Abstract:
The Gold Rush Lot project, located in Breckenridge, Colorado in the Rocky Mountains, involved earth retention systems to support excavation for affordable housing for Breckenridge Resort employees while maintaining the current overflow parking. The project site is in steep mountainous terrain and presented challenging subsurface and drilling conditions. The project consisted of approximately 1,000 LF of wall length with a maximum cut height of 30 ft.
The geotechnical information available for design was limited to the lower portion of the site and did not include borings along the excavation to be retained. Due to the limited subsurface data, a pre-construction test program was performed by Schnabel in the fall of 2024 to evaluate ground conditions and feasible earth retention systems. The test program included excavation near the retained area, application of shotcrete, and installation of trial soil nails at four locations along the proposed retention alignment. The testing focused on evaluating the overall ground conditions, grout loss, and constructability of different systems.
Subsurface conditions varied significantly across the site. Portions of the slope consisted of clayey soils with rock fragments, where soil nail walls were feasible and successfully constructed. Other areas consisted of talus deposits with cobbles and large rocks and boulders. These materials did not stand-up during excavation, and excessive grout loss was experienced during the installation of test nails through this area. In these areas, cantilevered soldier pile walls were selected as the primary means and methods.
Drilling through the talus deposits proved particularly difficult and required the use of segmental casing to advance holes and maintain stability. The final retaining system consisted of a hybrid approach including soil nail walls, cantilevered soldier pile walls, and soldier beam walls with tiebacks.
This case history demonstrates the importance of pre-construction testing, flexibility in design, and adapting retaining systems to variable subsurface conditions encountered in mountainous terrain with difficult drilling conditions.