DESCRIPTION OF THE AREA

The investigated area with a slope with an excavated crest constitutes the southern end of the municipality of Munkedal. In different contexts, it has also been called Torp övre, Kviström södra and the Åtorp area. The area is located on the west bank of the river Örekilsälven and extends from the Kviström bridge in the north to the Åtorp manor house in the south, Fig. 1.

The area is located in the Örekilsälven river valley, about 2 kilometres north of the river mouth in the Saltkällefjorden fjord in the northern part of the province of Bohuslän. The area was originally a plateau of sediments between the surrounding mountain ridges, which rise to a height of about 70 m above sea level on both sides of the plateau. The plateau nowadays has a level about 20 m above the mean sea level. Through the years since the plateau rose above sea level, the river has eroded its winding channel down through the sediments and created steep slopes on the riverbanks. This has resulted in numerous slides in these slopes. Most of these have been relatively shallow slips extending only a few metres in from the crest.

However, the topography of the area indicates that larger slides have occurred too, among them a large quick clay slide in the northern part of the area.

Just south of the Kviström bridge, the river bends from running in a south-easterly direction to flow towards the north-east. The topography of the ground south-west of this bend indicates that a quick clay slide has occurred with its outflow in the river bend. About a hundred metres further downstream, the river makes a sharp turn of about 120 degrees and starts running in a south-south-westerly direction. It then continues for about 700 metres in a smooth shallow bend towards a southerly direction past the Åtorp manor house, whereupon it turns towards south-west. The area of the investigations is thus a geographically restricted, slightly protruding peninsula into the river Örekilsälven. The seasonal variation in water transport in the river is large, and the maximum variation in water level is about 3 metres.

Fig. 1. Map of the investigated area in southern Munkedal from 1978. In this map, the river Örekilsälven is designated as Kviströmsälven.

Copyright Lantmäteriverket 2003. From The Property Map reference no. M2003/5268.

Valid to 2007–09–30.

The Bohusbanan railway line runs on the western side of the plateau, about 200–

300 metres behind the location of the slope crest before the excavation. On the other side of the valley runs the main road between Göteborg and Oslo, the European highway E6. The buildings on the plateau today consist of the Åtorp manor house, which is a hotel and conference centre, a carpentry factory and a few dwelling houses. In earlier times, as shown in the map, there was also a cement works and a larger number of dwelling-houses, but the works has been closed since the excavation took place and the houses were demolished in connection with the stabilising measures.

The stability in the area has been investigated to a limited extent in connection with different extensions of primarily the industrial premises and Åtorp manor house.

It has then been pointed out that the stability of the slopes was low and, already in the 1950s, an investigation by Caldenius suggested that no buildings should be placed closer than 100 metres to the crest for safety reasons. In that investigation, it was also proposed that the whole stretch along the river should be supplied with erosion protection to stop the ongoing erosion process. Later investigations cited and repeated these proposals, but nothing happened until a slide occurred in the slope directly beneath the main building of Åtorp manor house in 1980. This prompted an investigation that was carried out by Bohusgeo AB, which showed that the stability was clearly unsatisfactory in this part of the area. It was also pointed out that there were many sections in the area with more unfavourable geometrical conditions than in the investigated part. It was therefore apprehended that the stability was very low in other parts of the slope along the river as well.

This led to further investigations, which were carried out in co-operation between SGI and Bohusgeo AB (Swedish Geotechnical Institute 1985). Eventually, the results led to an investigation of the entire length of the slope towards the river in this area and also in a large area in the central parts of the municipality. The outcome of the investigations led to an intervention by the Swedish Rescue Service Agency in order to secure the safety in all of this extended area. The so-called “Munkedal works” comprised large excavations and earthmoving operations in the central part of Munkedal, where the whole river channel was moved (SVT 1985, NCC 1985).

They also comprised large excavations and construction of erosion protection on the riverbanks in the area of the present investigation, Figs. 2 and 3. The slope directly beneath the Åtorp manor house was reshaped and flattened by a combination of excavation and filling. In connection with these works, a number of landed properties were bought up and the buildings on these were demolished. The size of the excavations was partly limited by the existing cement works, which at that time

Fig. 2. Ongoing erosion and small slides before construction of the erosion protection.

Fig. 3. Carrying out of excavation works.

was still in operation. This industry has later been closed and these buildings have also been demolished.

In the southern part of the Torp area, there was a small wood of beech trees growing in the slope and the area just behind the crest. The wood was protected by an environmental decree but it became necessary to cut it down in order to enable the implementation of the stabilising measures. A plan for replanting was created in order to re-establish the character of the area with a leafy wood consisting primarily of beech trees. The plan sought to take into account both environmental interests of re-establishing the vegetation and geotechnical interests of avoiding trees catching a great deal of wind being placed at unsuitable locations in the area with regard to local stability.

2.2 GEOLOGY

No thorough geological investigation, like e.g. those in the Göta-älv valley and at Tuve (SGI Report No 11b), has been made in the Munkedal area. Nor is the area included in any recent geological mapping with an accompanying description of the area. It is only briefly mentioned in an old survey by Lindström (1902).

However, the general outline of the geological history is the same as for the Göta-älv valley and other valleys with marine clay deposits in the Göteborg and Bohuslän area.

The area was covered by inland ice during the last glaciation and the deposition of sediments started when the retreating ice front passed the area. This occurred about 12,400 years ago. Fine-grained particles in the melt-water then started to accumulate by sedimentation on top of the bedrock or on top of a layer of till which had been deposited below the ice cover. The soundings in the Torp area that have reached firm bottom layers indicate a layer of till on top of the bedrock, and the pore pressure measurements in the same section indicate that the layer is continuous. This section, designated Section A, is located in the northern part of the area, see Fig. 4. The firm bottom in this section slopes down towards the location of the river channel, and the maximum thickness of the sediments is here about 60 metres. In a section located 200 metres further south, none of the penetration tests has reached firm bottom and the maximum thickness of the sediments here can only be estimated to more than 70 metres. Later geophysical investigations have confirmed that the bedrock surface slopes downwards towards the south and have indicated that the thickness of the till layer varies from a metre or so to about 10 metres.

The sediments were precipitated in sea water, whose salt content and temperature varied with depth and the distance to the ice front. Normally, the soils in the lowest layers are coarser with infusions of silt and sand layers, but the clay content then starts to increase continuously upwards in the profile. The lowest layers also normally contain sulphides in sufficient quantities for the clay to be significantly banded, striped or flamed by black sulphide colour. At the start of the deposition, the sea level was more than a hundred metres above its present level in relation to the bedrock, but the depths gradually became shallower as the ice front moved away and the isostatic uplift of the land progressed. Postglacial sediments then started to overlay the glacial deposits. When the area had become even shallower, the river had been created in the higher areas to the north and the eroded particles transported by this started to deposit at and some distance away from the river mouth. The very fine-grained sediments then started to be overlain by new sediments, which became coarser and coarser as the shoreline and river mouth approached the present area. When these passed the area, a delta was probably created, similar to that which can be seen today at the outlet of the river into the fjord about 2 kilometres downstream. Varying water transports and water levels resulted in different kinds of lateral fluvial sediments being deposited over the area around the river mouth. The thickness and grain sizes of these sediments were in principle largest around the main river channel and decreasing sideways, but large variations occurred due to the shape of the delta and its variation during the time of deposition of delta and lateral fluvial sediments in the area. From a certain level, the clay in the deposits thus started to become gradually coarser upwards. They were then overlain by silt and sand and in some spots even by gravel. These upper deposits are normally layered. Deposits of silt and sand at the top with a thickness of more than 10 metres have been found within the investigated area. The normal picture before the excavation was that these deposits had a thickness of about 6 metres at the crest of the slopes and then gradually became thinner towards the valley sides.

However, large variations have been found between the two sections in this investigation and also in the different previous investigations. The delta and lateral fluvial sediments also contain varying amounts of organic material.

In this type of deposits, it is common that seams and layers of coarser wave-washed material are found embedded in the fine-grained material along the valley sides as a result of changes in climate and sea level during the period of deposition. This has not been reported in this particular area, which may be due to the fact that no investigations have been performed close to the valley side. However, in the southernmost section in this investigation, a continuous layer of coarser soil was found at a depth of 50–55 m below the original ground surface. This layer may be

assumed to be a result of a temporary re-advance of the ice front during the protracted period of deglaciation, as described by Stevens (1987).

Since the area rose above sea level, the river has eroded its channel down through the sediments and the river bottom now lies more than 20 metres below the level of the plateau behind the slope crest. A number of slides have occurred in the slopes created during this process. The slide debris has mainly been eroded away by the river and has not significantly affected the soil stratification. During the same time, leaching has gradually reduced the salt content in the pore water in the soil. This process is most pronounced for clay layers in which the distance to a draining layer is relatively short. The process has thus advanced furthest at the valley sides whereas the original salt content is better preserved in the thicker clay layers.

Leaching may entail that the shear strength becomes reduced, but primarily that its sensitivity to disturbance increases and that the remoulded shear strength decreases.

Quick clay has thus been found in parts of the Torp area where the clay layers are thinner, which here occur only at a certain distance from the river. However, the river runs very close to the valley side in the northern part of the area, and this is where indications of an old quick clay slide are found.

2.3 PREVIOUS INVESTIGATIONS AND