Soil, water, excavation: things worth knowing about the initial construction phase

According to the plans of the architecture firm Maki & Associates, a basically rectangular new building is planned with dimensions of approx. 65 metres along Rheinstrasse and approx. 45 metres along Wilhelmstrasse. The requirement here was to incorporate the historic entrance building of the Hotel Victoria. The building will have a full basement and be constructed on two floors above ground floor level. At its deepest point, the excavation will be about 10 metres deep. The built-up area adds up to 3,095 square-metres.

The soil
Beneath the artificial fillings that are approx. five metres thick (which may contain remnants of the former Hotel Victoria at this point) quaternary strata can be found in the Wiesbaden metropolitan area. This normally consists of loess loam and loess or flood plain clay over gravel and sands and they are underlaid by tertiary strata over large areas. These layers consist here of dark-coloured, silty marl clays, locally occurring limestone banks and intercalations of fossil sands, partly with embedded limestone banks. Due to archaeological issues, the underground construction work will be accompanied by archaeologists but an unexploded ordnance unit will also be on site.

The water
The site on which the Reinhard Ernst Museum is to be built is located in a state-approved spa protection area. It is about 500 metres away from the Wiesbaden thermal springs and around 750 metres from the Faulbrunnen fountain.
Depending on the season, the first aquifer that rises in the construction area (understood as a rock formation with cavities capable of conducting groundwater) shows a varied amount of flow, as it is mainly fed by precipitation. The main aquifer, referred to as the second aquifer, is located in the tertiary strata. Due to the alternation between groundwater-damming and groundwater-permeable strata, the ground water in inner-city areas is usually described as confined; this means the groundwater pressure level is higher in a water-conducting layer than the upper limit of this layer.

The construction pit
As the museum building will be embedded in the foundation and the geometric conditions of the construction site – which includes the distance to the roads and the depth of development into the earth – a waterproof, vertical construction pit enclosure is required. As such, either a sheet pile wall or an overlapping bored pile wall is used. In keeping with the cost-effective dimensions of the steel sheet piles and to reduce wall head deformations, temporary bolts are used. At various depths, the excavation work is interrupted and armature bores are carried out with flat inclinations. Steel cord bundles are inserted in these bore holes and the end sections are grouted with cement mortar over a defined length. This results in a cylindrical concrete grout. Forces can be introduced via this into deeper soil layers through the strands and an anchor head with a thread system – like with a screw. By implementing a waterproof pit enclosure, an impermeable trough is created; only the groundwater trapped in this trough then has to be removed from the construction pit. In addition, what is known as groundwater relief of the next lower aquifer is required. For this purpose, small wells are drilled in and around the construction pit, via which it is possible to lower the groundwater pressure level. After completing the excavation phase, a bed of gravel is laid, on which the reinforced concrete floor slab of the new museum building can be placed later on.