Bulletin of the National Speleological Society - ISSN 0146-9517
Volume 28 Number 3: 111-118 - July 1966

A publication of the National Speleological Society

Initiation of Ground-Water Flow in Jointed Limestone
Stanley N. Davis

Abstract

Dense unfractured limestone generally has a permeability of less than 10 -12 cm² and will not transmit significant amounts of water under natural hydraulic gradients. In contrast, limestone fractured by faulting may have a permeabiliity of more than 10 -6 cm² which will allow a natural circulation of ground water with subsequent development of large solution openings. Many limstone caverns, however, are found in areas devoid of extensive faulting. These caverns are commonly located along joints that must have been originally only hairline fractures. If this is true, then a basic question presents itself. Are the original fractures wide enough to tranmit significant amounts of water? In-place measurements of joint widths in otherwise sold rock have not been reported in the literature, but general observations of exposed joints strongly suggest that little water can flow though newly formed joints. Measurements in Wool Hollow Cave, California, indicate that the rocks on opposite sides of some joints are in constant motion probably caused by periodic stresses such as those that are produced by tidal forces ad seasonal temperature changes together with nonperiodic stresses such as those that are produced by world-wide seismic events. Differential movement along joints will both inhibit cementation and will increase the hydraulic conductivity of the joints by wearing away minor irregularities along the joint surfaces. Movement along the joints will also cause a reciprocating motion of the water in the joints, and it may even produce a net movement of water in one direction. Thus ground-water pumping produced by the movement of rocks may be an important driving force that supplements the driving force of the regional hydraulic gradient. Although partly speculative, this theory is supported by observations of tide-induced water-level fluctuations in deep water wells.

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