Königsteiner Höfe is the location of an innovative residential and commercial quarter which is being built in the middle of the Hessian town of Königstein. The 9,000m² site offers enough space for a total of eight buildings with around 75 apartments with 7,000m² of living space as well as other facilities. The general contractor Köster and the subcontractor Höfling Erdbau have started the first phase of this ambitious project which has to be completed by June 2024. This phase involves excavating the site and installing all the required ground support.

Bored pile wall 

After the ground breaking ceremony at the end of December 2022, the 65,000m³ construction site was first excavated to a depth of 15m on the slope side to a depth of 7m on the valley side, and the shoring was erected as a contiguous bored pile wall. By mid-February 2023, all 186 bored piles surrounding the excavation pit were completed. The piles then had to be anchored and lined by filling the space between each pile with steel mesh and shotcrete. To keep costs down, the shoring wall had to be profiled back beforehand with the surface being as flat as possible. 

Achieving the surface finish required would have been impossible in the difficult quartzite ground conditions using an excavator with breaker or ripper. For this reason, Daniel Korn, site manager at the contracting company Gerhard Höfling GmbH, chose a different excavator attachment, namely a rotary drum cutter. After initial trials with a different brand that did not yield the desired results, Enrico Trender, Kemroc’s sales manager, recommended the combination of a 25t excavator and a KR 120 (120kW) rotary drum cutter to complete this task.

Kemroc’s ‘KR’ range comprises of 19 transverse cutters with spur gears. They are designed to be robust and wear resistant and are said to be the ideal attachments for carriers with short booms in confined spaces. This feature is said to be advantageous in demolition and tunnelling as well as in trenching and pipeline work, concrete renovation, quarrying of soft rocks, underwater and profiling work. Daniel Korn from Höfling rented a KR 120 drum cutter for use on the company's own 25t excavator for the profiling work. With this combination, it was possible to mill the ground containing quartzite from between the bored piles with a smooth surface finish and line it with shotcrete.

Drainage trenches all around

Subsequently the contractor had to excavate a drainage trench 50–60cm deep and 50cm wide inside and at the foot of the shored wall for its entire length. After the experience of using milling equipment in these ground conditions, Daniel Korn decided to carry out this work using the combination of a 9t short tail excavator and a Kemroc EK 40 chain cutter (44kW). 

The ‘EK’ range of chain cutters are equipped with what is said to be a unique cutter chain running between the outer cutter drums. This is designed to loosen the material along the entire width of the cutter head without leaving a central spur as would be the case when using a conventional drum cutter without any sideways movement. In this way, it is possible to excavate trenches with a precisely defined width. Kemroc states that this operating characteristic saves time and up to 40% energy, is gentle on excavators and produces fine grained milled material that can often be used immediately as backfill.

On the Königsteiner Höfe construction site, however, the milled material was transported away for recycling. Backfill was not required because the sloping drainage trench was to be lined with a fleece material and filled with special drainage gravel so that when it rained, the water collected would flow down to the pump shafts. In retrospect, site manager Daniel Korn considered that completing the work with the equipment rented as a complete success: “In view of the difficult soil conditions with the presence of quartzite prevailing on our construction site, the choice of equipment was almost perfect for this task.”