Early Works

We were engaged as both Principal Contractor and Principal Designer on this project, demonstrating our experience in managing the design, health and safety and environmental obligations of this large-scale, complex project, in a busy part of Peterborough city centre. Prior to works commencing, we organised all the documentation, including the Section 80, ASB5s and service disconnections for the building. Due to the nature of the structure vast amounts of analysis and planning was needed to create a demolition methodology that was safe, robust and compliant.

The site was secured with a fully reusable kentledged hoarding system to prevent unauthorised access and defined working site constraints/boundaries. The hoarding was a proprietary system provided by “IP Jones Fencing” designed to resist wind and crowd loading, vital on the busy thoroughfare. The TW co-ordinator checked the assembly against the provided design for TW sign off in the TW register. Site hoarding was checked at the start of each shift as per the TW register and assessed for condition, which included reporting of unsecure panels, fastening and clips which were raised to the attention of the site supervisor (also the TW supervisor) for maintenance and repair or replacement.

During the mobilisation works site welfare had to be installed next to the building. However, the services had to be cut and capped in the road outside of the future site boundary in order to fully disconnect the structure. As a result of this a specialist plumbing team had to liaise with market traders, with the councils backing, in order to feed the cabins from a market based temporary water supply. With only one water supply on the eastern end of the market our team had to heavily modify the water main to allow for the feed of out cabin structures without impacting the market traders.

Stakeholders

The site was surrounded on all four sides by constraints all less than five meter from the base of the car park. The Peterborough indoor markets were adjoining the car park on the westerly side and presented the biggest challenge. The market needed to remain operational throughout demolition and the levels on noise pollution kept to a minimum. Due to the markets having high footfall from members of the public the client insisted that dust pollution be kept to a minimum for the project. The risk of collapse also presented a hazard for the markets, but this was mitigated with appropriate methodology.

On the Easterly side of the project was a large office development under construction, because of the high workforce traffic and the nature of the works dust needed to be kept to a minimum to ensure the damage to construction as kept to a minimum. Due to the open nature of the site next to the structure falling debris posed a hazard to construction operatives working in the car park of the development. As a result, a full scaffold screen was needed on this side of the car park also.

The other two sides of the structure were bordered by roads serving the centre of Peterborough. It was the council’s decision as to which road they would close to allow demolition. Due to high street retailers requiring loading bay access to the north this road had to remain open for the majority of the project. Therefore this road was limited to a weekend closure only. This meant site operatives only had a week to reduce the structures height to a level which could be demolished from the southern face reaching over.

Engagement with the high street retailers for this closure was of paramount importance. Deliveries had to be rescheduled to accommodate the road being closes but to ensure the shops remained stocked. Limited pedestrian access was also required to the rear of many of the shops to allow staff access to the structures. The high street also had several residential developments meaning out of hours work had to be kept to a minimum so that the residents were disturbed less.

In order to help our engagement with the client a live viewing system was installed allowing the various councillors and council site managers to log in remotely to check progress on the job and spot any perceived problems in real time. This system became invaluable during the Covid-19 Pandemic as councillors were able to watch progress whilst working from home and drastically cut down on site visits lowering the risk of transmission associated with the site.

Safety, Health and Environment

Due to the structurally unsafe nature f the car park from the onset safety became priority. The car park had to be demolished in a safe fashion to avoid any unintentional collapses. To achieve this vast structural surveying work took place on the car park to determine the true strength of the floors. Through experience with the British lift slab construction and from demolishing previously structurally condemned examples the team was well equipped to deal with the structure. Because of the need to fragment the car park into such small lumps, to avoid shear failure of the floor slabs through overloading, the risk of flying debris ricocheting out of the site boundary massively increased. As a result of this large debris screens had to be erected to prevent debris leaving the site boundary during demolition works.

Because of the city centre location of the job dust and noise pollution had to be kept to a bare minimum. A dust and noise monitoring station were established in the Peterborough market as this venue provided the greatest risk to the public. This monitoring system alerted site whenever there was a breach in the council agreed thresholds. In order to supress the dust, we mobilised 3 of our dust fighting fans to dampen demolition works reducing dust arisings. In order to avoid the environmental risks of spreading water born pathogens through the air in a city centre environment onto the public, we fitted UV treatment lights to the machines. This purifies the water that enters the machines to ensure no pathogens are being sprayed.

Due to the extensive use of scaffolding strict inhouse procedures were developed to appropriately inspect the scaffolding before work. The limit of untied scaffold lifts above the demolished structure was agreed and set at 2 lifts. This allowed us to work down 2 floors before needing to allow the scaffolders time to lower the scaffold. This number was heightened to 3 lifts if the site team removed the monarflex material.

Urban explorers had become a problem for the council whilst they managed the closed car park. As a result, we appointed a security team to prevent unauthorised access to the unsafe structure. This allowed us to confidently demolish the structure without the need to conduct a walkthrough of a condemned structure in person.

Learnings

The Northminster car park demolition project presented several unique challenges not previously encountered by Armac. The demolition of a structurally condemned structure in such close proximity to a busy public space such as the market. Through the correct environmental monitoring and the careful development of a safe phased plan of works the demolition was completed safely. Through frequent engagement with the council work was scheduled to take place in less busy periods and to avoid causing nuisance to the market traders and customers.

The staggering of the northern façade allowed the machine operatives to work up to the façade blind and at height with no risk of debris falling onto the road. The development of this method of works allowed us to reduce the closure of the northern road to just one weekend limiting the impact we had on the shops.

Because of the British Lift Slab construction of the car park great care was required when dismantling the structure. Overloading of the floors at any point could have led to the total collapse of the structure. As a result of this all lumps of concrete had to be reduced in size to that of a football to avoid overloading. The dynamic impact force of these lumps when falling was also calculated to ensure they did not punch through the slab or trigger it to collapse.

The Covid-19 pandemic began to gain traction and we faced a nationwide lockdown midway through the project. Because of the dangerous state of the structure and its potential to collapse, Peterborough City Council granted us ‘Key Worker’ status in order to complete the job. Learning how to operate within a pandemic was new to the team but very quickly the relevant protection measures were put in place to protect staff and the public. These included but were not limited to temperature checking, hand sanitation and staggered breaks to lower the number of people in the canteen at anyone time.

Method

The north of the car park featured small shop units associated with the next-door market. These shops were to initially be stripped out using a bobcat. Once these shops had been cleared and the bare concrete of the structure had been exposed the structural surveying team could attend site. After thorough analysis of the condemnation report and the assessment of the construction of the structure a sequence of works could be modelled.

The sequence of works needed to ensure the carp park remained in a state of equilibrium at all times the structure could not be allowed to become an extreme rectangle it had to remain as close to a square as possible in order to maintain stability.

Once the sequence of works was designed the two-scaffold screen were designed and installed. During the scaffold instillation works the rest of the structure was readied for demolition. This included extensive service disconnection diversion and protection. The roof of the structure featured a prefabricated CCTV controller’s office, this structure had direct links to all cameras in Peterborough City Centre. As a result of this there was a large number of fibre optics connecting to the structure. These had to be cut and capped in an appropriate manor to ensure the cut ends did not cause signal failure or noise within the fibre network.

Once the scaffold screens were erected the southern road was closed on a four-week closure. The hoarding was extended to encompass the new site area and road protection was placed to ensure that no damage was caused to the carriageway. It was at this point where a pair of custom-built high reach excavators were mobilised to site to begin works on the southern façade.

In accordance to the structural engineer backed plan of works the building first had to be cut in half from south to north to create two equally sized cubes. After one week of working through the car park the machines approached the northern façade. Due to the close proximity of the road the machines were to step the northern façade away from the road to prevent and debris leaving the site boundary.

The northern road was closed on a weekend long closure to allow for the reduction of the northern façade to a safe level. Once the carriageway was closed relevant roadway protection was installed and the hoarding removed temporarily for the weekends work. The high reaches assisted by 2 smaller excavators began to reduce the northern façade in size, focusing on the centre section to break through. Once the Northern façade had been reduced in height it reduced the risk of debris entering the road was eliminated.

The large prefabricated CCTV hut was cut into smaller pieces by the excavator and brought to the ground for recycling. This process required thorough reduction in size to avoid pieces overloading floors if they were to fall from the roof.

Once the car park had been broken into two sections each machine could now prioritise a section each. Working in an arcing motion removing a bay from the perimeter of each block slowly working towards the scaffold screens. When work approached the closes point to the markets a crane mounted dust suppression screen was used as extra protection for the members of public below.

Due to the height and construction of the lift shaft structures being greater than the rest of the car park a larger high reach excavator was brought to site to dismantle these structures. This machines increased height and tool bearing ability allowed it to slowly work the remaining two by two bay structure of car park on either side. Because of the compromised integrity of the structure the bays had to be reduced as a square to reduce the collapse risk of a full height slender structure.

Once the car park was fully demolished we retained a floor of kentledge supported scaffold screen on either side of the project during the foundation removal process. The concrete bases were broken from the ground and excavated. During this process a more permanent hoarding was constructed around the site perimeter. Once the bases had been removed from the ground the site was levelled to the agreed final levels with the client.