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According to Young (2013), project constraints are any factor, be it material or immaterial, that hampers the operations of the project team. To better understand these restricting factors, Chatfield (2015) proposed the project constraint model which features three major highly interdependent constraints, namely cost, time, and scope, at the 3 peaks of a triangle. If any of these restraints is altered in one way or the other, one of the other restraints must also be altered. While time and cost have been widely accepted by scholars, the scope has often been interchanged with terms such as goal, product, deliverable, and quality. Which such ambiguity, Brown (2009) added a fourth restraint to advance the triangle model into a diamond model, with the fourth restraint as quality. Further onwards, PMBOK (2009) advanced the model into a 6-point star that separated the input and output factors for the project from the factors dealing with the project process. As such, the first triangle had the original time, cost, and scope at the vertices, while the second triangle had risk, quality, and resources at the vertices.
In construction, there are 8 major constraints, according to Lau and Kong, 2006. These are outlined below.
Design constraints refer to issues that lessen the number of possible design solutions that can be used in a given project. Some of the most common design constraints include current technological endowment of a firm, commercial restraints like time and budget, regional weather and climate, level of infrastructural development in the locality, site condition, site accessibility, local building regulations, etc.
Technical constraints refer to factors that make it difficult to complete various construction activities. These usually relate to how practical it is to use certain methods of construction while at the same time conforming to the building regulations in force. Technical constraints include the order of carrying out certain construction activities, required space for construction workers to work safely, accessibility of the site, availability of storage facilities, client demands regarding the site.
Economic constraints refer to limiting factors in relation to the funds and resources allocated for the successful completion of the project. In the event that the budget is insufficient, the construction project will be negatively impacted in terms of quality, safety, and efficiency. Economic constraints also include the availability of cash when needed for payment of workers and the purchase of materials.
Management constraints refer to construction site policies that hamper the successful completion of the project. Such policies may include overtime guidelines, division of resources to different projects, safety practices, workplace policies, organizational culture, etc.
Legal constraints revolve around national and local laws that the project must be compliant with to avoid brushing shoulders with the authorities. These include health and safety regulations, employment law, environmental policies, building standards and regulations, and minimum wage regulations. It normally takes a lot of time to understand these laws and follow them to the letter, yet non-compliance may lead to heft penalties and permit withdrawal.
Time constraints refer to the inevitable delays that mean key project deadlines will not be met. Whereas the contractor may be responsible for the delays at times due to insufficient planning, there are other instances when the delays can’t be blamed on the contractor, such as when local authorities delay the issuance of permits.
Environmental constraints comprise those environmental concerns that a construction project must take into consideration for the successful completion of the project, including sustainability issues, the use of deleterious materials, carbon footprint, water and air pollution, noise control, disposal of construction waste, dust emission, etc.
Social constraints comprise issues likely to raise public concern over the carrying out of the construction activities and lead to protests or mass media action against the project. The most common form of social constraint is tagged “not in my backyard, wherein the general public tries to stop the construction of structures in their locality due to fears of insecurity or possible strain on essential services.
In the UK, several standards are used in guiding demolition contractors on the right practices and limitations as they go about their trade. The 4 main standards are the i) BS6187:2011 Code of practice for full and partial demolition, ii) BS5607:2017 Code of practice for the safe use of explosives in the construction industry, iii) Health and Safety: The construction (design and management) regulations 2015, and iv) HSE guidance on Establishing exclusion zones when using explosives in demolition. The provisions of each of these publications are discussed below.
This is a code of practice that addresses the demolition process, right from the design stage to the implementation stage. It covers both partial and complete demolition of all scales, large and small alike. It also gives recommendations relating to the efficient management of the demolition process, how to ensure structural stability during demolition, erection of temporary supports for structures under demolition, how to handle the intended collapse of structures, typical responsibilities in all stages of demolition, how to examine a given site for important information that will ensure smooth demolition, enforcing health and safety standards during demolition, risk evaluation and developing effective working plans, establishing and maintaining exclusion zones, and how to ensure the demolition activities remain environmentally friendly.
This code of practice gives recommendations on how to handle, store, and use explosives during various construction activities. Regarding demolition, the code of practice states that explosives can be used for both partial and full demolition concrete and masonry structures, steel structures, structures with a combination of concrete, steel and masonry, destruction of objects such as concrete foundations, beams, and columns, and detachment of erroneous structural elements. Explosives should typically be used to adequately weaken a structure so that it may collapse on its own. Explosives can be fed into shot holes to allow shattering or mounted on the surface of structures for cutting style demolitions. The two most effective explosives recommended for use are the nitroglycerine or emulsion type and high energy detonating cords.
These regulations by NFDC specify that demolitions of structures ought to be done in a way that prevents danger, or reduce danger as much as possible if it cannot be wholly prevented. Additionally, all plans for demolishing buildings must be written down on paper before the demolition works commence. Regarding the use of explosives, the guidelines require all stakeholders to take the necessary steps to make sure no worker or passerby or spectator is injured directly from the explosion or from the flying projectiles common during demolition by explosives.
This guide provides critical information regarding the establishment of exclusion zones during the demolition of structures using explosives. An exclusion zone is essentially an area of a given radius around a structure under demolition meant to shield people from flying projectiles. It is required that at the time of the blast, nobody should be within the exclusion zone apart from the person igniting the explosives. The exclusion zone should include 4 critical areas, i) the plan area of the structure under demolition, ii) the area where a greater volume of the structure is intended to collapse, iii) the area beyond the drop area where excess debris is predicted to fall, and iv) the buffer area between the predicted debris area and the peripheral of the exclusion zone. The extent of the exclusion zone is dependent on factors such as the designed collapse mechanism, the type and strength of explosives used, the detonation sequence, the structural form of the building, contractors previous experience with explosives, the location of neighboring structures, the site topography, and the soundness of the detonation system.
In the UK, the Construction (Design and Management) Regulations (CDM 2015) started operating on the 6th of April 2015, replacing the previous CDM 2007. It contains the most comprehensive laws regarding the maintenance of health, safety, and wellbeing of workers during construction projects. The major goals of CDM 2015 are i) to guide construction stakeholders on how to plan their work from the beginning to the end in a way that significantly minimizes risks, ii) guide contractors into hiring the right people for the particular job at hand, iii) help construction teams to achieve effective coordination throughout the construction project, and iv) raise awareness of possible risks associated with particular construction projects and how they can be best mitigated. Up to now, little research has been done on the effectiveness of the CDM regulations specifically on the demolition industry. The regulations seem to be more inclined to the construction industry. In 2004, the National Federation of Demolition Contractors lobbied for adequate coverage of demolition works withing the CDM regulations, but all subsequent revisions have failed to address the requests.
Other UK legislation applicable to demolition works include: i) Control of Substances Hazardous to Health Regulations (COSHH) 2002, ii) Health and Safety (Consultation with Employees) Regulations 1996, iii) Health and Safety at Work, etc Act 1974, iv) Lifting Operations and Lifting Equipment Regulations (LOLER) 1998, v) Management of Health and Safety at Work Regulations 1999, vi) Provision and Use of Work Equipment Regulations (PUWER) 1998, and vii) Work at Height Regulations 2005. All these are on a national scale. Being located in Shropshire, West Midlands, the demolition of the cooling towers at Ironbridge power station must comply with the regulation provided by the Shropshire Council Building Control.
Given the current study involves an exploration of the technical constraints experienced during the demolition of the cooling towers at Ironbridge power station, it was imperative to consider other cooling towers that have previously been demolished and how the demolition works progressed. Four demolition projects were considered namely, i) Calder Hall Nuclear Power Station, ii) Didcot Power Station, iii) Ferrybridge Power Station, and iv) Richborough Power Station.
Calder Hall, located in Sellafield, was the world’s first commercial nuclear power station. It was commissioned by Queen Elizabeth in October 1953 and successfully generated electricity up to 2003. The power station was decommissioned after increasing concerns of the inadvertent collapse of the cooling towers due to aging.
The power station had 4 cooling towers in total, each rising to a height of 88 m. Each tower comprised 64 raking legs of 7.3 m height with a pond structure beneath the shell. A project team was put together to assess the best demolition method that would be both safe for humans and the environment and be cost-effective. The team settled on the use of explosives. The preliminary exclusion zone was set to 70 m for people and 110 m for any glassed structure. On the day of demolition, the exclusion zone was extended to 200 m, fully manned by 61 sentries.
Surrounding utilities of significance were first identified. This was followed by the removal of all redundant plants. Other plants around the structure, such as the pump pits and water treatment plants, were demolished. All internals, including supporting columns, beams, 260 tons of timber framework, 6000 m3 of plastic packing, and 75 tons of asbestos pipes were then removed. Nearly 6,000 holes were drilled around the four cooling towers in a horizontal stitch pattern, as well as three 10 meter high vertical slots. After a successful test blast, explosives were loaded over a 10 day period and detonated. The two towers to the north were demolished simultaneously. After four minutes, the two towers to the south were demolished.
Some of the major constraints experienced in this project included i) protecting the nearby reaction towers from damage, ii) protecting the process waste line and interceptor sewer line next to the towers from debris damage, iii) protecting UK’s only nuclear Fuel Handling Plant which was only 40 meters away, iv) containing ground vibrations to protect other sensitive structures within the Sellafield site, and v) minimizing the amounts of radioactive waste released to the environment.
On the day of demolition, Saturday 29, 2007, each pair of the towers took about 4 seconds to fall to its footprint, as designed. Debris spread around the towers to a radius of 20 meters. On carrying out the post demolition checks, it was discovered that the windows of the reactors adjacent to the cooling towers had broken in the process.
Didcot power station, located in Sutton Courtenay, Oxfordshire used to generate electricity from coal and natural gas. It was rated the second most polluting establishment in Britain. It was closed in 2013 following heated protests by the public and environmental activists. The station had a total of 6 hyperbolic cooling towers. The first three were demolished in 2014 and the other three in 2019.
During preparations for the demolition in 2016, a large volume of the boiler house collapsed as it was being weakened for explosive demolition by cutting into the structure. The incident killed 4 men, injured 5 men, and made about 50 people inhale large amounts of dust. The rest of the structure that remained standing was brought down in a controlled demolition. The first three towers, each rising to a height of 114 m, were set for demolition between 3 am and 5 am on Sunday, July 2014, but slight delays saw it demolished a few minutes past 5.00 a.m. The other three towers demolished at 7.30 a.m. on 9th February 2020.
The detonation was carried out by demolition contractors Brown and Mason. The exclusion zone was set within the boundary fence of the power station. A small section of the nearby A4130 was closed for about 2 hours. The structure was washed down before demolition to reduce dust emission. Explosives loaded throughout the circumference were fired and the cooling towers successfully collapsed on their own footprints.
One of the major constraints experienced during the demolition was the protection of the nearby pylons and transformer station from damage During the demolition, the transformer exploded and caused more than 40,000 area residents to go without power for a few hours. Another constraint involved pressure from the public to shift the time of demolition to daytime hours to allow them to view the demolition, but the concerned authorities refused to give in to the pressure.
The demolition of the 6 cooling towers was largely successful, without major accidents and pollution concerns. The only incident that threatened the integrity of the project was the premature collapse of the boiler.
The Ferrybridge power stations are a combination of three plants powered by coal on the River Aire in West Yorkshire, England, at the junctions of the A1 (M) and M62 roads. The first station operated from 1920 to 1976, the second station from 1950 - 19990, and the third station from 1960 to 2016.
The power station comprised a total of 8 cooling towers. The average height of the cooling towers was 114 m. The first one was demolished on 28 July 2019. Later on 13 October 2019, another four cooling towers were demolished. The remaining three cooling towers are still standing up to date.
All the towers took about 10 seconds to collapse totally. On the demolition day, about 100 homes were evacuated to prevent any risk of injury arising from flying projectiles and undue vibrations. Roads were closed using rolling roadblocks. An exclusion zone was set to about 328 yards around the cooling towers.
The major constraint experienced during the demolition was the close proximity to major roads and residential homes. With such, the risk of injury was high if the demolitions went wrong. Containing dust was also difficult.
The controlled demolition of the cooling towers at Ferrybridge power station went on smoothly without major incidents and eventualities. The remaining cooling towers are set to be demolished by end of summer 2021.
The Richborough Power Station was located in northeast Kent, about 4 kilometers to the Southwest of Ramsgate and 3.5 km to the north of Sandwich. It was built next to the mouth of the River Stour and used to generate 336 MW of electricity. Having successfully operated from 1962 to 1996, the station was closed due to public pressure regarding the harmful environmental effects of the Orimulsion fuel used. The 3 cooling towers of the station were among the structures to be demolished. demolished on 11th March 2012.
The three cooling towers rose to a height of 97 meters. The demolition project was fiercely opposed by locals who termed the towers as a key part of the historical landscape given that they long acted as a landmark for boats navigating the River Stour. Preparation work for the demolition was carried out between May and June 2011.
The three cooling towers were first stripped of all mechanical plant. The removal of asbestos by professional asbestos contractors followed. To demolish the towers, the contractor first got rid of the two slots in the shell through the use of explosives to make way for a rotational collapse in one direction. Other techniques such as basting, use of wrecking balls, and use of high reach excavators were ruled out citing unacceptable health and safety risk to the workers at the site, including noise, vibration, and dust. An exclusion zone of was set and was patrolled by the Police and contractor sentries.
Some of the constraints encountered in this demolition project included i) the need to pump out adjacent water ponds and discharge to the River Stour, ii) the need to divert traffic to the A2 and A299 roads as alternative routes to Ramsgate and Dover, iii) containment of the 17,000 tonnes of waste debris, iv) preservation of reptiles, birds, bats, water vole, otter, great crested newt, and peregrine falcon that are common to the area, v) prevention of contamination of the nearby water resources and soil, vi) suppressing dust.
The three cooling towers were finally successfully demolished on 11th March 2012. They were detonated in sequence with a three-second interval between the collapse of each of the structures.
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