FROM CONSTRUCTION WATER RUN-OFF
MARTIN SHERIDAN from SSI Environmental writes about managing and removing silt deposits from watercourses as a result of construction works.
High levels of silts, fines, clays and colloids can enter natural watercourses as a result of natural events such as heavy rainfall and run-off water, man-made actions such as land drainage and, more challengingly, during major earthworks construction phases such as road building, excavation and remediation of new and old buildings.
Once they have entered into the watercourse, they precipitate and cover the river bed with fine layers of silts and fines, gradually suffocating the life of the river bed. Colloids, on the other hand, can, under some circumstances, agglomerate on the gills of fish and result in fish mortality. In nature, these silts are gradually washed away by successive rainfall events. The challenges facing engineering construction works are made harder by the potential of localised, heavy loads of spill entering water courses, potentially at several locations, with high frequency.
- Silts are generally fine-grained soils that do not include clay minerals and tend to have larger particle sizes than clay (2-4 μm).
- Clays are fine-grained rock or soil material combined with one or more clay mineral (silicates) with traces of metal oxides and organic matter containing variable amounts of trapped water with particle size less than 2μm.
- Fines can be considered to be an overlap between clays and silts exhibiting similarities in particle size and physical properties.
- Colloids (colloidal suspensions) may be regarded as the overall mixture of smaller, non-soluble particle clays and fines (1-0.001μm) dispersed in water. The resultant hydrocolloid is a mix of hydrophilic (negative electrically charged) polymers dispersed in water that do not filter readily or settle and would take a very long time to settle, (days, weeks, months) unlike the larger silt and fine type particles which settle rapidly (seconds, minutes etc.). Therefore, removal of larger particlesized fines, clays and silts can be achieved by appropriate management and use of ‘passive’ systems such as Hy-Tex Silt Fence, sedimentation lagoons, silt bags and Sedimat. To mitigate colloidal suspensions from watercourses an ‘active’ water treatment process is required whereby negative electrically charged clays and fines are ‘charged neutralised’ to allow them to ‘fall-out’ of solution. Although this can be achieved by addition of liquid chemical coagulants, polymers and pH neutralising chemicals, their environmental impact along with health and safety requirements for handling of these chemicals, can be significant.
January/February 2015 CONSTRUCTION 55