Water Table Profiles in the Vicinity of Horizontal Drains

Abstract

One of the most common and effective means of slope stabilization is lowering the water level within a soil mass. Frequently, horizontal drains are installed for this purpose. Computer-aided slope stability analyses are then used to evaluate the increase in factor of safety produced by drain installation. Critical to these analyses is the location and shape of the water table surface above the drain field. However, evaluation of the water table surface is complicated by its complex corrugated shape, with troughs corresponding to drain locations and ridges at the midpoints between drains. The objective of this research was to accurately describe the water table surface within a drain field using easily measured field and laboratory parameters. To accomplish this, physical and computer modeling of the water table along and between drains was conducted. The results of these analyses were compared to an analytical solution of the water table profile between drains that was derived by modifying groundwater equations developed for agricultural engineering applications. Based on these comparisons, a method was developed to describe the water table surface using the analytical solution and an experimentally derived correction factor. The method was confirmed by comparisons to field data. As a result of this research, water table surface heights can be approximated along and between drains. Additionally, an average water table surface height may be calculated and used in stability analyses, allowing accurate substitution of two-dimensional analyses for more complex threedimensional situations.

Streambank dewatering for increased stability

“…Maintaining lower levels of pore water pressures by dewatering may accomplish both. Bank dewatering occurs naturally through the transpiration of vegetation (Simon and Collison, 2002) and artificially by installing horizontal drains (Rahardjo et al, 2003;Crenshaw and Santi, 2004) or by capillary siphon systems (D. Gray, written commun., 2006). Vegetation is an important contributor to bank stability because it increases the apparent cohesion of the soil through root reinforcement.…”

Field-Scale Measurement of Groundwater Profiles in a Drained Slope

“…In general, Crenshaw and Santi's (Crenshaw, 2003;Crenshaw and Santi, 2004) work showed the groundwater surface in a drained slope to be corrugated in shape, with lows along the drains and highs between them (Figure 1). In the upper part of the slope, along the back half of the drains, the groundwater surface diverges from the drains, rising above them (Figure 2).…”

“…Accounting for the remedial drainage effect on a three-dimensional (3-D) groundwater profile in a slope-stability model can make a difference in the safety factor by as much as 10 percent (Santi et al, 2003). A potential method exists for estimating the drainage effect on piezometric heads within remedially-engineered slopes for use in stability analyses (Crenshaw, 2003;Crenshaw and Santi, 2004), but was not previously verified with field-scale testing.…”

“…Crenshaw and Santi (Crenshaw, 2003;Crenshaw and Santi, 2004) used three laboratory-scale physical tests and computer modeling to analyze various recharge scenarios that could be expected to occur in the field based on the hydraulic conductivity of the soil composing a slide mass. The tests showed that the groundwater profile varied primarily according to average hydraulic conductivity.…”

Groundwater flow modeling for effective implementation of landslide stability enhancement measures

“…According to Crenshaw and Santi (2004), the horizontal drains are a common method to dewater the hill slopes as a measure to increase the slope stability. Figure 3 shows the locations of some of the LSEM at the study site.…”