HORTUS, a pioneering project for sustainable construction

Hortus office building in timber construction with green surroundings

A construction project for an office building in Allschwil near Basel is taking on a pioneering role in sustainable construction. HORTUS thinks beyond ecology. The modern office building will offset the grey energy used for its construction within one generation. How will it achieve this? As a supplier of solar energy from the roof and from the facade.

But sustainability at HORTUS goes much further than that: the building is setting new standards by using natural materials from the region. The guiding principle of ‘love for the local area’ gave rise to the holistic approach and the obvious idea of using building materials sourced directly from the local area: earth and wood.

«When we intervene in a location, we leave behind at least as much value as the location had before our intervention.»

Johannes Senn, client and CEO of the SENN Group

Sustainability on all levels

Sustainability was a central theme in the project’s development. This was why it was essential to source the wood used locally. Under the leadership of Blumer Lehmann, the forest from which each piece of wood came was also clearly depicted and documented. For the total of 3,000 m³ of wood required, this meant ensuring availability with 5 sawmills and 30 forestry districts well in advance. In addition to the procurement of construction materials, the principles of the circular economy, cradle to cradle and second life were also major considerations. The components can thus be dismantled at the end of their service life, re-used or returned to the forest and the earth. Sustainability at the origin also means using as few processed materials as possible, such as solid timber and only minimally glued laminated timber.

«We have to be able to give everything a lasting and distinctive beauty. It's really a challenge, but it's also a delightful idea. Because it creates a new aesthetic in the world.»

Jacques Herzog, Architect, Herzog & de Meuron

The mock-up

The special thing about HORTUS was that the processes had to be changed. Herzog & de Meuron and ZPF initially focussed on the environmental aspects of the materials. Taking the material as the starting point, the next step was to develop the construction and architecture. Blumer Lehmann’s mock-up was also able to provide answers to questions about the choice of materials and construction, the architectural details and production processes.

The tinkerers and the makers

In collaboration with the project teams from Herzog & de Meuron and ZPF Ingenieure, the client SENN developed the foundations for the climate-friendly office building with around 12,000 m2 of usable space. Blumer Lehmann prepared the timber construction for execution and worked with Lehm Ton Erde on the clay construction. A dedicated production facility was set up on site for the project. This was used to fill the ceiling elements with the local excavations and process them into rammed earth.

Detailed view of a clay-wood composite ceiling in the HORTUS building

Wood-clay composite elements from the field factory

It doesn’t get more local than using excavation material from the construction site for the wood-clay composite elements specially developed for the slabs. The elements were manufactured from wood at our factory in Gossau. They were then filled and rammed with excavation material right next to the construction site in a tent serving as the field factory, which Blumer Lehmann developed together with Lehm Ton Erde, its earth construction partner.

The rammed earth formulation developed by Lehm Ton Erde consists of 76% local excavation material and 24% local marl. The mixture was made from the sieved and broken-up components. Specially developed mini feeders decanted the clay mixture into the prefabricated timber elements. The mixture was compacted into rammed earth using rammers and plate compacters.

Office space with networking and a healthy indoor climate

HORTUS should be ready for use in 2025. The office space on offer is primarily targeted at digital and technology companies. These companies should benefit from mutual exchange and proximity to the life sciences sector and the Switzerland Innovation Park Basel Area. The rental concept of the office building, with the advantages of networking and a healthy working environment, provides for the office space to be divided up and furnished by the future tenants themselves to suit their needs. Fully equipped communal areas are also planned.