Engineering properties of a rock considered suitable for rock toe benches in highway embankments are different than the properties required for fill material. A method of evaluating toe-bench material is needed to assess various rock strata that are encountered during preliminary site investigations for highways so that easy distinctions can be made between rock preferred for toe benches, rock favored for use as fill material, and rock that is not suitable for either. Rock quality is assessed on the basis of absorption, density, slake durability, unconfined compressive strength, freeze-thaw durability, and L.A. abrasion loss. These are considered important properties in defining the integrity of the rock, and provide for a basis for systematic evaluation of rock material. A rating classification for evaluating rock material from Carboniferous strata of western Pennsylvania for use as toe-bench material is presented herein. A series of laboratory tests were performed on three sandstone and two limestone rock units for differentiation on the basis of strength, durability, and overall usefulness as a fill material. Samples were collected from five Mississippian-Pennsylvanian strata consisting of low to high durability rock. Typically, low durability rock units such as shales, claystones, and siltstones, etc., are neglected considering their infrequent use as durable rock fill.
Specifications for acceptance of rock toe material do not currently exist within state and federal construction manuals. Therefore, research was conducted to determine the commonly specified engineering property values for rock fill applications in highway construction. These engineering properties were then used to determine rational cutoff boundary values for acceptance of rock material for use within a rock toe structure.
Values for various properties tested range from 0.26-4.7% for absorption, 2.46-2.67 for specific gravity, 154-170 pounds per cubic foot (pcf) (2.47-2.72 Mg/m3) for bulk density , 0.7-11.6% for porosity, 10,300 to over 25,000 pounds per square inch (psi) (71-172 Mpa) for unconfined compressive strength, 97.3-99.6% for slake durability index, 21.2-45.9% for L.A. abrasion loss, and 1.5-21% for freeze-thaw loss. Bivariate statistical analysis showed a lack of significant correlations within the data set. Compressive strength showed the best correlation with bulk specific gravity (r2 = 0.61) and exhibited modest relationships with L.A. abrasion loss (r2 = 0.52) and with the value of L.A. abrasion divided by bulk specific gravity (r2 = 0.58).
Based on the test data produced, an evaluation of rock material is conducted on the five rock units studied. Each rock unit is categorized as passing, marginal, or failing based on the previously determined cutoff boundaries. Predicted values of engineering properties from bivariate correlation equations produced similar results in the evaluation, although based only on compressive strength as estimated by other index properties. The use of empirical equations is considered to be marginally useful as only values of unconfined compressive strength, L.A. abrasion, and bulk specific gravity could reasonably be determined in this manner. Test data proved to be useful in determining the relative, and to a lesser extent, overall quality and durability of rock material for use in rock toe structures, as defined by other uses of rock fill material in highway and embankment construction.