Location of the new investigations

The new investigations in this project have been located in a section through the central parts of the excavated area. They have been performed at six points, at four of which detailed investigations with field tests and sampling were performed whereas only pore pressures were measured at two. The four points with detailed investigations were placed in such way that one is located between 20 and 25 metres out in the river, one is located in the riverbank, one is located at the centre of the excavated terrace and one is located in original ground about 25 metres behind the new upper crest. The desire was to find points with as well defined conditions as possible, which at the same time were easily accessible and did not require intrusion on private properties or obstruction to the ship traffic in the fairway. It was therefore not possible to place the points in a straight line perpendicular to the river. Instead, the points on land were placed in a line with an oblique angle towards the river. At the riverbank, this line bent backwards about 90° towards the investigation point in the river, Fig. 86.

The points were numbered from 1 to 6. Point 1 is in reality a short line about 20–

25 metres out in the river. A raft was borrowed from the fairway authorities and was moved along this line in such way that it did not interfere with the fairway in the deep channel. The raft was fixed in position along the river using cramp irons in the protruding point of the erosion protection at the old access road and the bollard holding the marker for the side of the deep channel. A large number of bottom anchors and ropes to firm objects ashore were also used to fix the raft sideways, Fig. 87.

Point 2 is located slightly inside the erosion protection at the former access road, which has been mentioned earlier. The shoreline here protrudes into a small cape and the point is located approximately in the line of a straightened-out riverbank, Fig. 88. All test holes at this point had to be pre-drilled about 2 metres to ensure that no stones or old fill material would damage the test and sampling equipment.

Fig 85.The area south of the remaining buildings in the Strandbacken area. The rocky hills, which constitute the western boundary of the Göta-älv valley, are seen in the background. a) Open farmland up to the main road and new houses on the other side of the road. The border of the remaining built-up area in Strandbacken is seen at the right side of the picture.

Fig 85.The area south of the remaining buildings in the Strandbacken area. The rocky hills, which constitute the western boundary of the Göta-älv valley, are seen in the background. b) The upper crest and open farmland towards the south.

Fig. 86. Plan of Strandbacken with the present excavated terrace, remaining buildings and investigation points.

Fig. 87.The raft and drill rig in position in Point 1 out in the river. a) View from the western riverbank. The bollard used for positioning is seen to the right in the picture.

Fig. 87. The raft and drill rig in position in Point 1 out in the river.

b) Preparations for a dilatometer test with automatic recording of test data in point 1. The bridge at Lilla Edet and the entrance to the lock at Ström can be seen in the background to the left of the drill rig.

Fig. 88. Point 2 at the riverbank. The pipes for the pore pressure measurement systems are seen on the left hand side of the area with flattened reeds.

The other investigations have been performed to the right of these pipes.

Point 3 is located about 4 metres inside the crest of the lower slope. Only pore pressures are measured at this point. The location is selected to place the systems close to the crest but at the same time in a protected position where they do not interfere with the road and trail. The location is shown in Fig. 81b, where the top of the pipes can be seen among the birch trees to the right of the road.

Point 4 is located at the middle of the flat excavated terrace. It is placed in one of the bare spots where no vegetation has been established. The spot is seen at the right side in Fig. 81c.

Point 5 is located at the toe of the upper slope from the rear of the excavated terrace to the natural ground surface. The position is shown in Fig. 89.

Fig. 89.The rear end of the excavated terrace with the pipes of the pore pressure measur- ing systems in Point 5 placed at the toe of the upper slope.

Point 6, finally, is located about 25 metres behind the crest of the upper slope in what until recently has been cultivated farmland. The only change that has occurred in this area is that a contractor has bought the land and that a few decimetres of topsoil have been scraped off and sold to gardeners. The pipes for the pore pressure measuring systems are seen in Fig. 85a near the side of the road, where they are out of the way. The other investigations at this point were carried out further out on the open field.

The points in this section are located between sections 37.43 and 37.50 in the Göta-älv investigation.

3.7.3 Field tests