PILING IMPLICATIONS FOR A NEW INDUSTRIAL DEVELOPMENT ON A CLOSED LANDFILL SITE By Michael Redfern

THE CASE STUDY It was proposed to redevelop a closed municipal landfill site in northern England for light industrial use. The site would comprise a number of small industrial units, as well as extensive hard paving areas for vehicle parking and loading. It was proposed to redevelop a closed municipal landfill site in northern England for light industrial use. The site would comprise a number of small industrial units, as well as extensive hard paving areas for vehicle parking and loading.

SOIL STRATIGRAPHY AND HYDROGEOLOGICAL SITUATION From the surface downwards: 1m of engineered clay liner to prevent further generation of leachate via rainwater 5m of domestic waste materials 7m of impermeable boulder clay Permo-Triassic sandstone beneath - a major aquifer Groundwater level 5 - 7m below base of clay Aquifer not used for local drinking water, but groundwater discharges into a river which supports a salmon fishery From the surface downwards: 1m of engineered clay liner to prevent further generation of leachate via rainwater 5m of domestic waste materials 7m of impermeable boulder clay Permo-Triassic sandstone beneath - a major aquifer Groundwater level 5 - 7m below base of clay Aquifer not used for local drinking water, but groundwater discharges into a river which supports a salmon fishery

EXISTING CONTAMINANTS AND CONTAMINATION SITUATION Monitoring undertaken for regulatory purposes, along with site investigation, confirmed the presence of typical landfill pollutants: solid waste, leachate and gas Pollutants were present in a leachable form No significant head of leachate at the base of the soil The groundwater in the underlying aquifer had not been affected by leachate; it was believed that attenuation of leachate within the clay prevented migration into the underlying aquifer. Monitoring undertaken for regulatory purposes, along with site investigation, confirmed the presence of typical landfill pollutants: solid waste, leachate and gas Pollutants were present in a leachable form No significant head of leachate at the base of the soil The groundwater in the underlying aquifer had not been affected by leachate; it was believed that attenuation of leachate within the clay prevented migration into the underlying aquifer.

POLLUTION HAZARDS ASSOCIATED WITH PILING THROUGH THE LANDFILL The following risks were considered most likely: –Breaching of the protective boulder clay by the piles, causing pollution of the aquifer –Disturbance and mobilisation of the perched leachate within the landfill, causing a greater pollution risk to water resources –Penetration of the clay cap, causing increased leachate generation through rainwater –Potential for the piles to allow migration of landfill gas into the surrounding atmosphere and nearby buildings The following risks were considered most likely: –Breaching of the protective boulder clay by the piles, causing pollution of the aquifer –Disturbance and mobilisation of the perched leachate within the landfill, causing a greater pollution risk to water resources –Penetration of the clay cap, causing increased leachate generation through rainwater –Potential for the piles to allow migration of landfill gas into the surrounding atmosphere and nearby buildings

DIFFERENT PILING TECHNIQUES AND THEIR APPLICABILITY 1) DISPLACEMENT PILING TECHNIQUES –Precast piles driven into the soil –Usually cause the soil to close up around pile, preventing the creation of contaminant pathways around the pile BUT: –Pile driving may cause cracking in the stiff clay layer due to upward expansion, which may create a pathway for contaminants into the aquifer –Possibility of disrupting protective clay covering via material heave - causes a pathway for landfill gas

–Protective clay layer may also crack due to upwards expansion - further pathway for landfill gas 2) NON-DISPLACEMENT PILING TECHNIQUES –Involve extraction of soil prior to pile installation –No soil disturbance, so no formation of contaminant pathways if the pile is placed or formed in direct contact with surrounding soil BUT: –Preformed holes through contaminated ground enable rapid flow of leachate through hole –Necessary to maintain a positive hydrostatic head to prevent water flowing into the hole, which normally involves adding water into the bore: this causes increased leachate disturbance and leaching of contaminants within the landfill

ALTERNATIVE TECHNOLOGIES 1) BORED DISPLACEMENT AUGER PILE –Uses a rotary boring method - no possibility of dragging contaminants into clay layer –Increases stress, causing soil to close up around pile - inhibits movement of contaminant BUT: –To facilitate boring, the base has to have a larger diameter than the drive tube, creating a pathway for contaminants in the peripheral zone of soil –Helical pile shape considerably lengthens seepage of contaminant along pile/soil interface –Possibility of a significant volume of contaminated liquid entering the clay layer

2) DRIVEN CAST-IN-PLACE PILES WITH CONCRETE CASING –Concrete forced by hydrostatic pressure to come into contact with surrounding soil when casing is removed, preventing contaminant seepage paths 3) CONTINUOUS FLIGHT AUGER PILES (non- displacement) –Rely on retention on auger flights to provide support to surrounding soil until auger is withdrawn and concrete is intruded –Intruded material must be placed under pressure at a rate consistent with that of the auger withdrawal to ensure the hole receives sufficient support –This may be difficult, especially near the ground surface

THE EVENTUAL STRATEGY Solutions based on end-bearing piles into sandstone and driven pre-cast piles were abandoned Continuous flight auger piles using temporary casing, terminating within the clay were adopted, as: –Auger use prevents leachate & gas migration –Auger provides support to the soil before concrete is added –Casing minimises leachate migration as concrete forced to come into contact with surrounding soil, preventing leachate and gas migrating –No penetration into sandstone aquifer, so no risk of contaminating the river and salmon fishery