3.6.2 Lowering a Water Table
The level of a static water table can be lowered to an extent that the structural elements of roads are not affected by either free water or capillary rise using two basic techniques. This is achieved by the use of formation drains parallel to the formation, and blanket drains beneath the pavement or formation. A combination of the two is sometimes employed. Some techniques for lowering a water table are illustrated in Figure 3.6.
Source: VicRoads (2004a)
Section 5.7 of the Guide to Roads Design – Part 5: Drainage Design (Austroads 2008b) details methods to design drain spacings to lower the water table. Alternatively, an empirical approach may be used based on a field trial either before or during the early construction phase of a project. Trenches can be dug, and flows and water table levels can then be measured. In cases where the material is relatively free-draining, pertinent observations may present quite quickly, but long periods of time might be required for slow draining materials. If an intuitively expected observation is not readily obtained the trenches can be deepened or more trenches dug at closer spacing.
Whilst this practical technique may provide more reliable answers for the conditions of the field test, it should be recognised that the worst condition may not have been encountered in the trial.