BUILDING AMONG BOULDERS – A CASE STUDY

Challenging building sites are increasingly commonplace in southern California, where accessible open land is at a premium. Creative solutions are needed to address the complex geological and geotechnical challenges posed by these sites, to better blend them into the existing landscape, and to preserve cultural resources. One such site is the proposed location of the ~50-acre (20 hectare) Agua Caliente Cultural Museum, near Palm Springs, California. The site is located on a debris flow fan near the mouth of Andreas Canyon, which drains from the rugged eastern slope of the San Jacinto Mountains. Bouldery debris flow deposits, some containing granitic and metamorphic megaliths up to 25 feet (7.6 m) across and weighing up to 200 tons (180 metric tons), comprise the fan surface. The museum complex, to be constructed on the northern half of the fan, will consist of two principal buildings, an amphitheatre, and appurtenant parking and landscape areas. Proposed construction alternatives include incorporating boulders into the décor and foundation of the facilities, rather than moving or fragmenting them.

Debris flows resurface roughly 10% of the fan every 300 years. Individual flows exhibit thicknesses of about 1.5 to 5 feet (0.5 to 1.5 m), and volumes of around 3.75 million ft³ (10⁵ m³). Recent deposition has been concentrated on the southern half of the fan. Over 300 years have elapsed since the last event. Various debris flow mitigation methods were evaluated, and a levee configuration adopted to route flows through the active channel, around the proposed facilities.

To aid in construction planning, we performed a statistical analysis of boulder sizes using a high-resolution aerial photograph of the fan. Measurement of maximum and minimum block axes along 6 parallel transects, spaced 250 feet (76 m) apart, permitted calculation of approximate boulder size and weight distributions for the entire site. A bimodal block size distribution was identified, centered at diameters of about 5 and 10 feet (1.5 and 3 m).