Comparison of Methodologies for Establishing Design Properties of Horizontal Drainage in Soft Cohesive Soils

Estimating the rate of settlement for foundation soils treated with vertical drains requires an understanding of the horizontal drainage behavior of the soil, because the time of consolidation settlement may be critical to the overall construction schedule and sequencing. This paper provides a case study comparison of the results of methodologies associated with obtaining design parameters for horizontal drainage for use with vertical drain design, including backcalculation of field settlement data, cone penetrometer testing for pore pressure dissipation, and laboratory Rowe cell testing, by means of the soft, cohesive Lake Bonneville soil deposits in Salt Lake City, Utah. Each of these methodologies has an inherent set of strengths and limitations that should be considered when vertical drains are being designed or time of consolidation settlement is being estimated. Backcalculation of field performance data is effective in identifying true in situ settlement behavior but is not always feasible. Rowe cell testing tends to provide values that more closely correspond with those obtained from backcalculation but is not often performed. Testing for pore pressure dissipation is the most used technique, but it can provide drainage values much higher than the other two methodologies.