Slake durability and engineering properties of Durham Triassic Basin rock (North Carolina)

The sedimentary rock of the Triassic Basin has been problematic for engineers due to the wide range of behavioral responses following excavation. The underlying cause of excessive settlement and slope failures in regional highway embankments appears to be a rapid degradation of these geomaterials in the foundation. The means to adequately identify this material for durability is lacking, creating concern for engineers required to design with this rock.; Previous research on shale durability has examined the correlation between strength and degradation potential; however, relationships varied considerably depending upon the material and geologic location from which the samples were gathered. This study focused on classifying the weaker sedimentary rocks of the Durham Triassic Basin and providing a possible relationship between durability and the strength and plasticity of these materials. Unconfined compressive, split tensile and point load strengths were examined and correlated with the two-cycle slake durability index (I_d2) for various types of rock. Plasticity using Atterberg limits and the activity of the clay matrix (Skempton, 1953) were also compared to the I_d2.; The 63 specimens recovered from the Basin consisted of hard, resilient sandstone interlayered with partially durable and non-durable siltstone. Compressive strengths for ranged from 4 MPa for the weaker material to over 62 MPa for the stronger rock. I_d2 values varied from 6.3% to 98.9%, the entire range of durability. Correlations for slaking with split-tensile and point load strengths were too weak to use in predicting the degradation potential of these rocks. Although the plasticity indices ranged from only 1.0% to 7.9%, the liquid limits did provide a method for durability classification.; In addition, field investigations of elastic modulus were conducted using a rock dilatometer. Although similar pressuremeters have been successfully employed to determine in-situ strengths of hard rock, an analysis on soft materials and their connection to durability has not been reported. The modulus ratio (E/q_u) and durability, derived in the laboratory, were compared to in-situ results. An empirical relationship for the slaking potential of Durham Triassic Basin rocks was developed.