INLAND OIL SPILL CASE STUDY

COMPLEX STRUCTURAL REMEDIATION AFTER A KEROSENE SPILL

Introduction

Dealing with inland spill incidents is often not as straightforward as you might think, with input required from numerous parties to ensure the safe completion of remediation works in compliance with a wide range of legislation. This case study highlights how complex inland spill incidents can be, and how when dealing with these types of incidents, you need to ensure you seek expert advice.

Oracle Environmental Experts Ltd were appointed to remediate a devastating heating oil (kerosene) spill that occurred in a 16th Century Grade II Listed propertyset in the West Sussex countryside. Internal kerosene impact was identified in the kitchen and hallway at the property requiring extensive excavation works. Additionally, the kerosene had migrated vertically and impacted two stone walls of the cellar located below the kitchen and dining room. A combination of detailed intrusive investigations, deep excavations and highly complex structural replacement were undertaken over the course of 4 months to restore residential amenity to the property.

Incident Summary

At the start of the summer in 2019, the homeowner noticed strong oil odours in her property, particularly within the kitchen. After contacting her local OFTEC registered engineer, an oil leak was discovered from a failure on the oil feed line where it entered the Aga in the kitchen. The homeowner then noticed that a large ‘damp’ stain had appeared on a wall in the hallway which emitted a strong oil

odour. Based on delivery and consumption records, it was estimated that over 1000 litres of kerosene had been lost as a result of the spill incident.

Initial Visit and Emergency Mitigation

Oracle Environmental Experts initially attended the property to assess the extent of kerosene impact and to install an air mover to reduce the elevated volatile organic compound (VOC) concentrations within the dwelling. The VOCs recorded in the ambient air of the property reached a maximum concentration of 51 parts per million (ppm) in the cellar prior to installation of an air mover. Lime plaster on the stained wall was removed to reveal a concrete block construction. It was evident from the VOCs released from the wall that it was saturated with kerosene, and the wall was subsequently covered and sealed with membrane to contain the vapours. Following OEE’s initial visit, it was clear that further intrusive investigation was required to delineate the extent of kerosene impact to the soils below the property and to the structures within the property.

Investigation

5 no. trial pits were progressed through the floor of the kitchen to establish the extent of kerosene impact and the nature of soils below. The first trial pit (TP-1) was progressed at the spill origin in the kitchen adjacent to the Aga. A corehole was initially progressed through a 0.1 m thick concrete floor slab and was screened for VOCs and revealed a concentration of 1104 ppm.

The concrete slab was underlain by a 0.05 m layer of screed, sand and brick fragments which returned a VOC concentration of 1419 ppm. The soils were then hand dug to 0.25 m below floor level (bfl) where a clay soil was encountered allowing a soil bore to be progressed through the base. At over 1 m bfl the soils were still found to be heavily contaminated with 1253 ppm recorded at this depth. A soil sample was recovered from a maximum depth of 2.5 m bfl where the VOCs were found to decrease to 444 ppm (Table 1).

A further 4 trial pits were progressed through the floor of the hallway. Much of the floor cover comprised of parquet flooring which when lifted, revealed a black adhesive layer that had been degraded by the kerosene. The cellar located below the kitchen and the dining room of the property was found to be constructed of four solid stone walls and a natural flagstone floor. Laterally drilled probeholes were progressed into the walls directly below the spill origin. A maximum VOC concentration of 472 ppm was recorded from a probehole through the stonework. From the extent of impact observed, it was evident that two of the stone walls in the cellar would need to be removed and replaced in their entirety.

Site Preparation

Owing to the Grade II listed status of the property, listed building consent was required to complete the remediation works. On 15 January 2020 listed building consent was granted to allow the remediation works to be undertaken. Given the extensive internal remediation works required and the age of the property, there was potential for asbestos to be present and a refurbishment survey was commissioned which identified the presence of asbestos containing materials in the work area. These were removed by a licensed contractor prior to anyremediation works commencing. The local

authority building control department were consulted and appropriate building notices placed to enable the works to proceed. Prior to intrusive works commencing, a surveyor was commissioned to provide a schedule of condition of the property.

Owing to the extensive nature of the works required, the homeowner moved into temporary alternative accommodation the duration of the works. The contents of the building were removed entirely including all furniture and personal belongings which were placed into temporary storage. All units and worktops in the kitchen and utility room were stripped out and disposed of.

The Aga was removed from the kitchen and the boiler removed from the cellar by an OFTEC registered engineer. This also included the removal of all associated pipework which was clipped to the cellar walls. Water pipes and electric cables were isolated where appropriate. The stained concrete block dividing wall between the kitchen and hallway was broken out over an approximate 2 m length and was removed from site for disposal. The wall was found to be non-load bearing but with no below ground blockwork supporting it.

Remediation Works

After establishing the extent of kerosenecontamination at the property, it was clear

that several challenges would be faced in order to complete the necessary remedial works safely. Factors including the requirement to undertake deep excavations in a property with minimal foundations and the removal of retaining structures, meant that the works would not be possible without detailed input from an experienced structural engineer.

OEE appointed a structural engineer to provide a detailed method statement specifying every step required to facilitate the scope of remedial works. OEE subsequently commenced remediation works at the property at the start of June 2020.

To carry out the deep excavations required, a wall in the hallway was underpinned with a mass concrete pad footing installed to support the wall. A series of acrow props were laced together and laterally braced under a timber beam in the kitchen. Steel beams were then laced together to allow the soils in the kitchen and hall to be excavated safely.

The concrete floor slab in the kitchen was broken out over an approximate 13 m2 area and the soils below were excavated to a maximum depth of 2.6 m bfl. The floor slab was broken out in the hallway over an approximate 6.26 m2 area and soils below were excavated to a depth of up to 1.4 m bfl. With the kitchen excavation at 1.35 m bfl, temporary works were constructed to support the cellar walls. This included 2 no. concrete pads installed into the kitchen and the cellar to allow 2 no. steel beams to be slotted through the cellar wall. The flagstones upon the cellar floor and clay drainage gullies were lifted and set aside for later reinstatement.

The soils below were excavated over an approximate 3.52 m2 area to a depth of 0.5 bfl. An approximate 2.2 m length of the stone cellar wall was broken out and replaced with new stone. Select stones considered suitable were individually broken on site to create a like for like appearance with the existing stonework. The method provided by the structural engineer included the construction of concrete block piers to support the cellar walls. A second stone wall was also removed over an approximate 1.5 m length. An approximate 0.5 m length of the oak sole plate exposed on top of the cellar wall located below the spill origin, was found to be impacted with kerosene and was subsequently replaced with clean oak by a skilled carpenter.

All excavations were backfilled with clean MOT Type 1 compacted in appropriate layers to minimize settlement. A hydrocarbon gas barrier membrane was installed across the excavations in the kitchen and hallway. A new concrete slab was cast in the kitchen and hallway. A new chimney surround for the Aga in the kitchen was rebuilt with clean bricks to match the existing specification. Through the remediation works, a total of 55 tonnes of kerosene contaminated soils and structures were removed from within the property for off-site disposal at a suitably licensed facility. The remedial works were completed by mid October 2020.

Reinstatement and Verification

On completion of the remediationworks, the site was handed over to the homeowner’s building contractor to complete final reinstatement works inside the property. This included a new kitchen and new carpets where required. All furnishings were replaced, and the property was redecorated in its entirety. The insured subsequently moved back into the property in February 2021.

A total of 10 no. verification soil samples were scheduled for independent laboratory analysis which returned TPH concentrations from <10mg/kg to 96 mg/ kg. The concentrations of the contaminants of concern in soils were compared to Generic Assessment Criteria to assess the potential risk to health from residual soil contamination. None of the concentrations of the contaminants of concern in soils exceeded the relevant assessment criteria. Ambient air samples were collected from the kitchen, dining room, upstairs landing and bedroom and were assessed against relevant air quality values. None of the concentrations of the contaminants of concern were reported at concentrations in excess of relevant air quality values. On completion of works a qualitative risk assessment confirmed that all of the originally identified risks had been reduced to an acceptable level.

Ultimately the investigation and remediation of the site was completed for approximately £140,000 plus VAT, but this figure excludes final reinstatement costs and any alternative accommodation costs.

Key points to remember:

Always complete a sufficiently detailed investigation to establish the full extent of kerosene impact on soils and building structures.

Ensure you have the appropriate listed buildings consents and permissions required for working on listed buildings.

Ensure you commission a condition survey of the building prior to intrusive works commencing.

Ensure you have the appropriate local authority building control notices and approvals.

Ensure any asbestos is identified at an early stage and removed by appropriately licensed contractors.

Seek advice from an appropriately qualified structural engineer where expert advice is required on removal and replacement of structures.

Ensure appropriate verification through the collection of sufficient laboratory analysed samples.

Author: Becky Wadley – Environmental Consultant – Oracle Environmental Experts Ltd

Correspondence email: bwadley@oracleenvironmental.com

Website: www.oracle-environmental.com