Through the numerical modeling of large-scale landslides, a drainage well with horizontal drains was configured with a different elevation, length, spacing and location, and steady/transient seepage analyses were performed to determine the effects on drainage efficiency and slope stability. The parametric studies indicated that the
lower is the installation elevation of horizontal drains or drainage well, the more efficient is the drainage and the more stable is the slope. The drain length, Ld, must be sufficient to pass through the potential sliding surface, PSS, to obtain economical and effective subsurface drainage and slope stabilization. Under identical conditions, the effect of the drain length, Ld, on the maximum groundwater drawdown, hwmax, is much greater than that of the drain spacing, S. Consequently,to improve the groundwater drawdown and drainage efficiency, it is more economical and effective to increase the drain length than to decrease the drain spacing (or increase the number of horizontal drains). Generally, the hwmax and discharge rate, Q, of subsurface drainage is similar in parallel and fan-array drains.
Key Words: drainage well, horizontal drain, subsurface drainage, groundwater drawdown, factor of safety.