Much has been written about the effects of concrete and cement on the climate. The sand-lime brick required for cement production is burned at 1,450°C to produce cement clinker. One third of the CO2 released in the process is fuel-related. Two-thirds of it is process-related, because CO2 is separated during the calcination of the limestone into quicklime. "While the fuel-related CO2 emissions can be reduced, for example, through higher biomass proportions in the fuels, the process-related CO2 emissions of clinker production cannot be reduced with currently available technologies," states the "CO2 roadmap for the German cement industry" published in 2020 by the German Cement Works Association (VDZ).
It is therefore up to the construction industry and its clients to use concrete more sparingly. While cradle-to-cradle approaches and resource-conserving construction are high on the agenda in building construction, the potential for "concrete-efficient" special civil engineering is naturally often hidden. Stefan Lechelmair is head of the Stump-Franki branch in Stuttgart and is familiar with all aspects of foundation engineering: "The CO2 footprint of excavations and foundations depends mainly on the materials used. As a technology leader, we face up to the responsibility of developing sustainable and resource-saving solutions. One focus of the research work at Stump-Franki is therefore fully biological building materials such as our environmentally neutral biosoftening gel for which building authority approval has already been granted."
Sustainable special civil engineering for a sustainable urban quarter
On the site of the former freight station in Bad Cannstatt, the new Stuttgart district Neckarpark is being built on 22 hectares. As a "model project for sustainable urban development", the mixed quarter for living, working and shopping will provide space for 2,000 people. Groundwater-protecting construction methods were mandatory, because mineral and medicinal waters flow beneath the area and must be protected from harmful environmental influences at all costs. Bad Cannstatt has the second largest mineral water deposit in Europe after Budapest, including a majority of state-recognised healing springs. Since 2002, they have been protected by a special ordinance.
With green roofs, clinker brick facades and inner courtyards, the building on construction site Q8 also follows the neighbourhood motto "urban, liveable and sustainable". In two 5- and 6-storey building sections connected by transparent bridges, it offers a total of 18,000 m2 of office, service and exhibition space. Due to the high level of mineral water, the underground car park was realised with only one level. For the project, Stump-Franki produced a Franki pile foundation with minimised embedment lengths and an environmentally compatible soft gel base.
The manufacture of a Frankipfahl NG®.
In 1909, the Belgian engineer Edgard Frankignoul applied for a patent for the Franki pile. The enlarged pile base brought a decisive advantage: the load-bearing capacity is considerably higher than that of other pile systems with comparable dimensions. To increase the load-bearing capacity of the soil, additional compaction can be carried out - as in the case of the Q8 project - without using concrete by tamping out gravel in a defined area below and above the pile settlement depth.
The Franki pile is a cast-in-place concrete driven pile with a reclaimed driving pipe, which is sealed watertight at the lower end with a plug of dry concrete or gravel sand. Inside the pipe, a free-fall ram strikes this driving pad, driving the pipe into the ground. Once the target depth is reached, the plug is loosened and the required amount of foot concrete is tamped out. In this state, the pipe is sealed by the surrounding footing material so that no water can enter. In the next work step, the reinforcement cage is inserted, the fresh concrete is poured in and the jacking pipe is pulled up again. Finally, the pile head is capped and the connecting reinforcement is produced.
In the classic Franki pile, the shaft is also made with tamped concrete. The optimised new-generation Franki pile, developed in the 1980s, differs in two essential process steps: the shaft is made with flowable concrete and the required volume for the expanded pile foot is determined as a function of the subsoil strength and the pile load to be supported with the help of footing design curves, which enables optimal adaptation to the subsoil. Franki piles displace the soil during driving, so there is no need to transport material away by lorry and dispose of it. In addition to the economic aspect, this is also a significant contribution to sustainability.
Use of the Frankipfahl NG® saved 650 t CO2 equivalent.
The top ground level of construction site Q8 was around +220 mNN. Between + 212.87 and +211.77 mNN, there is a protective layer of gypsum rock above the water-bearing strata, from which a distance of 0.5 m had to be maintained. The challenge for foundation work was to transfer the loads into the load-bearing soil layers of Quaternary and Neckar gravel with the shortest possible piles. Due to the widening of the footing, the total of 343 Frankipfähle NG® piles could be produced shorter than alternative large-bore piles while transferring the same loads, so that the sealing effect of the Keuper layer was maintained.
Two different types of Frankipfähle NG® with diameters of 51 and 61 cm and an average pile length of 3.65 m were installed at a depth of 217.65 mNN. This meant an average of just under 2 m3 of concrete per pile and, in the final result, a total quantity of 697 m3. In comparison, the possible alternative with 537 large-diameter bored piles with diameters of 120 cm would have resulted in a total of 2,134 m3 of concrete, whereby twice the amount of concrete would have flowed into each pile.
Considerable differences also emerge when considering CO2 emissions. The use of large bored piles would have caused around 650 t CO2 equivalents more. If you consider that a beech tree needs about 80 years to sequester one tonne of CO2, it becomes clear that every measure to reduce CO2 helps.
Soft gel sole with optimal CO2 footprint
To prevent hydraulic ground failure and keep the excavation pit dry, it had to be protected against vertically rising groundwater with a buoyancy-proof sealing base. Instead of a DSV base, Stump-Franki recommended a 0.5 m thick soft gel injection base. The low-emission method does not require cement to be injected into the ground and reduces the amount of lorry transport required. In addition, there is no return slurry.
A total of 608 injection holes were made for the installation of the injection lances at a fixed distance from each other. The soft gel was then injected into the pore spaces of the foundation soil to seal the soil horizontally against penetrating water. Instead of enclosing the entire excavation pit vertically, temporary sheet pile boxes were installed around each of the 57 individual foundations.
The silicate gels used, "Stump-Silitight 69", are approved by the building authorities. They are produced in a two-component process from 1/5 sodium silicate, 1/50 hardener and 3/4 water, whereby the exact composition depends on the soil conditions. Compared to a DSV base, the soft gel base avoided the input of 458 t of cement into the soil. If one compares the total material consumption of both methods, there is a saving of 250 t CO2 equivalents. In total, 900 t of CO2 were saved at Q8 through the use of environmentally friendly processes. This shows how important it is to incorporate sustainability as early as the planning stage.
