STRESS -RELIEF MODEL, A RELIABLE PREDICTOR OF ENHANCE SECONDARY PERMEABILITY AND POROSITY

Various authors support the stress-relief hydrogeologic conceptual model as a reliable predictor of enhanced secondary permeability and porosity. Other than well-known sheeting joints in granitoid rocks, the significance of this topographically related zone of weathering is not fully appreciated by the geotechnical community. Igneous, metamorphic and indurated sedimentary rocks all exhibit a degree of enhanced near-surface joints and fractures. Early observations were made for TVA projects and later within Appalachian Basin coal-bearing rocks. Previously, we provided four lines of evidence in support of this concept for an artesian sandstone with shallow cover along the flank of a syncline in the Allegheny Plateau, Central PA. These include: reduction in hydraulic gradient beneath a valley eroded oblique to strike v beneath adjacent walls and uplands, larger valley well yields, presence of no-flow hydraulic boundaries reflected in time-drawdown data and asymmetric drawdown along valley axes.

A new well field completed in the same aquifer nearby showed potentiometric contours to be a subdued replica of topography and less affected by the regional synclinal dip. Four separate pumping tests produced elongate cones of depression each parallel to the valley trend (first order regional control). These were aligned with suspected wrench faults unlike the other well field where one valley and its elongate cones of depression truncated wrench faults (second order control).

Springs at higher elevations were ephemeral and more persistent nearer to flood plains. They showed variable seasonal flows and responses to pumping and contained less mineralized water compared to the artesian sandstone. Specific-capacity data, normalized for exposed saturated rock penetrated varied from a mean of 0.06 for uplands and valley walls and 0.07 gpm/ft/ft for valleys. Although water-bearing fractures tend to decrease with depth, significant yields were obtained to depths of 400 ft at fracture-trace sites (third order regional control).