Reduction in Wick Drain Effectiveness with Drain Spacing for Utah Silts and Clays

Although decreasing the spacing of vertical drains usually decreases the time for consolidation, previous field tests have shown that there is a “critical” drain spacing for which tighter spacing does not decrease the time for consolidation. This critical spacing is thought to be due to disturbance effects from installation of the drains. Installation disturbance is thought to depend on soil layering (uniform vs. layered) as well as mandrel and anchor geometry. Installation disturbance can be modeled using a smear zone or by using a back-calculated consolidation coefficient (Ch/Cv) ratio. To validate these design methods and assess the importance of soil layer and mandrel geometry on drain effectiveness, full-scale field tests were conducted at Mountain View Corridor in Lehi, Utah. Tests were performed for triangular drain spacings of 5.8, 5.0, 4.0 and 3.0 ft with both rebar or plate anchors. Test results showed that installation disturbance is less important for thick clay layers than for thin interbedded clay and silt layers. Disturbance was found to be a function of drain spacing divided by mandrel-anchor diameter. Results also showed that the rebar anchor caused about twice the disturbance of the plate anchor. Using the smear zone model, with a Ch/Cv ratio of 1.25 and ds of 3.07 times dm, the time rate of settlement was reasonably well predicted, while the back-calculated Ch/Cv ratio, with no smear zone, also predicted the time rate reasonably well.