best: “If we thought we found an easy answer on this project, we were wrong.”

Planning began in 2015. Our work, along with that of RTA Architects and HCDA Engineering, focused on solving two primary problems: (1) overcoming issues related to excavating and building in the permafrost layer, and (2) designing structural materials that could perform at the 14,000-foot altitude, but could be partially constructed at a lower altitude to minimize wear and tear on crews and equipment.

Permafrost

When we started this project, we knew what not to do. The existing visitor’s center is a one-story facility constructed in 1963 using a hybrid foundation composed of a reinforced mat with steel beam grid. Due to costs and difficulty in excavation during the initial construction, the initial recommendation of 6 feet of crushed rock fill was changed to 3 feet of crushed rock and 3 feet of foam. The reinforced mat was an attempt to prevent the freezing temperatures from reaching the foundation, but it didn’t work. Construction of the facility on top of the permafrost layer created a zone of warmth that melted portions of the permafrost. In the first 9 months, the building settled 9 inches. It has been problematic ever since.

Alpine permafrost is relatively rare in the lower 48 U.S. states. The physical characteristics and mechanical properties tend to be highly spatially variable and extremely sensitive to changes in temperature. To accurately plan, we relied on a series of subsurface investigations conducted by the US Army Corps of Engineers (USACE) in 2015 and 2016. The CTL team then completed a series of reports based on the USACE investigations and other historic data. The reports were all in agreement that the subsurface conditions consist of an active layer of weathered Pikes Peak Granite, approximately 6 feet in thickness, underlain by an ice-rich permafrost layer and finally Pikes Peak Granite bedrock, the surface of which is significantly fractured in some locations. The thickness of the permafrost layer was unknown.

To account for these conditions – and keeping in mind the failed foundation of the existing Summit House – we recommended that foundations for the new facility be shallow, reinforced concrete spread footings bearing on the granite bedrock. We recommended foundation walls be insulated precast concrete panels, and floors on the lowest level of the facility to be slab-on-grade. Below the floors, the permafrost layer is to be removed and replaced with 3-inch minus, crushed bedrock fill.

Another key consideration was adequate drainage, which is critically important to avoid freezing runoff water from contributing to frost heave of the facility. Foundations were designed to be surrounded by free-draining fill materials and exterior drains that facilitate movement of the water away from the facility.

Building Materials

Building materials are a significant issue at this altitude and in the permafrost. To account for all possible issues, we employed thermodynamicist Bob Pintner from Alaska-based R&M Consultants. Together, CTL’s materials experts and the Alaskan heat engineers devised building materials that could withstand both the cold and altitude and be built in a facility in Colorado Springs, yet maintain integrity at the high altitude.

To manage installation of foundation micropiles, we plan to preheat drill holes, reinforcing rod and grout mix constituents, and admixtures, to ensure proper curing and bonding before the grout freezes.

The team is pre-manufacturing the entire building shell in prefabrication shops in Colorado Springs, including precast concrete structures, foundational formwork for cast-in-place footings, precast exterior walls, rebar cages and components for a hydro radiant-heated flooring system.

Once designed, the size of the transport load needs to be very carefully calculated, as materials need to be delivered and/or hoisted up the steep and winding, limited weight-bearing capacity road. It takes hours to haul heavy loads just a few miles. In some cases, as with the track hoe and rock crusher, the equipment can’t be hauled. Recently we had to drive a steel track hoe up and down the mountain – at speeds barely reaching 3 mph. It took six hours to get down the mountain, in the middle of the night to avoid any car traffic.

It’s not just equipment that has a much lower productivity output on the peak. Construction crews can only work 6.5-hour shifts, and workers need to pass a physical exam that ensures they can handle an oxygen content nearly half that of Colorado Springs. While these requirements don’t affect a geotechnical plan, they absolutely impact the overall construction schedule. Currently, the completion date is scheduled for December 2020.

Construction Resumes

Construction began June 2018 and resumed again this May. To date, crews have blasted 10 to 25 feet below grade with excavation to bedrock, supporting the building below the layer of permafrost. In addition, crews placed 43 cubic yards of concrete footings for the foundation of the center in 2018.

The next phase of the project involves pouring the foundations for the Summit House and the adjoining U.S. Army Corps of Engineers High Altitude Research Laboratory. The team hopes to install the lower portions of the foundation walls this season, along with part of the main structure for the House.

Crews will also place 430 precast pieces of below-grade, load-bearing members, composite veneer sandwich panels comprising 8 inches of structural concrete, 8.5 inches of expanded polystyrene insulation and 3 inches of nonstructural concrete veneer, according to Jack Glavan, manager of Pikes Peak–America’s Mountain.

We are hopeful that Mother Nature cooperates, confident in our planning, but only sure of one thing: We won’t get any easy answers.

William Hoffmann is Senior Principal Engineer, Business Development, Southern Region for CTL|Thompson. The firm is a full-service geotechnical, structural, environmental and materials engineering firm. Established in 1971, the firm employs 255 technical and nontechnical employees and provides expertise in small and large-scale projects in all areas of construction. CTL|Thompson is headquartered in Denver.