Hortus: Big-scale Earth and Timber

From Excavation to slab: Hortus, Timber and Earth Proven at Industrial Scale

In March 2024, silence returned to the field factory in Allschwil: after eight months of production, the small tent city was dismantled. A total of 810 prefabricated timber slab elements had been filled and compacted with local excavated soil before being assembled just 200 metres down the road. This achievement was only possible thanks to the courage, collaboration, and creativity of everyone involved. The tireless work of the dedicated teams from Blumer Lehmann and Lehm Ton Erde was supported by highly specialized machines developed specifically for the project. One week after production ended, the topping-out tree was placed on the gable, marking the completion of the shell of this innovative office building. The flagship project was inaugurated in June 2025.

The insights gained during the process (together with the decades of experience of the participating companies Lehm Ton Erde, gbd, and Blumer Lehmann) today form the foundation of Lehmit. We look back on an intense, challenging, and inspiring project.

Preparation and Development

When the contract for the project was awarded, only three months remained to design the field factory and develop the necessary machines. With creativity and openness toward the new team and material combination, the production concept was worked out in several workshops. In collaboration with Bertram Hartmann, a one-man mechanical engineering company from Liechtenstein, miniature feeders for filling the elements were developed and built. To avoid losing time, work proceeded in parallel: the mixing area was prepared, four tents were erected and fitted out, a turning gantry was developed, and the production team was scouted: a team bold enough to take on a type of production that had never been attempted before.

From Excavation to Building Material

While the field factory was being set up, excavation had already begun on the construction site. Beneath a thin layer of topsoil lay three soil types: clay layer, clayey gravel, and sandy gravel, excavated in successive ten-centimetre layers, then screened and crushed. In the proper mix ratio, defined and tested in advance by Lehm Ton Erde, these materials supplied 75% of the rammed earth mix, supplemented with regional marl. In Central Europe, more than 50% of excavated soil could usually be reused for rammed earth.

Industrial Production in the Field Factory

The timber frame elements were prefabricated from local wood at Blumer Lehmann’s workshop and transported to the field factory in Allschwil. There, the chambers of the frame elements were filled with the prepared raw material by machine and compacted into rammed earth using hand rammers, electric rammers, and vibrating plates. Immediately after compaction, the filled elements were turned and stored in a buffertent until they were installed on site. There they completed their drying process within the shell. This approach eliminated the need to wait for drying times and enabled production to supply the assembly just in time.

From Tent to Building

To transport the elements, each weighing around 3.5 tons, a tractor with a special trailer was used to be able to transport four elements per trip, covering the 200 metres to the construction site. There, the assembly team positioned the precisely fitting elements with a crane and screwed them together into a load-bearing slab structure. On peak days, up to 36 elements could be installed. The buffering-tent provided production with the necessary lead time to ensure a steady supply of elements to the site, even on such high-output days.

In six sections, like cake pieces to provide constant protection from the weather, Hortus grew upward. Floor by floor each section was installed, until one week after the last element was set, the topping-out tree was placed on the ridge.

Efficient Use of Materials: Building Resource Instead of Waste

A total of 3,000 tons of earthen material were processed for Hortus. One hundred percent of this soil avoided ending up as landfill waste and was instead transformed into the intelligent mass of rammed earth. This mass provides thermal inertia, regulates humidity, and contributes to fire safety and acoustics for the office spaces. By using excavated soil, timber consumption was reduced, since the earth protects the timber from fire exposure and eliminates the need to overdimension the wood sections for fire resistance.

Timber Construction + Earth: A Familiar Process in a New Rhythm

For Blumer Lehmann, Hortus was largely a conventional timber construction project. Apart from the massive solid spruce beams, the production processes, CNC machining, and logistics followed standard practice. The only addition was the rammed earth production in the field factory, which was slotted in between prefabrication at the workshop and on-site assembly. 

This decoupling proved successful: combining rammed earth with established timber construction techniques enables new scalability and efficiency in earth building. Rammed earth benefits from the tried-and-tested assembly and connection methods of timber construction, and drying times no longer need to be awaited. Timber construction, in turn, gains low-carbon mass and is protected from fire by the earth, without disrupting the usual on-site workflow.

Outlook

With Hortus, an industrially manufactured timber–earth slab system was realized at this scale for the first time. The machines, quality processes, and expertise developed in the process are now an integral part of Lehmit. At Lehmit, we look forward to future projects where we can contribute our knowledge and collaborate in creating innovative buildings in earth and timber.