Sanctus were contracted to carry out an enabling works package for the new Addington Valley Academy located at the former Timebridge Centre on Field Way, Croydon. The community centre that had previously occupied the site had reached the end of its operational life and the local authority had taken the opportunity to replace the old 1950’s style building with a new modern Free School for children with unique educational needs.
Sanctus mobilised to site prior to the completion of the demolition works, in order to carry out vegetation clearance. This included the removal of a series of semi-mature trees and a hedge row that had become grossly overgrown and a focus for fly tipping activities. The vegetation was removed from site and diverted from landfill to a suitably licensed repurposing facility for shredding and composting.
Following the completion of the vegetation clearance the topsoil across the site was stripped using a 21t excavator with a toothless bucket to ensure only the topsoil was removed and none of the underlying subsoil were disturbed. The topsoil was then placed into two separate stockpiles designed for intermediate and long-term reuse respectively. Stockpiling activities were undertaken in accordance with the DEFRA guidance ‘ Construction Code of Practice for the Sustainable Use of Soils on Construction Sites’ and BS3882:2015 Specification for Topsoil to preserve the soils chemical and physical structure essential for enabling its successful reuse as a topsoil. In total some 2,000m3 of topsoil was successfully stripped and stockpiled ready for reuse, preserving a valuable asset and avoiding additional disposal / import costs for the client and environment.
The stockpile for intermediate reuse, following formation of the new playing fields, was covered to prevent the production of nuisance dust and protect against infiltration of rain as the soils were due to be reused within the coming weeks. The second stockpile, which was to be used in the areas of soft landscaping following completion of the construction some 18-24 months later, required a more long-term management solution to prevent its degradation. Sanctus therefore opted to hydroseed the stockpile which comprised the application of a mix of grass seed, mulch, nutrients, and water across the entire surface of the stockpile to create a rapidly growing deep rooting ryegrass which would stabilise the material while preventing the production of dust and reducing the ingress of rainfall.
Facilitating the Reuse of Soils on Site
In order to facilitate the reuse of soils on site Sanctus declared a Material Management Plan (MMP) to CL:AIRE. Using this approach meant that all soils proven to pose no risk to the sites’ end users could be reused across the site instead of being classified as waste and requiring offsite disposal. This represented a significant cost saving through the reduction in disposal and import costs which would have otherwise been required to achieve the design levels of the new development.
As this approach required all soils to undergo verification sampling Sanctus carried out a trial pitting exercise across all areas where the reduced dig was to be carried out prior to excavation commencing and recovered samples from each of the materials present as per MMP and Remediation Strategy. This meant that the soils were verified as suitable for reuse before they had been excavated meaning that where necessary, the soils could be excavated and placed in their new locations directly thereby reducing the need for multiple material movements.
Reduced Level Dig
Once the demolition of the former Timebridge Centre had been completed and the demolition contractors fully demobilised, the main works could proceed. This comprised the excavation of soils to achieve the design levels of the new development, requiring the excavation and movement of some 4,450m3 of Made Ground and natural soils . These were excavated using a combination of 21t and 30t excavators using 12t front tipping dumpers to transport the materials via the designated vehicular transit routes installed to keep plant and pedestrians separate.
Any previously unidentified materials or materials demonstrating visual or olfactory evidence of contamination encountered during the excavation works were placed into separate stockpiles for additional testing. Soils that had been found to be unsuitable for reuse by the sampling programme were segregated, clearly signed and removed from site to a suitable disposal facility. Where unsuitable soils were identified and excavated, Sanctus validated the successful removal of these soils by carrying out chemical analysis on samples from the sides and base of the resulting excavation and removing any additional material until the samples had confirmed the suitability for reuse.
Previous site investigations had identified a number of locations where Chrysotile asbestos was identified as visible fragments of cement sheeting. In order to minimise the risk to the sites end users as well as to remove any potential legacy contamination Sanctus excavated these hotspots and removed the impacted materials from site. Prior to disturbing the impacted soils Sanctus produced and submitted to the HSE a Plan of Work and Discovery Strategy in accordance with the 3-year unconditional license granted by the HSE to work with asbestos and mobilised their industry leading asbestos controls to site to ensure that the removal of the soils could be done safely. These included a Sanctus bespoke mobile decontamination unit to site along with a dust suppression system.
In order to provide a cost saving to the client, all soils that had been found to contain visible fragments were passed over a picking station where trained and experienced operatives removed the visible fragments by hand. This processing operation allowed the reclassification of the soils and with that achieved a significant cost saving for the client. Following the removal of each contamination hotspot the successful removal of the contaminated soils was demonstrated through the collection of validation samples and the completion of an as-built survey for inclusion within the verification report. Once confirmed free of unsuitable materials the resulting excavations were backfilled with site won soils to achieve the required design levels. These remedial works were able to remove any potential sources of risk to the sites end users as well eliminating any legacy issues for the site going forward.
Retaining Walls and Piling Mats
With the site having a steep change in levels to the north, as well as the new build being positioned lower than the surrounding landscape, the new development required significant retaining walls to ensure a stable building platform. The design engineer had concerns that the original design, due to be seated within the superficial deposits of the Thanet Sand Formation, would not be sufficient to support the weight of the platform over the design life of the development. As such it was proposed that the wall should be seated on the underlying chalk bedrock, unfortunately the depth of the chalk had not been confirmed by previous investigations in the vicinity of the proposed retaining walls.
Sanctus were therefore tasked with confirming the depth to the chalk along the line of the retaining walls. This investigation comprised a series of large machine excavated trial pits determining the depth of the chalk to lie at c.5m below ground level, a significantly greater depth than the original proposed foundation depth of 1.5m bgl. Following receipt of this new information, the designers confirmed the need to seat the walls on the chalk creating a new challenge for Sanctus in the form of a significantly larger volume of arisings that required excavating, temporary stockpiling and either reuse or offsite disposal as well as the need to source sufficient aggregates to form the now much deeper sub-base required below the concrete foundation of to the retaining wall.
Sanctus were able to use their extensive approved supply chain to not only find a reuse location offsite for the majority of the additional material being excavated, preventing it from being sent to landfill, but also to identify a source of recycled aggregate locally of both quantity and quality that saved on the need to extract further virgin aggregates representing both a cost saving to the client and a reduction in the carbon footprint of the enabling works programme.
With the sub-base formed using recycled aggregates Sanctus were then able to form the reinforcing steel work on site and cast in-situ the foundation slab for the retaining wall. Once set the pre-cast retaining wall sections were lifted into place and installed in accordance with the designers instructions.
Despite the change in design scope occurring following mobilisation to site Sanctus were able to rapidly react, identifying and mobilising the required plant and materials to site delivering the works within the programme.
As the new development included a piled foundation solution, Sanctus were additionally tasked with installing the piling mat suitable for the equipment required to complete the piling design. Following completion of the reduced level dig and prior to the formation of the piling mat, Sanctus carried out pile probing across the entire footprint of the new building. This was to confirm that there were no previously unidentified obstructions in the ground that would affect the piling works. The probing also confirmed the absence of any unexploded ordnance, an essential factor to confirm as the site is located in close proximity to London with the area being heavily bombed during World War II and as such was at high risk of historic ordnance remaining on site. When constructing the mat Sanctus were once again able to identify a local source of recycled aggregate that would be suitable for use in place of virgin quarried aggregates. Once completed, the mat was tested in situ to confirm that the mat had the required bearing capacity for the piling rig due to be mobilised to site.
Re-use of Soils
Due to the nature of the works over-digging was required to create sufficient space to form structures such as the retaining walls, additionally the majority of the playing field was to be raised to create a flat and level playing surface of the soils excavated, approximately 75% of the soils were to be reused on site, in areas where levels were to be raised to the required levels and as for backfill behind the retaining walls following their installation.
As the western half of the site was to be used as a playing field the soils to be reused in this area were required to contain no materials greater than 125mm in diameter. Due to the majority of the soils excavated comprising Made Ground containing elements of brick and concrete, many of which exceeded 125mm, the soils were passed over a screener which removed the oversized elements and produced a material suitable for use across the playing fields. The oversize items removed from the soils were not suitable for reuse at the site and were therefore removed from site for recycling.
When forming the base of the new playing field Sanctus opted to spread the processed materials using a dozer unit equipped with a GPS blade control. This meant the soils could be spread quickly and efficiently but also accurately to the correct level by a single machine operator, a much more efficient system then the traditional approach of using an excavator and supervising engineer who is required to regularly check the levels.
Where backfilling was required for excavations and behind retaining walls, large items of brick and concrete were removed to create a more homogeneous geotechnical fill to make the reinstatement works simpler. Once placed the materials were compacted where possible with a twin-drum vibrating roller however due to the risk of overloading the retaining walls and causing a failure of the wall section the materials immediately behind the walls were compacted using a vibrating compactor wacker plate attachment for the excavator placing the materials .
By careful assessment and management of the site won soils and the required design levels Sanctus were able to facilitate the reuse of some 4,450m3 of the excavated soils with only 750m3 being removed from site. Of the material removed from site 350m3 required offsite disposal due to being chemically unsuitable with only 400m3 being surplus to requirements and unable to be reused on site representing a reuse rate of suitable materials in excess of 90%.