Earth blocks made from excavated clay: The Apex, King's Cross

How subsoil dug out of a London basement became unfired earth blocks at our family brickworks and went back to line the same basement walls.

Photo: Russ Avery

At a glance

The problem

Tribeca is a large life sciences development on the site of a former Royal Mail sorting office, between St Pancras Way and the Regent's Canal. The Apex, a seven-storey building with a basement, was the first of its buildings to be built, and Reef Group used it to try out ideas for the buildings that followed. One of those ideas was replacing concrete blocks with our unfired earth blocks, called STROCKS™ to cut upfront carbon.

A basement on that scale means a great deal of excavated subsoil. On most London jobs it goes to landfill, and landfill tax is paid on it. The basement walls would normally have been lined with concrete blocks. The design team wanted to know whether one could replace the other, and that is where we came in.

The idea

The idea came from Nikolay Shahpazov, the project architect at Bennetts Associates. He had come across our earth blocks during lockdown visiting a housing scheme in Walthamstow where site material had been used to make Site STROCKSTM and once he was project architect for Tribeca, raised it with VolkerFitzpatrick, who were piling on site at the time. Fresh soil from the site came to us, and we made sample blocks with Rowland Keable of Rammed Earth Consulting for the client to see.

As Nikolay put it at the time: "The clay, sand and silt remain in place. […] Here, we are storing resources for future use, which complements the circular economy."

 Locally grown straw from our family farm is added to the soil to create STROCKS™. Photo: Russ Avery

Testing

The Tribeca subsoil had more silt in it than the clay we use for bricks, but it was good enough for unfired blocks. We made three mixes, each with straw: 100% site subsoil, 75% subsoil with 25% sand, and a 50:50 mix of subsoil and sand. Tested, they reached about 2.8, 4 and 6.5 N/mm², respectively. The structural engineer chose the strongest, the 50:50 mix, which is roughly equivalent to a medium-dense concrete block.

The laboratory results were good enough for the team to swap concrete blocks for ours in the basement. The original plan was to use them throughout the basement, but questions about fire integrity on the dividing walls between rooms meant they were specified for the perimeter walls only. The blocks have since passed a fire test, and Reef Group said it intended to use them on the second phase, including internal basement walls.

Making

Approximately 1,000 tonnes of excavated subsoil came from King's Cross to our family brickworks 35 miles away at Bellingdon, near Chesham, and went back to site as finished blocks.

We start by putting the raw soil through a wet pan, which uses heavy rollers to crush any aggregate through slots in steel plates. We then mix it with locally grown straw from our family farm, at around 2% by mass, adding sand or clay as the mix requires. The prepared mix then goes to our block making machine, which presses about 700 blocks a shift into sanded wooden moulds.

We then dry the blocks rather than fire them, at 50°C for four to five days in driers that hold around 4,000 blocks each. The heat comes from our biomass boilers, which run on locally sourced wood. Any block that cracks is broken down and goes back into the next batch. All of our plant is second hand: machines from other yards that we have restored and recommissioned rather than see them scrapped.

STROCKS™ being dried on racks with heat from our biomass boilers. Photo: Russ Avery

Building

Working from the outside in, each basement wall starts with the in-situ concrete retaining wall, then a layer of mineral wool insulation and a set of upright aluminium rails. Our blocks are built in front of the rails in stretcher bond (laid lengthways, with each course offset by half a block), and metal ties at each course hold the blockwork to the rails. Our blocks line the inside of the concrete retaining walls and carry no structural load.

The bottom two courses, and the areas around service penetrations, are conventional blockwork, to keep the earth blocks clear of any damp or flooding. The mortar uses the same material as the blocks, with sand added for workability. There is no cement in the blocks or the mortar, and any mortar left over at the end of the day goes back in the bucket for the next morning.

We welcomed Turick's blocklayers to the brickworks for a training session, so they could show they were ready to lay the blocks. Many of the team were from Moldova, where earth building is common, and they found the blocks easy to lay. The blocks went up as quickly as concrete blocks, though pointing took longer to get the finish right. The walls were left fair-faced, brushed down once the mortar was dry, and finished with linseed oil to stop dusting.

Results

Our project partners have published the following results.

• VolkerFitzpatrick reports that using earth blocks for the basement perimeter walls "resulted in an 84% reduction in carbon emissions and saved 93 cubic metres of waste compared to traditional materials".

• Building reports that making the blocks produces a tenth of the carbon emissions of making concrete blocks. That figure covers manufacture only, not transport, construction or end of life.

• Jason Russell of Reef Group told Building that using the blocks on The Apex saved 10 tonnes of carbon.

• On price, the blocks were reported as similar to concrete blocks, but they were laid on their side, so more were needed. The savings came from not sending the subsoil to landfill and avoiding the tax on it.

• VolkerFitzpatrick says The Apex received a Gold Award in May 2024 for using blocks made from the site's excavated clay in the basement perimeter walls.

What we learned

Being first means building the evidence as you go. Alongside our own testing, the design team brought in academic engineers and other experts to show how the blocks would perform, both structurally and in a fire. That body of evidence is now there for the next project to build on.

On a commercial job, the client and the funder need to be confident in a new material. At The Apex, the test results gave them that confidence, and Reef Group went on to plan wider use of the blocks in the next phase.

We also learned that site soil can go further than you might expect. The team chose the strongest mix to be safe, but the testing showed that even the 100% site subsoil mix would have been strong enough for a non-loadbearing lining. On future projects we can match the mix to the application, which means using more of the site's own soil and less bought-in sand.

Distance matters too. The carbon case depends on how far the soil and blocks have to travel, and on very large schemes it may make sense to make the blocks on site instead.

“Using on-site material is a tried and tested approach and there is no reason for it to be viewed as inherently risky. It is sensible and practical, in terms of cost savings, performance, resource utilisation and waste avoidance.”

Jim Matthews, Managing Director, H.G. Matthews

Our STROCKS™ have extremely low embodied energy and can be used for a wide variety of applications, from internal load-bearing walls to structural base-to-earth floors. Photo: Russ Avery

What this means for your project

We think blocks made from your own site soil are worth looking at if your project has:

• a large basement or excavation, with clay-rich subsoil that would otherwise go to landfill
• walls where an unfired block suits the job, such as basement linings and internal walls kept clear of standing water
• enough time in the programme for samples and testing before the blockwork package is let (?)
• a site within reasonable haulage distance of Bellingdon, or the space and volume to consider making blocks on site

Not every soil will work, and we will say so if yours won't. For an honest first answer we need the site investigation report, a rough idea of the wall area or block quantity, the site location and your programme.

If you are assessing carbon, our standard STROCKS™ have a verified Environmental Product Declaration (NEPD-10568), issued in November 2025 and valid to November 2030. It gives a cradle-to-gate (A1–A3) GWP-total of −15.4 kg CO₂e per tonne, made up of +48.3 kg fossil carbon and −65.0 kg biogenic carbon stored in the straw, plus a small land-use figure. The end-of-life modules (C1–C4) add emissions back; we will send you the full table. The EPD covers STROCKS™ made from our own clay, not the Apex blocks made from Tribeca subsoil.

Get in touch

Have a basement or major excavation coming up? Send us the site investigation report and we will tell you honestly whether your clay could become blocks. You can also find our STROCKS™ EPD here.

Contact us today:

Phone: 01494 758 212 

Email: info@hgmatthews.com

Sources

This blog post draws on our own knowledge of the project and on these published articles:

• Architecture Today, “Earth works” (April 2023)
• Building, “The Tribeca brick: Made from earth and water but (crucially) no fire” (August 2024)
• VolkerFitzpatrick, “Apex Tribeca” project page