A case study of sustainable construction using the building of a large garden bridge as an example
This Paper is in two parts. The first part describes sustainable construction, why it is needed, and how to achieve it. The second part is a case study of the design and construction of a ‘Monet’ style bridge, the design of which was required by the Client to look like the bridge over the pond in the garden of the famous French impressionist painter, Claude Monet.
The first part of the Paper includes:-
- Sustainability – What is it?
- Natural Capital.
- ‘Think Global – Act Local’.
- The Sequential Test for Sustainability.
The second part of the Paper is a case study of the ‘Monet’ bridge.
Sustainability – What is it?
Sustainability came to the top of the political agenda in the early 1990’s when the world’s Governments met in Rio de Janeiro to discuss the ‘Caring for the Earth’ 1, 2 report on the future of life on Earth. The politically popular definition of sustainability is ‘ Development that meets the needs of the present without compromising the ability of future generations to meet their own needs’. This is a political nonsense. The real definition, as used in the ‘Caring for the Earth’ report is:-
‘Improving the quality of life whilst living within the environmental capacity of the Earth’.
The other relevant definition, from the same report, is:-
‘A sustainable activity is one that, for all practical purposes, can continue forever’.
Sustainability is not new. Historically there have been scrap metal merchants who recycled metals for many decades. As far back as the mid 1980’s the author was implementing whole life costing for buildings, and this has much in common with the concept of sustainability.
If we look into the future, currently only about 1% of infrastructure is replaced each year, giving a 100 year design and replacement cycle, whereas the planning system assumes a 50 year renewal cycle. The design life of a new house is currently 60 years. The design life for cement to British Standard 8500 is only (at least) 50 years. Most of the infrastructure that you would see in 2050 is already here, and most of it will be over 150 years old by then. Clearly we need to build more sustainably than in the past and at present. A fundamental part of sustainability is the prudent use of Natural Capital.
Natural Capital comprises everything that exists on and around our planet – the Earth. The understanding of Natural Capital cannot be taught – it has to be found, then understood, and the knowledge acted upon. It is like finding out how to balance when learning to ride a bicycle. We only have the one planet we live on, yet at present we are consuming our limited resources at a rate that requires THREE planets like ours to support our current use of these resources.
The term ‘embodied energy’ is often used in connection with sustainability. One definition of ‘embodied’ is defined as ‘to give a tangible, or concrete form to’, and there are other definitions from other sources. However, the use of ‘embodied’ in connection with sustainability is incorrect. The term ‘embedded energy’, defined as ‘to fix or become fixed firmly and deeply in a surrounding solid mass’, correctly describes the amount of energy used in the production and transport of materials which cannot be recovered. Steel has a low embedded energy because producing it from scrap takes only 25% of the energy required to make it from iron ore. Cement has a high embedded energy as all of the energy used in making it is lost in the manufacture and cement cannot be recycled like steel. Concrete, and in particular, reinforced concrete, is the most widely used structural material worldwide, and this is expected to continue because of its versatility. However, where more sustainable materials can be used they should be used instead of concrete, especially in masonry construction.
‘Think Global – Act Local’
The phrase ‘Think Global – Act Local’ came from the Rio Conference in 1992, and is the essence of sustainability. We need to understand Natural Capital, and Think Global (about our planet) and Act Local (buy and use our resources as locally as possible). Some resources have to be transported around the world, but where this is necessary it needs to be done as sustainably as possible which usually means by sea or rail. In contrast, the use of air freight for transporting out of season produce like flowers, fruit and vegetables is not sustainable.
Take, for example, hydrocarbon fuel, on which the developed word economy is based. In 1967 a gallon of petrol cost the equivalent of 23.75 pence. 43 years later at the start of 2010 it is now about £5.00 a gallon, a 21 fold increase! If we project forward another 43 years it would cost £105.00 a gallon at the historical rate of increase! However, world reserves of oil are expected to peak around now and then diminish with time, and the price will rise further with increasing scarcity.
The other big problem for the developed countries is the amount of waste we produce. Around 434 million tons of waste are produced in the UK each year. That amounts to 7.23 tons per person per year. Of that only half a tonne per person is domestic waste, and that amount is reducing as we recycle more. Too much ‘waste’ comprises items that were manufactured and then thrown away without even being used. Too many ‘goods miles’ are travelled as items are moved around the globe instead of being locally sourced. This results in the toxic pollution of our environment and the production of more carbon dioxide, methane and leachate etc. Construction waste is a substantial problem which we need to reduce drastically. We need to ‘Think Global’ about the effects on our planet and ‘Act Local’ to achieve a sustainable way of life.
The Sequential Test for Sustainability
The sequential test for sustainability is about the use of resources. It is a method used at every stage of the design process to ensure that each part of the process and the design right down to the smallest detail is as sustainable as it is possible to make it. It is used to ensure the best use of the finite resources that we have, (our ‘Natural Capital’). Each part of the design and construction should be as close to the top of the sequential test as it can be.
The test is used to assess the prudent use of Natural Capital for all aspects of a project. It has six levels as follows:-
- Not Used (Avoid the need to use . . . ). e.g. Primary aggregates, first use new materials.
- Re-Used (Use for same purpose / situation without re-processing). e.g. Secondhand bricks, paving slabs.
- Down Used (Use for lesser use without re-processing). e.g. Secondhand oversize steel beam section used instead of a new one of the ‘designed’ size.
- Recycled (Make new from existing). e.g. Steel, aluminium, glass.
- Downcycled (Use for a less sustainable purpose). e.g. Crushed structural concrete used as fill ; glass used in road surfacing.
- Waste Only what we cannot at present re-use, downuse, recycle, or downcycle. Waste needs to be minimised.
Some terms, such a ‘recycling’ are often misused. For example, people talk about using recycled concrete when in reality it is not currently possible to recycle concrete i.e. make new cement and aggregate from old concrete. So concrete is downcycled, or goes as waste, whereas steel and glass can be recycled again and again, and have been for many years.
A CASE STUDY OF THE ‘MONET’ BRIDGE
This is a case study of the state of the art sustainable construction of a ‘Monet’ style bridge. The bridge has been chosen as an example of the design and implementation of sustainable construction, to demonstrate the concepts of applied sustainability in practice, and to avoid the limited examples that would be found in a single building. In this way the concepts can be learned and applied to all buildings and structures.
The Client required the bridge to look like the one in the French impressionist painter’s garden even though it would be some 26m long. Sustainability was the key requirement for the design, construction, maintenance and eventual removal of the bridge at the end of its life. Full use was made of the sequential test for sustainability at all stages from the compilation of the project brief onwards. The prudent use of natural capital was the key to achieving sustainable construction, including the minimisation of embedded energy.
The site comprises a privately owned house, garden and paddock extending to a total of about 3.5 acres with the river running through the middle of it and separating the paddock from the remainder of the site. A bridge designed and built by the previous owner, (a ’builder’), four years previously was found upon inspection to be structurally inadequate in most respects, and one of the main supporting beams had cracked through much of its depth.
The Design Brief
The design brief was led by the requirements of the Client and statutory requirements. It comprised:-
A ‘Monet’ style bridge;
For use by pedestrians, a ride on mulching mower (like a quad bike), and for horses to be led across the bridge;
It would be a privately owned and maintained bridge;
The Client required a 30 year design life to ‘see them out’;
CLSPDBPFMO form of contract required;
The Environment Agency requirement for a clearance of at least 600mm above the level of the flood plain to the underside of the bridge;
From the design brief a list of design considerations was prepared. It comprised:-
An 11.250 metre main span, 26 metres long overall including the approaches;
Construction and maintenance over the river;
Access only through the garden of the property;
To be built during the winter;
The site is in a flood plain with a soft ground surface requiring the use of duck boards for pedestrian access;
The site is remote from the nearest road, which is a narrow lane about 80 metres and two 90 degree bends away.
The optimum use of natural capital;
A low amount of embedded energy;
The materials were to be sourced as locally as possible;
Simplicity of maintenance without the need for temporary access, e.g. scaffolding over the river;
Ease of dismantling and removal with re-use;
Waste minimisation during the whole life of the bridge;
The primary aggregates should match the pH of the local ground;
The bridge should have pinned and sliding bearings for the main beams to allow for movement. Design guidance was obtained from Footbridges in the Countryside3.
The Design Solution
Each part of the bridge (except the two main beams) can be individually replaced, including the foundations;
Removal and re-use of the bridge at the end of its working life at this location if required;
Ease of dismantling and further use of materials;
Waste minimisation to be an integral part of the design;
Little mechanical plant to be used, due to access restrictions and soft ground conditions. Work to be done mostly by hand.
The completed bridge
The bridge was built almost entirely by hand, and being built over a river Health & Safety was fundamental to every stage of the project. Particular precautions were taken in respect of the possibility of contracting Weil’s Disease due to the proximity of the river water. The bridge was built using a combination of a main and other contractors, with some volunteers (who did not have construction experience), under professional direction. Construction was carried out during the winter, mainly between the end of October and March of the following year. Landscaping was done between March and the end of June and the bridge was officially opened on 30th July 2005.
In order to safely dismantle the existing failed bridge a scaffolding platform was built across the river. The bridge was then carefully dismantled in order to save as much as possible for re-use where the materials were still suitable for that purpose.
The foundations were gabions filled with limestone of a similar pH to the surrounding ground, which is alkaline. Conventional concrete or concrete and brick foundations were not used because they require more energy intensive materials, are more difficult to construct below the high level water table, and would therefore have cost considerably more than the gabion solution. The use of large size crushed concrete instead of limestone was investigated but it was not available from the recycling firms at the time of construction. The gabions were wrapped in Terram geotextile up to just above ground level to prevent the surrounding soil from creeping in between the stones (making them dirty), and to prevent settlement at the bridge approaches which is a common problem with some bridges. The Terram also allows the stones to be re-used when the bridge is removed. The gabions were held together with spiral linking wires along all edges and have a divider mesh across the middle for strength.
On top of the gabions were cast two small reinforced concrete bearing and distribution beams to take and distribute the loads from the main steel beams of the bridge. To minimise the use and amount of energy-intensive concrete required, the main beams of the bridge were designed to sit right at the ends of the distribution beams which are the same length as the gabions. The concrete was prevented from entering the gabion stones by the use of Terram. This design allowed the bridge fixing bolts to be firmly anchored into the gabions. The reinforced concrete bearing beams were constructed with state of the art large base spacers to British Standard 7973 resting on the gabion wires.
The main beams were curved parallel flange steel channels with their toes facing inwards, with Warren plan bracing and diagonal cross bracing of steel angles at the ends and in the centre. The channels were chosen to give a plain vertical face to the outside of the bridge to match that of the rectangular timber beams in the original ‘Monet’ bridge. All of the steelwork and fixings were hot dipped galvanised to reduce the maintenance requirements and risks of working above the river. The steelwork (but not the fixings) was painted green to match the original ‘Monet’ bridge and the timber standards and handrails. The curved main beams have a pinned bearing at the paddock end (the least accessible end) and a sliding / rotating bearing at the (more accessible) garden end.
The deck, standards and some of the handrails were made from softwood timber from sustainable forests pressure treated with preservative. The centre and lower rails of the handrails were re-sawn hardwood from the old bridge. Fixings for the timber deck and handrails were all of stainless steel.
The sloping wingwalls were formed with EcoSleepers which are an environmentally friendly version of traditional railway sleepers. The approaches to the bridge were formed with downcycled crushed concrete wrapped in Terram to avoid soil penetration and to make it re-useable when the bridge is removed. Re-used Victorian diamond pattern blue paving bricks formed the non-slip surfaces at the top ends of the approach ramps.
The steel beams weighed approximately 700kg each and were offloaded by a lorry mounted HIAB. They had to be carefully manoeuvred off the lorry in the narrow lane between the house, the garage, and overhead cables. The first one was lowered onto three trestles made from timber from the old bridge, which ran on short scaffold tubes on a series of three scaffolding planks borrowed from the scaffolding over the river. Each beam could be easily moved by one person. Each beam was moved along the garden then turned through 90 degrees and out onto the scaffolding deck over the river. It was then raised using three chain hoists hung from an overhead scaffold, moved sideways until it was above the end of the distribution beam, and lowered around the fixing bolts. As each beam was curved upwards it was unstable on its own and was supported laterally at mid-span until the bracing had been installed. The two main beams were moved and placed in position and the bracing fitted within one day.
As soon as the steelwork had been fixed in position the timber deck supports and the handrail standards were bolted to the main beams. A rope handrail was installed for safety, and the deck timbers loose-laid across the bridge. The scaffolding could then be removed freeing it for re-use on its next project. The deck timbers were progressively fixed across the bridge, starting from the garden end.
The three rows of handrailing were then fixed. The lower rail was added to the original ‘Monet’ bridge design as a safety measure to prevent the horse’s hooves from going off the deck if they slipped, and for the mulching mower wheels for the same reason.
The end walls were built with engineering bricks for durability and capped with re-used blue Victorian diamond pattern bricks. These were laid on the slope to match the bridge deck and approaches and to give a durable transition from the grass of the approaches onto the timber deck.
The approaches were constructed by excavating a calculated amount of topsoil from the existing ground, laying Terram and filling with downcycled crushed concrete. The Terram covered the crushed concrete, kept it clean and prevented the topsoil surface from penetrating into the crushed concrete; had this occurred it would have resulted in the subsidence of the top surfaces of the approaches. The topsoil was graded and grass seed sown. Turf was not used as it would have had to be imported onto the site.
The main beams were painted with a two part epoxy paint for aesthetic purposes before the deck was fitted. The handrails were coated with a matching coloured wood preservative.
Waste minimisation was a key part of the project. No excavated material was removed from the site. Instead the ‘cut and fill’ design allowed all of the excavated material to be used to topsoil and landscape the bridge approaches. At the end of construction there was less than one wheelbarrow full of waste. This consisted mainly of timber offcuts from the joints and small triangular offcuts of Terram. Waste minimisation at the end of the life of the bridge is also a major consideration. To this end the only waste arising on removal of the bridge will be the four rubber bearing pads and eight nylon isolating sleeves which together would be less than half a bucket full. By the time the bridge reaches the end of its life it may well be possible to recycle or downcycle even these small items.
A comprehensive ‘Owners Manual’ containing the ‘as built’ drawings, a maintenance inspection plan, and a maintenance log were provided as part of the project. Lifetime maintenance can be done without the need for scaffolding, and the designed maintenance plan makes a great contribution to reducing the environmental impact and increasing the sustainability of the bridge.
Re- Use of Materials
The following materials can be re-used at the end of their life on the bridge:-
The structural steelwork;
The primary aggregate gabion fill;
The bridge timbers of various sizes;
The stainless steel fixings;
The Terram geotextile;
The Eco sleeper wing wall timbers;
The crushed concrete fill;
The topsoil and excavated material.
Down Use of Materials
The timbers not suitable for re–use can be used for other purposes e.g. around the Client’s garden.
The following materials can be recycled with currently available technology.
The gabion cages (steel);
The bearing fixings, nuts and washers (stainless steel);
The bridge fixing bolts, nuts and washers (steel);
The reinforcement (steel).
The downcycled materials comprise:-
The bricks from the end walls, which could be used as hardcore;
The concrete from the bearing pads.
End of Life – Waste
At the time of construction the following materials were the only ones which would produce end of life waste. However, by the time they reach the end of their lives it may well have become possible to recycle them.
- Rubber bearing pads.
- Plastic bearing sleeves.
Together they comprise less than one bucketful of waste. If the bridge had been designed and built using conventional methods it would have produced three large skips of waste.
As the bridge is privately owned the detailed cost is not available for publication. However, when considering costs the whole life cost is the most important one. This comprises the initial cost of construction together with the maintenance costs during the life of the bridge, and the eventual removal and disposal costs.
For this state of the art sustainable design the initial costs were about half of those normally associated with a conventionally constructed bridge.
The savings were made by:-
Using gabions for the foundations instead of concrete or concrete and masonry;
The gabions allowed ‘dry’ construction to be built easily even though they were partly below the level of the water table;
The substantial reduction in the amount of waste generated from the construction, including the tax on waste sent to landfill;
The removal of the scaffolding at the earliest point during construction;
Minimising the amount of concrete required for the bearing pads;
Re-using materials from the old bridge, sometimes several times during construction.
During the working life of the bridge savings will be made because:-
The maintenance inspections can be fully carried out without the need for access equipment;
Each part of the bridge can be maintained and, if necessary replaced (except for the two main beams), without the need for scaffolding for access. The main beams and associated steelwork is designed to last for the full design life without structural maintenance.
Costs during the working life of the bridge are estimated to be about 25% of conventional costs at current prices.
At the end of its working life the bridge can be removed and the materials re-used, downused, recycled, or downcycled with almost no residual waste.
Overall the whole life cost is expected to be less then half of that for a conventionally built bridge at current prices.
Sustainability in construction can be achieved;
It uses less resources, and especially energy, than conventional construction;
It produces less waste;
It is cheaper;
It can give a long design life.
The same techniques have been used for many years on building projects, utilising the whole life cycle costing techniques, the sequential test for sustainability, and waste minimisation to the fullest extent available at the time.
As resources become increasingly expensive and scarcer sustainable construction is the only viable long term solution. We can do it and we need to implement it now on every project.
The following suppliers embraced the sustainability of the project and supplied the materials. The list is given to assist others in sourcing materials sustainably.
Scaffolding – Fleet Roofing and Scaffolding Ltd.
Gabions – Hy-Ten Gabion Solutions, part of Hy-Ten Limited.
Stone for gabions – CED Ltd.
Terram – Sheffield Insulations Ltd.
Steel reinforcement and chairs – Tomasa Ltd.
Plastic spacers for reinforcement – Injection Plastics Ltd.
Bridge bearings – Ekspan Ltd.
Nylon sleeves – Aquarius Plastics Ltd and Brimic Engineering.
Lifting equipment – HSS Hire Ltd.
Bridge steelwork – Nu-Steel Structures Limited and The Angle Ring Company Limited.
Timber – Coomers (Oakhanger) Ltd.
Eco sleepers – Travis Perkins Ltd.
Fixings – Margnor (Fasteners) Ltd and TECO Building Products Ltd.
Bricks – Meakin Building Supplies Ltd.
Secondhand paving bricks – Woodland Farm Reclamation.
Crushed concrete – C G Comley & Sons Ltd and Hutchins & Carter Ltd.
Paint – Jotun-Henry Clark Ltd.
The following suppliers were found to be not interested in sustainable construction projects.
Scaffolding - Chequers Scaffolding Ltd, and Marion & Wake Ltd.
Gabions - Intermesh, (part of the Phi Group Ltd), Maccaferri Ltd, and Weldgrip Ltd.
Bearings - ACM Bearings Ltd, Freyssinet Ltd, Henderson Bearings Ltd, and Maclellan Rubber Ltd.
Steelwork - Leedsheath Ltd, and Pyramid Steel Ltd.
1. ‘Caring for the Earth. A Strategy for Sustainable Living’, IUCN / UNEP / W W F, (1991), Gland, Switzerland. Report ISBN 2 - 8317 - 0074 – 4.
2. ‘Caring for the Earth. A Strategy for Sustainable Living’, IUCN / UNEP / W W F, (1991), Gland, Switzerland. Summary ISBN 2 - 8317 - 0074 – 4.
Note: The Report and Summary are separate documents but have the same title and ISBN.
3. ‘Footbridges in the Countryside – Design and Construction’, by Reiach Hall Blyth Partnership, and published by the Countryside Commission for Scotland. 1981. Perth. ISBN 0 9022 2652 5.
Chris Shaw CEng FICE FIET MIStructE MCMI, Structural Engineer
For a list of local architects to help with construction projects click here.