MEETING THE CHALLENGES OF CHARACTERIZING SUBSURFACE CONDITIONS IN BOULDERY TERRAIN

The scarcity of undeveloped land in heavily urbanized southern California is rendering challenging building sites commonplace. One such site is the Agua Caliente Cultural Museum, currently under development near Palm Springs, California. The site is located on a debris flow fan at the mouth of Andreas Canyon, which drains from the rugged eastern slope of the San Jacinto Mountains. The museum facilities will occupy a ~50-acre (20 hectare) parcel on the northern half of the fan and will include a two-level Museum building with a 59,000 feet² (5,484 meters²) footprint. Boulder-laden debris flow deposits, containing granitic megaliths up to 25 feet (7.6 m) in diameter and weighing up to 200 tons (180 metric tons), compose the fan. Material screened from a sewer excavation along the property line indicates roughly 50% of the substrate is >8 inch (20 cm) diameter. To aid in construction planning, we performed a statistical analysis of boulder sizes for the site using a high-resolution aerial photograph for boulders ³2 feet (0.6 m). Measurement of maximum and minimum block axes along 6 parallel transects spaced 250 feet (76 m) apart allowed us to calculate approximate boulder size and weight distributions for the site. This analysis yielded a bimodal distribution centered at block diameters of 5 and 10 feet (1.5 and 3 m). Within the limits of the Museum building, all boulders ³5 feet in maximum diameter were field mapped to verify these results and permit estimation of the boulder density in the proposed construction area. This exercise located 136 boulders within a 43,000 ft² (4,000 meters²) portion of the building footprint. Construction alternatives under evaluation include: over excavation of the native alluvium and replacement with an engineered fill pad; excavation of shallow footings into the native alluvium; construction of a deep foundation system (i.e. piles or caissons); or construction of a mat-type foundation system by linking the surface boulders with steel reinforcement and concrete slurry.