Earth works

WordsJohn RamshawPhotosWill Claxton

While earth-based construction techniques are undoubtedly gaining traction within the construction industry, they have until now been mainly associated with modest-sized developments in rural settings. But this is set to change with Tribeca, an ecologically ambitious, large-scale development in London’s Kings Cross designed by Bennetts Associates for London-based developer REEF Group. The mixed-use scheme replaces a former Royal Mail sorting office, and comprises five structures accommodating mixed-tenure apartments, retail and leisure facilities, as well as offices.

One of the five buildings, The Apex, is set to become the UK’s first large-scale structure to use site subsoil as a construction material. Instead of being taken to landfill – as is commonplace on buildings of this type – the excavated soil is being transported to Buckinghamshire-based brickmaker HG Matthews, where it is being turned into high-performance earth blocks, before being delivered back to site for use in wall construction.

After initial sieving using a wet pan, the site material is loaded into a feed hopper and then passes through a set of rollers to grind down any remaining oversized deposits

Carbon reductionThe idea to use earth blocks is the brain-child of project architect Nikolay Shahpazov.He’d been interested in the technology for some time but saw an opportunity to use it on a large-scale project when he joined Bennetts Associates and started working within the practice’s dedicated sustainability group. For Shahpazov earth blocks are an obvious choice when it comes to reducing embodied carbon.

Each block contains only ten per cent of the embodied carbon of a traditional concrete block. They also store reusable site material within the building itself. “The clay, sand and silt remain in place,” explains Shahpazov. “These resources become finite if, for example, we make bricks, due to the difficulty of removing mortar for reuse. Here, we are storing resources for future use, which complements the circular economy. Added to this, the ‘living’ nature of the blocks enables them to regulate interior temperatures and humidity, as well as reduce airborne pollutants.”

The earth blocks are removed from their wooden moulds by hand. Approximately 700 units are produced per shift.

Making the dream a realityGetting from concept to reality with the use of earth blocks on The Apex has been a lengthy and at times demanding undertaking for Shahpazov. “I first got to know about HG Matthews’ earth blocks during lockdown,” he recalls. “I then tried to incorporate them into one of the practice’s current projects but it was already too far advanced. When I became project architect for Tribeca I discussed the possibility of using them with the contractor, VolkerFitzpatrick, who was piling on site at the time. I asked if we could take some fresh soil for analysis and then got in touch with Roland Keeble of Rammed Earth Consulting, who together with HG Matthews, made some block samples, which were presented to the client.”

Following various laboratory tests, it was decided that the earth blocks would be used in the basement in place of concrete blocks. But there were still a number of challenges – not least ascertaining the blocks’ structural and fire performance characteristics. Much of this information came from structural engineering professors and other specialists known to Shahpazov, and was fed back into the design and specification process.

“It was a long process, and there were several moments when it looked like we might not be able to use the blocks after all,” confesses the architect. “But in the end we managed to get all the necessary information, persuade everybody that it was a good idea, and even organise a training session at HG Matthews for the blockwork subcontractor, Turick, to prove that they had the necessary skills to lay the blocks on site! Luckily many of the guys on the subcontracting team are from Moldova and were familiar with and enthusiastic about earth-based construction.

While the blocks are undoubtedly fit-for-purpose, they are not warrantied, so most importantly of all the client and project funder had to be convinced. If you want to change the way projects are built, you really need to engage with and convince the big developers. To be honest the whole process nearly broke me and has cost more than it probably should have, but it’s the first time that it’s been done.”

The blocks are stacked on metal racks before being dried – rather then fired – for seven days at a temperature of 50°C

Getting the mix rightThe original intention was to use earth blocks throughout the basement, but issues over fire integrity relating to dividing walls between rooms led the team to specify them for the perimeter walls only. Given enough time and patience for testing, Shahpazov believes that the blocks would have fulfilled the original brief. The blocks are currently being used in a non-loadbearing capacity to line the inner face of the concrete retaining walls, but can be used structurally.

The earth blocks specified for The Apex have a strength of around 6.5N/mm2 – equivalent to medium-dense concrete blocks. “We experimented with three different block mixes,” explains Shahpazov. “The first was 100 per cent excavated subsoil mixed with straw. The Second was 75 per cent subsoil with 25 per cent sand and straw. And the third was 50 per cent subsoil and 50 per cent sand and straw. Following testing, the third mix was found to be the strongest and was consequently selected by the structural engineer for use on the project. In hindsight, the weakest mix would have been sufficient, as it’s still pretty strong and we are essentially using the blocks as a cladding material.”

In terms of price, the earth block are similar to traditional concrete blocks. However, they are being installed on their side, rather than on edge, so more blocks are required. “The financial saving comes from avoiding landfill in relation to site excavation, and the tax associated with this,” says the architect.

Following the drying process, any cracked blocks are broken down and reprocessed into another batch

Site installationThe 5.9-metre-high basement wall build up comprises insitu concrete, mineral wool insulation, vertical aluminium rails fixed back to the concrete substrate, then the earth blocks, which are laid in stretcher bond with horizontal metal ties connecting the courses back to the rails. At the base of the wall is a low plinth made from concrete blocks. This is designed to protect the earth blocks should the basement ever become flooded. The blocks can be laid as quickly as their concrete equivalents, although the pointing can take longer in order to achieve the desired aesthetic.

The mortar used to construct walls is made from the same material as theblocks, which is supplied wet on site. Crucially, there is no cement used in the block manufacturing or laying processes. The clay mortar is extremely strong and tough, although sand had to be added the mix to improve its workability on site. Unlike cement-based mortars, unused clay can be put back in the bucket and reused the next day, rather than thrown away when it hardens.

A fair-faced finish was specified for the walls. Once the mortar is dry, the blocks are brushed to remove loose particles of clay and dust. A layer of linseed oil will then be applied to the outer face to prevent further ‘dusting’ and provide a slightly darker finish.

The blocks are tied back to vertical aluminium rails, which are in turn fixed back to the concrete substrate

An earth block maker’s perspectiveCentral to the project is HG Matthews, an independent, family-run brickmaking business located in Bellingdon near Chesham. Partner Jim Matthews decided to evolve the business into producing more environmentally-friendly products some years ago. “We’d already taken steps to making the brick production process greener and were the first manufacturer in the country to switch to biomass for the drying process,” explains Matthews. “This saves around 30,000 litres of diesel a month. We then decided to go the whole hog and produce a natural, super-low carbon building material: the earth block.

One of the most energy-intensive parts of brickmaking is the firing process, so Matthews’ idea was to eliminate this and produce an unfired earth-based product. “There is nothing particularly new in this approach,” explains the manufacturer. “But where we have innovated is taking spoil from site, which has been dug out for basements and footings, turning it into blocks, and delivering them back to site. This stops the spoil from going to landfill, while also reducing the reliance on raw/virgin materials. The main issue relating to the overall sustainability of the process is how far the site material is transported. We would recommend a maximum distance of 30 miles to make it worthwhile.”

Loose particles of clay and dust are removed from the fair-faced walls prior to the application of linseed oil. The latter prevents further ‘dusting’ and provides a slightly darker finish.

Manufacturing the blocksIt is worth noting that all the plant used to manufacture the earth blocks is second hand and has been repurposed for the task. “It’s a bit of a Frankenstein’s monster,” laughs Matthews. “All the machines have come from different yards and we’ve restored and recommissioned them, rather than them going to scrap.” The block making process begins with a wet pan, a large industrial machine that sieves the raw site material using a series of steel plates. The newly refined spoil is then mixed with locally-grown straw. Clay or sand may also be added depending on the quality of the base material and the required design parameters of the blocks.

Sand is typically used if the blocks have a high clay content. “This helps to prevent shrinkage during drying, therefore stabilising the body of the material,” explains Matthews. “Sometimes the site material is very stony and sandy, so additional clay may needed to create the desired mix. We’ve experimented with varying ratios of sand and clay, and lab tested the blocks to ascertain their compressive strengths. We test before building and then during construction, depending on the engineer’s requirements.”

Each earth block contains only ten per cent of the embodied carbon found in a traditional concrete block

The earth mix is then loaded into a feed hopper and passes through a final set of rollers to grind down any remaining oversized deposits. From here it goes into the block making machine, which can produce 700 blocks per shift. The wooden block moulds follow a circuit: first they are sprinkled with sand to stop the blocks sticking to the inside, then they pass in front of a barrel where a bladed shaft pressurises the clay into them, and finally they are removed from the machine by hand. The blocks are frogged, which helps to ensure that the clay is dispersed to each corner of the mould when it’s thrown in.

The blocks are dried rather than fired at a temperature of 50°C for seven days in specially constructed driers containing around 4,000 units. The heat source comes from biomass boilers utilising locally-sourced renewable wood. Each earth block contains around 3.5 litres of rainwater (harvested from buildings on the site) prior to drying. Slow drying is essential in order to prevent cracking as the blocks shrink. Any cracked blocks are broken down and reprocessed into another batch. The blocks’ straw content (typically around two per cent by mass) not only acts as binder during the drying process, but also provides a form of structural reinforcement for the finished item.

Looking to the futureThe high, fair-faced earth block walls on The Apex impress with their elegant, roman-like proportions, warm colour and tactile surface. While the process from concept to realisation has been far from straightforward, the end result and the knowledge gained will hopefully enable other forward-thinking project teams incorporate this low-carbon technology into their buildings. “Building with earth blocks should be common practice throughout London and even beyond where there is significant site excavation,” concludes Shahpazov. “The process uses a tenth of the C02 of traditional practices so it’s a no brainer.”