Discontinuous flow of fracture water: a new technique of karst collapse prevention

A unique regime of groundwater flow in large limestone aquifers in China is presented in the context of variation of anisotropy of permeability tensor s with depth. The limestone aquifers are composed of an upper region where the permeability ellipsoid is upright or prolate and characterized by Kz>Kx and Kz>Ky, a transitional zone, and a lower zone whose permeability ellips oid is horizontal or oblate and characterized by Kz<Kx and Kz<Ky. The trans itional zone partitions the whole limestone into these two upper and lower parts. The principal permeability values Kx, Ky, and Kz in these permeabili ty tensors all decrease simultaneously and logarithmically with depth. When groundwater is discharged out of both the upper part and the lower part of the limestone aquifer by permanent mine drainage at a high yield, an initi al regional drawdown cone of groundwater is formed in the aquifer domain. T hus the karst collapse may occur catastrophically. However, a special disco ntinuous flow environment in this permeable continuum, which we refer to as "double water table flow", could appear in a large limestone aquifer if gr oundwater is further abstracted out of the lower part of the aquifer throug h mine drainage even at small yields. Under this condition - where a second water table appears beneath the upper high stage water table due to mine d rainage activity at the lower level of the aquifer - the previous regional drawdown cone disappears and the initial groundwater table reappears. Conse quently, the high stage of water table is re-established in the upper part of the aquifer, and the karst collapses come to a stop. During the process of tapping the lower aquifer zone, a local drawdown cone, separated from th e original groundwater table, gradually takes shape in the lower part of th e aquifer. This lower water table migrates downwards with depth as controll ed by mine drainage. In this case, the total yield of mine drainage does no t increase with depth and approaches a constant value. Although the flow ra te of groundwater is uninterrupted between these two parts of the aquifer, an unsaturated region of water cuts off the static pressure in the middle p ortion of the aquifer. Such an unusual phenomenon can be regarded as a spec ial discontinuous flow condition in a permeable continuum. "Double water ta ble flow" or DWTF is proposed as a new concept in applied hydrogeology and can also be developed into a new technique for karst collapse prevention. I t is completely different from some partial discontinuous appearances of gr oundwater pressure or separations of flow systems that result from a strati form aquifer combination on groundwater discharge. As the mine drainage cha nges from the upper part to the lower part in a large limestone aquifer, th e original continuous flow (a single water table flow) of fracture water ca n finally evolve into a discontinuous flow (a double water table flow). The occurrence mechanism of DWTF has been fully demonstrated by the hydraulic simulation of fracture water in an infiltration tank. The prerequisite to t he evolution of DWTF in a large limestone aquifer is that the permeability ellipsoid of the lower part of the limestone aquifer must be horizontal or oblate. The economic and environmental benefits of the discontinuous fractu re water flow are also briefly pointed out.