Modeling the Effect of Contact and Seepage Forces at Equilibrium on the Failure of Water Borehole

Abstract

There have been records of failures and quicksand conditions in boreholes in recent times impeding the performance and operation of boreholes which may have resulted from various factors ranging from construction problems, drilling inaccuracies, fitting and installation problems, some chemical effects within the aquifer medium etc, but it has been ignored that a factor of great benefit to the operation of water boreholes; seepage force could get to a considerable which becomes unsafe for the well operation thereby causing dislodgement of sand particles and sandstones resulting boiling. This research work has investigated the contribution of contact force and seepage force to the failure of boreholes. This necessitated the use of combined finite-discrete element method to generate model expressions from contact and seepage forces considered to be the major forces contributing to the flow of fluid through soil mass and boiling or quicksand effect results when seepage force becomes more in effect under critical hydraulic gradient and / or critical hydraulic head. A mathematical/laboratory model was used and an expression for calculating the critical hydraulic head causing critical seepage deduced as = and the equilibrium model has deduced an expression for the safe hydraulic head during well pumping as =. These have been verified using a laboratory investigation; borehole prototype well failure test. It has been established that there is strong agreement between model result and the laboratory study result from the correlation analysis conducted which has shown correlations of 1.00975 and 0.989879999701 for the critical state condition and equilibrium state condition respectively. For purposes of future calculations, borehole performance monitoring and designs, the standard critical hydraulic head of the system from Table 3 and Fig.7 is 2.92E-8 which has the strongest agreement with 2.59E-8 of the laboratory study with a deviation of 3.3E-9. The deduced models can be used to design and monitor the performance of boreholes. For safe pumping and corresponding yield in the bore hole system, inter-granular force between granular particles should equal the seepage force and this is achieved by ensuring that the deduced model expression is used to determine the safe hydraulic head. Finally, irrespective of the fact that an increase in hydraulic head increases discharge, the system should be operated at a head safe for the performance of the well and as long as the model hydraulic head expression deduced is used under the above conditions, safe pumping can be achieved at any voltage between 150volts and 240volts.

http://dx.doi.org/10.4314/njt.v34i3.32