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Hybride photogrammetric and geodetic surveillance of historical buildings for an urban tunnel construction Thomas Luhmann & Werner Tecklenburg Institute for Applied Photogrammetry and Geoinformatics University of Applied Science, Oldenburg, Germany E-mail: luhmann@fh-oldenburg.de Abstract The paper reports on photogrammetric and geodetic surveying for control measurements of historical buildings that are located in the neighbourhood of a large tunnel construction in the city of Bremen, Germany. The tunnel is about 800m long and 30m wide, and it is currently built by slotted wall method. Due to the ditch depth of about 20m it is likely that adjacent buildings might get significant deformations or damages which then have to be compensated financially. In this situation a three-dimensional object survey has to be performed that includes image documentation of damages. A total of 10 buildings had to be observed. The required object accuracy of ±1mm has been successfully achieved. Introduction In the 1980ies plans have been initiated for a large inner-urban street tunnel in the city of Bremen, Germany. The designed "Hemelingen Tunnel" was mainly planned as a connection between the freeway and the car factory of Mercedes-Benz, and to ease the traffic load within parts of the city. The location route mainly passes older residential areas in need for redevelopment, and some industrial zones as well (total length of location route approx. 2.3km). The tunnel is currently built in an open ditch, i.e. without underground work. However, some adjacent building have to be pulled down in order to dig the ditch. After finishing of the tunnel construction work the area above ground will be rebuilt as redeveloped residential and recreation areas.
The pure tunnel is about 800m long and 30m wide. In the neighbourhood of the construction site a number of older buildings exist, mainly built between 1900 and 1930. One of the buildings is the silver tool factory of Bremen listed for preservation, another one is a church. In some cases the distance between buildings and sheet pile wall is less than 1m. The condition of the buildings is very different, it varies from houses to be refurbished to recently renovated houses. An approximately 20m deep ditch has been dig for the construction of the tunnel (Figure 1, Figure 2). Due to possible ground movements and changes of the ground water table in the near surrounding of the tunnel, subsequent movements of the buildings can be expected that could lead to significant damages. During the initial project stage a financial volume of several million DM has been projected for possible renovation costs. In order to avoid claims for damages without good reasons, the project leader has placed orders for extensive evidence control measurements. These should document the existing damages on buildings before any construction work has been started. This includes all visible damages on interior and exterior parts of the buildings, e.g. setting breaks or damages bricks of the walls. Each damage is then measured by special gauges or tools, and documented by photographs (example in Figure 3). This work has been carried out by legally authorised experts. 
Figure 3: Documentation of existing damages One part of the evidence documentation dealt with the three-dimensional measurement of building geometries. In contrast to usual setting measurements performed by precise leveling only, one has also expected movements of the buildings that might occur not only in vertical direction. Investigations of the soil mechanics let assume that, in principle, the buildings might set non-regularly, thus leading to non-linear deformations. For this reason photogrammetric measurements have been carried out in addition to geodetic measurements, that will also be used as pictorial documents of the as-built status. In the past few years the IAPG has gathered experiences in the photogrammetric recording of large objects (Luhmann & Tecklenburg, 1997). The special technical problem of this project can be addressed by the location and inaccessibility of the buildings, and the definition of stable reference points used as a certain reference for future measuring epochs. In order to find a solution a combination of photogrammetric and geodetic measurements has been carried out, that will be discussed in the following paragraphs. On-Site Measurement Specifications The total project consists of 10 buildings in a distance of about 50m besides the tunnel construction site. For each building those parts of the façades have to be measured that face the tunnel, i.e. more or less all front sides of the buildings. The total area of façades amounts to ca. 9800m2. The façades have been prepared by signalised targets (Figure 4), that had to ensure a high reproducability in their position for future measurements. According to civil engineer experts the point density has been defined to ca. 4m, thus a total number of 250 reference points had to be measured for all buildings (overview in Table 1). The object point accuracy has been specified to 1mm in all co-ordinate axes. Table 1: Project data
 Figure 4: Point targeting
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