CHANGES IN SHEAR STRENGTH

An attempt was made to trace possible changes in the undrained shear strength because of the excavations by performing new field vane tests in those points where such tests had been performed in the investigations prior to the stabilisation works.

This was done at Points 2 and 3 in Section A, Point 9 in Section C and Point 22 in the northern part of the area.

Point S2 is located on the excavated terrace in Section A where about 5.5 metres of soil have been removed. A comparison between the tests before and after the excavation shows a certain decrease in shear strength down to a depth of about 5 metres below the present ground level, Fig. 52. The values are then about equal at greater depths. Below the level 0 metres, the values from the previous investigations are considerably lower. This is probably a result of a change to a smaller vane size.

When the values are corrected for the effect of a such change, which was measured in the special project concerning these effects, the values from the two investigations become about equal also below this level.

The corresponding comparison at Point 3 behind the upper crest in the same section shows very small changes, Fig. 53. The possible changes in this point are in the opposite way and the measured shear strength rather tends to increase after the performed actions. This is not quite impossible since the groundwater level has been somewhat lowered at this point, resulting in an increased effective overburden pressure, but the possible effect should be marginal.

An additional comparison was made in Section A. Field vane tests had previously been performed at the riverbank close to Point S1. In the new investigation, field vane tests were performed below the river bottom where another about 2 metres of erosion has occurred. The comparison shows marginally higher strength values below the riverbank, Fig. 54. However, the tests below the river bottom have been performed from a floating raft. The test levels are therefore not quite exact and a minor adjustment of the curves level-wise would bring them to almost coincide.

Point S22 in the northern part of the area is compatible with Point S2 since it is located on the excavated terrace and the thickness of the excavated soil layer was about the same. At this point, SGI had performed investigations before the excavation including field vane tests. Here, almost identical results were obtained in the new field vane tests, Fig. 55.

Fig. 52. Comparison between shear strength values measured by field vane tests before and after excavation at Point 2 in Section A.

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Fig. 53. Comparison between shear strength values measured by field vane tests before and after excavation at Point 3 in Section A.

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Before excavation and lowering of groundwater table After excavation and lowering of groundwater table " "

-Fig. 55. Comparison between shear strength values measured by field vane tests before and after excavation at Point 22 in the northern part of the area.

Fig. 54. Comparison between shear strength values measured by field vane tests below the riverbank and the river bottom in Section A.

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2m up on land, ground level +0.8 m Below river bottom, bottom level ca -1.3 m

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-Fig. 56. Comparison between shear strength values measured by field vane tests before and after excavation at Point 9 in Section C.

Point S9 in Section C is located on the upper excavated terrace where about 6.5 metres of soil were excavated. A draining base layer was then laid out and covered and the resulting difference in level before and after is about 6 metres. A clear difference in shear strength values between the tests before and after the stabilisation works is measured at this point, Fig. 56. The effect reaches down to about 6.5 metres below the present ground surface. The comparison is rendered somewhat difficult since the difference measured is located fully in the upper soil layer consisting of silty clay with infusions of sand and organic matter.

These comparative field vane tests do not show without doubt that a significant change in the shear strength has occurred as a result of the excavation. There are a number of indications that this has happened, but these are relatively weak and subjected to possible sources of error. At one point, there were no detectable changes at all. The uncertainty is partly due to the fact that field vane tests had only been made in single boreholes at each point in the previous investigations and no comparison of mean values and standard deviations can be made.

The excavations were fairly large, but the depth interval within which a measurable effect can be expected is still limited, among other things because of the adjoining lowering of the groundwater level. The soil also consists mainly of clayey silt and very silty clay with different kinds of infusions on those levels where a significant change ought to have occurred, which makes the interpretation more difficult.

However, together with all other comparisons between measured shear strengths at different points in the slope and with different methods, it can be considered established that a reduction in shear strength occurs when the soil is unloaded, particularly in superficial layers. The size of the measured reduction depends on what test method and what interpretation is used. The reason why different shear strengths are measured by different methods, particularly in the lower parts of slopes, is being investigated at present in a separate joint research project between the Swedish Rescue Services Agency, Chalmers University of Technology and SGI (Löfroth 2002).

2.9 STABILITY CALCULATIONS