GeoExpert ag - Seismic Investigastions

Geotechnical engineering: Assessment of a ground failure

Objectives of seismic survey:
A spontaneous road pavement rupture occurred in a residential area of the town of Altkirch (France) in November 2002. Beneath the pavement a 3 m deep and 2 m x 3 m wide cavity resulting from a ground failure and extending into the adjoining private property called for timely action to assess the cause. The Altkirch region is known to straddle an important fault system associated with the Upper Rhinegraben. The ground failure originated in Tertiary marlstone whose lower limit to the underlying limestone bedrock is unknown. A high-resolution hybrid seismic survey (plan - 70 kb), i.e. a combination of diving wave tomography and reflection seismic profiling, was carried out in order to image the geological structures and to assess the geometry of decompaction zones linked to the assumed fault system. Based on the results of the seismic survey, immediate action for geotechnical remediation were to be initiated.

Einbruch im Belag des Trottoirs

Fig. 1: Pavement rupture caused by the ground failure in the side walk of the street.

Der Einbruch erstreckt sich unter der Mauer

Fig. 2: The cavity produced extends beneath side wall into the adjoint private property.

Data acquisition parameters:

Lenght of profile	110 m (profile 1), 50 m (profile 2), 25 m (profile 3), 100 m (profile 4), 60 m (profile 5), 60 m (profile 6)
Receiver station spacing	0.5 (profile 1, 2, 4, 5 e 6) & 0.25 m (profile 2)
Source point distance	1 m
Number of recording channels	96 - 192
Source type	Hammer 8 kg
Recording time	250 ms
Date recorded	November - December 2002

Geophonauslage
Fig. 3 : Recording spread with 192 geophone stations spaced at 0.5 m along the street in close proximity to the ground failure.

Results:
The diving wave tomography evaluation (Fig. 4a) of the N-S profile crossing the street in close distance to the hole in the pavement cleary shows a funnel type subvertical vent filled with very loose material. It extends to a depth of at least 35 m where its width is an estimated 5 m. The vent is linked to a N-dipping fault with a vertical throw of some 10 m, as can be seen on the hybrid seismic section (Fig. 4b). On the seismic profile parallel to the W-E oriented fault (Fig. 5a & Fig. 5b) there is evidence that a) the axis of the channel hosting the vent dips 10° eastward, and that b) two ancillary faults intersect the main fault. Also to be noted are c) two reflectors being possible candidates for the top of the limestone bedrock. More detailed information is available only by bore hole(s) to be drilled for verification and calibration of the seismic data. The hybrid seismic survey, which produced results within two days, is crucial for the accurate positioning of the bore hole(s) and for remediation measures to be taken. The results of a total of six lines show that an E-W oriented fault produced a narrow channel filled with highly decompacted material (Fig. 6). It is assumed that the ground failure.was triggered by weathered marl collapsing into a karst cavity underneath the channel.

Refraktionsseimik (profil 2)
Fig 4a: Refraction seismic (profile 2)