Background
Leeds College of Art (LCA) is located close to Leeds City Centre, opposite the main entrance to Leeds University. The 0.27 hectare constrained site comprised a car park for LCA which had been constructed in 1985. Planning permission was granted for the construction of a new teaching block.
Phase I Geo-environmental work identified that a former school had been present on site, built in 1908 and demolished by 1978. A search of the West Yorkshire Archive Service found architectural records which showed the former school to have had two layers of below ground basements, the lower of which included a swimming pool, a boiler room and heating ducts. Ground investigation work, established that the two layers of basements were still relatively intact but had been infilled in places with demolition rubble but leaving large voids present and a foundational slab at 8-9m below ground.
WYG designed the proposed building to have a ground bearing floor slab, to which the voids would have posed a significant constraint, and also piled foundations, the installation of which would have been inhibited by the existing basement structure. Therefore, enabling works were required to remove the sub-surface structures and the uncompacted and voided made ground and to backfill to an engineering specification to provide a suitable founding stratum for the floor slab.
There were several technical issues which needed to be addressed:
• The heating ducts contained pipework that were lagged with asbestos. Removing this material is licensed asbestos removal works under the Control of Asbestos Regulations 2012. There were also fragments of asbestos containing material (ACMs) in the demolition rubble.
• Remediation of licensed asbestos in soils and made ground is a specialist area and only a small number of suitable remediation contractors hold a license as granted by the Health and Safety Executive. WYG led the procurement of the works and Sanctus was awarded the Contract.
• The excavation of several tonnes of highly degraded asbestos was required in a city centre environment, adjacent to housing, offices, and a main road.
• The basements extended off-site, in close proximity to the offices.
• A 8-9m deep excavation was required in immediate proximity to one of the main roads into Leeds City Centre, separated only by the pedestrian footway. Undermining the integrity of this road, used by HGVs, could have caused major network disruption. Substantial, large temporary works with trench boxes were utilised by Sanctus such that the excavation was completed safely.
• The three-storey college building was located immediately north of the site with gas and water mains crossing the site which needed capping and diversion.
• To the east of the site there was a 4-5m retaining wall which was leaning into the site, beyond which there were three rows of terraced houses with basements. The 8-9m deep excavation was required to close proximity to the leaning retaining wall.
• The project programme was extremely tight and of paramount importance to the client, meaning the piling / building contractor took possession of the site immediately upon vacation by the enabling works contractor meaning the validation report (a CDM handover document) needed to be provided in final format within a week of completion of site works.
Innovative Thinking & Adaptation
Traditionally works may have excavated the former basements, taken the material off-site and imported geotechnically and geo-environmentally suitable material to site for backfilling. That technique was not suitable at this site due to the costs of disposing of large quantities of asbestos impacted hazardous material.
Therefore, almost all material used for backfilling comprised site won material generated from the in-situ mix of historic building materials and made ground.
ACM’s were handpicked from site won material, or physically removed from the sub-surface building structure in compliance with CAR2012. This also minimised the requirement for disposal of soils as hazardous waste and the cost of importing material, both providing a major cost saving to the client. It also reduced the carbon footprint of the scheme and was a sustainable approach.
The presence of asbestos is still alarmist to many in the industry and particularly to the public. Undertaking a risk based approach to the asbestos contamination, considering CIRIA C733, rather than risk adverse measures which were unjustified, is therefore considered a relatively innovative approach. Remediation targets were defined and then re-negotiated with the Council during the works using a risk based approach, whilst still considering the perception of third parties.
The remediation strategy that was agreed with the regulator incorporated two different standards for the quantity of asbestos that was acceptable to be in the backfilled soils.
The asbestos gravimetric laboratory test was split between the Stage 2 visual inspection stage (for ACM’s and fibre bundles) and the Stage 3 PCOM microscopy stage.
The material being placed below 1m was required to show a result below detection in the Stage 2 part of the test, whilst for the upper 1m of soils (and piling mat) a below detection result for both the Stage 2 and the more stringent Stage 3 test was required. When a sample failed on the presence of asbestos fibres, the remediation target was re-negotiated with the regulator to allow for the presence of fibres at depth, outside of the piling area.
Onsite screening and crushing of recovered material produced sufficient suitably graded recycled aggregate to construct the piling mat and upper 1m of material containing no microscopically detectable asbestos material.
Sustainable Re-Use of Materials
The material in the ground at the start of the project contained substantial quantities of degraded asbestos. At the end of the project, the material that went back in the ground had been so thoroughly processed, only 100m3 was recorded as containing any asbestos fibres at all. (Stage 2&3 gravimetric test results were obtained by the contractor to monitor their work and the subsequent risk to the piling contractor for all backfilled material). That material was placed at depth in the area where the ground bearing slab is to be located, so that it would not be piled through.
The contract was written such that the savings from minimising off site asbestos disposal were shared between LCA and Sanctus, which provided an added incentive for the contractor to recover it. This financial bonus was subsequently passed on by the Contractor to the site staff who so effectively handpicked the material out.
Critically, given the city centre location, the maximisation of re-use of material on site saved circa 1,850 lorry movements against a disposal and import option. There was no turning space for HGVs on site, which meant any HGVs would have needed to have stopped the traffic to enter or exit site; this could have caused major impact to the road network on what is already a very congested main road.
The measure at the end of the project was that:
• 8,035m3 of material was excavated and broken out, with
• 120m3 of tarmacadam taken off-site for re-use;
• 10m3 of excess oversized concrete taken off-site for re-use elsewhere;
• 70m3 of topsoil was taken off-site for re-use;
• 9.7 tonnes of asbestos contaminated waste (including asbestos impacted PPE / RPE) disposed of;
• Only 0.1% of soils were disposed of at landfill (10m3), because (despite no visual ACMs and the lab sample recording asbestos below detection), the material had been in direct contact with degraded lagging, so it was considered safer to dispose of it.
• Therefore (excluding asbestos waste) 97.4% of material was re-used on site and 99.9% re-used / recycled in total.
Variations to the Geotechnical Specification
In order to facilitate material re-use and the increase the speed of backfilling, the specification for the engineered fill was varied on several occasions during the works.
The areas for the allowable re-use of different classes of material were relaxed in order to maximise material reuse whilst still meeting the engineered fill requirements.
The plate loading requirements were also relaxed once the performance of the material and the suitability of the contractors methodology had been demonstrated to avoid the need for Sanctus to frequently stop works to wait for testing to be carried out.
A key reason why the asbestos from the site was collected and not dispersed into the soils is because of the standard of the site works. ACM’s could be handpicked out of the excavated material prior to screening and crushing but it was the licensed asbestos removal from the ducts that was key to the success of the works.
There were large ducts approximately 3m below ground surrounding the swimming pool area and these contained lagged asbestos pipes which had not been stripped prior to demolition of the former building. The demolition predated all licensing regulations and asbestos at that time was still being used.
Sanctus undertook integrity surveys of the ducts during each phase where ACM was identified. Despite being over 100 years old, it was concluded that entry could be achieved providing a safe system of work was followed.
The contractor removed the soils from above the ducts which would have been imposing a load, broke out the top of the slab as so not to create a spread of asbestos and then entered the ducts, where safe to do so, from a lowered surface to remove the lagged pipework using wrap and cut methods.
The method was trialled using a small section of pipework with control measures in place to prove that fibre release was minimised, confirmed with Near Source air monitoring by an independent UKAS accredited contractor.
The pipework was wrapped in multiple layers of polythene sheeting, sealed and then removed in sections (without cutting as this would have mobilised fibres). 110v equipment was used only to cut the clean fixings to the wall.
The pipework was then placed into a sealed skip and taken off-site for specialist disposal. Approximately three sections of a total of 75m of ducting was treated this way, with suitably trained Sanctus staff implementing all the required control measures. As a result, fibre levels were not detected above the Clearance Indicator.
Re-use of Excavated Materials Under the CL:AIRE Definition of Waste
A CL:AIRE materials management plan was written, checked by a Qualified Person, and a declaration sent to the Environment Agency in accordance with the scheme.
The re-use of site won material was an intrinsic part of the options appraisal, sustainability, cost assessment and remediation strategy.
All works were carried out under the fodASB5 notification to the Health and Safety Executive for licensed asbestos removal works in addition to the deployment of an Environmental Permit and the F10.
