Ian Corne, CPESC, M.Land.Arch., B.E.S., Nilex Erosion Control Specialist
2016 conference of the Canadian Geotechnical Society
The use of rolled geosynthetic woven and non-woven geotextiles in the construction of civil engineering structures is state of practice. Ranging from filtration to soil reinforcement, geosynthetic use in civil engineering is prevalent.Where filtration governs in horizontal roadway applications, nonwoven geosynthetics have appropriately replaced well-graded aggregate filters in horizontal applications subject to vertical pumping under dynamic loading conditions. Where filtration governs in sloped gradient conveyance structures not subject to pumping, well-graded aggregate filters are required to carry non-storm-event nuisance and/or persistent sub-structure flows as they occur below the conveyance storm event rock armoring.In hydraulic conveyance applications, civil and hydraulic designers have inappropriately adopted wide-spread specification of nonwoven geotextiles in lieu of well-graded aggregate filters.
Civil and hydraulic structures are typically designed to hydraulic stability in a particular storm event. Conveyance channels are sized and armoured to carry predictable hydrologic events. Outside of critical structure design, common conveyance structure design rarely considers non-storm event water flow as it occurs by smaller rain events, persistent source water or grade water / piping.
The author surmises that nuisance and/or persistent sub-structure flows running below the geotextile manifests as erosion below the structure and/or reduced civil structure shear capacity of saturated subgrades. The result is failed conveyance structures where failure is initiated in the first rainfall after the geotextile is placed and ultimately presents in five to ten years after continual structure weakening and erosion below the geotextile by sub-structure water flow; the conveyance armouring finally yields to the additional weight of water in a ‘storm event’. The author surmises failure is incorrectly attributed to the storm event flow when the true failure is the long-propagating erosion gully below the geotextile. The high tensile strength geotextile spans the resultant erosion gully and carries the weight of the rock above. In time, the additional weight of water running in the channel causes the geotextile to break, presenting as an apparent ‘catastrophic failure’ that has been long propagated.