A COMPARISON OF METHODS FOR DETERMINING THE EXPANSION POTENTIAL OF EXPANSIVE CLAY BEDROCK

Expansion of clay-rich red-beds within newly exposed bedrock represents a significant design problem for engineering geologists working within southern California. Currently, three methods for determining the expansion potential of clay-rich bedrock exist and include: 1) swell testing of undisturbed bedrock, 2) swell testings of mechanically disaggregated bedrock that have been remolded to near-field densities, 3) direct comparison of the in-situ density of both weathered and un-weathered bedrock. Significant variation between the three test methods is common, with the first two methods significantly underestimating potential expansion. While the third method is considered the most reliable, it is also greatly limited in application, since it requires a single bed that transitions from weathered to unweathered be located on a single site. Day (1999) provided various laboratory testing to demonstrate the degree of heave within various bedrock samples increased during multiple cycles of wetting and drying. For this study, samples were obtained within a known, expansive, red-bed within the Pliocene age Saugus Formation. Five undisturbed bedrock samples as well as five remolded bedrock samples were placed in a swell apparatus and loaded to varying normal loads. The samples were then subjected to 25 cycles of wetting and drying. During each cycle, each sample was inundated with water for a period of 24 hours, afterward the reservoir was drained and the sample allowed to air dry for a period of 96 hours. Samples of undisturbed bedrock increased from an initial swell of 8.7 percent to a maximum swell of 11.4 percent after 15 cycles. Samples of remolded bedrock increased from an initial swell of 9.3 percent to a maximum swell of 11.0 percent after 7 cycles. Direct comparison of in-situ densities indicate a maximum swell of 12.6 percent. For this study, samples tested for multiple cycles of wetting and drying yielded results within 90 and 87 percent, respectively, of the maximum potential swell.