The Construction Challenges Of Keystone Hill

By Bruce Higgins | September 28, 2010

Improvements to the Keystone Hill segment of State Highway 145 — the sole route through the southwestern Colorado mountain resort community of Telluride — are on a winter hiatus now, but crews from Kiewit Western Co. will resume work this spring and complete the $11.4-million project next summer

Keystone Hill, some five miles northwest of Telluride, is a side-cut roadway on a steep incline, approximately three-quarters of a mile in length. The vast majority of traffic to and from Telluride — a resort community that attracts hundreds of thousands of visitors each year, in all seasons — funnels through this bottleneck, and the Colorado Department of Transportation developed the Keystone Hill Climbing Lane Project to add a climbing lane and 6-foot-wide shoulders to the existing narrow roadway. Kiewit Western began work on its contract on July 10, 2006, and, following the winter break, will complete the job by September 2007.

Though the physical length of the project is short, the monetary value of the work and the considerable time allotted for its completion are clear indicators that it is an unusual job. The time allowed for completion is largely due to the short mountain construction season making two work seasons necessary.

The construction is made particularly difficult by the steep mountainside into which the existing Highway 145 was cut. It is not possible in most places to simply add fill material to the outside of the roadway grade due to the steep drop-off angle of approximately 60 degrees into the valley below. An additional problem is brought about by the fact that the existing roadway curves sharply, following the mountainous terrain. These curves are being straightened for traffic safety. Finally, the absence of shoulders, combined with the curves, makes widening as well as straightening the roadway a necessity.

Excavating deeper into the mountainside equates to higher vertical wall exposure as the excavation goes farther into the face of the mountain, and in addition to substantial quantities of rock removal, could result in rockfall and landslide problems in the future. These problems above the roadway have been minimized by the use of other construction techniques, however. As the project was designed, the quantity of excavation is only 14,444 cubic yards, and the imported fill material totals just 20,434 cubic yards. The fact that both quantities are so small reveals that creativity has been maximized in the design.

The contour of the mountainside consists of four steep ridges or uphill cuts into the mountain, and five steep ravines. The ravines have drainage requirements that must be maintained during the construction as well as enlarged to accommodate the roadway widening. That enlargement or extension adds to the complexity of the construction, as most of the work involves widening in the ravines on the open or downhill side of the roadway. On two of the four steep ridges, retaining walls are being constructed against the mountainside slopes. And a third retaining wall will likely be added under a change order. The high cost of the project led to design changes to bring about cost reductions, but as the project advances, some of these changes are being reconsidered.

On the downhill side of the roadway, the first phase of construction involves underpinning the existing roadway, following the edge of the pavement with soil or ground nail anchors. This involves drilling 1,903 nearly horizontal shafts, each 6 inches in diameter. After a hole is drilled, an anchor bolt or epoxy-coated soil nail with spacers to assure centering within the grout, is placed in the shaft, and grout is pumped in to fill the hole around the nail. This horizontal drilling totals 48,750 linear feet in very limited working areas. Klemm, Tamrock and Atlas Copco track drills have been utilized for the work, with the Atlas Copco demonstrating for a single day.

A near vertical retaining wall is built along the existing roadway using shotcrete and double mat mesh reinforcing. Immediately after the shotcrete is placed the ground nail anchors receive plates and nuts in a wet-set operation to secure the retaining wall and confine the materials under the existing roadway once curing has taken place.

In two of the downhill segments requiring soil nails and shotcrete retaining walls, the drop-off is so extreme that the planned widening requires the use of a second level tier with soil nails supporting a lower retaining wall.

The next phase on the downhill side of the roadway is the installation of 341 vertical micropiles — 7-inch-diameter drilled shafts, reinforced and grout filled. These pilings total 17,050 linear feet and are being placed using the two Klemm track drills. Casings are used with the micropiling to provide a minimum of 15-foot free length to translate the load to the bonding zone, thus providing a deep foundation for the wall and widened roadway. On these piling foundations are placed structural concrete caps upon which the imported roadway fill material will be placed.

In conjunction with the placement of this fill material, the final retaining walls will be placed. These are Hilfiker mechanically stabilized embankment (MSE) walls in which horizontal friction mesh panels are placed atop each lift of compacted earth in the fill. The MSE walls will then be placed directly upon the structural concrete caps or upon soil where appropriate. Some 1,340 cubic yards of structural concrete is being placed. The actual Hilfiker retaining wall consists of rock-filled wire baskets held in place by the horizontal friction mesh panels. These walls enable the ravines to be filled with a system that is low risk for erosion and very stable in resisting settlement. The highest of the Hilfiker MSE walls is approximately 30 feet tall.

On the uphill mountainside work, the terrain requires some excavation. The three retaining walls above the roadway are shotcrete reinforced with mesh and utilize soil nails for support. Similar to the downhill walls, the ground nails are drilled at a slight downward angle into the mountain. The shotcrete walls are not vertical but rather sloped back toward the mountainside. Because crews can work from the existing roadway, this is significantly easier work than on the downhill side, but traffic is always a concern.

The project also includes removal and replacement of existing culverts with larger flow capacity reinforced concrete box culverts, removal of 2,370 linear feet of existing guardrail and installation of 3,450 linear feet of new guardrail, removal of 12,505 square yards of existing asphalt pavement and placement of 9,119 tons of new base course and asphaltic concrete roadway paving. With the new guardrail running the full length of the project on the open slope side, and the additional road capacity, the project will greatly increase safety for the traveling public.

Traffic control on the project is a real concern. Most construction projects have a detour route available, and while that route may add a little distance at reduced speeds, traffic can still keep moving. Such is not the case on this project, however, as the only alternate route adds no less than 80 miles and is simply not practical. When complete closures of the road have been necessary, traffic just has to wait for the road to be reopened, though whenever possible the closures have been confined to the 10 p.m. to 5 a.m. period. This has made a good public information program a necessity, and CDOT and the Kiewit Western staff have worked together to minimize the closures while keeping the public informed.

Key personnel on this important project include Ed Archuleta, resident engineer for CDOT; Joseph Colley, CDOT project engineer; David Valentinelli, CDOT assistant project engineer; Mike Monroe, superintendent for Kiewit; and Bill Howe, Kiewit project engineer. Strong team efforts project team coupled with good communications with the impacted communities are key to moving this challenging project forward with the least possible impact on tourist and commuter traffic through the area.

Author Information
About the author: Bruce Higgins has almost 39 years' experience in the construction industry, over 20 years as general manager of Tom Growney Equipment Inc. (John Deere Construction Equipment, Hitachi Construction and Mining Equipment, Bobcat, Dynapac, Sakai, and Broce Broom distributor) and over 18 years as manager/officer for two major contracting firms based in Albuquerque.