The test site

The experimental site is situated in the alluvial plain of Meuse river, near the centre of Liège city, Belgium (figure 6.20). At this location, the valley is about one kilometre wide and the river divides into two branches (the main stream and its derivation), delineating a lenticular island of 1.5 km long and 500 m large. The test site, which is a flat and unbuilt zone with a triangular shape of 200 m side, is located at the North-East end of the island (figure 6.20). It is surrounded by several streets and one main road on its South-East side along the Meuse derivation (figure 6.20).

The geology structure below the city centre is made of alluvial layers overlying a shaly Paleozoic basement. The layer geometry and properties are well documented on geotechnical maps (Fagnoul (1975)) gathering the existing information (mainly Cone Penetration Tests and

Figure 6.20: Local map of the test site. Th grey squares represent the locations of boreholes. The Cone Penetration Test is marked by a black cross inside a grey square. The North-South and East-West $P-S_V$ profiles are shown with thin black lines.

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borehole data). Twelve boreholes were drilled to the bedrock in the neighbourhood of the test site. They are reported in figure 6.20 and their logs are summarized in table 6.6. Three types of soft sediments are encountered from top to bottom: backfills of varying thickness (from 2 to 8 m), fluvial silts or clays of irregular distribution (down to 6 m depth, sometimes completely replaced by backfills), and finally a few metres of sand and/or gravel overlying the bedrock. The top of the bedrock was found at a depth ranging from 10.5 to 13 m. It is made of Wesphalian shales and sandstones with numerous coal veins, intensively exploited during the XX$^{th}$ century.

The bedrock depth reported by the borehole descriptions may not correspond to a sharp increase of the seismic velocity, due to the presence of a few metre thick weathered rock layer Jongmans and Campillo (1990). In the absence of specific information about the seismic properties at the test site, we conducted active seismic prospecting along two profiles oriented North-South and East-West (see figure 6.27 for location). Along each line were deployed twenty-four 4.5 Hz vertical sensors with a spacing of

Table 6.6: Borehole descriptions around the site (from geotechnical database). From B294 to B297, the silty layer is replaced by soft blue clays. Only B248 is included in the area investigated by arrays and geophysical experiments. Depths measured from surface are in metres.

Borehole	Backfills	Silts or Clay	Sand and gravel	Bed-rock	Water level
B248	0.0 to 2.7	2.7 to 6.0	6.0 to 10.0	-	-
B251	-	0.0 to 3.0	3.0 to 12.0	12.0	-
B252	-	0.0 to 2.5	2.5 to 10.6	10.6	-
B253	0.0 to 7.5	-	7.5 to 11.3	11.3	-
B254	0.0 to 5.5	-	5.5 to 10.3	10.3	-
B255	0.0 to 4.5	-	4.5 to 10.3	11.3	-
B294	0.0 to 3.6	3.6 to 6.2	6.2 to 13.0	13.0	-
B295	0.0 to 3.9	3.9 to 4.6	4.6 to 12.6	12.6	3.9
B296	0.0 to 2.0	2.0 to 4.4	4.6 to 12.7	12.7	3.5
B297	-	0.0 to 2.8	2.8 to 11.0	11.0	2.8
B298	0.0 to 2.7	2.7 to 4.9	4.9 to 11.3	11.3	-
B299	0.0 to 8.5	-	8.5 to 13.0	13.0	-

2.5 m and the waves were generated with a hammer and an explosive source. An example of signals generated by an explosive source is given in figure 6.21. P-wave arrival times and Rayleigh waves are clearly visible and were inverted to obtain $V_p$ and $V_s$ profiles, respectively.

Figure 6.21: Recorded signals for East-West $P-S_V$ refraction line, West source. First P-wave are visible on the left with small amplitudes. Surface waves develop between 0.1 and 0.4 seconds and constitute the most energetic part of the signal.

Figure 6.22: Refraction results obtained with travel time NA inversion for profile East-West. (a) to (d) $V_p$ profiles obtained by inversion for South, Central towards South, Central towards North, and North shots, respectively. (e) and (h) corresponding calculated traveltime-distance curves (in the same order). The black dots and the error bars are the experimental times picked on recorded signals.

Figure 6.23: Refraction results obtained with travel time NA inversion for profile North-South. (a) to (d) $V_p$ profiles obtained by inversion for West, Central towards West, Central towards East, and East shots, respectively. (e) and (h) corresponding calculated traveltime-distance curves (in the same order). The black dots and the error bars are the experimental times picked on recorded signals.