The effect of filter jacket clogging on the performance of prefabricated vertical drains in soft soils

Prefabricated vertical drains (PVD) are used to accelerate drainage and consolidation of soft sediment. An effective functioning of these PVD depends on its capacity to drain water from the subsurface. Among other causes, the clogging of filter jackets is thought to be of a considerable negative influence on the functioning of PVD. Current regulations prescribe a minimum ratio between the filters aperture size and the grain size distribution of the soil to prevent this clogging. Objective: The objective of this thesis was to evaluate the effect of filter clogging on the performance of PVD filter jackets, with regards to the soft soil conditions that are encountered in large parts of the Netherlands. Method: A literature review is performed on the principle of vertical drainage, soil types, filter types, clogging criteria, processes and testing methods. An experimental program was undertaken in the laboratory in which filter jackets were exposed to two different clogging phenomena, i.e. particle clogging and chemical clogging. Also filters from the field were evaluated. All filters were evaluated on their loss of permittivity with constant head measurements on filter pieces of three different filter types, the D165 and HS5417 from the Typar line of DuPont and the 30195 of Freudenberg. Particle clogging was evaluated by two different setups, the Oedometer and a long term filtration setup. Within the Oedometer tests the load and the moisture content of the clay sample varied. Chemical testing was done briefly, to see till what extend iron precipitation could induce filter clogging and if one filter would be more prone to clog than the other. Results: The permittivity tests on the filter pieces showed reproducible results. Repeated measurements on the same sample showed an average standard deviation of not larger than four percent of the samples mean. The initial permittivity also corresponded reasonable with the values provided by the filter manufacturers. With particle clogging, it was seldom observed that clogging ratios were higher than five. Relations between clogging and the applied load on the clay sample could not be found. Increasing moisture content showed a slightly higher clogging ratio. Another clogging indicator was found from work by Aydilek, initial permittivity. This is believed to be a better estimator; higher initial permittivity in general gave lower clogging ratios. In the chemical experiment it was observed that on the filter of Freudenberg large amounts of iron oxide precipitated. During filtration the filter seemed fully clogged. Permittivity tests on this filter showed an approximately four times larger clogging ratio, than that of the other ones. Conclusions: Based on literature and observations from both experiments and field data, it can be concluded that the permittivity reduction of filter jackets is always less than a factor of ten. Hence filter clogging is not a real issue, since the permeability of the soft sediments is several orders of magnitude lower than that of the clogged filters. Hence, the system permeability is mainly determined by the soil behavior.Geo-EngineeringGeotechnologyCivil Engineering and Geoscience