Final evaluation and restoration of the garden
(as of 29 August 2011)

Right after the initial assessment of the situation, the tunnel construction experts unanimously agreed that this subsidence does not pose any further risks to local residents. This has been confirmed after a thorough examination. The surface turns out to be stable. The experts involved speak of an atypical incident, for which there had been no prior indications based on the knowledge of the geological formation and on the analysis of existing tunneling protocols. Together with the tunneling experts of the participating construction consortium and the builder, a number of external experts worked intensively on elucidating the cause of the subsidence. They unanimously found that there are no voids in this place above the tunnel, and that the soil—which is made of sand—is very loosely packed in a narrow area beneath the hole. These particular soil conditions have a limited extension and start at a depth of about 12 metres below ground level. The tunnel excavation apparently triggered local ground movements, which led to the subsidence around four weeks after the tunnel boring machine had been working under that spot.
According to the engineers involved in the study, the results show a generally stable ground. Nevertheless, all parties have agreed to consolidate the region of loose sandy soil. This is done with so-called flushing lances, which have a diameter of about 6 cm and work from the bottom upwards. By slowly pulling the lance, the sand is washed down by water pressure, causing it to consolidate in the depths. The ground potentially lacking at the surface it then refilled. The result will be checked after a certain time in a depth sounding investigation.

First reaction (as of 13 July 2011)

Within an hour after notification, DESY's leading tunnel construction engineer was on site to analyse the situation and provide for the necessary first measures. The area was cordoned off and surveyed, and the hole filled. Until further notice, it is being inspected twice a day and measured. Since the occurrence of the subsidence, the tunnel construction engineers of the consortium ARGE Tunnel XFEL, DESY engineers and other experts have been working intensively on the elucidation of its nature and the search for its causes. Among others, they use special tools to carry out depth sounding of the soil (see "Depth sounding measures" and Photos). In addition, an independent geotechnical expert was consulted.
Once the investigations are completed and evaluated, a concept for the restoration of the garden will be developed, which may also include a special consolidation of the subsoil.
The ground above the tunnel is being routinely surveyed by the ARGE Tunnel XFEL consortium for possible subsidence. The time interval between the measurements depends on the location of the tunnel boring machine. The survey is done in a tight grid which also includes buildings, with the fixed measurement points located – with a few exceptions – in public areas (streets, sidewalks, etc.). Until further notice, the hole is now being surveyed twice a day. In addition and as a precaution, the landed properties located above the tunnel are being inspected in a wide area in front of and behind the hole, with special emphasis on the green areas.

Depth sounding measures

Since Monday, 4 July 2011, the nature of the subsidence and its surroundings have been analysed using various depth sounding methods, such as light and heavy dynamic probing and cone penetration testing. In addition, a magnetometer is used to detect metallic materials.
Light dynamic probing (DPL) involves driving a solid cone with a cross section of 5 cm² into the soil in several steps from the same drop hight. The number of strokes required to drive the cone to a determined depth is recorded. This number of strokes allows conclusions to be drawn on the density of the soil.
In heavy dynamic probing (DPH), a probe with a larger cross section (15 cm²) is used. As with DPL, the number of strokes required gives indications on the soil conditions (compactness). Compared to DPL, the DPH method enables deeper soil layers to be examined.
In cone penetration testing (CPT), an instrumented probe with a conical tip is pushed into the soil at a constant rate (usually 2 cm/s) using a rod system. The CPT instrument reports tip and sleeve friction along the probe. The tip friction allows conclusions to be drawn on the density of the soil, and the ratio of sleeve and tip friction gives indications as to soil properties.
A magnetometer is used to measure magnetic flux densities. One of its classical applications is measuring the earth’s magnetic field and its variations (geomagnetics). Local deviations of the magnetic field on the earth’s surface indicate the presence of ferromagnetic materials in the underground.
Smallest fluctuations of the Earth’s gravitational field can be measured using a microgravimeter. In a complex process, geophysicists can then extract information on the density distribution in the subsurface of the probed area.

Does the construction process present a risk for the residents and their houses?

The experts say that this tunnel construction method poses no danger for the residents. This is the reason why we have agreed to this construction method in the first place. On the other hand, no construction process and no building project is ever one hundred percent safe. Both the engineers involved in this tunnel construction and the external and independent experts we consulted are convinced that this subsidence and the one that occurred in November 2010 in a horse pasture are atypical. They still agree that another such subsidence is extremely unlikely.

To what extent have the neighbours been informed about the tunnel construction and possible risks?

Since the tunnel route was fixed in the plan approval process, that is since 2005, not only the owners and residents whose land is affected by the tunnel route, but also a broader community of neighbours along the tunnel route have been regularly informed about the project and the construction process. In this context, the great safety of the tunnel boring process was always pointed out, but also that no construction methods can be one hundred percent safe.
Key measures of neighbourhood work are the establishment of a neighbourhood office, which can be contacted by phone even outside office hours, public information events, newsletters and personal visits during which questions can be asked and answered directly.
In addition, an information point on tunnel construction was established at the shopping centre Bornheide from 21 February to 7 April 2011 and at the care centre Lupine from 9 May to 24 June 2011, where affected neighbours could inform themselves personally about the tunnel work.
In addition, notices are placed in appropriate places and press releases issued to local media. In addition, the neighborhood office distributed an information package to all affected households. This package included a DVD with the movie "The construction of the new X-ray laser tunnel under your residential area", flyers on the tunnel construction and the European XFEL project, as well as the business card of the neighborhood office.