PILING IMPLICATIONS FOR A NEW INDUSTRIAL

DEVELOPMENT ON A CLOSED LANDFILL SITE

PILING IMPLICATIONS FOR A NEW INDUSTRIAL

DEVELOPMENT ON A CLOSED LANDFILL SITE

By Michael RedfernBy Michael Redfern

THE CASE STUDYTHE 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

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

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

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

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