Modeling of the Effect of Critical Seepage Force on the Aquifer Soil Medium of a Borehole 1 Onyelowe, Kennedy C., 2 Okafor, Fidelis O. 1. Department of Civil Engineering, College of Engineering, Michael Okpara University of Agriculture, Umudike. P.m.b.7267, umuahia 440109, Abia State. e-mail: konyelowe@yahoo.com 2. Department Of Civil Engineering, Faculty Of Engineering, University Of Nigeria, Nsukka, Enugu State e-mail: fidelis.okafor@unn.edu.ng 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 the beneficial factor to the operation of water boreholes; seepage force could get to a considerable value that it becomes unsafe for the well operation thereby causing dislodgement of sand particles and sandstones from the wall of the borehole and the flow paths to the extent that sandstones experience boiling. The scouring of the particles collected at the wall of the transport pipe could damage the installations which is a huge financial loss to the owners of these facilities. Moreover, when soil particles flow, it makes the yield a poor one hence this research works to investigate the contribution of seepage force to the failure of boreholes. A mathematical/laboratory model was used and an expression for calculating the critical hydraulic head causing critical seepage deduced as ℎ () =0.000524 2 [ 5 (5 − 2  2 )]. Tables 2 and 3 and Figures 4, 5 and 6 have shown that there is strong agreement between the mathematical model and the laboratory check with closest agreement at the flow distance of 1.8m flow distance and a correlation analysis has shown a perfect correlation of 1.00975. It was also established that the well pump of 760watts power could be operated safely at 220volts beyond which the hydraulic head get more critical. 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. KEYWORDS: modelling, seepage force, failure, borehole, groundwater. I NTRODUCTI ON The present work is based on the study of the behavior and performance of water boreholes with respect to seepage force and it originated from borehole drilling engineering both for water and oil as fluids with different viscosities. A water borehole is a shaft usually vertical, excavated in the earth for bringing ground water to the surface for use [1]. An estimated 91% of total fresh water available to humanity is found as ground water which occurs in aquifers [1]. During water exploitation, few or several years after the drilling of a borehole, sand particles and small sandstone particles start to break away from the borehole surface. The number of particles which are transported by the fluid (water) - 1617 -