The rate of consolidation always slows down significantly toward the end of the consolidation period. The time between about 80 % and 95 % of primary consolidation can require as long time as that from start to 80 %, and the time from 95 % to, say, 98 % can take a very long time. It is not practical to design for a target completion level greater than an average degree of consolidation of 80 %. To reach even that level within a reasonable time requires a surcharge (“overload”) to be placed along with embankment. The surcharge is an extra embankment load (extra height) that is removed when the average degree of consolidation has reached the target level, usually about 80 % of the average degree of consolidation for the embankment plus surcharge. The magnitude of the surcharge load should be designed so that after removal, the consolidation of the remaining embankment is completed, resulting in more than a “100 % consolidation” for the embankment without the surcharge. The timing of the removal of the surcharge normally coincides with preparing the embankment for paving of the road bed.

The results of a vertical drain project must always be monitored. This means that the programme must include a carefully designed schedule of ground surface measurements of settlement as well as a good number of depth-anchors or similar gages to monitor the distribution of settlement. Piezometers to measure pore pressures are also required (the analysis of pore pressure records must be made with due respect to fact that the distance between a piezometer tip and a drain cannot be known with good accuracy). The time to remove the surcharge must depend on the measurement data. It is helpful to plot the settlement values a settlement versus log-time as the project proceeds. Such lin-log plots will show the settlement plot to appear in an approximately straight line on reaching 80+ % consolidation. Such plots should represent the development in individual soil layers, not just be from ground surface values.

A monitoring programme should include several stations measuring pore pressure distribution and settlement distribution with depth (not just settlement of the ground surface) and monitoring should commence very early in the project; immediately on preparing the ground and before placing the first lift of the fill.

It is absolutely necessary to realize that the purpose of the monitoring programme is not just to confirm that the project performed satisfactorily, that is, performed as intended and expected. The monitoring programme must be designed to respond to the key purpose of providing records for early discovery and analysis in case the project would not perform satisfactorily and, also, to provide data that will be sufficient for a comprehensive study of the conditions so that a remedial programme can be designed if needed (see also Section 4.4.10). To the same end, it is very worthwhile to include in the monitoring arrangement a station away from the drain area to monitor the performance for conditions of no drains but otherwise identical to those of the project. For a case history reporting an unsatisfactory performance of a wick drain project, see Fellenius and Nguyen 2013.

Figure 4.6 shows settlements measured in one point below a test embankment, where wick-drains were installed. The dotted lines indicate the reducing fill height due to the ongoing settlement. On removal of the surcharge (to half height), settlement essentially ceased (a small heave is expected to occur, provided full consolidation has had time to develop for the remaining embankment height)

On completion of the consolidation, the soil supporting the embankment is normally consolidated. This means that future settlement may occur due to small additional loading of the soil from, for example, a moderate raising of the elevation of the road bed or widening the embankment during future maintenance work, or, indeed, even from a load increase due to seasonal variation of the groundwater table (a lowering of the groundwater table will increase the effective stress and initiate—renew—the consolidation). For this reason, it is advisable that the project be designed so as to leave the soil underneath the final structure at a suitable level of preconsolidation stress. This means that the design of a vertical drain project should always incorporate a surcharge (i.e., an “overload”; an extra embankment load to be removed on completion of the consolidation).